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Microgeneration: Heat Generation Technologies

(Solar Thermal Hot Water, Ground Source Heat Pumps, Air Source Heat Pumps, Bio-energy)

The following is a description of the current state of technology.  For analysis of technology potential, please see 'Related Documents'.  For a list of current installers and  products, see the 'Accredited Installers and Products' page of the Low Carbon Buildings Programme (under 'External Links').

Solar Thermal Hot Water

There is a small but established market in the UK for solar water heating systems. Estimates put the total number of existing installed domestic systems in the UK at over 70,000 [Note 1], with about 5,000 [Note 2] new domestic systems installed each year. The main potential for this technology is in the domestic market. It is estimated that there is the potential for the number of retrofit installations to increase from this relatively low base (under favourable market conditions) to 50,000 new units installed per year by 2010, 300,000 by 2015 and 800,000 by 2020 [Note 3].

Systems comprise of solar collectors (evacuated tubes or flat plates), a heat transfer system (a fluid in pipes) and a hot water store (e.g. domestic hot water cylinder). A 3m2 collection area will provide between 50 – 70% of a typical home's annual hot water requirement [Note 4]. The cost of a professionally installed solar system for heating hot water can vary significantly, but a household system (4m2) could cost £2,000 for a new build and £2,800 for a retrofit installation [Note 5].

System savings range from around 454 kilowatt hours (kWh)/year/m2 of flat plate collector – 582 kWh/year/m2 for an evacuated tube system [Note 6]. This might amount to a saving of around £120 - £150 per year [Note 7] for electrically heated property or be as low as £36 - £46 [Note 8] for a gas-heated property. The pay back time on an average 3m2 household system would therefore be around 24 years for an electrically heated property and 80 years for a gas-heated property (based on current energy prices).

Ground Source Heat Pumps

The market for ground source heat pumps is currently small but growing. The total number of existing installation in the UK is estimated at 5 megawatt thermal (MWth) made up of around 600 - 700 units [Note 9]. The principal market for Ground Source Heat Pumps (GSHP) are domestic housing, commercial properties not connected to the natural gas network and commercial industrial properties with stable heat demand. It is estimated that there is the potential for the number of installations to increase from this low base to 10,000 units installed by 2010, 35,000 by 2015 and 55,000 by 2020.

Systems operate by circulating water (or another fluid) through pipes buried in the ground in trenches or in vertical boreholes. The pipes extract heat from the ground and a heat exchanger within the heat pump extracts the heat from this fluid. The compression cycle is employed (also used in refrigerators) to then raise the temperature to supply hot water to the building. They require electricity to work, although this can be provided by complimentary renewable energy sources. The cost of a typical household system is £4 - 6,000 [Note 10]. A typical system will provide 95 - 100% of a household's heating requirements.

For a domestic system with a total annual heat load of 30,000 kWh heated by natural gas [Note 11] the annual carbon emissions would be in the region of 6.3 tonnes of carbon dioxide (tCO2)/ year [Note 12]. Employing a 9 kilowatt (kW) (peak heat output) ground source heat pump with a coefficient of performance (CoP) of 3.5 and costing around £9,000 would require 8,570 kWh of electricity to operate the pump. Assuming a normal electricity tariff, the carbon dioxide emissions would equate to 3.7tCO2/ year equivalent to a net saving of 2.6tCO2/ year [Note 13]. If a 'Green' electricity tariff supplied the electricity for the pump, the carbon dioxide emissions could be reduced to zero.

GSHP is most likely to be an option where there is no access to natural gas and so the alternative may be oil or direct electric heating (storage heaters). In the case of the latter, financial savings could amount to around £640 [Note 14] per annum (assuming off-peak electricity). In the case of oil fired heating, the likely running and installation costs would be comparable.

Air Source Heat Pumps

These systems have yet to become widely available for the domestic market and continue to undergo minor development work. However they are likely to become commercially viable in the very near future.

They work in the same way as ground source heat pumps except that the source of the heat is the external ambient air. As external temperature is more variable than in the ground, coefficients of performance are likely to be lower, but so too are installation costs as no trenching or ground drilling is required.

