Ground Source Heat Pump
In this country the earth, a few meters below our feet, is at a constant temperature of about 11-12oC throughout the year.
Because of the ground's high thermal mass, it stores heat from the sun during the summer.
Ground source heat pumps (GSHP) can pump this heat from the ground into a building to provide space heating.
For every unit of electricity used to pump the heat, 3-4 units of heat are produced.
There are three important elements to a GSHP:
• Ground loop - Comprises lengths of plastic pipe buried in the ground, either in a borehole or a horizontal trench.
The pipe is a closed circuit and is filled with a mixture of water and antifreeze, which is pumped round the pipe absorbing heat from the ground.
• Heat pump - A heat pump works by using the evaporation and condensing of a refrigerant to move heat from one place to another.
In this case, the evaporator the takes heat from the water in the ground loop; the condenser gives up heat to a hot water storage tank, which feeds the distribution system.
A compressor, which uses electricity, moves the refrigerant around the heat pump.
It also compresses the gaseous refrigerant to increase the temperature at which it condenses, to that needed for the distribution circuit.
• Heat distribution system - Consists of underfloor heating or radiators for space heating.
Some systems can also be used for cooling in the summer.
The installed cost of a GSHP ranges from about £800-£1,200 per kW of peak heat output, excluding the cost of the distribution system.
Trench systems tend to be at the lower end of this range.
The installed cost of a typical 8kW system would therefore vary between £6,400-£9,600 plus the cost of the distribution system.
Horizontal loops are common in some areas.
A 1.2 to 2.0 metre deep trench is dug using a backhoe or chain trencher.
The pipe is laid out, sealed and pressure tested.
Then the trench is filled.
The land area required ranges from 75 to 150 square metres per ton (3.5kW) of refrigeration.
Vertical loops require less pipe than horizontal loops.
Well-drilling equipment bores small diameter holes 15 to 60 metres deep.
Two pipes joined with a U-bend are inserted into each hole.
The system can use one deep hole or several shallow ones.
Verticals can be installed almost anywhere.
The cost of drilling is often greater than trenching.
Pond loops reduce excavating costs by placing much of the loop in a pond, lake or stream.
In most cases, 1/4 to 1/2 acre of water surface and a minimum depth of 2 to 2.5 metres is required.
The Heating Cycle
In the heating cycle, the ground water, the antifreeze mixture, or refrigerant (which has circulated through the underground piping system and picked up heat from the soil), is pumped back to the heat pump unit inside the building. It then passes through the refrigerant-filled primary heat exchanger for groundwater or antifreeze mixture systems. In DX systems the refrigerant enters the compressor directly, with no intermediate heat exchanger.
The heat is transferred to the refrigerant, which boils to become a low temperature vapour.
In an open system, the ground water is then pumped back out and discharged into a pond or down a well.
In a closed-loop system, the anti-freeze mixture or refrigerant is pumped back out to the underground piping system to be heated again.
The reversing valve sends the refrigerant vapour to the compressor.
The vapour is then compressed which reduces its volume, causing it to heat up.
Finally, the reversing valve sends the now-hot gas to the condenser, where it gives up its heat to a water heating system.
Having given up its heat, the refrigerant passes through the expansion device, where its temperature and pressure are dropped further before it returns to the first heat exchanger or to the ground in a DX (direct expansion) system, to begin the cycle again.
Ground Source Heat Pump linked
toUnderfloor heating system
Ground Source Heat Pump linked toUnderfloor heating system
A heat pump can also be used for air conditioning in summer.
The cooling cycle is basically the reverse of the heating cycle.
The direction of the refrigerant flow is changed by the reversing valve.
This works best if a ground source to air heat pump is used instead of a ground source to water unit.
The refrigerant picks up heat from the house air and transfers it to the outside into a water body or return well (in the case of an open system), or into the underground piping (in the case of a closed-loop system).
Because heat pumps consume less primary energy than conventional heating systems, they are an important technology for reducing gas emissions that harm the environment, such as carbon dioxide (CO2), sulphur dioxide (SO2) and nitrogen oxides (NOx).
However, the overall environmental impact of electric heat pumps depends very much on how the electricity is produced.
Heat pumps driven by electricity from, for instance, hydropower or renewable energy reduce emissions more significantly than if the electricity is generated by coal, oil or gas-fired power plants.