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 | Electricity |
| Biomass (solid) | 2 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) |
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.
Todays' solar cells are mainly silica based and come in two kinds: poly-crystalline cells and mono-crystalline. With mono-crystalline cells a total conversion efficiency at 25% from solar radiation to electricity has been measured in laboratory conditions while the poly-crystalline cells peak at 20% efficiency under the same testing conditions. Solar cells on the commercial market exhibit at least 5% lower efficiency.
Thin-film cells tend to be cheaper than the poly-crystalline silica cells but the price paid is a lower efficiency, about 10-15% with the commercial modules. 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.
Ongoing research on multi-layer cells and on Grätzel cells show promising results and it may be expected that the efficiencies with commercial solar cells may reach 25-30% before year 2020.
A standard solar cell, 100 cm2, will produce direct current (DC). A typical voltage is about 0.5 Volt while the current is directly proportional to the incident radiation intensity and may peak at about 2.5 Ampere. Thus, the use of photocells requires that the cells be connected in series to provide a sufficient voltage – say 24 V – and that the direct current is then transformed into alternating current (AC/50 Hz) at 230 V for internal use in the building using an. Unless this is done, the electricity from the PV-array must be distributed using a separate, low-voltage DC network or can only be used to charge a set of batteries.