Thermal Energy Stores

Thermal Energy Stores Thermal Energy Stores 1 Introduction The Need for Clean Energy How Thermal Storage Work The UK Market 2 How Can Thermal Ene...
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Thermal Energy Stores

Thermal Energy Stores 1

Introduction The Need for Clean Energy How Thermal Storage Work The UK Market

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How Can Thermal Energy Be Stored? Phase Change Materials Underground Hot Bricks (Ceramics and Rocks) Water

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What Type Of Space Heating Is Thermal Storage Suited To? Gas Central Heating Electricity Renewable Energy (GSHP, Solar Collectors, Wood Chip) and CHP

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Thermal Energy Storage Systems Electric Heating Storage Ice Storage for Air Conditioning Renewable Energy Storage: o o o o o

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GSHP Underground Storage Systems Solar Water Heater with Accumulator Wood Chip Burner with Accumulator Solar Ponds CHP Buffering

Case Studies Domestic Scale Community Scale

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Products Available In The UK

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Useful Links And Source

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Introduction The Need for Clean Energy Heating your home is generally your largest carbon dioxide emission over the course of a year. The Energy Saving Trust has recently published figures stating that homes are responsible for 27% of UK carbon dioxide emissions, and heating is responsible for the majority of that. Improving the efficiency of the heating in our homes is a critical area for development if we are to curb our emissions in light of growing evidence supporting climate change. Improving our individual homes is something we can each take responsibility for, and for local government or designers to consider.

The Sun is the source of all energy on Earth: whether they are fossil fuels or renewable. Harnessing this abundant energy source is crucial to renewable energy technologies.

How Thermal Storage Works Thermal storage has been used in the past for storing ice to keep people cool in the summer. Large chunks of ice were transported down from the mountains to the warmer climate below, or it was broken up from frozen rivers in the UK wintertime and kept in underground ice houses to cool people in the summer. Heat storage has also existed in passive building design for millennia, in the form of heavy weight construction. Today, technological advances provide more active methods for heating and cooling: such as air conditioning, humidity control and building management systems.

Thermal energy storage, sometimes called TES, can consist of complex community-scale projects hooked up to renewable energy generation, CHP buffering systems for flats or small-scale stores for individual homes. Ice Houses at Culkein, Scotland The common purpose of a thermal store is as a buffer: between on/off peak energy prices or between supply and demand of energy. The kind of store can vary depending on what conditions it is being used in. The variety of applications and methods are outlined in the following sections. Thermal imaging photograph of heat escaping from a typical home

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The UK Market The UK is a worldwide leader in the research and development of new building technologies but has sometimes been slow to introduce these new advances into practice. This can perhaps be attributed to the cheap and abundant nature of North Sea natural gas reserves in the past. This, along with our temperate climate, means there has been little incentive until recently to adopt more efficient space heating technology or improve insulation. The UK has a sluggish uptake compared to other EU countries, such as Sweden, Germany or Denmark. As other European countries have realised, energy supplies are becoming less secure and are quickly seeking to become more efficient, sustainable and independent.

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How can thermal energy be stored? TES can be characterised by the type of material used. It may be useful to learn about the various strategies for storing thermal energy, along with their suitable applications. It’s important to realise your house or building is a ‘store’ in one sense: insulation in the walls and roof stop heat escaping and by adding more insulation you can passively store thermal energy. It should be stressed that insulation and energy efficiency are the first things to consider before investing in a renewable technology. A common name for an active thermal store is an “accumulator”. A variety of available stores are described below.

Phase Change Materials

Underground

Hot Bricks

Water

Phase Change Materials For practical use, only ‘solid-liquid’ phase change materials are used because both states are fairly manageable compared to a gas. Solid-liquid phase change materials act like any other when they are heated up: their temperature increases as they absorb the heat. But unlike other materials, when they reach a temperature at which they change state (water is solid below zero degrees Celsius, and liquid above) the material absorbs a lot of thermal energy without a significant rise in temperature. This thermal energy is released when the ambient temperature around the

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liquid falls again, and the material turns back into a solid again. The human comfort range is 20° to 30°C. In this range, phase change materials can store 5 to 14 times more heat per unit volume than conventional storage materials such as water, masonry, or rock!

