6 Direct heating using geothermal energy
In the same way that waste heat from conventional power stations can be used for direct heating of buildings, and in industrial production and horticulture, low-grade geothermal energy has considerable potential. Many existing developments, such as those in Iceland, use spent fluids from geothermal electricity generation. Areas of natural hot springs are an obvious target, but it is also relatively simple to exploit normal heat flow using either natural groundwater or a variety of heat-exchange systems.
Deep sedimentary basins associated with average heat flow can have useful geothermal potential, if they contain sediments with low thermal conductivity and therefore above-average geothermal gradients (Box 1). The Paris Basin beneath northern France contains several limestone and sandstone aquifers (Figure 9) in such a setting. At depths between 1 and 2 km, groundwater temperature is above 60 °C. More than 50 wells tap this low-enthalpy resource to use the hot water to heat domestic and industrial buildings. The corrosive, saline groundwater transfers heat to fresh water through heat exchangers, which then circulates through housing complexes at around 55 ° C, before returning to the heat exchanger. Such a system is little different from the circuit of a conventional gas or electrical heating boiler, except for its scale. The cooled geothermal fluid is then re-injected into the aquifer. One advantage of the Paris Basin is that the deep groundwater is semi-artesian (Smith, 2005), so that less pumping is needed. Each heating circuit typically supplies between 3 and 5 MW of power over its designed lifetime (30-50 years). Although this scheme is only marginally economic — during the oil glut of the early 1990s, development work all but shut down, the scheme saves the equivalent of 2 × 105 t of coal per year.
A similar scheme that supplies 1 MW of power was installed in Southampton in the 1980s to provide central heating and hot water for the city's Civic Centre, the surrounding area and a local swimming pool. A single well penetrates a Triassic sandstone at a depth of almost 2 km beneath the city, in which groundwater is at 70 °C. Semi-artesian conditions bring the water to a depth of about 650 m, thereby reducing the need for pumping. The spent water is discharged into the sea, the aquifer being naturally recharged where it is exposed at the land surface.
Between 1992 and 2000 the global capacity of direct geothermal heating increased fourfold from 4 GW to 16 GW. Compared with heating that uses fossil fuels, supplied very efficiently either by gas pipelines or electricity grids, direct geothermal heating is economic only when fossil fuel prices are high. The capital outlay on wells and distribution systems is the main financial burden, making direct geothermal heating unattractive for small-scale development. However, there is an approach that may become viable for a great many more individual consumers or small housing schemes.
In soil, even at shallow depths, geothermal heat flow and heat retained from summer solar warming ensures that subsurface temperatures are higher than winter air temperatures. This very-low enthalpy source can be exploited using a heat exchanger, known as a ground-source heat pump that uses upward and downward pipes about 100-150 m long set into the ground (Figure 10). A heat transfer fluid, usually water, circulates within the loop to transfer heat from the ground to the properties above. When demand is low during summer the underlying soil and rock reheat, ready once again for winter use. A variant on this approach is to use heat pumps — effectively reversible air conditioners — to pump excess heat downwards in summer and upwards in winter; at least some of the high energy cost of air conditioning is recycled.
Ground-source heat pumps can be deployed almost anywhere, and there has been a recent explosion in their popularity. The approach was developed simultaneously in the USA and in Europe, particularly Switzerland and Sweden. Installations in Switzerland are growing by 10% a year, and at an even higher rate in the USA, where there are already over half a million units. If 1000 residential units at the latitude of the UK were to have such geothermal heat pumps for winter heating and summer cooling, peak demand for electricity could be reduced by 1.7 MW, or gas demand by 0.7 TJ per year. The UK is a latecomer to this technology, but a new health centre in the Isles of Scilly now uses a 25 kW reversible ground-source heat pump to supply hot water, heating and cooling.