Thermal energy from the earth's crust*

Abstract

Abstract The characteristics of the Jarger and hotter natural hydrothermal systems are discussed, and several different models of heat source and heat‐transfer mechanism reviewed. It is shown that, with models depending on mixed conductive‐convective heat transfer from a magma chamber, the observed heat flows reauire either a contact area of several hundred square kilometres for the conductive transfer link, or very small separations between a chamber of circulating magma and the convective hot water system. None of the models is entirely satisfactory, and the possibility of heat transfer by magmatic steam is reconsidered in the light of the new isotope evidence, which indicates that most of the water discharged is of surface origin. It is suggested that it would be possible for this surface water to penetrate to and become absorbed by heated rock or magma at great depths and pressures, and for it to be ejected from the rock by subsequent heating, or to escape under reduced pressure to provide a very efficient high‐temperature heat‐transfer medium that would mix with ground water near the surface. The power potential of the various models is estimated by calculating the mechanical energy that could be obtained from an ideal heat engine taking heat from the rock or water and discharging the heat into a sink at a fixed temperature of 30°C. The results are plotted as graphs of available power per unit mass flow against temperature for saturated steam and water up to the critical temperature, and energy content per unit volume against temperature for steam, water, and rock. In nearly all cases, most of the stored energy is in the rock. The total energy content of the conductive and convective zones of the models discussed is estimated from their probable dimensions, and is found to be of the order of 10 6 to 10 8 megawatt years for the larger models. At the power drawoff rates of 250 to 350 megawatts involved in the Wairakei and Larderello power projects, these quantities of energy, if effectively drawn upon, would last at least several thousand years, and even the smallest probable model would supply these stations for some hundreds of years. If any of these models is in fact applicable to the areas concerned, it is unlikely that anv appreciable impression on the storage will be made over several decades; and neither the Wairakei nor the Larderello areas are yet showing any sign of exhaustion of their heat supply, though there are indications that the water supply may become a limiting factor. Eventually, both these and other hydrothermal power projects may gain great advantages from control of the water supply and circulation by injection of recharge water.