Your choices so far:
1 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 | 1 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 | Local heating (ind. house) |
Though high-temperature geothermal heat sources may be available in volcanic areas, most geothermal heat found in the European continent is low-temperature and demands a heat pump to be used for comfort heat.
Heat pumps are characterised by their COP (coefficient of performance). In the above example: input of 1 kWh of electricity plus 2 kWh of low-temperature energy produces 3 kWh of useful heat, the COP is 3. If instead the input had been 1 kWh of electricity plus 4 kWh of low-temperature heat and the output – consequently – had been 5 kWh, then the COP would have been 5. 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 district heating water) and the input medium (i.e. the temperature in the geothermal well), the lower the COP.
In district heating applications, where the forward temperature must typically exceed 70 °C, the return water may not fall below 55 °C and supposing the geothermal well to deliver at 20 °C, the COP is a bit more than 4.2. If the forward temperature could be reduced to 60 °C the COP would exceed 4.6. Saying then that the heat power delivered through the district heating system is 100 MW, a COP of 4.2 means that the heat pumps demand 23.8 MW of electricity while a COP of 4.6 reduced the need of electricity to 100/4.6 = 21.7 MW. The saving amounts to almost 10%.
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.