Assessment of a residual biomass micro-combined heat and power system based on an organic Rankine Cycle coupled to a boiler

Biomass-fuelled Combined Heat and Power Generation (CHP) systems can efficiently convert chemical energy contained in biomass into electricity and heat. Currently there is a large number of abandoned forests and biomass of agroforestry origin that is not being used. The use of residual biomass as a source of energy in CHP systems is presented as a particularly attractive alternative for energy obtention. This paper presents the results of a biomass boiler coupled to an Organic Rankine Cycle (ORC). Three residual biomasses, named pruning vine, pruning kiwi and gorse have been selected due to their potentiality and availability in the Galicia-North Portugal Euroregion. For this purpose, micro-cogeneration tests at bench scale were performed, based fundamentally on varying dissipation conditions in the low-grade temperature range (below 100 degrees C). Micro-cogeneration performance was assessed based on the electrical power and efficiency obtained, along with the global efficiency achieved (electrical plus thermal). Combustion measurements comprised gaseous emissions analyses and determination of the efficiency of the process. Micro-cogeneration results obtained show that differences in temperature between the hot and the cold source have a significant influence. The higher the temperature difference, the higher the electrical power and efficiency, as well as the higher global performance obtained, with values close to the maximum of the ORC employed (i.e 4 kW(e), 9% and 96% respectively). Thus, the technical feasibility of the system to simultaneously obtain heat and electricity from low grade heat sources in small-scale applications was demonstrated. Regarding to combustion, parameters regulated by European emission standards are under the limits using the biofuels (residual biomasses) studied. Thereby, the suitability of those biomasses in thermochemical valorization processes was demostrated.