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GEOTHERMAL SYSTEMS

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Scientists have discovered that “the cold, cold ground” is actually a valuable source of heat and energy that can be tapped relatively simply. Geothermal heat pumps (you will also see them referred to as ground source heat pumps) can extract this heat and use it for space and water heating in the home.

Visions of drilling down to the molten core of the planet immediately spring to mind, but it’s nothing as dramatic as that, thank goodness. The system is based on the simple fact that the soil holds heat from the sun, heat that can be harnessed in the same way as the heat from the traditional boiler in the home.

This is relatively new technology and it qualifies for attractive grants from the government’s Greener Homes Scheme, making it an option that is certainly worth investigation. On the other hand, maybe it’s because of its relative newness that the system “suffers from a disproportionate rate of customer dissatisfaction” (SelfBuild Ireland, Winter 2007 issue), not from the basic system itself but from the “low levels of understanding about soil heat fluxes and seasonal variations.” In other words, as with all other significant installations, it makes sense to go to established companies in this business.

There are three components to the geothermal heat pumps (GHP) system:

1, a system of plastic pipeworks that is buried several metres underground;

2, a heat pump for drawing the heat from ground for use in the home;

3, a heat distribution system that circulates the heat throughout the house.




1. The pipework system, known in the trade as the ‘horizontal buried pipe collector’ (HBPC), acts as a radiator in reverse: it draws in the heat (from the soil) rather than radiates it. The greater the difference in temperature between the warm soil and the cold pipes, the greater and faster the heat flow into the whole system. Which is why the fluid in the underground plastic pipes is normally water with anti-freeze; since the ideal temperature of the fluid is just above freezing point, the anti-freeze is needed to stop the water from freezing in the underground pipes.

2. The heat pump is, for all intents and purposes, similar to a conventional gas or oil boiler.

3. The heat is often in the form of hot water. This water is circulated around the house, either to radiators or as underfloor heating.

There are variations on the basic system that you should also be aware of: groundwater heat pumps, which literally bore downwards until striking a body of ground water, from which heat is extracted; a water-to-water system, which extracts heat from surface water in a lake or pond; and a slinky loop system, which curls the underground pipework into loops rather than using a HBPC method.

By all accounts, no matter which system is installed it would appear to be a sensible policy to doubt the expertise of any contractor in GHP until they prove their capability and knowledge. Do not assume that the contractor is totally familiar with the physics underlying the geothermal system. There are too many variables for a one-size-fits-all approach to planning and installation – variables such as seasonal changes in soil temperature, especially at shallow depths; the thermal properties of the soil; and the density, moisture content and type/size of soil particles, not to mention how the operation of taking heat from the soil will affect the ongoing capability of the system.


Cost (initial)

The initial capital cost of installing a GHP system is “usually higher than other conventional central heating systems,” according to SEI (Sustainable Energy Ireland, an independent body). However, it notes that grants under the Greener Homes Scheme will reduce initial costs significantly.


Cost (running)

SEI says in its literature on GHP that for every unit of electricity used to pump the heat, 3-4 units of heat are produced. In the United States, the EPA (Environmental Protection Agency) says the system can save homeowners 30-70% on heating costs (and, incidentally, 20-50% on cooling costs).

Cost (maintenance)

A heat pump system requires very little maintenance, says SEI. It suggests that anyone installing a heat pump should speak with their installer about a maintenance agreement.

Efficiency

“The system is among the most energy-efficient and cost-effective heating and cooling systems available.” (SEI)

Payback

Payback is typically about 8-10 years, according to SEI.

New houses/Existing houses

Geothermal equipment can be installed with equal ease in both new construction projects or in conversion projects.



Glossary

SEI

Sustainable Energy Ireland was set up by the government in 2002 as Ireland’s national energy agency. Its mission is to promote and assist the development of sustainable energy.



Flux

The rate of heat transfer. For geothermal heat pumping systems, it is the measurement of the flow of heat, which moves from hot to cold.