Your choices:
1 Local cooling (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 | 1 Local cooling (ind. house) | Transport |
| Residual oils/fats etc | Local heating (ind. house) |
Drilling a deep enough hole in the ground you will – sooner or later – hit any temperature you might desire for energy applications. But then:
- Drilling through rock is in itself quite expensive
- If we assume that what you want to do is to pump down water and then get hot water up to the surface again, you have to remember that the pressure needed to get the water up again will increase with about 1 bar/10 m. Thus, if you have a local geothermal gradient of 30 °C/km and you want to produce water at 100 °C starting from water at 10 °C your hole would have to be 3 km deep and your pumps would have to cope with a pressure of 300 bar.
At a depth of a few meters, the temperature will be approximately equal to the annual mean temperature at you location and this will often be sufficient for cooling on hot days. However, when geothermal systems are installed, they would usually be designed for heating purposes.
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 a 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, such a 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 ground water and bedrock conditions may such a "recharge" of the geothermal well also contribute to a better year-round performance of the system and also to a prolonged life.