What is Geothermal Power?
Geothermal power is utilising energy that comes in the form of heat from beneath the earth’s crust/ surface layer. Essentially this is utilising the same scientific principles as used in ground source heat pumps, but on an industrial scale. Geothermal power relies on large generators and infrastructure that can provide both heat and electricity to multiple dwellings and commercial properties.
Geothermal power is a renewable heat source that can provide energy for electricity production as well as heating for a number of applications and appliances. Currently in the UK there is one working geothermal power plant in Southampton, which is providing a local district heating solution rather than being used an electricity generating power plant.
Some areas of the world have much more geothermal activity (such as Iceland, West Coast of the US, Rotorua in New Zealand, etc.), and these are more obvious places to harness the earth’s geothermal power. However, if you dig deep enough, heat is available anywhere across the globe (including the UK) so we could definitely roll out this technology more, and build on expertise to harvest the heat in more effective ways.
The science behind geothermal power is relatively simple; heat is continuously flowing from the Earth’s core by conduction (travelling from hot to cold) to the surface and is therefore considered as renewable source as long as the Earth continues to have an active core. It is estimated that geothermal power could produce 44million MWs of power, so even if we could tap a very small percentage of this it would service most of our energy needs.
Geothermal Power for Electricity Generation
Utilising geothermal power requires accessing high temperature fluid that is heated deep underground. Historically this fluid has existed underground, formed by rainwater passing through cracks in the crust. The water is heated by hot rock underneath and compressed by pressure that maintain it its liquid form. The water potentially then finds a path through the Earth’s crust, and presents itself on the surface as hot water springs or geysers.
Nowadays, in addition to this naturally occurring phenomenon, we can also artificially mimic this by pumping cold surface water down into the earth’s crust, where it gets superheated and returns to the surface via circulation pumps. Existing Geothermal Power plants have either tapped into the naturally occurring process or mimicked this to produce the steam necessary to drive turbines to create electricity. As we can now have the technology to drill deeper than ever into the Earth’s crust, by utilising the man made process, we can implement geothermal power stations anywhere in the world.
Three common Geothermal Power generating systems
Dry Steam Geothermal Power
Dry steam geothermal power plants use geothermal steam directly to turn the electricity producing turbines. To have this structure in place requires steam directly travelling to the Earth’s surface, which is quite a rare phenomenon, so there are only few examples of this type of power station worldwide. The geothermal steam, which is superheated (above 1000C) is forced up through cracks in the ground under great pressure. The pressure of hot steam is driven through pipe shafts, which then rotate the turbines. The turbines drive a generator, which creates the electrical charge. Hot steam is then cooled in a cold heat exchanger and the cold water is then pumped back into the ground, which then kicks off this cycle again.
Flash System Geothermal Power
Flash steam geothermal power plants rely on highly pressurised, superheated water instead of steam. The highly pressurised liquid is pushed through a series of pressure tanks. In turn, these holding tanks reduce the pressure of the liquid, allowing it to turn into steam. This process is repeated several times in different depressurised chambers with the steam then collected and ’flashed’ through to drive a turbine generator system to create electricity. As with the dry steam geothermal power system, the excess hot liquid is cooled and / or condensed and then pumped back into the ground so the liquid is replenished.
Binary Cycle Geothermal Power
Binary cycle geothermal power plants use lower temperatures to produce energy and use technology much like that used in OTEC, using the heat gradient to turn a working fluid, such as ammonium and/ or propane into steam to drive the electro generating system. This type of system can be implemented using lower ground temperature as a working fluid has a lower boiling point than water. Once the working fluid passes through the turbine shafts, it is condensed back into the liquid and reused over and over again.
Geothermal Power for Heating
We have talked a lot about electricity generation in the above section, however geothermal power can also be used for heating solutions. Like ground source heat pump systems used in the home, a geothermal power system can be implemented as a district heating solution. In the UK, this would be optimal in areas that are not covered by the current gas grid. Using residual heat from the dry steam or flash system geothermal processes, local homes and businesses could be supplied with heat all year round.
The slight issue in the UK, is that the infrastructure requires investment by the generation companies to make this a feasible proposition, it would however allow them to supply both heat and power (CHP cogeneration) to consumers. These types of solutions are currently more common in Scandinavian countries, and therefore the UK is playing catch up as far as geothermal power goes.
Where is Geothermal Power now?
In the UK, there is enthusiasm about the prospects of geothermal power, but thus far DECC has not provided the additional support that the technology really needs to make a suitable foothold. Currently geothermal power qualifies for two Renewable Obligation Certificates (ROCs) per MWh of electricity generated, but the investment community believe this support should be closer to four ROCs, so that investment doesn’t go to other parts of Europe like Germany.
Geothermal power electricity is currently produced in 24 countries across the planet with the total combined installed capacity being approximately 10,715MW. The largest capacity is in the USA (3.1GW installed in 2010), however by 2015 this is expected to increase by 75%, taking installed capacity to 18.5GW. Geothermal power as a heating solution is much more widespread, and currently used in over 70 countries worldwide.
The largest geothermal power company in the world at present is the Calpine Corporation which taps geothermal electricity primarily in the geysers in California. The 19 geothermal power plants it has in this location provide 25% of the green energy to California. In the UK, the Eden Project in Cornwall was granted permission to build a hot dry rock geothermal power station in December 2010, which will power Eden and supply enough energy for 5000 additional houses in the surrounding area.