
56 Principles of Sustainable Energy Systems
© 2008 Taylor & Francis Group, LLC
1.8.1 Geothermal Energy
Geothermal heat originates from two sources: residual heat generated in the formation of the
planet by gravitational collapse, and heat generated by decay of radioactive isotopes. The core
of the Earth is estimated to have temperatures of 4500°C–6600°C. The solid plates that form
the crust of the planet are about 35 km thick and have relatively low thermal conductivity so
that the temperature at the surface is only about 10°C on average. The average temperature
gradient in the crust is 25°C–30°C/km. Commercially viable geothermal resources occu ...