Geothermal is an energy source that does not originate from the sun (along with nuclear) although the sun probably helped originally. The majority of the planet is very hot, molten rock with a thin, cool, solid crust. We can access this energy in certain spots on the surface of the planet, but the engineering issues regularly become too difficult with our current technology.
The heat mostly comes from the radioactive decay of various elements within the earth. Added to this is the heat from gravitation compressing the centre of the earth and the original heat from the planet’s formation (helped along by the sun). Solid rock holds its heat well (think of pizza stones) and is a poor conductor.
We don’t know a lot about the exact nature of the earth’s interior as we cannot access it. What we do know comes from interpreting geophysical data like the behaviour of seismic waves, density measurements and the fact that we have a magnetic field (most likely generated by a mobile, largely Iron core). The radioactive elements within the core are likely to be Uranium, Thorium and Potassium. Seismic studies suggest that the inner (central) core is most likely solid due to the immense pressures.
Some areas have easy access to hot rocks at or near the surface. Iceland sits on a spreading zone between two tectonic plates. Hot water pools abound and are used for many purposes including bathing. About 80 % of Iceland’s hot water requirement is satisfied by geothermal heat. Additionally about 26 % of the nation’s electricity comes from geothermal sources.
Denmark produces hot water accessed from sandstones from deep wells to provide heating. At Thilsted the system produces water from about 1200m at a temperature of 45 degrees C and is used as a community heating source. The Danes do no produce electricity from this water.
Worldwide, geothermal heating occurs in 70 countries while 24 of these also use geothermal heat to produce electricity. The US is the world’s largest geothermal electricity producer with about 28% of the total produced. New Zealand is another success story in producing electricity from geothermal heat.
The historic success stories have one thing in common. They access near surface, boiling water and produce electricity from steam. Steam turbines are still the major method of producing electricity with over 90% of US energy coming from steam driven systems. The heat source can be geothermal, gas, diesel, nuclear, coal etc. With geothermal the water temperature is limited to what is accessible at the time and producing this water inevitably leads to lower water temperatures as time goes by, unless carefully managed.
Initial geothermal systems were Dry Steam systems. Here the steam needs to be at 150 degrees C or greater to directly drive the turbines. These have been largely replaced by Flash steam systems. Here wells are drilled to access hot water / steam at high pressure. As this water comes to the surface at lower pressure it converts to steam and can drive the turbines. The remaining cooler water is pumped back into the reservoir at a different location and allowed to reheat. The colder water will lower the heat in the local rocks over time, so the amount of heat extracted needs to be managed.
Recently, Binary cycle systems have been developed which a lower the required water temperature to about 60 or 70 degrees C. These work by heating another fluid which has a lower boiling point. The secondary fluid is generally butane or pentane, although mercury systems have also been developed. While they can produce electricity from lower temperature ground water they are not very efficient.
Geothermal power has some problems, even in the best of systems. As discussed, water production will cool the system over time, unless new heat sources become available ( eg hot flowing lava). Water production can also allow dissolved gases to leak into the atmosphere (CO2, methane etc). These are generally low level compared to coal use for example.
Another problem is the reinjection of waste water. In Basel in Switzerland, a geothermal study project involved reinjecting water. Being in the highly stressed alps, this water injection created a series of minor earthquakes. As this is a similar technique to fraccing, it is not recommended in stressed, mountainous regions.
The major issue with expanding geothermal around the world is the problem of accessing hot enough water. There is plenty of hot water at depth, but it can be very expensive to access. In South Australia a number of projects were initiated about ten years ago but all of the Companies involved seem to have ceased geothermal activity. Various projects were touted to access hot rocks, eg granites 4-5 km below the surface, under the Cooper Basin, deep in the Flinders Ranges, deep in the Otway Basin near Penola and from hot artesian waters.
It would seem that engineering and hence cost difficulties have overcome these projects after spending many millions of largely government dollars. So while the shallow sources appear to work, deep hot rocks do not appear to be viable at this stage of technology.
Some of the hot rocks technology required drilling a deep well, then fraccing granites at about 5 km deep and pumping water through them to be accessed by a second well a kilometre or so away. Electricity was produced from this system for a brief period but production did not last long. The heat from these granites was enough to melt the bottom hole assemblies in nearby gas wells!
Water not hot enough!
The use of lukewarm water can also be problematic. Water is recommended to be over 65 degrees C in hot water systems, otherwise dangerous bacteria can build up. Outbreaks of Legionnaires disease are often from storing warm, rather than hot water. Solar hot water systems require a booster (gas or electric) to ensure water is hot enough.
Solar thermal systems use the sun’s energy, reflected from banks of mirrors to heat up a thermal mass (eg hot salt) and use this to create steam. If there is not enough heat stored in the system then a backup is required (generally gas).
A Heat Engine (ie a steam turbine) needs a heat source and a cooling procedure. The greater the difference between the two, then the more efficient the heat engine. This is called the Carnot cycle (or Rankine cycle). Steam turbines / engines are cooled by pumping cold water into the system or simply by using cooling towers (those big steam belching towers shown in climate videos). If the geothermal temperatures are low (ie at the 60 degree mark) then cooling temperatures must be even lower. This restraint makes low temperature systems inefficient.
In summary, geothermal is a great source of heat where it is accessible. Land use can become an issue where the resource is in a natural beauty area (often in National Parks). Production of electricity from hot rocks requires water that is hot enough, renewable and again easily accessible. It does not seem economically viable at this stage from deep heat reservoirs.