CHP – Cogeneration Power
CHP ‐ Cogeneration Power
CHP – Cogeneration Power ........................................................................................................... 4 1.Introduction .............................................................................................................................. 4 1.1 Preface I .................................................................................................................................................. 4 1.2 Preface II ................................................................................................................................................. 5 1.3 Preface III ................................................................................................................................................ 6 1.4 Energy and power measurement ........................................................................................................... 7
2.Basics ........................................................................................................................................ 8 2.1 What is a CHP system? ........................................................................................................................... 8 2.2 Energy balances ...................................................................................................................................... 9 2.3 Main advantages of CHP ....................................................................................................................... 11 2.4 Basic CHP schemes ................................................................................................................................ 12 2.5 Fuels ...................................................................................................................................................... 13
3.Technology .............................................................................................................................. 14 3.1 Internal combustion engine .................................................................................................................. 14 3.2 Steam turbine ....................................................................................................................................... 15 3.3 Gas Turbine ........................................................................................................................................... 17 3.4 Combined cycle ..................................................................................................................................... 18 3.5 Stirling engine ....................................................................................................................................... 19 3.6 Fuel cell ................................................................................................................................................. 21 3.7 Organic rankine cycle ............................................................................................................................ 23 3.8 Heat storage .......................................................................................................................................... 25 3.9 Qualitative comparisons among prime movers .................................................................................... 26 3.10 Quantitative comparisons among prime movers ............................................................................... 27 3.11 How to pick the proper technology ................................................................................................... 28
4.How to size a CHP plant ........................................................................................................... 29 4.1 Load profile I ......................................................................................................................................... 29 4.2 Load profile II ........................................................................................................................................ 30 4.3 Sizing approaches .................................................................................................................................. 33 4.4 An example of CHP sizing ...................................................................................................................... 35 4.5 The power‐to‐heat ratio ....................................................................................................................... 38
5.Micro/mini CHP ....................................................................................................................... 40 5.1 CHP sizes ............................................................................................................................................... 40 5.2 mCHP concept and advantages ............................................................................................................ 41 5.3 Smart grids and virtual power plants .................................................................................................... 42
6.Trigeneration........................................................................................................................... 45 6.1 What is trigeneration? .......................................................................................................................... 45 6.2 Compression chillers ............................................................................................................................. 46 6.3 Absorption chillers ................................................................................................................................ 47
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CHP ‐ Cogeneration Power
6.4 Comparisons ......................................................................................................................................... 49
7.District heating and cooling ..................................................................................................... 51 7.1 Main concept ........................................................................................................................................ 51 7.2 District heating (DH) .............................................................................................................................. 52 7.3 District cooling (DC) .............................................................................................................................. 54
8.CHP and RES coupling .............................................................................................................. 56 8.1 Reneweable sources available for CHP ................................................................................................. 56 8.2 Major advantages ................................................................................................................................. 57 8.3 Main aspects of biomass fired CHP plants ............................................................................................ 58
9.Feasibility ................................................................................................................................ 60 9.1 Implementation .................................................................................................................................... 60 9.2 Costs ...................................................................................................................................................... 61 9.3 Costs‐effectiveness ............................................................................................................................... 63 9.4 Integration of the CHP unit ................................................................................................................... 65 9.5 Control and risks ................................................................................................................................... 66 9.6 Ostacles and barriers ............................................................................................................................ 68
10.Main CHP policies and conclusions ........................................................................................ 70 10.1 Main incentive mechanism ................................................................................................................. 70 10.2 Conclusions ......................................................................................................................................... 72
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CHP ‐ Cogeneration Power
CHP – Cogeneration Power 1. Introduction 1.1 Preface I It is early in the morning, and I have to go to work. As soon as my nose peeks out from behind my front door, I realise that it´s a pretty cold day outside. I jump in my car, and the interior is almost frozen. I switch the engine on, wait a few seconds, then turn the heater on and allow the warm air to flow from the little fans in front of me. I am ready to go. No, not yet. There is too much silence. The CD of my favourite band is still in the player. It´s enough to press a button to play the music that makes me feel better. Ok, now I can leave. Probably not many realise it, but that morning my engine efficiency gained quite a few percentage points. That is, my car was performing better than during a warm day, when I don´t need to warm the interior up. That morning my car engine suddenly became a combined heat and power (CHP) system! Indeed, starting from the chemical energy contained in the fuel, it provided me with mechanical power to move, electricity to listen to my favourite band, and heating to warm me up. First bedrock! #combinedproduction A cogeneration system is a device capable of producing electrical, mechanical and thermal energy (from recovery) directly in the place in which it is installed. The winter is Siberia can be quite a hard problem to cope with. Temperatures fall very easily to values lower than ‐60°C. The Republic of Yakutia (Sakha), situated in north eastern Siberia, is a land of countless rivers and lakes, hundreds of glaciers and ice crusts. It is full of natural contrasts: it has the longest and coldest winters and, at the same time, the summers can be extremely hot. The capital of the Sakha Republic is Yakutsk. The company Sakha‐Torg is based here. It has a problem that is pretty common nowadays: it needs an independent and environmentally friendly source of heat and power. Sakha‐Torg decided to install a set of micro turbines running on natural gas to meet its demand. Reliable operation and low maintenance costs were the main factors in making the choice. Currently, five micro turbines with heat recovery modules produce electricity and heat for shopping malls, offices and warehouses in the region 1 . Second bedrock! #safe&clean CHP systems are proven as a safe and clean source of heat and power in cold and remote regions.
1 ‐ Sakha‐Torg, Yakutsk, Russia – Capstone, 2008
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CHP ‐ Cogeneration Power
1.2 Preface II The Natural History Museum of London, besides being a fantastic and unmissable place and one of the many reasons why millions of Londoners should be proud of their city, is home to life and earth science specimens comprising some 70 million items. Given the age of the institution, many of the collections have great historical as well as scientific value, such as specimens collected by the father of evolutionary theory, Charles Darwin. The museum is also particularly famous for its exhibition of dinosaur skeletons and ornate architecture . In the years 2006‐07, the Museum decided to undertake an important challenge: make significant cost savings, make significant greenhouse gas emissions savings, avoid the dumping of heat, lower the environmental impact and maximise future potential. The whole had to be realised without referring to any miracle from the sky. The miracle was actually a compact installation composed of a gas fired internal combustion engine, with an engine cooling circuit incorporated as a heating and driving force for an absorption chiller 2. As a result, the Museum now saves 11,000 MWh of energy each year (the standard annual energy consumption of ca. 550 British families3), reducing the amount of CO2 released in the atmosphere of more than 2,800 annual tonnes and saving a huge amount of money: £750,000 for any single year of operation. Third bedrock! #savings&cooling CHP plants may be a reliable way for saving money and cutting CO2 emissions. Cooling systems can be easily integrated to the plant.
2 ‐ Mark Howell ‐ Design, Build, Finance, Operate Tri‐generation for the Natural History Museum – Vital Energi, 2009 3 ‐ Typical domestic energy consumption figures ‐ www.ofgem.gov.uk
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1.3 Preface III 2012 ‐ October, 22nd‐31st: hurricane Sandy is the deadliest and most destructive hurricane of the 2012 Atlantic hurricane season. In Jamaica, winds left 70% of residents without electricity, blew roofs off buildings, killed one, and caused about $100 million in damage. Sandy's outer bands brought flooding to Haiti, killing at least 54, causing food shortages and leaving about 200,000 homeless; the hurricane also caused two deaths in the Dominican Republic. In Puerto Rico, one man was swept away by a swollen river. In Cuba, there was extensive coastal flooding and wind damage inland, destroying some 15,000 homes, killing 11 and causing $2 billion in damage. Sandy caused two deaths and damage estimated at $700 million in The Bahamas. In Canada, two were killed in Ontario and an estimated $100 million in damage was caused throughout Ontario and Quebec. In New York City, Governor Andrew Cuomo declared a wide state of emergency and asked for a pre‐disaster declaration on October 26th, which President Obama signed later that day. Following the onslaught of the storm, over 8 million customers in New York and New Jersey lost power, with many outages lasting weeks. Over 250 large buildings were also without power for several weeks, and in many cases months. Economists estimate the economic loss to New York City alone to be around $20 billion. Nevertheless, during those days the New York University (NYU) Greenwich Village Campus maintained essential services. How was this possible? In 2004, the NYU planned for future infrastructure needs and identified the following goals: improve overall reliability, achieve energy independence, maintain price stability and increase operating savings. Through the adoption of a CHP plant, the savings achieved included $5 million annual reduction of the University's energy costs and an estimated 43,400 tonnes per year of CO2 emissions reduction4. Among other things, and with all due respect to the victims, Sandy can serve as a further reminder of how important energy is in our daily lives. CHP took a front seat during hurricane Sandy, proving its critical value in supporting resilience and ensuring reliable power and heat supply. Examples like the NYU cogeneration plant highlight the value provided by CHP/cogeneration during major weather events. While the majority of Manhattan was without power, most of NYU’s Greenwich Village campus had electricity, heat and hot water. NYU was able to generate its own electricity and heat via a 15 MW cogeneration plant. Fourth bedrock! #reliability CHP may prove to be of critical value in supporting resilience and ensuring reliable energy.
4 ‐ CHP as a Reliable Energy Model. A Case Study From NYU – International District Energy Association, 2013
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1.4 Energy and power measurement In order to strengthen the conclusions coming from the themes included here, a lot of adjectives will be used, but also (and especially) numbers. Crucial questions will arise: how much energy could each technology deliver, and at what economic costs and risks? Of course, we cannot just say “CHP is a money pit” or “We have a huge amount of biomasse”. The trouble with this sort of language is that it’s not sufficient to know that something is enough or not: we need to know how the one “enough” compares with another “enough”. To make this comparison, we need numbers, not adjectives. The unit of measure that will be most widely used in this treatise are the kilowatt (kW) for power, and the kilowatt‐hour (kWh) for energy. Maybe a good way to explain energy and power is by an analogy with water and water flow from taps 5. By referring to the next figure, if you want a glass of water, you actually collect a volume of water – one litre, perhaps (if you’re thirsty and your glass is pretty capacious). When you turn on a tap, you create a flow of water – one litre per minute, say, if the tap yields only a trickle; or 10 litres per minute, from a more generous tap. You can get the same volume (one litre) either by running the trickling tap for one minute, or by running the generous tap for one tenth of a minute. The volume delivered in a particular time is equal to the flow multiplied by the time. A flow is thus the rate at which a certain volume is delivered. If you know the volume delivered in a particular time, you get the flow by dividing the volume by the time. Here’s the connection to energy and power: energy is like water volume, and power is like water flow. For instance, whenever a modern kettle is switched on, it starts to consume power at a rate of 2 kW. It continues to consume 2 kW until it is switched off. To put it another way, the kettle (if it’s left on permanently, but don´t do it!) consumes 2 kWh of energy per hour; it also consumes 48 kWh per day. Furthermore, electrical and thermal energy will often be distinguished. Therefore, the electrical kilowatt (kWe) or kilowatt‐hour (kWhe) will be used for the former, and kWt and kWht for the latter.
