Your choices:
1 Comfortable indoor climate; 2 Biomass (solid); 3 Local heating (ind house)
| What is your resource? | What do you want to deliver? | What is the service the customer wants? |
| Biomass (digestible sludge) | District cooling | 1 Comfortable indoor climate |
| Biomass (fermentable sludge) | District heating | Electricity |
| 2 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 | 3 Local heating (ind. house) |
For any building outside such areas where district heating and cooling systems are feasible, the only alternative will be to organise the climate control system locally.
Todays' air-conditioning units will provide air-borne heating as well as cooling in one single unit and often such units will be found in the individual rooms in single-family houses. For cooling purposes in single-family houses and also in larger building there is no realistic alternative to the use of electricity-demanding compressor cooling.
For the use of renewable energy in combination with AC-units there is then only one alternative, and that is to provide at least part of the electricity need for the house by local, individual, generation.
Theoretically local electricity production from solid biomass can be achieved by the use of micro-turbines, Stirling engines and a number of other technologies, all which have been thoroughly investigated during the last decades but they all suffer from the same fundamental weaknesses:
- First, the attainable electricity efficiency becomes low, 5-15%.
- Second, electricity will only be produced when the boiler is actually running.
Just assume an extremely cold day and the boiler running at 20 kW. With an electricity efficiency of 10%, the result would be 2 kW of electricity which will not even be enough to run the electric cooker. During summer days – when the boiler is not running – there will be no electricity production at all. The cost for the system will not be able to defend such a small added value. Hence local electricity production from solid biomass and the use of AC-units for climate control is not feasible.
Instead, the alternative is to use a solid fuel boiler. This can be fired with wood-logs or with pellets.
Pellet firing systems for domestic use in single-family houses generally fall into two different categories, namely
- Burners to be mounted in hot-water boilers
- Self-contained stoves for air heating, containing combustion chamber, burner and day storage tank assembled. For some units the day tank might be separate while quite often all the parts are assembled in one physical unit
Both are commercially available as off-the-shelf units. The reliability with the commercial systems is high and the need for maintenance is low.
In case a pellet system replaces old oil- or gas fired systems it is crucial to check the status of the chimney. Pellet firing will result in larger flue gas volumes and the chimney must be able to cope with that. In case a pellet burner is mounted in an old boiler the flue gas temperatures may also become higher.
While pellets are thus the recommended choice, both for simplicity and for environmental performance, for economic reasons, also wood-log firing is used in this scale. Provided proper treatment of the wood-log fuel, provided modern, down-firing boilers, provided heating systems equipped with proper-sized accumulator tanks and provided the house-owners have been properly trained, such systems may also have acceptable environmental performance.
With old domestic boilers, the walls of the combustion chamber were cold, with a temperature close to that of the water. Hydrocarbons and tar released from the wood logs will then escape from the combustion chamber and may cause severe air quality problems. With old boilers these problems cannot be eliminated.
The most modern wood-log boilers are designed for downwards combustion and lined with ceramics. In these, the gases from the coldest log will pass down, through the bed of already burning and glowing material, so that the gas is maintained at a high temperature throughout combustion, leading to very low emissions of hydrocarbons. These boilers are also designed for batch firing, so that a specific load of logs are input, ignited and allowed to burn out completely. Hence a modern boiler saves a lot of manual work since it does not require a continuous feed as did the old boilers.