Combined Heat and Power (CHP) systems Heat Engines & Boilers
Contents • Introduction • Features of cogeneration • Cogeneration with - steam turbines - gas turbines - internal combustion engines - ORC systems - fuel cells • Tri-generation • Economics of Cogeneration
Introduction • Cogeneration (Combined Heat and Power – CHP production) is the simultaneous generation of heat and power • CHP systems generate electricity (or mechanical energy) and thermal energy in a single, integrated system Four categories of CHP applications: small-scale CHP schemes: to meet space and water heating requirements in buildings, based on spark ignition reciprocating engines large-scale CHP schemes: for steam raising in industrial and large buildings, based on compression ignition reciprocating engines, steam turbines or gas turbines large scale CHP schemes for district heating: based around a power station or waste incinerator with heat recovery supplying a local heating network CHP schemes fuelled by RES: these may be at any scale
Features of Cogeneration • Purpose of CHP: the power generation besides heat generation (instead of the converse case)! • The existing heat is generated during power generation: the evolved energy of the fuel combustion in heat engines is utilized, thus thereby electricity is obtained while the remaining heat is extracted with heat exchangers. • The success of CHP depends on: “using recovered heat productively”, so the prime criterion is a “suitable heat demand”. • CHP is likely to be suitable where there is a fairly constant demand for heat for at least 4,500 hours in the year! • The timing of the site’s electricity demand is important as the CHP installation will be most cost effective when operating during periods of high electricity tariffs. • The most efficient CHP systems (exceeding 80% overall efficiency) are those that satisfy a large thermal demand while producing relatively less power. • The use of biomass fuels or waste materials in CHP schemes leads to increased cost-effectiveness and less need for waste disposal
Benefits of CHP when sized according to heat demand: • Large cost savings, thus additional competitiveness for industrial and commercial users, and affordable heat for domestic users. • An opportunity to move towards more decentralised forms of electricity generation, with a plant designed to meet the needs of local consumers, providing high efficiency, avoiding transmission losses and increasing flexibility in system use. • The improved local and general security of supply reduces the risk for consumers to run out of electricity and/or heating. • The reduced fuel need reduces import dependency • Provides an opportunity to increase the generation plant diversity, and competition. • Increased employment: the development of CHP is a generator of jobs. • An economically productive approach for the reduction of air pollutants through pollution prevention (traditional pollution control achieved solely through flue gas treatment provides no profitable output and actually reduces efficiency and useful energy output).
Technical principles of operation
Prime movers options
A Cogeneration plant consists of four basic elements: a prime mover (engine which drives the electricity generator). It may be: • a steam turbine, • a gas turbine or • a reciprocating engine • new technology options: micro-turbines, Stirling engines, ORC systems, fuel cells an electricity generator Heat engines a heat recovery system External combustion Internal combustion Spark Compression a control system Turbines Engines ignition ignition Steam Steam Gasoline Diesel Classification by heat turbine engine engine engine engine type Gas Stirling Gas engine turbine engine Combined Combined cycle cycle ORC system
Cogeneration with steam turbine - Steam turbines: • are the most common prime movers, particularly in industries and district heating; • are a well proven technology when there is demand for both electricity and large quantity of steam at high and low pressures; • range from a few kWs (for economy reasons mostly) to a few hundred MWs (turbines below 2 MW may be uneconomical, except where the fuel has no commercial value). - CHP with steam turbine is based on the utilization of steam: the steam first rotates the turbine by the turbine blades, and then the extracted heat from the remaining energy of the steam is utilized • Fuel used: any type of solid, liquid or gas fuel, depending on boiler requirements • Most widely used types of steam turbines: the backpressure, and the extraction-condensing, a rather new technology option is that of the boundary layer (Tesla) turbine.
Backpressure steam turbine A CHP system using a backpressure steam turbine consists of: a boiler, the turbine, a heat exchanger and a pump
3 different configurations, depending on the pressure (or temperature) levels exist: the simple backpressure; the extraction backpressure; the double extraction backpressure. • The total efficiency of a backpressure steam turbine CHP system is the highest. • When an efficient boiler is used, the overall thermal efficiency of the system can reach 90%.
