Experimental study of a low-temperature micro-scale organic Rankine cycle system with the multi-stage radial-flow turbine for domestic applications

The paper presents an experimental evaluation of the efficiency of a small-scale low-temperature organic Rankine cycle (ORC) as an mCHP (micro Combined Heat and Power) system. Ultimately, the cogeneration system is dedicated to individual households and should be able to supply them with electricity and heat. A high-speed four-stage radial-flow microturbine with a nominal power of 2.5 kW(e) was used as an expansion machine. The low-boiling medium under the trade name of HFE7100 was used as a working medium in the system. Experimental tests of the ORC system were performed with various loads and under various operating conditions. The main purpose of the work was to achieve the maximum power of the microturbine at maximum possible efficiency of the ORC system. For this purpose, the supply system of the microturbine was improved by using an additional hole through which the blade system is fed. After the improvement, the microturbine produced about 3.9% more electric energy; this was due to the increase in the rotational speed (by approx. 7.2%) as compared to earlier measurement results. The maximum electric power generated by the microturbine was 2.12 kW(e) and was achieved at a rotational speed of about 22,440 rpm. The maximum thermal efficiency of the ORC was about 6.5%. The cycle had the maximum isentropic efficiency of the microturbine at 71% and it was lower by 1% than the value obtained using a numerical model. The improvement of the microturbine's supply system enhanced the exergetic efficiency of the ORC system and its maximum value was about 21.5%.