A Geothermal Heat Pump uses the earth, or water from a pond, as a heat source. Also called a Ground Source Heat Pump (GSHP), the system can extract heat from the ground, and redistribute it in your home. In summer this process can be reversed, and so provide a cooling system. It is an extremely efficient process, requires no fossil fuels (such as gas or oil), and reduces your energy costs significantly.
How it Works
An array of pipes is laid underground, either horizontally or vertically. At a depth of 3-6 feet, the temperature of the ground, except in extreme permafrost areas, maintains a steady temperature year round. Refrigerant is piped through this array, and extracts heat from the ground, much in the same way as it would extract heat from a refrigerator. This heat is then re-distributed in your home heating system, either to heat the home or to heat or hot water, or both. Electricity is used to drive the pump to circulate the refrigerant, rather than to provide heat, so the electrical costs are in any case much lower.
Horizontal heat pump arrays are laid out, as the term suggests, about 6 feet under the ground, in a long horizontal trench. There is some seasonal variation in the heat available, and the effect of the sun in summer in important. Installation is very disruptive, as most of your garden needs to be dug up, although lawn turf can be easily lifted and then replaced. It is also generally cheaper (about half the cost of a vertical array), and requires no specialist installation. A vertical array is placed at a depth of 75-500 feet (23-150m) in a hole drilled by a specialist contractor. A typical home setup would need three or four holes. After the pipes are inserted, the holes are filled with bentonite grout to enhance thermal connection, and to prevent ground water contamination or artesian well overflow. The heat supply is more constant, and depends on the heat available in the geology or in the underground water reservoir.
How efficient is a Geothermal Heat Pump?
The technical term is ‘Coefficient of Performance'(COP), which simply is a measure of how many units of energy output do you get for each unit of energy input. For almost all geothermal heat pumps, the COP is 3.0 or better, which means that for every unit of energy (from gas, oil or electricity) that goes into the system, you get three units of heat provided by the system. You can see this clearly in the video below.
What about cooling in Summer?
In summer, the heat transfer process can be reversed, and the refrigerant extracts heat from your home, and deposits it into the ground, which acts as a heat sink.
Is a GSHP Environmentally Safe?
The refrigerant used is normally R410A, which is very ozone friendly, biodegradable and non-toxic. This has replaced chlorodifluoromethane, which has significant effects on ozone. The EPA has estimated that a typical household system will use 75% less electricity, and avoid CO2 emissions of 21,300 pounds and 45 pounds of NOx per year.
How does a Geothermal Heat Pump differ from other heating systems?
A GSHP is significantly cheaper to run than other heating systems. However, it does take longer to warm your house at the beginning of winter, so you would need to start it up a couple of weeks earlier. Once running, you will probably feel your radiators at a comfortable warm temperature, rather than piping hot, and so you would probably run the unit for longer during the day. Nevertheless, you will still save money, as power is used to circulate refrigerant with a small pump, not to actually provide heat.
A GSHP is more efficient than an Air Source Heat Pump (ASHP), which extracts heat from the air. An ASHP is really only efficient at temperatures above -6 deg celsius (21 deg F), but a GSHP can still provide heating at much lower outdoor temperatures. However, if you live in an apartment, you will obviously not have access to the ground space to install a GSHP, and so the best alternative is an ASHP.
Can I be energy independent with Geothermal Energy?
It is unlikely that a GSHP can meet all your energy needs. In cooler areas, you will need extra heating for your hot water needs. However, a combination of GSHP, and roof mounted solar water heater, and a solar panel array can definitely make you energy independent, and in some areas, you will almost certainly be able to sell power back to the grid.
How Long do Geothermal Heat Pumps Last?
A GSHP can last for 30 years or more. When you consider that you will probably recoup your investment in between 4 an 10 years, that gives you a long time with almost free heat.