Energy flow diagram of backpressure steam turbine application
Extraction - condensing steam turbine
• The heat extracted from the steam in extraction-condensing systems is optimised by exhausting the steam from the turbine at less than atmospheric pressures. • These turbines have higher power to heat ratio in comparison to backpressure ones. • Overall thermal efficiency < that of backpressure turbine system (exhaust heat cannot be utilized - normally lost in the cooling water circuit).
Cogeneration with gas turbine
Two main categories of gas turbines: • aero-derivative turbines (modified versions of the original aircraft turbines): Main characteristics: low specific weight, low fuel consumption, high reliability. Advantages: high levels of efficiency, compact design, easy access for maintenance. Disadvantages: relatively high specific investment cost, high quality fuel, a lowering in output and efficiency after a long period of operation. • industrial gas turbines (robust units for stationary duty and continuous operation)
Cogeneration with combined cycle
• CHP with combined cycle = Combination of different CHP types: The gas turbine - steam turbine combination is the most common one! • Supplementary firing can increase the flexibility of the system, but in case of application reduce electrical efficiency.
Combined cycle principals • The highest electricity generation efficiency can be reached nowadays by combination of gas turbine with steam cycle. • This means that Rankine cycle is heated by exhaust gas of a gas turbine.
Combined cycle principals
Efficiency of combined cycle:
Efficiency of gasturbine:
Efficiency of steam cycle:
tot
GT
=
.
Q in
=
steam
.
PGT + Psteam
PGT .
Q in
=
Where input heat to the steam cycle: Q transfer
Psteam .
Q transfer .
= Q stea min let
Total efficiency of combined cycle .
.
.
Q transfer = Q in PGT = Q in (1 .
.
.
Q stea min let = Q transfer
= Q in
GT
(1
GT
.
Psteam= tot
=
tot
=
steam
PGT +
Q stea min let = .
Q in
steam
(1
) .
Q in
steam GT
)
(1
GT
)
)
.
Q in
GT
1+
steam
steam
GT
steam
Examples:
GT
= 0 ,3
= 0 ,3
= 0 ,8
steam tot
= 0 ,47
GT
= 0 ,35
= 0 ,38
= 0 ,9
tot
= 0.575
Expected development of power generation
Additional firing application .
Input heat:
Q in = QGTin + Qadd Qadd fadd= Qin=QGT(1+fadd) QGT PGT + Psteam PGT + Psteam = tot = Qin QGTin (1 + f add )
• Additional firing decrease efficiency, • but increase operation flexibility of the system.
Combined cycle system schematics
Energy distribution in a combined cycle system
Single pressure non reheat HRSG
Energy transfer diagram of a simple pressure HRSG
Two pressure level system
Energy temperature diagram of a triple pressure HRSG
Connection of steam cooled bladed gas turbine
Typical triple pressure combined cycle
Combined power station example Újpest CC/GT power plant Gas
ST
70 bar 500C
Oil
70/6bar . 1.
70/25bar
SB
2.
70/1 bar
HRSG
Steam consumer Gas
cooling tower
Oil
HWB G.
GT
Heat consumer
Combined power station example
Combined power station example
0,958
0,917
0,875
0,833
0,792
0,75
0,708
0,667
0,625
0,583
GT
0,542
0,5
0,458
0,417
0,375
0,333
0,292
0,25
0,208
0,167
0,125
0,083
0,042
0
Operation schedule example of a combined cycle
MWe
80
ST
70
60
50
40
30
20
10
0
Cheng cycle • In this system steam generated from the heat of exhaust gas is fed back to the gas turbine. • In this system there is not needed extra steam turbine. • But the total steam quantity feed back to the gas turbine can cause operational and Hydrogen diffusion problems. That is why in this pure form it has not been applied. • But a small amount of steam injection into gas turbine can reduce peak temperature and reduce NOx emission meanwhile increase the power of the gas turbine. This solution is called partial Cheng cycle.
