A brief introduction to microgeneration, what it is and what it can do for you and the planet

An overview of different electricity and heat producing microgeneration technologies with links to further details on each

Before you look for ways to produce your own energy, it makes sense to minimise your energy needs.  An outline of some energy efficiency measures you can take.

MICRO CHP • MINI CHP • BIOMASS • HEAT PUMPS • SOLAR HOT WATER • SOLAR PHOTOVOLTAICS (PV) • MICRO WIND • MINI WIND • MICRO HYDRO

There are several types of heat pump, using a variety of thermodynamic processes, but all have a common characteristic; they transfer "low grade" heat from an ambient source to a useful application of "high grade" heat.  The two types considered here use the outdoor air, ASHP (Air Source Heat Pumps) or the ground, GSHP (Ground Source Heat Pumps) as a low grade heat source to provide hot water and space heating.  The latter type is sometimes also referred (incorrectly) to as a "geothermal" heat pump.  Both these types are able to produce hot water in suitable quantities and at suitable temperatures for use in conventional UK central heating systems.

Although these are the principal systems able to provide full, conventional central heating, there are many other types of heat pump including:

• Ventilation heat pumps which use the waste heat in the exhaust air from a house to provide domestic hot water and/or limited space heating
• Gas fired heat pumps (absorption and engine driven) equivalents of the electrically driven systems noted above
• Air to air heat pumps which provide direct air heating from the outdoor air, but do not provide hot water

Performance of heat pumps

COP (Coefficient of Performance)

The COP of a heat pump is a measure of its energy benefit; it is the ratio between the electrical energy input to the total heat energy delivered to the home or other application.  The COP is strongly influenced by the amount by which the heat pump has to raise the temperature; the smaller the temperature lift, the higher the COP.  In other words, it will operate more efficiently if the delivered water temperature is lower or if the heat source is warmer.

The COP of the ASHP is therefore dependent on the outdoor air temperature which can be as low as -10°C.  Unfortunately this is also when the home will require most heat.  The GSHP on the other hand, extracts heat from the ground which remains at a constant temperature throughout the year of around 5-10°C.  For a low temperature heating system (e.g. underfloor heating) designed for heat pumps, the delivered temperature can be around 30°C, so whilst the GSHP has to raise the temperature by 20°C, the ASHP will have to raise it by 40°C.  The difference between the two systems is less pronounced for retrofit or other high temperature (50°C) radiator applications, where the temperature lift will be 60°C and 40°C for ASHP and GSHP respectively.

Hot water production

In order to provide DHW (domestic hot water) for showers etc. the temperature will need to be around 50°C all the time, and to avoid the risk of legionella, must go through a pasteurisation cycle at regular intervals, typically once a week, during which the temperature must be significantly higher.  Some heat pumps (e.g. Daikin) use an electric heater element  to achieve this high temperature as the energy penalty is relatively low for the short duration; other manufacturers (e.g. Calorex) use the heat pump to provide all the necessary heat, although the COP will be very low and the process causes considerable stress to the compressor and other components. 

Meeting heat requirements in extremely cold weather

Heat pumps are rarely sized to provide the full heat demand of the home as this would mean that they would be operating sub-optimally for much of the year, although larger units sometimes use two compressors to allow stepped output.  An additional heat source is therefore required to meet peak heat demands; this can be a gas or oil boiler or an electric heater (with a COP = 1).  In general, North European and Scandinavian products tend to use a separate electrical element both for this and the pasteurisation of DHW, whilst products designed for the less extreme UK weather conditions, may use the heat pump itself to meet all heat demands.

Seasonal COP

It is therefore important to examine manufacturers' claims of COP carefully, taking into account any additional heat inputs needed to maintain comfort conditions throughout the year.  Often COP claimed may refer to specific (favourable) operating temperatures rather than the most important seasonal (or annual) COP.  For an ASHP, the seasonal COP may be around 2.5, whereas a GSHP may be around 3.5.

The role for heat pumps

In carbon terms, a heat pump can compete with a condensing gas boiler, if it can achieve a COP of 2.4; in financial terms it needs to achieve a COP of at least 3 for peak rate electric or around 2 for a well designed system using an off-peak tariff.  However, the very high capital costs currently make heat pumps uncompetitive where gas is available.  This situation is constantly changing and, as the carbon intensity of the grid reduces, and gas becomes more expensive relative to electricity, heat pumps will become increasingly competitive, particularly as the Building Regulations impose more stringent carbon limits.

GSHP products currently perform better than ASHP, but are complex and messy to install as they require either heavy drilling equipment to create very deep bore holes (around 100 metres) or disruption of a large land area for "slinky" pipe installations.  However, once installed, they are virtually invisible and very quiet, whereas ASHP can be both physically intrusive and rather noisy unless enclosed within an acoustic housing.

As homes become better insulated, the demand for space heating will fall both in absolute terms and in relation to ever increasing DHW demands.  In this case, the annual COP of the GSHP and ASHP will become similar as the ASHP will make use of a higher temperature ambient source in the summer, whilst the opposite will occur in the winter.  As a result, ASHP could become the clear choice for new homes.