Ground Source promises to deliver good year-round energy-efficiency, and can offer considerable long-term environmental advantages. The Ground Source Heat Pump (GSHP) extracts heat stored in the ground. This is occasionally referred to as ‘geothermal’. However, the heat is mostly from the surface (solar gain), and very little is from the earth’s core. (‘Proper’ geothermal is found in several parts of the world, including Iceland, Japan, New Zealand and even Southampton).
At depths of 2m and more, the ground temperature does not deviate very much from the average summer/winter surface temperatures (around 8° to 12°C [46ºF to 54ºF] in the UK depending on location). At this depth, there is an enormous store of heat that can be usefully tapped for heating in the winter.
The most practical way of extracting this energy is to bury a large amount of pipe in the ground. This is usually laid in horizontal trenches at depths between 1.2 and 2m depth. If insufficient land area is available, or if excavating a garden is impractical, then vertical boreholes are an alternative method that gives similar results. However, the borehole method is usually considerably more expensive to install.
Basic description of the component parts of a GSHP:
1. A heat pump packaged unit
Showing water-water or brine-water type. (approx. the size of a fridge). This has source-water (glycol) pipe connections and heated-water pipe connections. Sometimes the unit contains a domestic hot water cylinder (DHW), in which case the unit will be the size of a large fridge freezer.
2. The heat source
This is usually a closed-loop of plastic pipe containing a Glycol Antifreeze solution. This pipe is buried in the ground in the form of vertical bore holes or horizontal trenches. The trenches take either straight pipe or coiled pipe, buried around 1.5m below the surface. A large area is needed for this.
3. The heat distribution system, or Heat Emitter system.
This is either underfloor heating pipes or conventional panel radiators with a large surface area, connected via normal water pipes.
4. Electrical input and controls.
The system will require an electrical input, and the electric distribution operator DNO should be notified to verify that the building’s supply is adequate.
A specialised controller will be incorporated in the heat pump to provide temperature and timing functions of the system. A good controller can be key to a energy-efficient system. However, it is often said that a sophisticated controller set badly is worse than a simple control unit.
Ground Source Heat Pumps offer several advantages
- The water-water (or antifreeze-water) heat pump unit is a sealed and reliable self-contained unit.
- There are no corrosion or degradation issues with buried plastic pipes.
- The system will continue to provide good output even during extremely cold spells.
- The installation is fairly invisible. i.e. no oil tanks or outside unit to see.
- Very little regular maintenance required.