Externally fired gas turbine with partial Cheng cycle
Integrated Gasification Combined Cycle (IGCC)
Integrated Gasification Combined Cycle (IGCC)
Poly-generation system example
Cogeneration with Internal Combustion Engines (ICEs) • IC engines: are mostly used in low and medium power CHP units; have higher electrical efficiency compared to other prime movers, but the thermal energy produced is not easily used (due to its lower temperatures it is dispersed between exhaust gases and engine cooling systems). • IC engines can be: spark ignition (Otto-cycle) or compression ignition (Diesel-cycle) Advantages (relative to other CHP technologies): low start-up and operating costs; reliable onsite and clean energy; ease of maintenance; wide service infrastructure. Schematic diagram of cogeneration with an internal combustion engine
Compression ignition (Diesel) engine Categories of “Diesel engines”: • Two-stroke (or low-speed) engines: ignition takes place once every revolution, engine speed < 200 rpm, output of 1-50 MW, electrical efficiency: 45-53%. • Four-stroke engines: ignition takes place during every other revolution. They can be: medium speed engines: speeds between 400 and 1000 rpm, ratings between 0.5 and 20 MW, electrical efficiencies of 35-48%, or high-speed engines: speeds between 1000 and 2000 rpm, ratings between a few kW and about 2 MW, electrical efficiencies of 35-40%. • Fuels used: diesel, heavy / light fuel oil, LPG, natural / producer / digester gas, etc.
Spark ignition (Otto) engine • An electric spark from a spark plug ignites a mixture of fuel and air. • The SI engines used in power generation applications may be “gasoline engines” or “diesel engines converted to have spark-ignition operation”. • Ratings’ range: 20 kW to 1.5 MW. • Operation speeds: 750 - 3000 rpm. • Electrical efficiencies: 25 - 35%.
Gas Engine Connections
Energy flow diagram of the gas engine
Essential data mech
W& shaft = Q&
e
firing
br
PGE = Q& firing
PGE + Q& GE = & Q firing
e ,net
GE
PGE Pself = Q&
heat
PGE = = & QGE
heat ,useful
firing
Q& useful = Q& GE Q& self
sum ,net
=
PGE
Q& GE = Q& firing
e heat
Q& useful = Q& firing
Q& emergencycooling Q& exhaustgasbypass
Pself + Q& useful Q& firing
net
PGE Pself = Q& useful
Electrical efficiency variation at part load in case of different engines 43,00 42,00 41,00 40,00 39,00 Jenbacher 320 38,00
Jenbacher 316 Jenbacher 420
37,00
Deutz V20
36,00 0,5
0,6
0,7
0,8
0,9
1
Net total efficiency versus heat utilization sum ,net
=
PGE
Pself + Q& useful Q& firing
The cogeneration ratio versus net total efficiency
cog,net
el:
1,45 1,35 1,25 1,15 1,05 0,95 0,85 0,75 0,65 0,55 0,45 0,65
0,7
30%
0,75
35%
0,8
net
PGE Pself = & Q useful
40%
0,85
Net summa efficiency
45%
0,9
50%
0,95
Cogeneration with External Combustion Engines Steam (or Spilling) engine
• CHP production with the use of a steam engine is similar to the one with a steam turbine. • Currently available steam engines: 30kW - 500kW. • Fuel used: any type (solid, liquid or gas) depending on boiler requirements.
Stirling engine • Stirling engines operates by the temperature difference of the inside medium. • Three main groups of Stirling engines’ configurations: the Alpha, Beta and Gamma arrangements (see figures below)
Alpha Stirling
Beta Stirling
Gamma Stirling
Stirling engine • A Stirling engine is a closed-cycle regenerative heat engine with a gaseous working fluid. • "Closed-cycle" means the working fluid, the gas which pushes on the piston, is permanently contained within the engine's system. • This also categorizes it as an external heat engine which means it can be driven by any convenient source of heat. • "Regenerative" refers to the use of an internal heat exchanger called a 'regenerator' which increases the engine's thermal efficiency compared to the similar but simpler hot air engine. • Noted for its high efficiency, quietness of operation and the ease with which it can utilise what would otherwise be waste heat, the Stirling engine is currently exciting much interest as the core component of domestic combined heat and power (CHP) units, the widespread adoption of which could have a significant effect upon worldwide carbon dioxide emissions.
