Evaluation of heat transfer correlations for helical coils based on measurements of a solar organic rankine cycle operating at subcritical conditions

Abstract

Organic Rankine cycle (ORC) is an acknowledged method for utilizing low temperature heat sources for generating electricity. Among many applications at heat sources such as internal combustion engines and industrial facilities, solar power is a significant energy source. For solar boilers, helical coil heat exchangers are widely used. The design of these heat exchangers are made with conventional methods, which are mostly not validated for ORC conditions, namely larger tube diameters and working fluids. In order to analyse the accuracy of conventional helical coil heat transfer correlations in design, the geometry and performed measurements at subcritical conditions of a helical coil heat exchanger is taken as reference for the off-design. The helical coil is electrically heated for simulating the photovoltaic/thermal (PV/T) collectors, for testing the solar ORC concept. Then the ORC is coupled with the PV/T collectors on the field for the complete solar ORC system. The inlet conditions of the existing installation are used for sizing and rating via 9 two-phase heat transfer correlations existing in the literature for the tube-side of helical coil. The helical coil outer diameter is 33,7 mm, whereas the shell inner and outer diameter are 0,526 m and 0,674 m, respectively. The coil diameter is 0,6 m. Three measurements are made at changing ORC medium (R404a) mass flow rates, namely 0,1 kg/s, 0,17 kg/s and 0,24 kg/s. R404a’s inlet temperature changes between 21,9 °C and 33,6 °C at a pressure range of 17,5 – 31,6 bars (representing the saturation temperature). In all three cases, the heating water inlet conditions are fixed at an inlet temperature of 95,3 °C and a mass flow rate of 2,67 kg/s.