We are well aware that buildings and construction are the mayor hogs of energy consumption in the world. 45% of energy generated is used to heat, light and ventilate buildings…so it is no secret that the fight against climate change should begin in our own homes. These days we hear a lot about ways of cutting energy, and renewables are becoming more and more common in mainstream construction. ‘Geothermal’ is a very adaptable source of energy for heating and cooling buildings that can help reduce the impact the impact of energy hungry buildings. But even if you’ve heard of the term, do you really know what Geothermal is or how it works to heat a building?
If you ask Wikipedia what Geothermal heating means you get this…
† Geothermal heating is the direct use of geothermal power for heating applications. Humans have taken advantage of geothermal heat this way since the Paleolithic era and even today approximately seventy countries made direct use of geothermal heating in 2004. As of 2007 Geothermal heating capacity is believed to satisfy 0.07% of global primary energy consumption.
Ok, so now we know the earth is warm and cozy down below, but how do we bring that heat inside and use it in our homes?
There basically are three options are available for the ground source heat loop: borehole, straight horizontal and spiral horizontal (or ‘slinky’). Each has different characteristics allowing you to choose the most suitable for your property. Horizontal trenches can cost less than boreholes, but require greater land area. For slinky coil, a trench of about 10m length will provide for about 1kW of heating load.
Here is how it works
The Ground Source Heat Pump is an electrically powered system that takes advantage of the Earth’s constant ground temperature to provide heating for your home or business. I live in Scotland, and believe me when I say that most days your safe leaving the house with a heavy coat, but even here in the U.K., at several metres below the surface, the ground is always at a constant 11 – 13°C making it the perfect constant source of heat your house needs.
Ground source heat pumps (GSHP) can pump this heat from the ground into a building to provide space heating and, in some cases, preheating domestic hot water. For every unit of electricity used to pump the heat, 3-4 units of heat are produced. As well as ground source heat pumps, air source and water source heat pumps are also possible.
There are three elements to a Ground Source Heat Pump.
1 The Ground loop
this is made up of lengths of plastic pipe buried in the ground. The pipe is a closed circuit and is filled with a mixture and anti-freeze, which is pumped round the pipe to absorb heat from the ground. Three options are available for the ground loop: borehole, straight horizontal and spiral horizontal (or ‘slinky’). Horizontal trenches cost less than boreholes, but require greater land area. For slinky coil, a trench of about 10m length will provide about 1kW of heating load.
2 The Heat pump
although we may not know it, heat pumps are very familiar to us. Fridges and air conditioners are all examples. 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 (eg the squiggly loop in the cold part of your fridge) takes heat from the water in the ground loop; the condenser (the hot thing on the back of your fridge) gives up heat to a hot water tank which feeds the distribution system. A compressor, which uses electricity, (this is what makes the noise in your fridge) 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.
3 The Heat distribution system
this consists of underfloor heating (the most efficient way of using the heated water produced from ground source heat pumps) and a storage cylinder for hot water supply. Some systems can also be used for cooling in the summer. Heat pumps can be used with radiators by the addition of a buffer tank.
How do I know if my site or house is suitable? Basically if you have a bit of land available chances are your home or project will be suitable but you must consider a few things before you know this for certain:
- The type of heat distribution system. Is there potential for underfloor heating (although it is also possible to use with radiators)?
- Is there space available for a trench or borehole to accommodate a ground loop?
- Is the ground material suitable for digging a trench or borehole?
- Is the property off-mains gas? This will have an effect on the economics (ie payback period).
- Do you want to be 100% renewable? If so, purchase green electricity, or install PV (solar) or some other form of renewable electricity generating system to power the compressor and pump.
- What auxiliary heating will you use?
- Is there also a cooling requirement?
- Will the system be for a new building? There is considerable potential for reducing costs by combining ground loop installation with other works
Bear in ming that sizing of the heat pump and the ground loops is crucial to the operation of the system. Initially, it is a good idea to explore ways of minimising space heating and hot water demand by energy efficiency measures. A heat pump can be designed to meet 100% of space heating requirements. Sizing is a job for specialists, so don’t go trying to run your own calcs! Also bear in mind that a GSHP will usually only pre-heat domestic hot water. So top up heating (eg an immersion heater) will be required.
Are there any environmental impacts? Very few, but The main environmental impacts are:
• Pollution from using grid electricity generated through fossil fuel. Measures can be taken to reduce these impacts – for example, you could purchase dual tariff green electricity from a number of suppliers. However, even if ordinary grid electricity is used to run the compressor, the system will still produce less CO2 emissions than even the most efficient condensing gas or oil boiler with the same output.
• Use of refrigerants in the system. Refrigerants are present in GSHP systems and can pose a threat to the environment through being toxic, flammable or having a high global warming potential. However, new types and blends of refrigerants with minimal negative impacts are being developed. A correctly fitted system will also greatly reduce the potential for leakage, which is why using a professional installer is highly recommended.
