ABSTRACT
Geothermal energy is derived from the earth’s natural and continuous heat flow which has been estimated at some 2·75×1016 cal/h (thermally equivalent to 30 000 million kW). This massive flux can be related to a temperature differential of some 4000-4500°C at the centre of the earth compared with an average of 15°C at the surface of the crust. Considered on this global scale, geothermal energy may be regarded as a renewable source, but for practical purposes the energy available from any individual site will be exhausted within some two to three decades. It is generally accepted that thermal energy is derived primarily from the decay of long-lived radioactive minerals with a smaller, but significant, contribution from a slight cooling of the earth.1 Before the discovery of radioactivity and the realisation of its significance in the thermal history of the earth, physicists such as Lord Kelvin had calculated that the time required for the earth to cool from an original molten state to its present condition was only some 80 million years. It was subsequently realised that the internal heat source provided by the decay of the long-lived radioactive isotopes reduces the cooling rate sufficiently to accommodate the observed heat flow without the necessity for the full cooling hypothesis. However, Oxburgh and O’Nions2 have suggested, on the basis of observations on the occurrence and distribution of primordial and radiogenic helium, that the present heat loss from the mantle is grossly out of equilibrium with the helium loss. They deduce that ‘Either radiogenic helium is accumulated in the mantle whilst heat escapes, or current concepts for the bulk chemistry of Earth are in error and much of the terrestrial heat loss is non-radiogenic.’
