A number of energy sources can be used as the heat source for heating buildings. Most commonly, heat pumps draw heat from the air or from the ground.

The heat drawn from the ground is, in most cases, stored solar heat, and should not be confused with geothermal heat, though the latter will contribute in some small measure to all heat in the ground. Other heat sources include water; nearby streams and other natural water bodies can be used.

Ground array design is an important factor in the operation of a heat pump. Get it wrong and the heat pump could run out of energy mid winter just when you need it most. Get it right and the heat pump will provide low cost heating for its life.

Below 1m the ground stays at a fairly constant 8-10°C all year round. This provides an adequate energy source for a heat pump to generate enough heat to comfortably heat buildings. A mixture of glycol and water is pumped around pipe buried in the ground. This pipe absorbs some of the energy in the ground and transfers this energy back to the heat pump. 

There are a number of different configurations of pipe that can be laid in the ground.

1) Slinky Pipe
Slinkies are the most cost effective way of installing ground arrays as they reduce the amount of digging required. These were developed by the University Of Oklahoma, and are one of the world’s most popular heat sources for heat pumps. In simple terms, slinkies are a length of coiled plastic pipe buried in the ground using a digger. They can be installed on their vertical edge or horizontally.

2) Straight pipe
Straight pipe can also be used, however this can increase the amount of digging required by up to 5 times and hence the cost of installation. There is no difference in the amount of ground required as the energy source for slinkies or straight pipe and both generally have the same performance.

3) Borehole
If there isn't enough space to install slinkies or straight pipe then an alternative is to drill vertically down. Boreholes tend to be 60-100m deep and usually consist of a single pipe inserted within the borehole, which is then backfilled with thermal grout. The depth and number of boreholes will vary depending on the makeup of the ground the borehole is being drilled in. A specialist contractor is required to drill boreholes and hence they can tend to be an expensive alternative to slinkies. 

Boreholes become economically viable on large commercial projects.

4) Lake systems
If the property is next to a lake then it is possible to use the lake water as the energy source. This is generally the most cost effective means of installing arrays as digging is kept to a minimum. Lake systems can be either open source or closed source.

The most popular lake system is a closed loop system. In this system a glycol and water mix is circulated around the slinkies, which are sunk to the bottom of the lake. The slinkies are attached to a frame (pond mat) and will absorb energy from the surrounding water. The lake has to be sufficiently deep to avoid freezing around the slinky and to avoid any damage from passing boat traffic.

In an open source system, water is pumped out of the lake or borehole. It passes through the heat pump and is then discharged, either back to the lake or another acceptable discharge area. When planning an open source system you need to consider corrosion issues, filtration, extraction and possible freezing within the heat exchanger. It is also important to consider the electrical energy required to pump the water to the heat pump.

5) Air Source
With air source heat pumps, air is drawn over a heat exchanger (coils) and the heat is extracted from the air. They work all year round, even with temperatures as low as -15°C. However, their efficiency tends to fluctuate due to daily and seasonal air temperature variations and is lower during the heating season than a ground source heat pump. Air source heat pumps can tend to be quite noisy, however the installation costs are lower than ground source as no trenching is required.

About the author: John Barker-Brown is special projects manager at British heat pump manufacturer Kensa Engineering.

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