Drying effect of carrot in drying greenhouse combined with heat pump system

Abstract: Vegetables are more popular to be added into food after drying. But the cost of vegetable drying technology is relatively high, failing to meet the needs of the food industry at present. In this study, a combined system of drying greenhouse and heat pump was designed to dry vegetables at a low cost. Some structural parameters were designed in the dry greenhouse, according to the actual working environment of test sites, including the deviation angle, roof structure, roof inclination angle, and ventilation system. A three-stage heat pump was developed to combine the drying greenhouse. Specifically, the three condensers were taken as a whole, where the low-temperature air was heated to a high temperature through three stages, and then put into the drying area for vegetable drying. In three evaporators, the high-temperature air was cooled to a low temperature for the next cycle in the condenser. Carrots were selected as experimental materials, aiming to determine the performance index and drying effect of the system. A field experiment was carried out in Quanzhou City, Fujian Province, China from March 25, 2019 to April 25, 2019. The solar radiation intensity and the temperature inside and outside the drying greenhouse were firstly measured, then the performance indexes of the heat pump were evaluated, and finally, the quality indexes of the carrot were collected during the test. The experimental results showed that the dry greenhouse provided a better thermal insulation performance than before. Specifically, the average solar radiation indoor was 526 W/m2, the average temperature in the daytime was 6.4℃ (higher than that in the outdoor), and the average temperature at night was 4℃ (higher than that in the outdoor). Furthermore, the COP of the system increased by 26%-29%, compared with the heat pump individually. In system parameters, the drying temperature, loading capacity, and wind speed posed significant effects on the quality of dried carrots. Under the conditions of drying temperature of 60℃, wind speed of 2 m/s, and loading capacity of 2 000 kg, the quality indexes of dried carrots were better, with the average color (a * value) of 31.27, the average rehydration ratio of 5.42, the average vitamin C of 1.51 mg/g, the average total sugar of 12.36 %, and the average carotenoid of 0.68 mg/g. The economic indicators showed that the processing capacity of fresh carrots was 80 kg/h, the discharge of dry carrots was 18.8 kg/h, while the power consumption was 53.5 (kW·h)/h, and the production cost of dry carrots was 2.25 yuan/kg. In the whole, an attempt was made to explore new technology of vegetable drying during this time, particularly combining the greenhouse and heat pump. A better performance was achieved for the higher quality of carrots than before. The subsequent investigation can be focused on how to better retain the nutrients of vegetables, such as the updated drying medium of the heat pump unit for faster heat transmission during drying. The findings can provide an insightful theoretical idea and scientific support to the technical exploration on the food supply of bulk carrot after drying.