Energy solutions: Geothermal energy

Our Earth as a heater

Geothermal simply means 'Earth’s heat'. The centre of the Earth is extremely hot, about 5,500°C at the core just over 6,500 kilometres below the crust. This is about as hot as the surface of the sun.

Even at 3 metres depth the temperature remains constant at 10-16°C throughout the year. On average, the temperature below the surface increases by 3°C for every 100 metres of depth. 

Tapping geothermal energy

The relatively constant temperature of the top 15 metres of the Earth's surface (or groundwater), technically known as shallow geothermal technology, can be used to heat or cool buildings. A heat pump uses a series of pipes to circulate fluid through the warm ground.

In the winter when the ground is warmer than the buildings above, the liquid absorbs heat from the ground, which is then concentrated by means of heat exchangers or collectors, and transferred to the buildings. In the summer, when the ground is cooler, the pump transfers heat from the buildings back into the ground, or cools an air conditioning system.

Tapping the energy deep inside the earth, the so-called deep geothermal energy requires boreholes to great depths. If thermal groundwater is available, this can be used directly in hydrothermal power plants to generate electricity and heat. If not, water can be pumped between hot layers of rock and then brought back to the surface at high temperature by means of a second borehole.

Advantages

The use of geothermal energy requires no burning of fossil fuels. Geothermal power plants emit only excess steam and very few trace gases (1000 to 2000 times less carbon dioxide than fossil fuel power plants), they take up very little land compared to traditional fossil-fuel plants, and advanced drilling techniques minimise the impact of drilling wells. The electricity produced is also more 'available', as fossil-fuelled power plants produce electricity 65-75% of the time compared to 90% from geothermal power plants. While geothermal resources are not spread uniformly, geothermal heat pumps can be used almost anywhere.

Potential

By 2000, there were 8000 MWe installed electricity production and 15,000 MWt for direct thermal use (heating and cooling), according to the International Geothermal Association (IGA).

When a heat pump is used to provide domestic heating, the savings on electricity alone can outweigh the cost of installing and running the system. Where geothermal energy is used in agriculture (such as to heat greenhouses) heating costs can be cut by up to 80%.

Due to the availability and continuity of geothermal energy, electricity generation from geothermal sources is of great significance for an energy industry increasingly relying on renewable energy resources. However, the use of deep geothermal energy in power plants is still on its infancy. Exceptions are countries such as the Philippines (27% of total electricity generation!), Iceland, the United States, Indonesia, New Zealand, Mexico and Italy, which all have favourable geological conditions. In these countries geothermal energy is already well established, since the exploitation is fairly economical. Further research and development is required to reduce costs and optimise the technology involved.