Geo-thermal energy systems, which tap the relatively constant earth temperature a few metres underground, is gaining in interest and use as more architects, builders and homeowners are discovering the benefits of these systems.

How does it work?

The earth absorbs almost 50% of all solar energy and remains a nearly constant 10°C to 21°C (50°F to 70°F) temperature, depending on geographic location. Working with an underground loop system, geothermal heating and cooling systems utilize this constant temperature to exchange energy between your home and the earth as needed for heating and cooling.

Geothermal heating at Shettleston, Glasgow

New residential developments are beginning to include this energy source in their plans since it is easier to install during new construction and cost economies of scale can be had. Depending on the type of development, the same system may be used for multiple units. One such development is in Shettleston, Scotland, where John Gilbert Architects, Glasgow, completed a residential project that uses a combination of solar and geothermal energy to heat 16 houses. In the first year of use, the costs for heating and hot water supply averaged just £150 (€220) a house per annum.

The decision to use geothermal energy was made when the firm discovered a disused coal mine under the site. A scheme was developed which uses the preheated water in the flooded coal mine to heat the houses. The water in the coal mine, which is 100 metres below ground level, maintains a temperature of about 12°C (54°F) throughout the year. A well hole was drilled down to the mine and an in-line pump used to raise the warmed water, passing it through one of two heat pumps to boost the temperature to 55°C (131°F). The warm water is stored in a thermal storage tank and is then distributed to each house's radiators. Each flat has its own room thermostat and timer switch.

For hot water supply the thermal store is pumped through indirect coils in each hot water cylinder in order to heat the water to about 45°C (113°F). Although warm water can be taken from the cylinder, electric immersers were provided so that tenants who require it can boost the water to higher temperatures. Energy is still saved since less is required to raise water from 45°C to 60°C (113°F to 140°F) than in a system which heats water from a cold start. The advantage in having tenant control over hot water heating is that expensive metering controls are avoided. Feedback from tenants suggests that few people require the electric immerser water heating boost.

The Shettleston development also uses solar panels, providing additional heating to the thermal storage tank during times when tank heat is dissipated. The solar panels are so effective over the summer and autumn months that the heat pumps have not had to operate.

Benefits to the Pocketbook and the Environment

Aesthetically, living space is gained since the geothermal in-home components are all-in-one, relatively small units compared to traditional systems. But more enticing to home owners, the cost savings for heating, cooling and hot water supply have been reported at upwards of 40% per annum depending on the geographic and economic location. A luxury town house complex using geothermal energy to heat and cool its 320 units in Adelaide, Australia, estimates their total annual savings at over €125,500.

Additionally, geothermal technology produces 50-60% less carbon dioxide (CO2), 15% less sulfur dioxide (SO2), and uses 30% less energy then traditional systems, reducing residential energy's environmental footprint. The World Energy Council estimated in the WEC Survey of Energy Resources 2001 that total world use of geothermal power contributes energy savings of approximately 26 million tons of oil per year and 80 million tons in CO2 emission reduction per year, compared to equivalent oil-fuelled production.

And, with the earth acting as a constant, solar energy "sponge", geothermal energy is a viable, renewable energy source available 24 hours a day, all year round.