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Renewable heat
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Renewable heat is an application of renewable energy and it refers to the renewable generation of heat, rather than electrical power (e.g. replacing a fossil fuel boiler using concentrating solar thermal to feed radiators).
Many colder countries consume more energy for heating than electrical power. For example, in 2005 the United Kingdom consumed 354 TWh of electric power, but had a heat requirement of 907 TWh, the majority of which (81%) was met using gas.
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Encyclopedia
Renewable heat is an application of renewable energy and it refers to the renewable generation of heat, rather than electrical power (e.g. replacing a fossil fuel boiler using concentrating solar thermal to feed radiators).
Many colder countries consume more energy for heating than electrical power. For example, in 2005 the United Kingdom consumed 354 TWh of electric power, but had a heat requirement of 907 TWh, the majority of which (81%) was met using gas. The residential sector alone consumed a massive 550 TWh of energy for heating, mainly in the form of gas. Almost half of the final energy consumed in the UK (49%) was in the form of heat, of which 70% was used by households and in commercial and public buildings. Households used heat for mainly for space heating (69%) and heating water.
Renewable electric power is becoming cheap and convenient enough to place it, in many cases, within reach of the average consumer. By contrast, the market for renewable heat is mostly inaccessible to domestic consumers due to inconvenience of supply, and high capital costs. Heating accounts for a large proportion of energy consumption, however a universally accessible market is still in its early stages.
Leading renewable heat technologies
Solar heating
Solar heating is a style of building construction which uses the energy of summer or winter sunshine to provide an economic supply of primary or supplementary heat to a structure. The heat can be used for both space heating and water heating (see solar hot water). Solar heating design is divided into two groups:
- Passive solar heating relies on the design and structure of the house to collect heat. Passive solar building design must also consider the storage and distribution of heat, which may be accomplished passively, or use air ducting to draw heat actively to the foundation of the building for storage. One such design was measured lifting the temperature of a house to 24°C (74°F) on a partially sunny winter day (-7°C or 19°F), and it is claimed that the system provides passively for the bulk of the building's heating. The home cost $125 per square foot (or 370 m2 at $1,351/m2), similar to the cost of a traditional new home.
- Active solar heating uses pumps to move air or a liquid from the solar collector into the building or storage area. One application, solar water heating, works in conjunction with an existing water heater, and is based on solar panels fitted to the roof. In contrast to photovoltaic panels which are used to generate electricity, solar water heating panels are cheaper to manufacture, and capture a much higher proportion of the sun's energy.
Solar heating systems usually require a small supplementary backup heating system, either conventional or renewable.
Heat Pumps
Heat pumps use work to move heat from one place to another, and can be used for both heating and cooling. Though capital intensive, heat pumps are economical to run and can be powered by renewable electricity. Two common types of heat pump are air-source (ASHP) and ground-source heat pumps (GSHP), depending on whether heat is transferred from the air or from the ground. Air conditioning units are usually air-source heat pumps, and many models allow pump based heating and cooling, and continue to be effective with external temperatures as low as -15 °C. The efficiency of a heat pump is measured by the coefficient of performance (CoP): For every unit of electricity used to pump the heat, an air source heat pump generates 2.5 to 3 units of heat (i.e. it has a CoP of 2.5 to 3), whereas a GSHP generates 3 to 4 units of heat. Based on current fuel prices for the United Kingdom, assuming a CoP of 3-4, a GSHP can be a cheaper form of space heating than oil, LPG and electric storage heaters. It is however more expensive than mains gas unless the heat pump is linked to an interseasonal thermal store when the CoP can rise to 7 by extracting heat from warm ground .
Interseasonal Heat Transfer
Interseasonal Heat Transfer combines active solar collection to store surplus summer heat in thermal banks with GSHPs to extract it for space heating in winter. This reduces the "Lift" needed and doubles the CoP of the heat pump because the pump starts with warmth from the thermal bank in place of cold from the ground.
CoP and Lift
The CoP increases as the temperature difference, or "Lift", decreases between heat source and destination. The CoP can be maximised at design time by choosing a heating system requiring only a low final water temperature (e.g. underfloor heating), and by choosing a heat source with a high average temperature (e.g. the ground). Domestic Hot Water (DHW) and conventional radiators require high water temperatures, affecting the choice of heat pump technology. Low temperature radiators provide an alternative to conventional radiators.
