Heat Pumps

What are heat pumps?

Heat pumps are a relatively low energy way of heating or cooling buildings compared to conventional boiler systems.  Heat pumps transfer heat between zones of different temperatures to provide either heating or cooling, depending on what the requirements are at the time.  A heat pump is similar to a refrigeration circuit in that heat is transferred between a circulating fluid and its surroundings.  In the case of a refrigeration circuit heat is transferred from the refrigerator to the low temperature fluid circulating through the unit (the working fluid or refrigerant) thereby evaporating it.  The working fluid is then compressed (with a pump) and condensed (through cooling by air or water) and then enters the refrigerator again.  A heat pump in cooling configuration operates like a refrigerator, and a heat pump for heating operates like a refrigerator in reverse.

The following two main types of heat pump may be used to supply heat to a building:

  • Ground source heat pump
  • Air source heat pump.
Electricity inputSchematic of a heat pump circuit in heating mode

Electricity inputSchematic of a heat pump circuit in heating mode

Ground Source Heat Pumps

Ground source heat pumps (GSHP) move heat (or coolth) from the ground (heat source) to the building (heat sink).  The temperature below ground does not vary as much as the air temperature through the year with the result that the source temperature is closer to the sink temperature for ground source heat pumps.  In the winter (in the UK or other northern countries) the ground temperature is generally higher than the ambient air temperature, and so heat may be transferred efficiently between the ground and the air to heat buildings.  In the summer the ground temperature is generally lower than the ambient air temperature, thereby allowing cooling to be provided efficiently to buildings.

Air source heat pumps

Air source heat pumps (ASHP) during the heating season transfer heat from the outside air into the building.  The principle is the same as for the ground source heat pumps, with the main difference being that, with the outside air being much colder than the ground, much more work has to be done to reach the internal temperature required in the building.  This means more input energy is required and therefore the system is less efficient.  Similarly in cooling mode more work has to be done by air source heat pumps than ground source heat pumps in transferring heat from inside the building because the external air temperature is likely to be higher than the ground temperature.  Thermodynamically, more work has to be done in moving heat to a across a higher temperature differential. The efficiency of a heat pump is expressed in terms of the coefficient of performance (CoP), which is the ratio between the energy supplied (either heating or cooling) and the input electrical energy.  For a ground source heat pumps the CoP may be over 5, whereas for an air source heat pump the average is unlikely to be over 3 with currently commercially available technology.  In summer, in the UK for example, the ground temperature is lower than the target building internal temperature and in this case ‘free cooling’ is a possibility, whereby there is direct heat exchange between the working fluid and the building circulation without the need for operating the heat pump.  This allows much higher CoPs.

Installation and cost comparison

Ground source heat pumps

Ground source heat pumps require pipework to be installed underground. Heat is transferred between the ground and the working fluid through the walls of the pipework.  The most efficient ground source heat pumps require vertical boreholes, typically to a depth of around 100m.  This requires significant capital outlay and may be difficult where there is restricted space, for example in a row of terraced houses with small gardens.  There are also local geological considerations that may constrain the potential for boreholes.  The boreholes need to be far enough apart such that they do not interfere with each other in thermal terms.  For new buildings with pile foundations, the boreholes may be integrated into the piles.  This can reduce costs but allows no access to the pipes in case of failure.  Access is still likely to be a problem if the boreholes are introduced below a building even if they are not integrated into the piles.  If there is plenty of outside space available but capital is restricted, then horizontal pipes may be more suitable for heat exchange with the ground.  The efficiency of this kind of heat pump system will not be as high, since nearer the ground surface the temperature is more similar to surrounding air temperature than further down.

Air source heat pumps

The cost of air source heat pumps is lower than that of ground source heat pumps, and installation much easier.  However there will need to be space for a box next to the building.  This may also be unsightly and make some noise, so some consideration may need to be given to aesthetics.

Building heating system

For the reason that heat pumps operate most efficiently when there is the lowest temperature differential between heat source and heat sink, heat pumps are most suited to lower temperature heating systems.  Lower temperature heating systems require larger areas of heat transfer to provide the same internal air temperature and therefore under floor heating is most suited to heat pumps.

How do I go about it?

The first step might be to decide which suits best – ground source or air source heat pumps.  This will depend on a range of factors including:

  • Available space
  • Available capital
  • Heating demand
  • Whether your current heat supply is gas, oil or electricity and the associated cost
  • Available support tariff.

Wattcraft can help you work through this.