Study of heat pipe heat exchanger for low temperature waste heat recovery application

Abstract

Due to environmental concern and high price of fuel, waste heat recovery has become a very crucial issue for the industrial and commercial energy users. As a large portion of this waste heat comes out as a low-grade energy, conventional energy recovery technology can’t be employed to recover this waste energy economically. Heat pipe heat exchanger (HPHE) is an alternative technology that can be employed to recover this low grade waste energy. In this study, performance of an air-to-air HPHE has been studied experimentally. The HPHE consisting of number of heat pipes (thermosyphon) was constructed in a shop without any sophisticated equipment using copper tube and distilled water as the working fluid. Performance of the heat pipe was experimentally investigated for different working fluid, filling ratio and adiabatic length. Among the studied parameters, water was found superior over ethanol as the working fluid and heat pipe with 50% filling ratio shows better performance compared to that of other filling ratios. Shorter adiabatic length enhances heat transfer performance and the best performance was found for the heat pipe without adiabatic section. The best performer heat pipe with optimized parameters was selected for the construction of air-to-air HPHE. The constructed HPHE was placed between two ducts carrying hot and cold air. Temperature of the hot and cold air was varied to resemble two types of waste heat recovery applications: (ⅰ) low grade waste heat recovery applications where hot air was at ⁓70°C and cold air was at atmospheric temperature of ⁓35°C (ⅱ) HVAC waste heat recovery application where hot air was at atmospheric temperature of ~35°C and cold air was at ⁓25°C. Performance of the HPHE was evaluated by analyzing heat transfer rate, heat transfer coefficient, effectiveness and NTU under different mass flow rate of air. The results indicate that HPHE can recover waste heat from a small temperature difference in both low and moderate temperature applications. At moderate temperature application, about 800W energy was recovered by HPHE when hot fluid temperature was 70°C and cold fluid temperature was at 30°C and at low temperature application, about 200W energy was recovered when hot fluid temperature was at 40°C and cold fluid at 22°C. Financial analysis also has been conducted for the HPHE and average payback period has been estimated with the current energy prices as 3.3 years for the moderate temperature application and 8.2 years for the HVAC waste heat recovery application.