Direct geothermal energy could be key to our clean energy future

Direct geothermal energy provides more than half this Vienna building’s heating and cooling requirements. Energietecknik GmbH & Co, Vienna

While direct geothermal energy is extensively used in other countries, it is rarely encountered in Australia.

This might be because we have cheap sources of energy (although, regrettably, not the cleanest). We also haven’t had to worry too much about alternatives yet.

Our climate is also less demanding than the much colder, heavily populated areas of the northern hemisphere.

But it’s highly likely that this is going to change dramatically soon. A carbon price will push up the price of conventional power and greenhouse gas emissions will have to be seriously reduced.

Clean energy has to come to the rescue.

While all forms of renewable energy will play a part, the geothermal alternative must become a major player. It is abundant, totally renewable (it is recharged over enormous areas by the sun), it involves well established and reliable technology, and, unlike most others, it is available 24/7.

Energy use in buildings accounts for 26% of Australia’s greenhouse gas emissions. Heating and cooling accounts for over half of this.

Cheaper energy, lower emissions

The introduction of direct geothermal heating and cooling to Australia - even on a moderate scale - would have a significant impact on power requirements. There would be enormous economic and environmental benefits.

As virtually every building in Australia requires some form of heating or cooling, direct geothermal energy could influence every Australian and their carbon footprint.

For each kilowatt of electrical energy put into a direct geothermal system, about 4 kilowatts of energy is developed for the purposes of heating and cooling.

This means that outside of the capital costs of the installation, 75% of the power is free.

A significant amount of electrical power in Australia is generated with brown coal. Replacing 75% of this with a totally clean renewable energy source would reduce greenhouse gas emissions to as little as 25% of what occurs with current practice.

Clearly, this is a crude assessment of what is possible and other fuel sources are not taken into account.

But the figures do indicate some of the significant economic and environmental benefits that can be achieved directly and indirectly.

Tapping the earth’s potential

Outside the few volcanic regions of the world where it is readily available, geothermal energy can be extracted in two ways.

Electricity can be generated from the heat in rocks several kilometres below the surface. Down here, temperatures exceed 200ºC. Although this source of power has enormous potential, we are still several years from producing electricity from it on a commercial scale.

The other is the direct form, which is well established outside Australia.

Direct geothermal energy uses the ground within a few tens of metres of the surface. It acts both as a heat source and a sink to heat and cool buildings.

A geothermal pump will cool your house in summer, and warm it in winter.

It has been estimated that there are over 2 million direct geothermal installations in operation around the world, principally in northern Europe and North America.

The key element in any direct geothermal system is the ground source heat pump. In winter the pump extracts heat from water circulating in ground loops and delivers it to a building.

In summer, the reverse happens with the pump extracting excess heat from the building and dumping it to the ground.

The principal is the same as the heat pump on your fridge. A ground source heat pump is simply a powerful fridge capable of moving large quantities of heat.

The pump for an average house is about the size of a small bar fridge.

But there are many examples of much larger direct geothermal energy applications. The underground railway station pictured below is one such installation in Austria.

Geothermal can be used on a much larger scale than your home. Energietecknik GmbH & Co, Vienna

The capital costs of installing a direct geothermal system are still a little high. But with industry becoming better geared to needs, and with better systems of design and installation, prices should fall significantly over the next year or two.

This, combined with the likely major increase in the cost of conventionally derived energy, will mean that capital costs can be recovered in a few short years.

Making it cheaper

Although there are a large number of installations worldwide, these have been driven by the heating, ventilation and air conditioning industry. There has been little technical input from geotechnical engineers.

This has led to numerous, very approximate guidelines of what should be installed in the ground to provide the energy required.

These guidelines lead to installations that are generally over-designed. With better technical information, systems could be more cost effective and competitive.

The University of Melbourne is putting geothermal energy to use on campus. Stuart Colls

To get better technical information for Australian conditions, the Department of Infrastructure Engineering in the University of Melbourne’s School of Engineering has embarked on an extensive research project.

We have planned field performance studies for new and existing buildings around the state, and recently completed a Geothermal Test Facility on the university campus.

This facility comprises a number of different ground loop systems installed in vertical boreholes with diameters between 100mm and 600mm to depths of 30m.

The facility has been specifically designed to provide data on energy yield from the ground underlying the campus and the ground loop systems best suited to extract (or dump) this energy.

Detailed testing has recently commenced and the results to date indicate that direct geothermal energy has a very big future in Australia.

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