Micro-Encapsulated Phase Change Material At the moment, phase change materials are good for specialised applications: such as in sub-stations with expensive equipment, namely in tropical climates. The phase change materials keep the indoor air temperature below a certain point by absorbing the heat given off by the equipment. Regular cooling systems run on diesel generators, which can be used less where phase change materials are implemented. This can mean large energy savings and provision for short term power failure. Paraffin wax can also be considered as a phase change material. It has a high specific heat capacity which makes it good for storing thermal energy; it is relatively cheap but has low conductivity so a large surface area is required for thermal storage. Form-stable paraffin wax PCMs use the paraffin as the latent heat store and an inorganic silica gel polymer to encapsulate it. Low melting paraffin waxes can be engineered to melt between 0 and 50’c.

Underground The Earth is a natural thermal store that absorbs solar energy during the day. This energy dissipates throughout the ground at a rate dependent on the composition, amount of ground water and the material. Ground-source heat pumps take advantage of the stable temperature under the ground to either retrieve warmth when it is cold at the surface, or dump warmth in the ground when it is hot above. This is a well-tested technology and is a form of thermal energy storage.

The storage capacity of the ground is dependent on a few main factors: - Depth of the borehole - Surface temperature/solar availability - Material surrounding borehole: rock, soil, clay, sand or gravel - Fill between borehole and heat exchanger can aid or hinder dissipation

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These factors can be ascertained by drilling a test borehole. In favourable conditions, the heat in the ground will dissipate slowly enough to be of use.

An enhanced, solar-assisted ground-source heat pump can boost the amount of energy in the store by pumping hot water from solar collectors down into the boreholes. This can provide the buffer between supply and demand that is so often crucial with solar water heating. If conditions are favourable and the installation correctly engineered, then the store can last from summer through to winter. Drilling a borehole for a GSHP in Broxtowe

A ground-source heat pump extracts naturally stored heat from the ground. Using solar collectors on your roof, connected to the GSHP, can boost the ground’s natural ability to store energy.

Hot Bricks Clay bricks, rocks or other ceramic materials can be used for short-term heat storage. All of these materials have a high specific heat capacity, meaning they can hold a lot of heat within themselves. This is important for use with electric space heating systems with two-tariff metering. As a tried and tested method, it has proved to be a questionable technology: see Section 4 on electrical heating storage.

Water The most common method of storing thermal energy is with water. It’s cheap, abundant, inert, safe to handle and has a high specific heat capacity. It is also compatible with existing hot water central heating systems. One problem is with system corrosion and subsequent leakages. Water has a good heat-storage-to-volume ratio, five times greater than rock, and can easily be used as a transfer medium with solar collectors and ground-source heat pumps. Thermal energy storage is made practical by the large specific heat capacity of water: 4181 Joules per Kg of water per ‘C at room temperature. This is only beaten by ammonia, helium and hydrogen, all of which are harder to control as a liquid.

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What type of space heating is thermal storage suited to? Gas, Electrical and Renewable The type of thermal store suitable for a certain application depends on the type of space heating. The most common in the UK at the moment are mains gas powered central heating with water as the heat transfer fluid. Most of these systems are relatively reliable, cheap to run, safe and are suitable for individual homes or flats with a gas connection. The systems commonly have hot water cylinders for short-term storage of hot water, only for the convenience of having hot water available instantly. With this kind of heating systems, there is limited value in having thermal storage since demand and supply are generally in synch. Electrical resistance heaters are increasingly being used in new buildings in the UK; predominantly flats in London. These flats have no mains gas supply and the use of electrical space heating is argued to be environmentally friendly on the grounds that the supply is from a ‘green tariff’. This is not quite the full picture since studies have shown that electrical heating and hot water generate among the highest CO2 emissions. According to current research from the DTI, “up to at least the year 2050 and probably longer electric resistance heating will continue to be a waste of the earth’s resources. We would be better off burning gas or LPG for heating. In towns, a better option would be to use combined heat and power (CHP)”, from either biomass or natural gas. The largest problem with using renewable energy sources for space heating is the time difference between supply and demand. A typical situation is that during the day the Sun is at its hottest but the demand for space heating is at its lowest. This kind of delay is common with renewable technologies and so storage, or buffering, is the key to success in many situations. A lot of time and money is being invested in developing technologies for this purpose.

