ABSTRACT
The inner core of the earth reaches a maximum temperature of about 4000 C. Heat passes out through the solid submarine and land surface mostly by conduction – geothermal heat – and occasionally by active convective currents of molten magma or heated water. The average geothermal heat flow at the Earth’s surface is only 006Wm−2, with a temperature gradient of <30 Ckm−1. This continuous heat current is trivial compared with other renewable supplies in the above surface environment that in total average about 500Wm−2; see Figure 1.2. However, at certain specific locations, increased temperature gradients occur, indicating significant geothermal resources. These may be harnessed over areas of the order of square kilometres and depths of ∼5km at fluxes of 10-20Wm−2 to produce ∼100MW thermalkm−2 in commercial supplies for at least 20 years of operation. Geothermal heat is generally of low quality, and is best used directly
for building or process heat at about 50-70 C, or for preheating of conventional high temperature energy supplies. Such supplies are established in several parts of the world and many more projects are planned. Occasionally geothermal heat is available at temperatures above about 150 C, so electrical power production from turbines can be contemplated. Several important geothermal electric power complexes are fully established, especially in Italy, New Zealand and the USA. It is common to use heat from the near-surface ground or from lakes, etc.
as input to a heat pump. Although this may be interpreted as a ‘geothermal’ source, we do not include such systems as geothermal supplies for the purposes of this chapter. It is probably more meaningful to consider such sources as stored heat from sunshine, since replenishment will be more from the environment above than below. In Chapter 1 renewable energy was defined as currents of energy occur-
ring naturally in the environment. By this definition some sources of geothermal energy can be classed as renewable, because the energy would otherwise
