Farm Diversification

Basic Principles – “a fridge in reverse”
It is well known that heat pumps are refrigeration systems operated “in reverse”.   Refrigeration systems, such as those used in cold stores, transfer heat from air in a confined space to outside air.  This type of fridge has two heat exchanging coils each are a series of thin pipes fitted with fins to improve heat transfer. The evaporator coil absorbs heat into the fluid refrigerant passing through the pipes.  The refrigerant is pumped to the condenser coils externally where the heat is transferred to outside air being drawn through by fans.

To improve the efficiency of this process the refrigerant in the evaporator is at low pressure - a low temperature liquid.  The heat from the air passing through the coils makes the refrigerant boil, so absorbing a large amount of extra heat (latent heat of evaporation).  The vaporised refrigerant is then compressed, increasing its pressure and temperature.  The temperature of the refrigerant is raised well above the outside air temperature.  The refrigerant then easily looses heat to the air blown through the condenser coils.  When the pressure is reduced in the flow control the temperature drops and the refrigerant is ready to go round the circuit again. 

Key points about this type of refrigeration system, which also apply to heat pumps are:

• The system is self contained.  The refrigerant circulate continually transferring heat from one place to another
• By altering the pressure of the refrigerant it can be made to change temperature, evaporation and condensate to optimise the efficiency  of the system
• The electrical usage of the system is mainly that used to power the compressor
• The measure of the efficiency is called the Coefficient of Performance (COP).  This is usually about 3 - 5 meaning that the heat transferred will be  three to five times that of the energy used by the compressor

How does the theory transfer from refrigeration to heat pumps?
Heat pumps also transfer heat from the evaporator coils to the condenser coils.  They ‘pump’ heat from a cool area to an area that needs heating.  Sufficient heat will be obtained from the cool area so long as it is warmer than the refrigerant.  As the refrigerant will be well below freezing this is relatively easy to achieve by using residual warmth in the soil, air or nearby body of water.  The heat obtained can either be transferred to air and this may be a good option if there is an existing air based heating system.  Alternatively the heat can be passed though a coil in a water tank and the heated water used as hot water and for a standard central heating system..

There are two types of heat pumps:

Air source heat pumps: Outside air in the UK has plenty of heat in it to warm the refrigerant and as installation is relatively easy heat pumps that use air are becoming more common.  It is true that the efficiency will decrease as the temperature drops far below freezing and for this reason air source heat pumps may not be a good choice in, for example, Canada.  However the proportion of the year that outside air is too cold in the UK is small and at these times the heating can be supplemented by another energy source, for example an electrical immersion heater or a wood fuel system.
The warmer the air passing over the evaporator the more efficient the heat pump will be.  Heat pumps are therefore sometimes installed in conjunction with heat recovery systems, with the warm air exhausted from the building passing through the evaporator coils.
Although air source heat pumps are relatively easy to install they have slightly lower efficiencies than the other type of heat pumps

Ground source heat pumps: Using the temperature of the subsoil, which gains heat from the sun, to warm the refrigerant in the evaporator has the attraction that its temperature fluctuates less than the air, and the temperature never gets very much below zero.  This means that the system efficiency is improved.  However to access this source of heat pipes must either be laid in a series of trenches, or in vertical boreholes.  Ground source system therefore suit best installation in newly built properties which are close to usable land for pipe burial, or farm applications where cleared land is nearby.

An 8 kW domestic ground source heat pump will cost around £8,000 - £12,000, excluding internal heat distribution system, although installation costs vary widely.  Air source heat pumps are normally about 30% cheaper, than a ground source heat pump with a standard installation.  A grant may be available for householders from the Energy Saving Scotland Home Renewables Grant Scheme to cover 30% of the installed cost, up to a maximum of £4 000.

The running costs will depend on the cost and efficiency of other fuels, and on the way the heat pump is installed

• Heat pumps are a modern, energy efficient way of providing heating for well insulated buildings
• The capital costs are higher than conventional systems but the running costs may be lower which means heat pumps will suit some situations
• With recent improvements in design heat pumps are likely to be installed more widely as a more environmentally friendly alternative to conventional heating systems

No Information Available

No Information Available

No Information Available

No Information Available

Ground source heat pumps are eligible for payments under the renewable heat incentive (RHI). Currently (March 2012) air source heat pumps are not.

For further details please see SAC's summary of the RHI scheme or a full range of useful information on the RHI can be found on the Ofgem's RHI website.