Your choices so far:
1 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) |
| 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 | Local heating (ind. house) |
Before looking into how to use geothermal energy, it is important to realize that the energy source itself readily splits into several categories that affect the possible system solutions:
- Sources with such high temperatures in water confinements underground that they can be readily used for steam extraction and hence for electricity production via steam turbines.
- Sources with such high temperatures in the solid, dry, bedrock that they can be used to heat up a medium (brine) and this medium can then be used to generate steam via simple heat exchangers.
- Sources with sufficiently high temperatures to produce climate heating (50-120 °C is needed depending on season and on latitude) only via simple heat exchangers.
- Sources with sufficiently high (typically 10-20 °C) and stable temperature to supply long term, low-temperature heat extraction via a medium, the temperature of which is then "geared up" by a heat pump to provide climate heating.
Geothermal energy comes from two main sources, namely
- Heat conducted out from the hot zones in the inner part of our planet
- Radioactive decay in the crust
The crust of the earth is a thin (comparatively thin, that is: its 35-50 km thick below the continents and thinner, 5-15 km, below the oceans) solid layer with a comparatively low density, about 3600 kg/m3.
Below the crust is the mantle of the earth which is about 2850 km thick and has a successively increasing density from about 3600 and up to about 5800 kg/m3.
The central part of the earth has a radius about 3200 km and a density increasing from about 10000 up to more than 13000 kg/m3.
While the temperature in the centre of the earth is more than 4000 °C, then the temperature where the core meets the mantle is "only" about 3000 °C and when the mantle meets the crust it has dropped further down to about a very modest 1400 °C.
Thus, the temperature drop through the crust is about 1400 °C over a distance about 40 km which gives 35 °C /km. This is called the geothermal gradient and it becomes very different in different types of bedrock. The value just quoted – about 30-35 °C /km – is typical for younger, sedimentary type bedrocks, while the values may be significantly lower in primary formations such as the very old and thick granite shield underneath Scandinavia where values typically range between 7 and 22 °C /km.
These temperature gradients give rise to a heat flux which is – as a global average – 75 mW/m2. Since the temperature gradients are variable, then the heat flux also varies within wide limits depending on the location and typical values for northern Europe are in the range 35-75 mW/m2.
You might want to compare these fluxes – 0.075 W/m2 – to the energy flux incident from the sun which is in the order of 750 W/m2… Except for volcanic areas, the temperatures available from geothermal energy are low.
Thus, geothermal energy is characterized by low energy fluxes and by low temperatures – remember that we exclude the volcanic areas – and his has a pronounced influence on the technologies we may use and on the applications for which it is useful.
Drilling a deep enough hole in the ground you will –sooner or later – hit any temperature you might desire for energy applications. But then:
1. Drilling through rock is in itself quite expensive
2. 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.
Extracting energy from a volume in the bedrock will successively cool down the rock, unless the energy is replenished at – at the very least – the same rate it is extracted. Since the heat fluxes are limited, this replenishment is mainly supplied by water flowing in the bedrock and in many cases water-carrying layers in the bedrock, aquifers, are used as sources for geothermal energy. These aquifers may be of two different kinds, namely stationary, where the water is actually not flowing through but only serves to even out the temperature in the horizontal direction and dynamic, in which case warm water flow through the layer and thus supplies new energy continuously.
At about 700 m depth below Lund in southern Sweden, there is a stationary aquifer originally at 22-25 °C which has been used to provide district heating for 15-20 years and is expected to last another 15 years before it has been exhausted, i.e. has been cooled down to about 15 °C. So even stationary aquifers may well serve as energy sources.