Dry Steam Geothermal Power was the oldest use of geothermal power on Earth. It was also the simplest.
Geothermal hot springs had been used for bathing since Paleolithic times. The Romans used hot springs to feed public baths. They also used hot springs for heating houses. In the early 20th century, heating started to be based directly on geothermal energy. The first of these were used in greenhouses. By the 1930s, geothermal energy was used to heat houses. In the 1960s, geothermal energy took on electricity generation.
Tech Level: 9
In 1960, Pacific Gas and Electric created the first successful geothermal power plant. It took heat from earth's core and used it to heat a fluid. In this case, it was dry steam. This required a superheater to convert wet steam into dry steam. The dry steam turned a turbine which, in turn, turned a generator that generated electricity. The dry steam would then be condensed back into water. Dry steam power plants were the simplest design for a geothermal power plant. But there was a problem. The temperature at which this could be done was 150°C. By comparison, flash steam power plants were much hotter.