5 ‐ MacKay ‐ Sustainable Energy Without the Hot Air – UIT Cambridge, 2009
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2. Basics 2.1 What is a CHP system? Imagine an oil tanker navigating across a sea – e.g. the Mediterranean – heading from Arabia toward Europe for some re‐supply. How would you feel if you were told that almost three quarters of that oil is going to be totally wasted, discarded and never ever used? You would not actually be pleased about the amount of wasted fuel. Indeed, a traditional fossil fuel burning electricity plant has an efficiency varying between 25% and 45%, depending by the fuel usage and load conditions. According to IEA 6, in 2010 around 70% of the world’s electricity generation was fuelled by fossil sources (coal, oil, natural gas). There is something even worse: a typical car engine works mostly at part load, especially if it runs across a traffic congested city. Average efficiencies of such engines swing between 8% and 15%. These are the main problems that remain in the background of combined heat and power concepts. A cogeneration system is a device capable of producing electrical, mechanical and thermal energy, directly in the place in which it is installed. The term "cogeneration" takes its origin from the fact that a system is able to combine thermal energy recovery contingent on the electric energy production. A CHP system consists of a number of individual components – prime mover (an engine, for instance), generator, heat recovery system, and electrical interconnection – configured into an integrated whole. The type of equipment that drives the overall system (i.e. the prime mover) typically identifies the CHP system. As it will be shown in the next chapter, prime movers for CHP systems may include different kinds of reciprocating engines, diverse types of turbines and new technologies like fuel cells. These prime movers are capable of burning a variety of fuels, including natural gas, coal, oil, plus alternative fuels, to produce shaft power or mechanical energy. The mechanical energy from the driving equipment is most often used to drive a generator that produces electricity; but it may also be used to drive rotating equipment such as compressors, pumps and fans. Indeed, thermal energy from a CHP system can be directly used for heating purposes, or indirectly to produce steam, hot water or hot air for some specific processes. A further alternative is to use heat for producing chilled water for cooling purposes.
6 ‐ Key World Energy Statistics – IEA, 2012
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CHP ‐ Cogeneration Power
Basic scheme of a cogeneration system. Source: BKWK. Schrift größer. Unite usw.
2.2 Energy balances The aim of any cogeneration system is to self‐produce electrical energy and recover heat from the prime mover´s cooling fluids and exhaust gases and deliver it to users linked to the system. In respect to a traditional installation, advantages are many, from an energetic, environmental impact and economical point of view. In order to appreciate such advantages, it is necessary to compare the energy balances involved. As commonly accepted, the efficiency of electrical energy production of traditional power plants (powered by fossil fuels) is generally lower than 40%, while it is larger than 50% for new power plants based on combined cycles. Therefore, from 100 units of energy input and fuel in the system, about 40 units of electrical energy are produced. Even an electricity distribution grid will have an efficiency lower than 100%, so, amongst the 100 energy units, only about 30 electrical units arrive with users. Thermal energy produced by the system is generally wasted, except for rare cases of district heating (a technique for recovering part of thermal energy and reusing it for distribution in a neighbouring district), and ao not used for domestic heating or for producing domestic hot water. Conventionally, thermal energy is produced locally through a boiler, the efficiency of which is about 90%. In the case of cogeneration, heat produced by the prime mover and a great part of the heat of the hot exhaust is recovered. In this way, the system efficiency increases up to global values of about 85%, as schematically shown in the following figures. That is, by adopting combined heat and power, only a little bit more than one tenth of our oil tanker cargo is wasted (see 2.1).
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CHP ‐ Cogeneration Power
Traditional generation of heat and power, efficiencies and energy balances. Source: RENAC.
CHP generation, efficiencies and energy balances. Source: RENAC.
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2.3 Main advantages of CHP The main obtainable benefits when a CHP unit works properly are:
Increased efficiency of energy conversion and use. Lower emissions to the environment of all the main greenhouse gases. In some cases, biomass fuels and some waste materials, such as refinery gases, process or agricultural waste, can be used. These substances can easily fuel a CHP plant and, if available directly in the place where the system works, may largely increase the cost‐effectiveness, especially by reducing the costs for fuel supply and waste disposal. A real opportunity to develop more decentralized forms of power generation, where plants are designed to meet the needs of local consumers, providing high efficiency, avoiding transmission losses and increasing flexibility in system use. A real opportunity to increase the diversity in electricity generation and to provide competition in the power generation market.
On the other hand, the main disadvantages are mainly linked to accurate sizing, since the energy production needs to match as much as possible the energy requirements: the availability of an expert and reliable know‐how is crucial. Furthermore, a CHP installation is generally coupled with high capital costs and maintenance efforts. Strictly speaking, CHP is not a renewable technology, but just a more efficient way to generate energy. Nevertheless, a CHP system easily creates the basis for a decentralized energy structure that may even be fuelled by local renewable energy sources. For instance, stationary gas engines can run on a variety of fuels which can include biodiesel, plant oil, biogas or biomass possibly locally produced. If these alternative fuels are not available, a fossil fuel is used but nevertheless what is produced is a far more efficient solution, though not renewable. It is obvious to say that, if we are going to generate electricity, it is better to do it in the most efficient way possible. In essence, CHP can easily provide the initial framework towards a renewable energy power and heat system.
A CHP unit engine based. Source: Wikimedia.
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CHP ‐ Cogeneration Power
2.4 Basic CHP schemes Generally speaking, a CHP system can be schematized by means of two main cycles: the topping and the bottoming cycle. According to the next figures, in the topping cycle the electricity (or mechanical power) is produced first, and then heat is recovered to meet the thermal loads of the facility. It is generally found in facilities which do not have extremely high process temperature requirements. The basic Brayton and Rankine cycles work as topping cycles. Indeed, in the bottoming cycle the thermal energy is the main desired product and it is produced directly from the combustion of a fuel. This energy usually takes the form of steam that supplies process heating loads. Process heat waste is always present, can be heat recovered and used as an energy source for running a turbine, producing electric or mechanical power. Common systems that use this cycle are industrial applications with high temperature processes such as steel reheat furnaces, clay and glass kilns and aluminium re‐melt furnaces.
The topping cycle. Source: Energy Efficiency Indicators for Public Electricity Production from Fossil Fuels – IEA, 2008
The bottoming cycle. Source: Energy Efficiency Indicators for Public Electricity Production from Fossil Fuels – IEA, 2008
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2.5 Fuels The choice of a proper fuel is crucial for a CHP unit. Depending on the choice of technology, a wide range of fuels can be used. Several aspects have to be considered, like present and likely future costs, availability, possibility of storage and transportation, energy content, emissions, etc. Identification and specification of the intended fuel is essential at the outset and the system must be designed around the available fuel supply, also taking into account any variation in quality. In particular, the energy content of a fuel is expressed through its Higher Calorific Value (HCV). This is the total amount of heat per unit of mass or volume produced by the direct combustion of a fuel when considered as totally dried (the water of combustion is entirely condensed and the latent heat contained in the phase change from vapour to water is recovered). Less used is the lower calorific value, where the products of combustion contain the water vapour plus the unrecovered heat in the water vapour. As is evident, a fuel with a higher HCV will provide more energy per unit of mass (or volume).
Calorific value, density and emissions of the most common fuels Fuel
HCV (kWh/kg)
Density (kg/m³)
CO2 content (%)
8
160‐200
13
Coke
7.9
360‐470
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Wood chips (50% moisture content)
2.4
350
14
Peat (50% moisture content)
2.5
400
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Natural gas
13.3
0.81
11.7
Hydrogen
33.3
0.09
0
Kerosene
11.9
800
12
Fuel oil
11.9
840
12
Charcoal
Source: http://www.erab.com/skiss/uk68.pdf
As already mentioned, it is not always possible to pick the fuel with the highest energy content or the lowest CO2 content. Diverse aspects play a role as also local availability and the costs involved have to be taken onto consideration. CO2 generation during the burning of fossil energy sources, in kg CO2 per kWh of fuel consumption. Source: ASUE.
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3.Technology 3.1 Internal combustion engine An internal combustion engine, which is what almost all cars and trucks on the roads nowadays have, uses a four stroke (or two stroke in some cases, but it is not common for CHP plants) process to convert fuel into motion. The four stroke processes of an engine are: intake, compression, combustion and exhaust. The operation is actually pretty similar to that of a human heart, except for the combustion part. The intake stroke is when the intake valve opens and the piston moves down to allow a mixture of fuel and air into the cylinder. When the piston moves upward and reduces the area in the cylinder, thus compressing the oxygen and fuel mixture and giving the explosion from the ignition power, the compression stroke is completed. In the combustion stroke the piston reaches the top of the cylinder and the spark plug emits a small spark able to ignite the oxygen and fuel mixture. This results in a little explosion occurring in the cylinder, which pushes the piston downward. Once the piston reaches the bottom of the cylinder the exhaust stroke begins: the exhaust valve is open and the exhaust gases leave the cylinder, thus completing the cycle. Internal combustion engines use traditional spark‐ignition engines (as used in cars and small electricity generators) to provide a certain amount of power. In many CHP applications these are converted to operate on natural gas or biofuels. For CHP installations, at full load the electrical efficiency is typically 25‐40%, with efficiency increasing with size. The heat produced is usually hot water, rather than steam, and they generally produce 1‐2 units of heat for each unit of electricity, with the ratio of heat to power generally decreasing with size . Internal combustion engines for CHP are typically 70‐1500 kWe in size (but are available up to about 5 MWe and down to 5.5 kWe) and are best suited to non‐industrial smaller sites where most of the demand is for hot water. They are usually used in packaged CHP units, along with heat exchangers to recover heat coming from the numerous waste heat sources. These essentially include: engine cooling circuit, engine exhaust gases (that provide the highest temperatures) and oil circuit 7.
Typical efficiency and losses of an internal combustion engine. Source: Schaumann, Schmitz ‐ Kraft‐Wärme ‐ Kopplung, 2010
7 ‐ Introducing Combined Heat and Power – Carbon Trust, 2010
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Example of CHP scheme with an engine as prime mover. Source: ASUE.
3.2 Steam turbine Steam turbines are slightly different to other CHP prime movers, as they require a separate boiler to create a working fluid in the form of steam. In CHP applications, a boiler generates steam which is put through a steam turbine. The steam turbine produces electricity and the remaining exhaust steam can be used to provide energy to generate hot water or for heating/cooling purposes. The basic process behind steam power generation is the Rankine cycle. Water is heated in a specific boiler. The temperature and pressure are such that the water turns into steam. The steam is then transferred to a turbine where the pressure of the steam is reduced (usually to sub atmospheric pressures) by expansion over the blades in a condenser. If the turbine is coupled to a generator, this process produces electricity. The exhaust steam is condensed back to a liquid. This water, referred to as return water, is mixed with new water, referred to as feed water, and pumped back to the boiler in order to repeat the cycle. Such turbines are particularly appropriate for CHP when steam is needed, or where the fuel available cannot be burned directly in the prime mover. They are typically suited to large‐scale applications or where the amount of heat required is much greater than the amount of power. Waste heat from a steam turbine can be used for space heating or cooling (see trigeneration), for processes, or it can be used to create chilled or hot water. The huge steam turbines used in large coal and nuclear power stations have average electrical efficiencies of about 36‐38%.
