Environmental impact

Despite claims to the contrary, it is virtually impossible to produce and transform energy without some carbon dioxide emissions. A survey of over 5000 MW installed capacity of geothermal emitted an average of 65 g CO2 per kWh. This compares with 450 g/kWh for gas, 906 g/kWh for oil and 1042 g/kWh for coal, excluding the embodied energy in plant and equipment as well as carbon miles for the transportation of oil and coal. So, geothermal advocates are justified in their claim that it is one of the cleanest sources of energy.

At a guess, geothermal-boosted ground-source heat pumps will lead the field in low-energy technology for buildings. It is relatively low cost, economical to run, insulated from the vagaries of weather and climate and changes in temperature and reliably produces heat in winter and cooling in summer. If PVs with battery backup provide the power for pumps and compressors then it really is a zero-energy system in operation.

Heat pumps are an offshoot of refrigeration technology and are capable of providing both heat and cooling. They exploit the principle that certain chemicals absorb heat when they are condensed into a liquid and release heat when they evaporate into a gas.

There are several different refrigerants that can be used for space heating and cooling with widely varying global warming potential (GWP). Refrigerants which have an ozone-depleting potential are now banned. Currently, refrigerants which have virtually zero GWP on release include ammonia, which is one of the most prevalent.

The heating and cooling capacity of the refrigerant is enhanced by the extraction of warmth or 'coolth' from an external medium - earth, air or water.

The most efficient is the geothermal heat pump (GHP), which originated in the 1940s. This is another technology which goes back a long way but which is only now realizing its potential as a technology for the future.

It exploits the relative warmth of the earth for both heating and cooling. The principle of the GHP is that it does not create heat; it transports it from one area to another. The main benefit of this technology is that it uses up to 50% less electricity than conventional electrical heating or cooling. A GHP uses one unit of electricity to move between three and four units of heat from the earth.

Most ground-coupled heat pumps adopt the closed-loop system whereby a high-density polyethylene pipe filled with a water and antifreeze mix, which acts as a heat transporter, is buried in the ground. It is laid in a U configuration either vertically or horizontally. The vertical pipes descend to about a 100 m depth; the horizontal loop is laid at a minimum of 2 m depth.

The horizontal type is most common in residential situations where there is usually adequate open space and because it incurs a much lower excavation cost than the alternative. The only problem is that, even at a 2 m depth, the circuit can be affected by solar gain or rainfall evaporation. In each case the presence of moving ground water improves performance.

Ground Coupled Heat Pumps (GCHP) a.k.a. closed loop heat pumps

Ground Coupled Heat Pumps (GCHP) a.k.a. closed loop heat pumps

Groundwater Heat Pumps (GWHP) a.k.a. open loop heat pumps
Surface water Heat Pumps (SWHP) a.k.a. lake or pond loop heat pumps
Figure 4.2 Heat pump variations (source: Kensa Engineering)

Usually the lowest cost option is to use water in a pond, lake or river as the heat transfer medium. The supply pipe is run underground from the building and coiled into circles at least 1.75 m below the surface (see Fig. 4.2).

Dr Robin Curtis (GeoScience Ltd) considers heat pumps to be analogous to rechargeable batteries that are permanently connected to a trickle charger. The battery is the ground-loop array which has to be large enough, together with a matched compressor, to meet the heating/cooling load of a building. The energy trickle comes from the surrounding land which recharges the volume of ground immediately surrounding the loop. If the energy removed from the ground exceeds the ground's regeneration capacity, the system ceases to function, so it is essential that demand is matched to the ground capacity.

At present, ground-coupled heat pumps have a coefficient of performance (COP) between 3 and 4, which means that for every kilowatt of electricity, they produce 3 to 4 kilowatts of useful heat. The theoretical ultimate COP for heat pumps is 14. In the future a COP of 8 is possible.

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