Your choices:
1 Biomass (solid); 2 District heating; 3 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) | 2 District heating | 3 Electricity |
| 1 Biomass (solid) | Electricity | Process cooling (< 0 °C) |
| 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) |
Whenever the scale so permits shall heat production be combined with electricity production. The use of electricity should always be restricted to such services where it cannot be replaced by other energy carriers. One main feature of electricity as compared to other energy carriers is its thermodynamic quality as expressed as its exergy content or its fraction of exergy. The higher the fraction of exergy, the more flexible is the energy carrier and electricity is almost 100% exergy.
This property is inherent with the electricity as such – regardless of how it has been produced – but like energy, exergy cannot be created.
For wet solid biomass (45-50% water), the exergy content is comparatively low, maybe down to 50%, while for dry solid biomass (less than 20% moisture) it is very high (well exceeding 80-85%).
To illustrate: Assume a wet biomass to have a fraction of 50% exergy and further assume that 100 kWh of energy is available in the form of fuel. Then it will be theoretically impossible to produce more than 50 kWh of electricity, simply because there is no more than 50 kWh of exergy in the fuel. In reality, not even this will ever be achieved, but the real efficiency in electricity production from solid fuels using the common steam-cycle technology will only scarcely exceed 45%. Apart from a bit of losses, the remaining energy will be present in the form of heat.
Therefore: If electricity is desired by the end user and the energy source is solid biomass, then CHP technology should be used.
In case the sale of electricity is not a primary business idea but the most important is to make the energy-plant self-sustained with respect to electricity, so that it may be operated also during a major black-out, the a low-efficiency steam turbine, a screw expander or a reciprocating steam engine aimed to operate on saturated steam may be sufficient. With such systems, the total efficiencies will usually be less than 15% while the robustness of the system and the low maintenance costs may still make them interesting.
Such systems may also be of interest in case the district heating system is too small to carry the costs for superheating and for more advanced steam cycles.