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Heat Pumps

Heat pumps

Introduction
In the UK, the earth - a few metres below our feet - keeps a constant temperature of about 11-12C throughout the year.
Because of the ground's high thermal mass, it stores heat from the sun during the summer. Ground source heat pumps (GSHP) transfer some of this heat from the ground into an adjacent building to provide space heating and, in some cases, pre-heating domestic hot water. For every unit of electricity used to pump the heat, 3-4 units of heat are produced. As well as ground source heat pumps, air source and water source heat pumps are also possible. Air source heat pumps are particularly cost effective in locations where frosts are infrequent, such as coastal locations.

How does it work?
There are three important elements to a GSHP:
• Ground loop - comprises lengths of pipe buried in the ground, either in a borehole or a horizontal trench. The pipe is usually a closed circuit and is filled with a mixture of water and antifreeze, which is pumped round the pipe absorbing heat from the ground. A ground loop is not required when installing an air-source heat pump, which can reduce the overall installation cost by thousands of pounds.
• Heat pump - although we may not know it, heat pumps are very familiar to us - fridges and air conditioners are both examples. A heat pump has three main components:
Evaporator (e.g. the cold bit at the back of your fridge) - takes the heat from the water in the ground loop.
Compressor (e.g. the bit that makes the noise in a fridge) - moves the refrigerant round the heat pump and compresses the gaseous refrigerant to the temperature needed for the heat distribution circuit.
Condenser (e.g. the hot bit at the back of your fridge) - gives up heat to a hot water tank which feeds the distribution system.
• Heat distribution system - consists of under floor heating or radiators for space heating and in some cases water storage for hot water supply. Underfloor heating is much more efficient and is ideally suited to heat pumps because they produce water at a lower temperature than gas or oil fired boilers.

What are the options (for a ground source heat pump collector)?
Three options are available for the ground loop: borehole, straight horizontal, multi-pipe collector and spiral horizontal (or 'slinky'). Each has different characteristics allowing you to choose the most suitable for your property.
Horizontal trenches can cost less than boreholes, but require greater land area. For slinky coil, a trench of about 10m length will provide for about 1kW of heating load.

Sizing
Sizing of the heat pump and the ground loop is crucial to the operation of the system and will depend on your heating requirements. At the outset it is sensible to look at energy efficiency measures such as wall and floor insulation to reduce your heat demand.
When sizing a system it’s important to consult a professional installer for expert advice. A heat pump can be designed to meet 100 per cent of space heating requirements but it will usually only pre-heat domestic hot water so top up heating (e.g. an immersion heater) will be required. Generally heat pumps are sized to provide less than 100% of space heating requirements, needing you to provide a form of supplementary heating for the few coldest days of the year (either by immersion heaters in the heat pump, or some other form of heating such as a wood buring stove).

How much does it cost?
• Installation costs: The installed cost of a GSHP, for a professional installation, ranges from about £1000-£1,600 per kW of peak heat output, excluding the cost of the distribution system.
Trench systems are cheaper so tend to be at the lower end of this range, and the price per kW gets lower as the systems get larger. The installed cost of a typical 8kW system would vary between £8,000-£12,000 plus the cost of the distribution system. The installed cost for an air-source heat pump can be 20-40% cheaper than for a ground-source system.
Note that costs are dependent on property and location, so the cost for a system for your home may differ.
• Running costs: The efficiency of a GSHP system is measured by the Coefficient of Performance (CoP). This is the ratio of the number of units of heat output for each unit of electricity input used to drive the compressor and pump for the ground loop.
Typical CoPs range between 2.5-4. The higher end of this range is for underfloor heating, because it works at a lower temperature (30-35oC) than radiators.
Based on current fuel prices, assuming a CoP of 3-4, a GSHP will be a cheaper form of space heating than oil, LPG and electric storage heaters. However, the amount saved will depend on the fuel replaced. The most efficient system may even be cheaper to run than mains gas at 2007 gas prices. If grid electricity is used for the compressor and pump, then an economy 7 or economy 10 tariff usually gives the lowest running costs. However, where a heat pump feeds radiators, for which the heat pump has to operate for a greater proportion of time on peak rate electricity than for underfloor heating, you may find that shopping on a regular basis for the cheapest standard tariffs ends up cheaper (and doesn't lock you in to any one supplier).

Are there any environmental impacts?
The main environmental impacts are:
• Pollution from using grid electricity generated through fossil fuel. Measures can be taken to reduce these impacts - for example, you could purchase dual tariff green electricity from a number of suppliers. However, even if ordinary grid electricity is used to run the compressor, the system will still produce less CO2 emissions than even the most efficient condensing gas or oil boiler with the same output.
• Use of refrigerants in the system. Refrigerants are present in GSHP systems and can pose a threat to the environment through being toxic, flammable or having a high global warming potential. However, most heat pumps now use types and blends of refrigerant with minimal negative impacts. A correctly fitted system will also greatly reduce the potential for leakage, which is why using a professional installer is highly recommended.

Is my house suitable?
You should consider the following issues if you are considering a ground source heat pump. An accredited installer will be able to provide more detailed advice regarding suitability.
• The type of heat distribution system. GSHPs can be combined with radiators but under floor heating is better as it works at a lower temperature.
• Is there space available for a trench or borehole to accommodate a ground loop?
• Is the ground material suitable for digging a trench or borehole?
• What fuel is being replaced? If it is electricity, oil, LPG or any other conventional fossil fuel the payback will be more favourable. This makes heat pumps a particularly good option for off gas grid areas.
• Do you want to be 100 per cent renewable? If so, purchase green electricity, or install solar PV or some other form of renewable electricity generating system to power the compressor and pump.
• Do you require a back up heating system?
• Is there also a cooling requirement?
• Is the system for a new building development? Combining the installation with other building works can reduce costs.
• Can you incorporate insulation measures? These measures include wall, floor and loft insulation, as these will reduce your heat demand.

Are there grants available?
Yes, householders and community groups in Scotland can access grants from the Scottish Executive. These grants are managed jointly by the Energy Saving Trust and the Highlands and Islands Community Energy Company. Householders can access capital funding of up to £4,000 (ex-VAT) and community organisations can access up to £10,000 for feasibility assistance and up to £100,000 for capital assistance.
For more information call the Energy Agency on 01292-521896 or the EST helpline on 0800 138 8858.

Energy Agency Case Studies

To view community heat pump installation projects click on the following:

Beattock Hall

Ballantrae Rural Initiative for Care in the Community

Auchencairn Enterprise Centre

New Cumnock Swimming Pool Air-Source Heat Pump