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But in CHP applications, where the steam extraction reduces their electrical output, they have typical electrical efficiencies of 11‐20%. However, the overall efficiency of a steam turbine based CHP system ranges between 78‐83% 8.
CHP plant scheme with a steam turbine as a prime mover. Source: http://www.retscreen.net/ang/equipment_for_combined_heat_and_power.php
8 ‐ Introducing Combined Heat and Power – Carbon Trust, 2010
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3.3 Gas Turbine The main elements of a gas turbine are a compressor, which compresses incoming air, a combustion chamber where a fuel is burned with the incoming air, and a proper turbine to convert the energy of the hot and high pressure exhaust gases into power. The gas turbine can be seen as an internal combustion engine employing a continuous combustion process, instead of an intermittent one. The basic description of a gas turbine operation is the Brayton cycle. Incoming air at ambient conditions is drawn into the compressor, where its temperature and pressure are raised. The next step is the combustion chamber, where the fuel is burned so that the air arrives at high temperature and pressure. The resulting exhaust gases then enter the turbine, where expanding they move the blades, producing power. The exhaust gases leaving the turbine are not recirculated, causing the cycle to be classified as an open cycle. Nevertheless, there are some applications where this open cycle can be modelled as a closed cycle: since the temperature of the exhaust gas leaving the turbine is often considerably higher than the temperature of the air leaving the compressor, the latter can be heated by transferring heat to it from the hot exhaust gases. This is normally done through a proper heat exchanger, which is also known as a regenerator or a recuperator. According to the CHP concept, heat from a gas turbine’s exhaust gases can be easily recovered and used for space or process heating. This technology is usually employed in large‐scale schemes (larger than 1 MWe) although there are small‐scale turbines of between 80 kWe and 100 kWe available as packaged CHP systems. Their electrical efficiency ranges from around 21% for smaller turbines, to 25% for standard turbines of around 1 MWe, and up to about 36% for very large turbines (above 100 MWe). Even if gas turbines normally have a higher electrical efficiency than steam turbines, they require a cleaner fuel. However, they typically have lower electrical efficiencies than internal combustion engines, but are smaller and require less maintenance 9. A CHP plant scheme with a gas turbine. Source: Enertwin.
9 ‐ Introducing Combined Heat and Power – Carbon Trust, 2010
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Scheme of a gas turbine. Source: ASUE.
3.4 Combined cycle The combined cycle is one of the most used nowadays in power plants, and forms a hybrid which includes the Brayton Cycle on the topping portion and a standard Rankine Cycle on the bottoming side. The high temperature exhaust from a gas turbine is reused to generate high pressure steam which then passes through a steam turbine to generate more power. Clearly, this combination provides higher efficiencies than single cycles, up to 55%, and is typically used in large‐scale power generation. Heat can still be recovered from the steam turbine cycle for further applications, exactly as in simple steam turbine installations. For the higher efficiency and power generated, of course combined cycles are very attractive from an economic point of view. Indeed, in the last forty years combined cycle power plants have greatly influenced the power generation industry. Presently, about 90% of the newly constructed power plants are combined cycle power plants. By recovering useful heat at different stages, all the combined cycle power plants can be ideally applied for cogeneration to further enhance their fuel utilisation. Since the efficiency of the turbines can drastically decrease if heat is recovered at an early stage (i.e. from the incoming hot flow), here it is particularly important to properly judge the optimal solution for providing electric power and heat.
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Scheme of a CHP plant with combined cycle (HRSG stands for: heat recovery steam generator). Source: retscreen.
3.5 Stirling engine In addition to the more established types of prime mover, Stirling engine, fuel cell and ORC based CHP are emerging in the global market, but are still under development. The concept of this Stirling engine is fairly simple: referring to the following figure (where the 4 different phases are examined), the engine consists of a heat exchanger, an enclosed cylinder, two pistons, and a flywheel connecting the two pistons by a set of linkages. The main idea is that the heated gas expands the enclosed cylinder and moves the heat piston. While the heat piston is moving, the other cold piston is also moving and compressing the cold gas. The linked flywheel is set to motion by the connected pistons thus converting the thermal energy captured by the air into mechanical motion. The flywheel then stores this mechanical energy and allows the mechanical energy to be transferred to the generator to produce electricity. The geometry of the linkages determines the relationship between the motion of the cold piston and the heat piston.
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One of the advantages of a Stirling engine is its distinctive combustion process. It is external combustion that allows the engine to run on a multitude of fuels. Essentially, anything that burns can be thrown in the combustion chamber for fuelling a Stirling engine. Nevertheless, the most widely used fuels still include gasoline, diesel, natural gas, propane and biogas. Another advantage of an external combustion chamber is that the fuel does not have to be refined as it does for other types of engine with internal combustion. The external combustion is a process that also allows a complete combustion, resulting in less unburned hydrocarbons emitted with the exhaust gases. Concerning CHP technology, Stirling engines are one of the most cited prime movers in small‐ and micro‐scale CHP. As any other CHP system, even Stirling engine micro CHP installations convert primary energy into electricity and heating simultaneously but, given their size, only for residential purposes. While the energy conversion from fuel energy to electric power is done by a proper Stirling engine, the heating of water and space is done by utilising waste through a conventional heat exchanger at the cold piston. The micro regime is typically designated to systems where power generation is less than 3 kW.
Diagrams of a Stirling engine. Source: http://www.animatedengines.com/vstirling.html
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Figure 2K – Scheme of a general CHP plant with Stirling engine. Source: http://asue.de/themen/blockheizkraftwerke/grafiken
3.6 Fuel cell A fuel cell is conceptually simple. The basic electrolysis reaction of water into hydrogen and oxygen through the application of an electric current (2H2O + e‐ ‐> 2H2 + O2) can be reversed to produce water and electricity (2H2 + O2 ‐> 2H2O + e‐). In its general form, a fuel cell consists of a porous anode and a porous cathode. These two electrodes are separated by an electrolyte. An oxidant is fed to the cathode to supply oxygen, while a fuel is fed to the anode to supply hydrogen. The electrolyte supports the transfer of ions between anode and cathode to support the reverse electrolysis reaction. Fuel cells operate with a continuous stream of air providing oxygen and a source of hydrogen (a fuel). While pure hydrogen (H2) can be used, this is not a very convenient fuel (it is very difficult to obtain, store and carry). In general, fossil fuels, such as methane, methanol, naphtha, coal gas and other hydrocarbons, are chemically broken down (not burned!) to provide hydrogen. Although the only output of reverse electrolysis itself is water, the fact that such hydrocarbon fuels are broken down to obtain hydrogen means that fuel cell systems generally exhaust carbon dioxide and other pollutants along with the water. Nevertheless, when compared to other prime movers, fuel cells are relatively non‐polluting, and are in principle quiet, easy to maintain (they have no moving parts) and with conversion efficiencies of roughly 50%. CHP systems based on fuel cells can approach overall efficiencies of up to 80% 10. Since an individual fuel cell generates from 0.6 to 0.8 volts of electricity, a large number of them have to be stacked in a fuel cell system and connected in series to provide a useful power output. Furthermore, a workable fuel cell
10 ‐ http://www.vectorsite.net/tpchem_13.html
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system always includes: a power output subsystem to provide electrical power at the proper DC or AC voltages required; and a subsystem for converting the hydrocarbons from fuels into hydrogen gas. In order to recover part of the heat produced, the anode and cathode may have channels to allow the distribution of coolants, such as water. The waste heat provided by fuel cells that operate at high temperatures can be used for heating. Unfortunately, in many cases a catalyst is needed to help accelerate the reverse electrolysis reaction, particularly in fuel cells that operate at low temperatures. This factor turns into a problem since the catalyst is made of platinum for some types of fuel cells, thus strongly influencing their cost.
Schematic drawing of a hydrogen/oxygen fuel cell. Source: http://www.baxi‐innotech.de
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Scheme of a CHP plant with fuel cells fuelled by natural gas. Source: http://www.bios‐ bioenergy.at/en/references/all‐projects/lienz.html
3.7 Organic rankine cycle The Organic Rankine cycle (ORC) is the cycle behind a technology that can convert thermal energy from a source at relative low temperatures (in the range of 80 to 350°C) to electricity. Even if the basic principles are similar to the operation of a conventional steam turbine power plant, the operation at lower temperatures opens up the possibility to exploit lower grade heat that otherwise would be wasted. It is a technology that can play an important role in improving the energy efficiency of new or existing energy intensive applications, by recovering thermal energy at the lowest stages. The problem that has led to the development of the ORC is the following: if the expansion in the turbine goes too far, the super‐heated steam turns into wet steam, which can erode the turbine blades due to the impact of tiny water droplets. In a conventional steam plant, the water steam cycle is suitable for turbine inlet temperatures above 350°C. At lower temperatures the efficiency significantly decreases and the danger of erosion due to condensation of droplets within the turbine increases because the expansion goes too deep and part of the steam turns into water. ORC overcomes these problems by using an organic fluid instead of water (therefore it’s called organic Rankine cycle). Organic fluids have lower boiling temperatures than water, which make them suitable to explore heat potential with temperatures below 350°C. By adopting a working fluid that can be converted into useful steam thanks to an existing source of waste heat, higher efficiencies can be achieved than with a conventional steam cycle (similarly, as already seen, in the combined cycle the source of waste heat was the exhaust of the gas turbine in the bottoming part of the cycle).
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In the ORC, many organic working fluids are such that the expansion in the turbine always ends within the dry (super‐heated) region. In this way the danger of blade erosion is excluded and low maintenance costs are assured. In general, the ORC technique is characterized by its robustness, compact design, high ability for automation and the comparatively high efficiency. The efficiency of the sole ORC may actually be only around 25%, but when such a system is integrated in some industrial or CHP plants, the overall efficiency of the whole installation can reach 85%.
Scheme of a CHP plant with ORC. Source: GMK.