Electrical resistance heater vs. gas-powered central heating radiator

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4 Thermal energy storage systems Electric Heating Storage This can be useful for electric heating systems with two-tariff electrical metering, where the heater can be switched on at night (using cheap off-peak electricity) to ‘charge’ the hot bricks or rocks, and the next day the heat can be released.

Since the National Grid operates continuously, a cheaper supply is maintained during the night since demand is low. During the day when demand is high, the Grid responds by generating more electricity. Within this system, as long as you have a two-tariff contract, the electricity is cheaper between 00:30 and 07:30. This kind of tariff is called either Economy 7 or White Meter in the UK and is designed to power such storage heaters or immersions. The popularity of using this kind of thermal store has declined at lot recently because of the numerous drawbacks and because of the increased popularity of cheaper gas powered heating. Using hot bricks for thermal storage has proved itself to be inefficient, hard to manage and not particularly environmentally friendly. Electrical and gas space heating are talked about in Section 3

Drawbacks: - Hot brick storage heaters are cumbersome and very heavy due to the material used. - If too much heat is stored, it will eventually escape into the building anyway. - If too little heat is stored, i.e. on a surprisingly cold day, then full-price electricity is required to compensate. - The environmental benefits are uncertain since using high-grade electricity for generating heat is inefficient. Two-thirds of the initial fossil fuels used in the centralised power station are lost through generation inefficiencies and transmission losses.

Positives: - If controlled properly, it can make space heating a bit cheaper since it takes advantages of off-peak electricity. Two-tariff prices are currently around 12.95p kWh Peak and 4.81p kWh Off-peak (Ecotricity: April, 2010). We would not advise using this kind of storage heater unless there are no alternatives to electric space heating. For remote situations where there is no gas connection, we advise

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considering an LPG, bio-diesel, biomass or solar water heater system.

Ice Storage for Air Conditioning In America where air conditioning is common, new technologies are emerging for storing ice. This is similar to the idea of charging up the hot bricks at night when electricity is cheap for use the next day. Storing ice like this yields economical savings, but does not produce significant reductions in CO2 emissions, similar to the hot bricks. For large applications, such as universities or hospitals, a saving in kWh of up to 15% can be achieved.

Renewable Energy Storage GSHP Underground Storage Systems Borehole thermal energy storage systems can store large quantities of solar heat collected in summer for use in the winter, or from the day through to the night. For more information on ground-source heat pumps look at the guide on the NEP website. Once the borehole has been drilled, a plastic U-shaped pipe is inserted into the hole. The surrounding space around the plastic pipe is filled with a highly conductive material, usually bentonite. The storage of thermal energy underground fits perfectly with the use of a solar water heater or a cogeneration system. Both of these renewable technologies, as mentioned earlier, often have an inequity between the availability of energy and the demand for it. Using the ground as a thermal store means the energy can be stored when it is available (a sunny day for solar collectors, and unused excess heat for CHP) and used usefully later on. This can result in a highly efficient system. Typical borehole U-tube heat exchanger See the Drake Landing Solar Community in Section 5 for a thorough example of this at work on a large scale. To optimise the efficiency of the system and to offset the large amount of energy exerted to drill the boreholes, this kind of thermal store may be better suited to

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larger, community-scale applications. In these situations, there may be a network of solar collectors on all the roofs in a neighbourhood, or a cogeneration system, all connected together. And all this will be linked to an array of boreholes that act as the store. Alternatively, when larger buildings require pile foundations the ground loop can be embedded into these during construction.

