Your choices so far:
1 Biomass (solid); 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 |
| 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) |
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.
The production of electricity from heat – as is the case in fuel firing and in nuclear reactors – suffers from thermodynamical constraints limiting the amount of electricity that can be produced as compared to the energy input by the fuel. The part of the fuel energy that is not converted into electricity, about 55-60% of the energy in most condensing, coal-fired power plants throughout Europe, is lost in the form of heat.
Solid, fossil fuel fired power stations (i.e. coal fired) will typically be large-scale with power (i.e. electricity) outputs in the range of GW's. Such large installations will never be of interest when it comes to biomass firing since the area from which the biomass would have to be collected would become vast and the hauling distances for the fuel would make it un-economical. Thus, biofuel-fired power stations will only scarcely be larger than 500 MW thermal (equal to about 200 MW electricity). This has a pronounced effect on the preferred technology and on the localisation of the plants.
While coal-fired power stations will be localised close to the coalfield and will – unless there happens to be a major river nearby – be equipped with cooling towers to fan away all the heat produced will the smaller, biofuel-fired, plants be small enough that the surplus heat can be accommodated in district heating systems. Thus biofuel-fired energy production plants are located in the outskirts of cities and they make use of co-generation technology.
At the same time, it is important that the biofuel-fired plants do not get too small. Solid biomass is more variable in quality than coal and though a modern, large-scale (15 MW thermal or lager) boiler may show a certain degree of fuel flexibility, there will still be limits. A boiler designed for wood-fuels will not necessarily be able to cope with agricultural residues, for example. However, the tolerance with respect to fuel quality becomes larger as the boiler becomes larger. The smaller and cheaper the boiler – the higher the demands on a uniform fuel quality. To keep the operational cost (i.e. the fuel cost) down it is therefore necessary that boilers for biofuel applications be at the very least a few MW. That size will usually be too small to host co-generation. So if electricity is the desired product and solid biomass in the resource, the boilers will have to be at least some 15 MW thermal.