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Following the increase in oil prices in the mid‐1970s, Britain assessed its geothermal resources. Low‐temperature, hot‐water resources, in the range 40°C to 100°C, occur in Permo‐Triassic sandstones in several deep sedimentary basins. In total these resources are estimated at 69.1 × 10 18 joules (J) (2576 million tonnes coal equivalent). Resources also occur in Upper Palaeozoic aquifers but, as the permeability of these aquifers depends upon fissures, exploitation is difficult. The only surface manifestations of hot water at depth are the warm springs at Bath and Bristol and in the Peak District and Taff Valley, which issue from the Carboniferous Limestone. The potential of radiothermal granites for Hot Dry Rock (HDR) development has also been investigated, particularly in the Carnmenellis granite in Cornwall. Three boreholes drilled in the granite to depths of over 2 km have been connected by developing natural fractures. Water circulation between the boreholes and through the fractured rock has been successful.

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