Scientists from the South China University of Technology have developed a passive solar module cooling technology that utilizes a hygroscopic hydrogel as the cooling agent. Hygroscopic hydrogels are a new type of sorbent materials that offer extreme water uptakes, efficient water capture and release kinetics, and low desorption enthalpies.
In the study “Hygroscopic hydrogel-based cooling system for photovoltaic panels: An experimental and numerical study,” published in Applied Thermal Engineering, the research team said this is not the first attempt to use hygroscopic hydrogels to reduce PV module temperatures and added the novelty of their approach consisted of assessing the impact of the improved thermal conductivity on heat and mass transfer during the hydrogel evaporative cooling process.
“We developed a mathematical model to simulate heat and mass transfer processes in real-world environments,” the research's corresponding author, Li-Zhi Zhang, told pv magazine. “We analyzed the operational mechanisms and factors responsible for controlling the system from the perspective of the heat and mass transfer resistance.”
The group investigated, in particular, a hygroscopic hydrogel composed of the polyacrylamide (PAM) polymer and lithium chloride, which is a moisture sorbent able to absorb and release moisture depending on its sorption characteristics, ambient temperature, and humidity. The hydrogel is heated by the panel during the day, while at night it absorbs water from the air, to regenerate itself, through an evaporation-sorption cycle.
The researchers placed the hydrogel with a thickness of 6 mm on the backside of 10 cm x 10 cm solar panels installed with a tilt angle of 30 degrees. They then conducted a series of simulations for 96 h to compare the cooling and electrical performance parameters of the panels with that of reference panels without cooling.
Their analysis showed that the hydrogel can considerably reduce PV module temperature between 8:00 and 4:30 pm and by up to 7.5 C at noon. Furthermore, the hydrogel was responsible for increasing the power yield of the cooled system by up to 2.1% compared to the uncooled one.
“Subsequently, the hydrogel began to sorb moisture from the air, whereupon the temperature of the PV panel in the cooled system surpassed that of the solely PV panel, owing to the heat released during sorption,” the scientists explained. “The combined effects of the decreasing light intensity and declining water content of the hydrogel adversely affected the cooling performance of the cooled system.”
The simulation also showed that the external heat transfer resistance, the internal mass transfer resistance, and the water content of the hydrogel were the key factors for determining the overall system performance.
“Increasing the thermal conductivity of the hydrogel could improve the system performance by reducing the internal thermal resistance,” the academics stressed. “Once the thermal conductivity reaches 1 W/m/K, the internal thermal resistance becomes negligible compared to the external thermal resistance, and further increases in thermal conductivity, which has a diminishing effect on system performance.”
Looking forward, the group is planning to assess the technology's technical and financial viability.
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