Renewables spotlight: geothermal energy

Derived from the Greek words geo (earth) and therme (heat), ‘geothermal’ refers to drawing heat from the earth’s core. The molten core of the earth is toasty, roughly as hot as the sun at a sizzling 6,000 degrees Celsius. This heat is consistently replenished by the decay of naturally occurring radioactive elements within the earth’s crust. Humans have been taking advantage of this natural and consistent source of heat for thousands of years.

While paleolithic humans lounged in hot springs after a hard day of hunting, our use of geothermal energy today has grown in sophistication and variety. We now use geothermal energy to heat buildings (including entire towns), grow plants in greenhouses, dry agricultural products (such as berries, tea, coffee or rice), heat water at fish farms, and support certain industrial processes such as pasteurising milk. We even pipe geothermal water under roads to melt snow.

A major use of geothermal energy today is to generate electricity, via geothermal power plants. These plants come in three different designs: dry steam, flash steam and binary cycle. Dry steam, the oldest type of power plant, draws steam directly from fractures in the ground to drive a turbine, which in turn generates electricity. Flash steam plants draw high pressure water from underground and mix it with cooler, low pressure water to create steam, which in turn drives a turbine to generate electricity. Binary cycle power plants – the most popular today and considered the way of the future – use hot water passed through a secondary liquid that has a lower boiling point than water. This secondary fluid transforms to vapour, which in turn drives a turbine to generate electricity.

As a renewable energy source, geothermal energy has huge potential to power our world in a sustainable, reliable and environmentally friendly way. Producing just one-sixth of the carbon dioxide emitted by a clean natural-gas power plant, geothermal energy is a sensible choice for a world looking to reduce its carbon emissions. Unlike other renewable energy sources like the sun and the wind, geothermal energy does not fluctuate – it is consistently available. This ‘round the clock’ reliability makes it the perfect complement to other renewable energy sources as it can fill in the gaps when the sun isn’t shining and the wind isn’t blowing, ensuring a stable power grid.

Due to being underground, geothermal energy is resilient to most weather and man-made disasters, offering further stability. We also don’t need to worry about it running out any time soon. As noted by National Geographic: ‘geothermal energy isn't entirely renewable – it will run out when the earth does. In about five billion years.’

While there are many advantages to be gained from geothermal energy, there are significant challenges holding it back from widespread use today. A key challenge is that it is location specific: geothermal plants are limited to areas near tectonic plate boundaries, in order to access the heat from deep underground. Geothermal power plants can run the risk of triggering earthquakes, as they force water into the earth’s crust to open up fissures, which in turn alters the stability of the earth’s structure. However, most geothermal plants are located away from populous areas to mitigate this risk. This is probably a good thing on another account, as geothermal plants may release hydrogen sulphide, a gas that smells like rotten eggs.

There are also environmental impacts to consider: greenhouse gases stored beneath the earth’s surface are released into the atmosphere during digging. While the emissions rate is much lower than that associated with fossil fuels, it is still important to consider. Geothermal energy is also an expensive resource to tap into, with power plants costing millions of dollars and home geothermal pump systems setting you back as much as $30,000, depending on where you live.