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3.8 Heat storage CHP technologies are commonly slow to respond to changes in heat demand and are not well suited to frequent turning on and off or partial operation. Whereas the prime movers technologies used in CHP can respond relatively quickly to changing electricity requests, thermal energy response is slower and much less capable, particularly when the CHP unit is started from cold and/or fired by solid fuels. One approach is to use heat rejection radiators to disperse excess heat generated during peak power demand periods. However this wastes energy and reduces both energy savings and economic benefit of the plant. A far better approach is to install heat accumulators which can be used to store excess heat generated during periods in which electricity demand is high (and the CHP unit has to run at full load) but heat demand is low. Such stored heat can be used as an additional supply at times of peak heat demand. This decoupling of heat production from heat demand improves the operational flexibility of the CHP plant. Among the main and most evident advantages, this technique enables CHP plants to operate at times when electricity tariffs are high and allows the cogenerated heat to be consumed later when electricity revenue is not as favourable. Heat accumulators are effectively large water tanks; as heat is absorbed the temperature of the internal water rises and as heat is extracted the temperature decreases. Hot water storage tanks are especially used in the residential sector, where they can be easily integrated with solar energy systems, and can generally perform within the following ranges: Temperature: 60‐100°C Capacity: 0.1‐6000 m³ Energy storage density: 60‐80 kWh/m³ Long duration storage only partly possible Costs between 0.5 and 7 €/kWh (in EU) As it is evident, the heat stored in heat accumulators is not of appropriate grade for power generation (the temperature is too low). The main function of heat accumulators is to assist in balancing power demand variations, improving significantly the ability of any CHP unit to respond to variations in heat demand and increasing its operating hours.
Heat storage tanks. Source: Buderus.
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3.9 Qualitative comparisons among prime movers Summary of CHP technologies. Source: U.S. Environmental Protection Agency, Combined Heat and Power Partnership – Catalogue of CHP Technologies – 2008
Engine
Advantages
Gas Turbines High reliability.
Micro Turbines
Limits
Low emissions. High grade heat available. No cooling required.
Require high pressure gas or in‐house gas compressor. Poor efficiency at low partial loads. Output falls as ambient temperature rises.
Small number of moving parts.
High costs.
Compact size and light weight. Low emissions. No cooling required.
Relatively low mechanical efficiency. Limited to lower temperature cogeneration applications.
Internal High power efficiency with part load Combustion operational flexibility. Engines Fast start‐up. Relatively low investment cost. Can be used in island mode and have good load following capability. Can be overhauled on‐site with normal operators.
High maintenance costs. Limited to lower temperature cogeneration applications. Relatively high air emissions. Must be cooled even if recovered heat is not used. High levels of low frequency noise.
Operate on low pressure gas. Steam Turbines
High overall efficiency.
Slow start up.
Any type of fuel may be used. Ability to meet more than one site heat grade requirement. Long working life and high reliability. Power to heat ratio can be varied.
Low power to heat ratio.
Combined Cycles
High electrical efficiency, good flexibility in modulating electrical and thermal efficiency.
High costs.
Fuel Cells
Low emissions and low noise.
High costs.
High efficiency over load range. Modular design.
Low durability and power density. Fuels requiring processing unless pure hydrogen is used.
Source: U.S. Environmental Protection Agency, Combined Heat and Power Partnership – Catalogue of CHP Technologies – 2008
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3.10 Quantitative comparisons among prime movers Summary of typical cost and performance characteristics by CHP technology. Source: U.S. Environmental Protection Agency, Combined Heat and Power Partnership – Catalogue of CHP Technologies – 2008
Steam Turbines
Int. Comb. Engines
Gas Turbines
Micro Turbines
Fuel Cells
Electrical Efficiency
15 – 38%
22 – 40%
22 – 36%
18 – 27%
30 – 63%
Overall Efficiency
80%
70 – 80%
70 – 75%
65 – 75%
55 – 80%
Typical Capacity (MWe)
0.5 – 250
0.01 – 5
0.5 ‐ 250
0.03 – 0.25
0.005 – 0.2
Typical Power‐ to‐Heat Ratio
0.1 – 0.3
0.5 – 1
0.5 – 2
0.4 – 0.7
1 – 2
ok
ok
poor
ok
good
430 – 1,100
1,100 – 2,200
970 – 1,300
2,400 – 3,000
5,000 – 6,500
O&M Costs ($/kWhe)
50,000
25,000 – 50,000
25,000 – 50,000
20,000 – 40,000
32,000 – 64,000
1 hr – 1 day
10 sec
10 min – 1 hr
60 sec
3 hr – 2 day
Fuels
All
Natural gas, biogas, propane, landfill gas
Natural gas, biogas, propane, oil
Natural gas, biogas, propane, oil
Hydrogen, natural gas, propane, methanol
Noise
High
High
Moderate
Moderate
Low
Power Density (kW/m²)
> 100
35 – 50
20 – 500
5 – 70
5 – 20
Part Load Behaviour CHP Installing Costs ($/kWe)
Start‐up Time
A fuel cell unit. Source: www.ceramicfuelcells.de
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3.11 How to pick the proper technology Neither technology is better than the other. Instead, each has attributes that make it the most suitable for specific conditions of fuel type and quality, electric and heat load profile, physical space, ambient conditions, etc. The final choice will depend on the specific and different conditions occurring in each single case. However, a certain procedure has to be followed, and some basic parameters considered. The main steps are the following: 1. Identify and analyse energy consumption Type and quality of energy sources Temperature and pressure levels Quantity of energy consumption: amount of energy (kWh per year), maximum power (kW), and load profile and base load (kW) Time structure of energy consumption 2. Pre‐selection Compare relevant parameters such as: specific costs, efficiencies, emissions, space requirement Consider availability and financial budget 3. Technical characterization of the supply options Technical influences: part load behaviour, weather influences, etc. Design calculation and financial calculation in detail Tables and graphs for useful comparisons are provided. Topics like power‐to‐heat ratio appear. They will be taken care of later (see 3.5). Such graphs provide an area of values that fit a certain technology, and can be helpful in diverse occasions. Electrical vs.thermal efficiency for the main CHP prime movers. Source: Webster – Wiley Encyclopedia of Electrical and Electronics Engineering – John Wiley & Sons, 1999
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Electrical efficiency vs. electrical power for the main CHP prime movers. Source: BKWK.
4. How to size a CHP plant 4.1 Load profile I The first step in the design of a CHP installation is to analyse the energy consumption profiles. The energy profiles are indispensable for a good and reliable feasibility study. Typical daily, monthly, or yearly profiles of the different loads include electricity, thermal and cooling power. The main parameters to be kept under control are:
Peak and average demand (in kW) Load factor (ratio between average and peak demand, in %) Annual energy consumption (in kWh/year) Load demand duration curves (graphs) covering different periods
In particular, load demand duration curves reveal the characteristics of a building’s energy consumption. They illustrate peak load demand and the frequency of the other demand levels in hours per year, which is important information for sizing CHP. Of course, the more detailed a load curve is (say, on a daily basis instead of annual), the better the feasibility study will become. It is evident that diverse types of users can have very different energy profiles. For instance, while hospitals are energy intense facilities with long operating hours and more stable and predictable load profiles than
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other buildings, commercial buildings present fuzzy profiles, fewer operating hours per year if compared to industrial plants and seasonal variations. Evidently, energy profiles can be influenced by various parameters, such as seasonality, geographical location, users´ behaviour and technical restrictions. Starting from such curves, gearing the energy supply of a CHP installation to the user’s electricity and heat (and cold) needs is a complex matter. First of all, the number of energy streams differs per sector. For instance, the industrial sector often uses various steam types, cooling at various temperature levels, ventilation, tap water and space heating in addition to the provision of electricity. Glasshouse cultivation mainly requires greenhouse heating, electricity and CO2, whilst office buildings need heat, electricity and cooling. The desired temperature at which heat is to be provided must also be considered. Besides the inventory of the various energy streams, it is also important to know the capacity and exactly when demand occurs. Peak hours for electricity are commonly between 7:00 am and 11:00 pm on working days. Off‐peak hours are all other hours and weekends.
Example of electrical load daily profile. Source: RENAC.
4.2 Load pofile II A further important parameter for assessing the profitability of a CHP installation is the number of full load hours. The number of full load hours is the total energy use expressed in kWh divided by the installed capacity expressed in kW. A high number of full load hours are desirable for employing CHP, whereas a low number suggests the installation is used for only part of the year. Thus, CHP turns out to be particularly suitable for shopping malls, offices, and housing projects only if their scale is large enough [2]. In the diagrams the capacity (the vertical axis) is plotted against the number of hours by which this capacity is exceeded. The surface area in the diagram (kW times hours) is an indication for the energy demand. For example, as it is evident from the diagram, offices are not characterized by a large number of operation
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hours, so that here CHP installations can only be successfully implemented if properly dimensioned and ingeniously combined. Based on curves like these, rules of thumb can always be used for a preliminary estimate of the most efficient capacity. Each load duration curve is unique and major differences can be seen also within the same sectors. Therefore, hourly energy consumption data must always be requested or measured before assessing feasibility. In principle, two situations can be distinguished:
Buildings for which historical annual consumption data are available: consumption in existing buildings has usually been established over a number of years, and users see this information on their annual energy statement. Energy data can come from a range of sources: heat demand can come from metered fuel use or other existing heat meters, and electricity demand from monthly bills. Quarter hourly electricity demand data can be obtained from the building’s electricity supplier.
New buildings for which no energy consumption data are available: users of new buildings do not have annual data at their disposal and must therefore make a rough estimate of their annual consumption (through calculations or benchmarks, see exercises in §4), and normally energy saving measures are already incorporated in the new buildings. Building simulation software can provide a valuable tool in establishing likely heat demand profiles. Electricity demands are usually easier to determine through monitoring main utility meters. Reliable energy demand profiles have proved to best come from detailed data gathering 11.
11 ‐ Heat and Power. Combined Heat and Power: An Overview and Guideline – GasTerra, 2010
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Example of load duration curves for different energy uses in some sectors. Source: Ruan, Liu, Zhou, Firestone, Gao, Watanabe – Optimal option of distributed generation technologies for various commercial buildings – Applied Energy 86, 1641–1653, 2009 Energy Curves The duration curves show the load demand characteristics of the energy consumption of the building. They show the burden of peak demand and each range level of demand with the number of hours per year; this information is important in making the decision of the ability of a CHP.
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4.3 Sizing approaches The capital investment in CHP plant may be substantial, so it is important to run systems to achieve maximum returns. Idle plant accrues no benefits, so it is important that a CHP system operates for as many hours as possible. Basically, this means matching CHP capacity to base heat and power loads. As a rule of thumb, applications which have a simultaneous demand for heat and power for more than 5,000 hours per year will be worth investigating in detail [1]. According to the energy profiles, different sizing approaches can be adopted. The 3 main sizing criteria are: 1. Sizing based on minimum internal thermal and electric loads. The system is designed for maximum operating hours at full load. All electricity requirements exceeding the design production are met with purchased power. The purchasing price of electricity is typically relatively low, and it is not convenient to sell a certain amount of electrical power back to the grid. Sizing based on minimum internal thermal and electric daily loads. Source: RENAC.
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2. Sizing based on thermal load and selling excess electrical output. In these facilities, production follows thermal demand; in this way the CHP system can provide thermally efficient generation and more/less electrical power than the facility requires (depending on the time). The larger capacity system generally results in lower capital and maintenance costs per installed kW and greater thermal fuel efficiency. The price of the fuel is relatively low, and it is convenient to sell the surplus electricity back to the grid. Sizing based on thermal daily load and selling excess electrical output. Source: RENAC.
