As a source of heat and hot water, geothermal energy has been around for millennia. The first hot springs and health spas date back to Roman times or perhaps even before.
As a source of high temperature heat that can be harnessed to generate electrical power, the technology is more recent. The first recorded use of geothermal steam was in 1904 in Larderello, Italy, when a steam vent was harnessed to a simple turbine and generator generating enough electricity to light four light bulbs. In 1911, the world’s first commercial geothermal power plant was constructed on the same site. For more than 45 years there was little further development until, in 1958, the Wairakei geothermal power plant started operation in New Zealand.
Two years later, Pacific Gas and Electric, PG&E, started up the first geothermal power plant at The Geysers in northern California. The Geysers now consists of an interlinked network of geothermal plants that in total generate more than 700 MW.
Geothermal resources can provide both heat and electrical power. In spite of its inherent advantages, the global development of the technology is growing only slowly. Installed capacity worldwide has stayed almost flat at around 13 GW since 2014—that’s only a small fraction of the total global installed renewable energy capacity—which was over 2200 GW in 2018.
Nevertheless, there is continuing interest in the technology because once up and running, the fuel is free. But operation and maintenance costs can be substantial.
In 2017, Indonesia and Turkey continued to lead in terms of new installations, and the two countries accounted for most of capacity additions during that year. Other countries adding capacity were Chile Iceland, Honduras, Mexico, the US, Japan, Portugal and Hungary. However, the US remains the global leader in terms of geothermal power output. The chart shows the rated power in 2016, and added capacity in 2017 for the top ten geothermal power countries . 
Although the US leads the field in terms of capacity, the Philippines, Indonesia, and Turkey are heavily invested in the technology. In May 2018, it was reported that the Sarulla geothermal plant in Indonesia had reached full capacity, with all three 110 MW units up and running. 
Ethiopia is also the target of some serious money invested in geothermal energy. Reykjavik Geothermal, an Icelandic company, is working on two geothermal projects in the east African country. Planned to come on line in 2025, they will reportedly generate over 1000 MW in total power.
Iceland is famous for its geothermal resources. The country derives over a quarter of its electricity from geothermal energy and has over 700 MW of installed geothermal power capacity—much of which provides cogeneration heat for district water and space heating.
How it works
There are basically three ways that electricity can be generated from a high temperature geothermal resource. If the well produces dry steam, it can directly drive a turbine and generator.
Dry steam power plants are the simplest design. The steam is fed directly to a turbine where it generates electricity; the steam is then condensed and the condensate injected back under ground to replenish the aquifer producing the steam. At the Geysers in California, after the first 30 years of operation, the steam supply was becoming depleted and power generation was reduced. The solution was to reinject the condensed steam back into the reservoir 
If the geothermal resource is liquid phase, hot brines are brought to the surface—either under their own pressure or by a variety of techniques that force the hot liquids to the surface. If hot brine under pressure is the geothermal resource, steam can be produced from the brine in a flash separator. The steam powers a turbine, and the condensate and water from the flash separator are reinjected into the geothermal aquifer.
The third technology is more complex. Called a binary cycle system, it uses the geothermal brine to heat a secondary fluid that boils at a lower temperature than the temperature of the geothermal brine. The pressurized vapor can power a turbine and generator before being condensed and returned to the evaporator as a liquid in a closed loop system. The geothermal brine, now several degrees cooler, is reinjected into the geothermal aquifer. Binary cycle systems permit lower temperature geothermal sources to be exploited—down to about 150°C.
Other configurations are possible. Hybrid power plants allow for the integration of several generating technologies. In Hawaii, the Puna flash/binary combined cycle system is designed to optimize both flash and binary cycle technologies. 
Like all renewable energy technologies, geothermal power can be built and operated on a small scale. In 2017, a 4 MW binary cycle plant came online on the Portuguese island of Terceira in the Azores. Although small, the plant provides 10% of the island’s electricity. Hungary’s first geothermal plant—a 3 MW plant at Turawell, produces both electrical power and heat.
Several islands in the Caribbean have geothermal potential: Dominica, Montserrat, and the Grenadine islands in the Caribbean are volcanic islands with probable geothermal resources. But the technology is capital intensive and complex. In Djibouti for example, where geothermal resources are reckoned to be exceptionally good, the technology has been studied for over 30 years. Test wells have been drilled, drilled again, then drilled elsewhere. The results are promising and the economics look good—on paper. But donors are wary of investing in a technology with high upfront costs, long lead times, complex engineering, and uncertain environmental impacts.
In 2015 the Green Climate Fund approved a program called: Sustainable Energy Facility for the Eastern Caribbean, which supports the development and exploitation of potential geothermal sites on Dominica, Grenada, Saint Kitts and Nevis, Saint Lucia, and Saint Vincent and the Grenadines. It remains to be seen whether geothermal energy at these sites will be possible but given the high cost of electricity on these islands, there is a good chance that electricity generation from geothermal energy will prove to be an economic proposition.
This funding was supplemented in late 2017, when the European Union provided $14 million in grants to support geothermal development in the eastern Caribbean. Exploratory drilling on the island of Nevis was underway in late 2017.
The big question for geothermal power generation is whether it can compete with utility-scale solar photovoltaic plants and megawatt wind farms. Power from solar and wind is already less expensive than geothermal electricity, and the cost of electricity from solar and wind is expected to continue to fall—whereas geothermal costs almost certainly will not. But if geothermal energy is exploited for both power generation and district heating, the economics look a lot more favourable.
For several decades, geothermal heat from the Dogger aquifer has provided district heating to the Paris metropolitan area. At least four district heating systems in the Paris area expanded their geothermal capacity during 2017.
Space heating and district heating is one of the largest and fastest growing sectors for the direct use of geothermal heat, although swimming pools and other public baths may still be the single largest end-use category. Other uses of direct heating include domestic hot water supply, greenhouse heating, industrial process heat, aquaculture, snow melting, and agricultural drying. And Iceland is reportedly considering using geothermal heat and power to grow fresh vegetables close to the Arctic Circle.
China is the most significant user of direct geothermal heat, followed by Turkey, Iceland, Japan, Hungary, the US and New Zealand. Together these seven countries account for three quarters of direct geothermal use.
The term ‘geothermal energy’ generally refers to exploiting high temperature geologic resources that are several hundred meters underground. When the earth near the surface is used as a thermal reservoir—for ground-based heat pumps or for storing seasonal heat for several months at a time (like the Drake Landing installation in Alberta, Canada), these technologies are not usually filed under the ‘geothermal energy’ rubric—even though they are in a sense geothermal systems.
For more information check out these sources:
 See: Renewables 2018 Global Status Report. Op.cit.
 See: Sarulla geothermal plant I Indonesia reaches full capacity. At //www.renewableenergyworld.com/articles/2018/05/sarulla-geothermal-plant-in-indonesia-reaches-full-capacity.html/
 See: Ethiopian geothermal is private equity’s next $4-billion bet. At //www.renewableenergyworkd.com/articles/2018/ethiopian-geothermal-is-private-equity-s-next-4-billion.bet.html
 See the Wikipedia article: Geothermal Power, at //en.wikipedia.org/wiki/Geothermal_power/
5] See the Geothermal Energy Association’s website. At //www.geo-energy.org/basics.aspx
 See Renewables 2018 Global Status Report.at: //www.ren21.net/gsr-2018/
 See Renewables 2018 Global Status Report. Op.cit.