Your choices so far:
1 Sunshine; 2 Electricity
| 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 | 2 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) |
In many cases, electricity is assumed to be the main energy carrier desired by the end users, but this is not necessarily true.
For the end user, the unique thing with electricity is its flexibility or, in thermodynamic terminology, its high share of exergy. Electricity can be converted into mechanical work, into illumination, into extremely high or low temperatures, into pressure, into radiation of different wavelengths; it can be used to run home electronics, for transportation and – basically – any number of applications. Because of the high quality and the high availability with electricity it should be priced accordingly and the use of it should be limited to such applications where the unique features are fully valued.
Electricity production by solar energy can be achieved mainly in four different ways:
- Solar energy is used for steam raising and the steam is used in a steam turbine
- Solar energy is used to heat some other medium and this medium is then used in a closed-cycle Rankine cycle, i.e. in a gas turbine
- Solar energy is used to heat a medium and this medium is then used in some type of heat engine other than a turbine such as a piston steam engine, a Stirling engine or alike
- Solar energy is directly converted into electricity by the use of photovoltaic solar cells
Of these, only the fourth alternative is the one commercial today.
The most common solar cells of today are mainly silica based poly-crystalline cells peak at about 15% efficiency or thin-film cells with slightly lower efficiency.
With the cells that are on the market today, the life-span is about 20-30 years. So even though the efficiency is low will the depreciation time be long and hence the investment may be profitable.
For isolated houses, the installation of a separate, 24 V DC system may sometimes be worthwhile. Otherwise, solar electricity will only be a complement, preferably converted to 230 V AC and connected to the internal electricity system in the building. With such a connection "behind the meter", the electricity produced will be automatically accounted for and will provide an annual saving.