Development of biomass-fired micro-scale CHP system with modified Rankine cycle (RC)

Nowadays, many research activities are devoted to the development of renewable energy-based micro cogeneration systems dedicated to domestic, commercial, and agricultural sectors. This paper illustrates the prototype micro CHP system developed at the AGH UST in Krakow. The system is powered by a 100 kWth biomass-fired batch boiler and operates according to a modified Rankine Cycle. Steam is generated and superheated in two shell and tube heat exchangers (working as evaporator and superheater, respectively) and powers a steam engine. Next, it is condensed in another one shell and tube heat exchanger (i.e. condenser). The condensate is pumped to a degasser and finally – to an evaporator. Electricity is generated in a power generator connected to the steam engine. Operation of the proposed micro CHP system is controlled by a specially developed control and measurement system recording the following parameters: temperature, pressure, and medium flow (in the boiler, oil circuit, steam/condensate circuit, and cooling water circuit) and the following parameters are continuously adjusting during the system works: pump power, air fan power, steam valve opening, and the level of power taken from the power generator. During presented tests, selected parameters in oil and steam-condensate circuits were analysed, including: (i) oil temperature and boiler power variations in case of single and continuous combustion processes, (ii) oil, condensate, and steam temperature variations in the evaporator and superheater, (iii) steam pressure variations and steam flow variations in the steam bus, (iv) voltage current and power generated in the power generator. Actual configuration of the developed system allows for power generation only at a level of ~1,15 kWel. Such a low value results, e.g. from the limitations in maximum oil temperature, not optimised evaporator heat exchange surface, large pipe capacity, and low power generator capacity. These limitations were discussed and guidelines for further improvements of the system were elaborated.