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A novel heat pump drying system with auxiliary solar source and ejector using a variable speed compressor has been experimentally investigated. The influences of compressor speed on the performances of novel system were comprehensively evaluated and compared to the basic heat pump drying system. Moreover, the relationship between the compressor speed and the drying temperature under different solar radiation intensity was clarified. The experimental results indicated that the novel system achieved the higher heating capacity and drying temperature because of the input solar energy. However, the excessive heat input to the drying system also would increase the electrical consumption. To solve this problem, the novel system using a variable speed compressor could obtain the same drying temperature as the basic system at the lower compressor speed. When drying at the same temperature of 70 °C, compared to basic system, the novel system using a variable speed compressor improved the moisture extraction rate by 6.8 %–22.4 %, the specific moisture extraction rate by 23.7 %–31.4 %, and the COP by 26.5 %–42.3 %. The results could provide the guidance for combining the variable speed compressor and the heat pump drying system with auxiliary solar source. [ABSTRACT FROM AUTHOR]

• An experimental facility of novel heat pump dryer system is designed. • Dynamic drying characteristics of proposed system are experimentally studied. • The optimum operating parameters of proposed dryer system is studied. • The proposed dryer system shows great potential performance advantages. A new designed solar assisted ejector enhanced heat pump dryer system (SE-HPD) with closed-loop drying chamber using the R134A refrigerant has been experimentally investigated. In the SE-HPD, a hot water cycle system has been designed to simulate the solar collector. The dynamic drying characteristic of SE-HPD and the conventional heat pump dryer system (CHPD) were compared. Further, various drying characteristic parameters and the ejector performances of the new system under different solar radiation intensities, valve opening (OP) of electronic expansion valves and by-pass return air ratios (BR) were studied. The experimental results revealed that the exergy efficiency and moisture extraction rate (MER) of SE-HPD were always better than those of the CHPD. The exergy efficiency and MER of SE-HPD were improved by 28.7% and 54.3% compared with those of CHPD under the given operating condition. It was found that higher solar radiation intensity could significantly improve the pressure lift ratio of the ejector and contributed to the higher heating performance of SE-HPD. The optimum OP and BR to obtain the maximum MER for SE-HPD were also found to be 50 % and 70 %, respectively. The performance advantages of the proposed system have been verified through this experimental research. [ABSTRACT FROM AUTHOR]

The solar-assisted heat pump (SAHP) systems are increasingly gaining attention for their potential feasibility in various fields due to their energy saving benefits. However, the available research shows that SAHP still has potential for improvement in drying filed, especially in terms of combining vapor compression heat pumps with solar energy. Consequently, this paper aims to summarize the development of SAHP technology and provide guidance for the application of SAHP technology in the drying field. The development of the SAHP systems is reviewed in this paper, including four ways to enhance the performance of SAHP systems are discussed: photovoltaic/thermal systems, collector optimization, energy storage and ejector enhancement. The technology of energy storage and ejector enhancement will inspire the development of SAHP technology. Additionally, research on optimizing the performance of hybrid heat source heat pump systems is also discussed, and a review of alternative refrigerants that could be used in SAHP systems. It could be found that R134a is the most widely used in SAHP systems currently, but it needs to be replaced soon for its high GWP. HFOs refrigerants are a good alternative. Furthermore, the application prospect of SAHP systems in drying field is also analyzed. Finally, based on the current research status of SAHP technology, the future development trend and research directions of solar-assisted heat pump drying (SAHPD) are proposed: developing suitable refrigerants for heat pump systems; researching high-efficiency heat storage technology; developing multi-heat source assisted heat pump systems; and optimizing the operation strategy of SAHPD using artificial intelligence. • Four ways to enhance SAHP systems are reviewed. • Study on optimization of hybrid heat source heat pump systems is summarized. • The alternative refrigerants in SAHP systems is investigated. • The application prospects of SAHP systems for drying field are analyzed. • The future development trend of SAHPD systems is proposed. [ABSTRACT FROM AUTHOR]