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Saffron is the most expensive spice and Iran is the largest producer of this crop in the world. Saffron quality is profoundly affected by the drying method. Recent research has shown that hybrid photovoltaic–thermal solar power systems are more efficient in comparison with individual photovoltaic and thermal systems. In addition, heat pump dryers are highly energy efficient. Furthermore, they are suitable for heat-sensitive crops such as saffron. Therefore, in the present study, the performance of a hybrid photovoltaic–thermal solar dryer equipped with a heat pump system was considered for saffron drying, in order to obtain a high-quality product and reduce fossil fuel consumption. The effect of air mass flow rate at three levels (0.008, 0.012, and 0.016 kg/s), drying air temperature at three levels (40, 50, and 60°C), and two different dryer modes (with and without the heat pump unit) on the operating parameters of the dryer was investigated. The results of the investigation showed that total drying time and energy consumption decreased as air flow rate and drying air temperature increased. Applying a heat pump with the dryer led to a reduction in the drying time and energy consumption and an increase in electrical efficiency of the solar collector. The average total energy consumption was reduced by 33% when the dryer was equipped with a heat pump. Maximum values for electrical and thermal efficiency of the solar collector were found to be 10.8 and 28%, respectively. A maximum dryer efficiency of 72% and maximum specific moisture extraction rate (SMER) of 1.16 were obtained at an air flow rate of 0.016 kg/s and air temperature of 60°C when using the heat pump.
Mortezapour et al. (Sat,) studied this question.