Solar Water Heater with Accumulator As mentioned previously, the purpose of a thermal store for a renewable energy source is to act as a buffer between supply and demand. Solar water heating systems integrate in with a ‘wet’ central heating system to supply hot water and heating. The thermal stores resemble typical hot water cylinders but are larger. These are readily available and are often installed along with the roof-top collectors. The typical store is a large hot water cylinder with lots of insulation surrounding it to minimise heat loss. There are two coils inside the cylinder to allow for two heat inputs: solar and gas. As shown in the diagram below, the lower coil provides heat from the solar collectors and the top coil boosts the temperature when needed. This means that an adequate temperature is always achieved.

Viessmann solar water heating setup with dual cylinder water heater

Wood Burner with Accumulator Biomass burners can include log, wood pellet or wood-chip burning central heating systems. All of these are available for single homes, but their efficiency improves at larger scales or with an accumulator. Log burning systems need to be loaded by hand and so could be unsuitable for some people. With a decent accumulator they may only need to be fired once a day or less. Whilst pellets and wood chips can use automatic feeds and cleaning which tend to be expensive.

Whichever the form of the wood, the boiler can usually be installed with an integrated

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accumulator or thermal store. These store water up to 90’c which acts as a buffer between the energy released when the wood is burnt and the time when the hot water is required. Without a well-insulated thermal store, a part of the energy released from the biomass is lost to the surroundings forever. If you are considering a biomass boiler, it is definitely worth investing a bit extra in a decent store. A lot of installations have lent themselves to larger applications, such as schools, large houses, warehouses, libraries or flats. Smaller systems exist but still require space for the wood store and machinery. Accumulators, or thermal stores, can be either vented or unvented. A vented store will have a water tank above to allow for expansion: it is open to the atmosphere. The pressure may be lower than required and may need a pump. An unvented store is pressurised, typically to about 6 Bar, and will have an emergency pressure release. With an unvented store you can have high pressure hot water and safety from contaminants (vermin). Unvented stores are usually more expensive since they must deal with higher pressures. Burrough Court, FireMatic 150)

Leicestershire

(Herz

Solar Ponds Solar ponds are large-scale collectors and stores of the Sun’s energy. They can be used for a variety of applications, including: refrigeration, space heating, water desalination, drying and even electrical generation.

Salinity gradient solar pond in El Paso, Texas It consists of a body of water (whatever size available) that absorbs solar energy similar to any other pond or lake. It differs by having layers of salt solution at increasing concentrations. Because of this the pond must be lined with either a clay bed or plastic

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sheeting. The water has three prominent layers: a top layer of low salt content, a bottom layer with a high salt content and highest density, then a middle layer which acts as the insulation layer and salt gradient. This configuration of water densities means the pond stratifies, and the natural heat convection is stopped that usually allows heat to escape quickly. Solar radiation penetrates the water at the bottom layer, where it is trapped. This layer can reach temperatures of up to 90’c where the surface temperature is only 30’c! A heat exchanger runs along the bed of the pond and transfers the stored heat to the application site, generator or heat pump. They are very useful for rural areas where space is available, developing countries since the setup costs are low, or countries with a lot of Sun. They have been used and tested around the world, namely Israel, the US, India and Australia.

CHP Buffering TES systems that can be charged with heat are very well suited to cogeneration systems (i.e. CHP). Exhaust heat from the cogeneration system can charge the system, and be released on demand. Take the example of a hotel, hospital or residential care home with a cogeneration system setup to provide electricity and heating. During parts of the day, all of the available heat from the CHP may be required. However, when the heat demand is low then the heat store can be charged with the excess heat. Later on, when air conditioning or heating is at peak load, the store can be discharged to provide the supplementary cooling (via an absorption chiller) or heating. This supplementary heating or cooling will reduce the required installed capacity of the CHP and increase the overall efficiency by maximising the heat output from the CHP. Using thermal stores for community CHP in the UK can improve the economics of the setup and can help to absorb the output of fluctuating renewable energy sources, such as wind. DESIRE (Dissemination Strategy for Balancing for Large-Scale Integration of Renewable Energy) is a UK research project. The full report is available via a link in Part 7. Community scale CHP with a thermal store allows for the independent generation of electricity when grid prices are high. For example, the CHP will run during the day and not at night. The heat generated can be stored when it is not needed, and when grid electricity prices are low, the CHP can stop generating and still service the heat demand from the stores. In this way, the community can ‘play the market’ and make considerable savings on their electricity bills. At the moment, most existing community-size CHP plants in the UK do not have thermal stores. They are usually quite small and are only sized to minimum heating demands in order for the plant to run efficiently. In Denmark, larger installations with large stores have proved to be very successful. In order to integrate an increasing proportion of renewable energy beyond 20%, the UK must consider large-scale balancing methods. This is in order to counter the fluctuating supply of energy from renewable sources: for wind turbines, photovoltaics, etc. Technology such as gas-engine CHP with thermal stores can come on- and offline quickly to act to balance the uneven supply.

