Geothermal Energy: An Alternative Resource for the 21st Century

My nephew, David, lent me a book about geothermal energy by Harsh Gupta, and Sukanta Roy.  I learned a lot when I read it.

Geothermal energy is energy found in the earth.  It is created by natural decay of radioactive rocks, but principally by subduction of the earth’s crust where the continental plates push the ocean’s plates down, creating heat.  Where the heat is sufficiently hot, and where the warmth is near the surface, it is viable to produce electricity.

The warmth near the surface of the earth will depend on how close the warm rock is to the earth’s surface.  The warmth will also depend on the type of rock – some rocks act as better insulators than others.

Electricity from geothermal sources has been around a long time, beginning in 1904 in Lardello, Italy.  Today, geothermal electricity is generated in 21 countries, and has an installed capacity of 8000 MW.  This would be equal to the output of 2 Darlington plants, enough to supply 20% of Ontario supply, but it is small compared to the world’s installed capacity of wind, which is over 80,000 MW.  Geothermal heat is used in another 37 countries for direct uses like space heating, hot water, and fish farming.  Countries like Iceland, New Zealand, Costa Rica, Italy, the Phillipines, Japan, and Indonesia and the state of California all have significant installations,  obtaining as much as 15% of their electricity from this benign energy source.
An MIT study estimated that the US alone could install 100,000 MW of geothermal electricity generation, enough to supply 10% of US supply.

Electricity is made by either using water that is already present deep in the ground, or by injecting water from the surface, to make steam to run the generator.  Once used, the water is condensed, and re-injected into the rock below.  For rocks that are less hot, hot water can be circulated instead of steam, and the heat in the water can be used to run a binary cycle generator, that typically uses an ammonia/water mix, which has a lower vapour temperature, creating ammonia gas that can run a generator.  This fairly recently deployed method allows utilization of lower temperature resources (~100 degrees C).

A most interesting application using temperature differences between warm tropical surface water, and cooler deep ocean water was also described.  There are several islands that have installed this type of system, with an important side benefit that they create fresh water for people or agricultural use.

There was a lengthy discussion about exploring for hot rocks.  Much like finding oil and gas, there are numerous exploration techniques for improving the estimate of the extent and depth of the heat.  Ariel surveys, chemical surveys, limited shallow drilling, electrical resistivity tests, gravity variations, seismic tests are all tools used to establish the existence and extent of geothermal resources.
The book is highly technical, and has many mathematical formulas, that I have to confess lost me in many cases.  But still, an overall impression was left that there is substantial opportunity to expand our use of geothermal heat many fold, and that geothermal energy should play an important part in a sustainable energy future.  Anyone interested in getting into or investing in the geothermal business should read this book.

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