When most people think of energy production, they probably tend to think of digging holes or drilling to find coal, gas, or oil supplies beneath the ground, or maybe even uranium deposits. And when we think of renewable energy, above-ground sources of energy are probably what spring to mind: solar, wind, and water power. But what if digging in the ground could provide us with a renewable source of power that met all of our energy needs?
Geothermal energy is literally “heat from the earth”, and could theoretically be tapped almost anywhere in the world. The outer layer of the earth’s crust is variable in thickness, which means the molten parts are closer to the surface in some places. That’s obvious when we see volcanoes, or the famous hot mud pools at Rotorua in New Zealand. Early cultures may have worshipped the potentially destructive powers of volcanoes, and even used hot springs for their health benefits, while the ancient Romans had figured out ways to heat their homes with geothermal energy. But there are more modern ways to harness the free energy from the earth, and they don’t necessarily require a local volcano.
The basics: Hot rocks, dry rocks and wet rocks
The earth contains a whole lot of energy. Around 20% is left over from when the earth formed, and the remaining 80% is being generated by the decay of radioactive elements within the planet itself. What this amounts to is a great deal of heat which eventually makes its way to just below the crust. It’s the heat that is almost at the surface that is available, though of course, we don’t need to get to the molten lava to access it.
Rocks naturally absorb and conduct heat, and as you will know if you have ever put your hand on a large rock after the sun goes down, it retains the heat absorbed during the day. So the rocks beneath our feet retain and transmit the heat from below. Some of it comes close enough to the surface that we can drill down into it. Some of these hot rocks are dry, meaning there’s no water present around them, but some are wet rocks, which are either naturally porous, or have fractured enough to allow water to pass around and through them. Water is also an excellent conductor of heat, and these wet, hot rocks are the most useful for accessing geothermal energy. In some places, water gets heated to 370° C in the crust, which is much higher than it needs to be in order to be of use as an energy source. So how is it harnessed?
Direct thermal energy
Where temperatures reach around 30° Celsius within a few hundred metres of the surface, which includes much of mainland Australia and Tasmania, the heat from the earth can be used directly for heating houses, greenhouses, factories and other buildings. By installing deeper channels to get warmth as well as shallower ducts that provide cool air, buildings can be heated and cooled by alternately opening and closing the appropriate vents at different times of year. Such climate control systems are effectively emission free, and could be used in most parts of Australia.
Other uses for the direct heat of geothermal supplies are for more industrial and agricultural processes: from the drying of agricultural products such as vegetables, meat and fish, to the use of warmed water for fish and mushroom farming. The heat could also be used for de-icing in cold climates for road surfaces and in mining operations. The heat required for these types of activities will normally be harnessed by means of water, either injected into the ground or naturally occurring where rocks are suitably hot and porous, and then recovered at the surface and piped as needed. There is some loss of heat due to this kind of transport, so applications may be limited by the average temperature water can be heated at the geothermal source.
In areas where temperatures of the geothermal hot rocks are higher, water can be pumped down, then recovered at the surface as steam, due to greater heat and pressure. The steam could then be used to drive turbines in much the same way as electricity is generated in coal fired or nuclear-fuelled power stations. Geothermal electricity generation is already a reality in many countries, New Zealand generates around 10% of their national supply this way. El Salvador produces a quarter of their electricity from geothermal plants, while Iceland has about 30% of its supply coming from the ground.
In Australia, a number of geothermal electricity projects are underway, the most promising being in South Australia, where the Cooper Basin has been claimed to have the “hottest rocks on earth”, suggesting that they can access several thousand megawatts of electricity from the project. Australia’s total installed electricity generation capacity is over 50,000 MW, which is in excess of any single geothermal source at present. But unlike other renewable sources, geothermal electricity can provide the essential baseload supply, which is a major drawback with solar and wind based projects.
Further projects in multiple locations in Victoria, Western Australia, Tasmania and the Northern Territory demonstrate that the potential for geothermal electricity generation is far less reliant on geographic location than fossil fuel deposits. The use of direct geothermal energy for low level heating also has the potential to reduce electricity consumption and the use of limited natural gas supplies for heating. This can help reduce greenhouse gas emissions from coal burning electricity generation, as well as free up the cleaner burning gas for other purposes.
Greater government and private sector investment in geothermal exploration and technology will certainly pay dividends into the future, not just in Australia, but as an export market for local expertise and equipment. While geothermal energy may not quite fulfil the promises of an inexhaustible supply of energy, it certainly offers a clean, safe, renewable source of energy that should have a place in the future energy mix for Australia and the rest of the world. To begin making savings on your energy bills and to select a plan that suits the needs of your family, compare energy providers today.