Your choices:
1 Local heating (ind. house); Comfortable indoor climate; 2 Geothermal
| What is your resource? | What do you want to deliver? | What is the service the customer wants? |
| Biomass (digestible sludge) | District cooling | Comfortable indoor climate |
| Biomass (fermentable sludge) | District heating | Electricity |
| Biomass (solid) | Electricity | Process cooling (< 0 °C) |
| 2 Geothermal | Fuel: Gaseous | Process heat/steam (50 - 150 °C) |
| Sunshine | Fuel: Liquid | Process heat (150 - 1000 °C) |
| Water | Fuel: Solid | Process heat (> 1000 °C) |
| Wind | Local cooling (ind. house) | Transport |
| Residual oils/fats etc | 1 Local heating (ind. house) |
Except for a few areas, the geothermal resources found in the European continent are low-temperature. To produce comfort heating from such sources, heat pumps are used.
A heat pump is a device that can take (just as an example) 2 kWh of energy from a source at say 15 °C, it adds 1 kWh of pure exergy (i.e. electricity) and then delivers 3 kWh of energy at an elevated temperature, say 100 °C. A heat pump is not just any ordinary electrical heater but is in itself a sophisticated piece of equipment that performs an intricate thermodynamic operation – but heat pumps are today commercially available at a variety of scales and the life-spans are typically more than 10 years even for those that are manufactured in household scale.
The typical geothermal system for an individual house would consist of a hole – the geothermal well – some 50-200 m deep into which cold water (say 5 °C) is pumped. The depth of the hole will have been chosen so that the bottom of it holds a suitable temperature and will depend on local conditions. As the cold water again emerges from the hole it will therefore have a higher temperature, say 15 °C. The water then passes through the heat pump where a bit of electricity is added, energy is extracted from the water and it is cooled down to 5 °C again and then returns down the geothermal well for a new cycle. Part of the energy extracted from the 15 °C water, together with part of the electricity added, is transferred either to air or to another closed water loop (the radiator circuit) to provide space heating. The remaining energy from the cooling of the water, together with the remaining electrical energy that was added, is transferred to cold water in an open circuit to provide tap water.
To provide local cooling, this system needs only a very minor modification: On hot days, when cooling is desired, allow the 15 °C water emerging from the well to enter a heat exchanger through which the outdoor air for ventilation is taken. The outdoor air may then be cooled down to 15 °C before being distributed in the house and the water pumped down into the well may be heated to a temperature close to the outdoor temperature.
Depending on the size of the building, the system may require more wells, but the general layout will always be the same.
Heat pumps are characterised by their COP (coefficient of performance). Just assume the COP to be 3. Then input of 1 kWh of electricity plus 2 kWh of low-temperature energy produces 3 kWh of useful heat. If instead the COP was 5, the input had been 1 kWh of electricity plus 4 kWh of low-temperature heat and the output – consequently – had been 5 kWh. So the COP is the output divided by the electricity input. Hence, the COP is a measure of efficiency.
The bigger the temperature difference between the product medium (i.e. the outgoing water) and the input medium (i.e. the temperature in the geothermal well), the lower the COP. For sanitary reasons, tap water must be heated above 50 °C and hence, the COP for the production of tap water cannot exceed a certain value.
However, the temperature of the water used for comfort heating is system dependant: With old, wall-mounted radiators, there may well be a demand that the temperature exceeds 60 °C, with modern, wall mounted radiators 40 °C is enough and with a floor heating system the temperature is further reduced down to 30 °C.
So the total electricity consumption will be lowered if a floor heating system is used in conjunction with geothermal energy.