Heat Pump Overview

Heat Pumps:

A heat pump is a device based on the refrigeration principle for drawing heat from air, water or the ground, and making it available to heat a room space or water.

Regardless of the source of the heat, electrical energy is required to drive a compressor to concentrate low-grade heat to a level where it can be useful.   The low-grade heat source can be the air, the ground, lakes, wells, rivers or the sea.  If the ground is used, then generally there will be a loop or series of loops of pipework buried in the ground, normally at a depth of about a metre.  The sun has warmed up the ground at this depth and heat is available throughout the year at about 10°C.  As a rule of thumb, to obtain 10kW of useful heat pump output, you would need approximately 600 metres of buried pipework.

If the heat in the air is used, then a “packaged” unit is installed that looks the same as a conventional external air conditioning unit.  This system takes low-grade heat from the outside air and concentrates it to provide a warm internal air.  Units are available to upgrade the heat in the air to provide water at 55°C for domestic hot water, for under floor heating systems or to supply conventional air handling units.

A common misconception is that there is no heat available below zero degrees.  This is obviously not true as -20°C is colder than zero, so zero must be warmer than -20°C.  The problem is that water freezes at zero and our initial gut feeling is that if there is ice or snow outside how can there be heat available?  Air source heat pumps can still work efficiently at -20°C.  The advantage of a ground source heat pump is that the low-grade heat usually does not drop below 10°C, so the efficiency levels are normally higher than an air source heat pump.  Conversely, all the time that the outside temperature is above 10°C the air pump is more efficient.

Efficiency of all types of heat pump will vary based on the temperature of the available low-grade heat and refinements, such as high efficiency inverter compressors, though generally a C.O.P. of between 3 to 5 can be expected.  This means that for every kilowatt of electrical energy used to drive the heat pump, 3 to 5 kilowatts of heat can be generated.

As regards Low to Zero Carbon Technology, the air conditioning / heating industry was of the opinion that all types of heat pump which qualify for inclusion on the Energy Technologies List would be classified under the Part L building regulations.  However, due to a misunderstanding, only ground source heat pumps were included.  The Low Carbon Buildings Programme is now considering all heat pumps, and it is fully expected that amendments will be made by early 2007.

Geothermal Energy:

Geothermal energy is energy extracted from the earth’s natural heat, ie. from hot springs and certain types of rock.  This is energy, which can be used via heat exchangers to provide virtually unlimited heat with no direct use of fossil fuels, and only electricity to drive hot water circulating pumps.  Unfortunately it is only available in specific geological areas such as Iceland, or at enormous uneconomic depths.