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3. Sizing to maximize electric production. In some cases, these systems are designed to provide just enough thermal output to the process. These are often combined‐cycle plants in special industrial sectors, with some type of fixed or variable steam extraction pressure and flow rate. Sizing based on electrical daily load to maximize electric production. Source: RENAC.
4.4 An example of CHP sizing Let´s consider an example of CHP sizing [1]. We use daily load profiles for both electrical and thermal demand. In the next figures it is shown how the CHP unit output (green line) would interact with the demand curve, the red curve standing for the thermal load and the violet curve for the electrical load. In particular, in the first figure the behaviour of a CHP unit whose output is 500 kWt and 300 kWe is shown. The machine runs continually after it switches on at 7:00 am. In some hours of the day, there will be a surplus of heat generation, as it is evident from the left side of the figure. If the machine does not modulate its output, heat will be rejected during the afternoon because there is not enough heat demand. On the other hand, electricity surplus is not expected. Indeed, users will have to buy further electrical energy from the public grid in order to match that which is not covered by the CHP system. Indeed, in the second figure the behaviour of the same CHP unit is shown, but now the machine often works at partial
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load and switches on and off several times during mid‐afternoon, in order to follow the heat demand and not to waste further heat. Due to the new CHP output, the amount of electricity to be bought from the public grid is even larger than in the previous case. In the absence of crucial information, it is extremely hard to say whether the first or the second solution will perform better. Among other things, there is an essential dependence upon the type and the price of the fuels involved, the price of electricity and the type of CHP unit, since some prime movers, like turbines, cannot switch on and off so easily.
Load profiles and CHP output if the unit runs continually from 7:00 am. Source: Good Practice Guide. Combined heat and power for buildings – Action Energy, 2004
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Load profiles and CHP output if the unit switches off during the day. Source: Good Practice Guide. Combined heat and power for buildings – Action Energy, 2004
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4.5 The power‐to‐heat ratio In order to select and size the proper technology, it is also essential to correlate the electrical and thermal (both heating and cooling) demand, assessed with:
Power‐to‐heat ratio, defined as the ratio of useful electrical energy production (or demand) to that of thermal energy production (or demand) Frequency distribution of power‐to‐heat ratios
Referring to the first figure, in most of the cases a very high power‐to‐heat ratio indicates limited cogeneration application potential (even if some compromise solutions are still possible). A high power‐to‐ heat ratio indicates suitability for a system with relatively low thermal efficiency or an application that features exportation of on‐site generated power. A low power‐to‐heat ratio indicates suitability for a system with a higher thermal efficiency. Moreover, the power‐to‐heat ratio may help to pick the optimal technologies for running a CHP unit on the basis of existing plants or other previous experiences
Typical power‐to‐heat ratios for diverse sectors. Source: Cogen Designs, Inc.
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Typical power‐to‐heat ratios for diverse technologies. Source: Webster – Wiley Encyclopedia of Electrical and Electronics Engineering – John Wiley & Sons, 1999
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5. Micro/mini CHP 5.1 CHP sizes In total analogy to general CHP, micro/mini combined heat and power (referred to as mCHP) is a technology that mixes conventional small‐scale electricity generation and heat recovery, since the waste heat from cooling the prime mover and from the exhaust flue is harvested and used to heat space and water. However, unlike general CHP, here the main output is heat, and electricity is only generated when heat is needed. The two main prime movers for mCHP commercially available are the Stirling engine and the internal combustion engine, although also fuel cells, especially in Japan, are widely used. The typical ratio of heat and electricity production from a domestic scale CHP is 2:1. Even if there is nothing new to say about the idea and the technology behind mCHP, it is worth to dedicate a separate chapter to this topic, referring to the crucial role that mCHP plays in managing smart grids and virtual power plants. In order to better contextualize mCHP, the commonly accepted framework for CHP sizes is presented:
Micro CHP (up to 5 kWe): fuelled mostly by natural gas; use especially Stirling and fuel cells; can be applied for residential or small commercial buildings. Mini CHP (10‐50 kWe): based mostly on combustion engines (gas or heating/plant oil) and micro turbines (30 kWe); can be applied for commercial enterprises, swimming pools, apartment blocks. Small CHP (50‐200 kWe): based mostly on combustion engines (gas or heating/plant oil), micro turbines (60‐200 kWe) and steam engines; are often used in modular arrangement (e.g. base load CHP plant plus 2 peak load boilers). Medium CHP (200 ‐1000 kWe): based mostly on combustion engines (gas or heating/plant oil), small gas turbines (typically 500 kWe) and steam engines (often propelled with solid fuels as wood, etc.).
Large CHP (more than 1 MWe): based mostly on gas/steam turbines and large combustion engines (marine diesel). CHP unit in Monheim (GE), 2MWe, natural gas fired engine. Source: RENAC.
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5.2 mCHP concept and advantages When we talk about mCHP, we refer to the scale installed in domestic properties. Micro/mini CHP systems should always be installed and run to meet the heating needs of the building, rather than to generate more heat than is needed just to meet electricity demand. These systems are most suitable in a building where heat demand is quite high and consistent, for example very poorly insulated properties which cannot be improved using available technology. But these kinds of CHP installations may easily meet the needs of new or existing buildings. Most of the time it is possible to integrate them within an existing heating system, enabling existing boilers to meet the peak loads. mCHP systems work as a supplement to installations based on renewable energies, when the sun stops shining or when the wind stops blowing, the energy demand can still be met with a low‐carbon technology operating locally, like mCHP. “Citizens who use micro CHP are saving energy, reducing pressure and energy losses on the local electricity network. In the end they are helping balance the intermittency of renewable electricity” 12. Micro/mini CHP, like general CHP, provides thus the following key benefits:
Supply of both heat and electricity from a single energy source. Reduction of pollutants emissions by increasing the efficiency (and therefore reducing the losses) of the generation plant. Economic savings generated by reducing the amount of imported electricity and by selling the surplus of electricity back to the grid (though this latter operation is not always possible or convenient). Concretely this means lower energy bills for energy customers.
Security of supply is greatly enhanced by reducing reliance on centralised power production and by obtaining full control of a local production station.
12 ‐ Micro CHP: Empowering People Today For a Smarter Future Tomorrow – COGEN Europe, 2010
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A scheme of an mCHP plant for domestic use. Source:http://www.energysavingtrust.org.uk/blog/2013/02/20/maximising‐micro‐machines
5.3 Smart grids and virtual power plants CHP systems are usually connected to the national distribution network, in order to buy public electricity when the unit does not cover the entire power load, or to sell surplus power when necessary. All these new connections represent crucial developments that imply a shift away from traditional power grids, with their hierarchical top‐down structure, towards more diffuse bi‐directional networks capable of accommodating major fluctuations in both supply and demand. Energy companies, network operators and governments consequently must expect to face a variety of new social, technical and economic challenges. Important questions arise: how can the demand for electricity be met without compromising comfort and convenience, cost‐efficiency or security of supply? What is the best combination of technologies? Can local network overloads be prevented by using smart standardized techniques? Satisfying answers cannot be found without rethinking and rearranging the entire operation of the national grid and implementing intelligent solutions throughout including the parts of the grid that go inside users´ homes. There is talk about a “smart grid” when all the limits of an old electricity grid are overtaken and where IT (information technology) forms an essential control mechanism, matching supply and demand in the most economical way.
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A smart grid generally refers to a class of technologies upgrading electricity delivery systems, using computer based remote control and automation. Generally speaking, a smart grid, besides providing systems for energy storage and distribution, is characterized by:
Devices for fully monitoring demand response and consumer energy efficiency Advanced metering infrastructure Systems for improving communication throughout the network and its security
It is evident how mCHP is a key component of an emerging smarter energy environment and a player in the massive development of smart grids. Providing a vast number of decentralized and technologically advanced power stations connected to a single national grid, such systems may balance the inherent intermittency of some renewable sources, but they could also help the grid to cope with any capacity constraint. mCHP plants can definitely be considered as a highly reliable electricity generation solution. A clustering of distributed mCHP units controlled and remotely operated by a central entity allowing power generation to be modulated in reference to the instantaneous demand is also known as “virtual power plants”.
Traditional grid. Source: Introduction to Smart Grid – Department of Electrical & Computer Engineering, Texas Tech University, 2012
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A smart grid. Source: Introduction to Smart Grid – Department of Electrical & Computer Engineering, Texas Tech University, 2012
Scheme of an mCHP virtual power plant. Source: The Role of Micro CHP in a Smart Energy World – Ecuity, 2013
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6. Trigeneration 6.1 What is trigeneration? A cogeneration system is efficient when thermal users are present and when thermal and electrical demand is simultaneous. Cogeneration plants used for the domestic or tertiary sector only work efficiently during a limited period of the year, when buildings heating is necessary. That means that during warmer months the cogeneration system has to be arrested or kept working, wasting the heat produced; both these conditions are to the detriment of economic feasibility and global efficiency of the installation. Indeed, during summer periods consumers usually demand cooling energy (i.e. fluids at a low temperature) for a building’s air‐conditioning. Such cooling fluids are usually provided by using vapour compression refrigeration cycles, through systems in which a compressor is activated by an electrical engine, with high absorption of electrical energy. This is the reason why during warmer summer days the highest levels of electrical energy consumption are reached, bringing national electrical systems close to the blackout threshold in many countries. Absorption refrigeration systems are commercially available, and make it possible to generate cooling energy by using heat as a primary energy source, instead of electricity. Such systems certainly embrace a cogeneration system, as they make it possible to use an installation even during summer months; the thermal energy produced by the cogeneration unit can be used for activating the refrigeration system. And in this case we talk about "trigeneration". A trigeneration system is an installation able to produce three distinct useful energy forms – electrical, thermal and cooling energy. Trigeneration systems are based on the same engine installations used for cogeneration, but the temperature of the hot fluid that has to be provided to the absorption machine needs to be at least 90°C for a single effect machine. Generally speaking, there are actually two basic refrigeration techniques most commonly used for CHP systems:
Vapour‐compression chillers: indirect, electrical activating power
Vapour‐absorption chillers: operation with hot water or steam, direct heating through combustion The idea of trigeneration with absorption chillers. Source: http://esisrl.eu/demoeng/index.php?op tion=com_content&view=article&id=16 &Itemid=119
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6.2 Compression chillers The operation of a compression chiller is based upon the standard vapour compression refrigeration cycle. The elements of the basic system are: condenser, evaporator, valve (restrictor) and compressor (driven by electricity). Referring to the first figure, let´s start from the evaporator, where a refrigerant substance enters in the form of a cold and low pressure mixture of liquid and vapour. A certain amount of heat (Q0) is transferred from a relatively warm source to the refrigerant, causing the latter to boil. The resulting vapour moves then from the evaporator to the compressor, driven by electricity (P), where its pressure and temperature increase. This hot, high pressure refrigerant vapour leaves the compressor and enters the condenser, where it transfers a part of its heat (Q) to ambient air or water at a lower temperature. As a result of such transfer, the refrigerant vapour condenses into a liquid again. This liquid refrigerant then flows to an expansion valve, which creates a pressure drop that brings the pressure of the refrigerant back to that of the evaporator. At this low pressure, a small portion of the refrigerant boils (or flashes), cooling the remaining liquid refrigerant to the desired evaporator temperature. With the adjacent phase of evaporation a certain amount of heat (Q0) can be collected and expelled from a certain ambient. The cycle then restarts. In CHP units, compression chillers operate in electrical bottoming cooling production mode: the compressor is powered by the electricity produced by the cogeneration unit.
