Your choices so far:
1 Sunshine
| 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) |
| Geothermal | Fuel: Gaseous | Process heat/steam (50 - 150 °C) |
| 1 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) |
Solar energy is available in the form of electromagnetic radiation, which is emitted from any object with a temperature exceeding 0 K. Solar radiation hits the outer edge of the atmosphere at a rate of 1.39 kW/m2 but as it passes through the atmosphere it is reduced and at ground level, and on a surface perpendicular to the direction to the sun, approximately 800 W/m2 may be registered at noon on clear summer days in normal conditions.
This value is approximate; it is latitude and weather dependant and will vary with the time of day. Since the absorption is also depending on the thickness of the atmosphere, it is obvious that the value will depend on the altitude and since the quality of the atmosphere is also involved will also local pollution affect the total value. So you will realize that in a heavily polluted city at sea level will there be much less incident solar radiation than at a mountaintop.
Systems for the use of solar energy are commonly separated into three different types:
- Passive systems: These are usually systems for low-temperature house heating where there are no special collectors for the solar energy but where the building architecture in itself provides the heating. Windows, extended roofs and – in some cases – massive building elements to store heat may be parts of such a system. But passive solar heating is not only to plan for solar energy to enter the building – it is also to hinder it when it is excessive and to maintain it in the building to the extent that is desired. The energy service is to provide a comfortable indoor climate – not to accumulate the most solar energy.
- Active systems: Active solar heating involves concentrating the irradiation by aid of mirrors, lenses or specially designed collectors to provide higher temperatures. But these systems will usually still be designed for space or tap-water heating. Also heat pumps may be used to achieve higher temperatures and such systems would also be classified as active.
- Electricity production: Modern systems for electricity production will almost exclusively be designed to use photovoltaics ("solar cells" or "PV") for the direct transformation of solar energy into electricity but alternatives such as sun-powered Stirling engines are also available on the market. Solar thermal systems, where the solar radiation is concentrated to achieve high-temperature steam or air and then use this medium in a turbine are on the experimental stage but are not commercial.
The most widely use of solar energy is, however, for illumination through windows. Modern buildings should – but do not always – contain automatic control systems to switch off the artificial illumination when sufficient light enters through the windows. Such systems are commercially available and need only be installed.
Because of the nature of solar irradiation – its relatively low intensity, its seasonal and daily variation and its notorious un-predictability – solar energy is only a complement to other renewable energy sources and cannot be trusted for baseline production.
Though solar energy may thus play a role in a district heating system or in the electricity grid, it will not be treaded here as a source for industrial process energy.