Ground Source Heat Pumps
How They Work
The sun radiates energy onto the surface of the earth which the ground absorbs and retains. Ground source heat pumps capture this energy using a network of pipes laid either vertically, in boreholes, or horizontally in trenches. This network of pipes is filled with a mixture of water and anti-freeze and absorbs thermal energy from the ground by conduction.
The ground maintains a steady temperature of 12-14° C all year round and so GSHPs are just as effective in January as they are July – a key advantage over an air source heat pump.
The warmed mixture in the ground loops then passes through a heat exchanger and heats a refrigerant. The process of compressing the warmed refrigerant without allowing it to occupy a greater volume increases both its pressure and, most importantly, its temperature. This compressed and heated refrigerant then passes through a heat exchanger where it transfers its thermal energy to the heating system inside the house, often via a buffer vessel.
As with all heat pumps you will typically, by comparison to a gas-fired boiler, require a little more space within the building itself. This is to house a water cylinder and a buffer tank for heating water. The size of these components depends very much upon the size of the space to be heated and the anticipated occupancy of the building.
The water cylinder is just that and provides water to your taps and showers when called for.
The buffer tank serves to satisfy the heat pumps requirements for minimum flow rates and total volume of water in the circuit. Keeping the heat pump happy in this way increases its efficiency and lifespan.
Outside, you will need to consider whether you extract heat from the ground by using boreholes or trenches.
With trenches you need a large area – typically 2.5 times the footprint of the building to be heated. Experience has told us that despite presenting a lesser initial outlay, the creation of trenches for the ground loops causes huge disruption with which there is an associated cost. The ground will require at least three years to re-settle and delivers a result that is not as efficient as boreholes. This is because the ground loops sit much higher in the earth where the ground is cooler.
Boreholes are more expensive to install, however the efficiency offered in return is much higher and causes less disruption during the construction phase. However, the procedure does still create quite a mess and this needs to be considered. Typically, a 100-metre-deep borehole will produce approximately 6 kW of energy depending on ground conditions.
Ground Source Heat Pumps: