Heat recovery and dehumidification of exhaust air for improvement of thermal performance of a heat pump drying system.
Ngalonkulu, Solomzi
Ngalonkulu, Solomzi
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Abstract
South Africa generates about 85% of its electricity from coal combustion, making it responsible for 42% of emissions in Africa, which is the largest emitter in Africa, the world’s 11th most significant greenhouse gas emitter, and the most carbon-intensive non-oil-producing developing country globally. The drying of agricultural products, including fruits, vegetables, and nuts, constitutes a major energy consumption process. In developed nations, energy used for food preservation accounts for approximately 7–15% of total energy consumption, while it represents about 10-15% of global industrial energy usage. The use of heat pump technology for the preservation of biomaterials is a well-developed technology in countries such as Turkey and China, and most of the recent studies conducted are focused on optimising drying kinetics since the heat pump drying (HPD) technology has been well-examined for their climatic conditions. Nevertheless, over the past few years, the South African market has mainly focused on applying heat pump systems for space heating and cooling, and very little research has been done on heat pump technology for drying agricultural produce. As a result, these energy-efficient and environmentally friendly systems have not been widely used as they should be in South Africa. Therefore, in this study, an air source heat pump drying (ASHPD) system operating on the open, semi-closed, and fully closed loop was designed, constructed, and analysed. The primary objective of this study was to conduct a comprehensive investigation of the thermal performance of three (open loop, semi-closed loop, and fully closed loop) HPD system configurations to determine the most appropriate system for the prevailing climatic conditions in South Africa. This was done by conducting experimental investigations to examine the impact of various critical parameters on the thermal performance of an HPD system, such as the size of the orifice, the amount of refrigerant charge, the speed of the evaporator and condenser fans, and the variation of ambient temperatures. The results demonstrated that the increased refrigerant charge in the HPD system increased the refrigerant mass flow rate, the heating capacity, and the overall COP to a peak point at less than 100% charge. Nonetheless, the fully closed configuration reached the optimum charge at refrigerant charges less than those of the semi-closed and open configuration. Also, the change in fan speed and the ambient temperatures play a significant role in the operating conditions and the thermal performance of the three heat pump configurations. Increasing the condenser fan speed increased the heat rejection to the sink to a peak at 90% fan speed, while increasing the evaporator fan speed increased the thermal energy input from the heat source. As a result, the compression ratio decreased, leading to the increased refrigerant mass flow rate, heating capacity, and overall COP to a maximum at an optimum fan speed of 90%. On the other hand, the decrease in ambient temperature significantly decreased the suction pressures more than the discharge pressures. Subsequently, the refrigerant density dropped remarkably with the decrease in ambient temperature, resulting in lower refrigerant mass flow rates and negatively affecting the efficiency of the heat pump drying system The results show that the HPD system configuration significantly influences thermal performance. Recovering waste heat from the drying chamber to the evaporator improved the overall heating coefficient of performance (COP) by 4.4% and 17.3% for the semi-closed loop and closed loop, respectively; however, when the ambient temperature decreased from 20 C to -10 C, the COP degraded by 29.6% and 30.6% for the open and semi-closed loop, respectively. For the closed loop configuration, the COP decreased by 20.1% when the ambient temperature decreased from 10 C to -10 C. These results suggest that the heat pump drying systems are viable in South African climatic conditions. However, the open-loop and semi-closed loop configurations are best for warm regions or warm seasons, whereas the fully closed configuration is best for cold regions or cold seasons.
Description
Submitted in partial fulfilment of the requirements for the degree Doctor of Engineering: Mechanical Engineering at the Department of Mechanical and Mechatronics Engineering in the
Faculty of Engineering and the Built Environment at the Tshwane University of Technology.
Date
2024-08-01
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Tshwane University of Technology
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Keywords
Heat pump, Thermal performance, Heat recovery