Exergetic, environmental and economic analysis of a biomass cogeneration plant connected to a district heating network.

When a district heating network is fed by natural gas boilers (or biomass boilers), the selection of the network temperature level has a slight influence on the boiler efficiency which means that producing at low temperature allows only to decrease the heat losses through the network. However when a cogeneration plant based on a Rankine cycle is used, lowering the network temperature not only decreases the heat losses but also increase the electrical efficiency while decreasing the thermal efficiency. This aspect is very often overlooked when the quality of a cogeneration plant is assessed by the cogeneration efficiency namely the sum of the electrical and thermal efficiency which is misleading for decision makers.
In this contribution, an existing cogeneration plant connected to a district heating network installed on the University Campus in Liège (Belgium) is used as an application test case. Based on actual operational and economic data, a detailed economic evaluation is carried out which is supported by a calibrated simulation model of the whole installation in order to study the influence of the network temperature level. As a second step, the simulation model is used again to perform a detailed exergetic analysis intended to give physical insights to the economical study.
This results in a broader discussion on the several methods to assess the quality of cogeneration plants and the opportunity to develop such a technology together with district heating networks.