Scheme of the vapour compression cycle. Source: RENAC.
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A trigeneration system with compression chiller (COP = 3 in this case). Source: RENAC.
6.3 Absorption chillers Similar to the vapour compression cycle, a refrigerant in the absorption cycle flows through a condenser, expansion valve and an evaporator. What is really different in the absorption cycle are the kind of refrigerants used and the method of its compression. In fact, absorption refrigeration systems replace the compressor with a generator and an absorber (driven not by electricity but thermal energy). Referring to the first figure, refrigerant enters the evaporator in the form of a cool, low pressure mixture of liquid and vapour. Heat (Q0) is transferred from a relatively warm external source, is cooled at the refrigerant, causing the liquid refrigerant to evaporate. Using an analogy of the vapour compression cycle, the absorber acts like the suction side of the compressor: it draws in the refrigerant vapour to mix with the absorbent. The pump performs the compression process itself: it pushes the mixture of refrigerant and absorbent up to the high pressure side of the system. Eventually, the generator acts like the discharge of the compressor: it delivers the refrigerant vapour (produced by using a certain amount of heat QH coming from an external source) to the rest of the system. The refrigerant vapour leaving the generator then enters the condenser, where heat (Q) is transferred to an external heat sink at a lower temperature, causing the refrigerant vapour to condense into a liquid. The liquid refrigerant then flows to the expansion device (restrictor), which creates a pressure drop reducing the pressure of the refrigerant to that of the evaporator. Just as for compression chillers, the refrigerant evaporates and through this phase in the evaporator a certain amount of heat (Q0) is collected and expelled from a certain ambient that is to be cooled. The resulting mixture of liquid and vapour refrigerant travels to the evaporator to repeat the cycle. In CHP units, absorption chillers operate in thermal bottoming cooling production mode: the cooling side is powered by the thermal power produced by the cogeneration unit. This is the classic case of trigeneration as known from literature and as can be seen in most installations.
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Scheme of the absorption cycle. Source: RENAC.
An example of a trigeneration system with absorption chiller. Source: RENAC.
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6.4 Comparisons Summary: comparison between electric and absorption chillers. Item
Electric Chiller
Absorption Chiller
Energy Source
High electricity.
Gas, oil, steam, hot water.
Expensive power receiving facility.
Small electricity.
Refrigerant: CFCs, HCFCs, HFC.
Refrigerant: H2O, ammonia endangering humans and animals. Safe and harmless.
Refrigerant
Environmental pollution.
Principle
Dynamic process. Noise and vibration.
COP
≈ 4
Energy Efficiency
≈ 20 %
Static process. Low noise and vibration. 0.7 – 1.2 14 – 20 %
Source: http://www.scielo.org.co/scielo.php?pid=S0012‐73532011000400010&script=sci_arttext
Similarities of absorption and vapour compression refrigeration:
Both cycles circulate a refrigerant inside the chiller to transfer heat from an internal source to an external sink. Both cycles include a device to increase the pressure of the refrigerant and an expansion device to maintain certain pressure conditions which are critical to ensure controlled evaporation and condensation, which is critical to the overall heat transfer process. Refrigerant vapour is condensed at high pressure and temperature, rejecting heat to the surroundings. Refrigerant vapour is vaporized at low pressure and temperature, absorbing heat from an internal ambient or fluid.
Differences between:
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The absorption systems are powered by heat energy in the form of steam, direct fuel firing or waste heat. The absorption cycle uses a pump, not a compressor, to create the pressure rise between evaporator and condenser. Pumping a liquid is much easier and cheaper than compressing a gas. However, there is a large heat input in the generator. So, the system basically replaces the electrical input of a vapour‐compression cycle with a heat input. The absorption cycle uses different refrigerants, for example lithium bromide, and absorption system uses distilled water, which is harmless, or ammonia which is dangerous for human and animal life,
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as the refrigerant. The vapour compression refrigeration cycle generally uses more dangerous fluids (such as HCFCs) as the refrigerant with attached environmental hazards, like ozone depletion or global warming potential. Compared to compression chillers, absorption systems contain very few moving parts, offer less noise and vibration, are compact for large capacities and require little maintenance.
Compared with electrical chillers, absorption systems have a low coefficient of performance (COP = chiller load/work input). However, absorption chillers can substantially reduce operating costs because they can be powered by low‐grade waste heat. The COP of an absorption chiller is not sensitive to load variations and does not reduce significantly at part loads.
An absorption chiller. Source: www.colibri‐bv.com
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7. District heating and cooling 7.1 Main concept District heating and cooling is a technological concept comprising infrastructure for delivering heating and cooling services to customers. The basics are similar to those of CHP and trigeneration: the fundamental idea is to use local heat and fuel sources that under normal circumstances would be lost or remain unused. Once again this comprises a flexible infrastructure that is able to integrate a wide range of traditional and renewable energy sources. Heat and cold are delivered directly to consumers, and because of this no boilers and burning flames are needed inside users’ buildings. Individual substations for delivering thermal energy (heat exchangers) are small and silent, which is much more convenient than the conventional CHP solutions requiring individual heating and cooling equipment in each building. Particularly in the case of district cooling, the aesthetics of buildings can be enhanced as air conditioners are eliminated from façades, saving valuable commercial space. Since benefits are most apparent in areas with high energy demands, district heating and cooling represent the most suitable energy solutions for satisfying urban heat and cold demands. But energy demands from industry and intensive agriculture are also quite suitable for this technological concept. The most important aspect is probably that district heating and cooling can reduce potentially unstable dependency on energy imports by reducing the overall need for primary energy and by replacing imported energy with local resources. As practically any type of fuel or energy source can be utilised, a relatively quick switch to other fuel sources can be achieved: imported energy can be substituted with local resources, e.g. biomass. The district cooling market has emerged quite recently and is consequently less developed than the district heating market. It has, however, grown fast within the last decade. To get an idea, there are more than 5,000 district heating systems in Europe currently supplying more than 9% of total European heat demands with an annual turnover of €19.5 billion and 556 TWh of heat sales . Nowadays, Europe is once again leading the world in district energy technology. Increased interest for such solutions is particular in the Middle East, Asia and North America 13.
District Heating Plant in Köln (GE), serving 80,000 homes. Source: Rheinenergie AG.
13 ‐ District Heating and Cooling – A Vision toward 2020, 2030, 2050 – DHC, 2009
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7.2 District heating (DH) DH refers to a system where a number of buildings or dwellings are heated from a central source. The heating fluid is conventionally water, distributed through a double pipe network and then transferred to buildings for use in space heating, hot water generation and process heating. Often, DH systems cover large areas and are very complex systems serving many stations and thousands of consumers. A system may include more than one CHP plant. A DH system has three main elements: the heat producing station, the distribution system and the customer interfaces. In large systems, the distribution system may be separated in a transmission and a distribution system. The transmission part is responsible for transporting large amounts of heat energy over long distances, and a dedicated distribution system serves each building connected to the system. A transmission system may serve an entire region and cover several distribution systems in different towns. In more detail, when compared to owning and operating an own on‐site boiler, conversion to DH can benefit the user through many further advantages:
Building space can be used more productively. The customer interface requires far less floor space as required by a conventional system (boiler station for example).
DH is a more convenient way to heat a building because it eliminates the need to operate and maintain a conventional boiler plant. This results in savings in staff, administration, electricity and supply costs. In larger buildings (industrial or agricultural sector), the savings in staff can be significant.
Since users usually pay only for the heat that is actually used, they do not have to pay for the inefficiency of the system. Conventional boilers are often quoted at having efficiency well above 80%. Cyclical firing, under part load conditions, means that realistic efficiencies are somewhat lower. Global efficiencies on a yearly basis, within a range of 45‐65%, are not unusual. Such inefficiencies of individual boilers can entail significant costs for users.
The combined generation of heat and power substantially reduces fuel consumption.
A building’s appearance can be improved because no flue or chimney is required. Also, the DH operator takes responsibility for the control of any air emissions.
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Scheme of a DH network. Source: RENAC. References: [1] District Heating and Cooling – A Vision toward 2020, 2030, 2050 – DHC, 2009 [2] Skagestad, Mildenstein – District Heating and Cooling Connection Handbook – IEA
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7.3 District cooling (DC) In complete analogy to DH, DC comprises a system in which chilled water is distributed in pipes from a central cooling plant to buildings for space cooling and process cooling. A district cooling system contains three major elements: the cooling source, a distribution system and customer installations, also referred to as energy, or heat, transfer stations. Chilled water is the typical energy carrier, and is generated at the district cooling plant by compressor or absorption chillers. District chilled water is distributed from the cooling sources to the customers through supply pipes and is returned after extracting heat from the building’s secondary chilled water systems. Pumps distribute the chilled water by creating a pressure differential between the supply and return lines. Among the main advantages:
If a building owner is allowed to eliminate an on‐site chiller, s/he no longer needs to purchase utilities, operate and maintain the system, and replace components at the end of their life cycles. As for DH, because of the higher efficiencies that district cooling systems provide and their ability to utilise inexpensive or waste energy sources, the building owners can expect more stability in their energy costs into the future. For new buildings, the overall capital costs are reduced, because the cost of the chiller room is eliminated. A DC system also provides significant benefits to the municipalities where they are built. The most obvious benefit to the municipality is the significant amount of infrastructure that will be added, enabling a sustainable and reliable provision of energy. A benefit that is often overlooked is the ability of a DC system to capture cash flows that were previously leaving the community. Typically, energy expenditures leave the community to pay for the natural gas and electricity that is imported. A DC (but even DH) system service expands the opportunities of using local energy sources like CHP to keep more of the money, currently being spent for imported energy, circulating within the community.
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Scheme of a DC network. Source: RENAC.
References: [1] District Heating and Cooling – A Vision toward 2020, 2030, 2050 – DHC, 2009 [2] Skagestad, Mildenstein – District Heating and Cooling Connection Handbook – IEA
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8. CHP and RES coupling 8.1 Reneweable sources available for CHP Energy from renewable sources (RES) plays a vital role in the mitigation of greenhouse gas emissions. Moreover, it has several other well recognised benefits such as improved security of supply, contributing to improve air quality and creating new jobs and businesses – many of them able to revitalize industry and to improve local living standards in rural areas. But how can a CHP approach benefit renewable energy sources? CHP is a valid approach for all fuel sources that use thermal electricity production. Hence the CHP principle can be applied to bioenergy and, very rarely but with growing interest, concentrating solar and geothermal. The main CHP and RES coupling applications are:
Biomass for heat production. Combustion of solid biomass (wood derived) for heat production is the main CHP bioenergy route in the world, and has proven to be an important factor for improving efficiency and reducing pollutant emissions. Firewood, pellets, wood waste and garbage derived fuels are typically burned. Heat produced on a medium or large‐scale system through cogeneration can also be used to generate steam for industrial processes or to supply heating to networks.