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Case Studies Domestic Scale Solar Water Heater and Log Fired Boiler When considering home refurbishment, improving energy efficiency should be kept in mind. In today’s market, this is increasingly becoming a selling-point and worth the investment. The EST has a case study of a solid-walled Victorian semi that is a good example of a thermal store application. After improving the general efficiency of their home, the owners invested in some renewable technologies to help make their home even more environmentally friendly. The installation included: roof-top solar hot water collectors (4m2 flat plate) and a biomass boiler (14-28kW log boiler) which both provide space and water heating. To accompany these there is an 1100 litre thermal store. The solar collector connects to the first, bottom coil of a 200 litre twin-coil factory-insulated hot water cylinder, and the thermal store connects to the second, top coil in the cylinder. The store allows for the integration of the log fired boiler: it acts as a buffer which allows the boiler to function at peak capacity, at its highest efficiency, for prolonged periods of time. The store minimises the inequity between supply and occupant demand. The cost of the solar system (including the 200 litre cylinder) was about £2,500. The log fired boiler with accompanying accumulator cost between £7,500 and £8,000 before grant funding.

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Log fired boiler (left) and roof-mounted solar collectors (right). See Ruralenergy.co.uk for more case studies of biomass boilers with accumulators.

Beaufort Court, Head Office for Renewable Energy Systems Completed in 2003, this old farm is now a high-tech head office for RES. The offices are an exhibition ground for the technologies the company specialises in. Included are: • • • • • • •

a biomass crop field: 5 hectares of Miscanthus, annually harvested in winter 100kW biomass boiler, with backup gas-fired boiler a 225kW wind turbine: 36m hub height, 29m rotor diameter, expected 250MWh annual a 75m deep borehole used for cooling: a local chalk aquifer water supply at 12’c advanced PV cells with waste heat capture: 54m2, copper heat exchanger on the back 116m2 of solar hot water collectors a large underground seasonal thermal store: 1400m3 body of water with a 500mm piece of floating insulation on top.

Any excess, unneeded heat from the advanced PV cells and SHW is channelled to the store. During the summer and parts of the autumn, heat is feed into the store. The water in the store will gradually heat up (this could take a few seasonal cycles to reach optimum temperature) and can be used in the winter. An air distribution network is used to heat the buildings with the stored heat.

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They estimate the store will lose 50% of its heat over the course of the year. The stored heat will be low-grade, but will still be useful to preheat incoming air since the outside air in winter is significantly colder the stored water. The hierarchy of sources for heating goes: direct from solar collectors if available, thermal store, biomass and then gas backup.

COMMUNITY SCALE UOIT, University of Ontario Institute of Technology Completed in 2004, the geothermal site provides heating and cooling the University throughout the year. The 384 boreholes, each at 213 metres in depth, tap into a natural geothermal supply. In winter, the fluid circulating through the borehole heat exchangers collect heat from the earth and supply it to the buildings above ground. In the summer, the system reverses and dumps heat from the buildings deep down under the ground. The system has proved to significantly reduce energy consumption in the heating and cooling of the university buildings, as well as providing on-site education for students.

