The term ‘geothermal energy’ describes all forms of heat stored within the Earth. The energy is emitted from the core, mantle, and crust, with a large proportion coming from nuclear reactions in the mantle and crust. It is estimated that the total heat content of the Earth, above an assumed average surface temperature of 15◦C, is of the order of 12.6×1024 MJ, with the crust storing 5.4×1021 MJ (Armstead, 1983). Based on the simple principle that the ‘deeper you go the hotter it gets’, geothermal energy is continuously available anywhere on the planet. The average geothermal gradient is about 2.5–3◦C per 100 metres but this figure varies considerably; it is greatest at the edges of the tectonic plates and over hot spots–where much higher temperature gradients are present and where electricity generation from geothermal energy has been developed since 1904. Geothermal energy is traditionally divided into high, medium, and low temperature resources. Typically, temperatures in excess of 150◦C can be used for electricity generation and process applications. Medium temperature resources in the range 40◦C to 150◦ C form the basis for ‘direct use’ i.e. heating only, applications such as space heating, absorption cooling, bathing (balneology), process industry, horticulture, and aquaculture. The low-temperature resources obtainable at shallow depth, up to 100–300 metres below ground surface, are tapped with heat pumps to deliver heating, cooling, and hot water to buildings. The principles of extracting geothermal energy, in applications ranging from large scale electrical power plants to smallscale domestic heating, are illustrated in Fig. 3.1. Geothermal energy can be utilized over a temperature range from a few degrees to several hundred degrees, even at super critical temperatures. The high temperature resources, at depth, are typically ‘mined’ and are depleted over a localized area by extracting the in situ groundwaters and, possibly, re-injecting more water to replenish the fluids and extract more heat. Although natural thermal recovery occurs, this does not happen on an economically useful timescale.
Revitalization of Renewable Energy Generation in Nusa Penida