Operation of two-cylinder Stirling engine
Stirling engine for biomass firing
ORC process based co-generation
Advanced ORC process based cogeneration
Energy flow chart of the ORC process
Total system chart
Annual operation of CHP system
Fuel cells • They generate electricity by electro-chemical reaction directly from the fuel based on the oxidation of H2 • A typical single cell delivers up to 1 V. • The fuel cell generates heat also, which can be utilized. • Electrical efficiency can reach 40-70% depending on cell type. • End product is pure water
Fuel cell principal
Problems with fuel-cell application • H2 is used as fuel can be derived from natural gas, propane or coal, but these are fossil fuels • Or it can be gained from biomass, • Or through electrolysis from wind or solar energy • So hydrogen is not an original energy resource, only an energy storage medium • Further problem is that Hydrogen storage and supply can not be handled with existing fuel supply systems, new supply system has to be developed.
Tri-generation systems CHCP-Combined Heat, Cooling & Power production: • For space cooling of buildings in the residential, commercial or industrial sector. • Heat-driven district cooling, requiring heat mainly in summer, can help to balance the seasonal demands for cogenerated heat. • This increases the overall efficiency of the system.
Tri-generation with absorption chiller • The absorption refrigerator is a refrigerator that utilizes a heat source to provide the energy needed to drive the cooling system. • Absorption refrigerators are a popular alternative to regular vaporcompression refrigerators where electricity is unreliable, costly, or unavailable, where noise from the compressor is problematic, or where surplus heat is available (e.g. from turbine exhausts or industrial processes).
Tri-generation with ejector cycle
Operation principal of the ejector
Technical aspects having an impact on cash flows Selection of the appropriate technology CHP technology characteristics System
Electric power (MW)
Steam turbine Open cycle gas turbine Closed cycle gas turbine Joule-Rankine combined cycle
0.5 – 100*
Diesel engine
Annual Electric efficiency (%) Total average efficiency Load Load availability (%) 100% 50% (%) 90 - 95 14 - 35 12 - 28 60 - 85
Heat to power ratio 2 - 10
0.1 - 100
90 - 95
25 - 40
18 - 30
60 - 80
1.25 - 2
0.5 - 100
90 - 95
30 - 35
30 - 35
60 - 80
1.25 - 2
4 – 100*
77 - 85
25 - 35
25 - 35
70 - 88
1.67 - 5
0.07 - 50
80 - 90
32 - 40
32 - 40
60 - 85
0.42 – 1.25
Reciprocating 0.015 - 2 80 - 85 25 - 35 25 - 35 60 - 80 1.43 - 2 ICE Fuel cells - 50 upper limit 90 -for92industrial 37 - 45 37 - 45 85 can - 90have higher 1 – 1.25 *The value of 100 MW0.04 is a usual applications. Systems of this type capacities too 0.59 – Stirling engines 0.003 – 1.5 85 - 90 34 - 49 34 - 49 60 - 80 0.83
Fuel supply options Commercial fuels - fossil fuels extracted and treated or refined and sold nationwide: Coal: it has been long used in CHP plants (e.g. in large district heating schemes in Eastern Europe and Denmark); many coal-fired CHP schemes are old, relatively inefficient and polluting, but some new plants embody advanced coal combustion technology. Heavy and extra heavy fuel oils. Gas oil (diesel). Natural gas: increased use in power generation since the 1980s; it is cheap, flexible and it releases less CO2 / MJ delivered than coal and oil. LPG, naphtha, etc. Waste fuels - by-products or adjuncts of processing: Solid: wood off-cuts from furniture manufacturers, forestry and farming biomass, domestic refuse collected by municipalities, waste tyres. Liquid: black liquor from wood pulp manufacture. Gaseous: biogas evolved from e.g. sludge digester gas in sewage treatment works or landfill gas or synthetically produced biogas from: solid and liquid waste, or tail gases (from iron and steel works, chemical plants, refineries).
Heat recovery options • The heat recovery boiler is an essential component of the CHP installation: the simplest one is a heat exchanger Exhaust gases pass through it and heat is transferred to the boiler feedwater to raise steam; then, the cooled gases pass on the exhaust pipe or chimney and discharged into the atmosphere. Not all of the heat of exhaust gases can be recovered in a boiler because: for effective heat transfer, the temperature of the exhaust gases must remain above the temperature of the fluid to be heated (DT J 30°C); exhaust gases must not be cooled to a temperature at which their buoyancy prevents their proper dispersion under all weather conditions; exhaust gases must not be cooled to a temperature at which acid condensation could occur (sulphur contained in oil fuels can then be condensed into sulphuric acid); the latent heat of the water vapour in the exhaust gases can only be recovered by reducing the exhaust gas temperature to below 100°C. Exhaust heat boilers must be designed for the particular exhaust conditions of the specified turbine or engine.