Biogas fuelled applications. Biogas production through anaerobic digestion (either via conversion of bioenergy or capture from garbage in landfills) is growing. Drawn by EU legislation, for instance, many small‐medium sized CHP are now operating on biogas. Biogas, like any other biomass derived fuel, produces no net carbon emissions.
Biofuel fuelled applications. Today biofuels are made from biomass such as vegetable oils. Among these, the most important are bioethanol and biodiesel and it seems that biodiesel could become price competitive in the future.
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The Piesteritz (GE) biomass combustion plant. Source: Econcern. References: [1] Sustainable Bioenergy Development in UEMOA Member Countries – 2008 [2] Biomass for Power Generation and CHP – IEA, 2007 [3] Biomass Heating, a Practical Guide for Potential Users – Carbon Trust, 2009 [4] Planning and Installing Bioenergy Systems – DGS, 2005
8.2 Major advantages The major drivers for implementing a biomass/biofuel heating systems are as follows:
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Significant carbon savings. Producing no net carbon emissions, biomass fired systems can play a major role in reducing carbon footprints. In many industrialized countries, many organisations now have commitments or requirements to reduce their overall emissions and improve their environmental performance. Biomass can significantly help to meet such requirements. Operational cost savings. The costs of biomass fuels, if considered as cost per kWh, are typically lower than that of common fossil fuels, especially if the source is locally available. The amount of savings depends on the price of the fossil fuel being replaced and the cost of the biomass fuel used. In those cases when biomass is directly available in the place where a CHP plant is installed, the supply cost may even be null, and also costs for disposal are drastically cut. In fact, using certain biomass resources as fuels can divert them from becoming wastes and being sent to landfill. Disposing of such wastes normally has a considerable associated cost.
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Reduced fuel price volatility. Security of energy supply is a recurrent concern when dealing with fossil fuels; geopolitical instabilities in oil and gas producing regions can threaten availability and lead to unexpected price changes. While biomass fuels will be subject to changes in price over time, these are likely to be less extreme than those occurring with fossil fuels and may also be more manageable and predictable if the biomass is sourced locally.
A small biogas plant for domestic purposes in India. Source: ADATS. References: [1] Sustainable Bioenergy Development in UEMOA Member Countries – 2008 [2] Biomass for Power Generation and CHP – IEA, 2007 [3] Biomass Heating, a Practical Guide for Potential Users – Carbon Trust, 2009 [4] Planning and Installing Bioenergy Systems – DGS, 2005
8.3 Main aspects of biomass fired CHP plants Generally speaking, when considering all the types of biomass, including biogas and biofuels, there are three basic configurations of biomass fired CHP plants that can be realised:
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Biomass co‐fired in existing boilers. Particularly in the industrial sector, with a CHP unit based on a steam turbine or a combined cycle where boilers respecting the levels of temperature and pressure requested already exist, it is usually not cost‐effective to install new biomass fired boilers with the same output, unless a large steam addition is foreseen for further processes. Moreover, in several cases it may be technically difficult to combine the new equipment that biomass requires with the existing installations. These include new fuel handling facilities (receipt, storage and preparation) and several modifications to furnaces to allow for the different characteristics of the chosen range of biomass fuels.
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Biomass co‐fired in a new boiler. Remaining in the industrial sector, the possibility of installing a new, free‐standing biomass fired boiler can offer the benefits of fuel‐specific design and an independent operation of both the traditional and biomass boilers. In this case, the technical issues associated with connecting a new steam supply to an existing system are much reduced but, of course, the costs of this option are substantially greater, since the boiler is the main cost in a biomass fired scheme [22]. Biomass fired new plant. It is evident that this option offers the greatest flexibility and opportunity for fuel‐specific design, and maximises the improvements in supply reliability. Any technology can be chosen, according with the characteristic of the particular situation. However, a new plant evidently comes at the greatest capital cost and complexity. Biogas/biofuels co‐fired in a combustion chamber. In cases where the prime mover of a CHP plant is a combustion engine or a gas turbine, biogas or any kind of biofuel can be directly burned in the combustion chamber to run equipment. Biofuels, in particular, can also be mixed with conventional fuels and fired without modifying anything in the prime mover. Some precautions may be necessary in order to avoid the emission of special substances present in the fuel. Fuel storage and delivering facilities account for a significant proportion of the overall capital cost of biomass projects and careful consideration needs to be given to their design and functionality. A well‐ designed system for delivering, storing and transferring solid biomass is essential to ensure a smooth‐ running biomass plant. Main common problems to be addressed are:
Allow delivery by vehicles of different size and weight Prevent the ingress of water but also allow moisture to dry off from stored fuel
Meet necessary building regulations and safety requirements
Biogas plant in Shuby (Germany). Source: BioConstruct
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Scheme of a CHP plant with biogas. Source: RENAC. References: [1] Sustainable Bioenergy Development in UEMOA Member Countries – 2008 [2] Biomass for Power Generation and CHP – IEA, 2007 [3] Biomass Heating, a Practical Guide for Potential Users – Carbon Trust, 2009 [4] Planning and Installing Bioenergy Systems – DGS, 2005
9. Feasibility 9.1 Implementation Once the CHP energy potential has been assessed, and a decision made to move forward, it’s time to complete the project development phase of the CHP plant. This can be a very complicated and time‐ consuming process. This chapter outlines some of the basic steps in developing a project, highlighting the main problems occurring when a feasibility study is conducted and when looking into, among other things, environmental assessment, permissions, risks and liabilities, connecting to the grid and economic aspects. Generally speaking, implementation of actions and interventions on buildings or technical equipment, such as the installation of a new CHP unit, the aim of which is to achieve concrete and measurable energy savings, follows diverse main steps, summarized in the figure. Both technical and economic factors are considered in a proper feasibility study, as well as potential obstacles. Overall, the study should indicate whether or not the project should move forward, as well as providing the data and assumptions leading to that conclusion. Some of the technological factors include: energy profiles, plant design, operational strategy, environmental aspects, availability of technical services, etc. Some of the economic factors include: investment costs, fuel costs, operation and maintenance (O&M) costs, revenues and energy savings, as well as, electricity tariffs.
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Flow chart of executive and decisional phases. Source: RENAC.
9.2 Costs One of the first and most important phases is to assess the costs involved in the project. When planning a new CHP plant, costs are diverse. The main ones include the following: Capital costs. Include anything necessary for installing and starting the CHP unit. Whilst the capital and installation costs of CHP are significantly higher than for conventional boiler heating plant, CHP can yield very considerable savings in running costs. In the right applications, it can provide important economic returns on investment. Running costs. Fuel is one of the main running (operational) costs. When deciding upon a supplier, consideration should be given to price as well as to security of supply. In particular, a forecast of the future price of fuel is a very important aspect to consider, since the economics of CHP enable a sustainable and reliable provision of energy highly sensitive to changes in fuel costs.
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Maintenance is the other major running cost. Once a CHP unit has been installed and started, it is important to plan and carry out regular maintenance to ensure a high number of operating hours. Electricity tariffs. Electrical energy fees play a very important role in the decision‐making process of an appropriate CHP system. The profitability and optimization of a CHP investment is heavily dependent on the structure and the pricing of electricity (selling and buying) that are applied before and after the installation of such a plant. There are three kinds of fees when purchasing electricity:
Fixed fees: these are applied to all consumers and cannot be changed or improved Volumetric fees: these are computed in proportion to the electricity consumed each month Demand fees: these depend on maximum power demand during the month, regardless of how often it occurs There are 3 basic possible arrangements for selling surplus electricity produced back to the grid:
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Sale of excess power only. The CHP plant produces a fixed output and either purchases power or exports power, depending on the level of varying on‐site loads. Sale of a contracted amount of power. The facility takes on an obligation to provide a specified amount of power to the grid. This is usually done at facilities with fairly constant loads that are significantly below generating capacity. Sell‐all/buy‐all. A facility enters into an agreement with the utility or a third party to export all of the power generated on‐site and to purchase all of the power required by the facility. This arrangement may occur when the value of exported power is greater than the price of purchased power, or when the host facility only has a marginal electricity requirement compared with the amount it can generate.
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Main costs in a CHP Project. Source: RENAC.
9.3 Costs‐effectiveness Relatively small investments in energy management may be authorized without detailed financial scrutiny, but if the energy audit points to substantial investment as the only way to achieve meaningful long‐term savings (this is the case of CHP), those in control of the finances in an organisation will need to be convinced that it is worthwhile. A number of standard techniques are in use in order to assess the cost‐ effectiveness of a project and to filter out unpromising proposals. Generally speaking, such methods may be divided into non‐discounted and discounted. The main difference exists in the purpose of discounting: i.e. to take into account the time value of money, according to a certain discount rate, defined as the interest that has to be paid to acquire the capital to invest in the project 14. Several and diverse methods exist, and two among the fundamental and most used ones are explained in the following:
14 ‐ Harris – A Guide to Energy Management in Buildings – Spon Press, 2012
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The payback period method is the simplest of all to understand and to calculate. The total amount of the initial capital cost of the project is simply divided by the expected annual savings, resulting in a value in years. The advantages are that it is easily understandable, and a simple calculation. The disadvantages are that it does not take into account future aspects, as the timing of costs and benefits. The discounted cash flow (DCF) method acknowledges the importance of timing: it involves discounting future outflow and inflows of cash back to present day values, thus establishing a common base for the comparison of investment alternatives involving different periods of time. The main idea is that funds invested in the future have less impact than funds invested now, and the funds received early on in the lifetime of a project are worth more than funds received later. The present value of the funds invested in the start‐up phase is compared with the value, reported to the present, of the net cash flows expected to be generated over the life of the investment. Different approaches can even be merged together to produce quick and easy‐to‐read results. Either way, all the methods follow a common path: the first step is to identify and enumerate the total cost impact of an efficiency measure. One framework for this is known as life cycle cost analysis. Such analyses capture the total sum of expenses and benefits associated with an investment. The result (a net gain or loss on balance) can be compared to other investment options.
