Environmental Engineering Reference
In-Depth Information
Geothermal resources, particularly those at intermediate and low temperatures (between
60°C and 180°C) can also be used to supply direct heat. Heat exchangers or heat pumps
transfer the heat from the hot water to a secondary fluid to heat residential buildings and
greenhouses, or for industrial processes that require heat, such as food processing (IEA
Ground-Source Heating and Cooling
There is something perverse about expending vast amounts of electricity to cool buildings,
when there is an excellent source of cool air and water right beneath our feet.
Ground-source heat pumps (GHPs), also known as geothermal heat pumps, exploit
relatively constant underground temperatures and may be used to provide spatial heating,
cooling,anddomestichotwaterforalltypesofbuildings.GHPsdonotrelyonnear-surface
volcanic activity and can therefore be installed anywhere on Earth.
In temperate zones, where most of the world's energy is consumed, there is a constant
temperature of around 15°C at a depth of ten metres, all year round. This stability may be
exploited in winter to preheat water for conventional electric or gas-fueled boilers. If, for
space-heating radiators, we need water at 55 degrees Celsius, but the water entering our
home through municipal pipes is at 5 degrees, it makes sense to take advantage of the 15
degrees available just a few metres beneath our house. By raising the water temperature
by just 10 degrees, we can shave 20 per cent off our gas or electricity bill. The way this
works is remarkably simple. Water (or a secondary fluid) is injected into pipes that follow
a loop to a depth of 5-10 metres. The fluid returning from the loop is conveyed to a
heat pump, an electrical vapour compression unit that works like a fridge when cooling
or a reversed fridge when heating. In summer the underground stable temperature will be
used for cooling applications. In terms of geology, GHP systems bear little relation to the
recovery of volcanic heat at sites like Larderello. Strictly speaking, GHPs do not exploit
geothermal energy,butrathersolarenergystoredunderseveralmetresofinsulating earth.
27
There are two main types of GHP system: closed-loop and open-loop. In closed-loop
systems, a circuit of plastic piping is placed in the ground, either horizontally at a depth of
1-2 metres or vertically at 50-250 metres. A water-antifreeze solution is circulated through
the pipe, gaining heat in winter and losing it in summer. An open-loop system pumps
groundwaterorlakewaterdirectlyfromitssourceandthendischargesit,eitherintoanother
