High efficiency from Ground Source Heat Pumps

by John Findlay, GSHPA

The UK's first 'modern' closed loop borehole Ground Source Heat Pump installation has recently turned 20 years old with both heat pump and boreholes going strong, and expected to do so for many years to come. Many systems outside the UK are much older than this.

The major direct heat source for a GSHP is the ground, although ultimately it is, of course, from solar energy which is stored in the ground. A well-designed and maintained borehole, or trenched ground array, will provide continuous operation for 100 years or more – provided nobody digs it up!

As such the ground array acts as a very long term capital asset for the system owner. A low temperature GSHP scheme provides the most efficient means to obtain renewable heating and cooling. Over three units of heat (or cool) is provided for every unit of input electrical energy. Over the course of a year, this factor of GSHP efficiency is termed the 'seasonal performance factor' (SPF). A system providing a building over the course of a year with a balance of heat and cool will have a SPF of 5 or often much more. Combustion systems such as biomass or gas of course cannot reach an SPF of 1.

The UK's long term aim for the provision of renewable heating is to utilise heat pumps powered by low, and ultimately zero, carbon electricity. Great strides have already been taken to reduce the 'carbon intensity' of mains power – and so heat pumps provide a source of efficient renewable heat (and cool) with ever-decreasing CO2 emissions.

Great flexibility of design of the ground array is possible, with constraints on array design including site location, layout, underlying soil and rock characteristics and proximity to water. Renewable heat can be sustainably extracted from deep 'closed-loop' boreholes, shallow trenches, water supply boreholes, rivers, lakes or the sea. A GSHP system is the only technology that can store heat or cool for use at another time. This can provide a huge boost to system efficiency.

Ground arrays can provide heating and cooling to a single building or alternatively a number of buildings can be supplied by a common ground array. GSHP systems also offer an excellent means to provide district heating. For example, a 'source-side' system linked to heat pumps in each building on the heating circuit offers the most efficient means to obtain renewable heating; and the most efficient means of providing renewable cooling.

A GSHP can be designed to provide heat from as little as a 3kW domestic application up to multi-megawatt projects for hospitals, schools, colleges and large retail facilities. Many examples of all scales are operating efficiently in the UK today.

The number of projects considering GSHP technology is rising due in part to degression of biomass tariffs. RHI income from GSHP systems is now significantly higher than the equivalent biomass system – at all scales.

The Ground Source Heat Pump Association supports the growth and development of the GSHP industry in the United Kingdom by: