Wednesday, December 29, 2010
HEAT PUMPS by SOLAR SERDAR
RENEWABLE ENERGY CENTER SOLAR SERDAR (CRECSS)
Heat pumps use the same technology as fridges or air-conditioning units, but instead of taking heat out of the house, they bring heat in from the outside air or from the ground.
A heat pump will use electricity, so as long as grid electricity is generated by fossil-fuel and nuclear power stations it will not be a completely renewable heat source. It is important to make sure a heat pump operates very efficiently if it is to be a better option than gas or oil fired heating. A recent field trial report on heat pumps revealed that heat pumps installed in the UK in recent years often only reach low levels of efficiency, often offering little or no environmental or financial advantage compared to, say, gas heating. However, it seems that the problem is primarily that many systems were badly installed, or put into buildings that aren't suitable. There is evidence from both continental Europe and the UK that heat pumps can reach high levels of performance if specified and installed correctly.
The key thing is to have a well-insulated home, a correctly-sized heat pump and a low-temperature heating system - ideally underfloor heating running on water heated to only 35 degrees C.
Our information sheet gives more advice on how to evaluate the efficiency of a heat pump, how the different types work, and what the costs are likely to be.
What is a heat pump?
You’ll almost certainly have a heat pump in your home already, as they are used to keep fridges cool - basically by ‘pumping’ heat out. Most air conditioning units are also heat pumps. Both of these are generally airsource systems.
A ground source heat pump will usually be the more efficient option for home heating – giving the lowest running costs. Air-source systems are cheaper to install, but their efficiency will drop as air temperatures drop (when you need heat most). Also, heat transfer from air is more difficult than from other sources. Water-source systems can be very efficient, but they’re not common because you need a water source that will not freeze (such as a spring).
Are air-source heat pumps (ASHPs) noisy?
There will be some noise - check technical literature on manufacturers' websites for figures. Worcester Bosch quote 65 decibels (db) for the noise level at 1 metre from their unit. By comparison, normal conversation may be at a noise level of 50db, a busy office about 60db, and a busy street about 70db. The external part of an ASHP is basically the same as an air conditioning unit, but they do vary a lot - so don't judge all ASHPs by the noisiest air conditioner.
Are heat pumps environmentally friendly?
To be environmentally beneficial, the whole heating system must be properly specified and the house very well insulated (to a level above that specified by current UK Building Regulations). You can vastly improve the efficiency of your existing property with simple energy conservation measures - see our ‘Energy Conservation’ section for advice.
The efficiency of a heat pump is given by its coefficient of performance, or COP. A system operating at COP3 will give out 3 units of heat energy for each unit of electricity used. However, you need to bear in mind that most grid electricity is generated from fossil fuels or nuclear power, at efficiencies of only 30 to 40%, so you’ll need a very good COP to outweigh this. If a system has a poor COP you’d be better off with a gas boiler, which would give off less carbon dioxide and make a smaller contribution to climate change. Electricity generation also causes other forms of pollution: sulphur & nitrogen oxides (that cause acid rain), particulates, mining impacts, and nuclear waste.
An Energy Saving Trust field trial of 83 heat pumps in the UK found a wide variance in performance - only a few reached a COP of 3 or more. The average COP for air source heat pumps was 2.1, while the average for ground source systems was 2.3. Many were early installations, and as installers gain experience, performance should improve. To avoid a low COP, ensure that a home is well-insulated, has a low-temperature heating system and good heating controls, and that the ground loop or air-source unit has been adequately sized. See other questions and our information sheet for advice.
You can sign up for a ‘green tariff’, and have your electricity use allocated to renewable sources such as wind or hydro power. This is an excellent way to help promote the growth of the renewable energy industry, but it’s not a green light to use loads of electricity! Doing so will increase overall electricity demand, and until more renewable energy technologies are ready, this will increase the use of fossil fuels and nuclear power.
If you have a stream that would be suitable for a micro-hydro system, then a renewably-powered heat pump could be feasible and beneficial. See our Micro-Hydro information sheet for initial advice on this technology.
A heat pump contains about 2 kilos of refrigerant, usually hydrofluorocarbons (HFCs). These are potent greenhouse gases (about 1600 times more powerful than carbon dioxide) and a leak during or after the system’s life will have a damaging impact. Some suppliers use hydrocarbon refrigerants such as R290 or R600a (propane & isobutane); these will have a much lower impact if accidentally leaked.
Do I need a lot of land space for ground-source heat pump (GSHP)?
Trenches should be at least two metres deep to harness a consistent year-round heat source. Trenches will need 50-80 metres of pipe per kilowatt (kW), or 10 metres of ‘slinky’ coiled pipe per kW, with at least 5 metre distance between trenches with coils. So a typical 8kW heat pump requires around 400m2 of ground area for slinky coils. Note, however, that this will depend on a number of factors, including ground conditions.More info at http://solarserdar.wordpress.com/.
Boreholes need 20-50 metres of pipe per kW, and will usually be 100-150 metres deep. You may need 2-4 pipes per borehole, or more than one borehole. The Pipe diameter should be 20 to 40mm for best performance: large enough to reduce pumping power but small enough to increase flow velocity and cause ‘turbulent flow’ (giving better heat transfer). Bear in mind that installers trying to reduce costs might skimp on the length or bore of pipe, or the depth of the trenches.
What's the difference between an air-source and a ground-source heat pump?
Ground-source heat pumps (GSHPs) draw heat from under the ground using either a borehole or a series of pipes laid a few metres below the surface. This
heat is mainly solar energy that has been absorbed by the ground. The ground below a couple of metres down is protected from extremes of heat or cold and stays at about 10°C all year. This is of course not warm enough in itself - the heat pump ‘boosts’ it to the level needed for heating a home.
Air-source heat pumps (ASHPs) do the opposite of air-conditioning units. Instead of taking heat out of a building, they take heat from the outside air and uses it to heat a house. See our related questions to see which one is right for you.
How efficient are heat pumps?
A heat pump operates most efficiently when the temperature gap between the heat source and the heat demand is minimised. To reach COP4 you must have a very well insulated house - usually a new-build or an extensive renovation. Heat pumps will not be able to heat water to 75°C for standard radiators, so you’ll need a low temperature system. Large radiators can be run on water at about 50°C, whilst underfloor heating can be run at only 35°C. Underfloor heating can be fitted under solid or suspended timber floors, but thick carpets should not be used – they’ll stop the heat coming through.
So that a heat pump can work at maximum efficiency all year round it is usually sized to meet about 90% of the heating demand. This means some form of backup heating is likely to be needed during very cold spells – for example a wood-fired room heater. Heating domestic hot water to 60°C will further diminish efficiency, so an immersion heater is often used - which means more electricity use and higher costs. Solar water heating is a good alternative for hot water - see our solar water heating section for more advice.
High quality heating controls are needed to run the system as efficiently as possible. Weather compensation controls regulate internal heating according to the outside temperature. For example, in cool weather underfloor heating may need water at only 25°C to provide suitable comfort.More info at http://solarserdar.blogspot.com/.
Free, independent and impartial advice on renewable energy and sustainable living provided by the RENEWABLE ENERGY CENTER SOLAR SERDAR (CRECSS)
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