Survey of Energy Resources 2007
Geothermal - Direct Utilisation
Fig 11-7 shows the direct applications of geothermal worldwide by percentage of total energy use. The main growth in the direct use sector has, during the last decade, been in ground-source heat pumps. This is due, in part, to the ability of geothermal heat pumps to utilise groundwater or ground-coupled temperatures anywhere in the world.
Space heating is among the most important direct uses of geothermal energy. Preferred water delivery temperature for space heating is in the range 60-90°C and commonly the return water temperature is 25-40°C. Conventional radiators or under-floor heating systems are commonly used, but air-heating systems are also possible. If the temperature of the resource is too low for direct application, geothermal heat pumps can be used. This will be discussed in more detail below.
Open loop (single pipe) distribution systems are used for both private schemes and in district heating systems. In this case, geothermal water is used directly for heating and the spent water from radiators is discharged to waste. This type of system is only possible where the water quality is good and recharge into the geothermal system adequate. More commonly, closed loop (double pipe) systems are used. Then heat exchangers are used to transfer heat from the geothermal water to a closed loop that circulates heated freshwater through the radiators. This is often needed because of the chemical composition of the geothermal water. The waste water is disposed of via wells which are called re-injection wells. Closed loop systems are more flexible than open loop systems and they allow substitution of the geothermal water by other energy sources. Both of these two main types of district heating system are shown schematically in Fig. 11-8.