Block A and B are collectively heated with one wood pellets boiler of 100 kW and
a gas fired boiler of 100 kW. For the determination of the heat demand the
measurement results of an other similar project with 12 dwellings were
extrapolated to these 26 dwellings.
The pellets boiler is used as soon as the heat demand is more than 20 kW (e.g.
when the outside temperature is less then 16°C). If the heat demand is less than
20 kW only the gas fired boiler delivers the heat. Heat demands above 100 kW are
fulfilled by the pellets boiler (100 kW) in combination with the gas fired
boiler.
For the pellets boiler a thermal efficiency of 90% is supposed. From the
measurement reports of research institutes and testing organisations most wood
pellets boilers with a thermal power of 100 kW have at full and part load an
efficiency of more than 90%.
For the gas fired boiler a thermal efficiency of 90% is supposed.
The pellets have a lower caloric value of 4.8 kWh/kg and a storage density of
680 kg/m³. Wood pellets are a CO2-neutral kind of energy. For the
production and the transport of the pellets most studies assign a fossil energy
consumption of 5% to the pellets consumption.
The figure shows the expected heat load curve for the 26 dwellings in the
project.
The table below shows the results of the calculations. The total heat demand for
the 26 dwellings amounts to approx. 200 MWh/year of which about 90% can be
covered by the wood pellets boiler. Assuming a pellets boiler efficiency of 90%,
the pellets consumption will be 41,6 tons/year. Compared to the reference
situation (only a gas fired boiler) the reduction of the primary energy
consumption will be 85% or 190 MWh/year.
The CO2-emission reduction of the pellets boiler would be about 38
tons per year or 85% compared to the reference situation. For the calculations a
CO2-emission factor for natural gas of 56.1 gr/MJ was taken.
Table 1: Results of the calculations
| Duration of heat demand | 6.054 h/year |
| Running hours of pellets boiler | 4.246 h/year |
| Running hours of gas boiler | 1.831 h/year |
| Heat production pellets boiler | 180 MWh/year or 90% |
| Heat production gas boiler | 20 MWh/year or 10% |
| Total heat demand | 200 MWh/year |
| Pellets consumption | 200 MWh/year |
| Pellets consumption | 41,6 ton/year |
| Pellets consumption | 61,1 m³/year |
| Natural gas consumption | 22 MWh/year |
| Natural gas consumption | 2.400 m³/year |
| Primary energy consumption reference | 222 MWh/year |
| Primary energy consumption project | 22 + 10 MWh/year |
| Reduction of primary energy consumption | 190 MWh/year or 85% |
