STATUS OF BIOMASS FIRED DISTRICT HEATING SYSTEMS IN IEA COUNTRIES

Since district heating(DH) enables the use of biomass as energy source, the IEA Bioenergy Task 32 presents a survey of DH systems with respect to efficiency and costs. The evaluation describes the annual heat losses, the linear heat density in MWh per year and meter of pipeline, and the connection load for 800 district heating systems in Austria, Denmark, Finland, Germany, and Switzerland. The survey covers a broad range of connection loads from smaller than 1 MW to larger than 1000 MW. The evaluation reveals a strong dependence of the heat losses on the linear heat density. Thus the recommendation of a minimum linear heat density is confirmed. For a linear heat density of 1.8 MWh/(a m) as recommended as minimum value for district heating based on automatic wood combustion by QM Holzheizwerke (R), typical heat losses of 13 % are achieved in average compared to the expected target value of 10 %. Although the linear heat density is confirmed as an important parameter, the survey shows that the heat losses are distributed over a range of more than a factor of three at given linear heat density. This is due to the fact that additional parameters also influence the heat distribution with the following trends: The pipe diameter strongly affects the heat distribution losses as well as the capital and the pumping costs. Therefore, the pipe diameter is a key parameter for an economic system optimisation. An analysis of individual line sections of five district heating networks reveals that 80 % of the investigated sections are oversized by one or more diameters compared to the smallest allowable pipe diameter. Due to an economic assessment of a 1 MW network, the heat distribution costs achieve a minimum at the smallest allowable pipe diameter. Therefore a theoretical potential for cost reduction can be derived. This reveals a potential to reduce the heat distribution losses by up to 20 % and the costs by up to 30 %. In addition to the pipe diameter, the network layout, the temperature spread, the temperature level, the insulation class, and the full-load hours of the heat consumers also affect the heat losses and the heat distribution costs.