Daily Journal

Day 17 | August 3, 2005

Power from the Earth

Production of liquid silicate rock - or magma - occurs deep in the subduction zone. Its transport to the near surface has two direct consequences that we observed in today's visit to a geothermal power plant on the island of Hachijojima. Most obviously, we can see how the magma can give up its heat which is used for the production of electricity. Secondly, and the reason we are here, is that we can sample the volatiles released from the magma and transferred to the surface via the geothermal well.

There are 3 wells drilled to depths >1500m at the geothermal plant. At present, however, only one is in production. The operation takes advantage of injected surface water which boils as it circulates in the vicinity of the wells. The resulting vapor - at high temperature and pressure - is forced through a steam separator and onward through a series of pipes to a turbine system where it produces electricity. Well HT-1 can produce 3.3 MW of power, or about 20% of the island's power needs.

Interestingly, there is one wind-mill at the site which can be used to supplement the energy output of the plant on windy days. It is rated at 0.5 MW and wind speeds of 14 meters per second or greater are needed for optimum power production. The accompanying graph, although labeled in Japanese, gives the details.