Geothermal Energy: Renewable or Not?
Heat energy resource or geothermal resource below Earth’s crust can be utilized for electric power generation and for water heating purposes. Based on the record, the United States is the world’s largest producer of geothermal energy, and followed by the Philippines.
Heat energy resource beneath the earth’s surface can be as close as a few kilometers below the surface to heat-up the water sipping in a permeable rocks or fault line to several hundreds of degree Celsius, enough to turn a steam turbine and generates electricity.
But is this geothermal resource a renewable energy source or not?
Unlike wind and water resources that can be use for power generation as it cycles for an infinite number of times, trapped heat energy inside the Earth can cool down and died-up. But the rate that this heat is released is very slow (an average of about 60 milliwatt per square meter) that it would take millions of years to cool down the earth’s core. For earth’s people’s point of view, this heat source is almost infinite.
However, geothermal energy sources that could be tapped and used for power generation are only those heat reservoirs that are a few kilometers below the earth’s surface. These reservoirs are relatively small compared to Earths main heat source hundreds of kilometers underground. Thus, these heat reservoirs can decrease its temperature and pressure due to massive release of its heat through steam.
According to geoscientists, “When heat extraction rates exceeds the rate of recharge, reservoir depletion will occur, but on shutdown, will undergo asymptotic recovery towards original pressures and temperatures”
It means that if the heat extracted from the geothermal reservoir is so high that the reservoir cannot anymore keep its expected temperature and pressure, it will cool and die down. For this instance, the geothermal resource is not renewable. It would take around 50 years to recover the original pressure and temperature of the reservoir and could be used again. During this recovery period, the power plant, of course, has to be shut down.
Filed in: Renewable Energy - 
