Your choices:
1 Electricity; 2 Biomass (digestible sludge); 3 Comfortable indoor climate
| What is your resource? | What do you want to deliver? | What is the service the customer wants? |
| 2 Biomass (digestible sludge) | District cooling | 3 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) |
| 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) |
A comfortable indoor climate is maybe the most common energy service and is one of the most important ones.
Since the indoor climate is a low-temperature service, temperatures ranging from room temperature up to 50-70 °C in tap water and at the most 80 °C in radiator water, the demands on the energy carrier are low and low-exergy carriers such as district heating can be used. There is one exception to this, and this is the energy supplied for illumination which is best supplied as high-exergy, i.e. electricity. Typically, this results in a total demand for energy where low-exergy sources are dominant.
In the case of farm-scale biogas production the individual farm may take full responsibility for its own heat production using its own gas. The individual farm may well use an internal combustion engine producing not only heat but also electricity. If a cluster of farms form a co-operative or in case geographical or other factors are favourable, a small district heating system may be established connecting the farms and supplying heat to all the farms from one central boiler.
Like with the smallest scale wind turbines may a small electricity generation device – i.e. an IC-engine running on local biogas – be connected to the 230 V/single-phase grid "behind" the meter. The meter will then automatically discount the produced electricity from the one bought from the main grid and such a system may thus provide a saving though it will not provide any major income.
Therefore: If electricity is desired for climate control 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 may be more inclined to accept electricity deliveries from biogas generation than from small wind-power generators.