Abstract
Worldwide interest in low grade heat valorization using organic Rankine cycle (ORC) technologies has increased significantly. A new small-scale ORC with a net capacity of 3 kW was efficiently integrated with a concentrated solar power technology for electricity generation. The excess heat source from Photovoltaic (PV) collectors with a maximum temperature of 100 °C was utilized through a supercritical heat exchanger that uses R-404A as working medium. By ensuring supercritical heat transfer leads to a better thermal match in the heat exchanger and improved overall cycle efficiency. A helical coil heat exchanger was designed by using heat transfer correlations from the literature. These heat transfer correlations were derived for different conditions than ORCs and their estimated uncertainty is ~20%. In order to account for the heat transfer correlation uncertainties this component was oversized by 20%. Next, a prototype was built and installed in an integrated concentrated photovoltaic/thermal (CPV/T)/Rankine system. The results from the measurements show that for better estimation of the sizing of the heat exchanger a more accurate correlation is required in order to design an optimal configuration and thus employ cheaper components.
Highlights
An increased demand for energy and environmental issues on a worldwide level have stimulated a number of researchers to work on improving the efficiency of thermodynamic cycles and look for ways of utilizing renewable energy sources
One way to improve the cycle efficiency is by ensuring supercritical heat transfer in the heat exchanger
Three heat transfer correlations available from the literature were employed for the design of the heat exchanger
Summary
An increased demand for energy and environmental issues on a worldwide level have stimulated a number of researchers to work on improving the efficiency of thermodynamic cycles and look for ways of utilizing renewable energy sources. The organic Rankine cycle (ORC) is a suitable technology for employing low grade temperature heat from several renewable energy sources such as biomass, geothermal and solar. A way to enhance the overall efficiency of an ORC can be achieved by ensuring supercritical heat transfer between the heat source and organic fluid in the heat exchanger [1]. In order to be able to design an optimal heat exchanger suitable to operate at supercritical conditions, proper heat transfer correlations need to be used. To provide accurate correlations for designing a heat exchanger, the heat transfer process of the working fluids under supercritical conditions has to be studied. At supercritical state there are significant variations of the thermophysical properties such as the specific heat capacity, density, Energies 2016, 9, 432; doi:10.3390/en9060432 www.mdpi.com/journal/energies
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