Types of Renewable Energy Heat Pump

Below 1.5metres, the ground temperature is a constant 8–12^oC all year round.This low-grade heat can be taken from the ground using a buried pipe known as the ground loop, around which a mixture of water and anti-freeze is pumped. The heat is then transferred from the liquid within the loop by the heat pump and upgraded to a higher temperature for heating.

Groundloops can be either closed or open, and installed vertically as a borehole or
horizontally in a trench. The type selected will be dependent on the ground area available, as well as access and geological conditions. The British Geological Survey offers further information on site geology.

Contrary to what you may think, you don’t necessarily need a large space, but you will need land near your home suitable for digging trenches or drilling boreholes. You will need space inside your home for the indoor heat pump unit, which contains key components. The inside unit often contains the hot water cylinder and is roughly the size of an American style fridge.

Most homes in the UK use radiators or underfloor heating to circulate hot water, which is known as a ‘wet system’. Ground source heat pumps need a wet system too. If you don’t currently have a wet system, you will have to decide whether you’d like to install one. This is a great opportunity to make sure the wet system is optimised for a heat pump, resulting in lower running costs. Air-to-Water heat pumps make the best wet system, but if you don’t want or can’t have a wet heating system an Air-to-Air heat pumps are a cost and energy efficient option.

Boreholes are normally drilled where there is not a large land area available for a horizontal collector loop. The loops are usually installed into 100 and 150mm diameter boreholes, 15 to 120 metres deep. Water with anti-freeze (or brine) is pumped around these loops. Boreholes must be correctly spaced. If they are too close together, the net cooling effect on the ground will reduce the efficiency of the system. The cost and efficiency of the system will also be highly dependent upon location.

An alternative to using a borehole is to create a horizontal or 'trench' loop. These are commonly laid in trenches 1.2 to 2.5m deep. Horizontal loops typically require a larger land area than borehole loops. Another way of laying horizontal loops is to use a ‘slinky’ coil of pipe, so that the pipes overlap and less area is required. Horizontal loops can be considerably cheaper than vertical boreholes. They are also usually slightly less efficient than boreholes due to the fluctuating ground temperature near the surface.

Closed loop is the most common type of ground source heat pump. A sealed loop of pipe is filled with a mixture of water and anti-freeze. The liquid increases in temperature as it passes around the loop through the warmer ground. This heat is then transferred from the loop by the heat pump.

Open loop systems are not often used. Groundwater is pumped from boreholes drilled into underground water sinks (aquifers) or water is extracted from lakes, rivers and the sea. Heat is transferred from the water via a heat pump, with the water re-injected into the ground through another borehole. The re-injected water should not mix with the extracted water, or the system will be less efficient. This mixing is known as ‘short-circuiting’.

Installation of GSHPs involves digging large trenches or boreholes with large plant machinery. Consents will be needed if the works are in the setting of a listed building or affect a scheduled monument, or the site is in a Conservation Area. Possible harm to wildlife and habitats will also need to be considered. There may be other considerations too, such as existing underground services and cables, and access to the site.

Before starting work on-site, it is important to assess the possibility of unrecorded buried archaeology. In cases where there is known or suspected buried archaeology present, an archaeologist should be commissioned to undertake a Watching Brief during the groundworks.

Pipes will need to run between the boreholes or loops and the heat pump, and also through the building’s external wall. Care will be needed not to damage historic fabric. Generally, GSHP pipework enters the building below ground level. The design of the system will also need to ensure that the manifold chamber provides sufficient access for maintenance and adequate drainage. Additionally, there should be sufficient space for the indoor plant room and access to and maintenance of all the items of the GSHP plant. Documentation of the GSHP loop should be kept, as it could be easily damaged by later work on-site if the location is not known.