Geothermal Heating and Cooling

The Earth’s crust is broken into pieces called plates. Around the edges of these plates magma comes close to the Earth’s surface causing the ground and water in these areas to become extremely hot.

These areas are known as geothermal reservoirs and are usually deep underground. Sometimes there are visible signs on the surface of their existence in the forms of hot springs, geysers, volcanoes or fumaroles.

Areas that are close to these tectonic plate boundaries can make use of the very hot ground, water or steam found in these areas to produce electricity or provide heating for buildings and industry. 

Ground does not have to be close to tectonic plates to contain thermal energy. The average temperature of the ground in the United Kingdom is between 11 and 13 degrees centigrade throughout the year. This geothermal energy can be used to heat our homes when it is cold and cool them when it is hot.

There are a variety of methods used to extract heat from the ground, most commonly they involve a length of pipe buried vertically or horizontally into the ground. A liquid (heat transfer liquid), that is usually a mixture of water and antifreeze, is sent from the heat pump and circulated around these underground pipes. These plastic pipes are surrounded by ground that in the UK has an average temperature throughout the year of between 11 and 13 degrees centigrade. When the heat transfer liquid leaves the heat pump it will have a low temperature of around -10 degrees centigrade. As the heat transfer liquid travels through the underground pipes it absorbs thermal energy from the surrounding ground and increases in temperature. The liquid’s temperature could rise to +10 degrees by the time it finishes the underground journey and arrives back at the heat pump ready to be used.

The heat pump uses the same technology as a refrigerator. Inside the heat pump are a series of components, the evaporator, compressor, heat exchanger and expansion valve. Passing through these components is a special refrigerant liquid that has a boiling point of about -40 degrees centigrade. The refrigerant is used to absorb heat energy from the heat transfer liquid and then transfer this heat to water that will be used to heat the house and domestic hot water.

The Evaporator – The liquid refrigerant, at a temperature of -40 degrees centigrade or colder, enters the evaporator where it is warmed by absorbing heat from the heat transfer liquid from the ground loop. This causes the refrigerant to boil and evaporate, therefore turning it from a liquid into a vapour.

The Compressor – The vapour is then drawn into a compressor, where compression causes the vapour’s temperature to rise to over 100 degrees centigrade.

The Heat Exchanger – After leaving the compressor the vapour enters a heat exchanger. Heat from the vapour is absorbed by water that will be used to provide heating and domestic hot water for the house. As heat is absorbed the refrigerant vapour’s temperature reduces and it condenses back to a liquid. This process also releases a considerable amount of additional heat energy into the heat exchanger, which will also be absorbed by the water and used for house and hot water heating.

The Expansion Valve – The refrigerant leaves the heat exchanger as a very cold liquid at high pressure and enters the expansion valve. Here the pressure of the liquid refrigerant is reduced before it is sent back to the evaporator to repeat the whole process again.

Similar to gas and oil boilers a heat pump provides hot water that can be used with underfloor heating, radiators or a hot air heating system, to heat the house and provide domestic hot water.

When the cooling function is in operation the heat pump works in reverse and provides a source of cold water for the radiators or underfloor heating system. As this cold water feeds the heating system, the water absorbs heat energy from the house and therefore causes the house to cool. Since the heat pump is working in reverse the transfer liquid is now hotter than the ground it is circulated through. Therefore heat is absorbed by the ground and the transfer liquid is cooled.

The extent of cooling that can be achieved is limited when using radiators and underfloor heating due to condensation. Condensation will form on the surface of the radiators or the floor if they become too cold. Condensation dripping off the radiators and a wet slippery floor is obviously undesirable. To prevent this, relative humidity sensors are used to monitor the ambient dew point and regulate the temperature of the cooling surfaces, which does restrict how cold the system is able to operate at. 

Closed loops used a collection liquid made up of a mixture of water and antifreeze that is circulated out of the heat pump around collection pipes and then back to the heat pump.

Collection pipes are laid horizontally in trenches 1 to 2 metres deep and from 30 to 150 metres in length. This collection method is ideal if there is enough space available in the ground is of the property. The collection pipes are best laid under the lawn away from trees. Ahe trenches are easily dug by mini diggers, the pipes or coils on are then laid in the bottom of the trenches and then the trenches are filled in.

This method requires boreholes to be drilled that depending on the requirements of the system and the composition of the ground will be to a depth of between 50 and 150 metres. A collection loop is then fed down the borehole and the borehole is then in. This method only requires a small amount of outside space so is ideal for properties with small gardens or where digging large trenches is undesirable. Because temperature in the ground increases with depth this system benefits from higher ground temperatures than if a horizontal loop is used. The drawback to this method of collection is that it is expensive to drill a borehole.

If a Lake or other similar water source is available then this could be an excellent heat collection option even if the water is under ice during the winter. A trench from the heat pump to the water will be required then coils are placed on the bottom of the lake or water source and secured. The benefit to this method if a suitable water source is conveniently located is that it can significantly reduce excavation costs.

Open loops are used if there is a suitable water source like a ground well, pond, lake or river nearby. An open loop draws water from its source and either returns it back to the source or discards it in to another location.

When using the water from a lake water would be pumped to the heat pump where thermal energy is absorbed and then the water would be returned to the Lake. If drawing straight off groundwater or wells then after the water has been pumped to the heat pump it is discarding in another location.

In ideal conditions open loops are the most economic collection method for geothermal systems.