Experimental study on irrigation water warming system based on solar photovoltaic/thermal technology

The purpose of this study was to present a dedicated experimental investigation on the performance of a novel irrigation water warming system using solar photovoltaic/thermal technology. A combination was designed, including a flat-plate PV/T collector, the insulation barrels, the submerged pump, the frequency changer, the tungsten halogen lamp, and the system pipe. The cold irrigation water absorbed the heat through the flat-plate PV/T collector, where the temperature of photovoltaic modules was reduced for higher photoelectric efficiency. The experimental platform for the irrigation water warming system was built, where a series of tests were conducted under different working conditions, including the liquid mass flow rates 0.01-0.08 kg/(s•m2), initial water temperatures (5,7.5, 10℃), and the radiations (320, 465, and 650 W/m2). All 72 working conditions in total were conducted in the test, where the door and windows were closed in the room, and the temperature was controlled at 20℃ by the air conditioner. An analysis was made on the irrigation water temperature, the extent of temperature, photoelectric efficiency, solar thermal efficiency, and practical energy performance of the system. The results show that the outlet water temperature and the increasing extent of temperature reached 20.9 and 12.5℃, both of which were negatively correlated with the mass flow rate, particularly under the fixed condition, where the initial water temperature was 7.5℃, and the radiation was 465 W/m2. Nevertheless, the solar electric and thermal efficiency were positively correlated with the mass flow rate. The practical energy efficiency increased first and then decreased with the mass flow rate up rating, reaching a maximum of 0.484, where the inflection point was 0.03 kg/(s•m2). Besides, the irrigation water temperature, the extent of temperature, and energy performance of the system were compared under the middle radiation and different initial working condition of water temperatures. The heat transfer was much more intense, as the initial water temperature decreased, thereby causing the higher increasing extent of temperature. The initial water temperature was still an important factor to determine the water temperature of the outlet, indicating a positive correlation, and the maximum water temperature of the outlet of 22.6℃. Additionally, the solar electric efficiency, solar thermal efficiency, and practical energy efficiency were negatively correlated with the initial water temperature. Furthermore, irrigation water temperature and extent of temperature were relatively larger, when the system was under the higher solar radiation, but the increase of mass flow rate weaken the influence on the two indexes. The outlet water temperatures were maintained at about 10℃ when the system operated at 0.08 kg/(s•m2) with 7.5℃ irrigation water under different solar radiation. The solar electric efficiency and solar thermal efficiency were also negatively correlated with the solar radiation, where the maximum values were 0.096 and 0.417, respectively. Moreover, the practical energy efficiency was negatively correlated with the solar radiation, when the mass flow rate was less than 0.06 kg/(s•m2). It was just the opposite trend, when the mass flow rate was greater than 0.07 kg/(s•m2). In terms of application prospect, the initial investment of irrigation water heating system was higher using solar photovoltaic technology, compared with the traditional greenhouse photovoltaic roof, but the whole life cycle cost was smaller, and the LCC was reduced by 17.6%, considering the comprehensive energy benefits. The system was still in the stage of research and experimental study. The actual installation area can be further addressed, according to the actual outdoor irradiance, irrigation amount, irrigation time, required irrigation water temperature. © 2021, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.