HEAT PUMPS

Heat Pumps

What is a Heat Pump?
Heat pumps work on a very similar principle to the common refrigerator in that they pump heat from one location and into another. At about one metre beneath our feet, the earth maintains a constant temperature of around 10-12 Celsius. This is because heat from the summer sun is stored in the ground and gently released back to the environment during colder periods.

Heat pumps tap into this energy, transferring the solar energy stored in the ground to provide heating and hot water for the home, in much the same way as your refrigerator transfers heat from inside the fridge and into the room. At the end of the heating season, the sun allows the temperature of the ground to recover again.

Types of heat pump
Three main types of heat pump exist:
•Ground source heat pump (GSHP); collector consists of a buried, closed loop of plastic piping to use the ground as a heat source
•Air source heat pump (ASHP); uses the heat content within the air external to the building as a heat source
•Water source heat pump (WSHP); uses either flowing water (stream, river, etc.) or a large body of water as a heat source

The types listed above refer more to the type of collector than to the actual heat pump itself, which is essentially the same for each.

GSHPs collect the heat from the ground either through a horizontal ground loop, or a vertical borehole.

How Does a Ground Source Heat Pump Work?
There are three main elements to the functioning of a heat pump. These are:

•The collector, where heat is gathered from the surroundings and made available to the heat pump
•The heat pump unit: just like in a fridge this consists of an evaporator (removes heat from the fluid in the collector), compressor (compresses the refrigerant to the required temperature) and a condenser (the hot part of the system) connected to the central heating and/or domestic hot water supply
•Heat Distribution system: either on the wall radiators or under floor heating and/or domestic hot water

For GSHPs, each kilowatt hour of rated output might require up to 30-50 metres of underground piping depending on ground conditions, and the overall size of the system depends on your heating requirements.

The performance of a heat pump system is measured by its Coefficient of Performance (CoP). This ratio compares the energy input (electricity to the compressor) to the energy output (useable heat).

Due to the nature of the compressors in heat pumps, they are best suited for long periods of operation rather than regular switching on and off (cycling) as typically happens with, for example, a condensing gas boiler. Therefore heat pumps run continuously for up to a few hours, storing their heat gradually in a thermal store ready for use when space or water heating is needed.

How much energy can I expect my system to produce?
For every unit of energy input there should be an output of between 2.5 - 4 units (CoP = 2.5 ? 4) of heat returned depending on the nature of the collector type and heat distribution system; with under floor heating allowing the greatest efficiencies.

The smaller the difference in temperature between the heat source and the heat distribution system, the higher the CoP, hence under-floor heating is more effective for this technology than wall mounted radiators as they operate at a lower temperature.

A well designed system should provide all or nearly all the heating requirements for your home, and act as a pre heater for domestic hot water; heat pumps cannot heat water up to the 60 Celsius recommended for the killing of harmful bacteria in potable water. The temperature of domestic hot water can be topped up via an electric immersion heater or other secondary boiler.

What are the benefits of heat pumps?
•Long operational lifetime; in excess of 50 years for some components
•Well established in other countries
•No safety or pollution issues relating to the combustion of fuel for heat
•Energy output of the system is greater than input
•Some heat pumps can be operated in reverse; offering air conditioning in warm weather. For GSHPs this also increases the ground temperature, adding to the available energy for the heating season
•Easily incorporated into current heating system and fully automatic
•Suitable for use with Economy 7 and Economy 10 tariffs

Suitability
Some homes are more suitable for heat pumps than others, and the range of collector types increases the number of options available. A site survey will reveal the most suitable system for your needs. Homes of low thermal mass are less suited to heat pumps.

Additionally, the strength of your electricity supply may need to be checked to ensure it can accommodate the energy demand of the system when operating. When compressors switch on they draw a very large electrical current for a short period of time, which may go beyond the capacity of your electricity supply.

Maintenance
Heat pumps require little maintenance beyond an annual check of the system by you, and any fluid circulating in a ground loop by an engineer every 3-5 years.

Installation Costs
These can vary significantly depending on the location and customer requirements. A typical installation for a horizontal trench ground source heat pump will likely cost between £900 and £1400 per kW output, or £6,000 - £11,500 for the average fully installed system.
Oversized heat pumps will cycle between on and off modes more frequently than a correctly sized system, reducing its efficiency and causing unnecessary wear.

© 2008 West Wales ECO Centre