| Pump type and source | Typical use case | CoP variation with Output Temperature |
|---|
(e.g. heated screed floor)
! 45 °C
(e.g. low temp. radiator or heated screed floor)
! 55 °C
(e.g. low temp. radiator or heated timber floor)
! 65 °C
(e.g. std. radiator or DHW)
! 75 °C
(e.g. std. radiator & DHW)
! 85 °C
(e.g. std. radiator & DHW)> | High Efficiency ASHP air at -20 °C | | 2.2 | 2.0 | - | - | - | - | | Two Stage ASHP air at -20 °C | Low source temp. | 2.4 | 2.2 | 1.9 | - | - | - | | High Efficiency ASHP air at 0 °C | Low output temp. | 3.8 | 2.8 | 2.2 | 2.0 | - | - | | Prototype Transcritical (R744) Heat Pump with Tripartite Gas Cooler, source at 0 °C | High output temp. | 3.3 | - | - | 4.2 | - | 3.0 | | GSHP water at 0 °C | | 5.0 | 3.7 | 2.9 | 2.4 | - | - | | GSHP ground at 10 °C | Low output temp. | 7.2 | 5.0 | 3.7 | 2.9 | 2.4 | - | | Theoretical Carnot cycle limit, source -20 °C | | 5.6 | 4.9 | 4.4 | 4.0 | 3.7 | 3.4 | | Theoretical Carnot cycle limit, source 0 °C | | 8.8 | 7.1 | 6.0 | 5.2 | 4.6 | 4.2 | | Theoretical Lorentz Cycle limit ( pump), return fluid 25 °C, source 0 °C
| | 10.1 | 8.8 | 7.9 | 7.1 | 6.5 | 6.1 | | Theoretical Carnot cycle limit, source 10 °C | | 12.3 | 9.1 | 7.3 | 6.1 | 5.4 | 4.8 |
Wood-pellet heating
Wood-pellet heating and other types of wood heating systems have achieved their greatest success in heating premises that are off the gas grid, typically being previously heated using heating oil or coal. Solid wood fuel requires a large amount of dedicated storage space, and the specialised heating systems can be expensive (though grant schemes are available in many European countries to offset this capital cost.) Low fuel costs mean that wood fuelled heating in Europe is frequently able to achieve a payback period of less than 3 to 5 years. Because of the large fuel storage requirement wood fuel can be less attractive in urban residential scenarios, or for premises connected to the gas grid (though rising gas prices and uncertainty of supply mean that wood fuel is becoming more competitive.)
Wood-stove heating
Burning wood fuel in an open fire is both extremely inefficient (<5%) and polluting due to low temperature partial combustion. In the same way that a drafty building loses heat through loss of warm air through poor sealing, an open fire is responsible for large heat losses by drawing very large volumes of warm air out of the building.
However modern wood stove designs allow for extremely efficient combustion and then heat extraction (>95% efficiency is possible), and draw only small volumes of warm air from the building. High efficiency stoves meet the following design criteria:
- Well sealed and precisely calibrated to draw a low yet sufficient volume of air. Air-flow restriction is critical; a lower inflow of cold air cools the furnace less (a higher temperature is thus achieved). It also allows greater time for extraction of heat from the exhaust gas, and draws less heat from the building.
- The furnace must be well insulated to increase combustion temperature, and thus completeness.
- A well insulated furnace radiates little heat. Thus heat must be extracted instead from the exhaust gas duct. Heat absorption efficiencies are higher when the heat-exchange duct is longer, and when the flow of exhaust gas is slower.
- In many designs, the heat-exchange duct is built of a very large mass of heat-absorbing brick or stone. This design causes the absorbed heat to be emitted over a longer period - typically a day.
Renewable natural gas
Renewable natural gas is defined as gas obtained from biomass which is upgraded to a quality similar to natural gas. By upgrading the quality to that of natural gas, it becomes possible to distribute the gas to customers via the existing gas grid. According to the Energy research Centre of the Netherlands, renewable natural gas is 'cheaper than alternatives where biomass is used in a combined heat and power plant or local combustion plant'. Energy unit costs are lowered through 'favourable scale and operating hours', and end-user capital costs eliminated through distribution via the existing gas grid.
Energy efficiency
Renewable heat goes hand in hand with energy efficiency. Indeed renewable heating projects depend heavily for their success on energy efficiency; in the case of solar heating to cut reliance on the requirement supplementary heating, in the case of wood fuel heating to cut the cost of wood purchased and volume stored, and in the case of heat pumps to reduce the size and investment in heat pump, heat sink and electricity costs.
Two main types of improvement can be made to a building's energy efficiency:
Insulation
Improvements to insulation can cut energy consumption greatly, making a space cheaper to heat and to cool. However existing housing can often be difficult or expensive to improve. Newer buildings can benefit from many of the techniques of superinsulation. Older buildings can benefit from several kinds of improvement:
- Solid wall insulation: A building with solid walls can benefit from internal or external insulation. External wall insulation involves adding decorative weather-proof insulating panels or other treatment to the outside of the wall. Alternatively, internal wall insulation can be applied using ready-made insulation/plaster board laminates, or other methods. Thicknesses of internal or external insulation typically range between 50 and 100 mm.
- Cavity wall insulation: A building with cavity walls can benefit from insulation pumped into the cavity. This form of insulation is very cost effective.
- Programmable thermostats allow heating and cooling of a room to be switched off depending the time, day of the week, and temperature. A bedroom, for example, does not need to be heated during the day, but a living room does not need to be heated during the night.
- Roof insulation
- Insulated windows and doors
- Draught proofing
Underfloor heating
Underfloor heating is significantly more energy efficient that traditional methods of heating:
- Water circulates within the system at low temperature (35 °C - 50 °C) making gas boilers, wood fired boilers, and heat pumps significantly more efficient.
- Rooms with underfloor heating are cooler near the ceiling, where heat is not required, but warmer underfoot, where comfort is most required.
- Traditional radiators are frequently positioned underneath poorly insulated windows, heating them unnecessarily.
See also
External links
- Heat pumps based on R744 (CO2)
- - Information from Heat Pump & Thermal Storage Technology Center of Japan
- Department of Trade and Industry, 2005 study on
- combining asphalt solar collectors, thermal banks and ground source heat pumps.
- Energy Saving Trust information on
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