Your choices:
1 Process cooling (< 0 °C); Electricity; 2 Water
| 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 | 1 Process cooling (< 0 °C) |
| Geothermal | Fuel: Gaseous | Process heat/steam (50 - 150 °C) |
| Sunshine | Fuel: Liquid | Process heat (150 - 1000 °C) |
| 2 Water | Fuel: Solid | Process heat (> 1000 °C) |
| Wind | Local cooling (ind. house) | Transport |
| Residual oils/fats etc | Local heating (ind. house) |
The energy found in moving water is basically exergy and since this is also the form of energy in electricity, water shall be used to produce electricity and basically nothing else.
The main options for hydropower production of electricity are:
- Power stations with a dam. This is the "traditional" hydropower station where a dam blocks, partly or completely, a river flow so as to (1) provide a water reservoir with a sufficient volume and (2) maintain a constant height (head) so that there is a height difference across the turbines. The total efficiency in this type of hydropower installations – i.e. the ratio of produced electricity to latent energy in the water – often exceeds 95%.
- Submerged power stations. This type of hydropower stations can be described a as a chubby submarine anchored at a river bed. Just like the common wind power stations, these make use of the flowing water in the river and may be virtually invisible. Like with common wind turbines, these will suffer from the limitation known as "the Betz limit" and the theoretical maximum electricity output amounts to approximately 60% of the total energy in the flowing water.
- Pumped hydropower stations. With these, water is pumped from low level to a high level reservoir during the part of the day when the demand for electricity is low. As the demand then peaks, the water is again allowed to flow down from the reservoir through the pumps – which then act as turbines – and electricity is produced. Though these may be used to "store" surplus electricity from e.g. wind power stations they have so far not come into widespread use.
The main characteristic for hydropower electricity production using the traditional technique, with a dam, is that it is fast and simple to control. The flow of water through the turbines may be controlled within seconds or minutes and therefore hydropower stations are used to balance the electricity grid.
The dam may in itself cause a severe environmental impact depending on the local conditions but it may also be turned into a positive thing if only proper care is taken to install fish ladders and the water level in the dam is closely controlled. Examples can be found where the dam itself has successfully been marketed as a fishing lake and where tourism flourishes around a hydropower station. Also, micro-power stations – ranging only a few hundred kW or up – can often be integrated in old water-mills and hence also contribute to the maintenance of cultural heritage.