Your choices so far:
1 Geothermal; 2 Local heating (ind. house)
| 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) |
| 1 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 | 2 Local heating (ind. house) |
The versatility, the life-time and the cost of modern, small-scale heat pumps has made individual house heating using geothermal heat a major competitor to individual house heating by wood-log or pellet firing.
From an environmental point of view, this is beneficial since the heat pump does not give rise to any exhaust. Therefore, heat pump installations may be promoted also in more densely populated areas. However: Drilling a number of geothermal wells in a limited area may well overload the replenishment of geothermal heat so that the bedrock is cooled down and so that – though the heat pumps may last for 15 years – the geothermal wells last only a few years.
For isolated hoses or in sparsely populated areas, though, this may be a very attractive alternative for home heating.
For apartment houses, the individual house will demand a number of geothermal wells since, in normal cases, the individual well will have a maximum capacity less than 30-50 kW.
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.
For a larger building the system would basically stay the same but there would be a need for multiple geothermal wells.
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.
Though this is not geothermal energy it should also be considered, in warmer climates such as the European continent, the use of outdoor air as the main energy source. Again, the utilisation of this low-temperature energy takes place via a heat pump. Using the outdoor air will eliminate the competition about the energy source that might arise if too many households in a limited area would go for geothermal heating.