title: Study on a thermally driven refrigerator based on an organic Rankine cycle and vapor compression refrigeration combined by single rotor expander-compressor
Creators: name: Alshammari, Saif Fraih K.
Publisher: name: University of Glasgow
Year: 2024

description: Environmental degradation is still a major global concern as a result of the widespread usage of high-grade energy derived from fossil fuels, which emit greenhouse gases in abundant. Countries and companies all over the world are dealing with these issues, looking for sustainable and environmentally beneficial solutions. One such promising path is using the organic Rankine cycle (ORC) to capture heat, particularly from low temperature sources. The ORC's ability to convert otherwise wasted energy into useable power places it at the forefront of strategies aimed at reducing emissions and combating environmental impact. Given this context, this study goes thoroughly into the novel single rotor expander-compressor device's innovative potential. This device, which is intended for use in both a VCR cycle and an ORC, is positioned as a transformative tool in energy optimization and sustainable refrigeration. A detailed and comprehensive numerical model of the ORC-VCR was constructed to accurately describe and assess the performance of this integrated system. This model serves as a predictive tool for practical use cases to ensure its accuracy and reliability. The study rigorously evaluated the thermal efficiency of this integrated cycle. Thorough evaluations were conducted, taking into account the varying temperatures at which evaporation occurs in both the ORC (62.75 °C – 89.7 °C) and VCR (-20 °C – 5 °C) systems, the range of temperatures at which condensation occurs in the ORC system (20 °C – 45 °C), and the rotor speed (500 – 3000 rpm), all while keeping the heat source temperature constant (95 °C). These characteristics played a crucial role in understanding the operating limits and effectiveness of the combined system. The maximum cooling performance achieved was 5.38 kW, with a heat to cooling efficiency of 56%, attained at an ORC evaporation temperature of 62.75°C and a VCR temperature of -5°C, alongside an ORC condensation temperature of 20.5°C. it was observed that cooling performance consistently improved with increasing rotor speeds, whereas rotor speed changes did not affect the heat-to-cooling efficiency. Other than energy analysis of the system, exergy analysis was conducted for the same system. By analyzing the exergy destruction contributions of each component within the ORC-VCR system, it was found that the ORC evaporator had the most substantial impact on total exergy destruction. However, the VCR evaporator's contribution could significantly increase if the evaporation temperature in the VCR were further reduced. The highest overall exergy efficiency recorded was 63%, with ORC and VCR evaporation temperatures set at 62.75 °C and -5 °C, respectively, and an ORC condensation temperature of 20.5 °C. Notably, overall exergy efficiency remained unaffected by variations in rotor speed.

In addition, this study explores the field of refrigerants, assessing other prospective refrigerants such as R245fa, R123, R134a, R1234ze(E), R1234yf, and Butane. Their performance is evaluated based on important indications, namely the heat-to-cool ratio efficiency, exergy efficiency, and the overall system efficiency. After this in-depth investigation, an examination of the VCR-ORC integration's potential influence on the environment was carried out, with a particular focus on aspects including consumption of fuel and CO2 emissions. In its essence, this research offers a perspective on the environmentally responsible integration of VCR and ORC systems. By combining these cycles, not only demonstrating the potential for improving energy efficiency, but also propose ways to address the urgent problem of emissions of greenhouse gases.