GROUND SOURCE HEAT PUMPS
Tim Pullen explains the types, costs and efficiencies of ground source heat pumps
Ground source heat pumps (GSHP use electrical energy in a highly efficient way to produce heat, significantly reducing heating running costs and CO2 emissions. While they require electricity to operate, they can also be combined with renewable electricity sources such as solar PV panels.
HOW DO THEY WORK?
GSHP use a series of buried pipes that extract the energy from the sun that warmed the ground. The heat pump then amplifies low grade heat into useful heat in the home.
The amount of heat that can be collected will be dictated by the soil conditions – for example, clay holds more heat than sand – and the amount of unshaded land available.
There are two main elements of a GSHP system:
● The ground array, which can be horizontal or vertical (more on this later). In either case the extent of the ground array will be determined by the size of the heat pump and the soil conditions.
● The heat pump itself, which is installed in the house. Some of these include a hot water cylinder and can be the size of a large filing cabinet. The smallest pumps, such as those in the Kensa Heat Pump range, can be fitted in an under-sink kitchen cupboard; however, they’re often located in dedicated plant rooms for easy access.
TYPES OF GSHP
A horizontal ground array consists of long lengths of pipework (also known as slinkies) buried in trenches around 1.2 to 1.5metres deep over a wide area of garden.
A vertical array will have boreholes drilled into the ground and connected across their tops. The number and depth of boreholes will be dictated by the size of the heat pump and the geology. For instance, an 8kw heat pump is likely to need at least three boreholes around 70m deep. They do not need a large area of land and if the price is right, a vertical array is a good option.
A water source heat pump is a system where pipes are installed via pond mats submerged into the water.
IS A GSHP RIGHT FOR MY HOME?
If you are considering a ground source heat pump, bear in mind:
● A heat pump works best in a well insulated home. A heat pump would still work in a home with a high heat demand but it would be more expensive to install. The point is that the difference in price between a, say, 15kw and 30kw gas boiler might be £1,000, including installation. The difference between a 10kw heat pump and a 20kw might be £10,000, plus the extra installation costs. Investing in insulation has a significant impact on the capital cost of the GSHP.
● A horizontal array needs garden space — around 50m2 will
be needed per kilowatt capacity of the heat pump. Therefore a 10kw heat pump would typically require around 500m2 of clear, unshaded ground. That said, a borehole installation is an option for those short on space, although they’re more costly to install. You will also need to consider access to the site for excavation machinery.
● It is most efficiently paired with underfloor heating as this requires much lower water temperatures than radiators. The alternative is to upgrade existing radiators to cope with the lower flow temperature.
WHAT ARE THE COSTS?
A basic ground source heat pump costs between £2,000 to £15,000 depending on size and brand. This cost is likely to be three to four times more expensive than a gas combi boiler and double the cost of an air source heat pump.
The quality of the heat pump equipment can add a significant amount to the cost of materials, effectively doubling it in some cases.
HOW EFFICIENT ARE THEY?
Efficiency is stated as the SCOP and a typical figure for a ground source heat pump would be something over 4.0. This means that for each 1kw of electricity used to drive the heat pump, it will produce 4kw of heat. It is primarily this efficiency that make heat pumps a good idea. Combined with storage such as a heat battery, they can provide the greenest heating system — they are the most efficient, and they can operate just as efficiently at night as during the day.