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This paper presents the results of thermodynamics modelling studies of a 2 kW (e) biomass-fired CHP system with organic Rankine cycle (ORC). Three environmentally friendly refrigerants, namely HFE7000, HFE7100 and n-pentane, have been selected as the ORC fluids. The thermodynamic properties of the selected ORC fluids which have been predicted by commercial software (EES) are used to predict the thermal efficiency of ORC. The results of modelling show that under the simulated conditions (1) the ORC thermal efficiency with any selected ORC fluid is well below (roughly about 60% of) the Carnot cycle efficiency; the ORC efficiency depends on not only the modelling conditions but also the ORC fluid – the highest predicted ORC efficiency is 16.6%; the predicted ORC efficiency follows the following order: n-pentane > HFE7000 > HFE7100 (2) both superheating and sub-cooling are detrimental to the ORC …