Operational Analysis of a Wood-Fired CHP Plant Feeding an ORC System and a DH Network

Wood-fired Combined Heat and Power (CHP) plants connected to District Heating (DH) networks can represent an efficient way to exploit biomass. Medium and small plants in rural areas can be supplied with local biomass and can contribute to increase the production of renewable energy in a sustainable way. In the last years, according to EU targets and thanks to the incentives provided on energy production from renewables, the number of biomass-fired CHP plants has continuously increased. In order to assess the behaviour and performance of a whole plant and of its various components it is essential to analyse operational data and parameters over the time. Real operation data can differ from nominal data because of their dependence on different parameters, i.e. biomass quality and moisture content, outdoor temperature, heat load, etc. The performance of each component has to be evaluated in order to assess the overall efficiency of the system and to find possible improvements. In this work the operational data of a wood fired CHP plant coupled to a DH network supplying users in a mountainous area are analysed. The DH system is supplied by two hot water boilers of total heat output of 8 MW and by a CHP system. The CHP system is made of a thermal oil boiler with nominal heat output of 6,5 MW, an ORC unit with nominal electrical power 650 kW, a flue gas cleaning systems with a multicyclone and an electrofilter and two heat storage tanks. The DH network is about 20 km with almost 500 supplied users. The ORC unit is used at maximum power in the winter season, and at partial load in summer when the heat load mainly provides domestic hot water. The analyses performed over a whole heating season allowed to define operational patterns of the CHP unit with the overall DH operation. A good electric efficiency at partial loads has been assessed, accordingly to the data provided from the manufacturer. The heat provided by the ORC unit covered a significant share of the heat required from the users, thanks also to the presence of the heat storage systems. The correlation between relevant parameters that describe the behaviour of the system can be useful to analyse other CHP and DH systems with similar layout and operation.