Life cycle cost analysis. Source: http://availagility.co.uk/2012/02/02/the‐science‐of‐kanban‐economics
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9.4 Integration of the CHP unit After a proper feasibility study has been conducted and accepted, the CHP system has to be installed, and new problems may arise. CHP cannot be considered in isolation and requires good integration with other energy systems on‐site. It is unlikely that all the power and heat requirements will be supplied by the CHP unit at any moment. CHP units usually work alongside existing boilers, with the boilers providing top‐up heat to meet peak demands. In fact, a CHP installation should always operate as the leading boiler, if savings are to be maximised. The electricity generated is best utilised on‐site, as greater value is realised for it (the selling price of surplus electricity is usually very low). However, in some cases, particularly where on‐ site electricity demand is low, it may still be worthwhile to export electricity back to the public grid. There are essentially two ways of connecting a CHP plant with a conventional boiler plant as shown in the figure: in series or in parallel with the boilers. Connection in series is most frequently used when a new CHP system is installed in addition to an existing heating system, as it creates the minimum interference with existing equipment. Connection in parallel is more common in completely new installations, especially where the CHP plant is likely to supply a significant proportion of the total heat load. In order to operate the CHP in parallel with the grid, technical approval must be obtained from the local distribution network operator: it will be necessary to ensure that the CHP unit can be isolated from the public grid in the event of a failure of either the CHP or the grid. In cases where the excess power can be sold back to the electricity supplier, special meters will need to be installed in addition to the existing ones. When exporting back to the grid, care should be taken to ensure that feed‐in tariffs are high enough to justify doing this. Most small‐scale packaged CHP units incorporate continuous monitoring devices as part of the control system. Such devices may significantly improve CHP reliability. The main function of a control system is to maintain optimum performances, by ensuring that the CHP unit operates correctly with other energy systems on‐site and by continuously monitoring the system to detect faults, malfunctions or under‐performance, so that corrective action can be taken before a system failure.
CHP units fitted in series may well be placed before the boilers. The CHP unit provides the base load and temperature. The following boiler tops‐up to provide for peak demand and higher temperatures.
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References: [1] Good Practice Guide. Combined heat and power for buildings – Action Energy, 2004
9.5 Control and risks Controls to be carried out during the construction phase should include:
Control of contracts: control of compliance of offered components, control of use of offered and appropriate materials/components, control of deadlines and procedures. Control of compliance: control of compliance with regulations for construction, control of compliance with permitting condition, and control of compliance with further agreements.
Control of costs: control of progress and billing, control of additional tasks and unforeseen administration of contracts and subcontracts. Of course, there are usually some elements of risk involved, both technical and financial. The following tab summarises the main risks that may occur when planning and installing a CHP system.
Construction of a new CHP plant. Source: RENAC.
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Main risks when installing a CHP system. Source: RENAC. Risk
Description
Potential Mitigation
Planning Risk
CHP unit oversized.
Long and thorough planning phase with experts involving energy efficiency measures.
Supply Risk
E.g. supply of natural gas from Russia.
Search for multi fuel capable technology (natural gas/LPG).
Market and Distribution Risk
The produced products or services (electricity, heat) cannot be sold in the expected amount and/or price.
Long‐term contracts with solvent buyer.
Technology Risk (in the broader sense)
The chosen proven technology might get Thorough technological analysis to outperformed by competing innovative choose the best available technology technology. with a proven track record but also with a competitive edge.
Malfunction, damage.
Fixed feed‐in tariff provides the best risk mitigation
Set liabilities in purchasing contracts. Uphold maintenance plan from supplier Check operation records regularly.
Stability Risk of Regulatory Framework and Country Risk
Change of framework conditions (e.g. feed‐in tariffs, tax breaks, etc.) during the life time of a project. Legal uncertainty. Unclear ownership rights, etc.
For investors: export guarantees; investment preferential in countries with a reliable political framework.
Change on environmental regulations.
Select technologies/fuels according to strong regulations (“one step ahead”).
Currency Risk
Currency devaluation.
Hedging with respective financial derivatives.
Interest Rate Risk
Increase of interest rate.
Hedging with respective financial derivatives.
Inflation Risk
Increase of inflation beyond expectation. Hedging with respective financial derivatives.
Force Majeure Risk
Unforeseeable events with negative impact on the project (e.g. earthquake, fire, flood, war).
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For governments: provide reliable conditions to attract investments.
Insurances where feasible.
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9.6 Ostacles and barriers The typical obstacles faced by CHP are due to market and users´ conditions. Particularly when we consider gas as the main fuel for producing thermal energy, for instance:
Unfavourable and unstable gas and electricity prices
Uncertainty about how a particular site’s heat demand will evolve over time
The need for high initial capital investment The balance between gas and electricity price along a certain period of time may play a crucial role: a change can reduce the return on investment in CHP and erode the advantage over conventional generation. In certain cases a conventional heat generating plant may still be preferred, as it requires lower capital investment and is seen as less risky. The main parameter that impacts upon the attractiveness of investment in CHP is probably the spark‐gap, which is the difference between the price of electricity and gas. The larger the spark‐gap (higher electricity price and lower gas price) the more favourable are the conditions for operating a CHP unit15. CHP also has higher associated operation and maintenance (O&M) costs than conventional generation. This is a consequence of the higher technical sophistication when compared with conventional methods of heat generation. In summary, the volatility in fuel prices, the relative prices of gas and electricity, uncertainties regarding a particular site’s energy demand in the future and the high investment attached to CHP installations are all considered obstacles to the installation of new CHP. Some of these are characteristics of CHP technology, while others are characteristics of the present state of the energy market. But besides operational and economic factors, obstacles and barriers are often due to psychological effects and knowledge deficits. The next figure summarizes the main motives and obstructions of rational energy use.
15 ‐ Analysis of the UK potential for Combined Heat and Power – www.defra.gov.uk, 2007
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Common motives and obstructions of rational energy use. Source: RENAC
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10. Main CHP policies and conclusions 10.1 Main incentive mechanism Incentives provided for CHP. Source: Legal Guide Energy Efficiency – RENAC, www.renac.de Measure
Description
Where?
Priority access Priority access means that connected CHP plants to the grid are sure that they will be able to sell and transmit their electricity: all electricity from CHP sold and supported can be fed to the grid anytime.
Several countries in EU.
ESCOs (Energy ESCOs can provide a wide range of measures to Service improve energy efficiency through the Companies) implementation of energy savings projects; they also help to overcome financial constraints and can help to pay off initial costs through the energy savings resulting from reduced consumption. An ESCO can be commissioned to directly implement (and operate) energy saving measures in the company in question. The contractor carries out the promised energy saving measures and, in return, receives a percentage of the saved energy costs.
In the United States (U.S.), ESCOs started to operate in the 1970s. In the United Kingdom, the commercial sector is the most important ESCO market. In Germany, energy contracting is promoted through the exemption of various levies and the promotion of combined heat and power.
Legal minimum feed‐in fees for electricity from CHP
As of 2010, feed‐in tariff policies had been enacted in over 50 countries, including Algeria, Australia, Austria, Belgium, Brazil, Canada, China, Cyprus, the Czech Republic, Denmark, Estonia, France, Germany, Greece, Hungary, Iran, Republic of Ireland, Israel, Italy, Kenya, the Republic of Korea, Lithuania, Luxembourg, the Netherlands, Portugal, South Africa, Spain, Switzerland, Tanzania, Thailand, Turkey and the United Kingdom 16.
Under a feed‐in tariff, eligible CHP plants are paid a cost based price for the electricity they supply to the public grid. This enables diverse technologies to be developed and provides investors with a reasonable return. The tariffs may differ by CHP technology, location, size and region.
16 ‐ http://en.wikipedia.org/wiki/Feed‐in_tariff#By_country
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Legal minimum energy saving quotas
Subsidies and investment incentives
Tax exemptions
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The basic idea of energy saving quotas is to oblige energy suppliers, utility companies or grid system operators to achieve a certain amount of energy savings (usually expressed in kWh) among their customers and to provide proof of the saving. Energy saving quotas can apply to all forms of fossil energy, e.g. electricity, natural gas and oil. Any revenues from a buy‐out option or penalties should be restricted to energy efficiency programmes. A major challenge of this approach is to determine the actual energy saving achieved by energy efficiency measures. This requires the obliged parties to determine the actual energy consumption and to compare it to the estimated hypothetical energy consumption without the energy efficiency measure. Direct financial incentives are generally set up by public banks to operators of industrial and commercial power plants. The source of the funding can be public or private.
Energy saving quotas have so far been introduced in the following countries and regions (among others): Italy, United Kingdom, France, Belgium, the State of New South Wales (Australia), the States of Connecticut, Pennsylvania, and Nevada (U.S.).
Some U.S. states such as Connecticut and Pennsylvania have energy efficiency funds. The Southeast Europe Energy Efficiency Fund provides funding for Southern Europe and Turkey. Other instances exist in: Latin America, Asia, sub‐Saharan Africa, including the Caribbean and Pacific Island States, North Africa and other EU neighbouring countries. In general, taxation is a powerful tool to influence Examples are: Germany and the behaviour of taxpayers, even at short notice: other EU countries, India, by taxing the amount of primary energy used when Costa Rica. generating energy, such as fossil fuels,, or by taxing the amount of energy consumed by the end‐user at the price of energy rises. Since electricity companies as well as most end‐users (especially energy‐intensive companies) are highly susceptible to higher costs, tax exemptions create a very favourable economic incentive to conserve energy.
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10.2 Conclusions Management of energy is the result of the combination of several elements, including technology, experience, knowledge, operative efficiency and investments. The goal of any CHP system is a reduction of cost via the right allocation of resources. For an increase in the cost of all energy forms and a variation of the availability of the majority of them to be predictable, it is necessary to work with the aim of reducing energy consumption in buildings without compromising the quality of life. Yet the individuation of new possibilities for saving energy is a creative activity, a branch of engineering for which fundamentals can be sketched only at large. It is common knowledge that universal recipes do not exist. Each case is different from the other, only a deep knowledge of all the technical installations involved in a specific structure, a right understanding of their operation and, of course, a solid basic preparation and experience can help define the best feasible interventions for any specific case. Having said that, this treatise has engaged the main fundamentals of the combined generation of heat and power, a technology better known as cogeneration, or CHP. CHP integrates the production of usable heat and power (electricity), in one single, highly efficient process. It is a technology that generates electricity whilst also capturing usable heat that is produced in the process. This contrasts with conventional ways of generating electricity where vast amounts of heat are rejected and simply wasted. This report is not to be considered thorough: only general aspects and fundamentals of the main topics related to CHP are dealt with. For this reason the reader, does not need to hold any specific background. Of course, people with a technical education will find everything easier here. After having reached and read this page, the reader should be able to describe a CHP system, to properly contextualize it and to understand the basic principles behind its operation. The most common ways to fuel a CHP system with a renewable source should be clear, as well as the way to use CHP for cooling (trigeneration). The reader should be familiar with the main basic technologies, the main international norms and all the principal problems that may arise while developing a feasibility study for a new CHP plant. Furthermore, after having understood and carried out the exercises presented, the reader will be also able to conduct a first complete assessment of benefits and profits arising from the adoption of a CHP system, and to properly interpret data and results of other and diverse projects regarding this technology.
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CHP share of total national power production per country. Source: CHP: Evaluating the Benefits of Greater Global Investment – IEA, 2008
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