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Geothermal heat pumps - How it works.

The earth is absorbing energy all the time. At the surface this is mainly solar energy, but at greater depth factors such as rainfall, groundwater and heat transfer through the bedrock all play important roles. This energy is entirely renewable and as a result, the ground remains constant at approx. 10 deg. C when you go deeper than a few metres. It is this constant, renewable source that geothermal heat pumps use to provide heating in your building.
From this example you can see that by cooling a large body of water by a few degrees, it is possible to heat a small body of water to very high temperatures. This is the main principal behind geothermal heating. From the large body of water ( in the pipes underneath your garden ) heat is transferred by means of the heat pump to increase the temperature in the smaller body of water ( in your underfloor or radiator heating pipes ).

There are two principal locations in the transfer of heat; the place where heat is absorbed, (the source), and where it is ejected, (the destination). we see that all a heat pump is really doing is moving heat energy from one body of water to another
The purpose of the geothermal collector (source) is to absorb stored energy from the ground and transmit this energy to the refrigerant at the evaporation stage, through a heat exchanger. This can be achieved by circulating brine, a mixture of anti-freeze and water, through coils of absorber pipe laid in the ground. A simple sum of 4 degrees of temperature reduction in 10 X times the liquid in the source pipes = a 40 degree raise in temperature of the 1 unit of liquid in the destination pipes : i.e. your radiators or underfloor heating system. So how is the reduction in the ground liquid turned into creation of heat in the home?
At the heart of a modern heat pump is a refrigeration system. Paradoxically, the refrigeration cycle is an efficient provider of heat as well as cooling.
To better understand firstly lets look at ‘Latent Heat’ - the change of state from liquid to gas (evaporation/vaporisation), we will get a better idea of how a heat pump works. The specific latent heat of vaporization is defined as ‘the amount of heat required to convert a unit mass of a liquid into the vapour without a change in temperature’. For water at its normal boiling point of 100 ºC, the latent specific latent heat of vaporization is 2260 kJ.kg-1. This means that to convert 1 kg of water at 100ºC to 1 kg of steam at 100ºC, 2260 kJ of heat must be absorbed by the water. Conversely, when 1 kg of steam at 100ºC condenses to give 1 kg of water at 100 ºC, 2260 kJ of heat will be released to the surroundings. Very simply

When something evaporates, heat is taken in e.g.
• You feel cold when you get out of a swimming pool. The water evaporating off your skin takes extra heat from your body and makes you feel cold even though the pool hall is hotter than a normal room.
When something condenses, heat is given out e.g.
• When you get steam from a kettle on your hand, it burns so badly. The steam condenses on your hand and gives out extra heat.
These two processes are the foundation of refrigeration and heat pump technologies.

Refrigeration Cycle
The above diagram shows the refrigeration cycle inside a Waterkotte heat pump. Instead of water however, the heat pump uses a refrigerant gas with a very low boiling point (-40oC and lower, depending on the refrigerant used). This allows it to evaporate very easily at low temperatures.

The refrigeration cycle is made up of 4 steps
1) The refrigerant is allowed to evaporate in the evaporator. This draws heat from the heat source water into the refrigerant

2) The refrigerant is compressed using a mechanical motor. This increases the energy per unit volume and allows the heat pump to produce higher output temperatures. This is the largest electrical input into the heat pump.

3) The compressed gas is allowed to condense in a condenser. This gives out the heat to the heating water. The more compressed the gas, the higher temperatures can be reached

4) The gas is allowed to uncompress as it goes through an expansion valve and the cycle starts again To achieve maximum efficiency, the temperature of the heat source must be as high as possible and the flow temperature on the heating side must be as low as possible. In this way the compressor will not have to compress the refrigerant gas as much and less electricity will be used. This again highlights the importance of the design of the geothermal heat source and the heat distribution system.

With the correct design from heat source to heat distribution, Waterkotte heat pumps can achieve an efficiency of more than 450% (a co-efficient of performance of 4.5), i.e. for every 1kW of electrical power used in the compression stage, there is 4.5kW of useful heat output, 3.5kW of which comes free from the ground.