Your choices:
1 Biomass (digestible sludge); 2 District heating; 3 Electricity
| What is your resource? | What do you want to deliver? | What is the service the customer wants? |
| 1 Biomass (digestible sludge) | District cooling | Comfortable indoor climate |
| Biomass (fermentable sludge) | 2 District heating | 3 Electricity |
| 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) |
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 raw biogas, the exergy content is comparatively low.
To illustrate: Assume raw biogas to have a fraction of 50% exergy and further assume that 100 kWh of energy is available in the form of raw biogas. 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 gas. In reality, not even this will ever be achieved, but the real efficiency in electricity production from biogas using the common IC-engine technology will only scarcely exceed 30%. 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 a digestible biomass, then CHP technology should be used. In the specific case of biogas, this may be accomplished on a single-farm scale. This is unique for biogas as compared to the other biomass sources, and is based on the fact that the output from a digester is a gas with a sufficiently high quality to be used in an (adopted) internal combustion engine.
The investment cost for such a unit is low enough to make CHP-production feasible in single-farm scale so that the individual farm may use the hot cooling water from the engine for comfort heat and that the electricity may be used locally or sold onto the low-voltage electricity grid.
Opposed to the case with small-scale wind-power production, electricity production from biogas will be much more stable and the risk for flickering on the grid will be minimised. Hence, the local grid operator will usually be more inclined to accept electricity deliveries from biogas generation than from small wind-power generators.