It is located on the Hengill volcano. This volcano is still active, as evidenced by numerous hot springs and fumaroles (openings where hot sulfurous gases emerge), but the last eruption occurred approximately 2,000 years ago. Here is the entrance (and far left above).
Currently, Hellisheidi is the third largest geothermal power plant in the world and the largest of five on this island. It powers about 30% of Iceland's total energy in the form of electricity and hot water, and virtually all the energy for the greater Reykjavik area. We saw tiny "personal" sized power plants occasionally along our drive in the outlying areas.
This exhibition was fascinating, although different than the Lava Center. By that I mean not so much show-and-tell or interactive-ness. Much of the info came from our guide or perusing colorful graphic displays. That took a little more time and effort to read.
We started with a guide who explained the geothermal process. Simply stated, it rains or snows and water soaks into the ground. The water is warmed by the internal heat of the earth's crust to between 225-400C (or 437-752F). Water is extracted through drilling. The hot water or steam energy is transported to places of use.
We saw part of the turbine processing area. We could feel the engine room vibrating under our feet.
Then we saw a short film reiterating the energy producing process and offering a little more history (plant constructed in 2006) and other facts. At full capacity the plant produces 303 megawatts of electricity and 134 megawatts of hot water. One megawatt can provide electricity for 1,000 people.
After the movie we were on our own to absorb the rest of the display information. This one shows the progression of the use of geothermal activity.
There was also an Icelandic mineral collection and samples of tools used to harness the energy. These drill bits were huge, each of the three were maybe the size of a cantaloupe.
Geothermal is used to generate electricity and heat buildings, including business, homes and a number of greenhouses we saw in our travels. We learned that ninety percent of houses in Iceland are heated this way. Many sidewalks in Reykjavik are heated by a snow melt system. People recycle hot "gray" water in their homes to melt snow in their driveways.
An aside: Mike and I talked about renewable energy today and did not remember seeing wind turbines in Iceland other than the small weather / earthquake monitors we saw along the Ring Road. So googled it and here's what we found: https://www.google.com/url?sa=i&source=imgres&cd=&ved=2ahUKEwjh57r47IzbAhVG3IMKHSdsBDsQjxx6BAgBEAI&url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DJzGnrtGGTw0&psig=AOvVaw1uT7PC88kXliNj8Pd0qjjZ&ust=1526650056121787.