Introduction to the Domestic Use of Geothermall Energy
Low level geothermal energy is a source of renewable energy found a few meters under the soil and has a near constant temperature of around 12 degrees C. It receives this heat from solar energy during the summer months which it stores, maintaining this heat in the soil all year round.
This type of geothermal energy can be used in a number of domestic applications, the most popular being to supply thermal energy to an under floor central heating system. This is a very efficient means of heating the house using a ground source heat pump supplied from an arrangement of underground pipes that are normally located within the property’s boundaries and being vertically or horizontally positioned under the soil.
This is another article on the use of renewable energy in domestic applications. Here we shall examine the installation and operation of a geothermal domestic heating system, and go on to have a look at an under floor heating system.
We begin with a description of the extraction of the geothermal energy from under the soil in your garden.
Introduction to the Domestic Use of Geothermal Energy
Geothermal Closed Loops
The two popular types of closed loops are horizontal and vertical.
- The horizontal Loop
Once the area required for the installation of the loops is known, a trench to accommodate the loops is excavated to a depth of about two meters. A track of the same depth is excavated between this trench and the outside wall of the house. The loops are then carefully installed and the tails led along the track to the house, where they are brought to the surface.
The system is then pressure tested to detect any leaks and once this is confirmed the trench and track is backfilled and lightly tamped. The tails are the connected to the heat pump.
- The vertical loop
The holes are bored in the ground up to a depth of between 50 and 80 metres, and the vertical closed loop pipe system is lowered carefully into the hole, leaving the insulated tails above the ground. The piping system is pressure tested as per the horizontal loop system and the hole back-filled with soil. The area from which the pipes emerge from underground is carefully grouted with a cement mixture. The loops are then connected to the heat pump.
The Ground Source Heat Pump
This comes as a complete unit which is usually located in a convenient room inside the house, but can be located outside if required. A ground source heat pump operates in the following manner.
A refrigerant is circulated through the heat pump system. Heat from the water circulated through the loops is transferred to the refrigerant via a heat exchanger (evaporator) which converts the liquid refrigerant into a gas. A compressor triples the refrigerant gas pressure, further raising its temperature before it is forced through another heat exchanger (a condenser) which transfers its heat to the domestic water system. This converts the gas back to a mixture of liquid and gas in the process.
From here the mixture passes through an expansion valve which expands the gas and liquid mix back to its original pressure and liquid properties. From here it passes back to the evaporator to continue the cycle.
The only electrical power expended is by the heat pump compressor and the loop water circulating pump and these can be powered using a renewable energy device such as PV panels or small wind turbine being backed up by the grid system.
Transfer of Thermal Energy to the Domestic Heating System
As we have seen above, the heat pump has transferred the heat from the refrigerant into the domestic central heating system, via a heat exchanger.
Inside the heat exchanger the refrigerant now in the form of a high temperature and pressure gas, is forced through a closed circuit set of coils. The domestic water is pumped through the under floor heating coils, through the exchanger and around the refrigerant coils, being heated in the process.
Sketches of geothermal loops and heat pump