Outside of the professional industry circles, only a few people actually know what geothermal energy is all about. As a result, there is significant confusion, leading people to think that geothermal energy refers to just to those large geothermal power plants set up next to geysers and hot springs.
Not the case.
Let us first look at what geothermal energy is all about.
Below Earth’s crust, there is a layer of hot and molten rock, called magma. Heat is continually produced in this layer, mostly from the decay of naturally radioactive materials such as uranium and potassium. The amount of such heat within 10,000 meters of Earth’s surface is quite massive: some estimates suggest that the amount of heat could be equivalent to 50,000 times that of energy from all the oil and natural gas resources in the world!
Now, what needs to be understood is that there is a heat gradient, and you get different geothermal temperatures in different regions.The areas with the highest underground temperatures are in regions with active or geologically young volcanoes. These “hot spots” occur at tectonic plate boundaries or at places where the crust is thin enough to let the heat through. Earthquakes and magma movement break up the rock covering, allowing water to circulate. As the water rises to the surface, natural hot springs and geysers occur, with water temperatures in these systems rising to over 200°C, sometimes much higher.
But such seismically active hot spots are not the only places where geothermal energy can be found. There is a steady supply of milder heat—useful for direct heating purposes—at depths of anywhere from 10 to a few hundred feet below the surface virtually in any location on Earth. Even the ground below your own backyard or local school has enough heat to control the climate in your home or other buildings in the community.
With such a range of temperatures available, geothermal energy can be used either for just heating or for power generation.
Typically, geothermal energy is used mainly for heating in locations where the temperature available under the earth’s surface is only moderately high – a few degrees higher than the surface temperature in winter and a few degrees lower than the surface temperature in summer. Equipment called geothermal heat pumps enable heat energy to be recovered from these locations. As you can guess, many regions in the world qualify for this type of use of geothermal energy.
In select locations where there is significantly higher temperature below the surface, and which could in turn manifest in the form of geysers or hot springs, or in the form of hot gases, geothermal energy can be used to generate power. Unlike in the case of geothermal heating, which is applicable for many regions worldwide, there are relatively fewer locations worldwide that qualify as suitable for generating geothermal power.
Thus, geothermal energy is an umbrella term, and power is only one of the two main output recovered from it, the other one being heat.
Not long back, a prominent Indian business group was keen on entering into geothermal – they wanted to be a pioneer in a cleantech sector and they felt that the geothermal energy sector was far less congested than most others (say solar). When they asked my opinion on whether they should invest in geothermal energy, I asked them back: Geothermal Heating or Geothermal Power? They said: Well, we are just keen on geothermal energy, it can be either power or heat. It took me some time to convince them that these two were quite different beasts.