Economics of Cogeneration Plants Capital costs Definition: “cogeneration unit(s) and associated plant, the expenditure required for the establishment of an operational CHP on the site”, including: installed, tested and commissioned; fuel supply, storage and handling; connection charges including reinforcement of local/national electricity networks; all associated mechanical and electrical services, installed and commissioned; new buildings, modification to existing ones, foundations and support structures; operator training, first set of spare parts or special tools for servicing and repair; engineering design: compliance with planning and building regulations environmental requirements, fire prevention and protection etc, and external professional services engaged to handle these matters. The investment costs are affected by: the local conditions; the heat utilization mode and location (e.g. existing heating system, distances).
• Gas turbine and large reciprocating engine CHP plant: Prime mover/generator and associated equipment: 40-60% of total installed cost. Heat recovery equipment and associated equipment: a further 15-30% of the costs. Electrical switchgear and protection equipment: 5-15% of costs. Investment cost in a micro gas turbine: 400 - 800 €/kWe Investment cost of a power station gas turbine: 250 - 750 €/kWe • Small-scale CHP plants based on spark ignition gas and dual-fuel diesel engines: Generator, heat exchangers, control equipment: 50-60% of total installed cost Investment cost for a gas engine: 700 to 1350 €/kWe • Steam cycle CHP plant: Investment cost of a boiler/steam turbine unit: 620-800 €/kWe Cost of an additional steam turbine to an existing gas turbine or steam boiler: 300-700 €/kWe Investment cost for Tesla turbine unit: 1000 - 1200 €/kWe • Steam engine unit: Cost of steam engine and generator system: 1300 to 2550 €/kWe Investment cost for an ORC system: 2040 - 2710 €/kWe Investment cost of Stirling-engines: 600 to 800 €/kWe
Operating costs Annual costs of operating a CHP plant: Fuel for the prime mover, and for supplementary and auxiliary firing if applicable. Labour for operating and servicing the plant. Maintenance materials and labour: scheduled maintenance by the manufacturers. Consumables: e.g. lubricating oil, feed water treatment chemicals, cooling tower dosing, as applicable. Back-up electricity prices and top up and export electricity prices. • Start-up cost: the costs related to the starting of the system (pre-warming, mediumfilling, etc. costs). It depends on the size and type of the system. • The system start-up time is a cost Type of Heat engine (or Typical ranges of “O&M” costs factor, which is: CHP plant) (€/kWh) for ICEs: 10 sec, for steam Natural gas (spark 0.0075 – 0.014 turbines: 1 hr to 1 day, for power ignition) engine ignition station gas turbines: 10 min to 1 hr, Compression 0.005 – 0.007 ICE for micro gas turbines: just 60 sec Steam turbine CHP plant around 0.004 Some typical O&M costs
Gas turbine Micro - turbine Fuel cell
0.002 – 0.007 0.002 – 0.01 0.003 – 0.015
Overall economics of Cogeneration Plants • •
•
CHP projects can result in simple payback periods of 3 to 5 years, or less. The economics of CHP projects are much more sensitive to changes in electricity prices than in fuel prices: e.g. a 10% increase in electricity prices might reduce the payback period by 15%, whereas a 10% reduction in fuel price would reduce the payback period by only 6%. A sensitivity analysis must be part of the feasibility study, no matter which method of economic analysis is employed…
Factors favouring short payback periods include: low investment costs low fuel prices high electricity prices minimum cogeneration fuel price premium (compared with boiler fuel) high annual operating hours high overall thermal efficiency
Economic comparison of various generation technologies
Typical efficiencies of various types of plants
Economic and operation characteristics
Summary You are already familiar with • Features of cogeneration • Cogeneration with - steam turbines - gas turbines - internal combustion engines - ORC systems - fuel cells • Tri-generation • Economics of Cogeneration
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