An organic Rankine cycle (ORC) system is a mature technology used to convert low-grade heat (e.g. geothermal, solar, waste heat) into electricity. However, its adoption rate into practical installations is still rather limited due to its rather low efficiency. Operating part of the cycle in the supercritical region can improve thermal efficiency and decrease heat transfer losses. This system is called a trancritical ORC system. However, in the supercritical region, several phenomena such as buoyancy and flow acceleration occur, resulting in ‘atypical’ heat transfer behavior, making the well-known single-phase heat transfer correlations such as the Gnielinski or Dittus-Boelter correlation unsuitable to design the heat exchangers in these systems. Therefore, the knowledge on this type of heat transfer, especially for larger diameter tubes, needs to be extended. For this purpose, an experimental test rig has been developed (see figure).
The objective of this thesis is to collect heat transfer data on low-GWP refrigerants in their (near)supercritical region. An experimental test rig has been designed and is currently under construction. To ensure reliable operation, a LabVIEW program to steer the test rig still has to be programmed and tested. In addition, calibration and validation of the test rig has to be performed as well. Finally, the heat transfer (coefficients) of several selected low-GWP refrigerants needs to be measured for a large temperature and pressure range, with the focus on the supercritical region.
This project fits into the Sustainable Development Goals (SDGs) adopted by the UN. The generated data will be used to develop a heat transfer correlation for refrigerants in the (near)supercritical region. This way, heat exchangers can be designed for trans- or supercritical ORC systems which have higher efficiencies than subcritical ones, making ORCs more interesting for their application in converting heat from low-grade heat sources such as solar, geothermal and waste heat.