Direct and indirect utilization of thermal energy for cooling generation: A comparative analysis

Low-grade thermal energy can be either directly or indirectly utilized to generate a cooling effect. The present study provides air-conditioner designers with a roadmap to select the economically viable solution for the utilization of waste heat or available renewable energy. An ejector cooling cycle and a vapor absorption cooling cycle are selected as the direct cooling methods. For the indirect method, the ORC is chosen as the power cycle that should generate the electricity needed to drive the refrigeration machine. Based on thermodynamics principles, the theoretical models are built and then solved by the EES software. The cycle's COP, cooling capacity, and exergetic efficiency are the comparison criteria that are studied under certain operating conditions of the heat source, condenser, and evaporator temperatures. The analysis shows that, at source temperatures higher than 165 degrees C, indirect cooling is recommended, where it shows a higher performance than the direct method in the ejector cooling cycle at a condensation temperature of 30 degrees C as long as the desired evaporator temperature is greater than 5 degrees C. However, at lower source temperatures less than 100 degrees C, indirect cooling is recommended in areas where the ambient temperature is low, where direct methods start showing higher performance at temperatures higher than 30 degrees C. (c) 2021 Elsevier Ltd. All rights reserved.

Automated summary

This study examines the utilization of low-grade thermal energy to generate a cooling effect, providing a roadmap for air-conditioner designers to select economically viable solutions using waste heat or renewable energy. By establishing theoretical models and comparison criteria, it is found that under certain operating conditions, indirect cooling can outperform direct cooling in some scenarios.