Systems are often installed on an external wall, and may give rise to noise issues in high-density housing developments.

Bio-Energy

There are a range of small-scale biomass heating systems commercially available in the UK across a wide range of sizes, combustion technologies and fuel sources. These range from single room heaters hand fed with logs, through to large scale industrial units with fully automated fuel handling systems using wood chips for large scale steam or Combined Heat & Power (CHP) operation. It is estimated that the existing number of domestic wood burning installations produce around 2.38 terawatt hours (TWh)/year [Note 15]. The principal market for domestic scale biomass heating will be in more rural locations where there is the space to accommodate the boilers and access to fuel is easier. The potential market size is 1.1M houses with an energy potential of 19.6 TWh/year [Note 16].

Biomass systems do emit carbon dioxide. However, as the biomass fuel is cultivated, it absorbs the exact same amount of carbon dioxide as is released when burnt. There will however be carbon emissions associated with, for example, any fertiliser used in production of the biomass fuels. As such, bioenergy systems are considered to be broadly carbon neutral and will almost always save more than 95% of the carbon from fossil fuels displaced [Note 17].

The cost of a typical household system is between £2,400 - £2,600 for a single room heater or £200 - £600 per kilowatt thermal (kWth) installed for a boiler system [Note 18], with fuel costs of around £15 – 30/MWh for wood pellets [Note 19].

For a typical domestic system with a total annual heat load of 30,000 kilowatt hours (kWh), a 9kW biomass system could deliver the heat required. In addition to the initial capital outlay, there would be an annual cost for fuel and maintenance [Note 20]. Overall the running costs would be comparable to gas or oil heated properties. But there would be net carbon savings of around 6.3 tCO2/year [Note 21] for a gas heated property and 8.7 tCO2/year for an oil heated property.

Footnotes:

[Note 1 - ref] BRE
[Note 2 - ref] BRE
[Note 3 - ref] Draft Renewable Heat and Heat from Combined Heat and Power Plants – Study and Analysis – AEAT April 2005.
[Note 4 - ref] STA figures.
[Note 5 - ref] As for footnote 3.
[Note 6 - ref] As for footnote3.
[Note 7 - ref] The price assumes the import cost of electricity of around 6 – 7p/kWh.
[Note 8 - ref] The price assumes the cost of natural gas around 2 p/kWh.
[Note 9 - ref] Draft Renewable Heat and Heat from Combined Heat and Power Plants – Study and Analysis – AEAT April 2005.
[Note 10 - ref] Clear Skies website.
[Note 11 - ref] This assumes with a new gas condensing boiler with Seasonal Efficiency of Domestic Boilers in the UK (SEDBUK) efficiency of around 90%.
[Note 12 - ref] Calculated on basis of 30,000 kWh/ 0.9(boiler efficiency) x 0.19 kgCO2/kWh.
[Note 13 - ref] Calculated on basis of total gas equivalent CO2 emissions (6.3 tCO2/ year) – electricity equivalent CO2 emissions to operate pump (8,570 kWh x 0.43 kgCO2/kWh.
[Note 14 - ref] Calculated on basis that direct electric heating would cost (assuming 3p/kWh might-time rate) 30,000 kWh x 3p = £900 per annum and GSHP would cost 8,570 kWh x 3p - £257 per annum.
[Note 15 - ref] Draft Renewable Heat and Heat from Combined Heat and Power Plants – Study and Analysis – AEAT April 2005.
[Note 16 - ref] See footnote 13.
[Note 17 - ref] See footnote 13. Carbon savings are 0.058 kgC/MWh where gas derived heat is substituted.[Note 18 - ref] Clear Skies website.
[Note 19 - ref] See footnote 13.
[Note 20 - ref] Assumes a fuel cost of £15 - 30 per MWh.
[Note 21 - ref] This assumes comparison with gas or oil boilers with Seasonal Efficiency of Domestic Boilers in the UK (SEDBUK) efficiency of 90% and 0.19kgCO2/kWh where gas derived heat is substituted and 0.26kgCO2/kWh where oil is displaced.