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Swedish Solar Heated Residential Area with Seasonal Thermal Storage The initial results for a partially solar heated estate of 50 residential units are so far quite positive. The estate is in Anneberg, Sweden, and has low-temperature space heating coupled to a seasonal ground store of solar heat. The heat is supplied by 2400 squaremeters of solar collectors, and individual electric heaters are used for supplementary heating in the homes. The store consists of roughly 60,000 cubic-meters of crystalline rock with 100 boreholes at 65 metres depth. Typically, double U-tube heat exchangers are used. The estimated heat loss from the heat store is about 40% of stored solar heat and the average solar fraction is roughly 70% after 3-5 years of operation. The initial results show that after 2 years of operation, the problems encountered have led to unexpected inefficiencies but the system idea works as intended. The full report is written by M.Lundh and J.O.Dalenback of Uppsala University in Sweden, and is available from Sciencedirect.com

Drake Landing Solar Community, in Canada This is a very interesting example of how a new master-planned housing estate can work towards self-sufficiency and generate its own heat and power from mainly renewable sources.

(dlsc.ca) Diagram showing the system layout

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The site boasts an improvement of 30% on conventional house efficiency standards and the ability to generate 90% of the houses space heating needs from solar thermal.

The estate has 52 homes, each with a separate garage covered in solar collectors. There are 800 in total and are all networked into a seasonal AND short-term store. This allows for storage from day-night and summer-winter. Borehole thermal energy storage provides the long-term storage, and water tanks provide the short-term storage. These tanks also act as the hub between the collectors, the houses and the underground store. The individual solar collectors transfer heat along underground pipes to the short-term storage tank at the central hub. During the hot summer days, excess heat from the short-term store is transferred to the underground store: a collection of 144 holes, 37 metres deep arranged in a grid array 35 metres in diameter. By the end of the summer, the ground will reach about 80’c. It is thought it will take a few years for the ground to fully charge to this temperature. This energy is then released during the winter. The centre has a backup gas boiler in case the supply cannot meet demand.

Portsmouth Charles Dickens Estate The city council secured £425k of funding from the Community Energy Programme, for a CHP unit and thermal store. The system saves about 420 tonnes of CO2 per year, and exported electricity is used in other council buildings saving about £112k. The scheme covers over 500 flats, a local primary school, a nursery and leisure centre. The CHP is 0.6MWe and 2.2MWh, with a 70 cubic metre store (pictured right).

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Products Available

Company

Technical Details

SunGain

Full solar water heating system: including thermal store, collectors, controls…

£3,000

NEP website, or 01159859057

Green Heat Team

Full solar water heating system, wood pellet boilers and log fired boilers (all with accumulators)

On request

www.green-heatteam.co.uk

Energy Shop

Standard vented twin-coil copper cylinders for use with solar water heating. 123 – 210 litre.

£380 - £495

www.energysaving.me.uk

Consolar

Thermal stores for solar water heating, wood fired boilers or heat pumps.

On request

www.consolar.co.uk

On request

www.gledhill.net

Gledhill

Viessmann

Rural Energy

Approx. Cost

Thermal stores for solar water heating, heat pumps or microchip. Vented or sealed Twin-coil cylinders for solar installations. Biomass boilers, heat pumps, photovoltaic cells and solar water heaters Buffer tanks, predominantly for woodboilers but with option second coil for solar water heating. 300-5000 litre. Unvented

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On request

300L = £1017 (1 coil) £1280 (2 coil)

Contact Details

www.viessmann.co.uk

www.ruralenergy.co.uk

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Useful links & Sources www.dlsc.ca Drake Landing Solar Community www.lowcarbonbuildings.org.uk Low Carbon Buildings Grant Scheme www.sciencedirect.com Source of Scientific Research www.acca.org/tes AC Contractors of America www.engineering.uoit.ca/facilities/borehole.php University of Ontario, UOIT www.energysavingtrust.org.uk Energy Saving Trust www.energysavingtrust.org/housingbuildings/publications EST Tech. Guides www.ierp.bham.ac.uk/publications/desiresummaryofresearchfindings.pdf CHP with thermal energy storage www.renultd.co.uk Renewable Nottinghamshire Utilities www.beaufortcourt.com Beaufourt Court

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