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| Fig. 1: Image of a Hydrogel used in absorption of water. (Source: Wikimedia Commons) |
Solar-panel efficiency is significantly affected by temperature: for every °C increase in panel surface temperature, the efficiency drops by approximately 0.2% to 0.5%. [1,2] In hot environments such as deserts, this can translate to efficiency losses of up to ~18%. [3] A promising passive solution to this thermal issue uses hydrogels: polymeric, water-absorbing materials that can be applied to the backsides of solar modules to provide evaporative cooling without active energy use.
Hydrogel has multifaceted uses for the energy industry. Hydrogel is a unique material that is semi solid and liquid, allowing it to demonstrate properties of both under different conditions (Fig. 1). Among many use cases, hydrogel helps with improving energy storage as a supercapacitor, acting as a catalyst for hydrogen production, or providing thermal regulation where integration with photovoltaic (PV) cells can improve PV cell efficiency. [4-6]
Hydrogel cooling occurs via hydrogel's dual response to water vapor according to temperature. At night or during cooler periods, the hydrogel layer absorbs moisture from ambient air, absorbing thermal energy to form new bonds. During the day, as the solar panel heats up, the hydrogel releases water via evaporation. The latent heat of evaporation removes thermal energy from the hydrogel-PV system, thereby lowering the solar panel temperature. This is a passive cooling system not requiring fans, pumps, or externally-powered devices. As such, maintenance is minimized, no additional energy consumption is required, and no additional carbon cost is genreated. This makes hydrogel a great passive coolant for solar panels, especially in areas where it is hard to provide constant maintenance, like in the desert.
Research from the Vidyasirimedhi Institute of Science and Technology (VISTEC) in Thailand has demonstrated a hydrogel system which reduces solar panel temperature by about 23°C in testing (from ~70°C → ~47°C) [7]. The paper argues that a combination of PV and the temperature decrease translated into an approximate 12.3% boost in efficiency under the test conditions. The hydrogel had a specific cooling power of 1.86 W/g and added only ~5.1 kg m-2 in weight to the PV system. Hydrogels offer a substantially lighter (~80% reduction) in weight of many competing thermal regulation solutions, including the use of phase-change materials. [7]
Another research work at King Abdullah University of Science and Technology (KAUST) similarly achieved an average temperature drop of ~9.4°C on average in the desert conditions [8]. The hydrogel provided 175 W m-2 of cooling power, resulting in an increase in PV efficiency by ~10.2% on average. In a controlled lab setting, the team discovered that the cooling power of hydrogels peaks with ~14.1°C decrease in PV temperature when the system is in a 38°C ambient condition. This implies that the hydrogel passive cooling system works best if integrated with PV cells in a tropical climate. [8] Importantly, long-term durability tests indicated that panels with the hydrogel cooling layer lasted over 200% longer than uncooled panels in the same test-bed conditions. KAUST estimates this cooling approach could reduce the levelized cost of energy (LCOE) by approximately 18%. [8]
The benefits of hydrogels seem plentiful. Hydrogels lower the operating temperature and improve the efficiency of energy output for PV panels over their lifetime, while extending the PV lifespan, with little additional carbon footprint or cost generated. The lightweight implementation of hydrogels on the back of PV panels also reduce the structural burden on rooftops or solar farms. Additionally, potential cost reduction in LCOE makes the hydrogel integrated PV system promising from an economic viewpoint for large-scale installations.
Yet, upon closer inspection, benefits of integrating hydrogels with PV cells might be overstated. The results of cooling the power that hydrogels can provide is not truly sustainable. The team at VISTEC measured how strongly each hydrogel cools by measuring how much extra heat a machine had to supply to keep the sample at 60°C, with more supplied heat implying stronger cooling power [7]. Looking at Figure S5(a) of their supplement, we see that the specific cooling power of 1.86 W/g was obtained from the heating test over a duration of about 20 minutes. Given that the latent heat of evaporation of water is 2260 J/g, a hydrogel that is assumed to be fully made of water can only supply the given specific cooling power of 1.86 W/g for about 20 minutes, as shown in the calculation below, which spans the duration of their test.
As calculated above, the specific cooling power of hydrogels is only maintained at the peak cooling power stated in the VISTEC study [7] when the hydrogel layer is fully moisturized. Evaporative cooling works best in dry, hot climates with low humidity. Similarly, in cold climates with freeze-thaw cycles, hydrogels might not act as a good regulator of temperature, and might not be a universal solution for PV thermal regulation. Thus, the team at VISTEC has demonstrated the ability to produce high percentage of water by weight in their hydrogels, with the right chemical structure to almost fully optimize cooling via evaporation. Current research in the field have not yet demonstrated cooling capabilities that are sustainable over long periods of time, nor cooling capabilities that are robust against varying temperatures and humidities - which affect evaporation.
Additionally, scaling up the hydrogel-PV system incurs further challenges. Commercialization involves specialized manufacturing of optimal hydrogel types (as reflected in the study from VISTEC [7]), developing a supply chain, scaling, and ensuring the durability of hydrogel integrated PV systems under varied climates and prolonged duration. On a pragmatic standpoint, the increase in efficiency provided by hydrogels might not be huge enough to convince solar farms or homeowners to make the switch.
The use of hydrogels as passive cooling layers on solar panels presents a compelling opportunity to improve both efficiency and longevity of PV installation, but the idea still requires more investigation. While promising, challenges remain in ensuring the durability, cost-effectiveness, and climate-adaptation of hydrogel-PV systems. If these challenges can be addressed, hydrogel cooling could become a valuable component of next-generation high-efficiency, long-lifetime solar systems. These cooling hydrogels could become a relatively low-maintenance retrofit option for older solar panels, boosting output and extending usable life without replacing the panel itself. Large solar farms in hot climates stand to benefit most, given the greater efficiency losses due to heat. If deployment cost falls and durability is proven, the adoption of hydrogel integrated PVs could become commonplace. Nonetheless, the hydrogel cooling concept also links to broader themes of thermal management in energy systems, and it is an interesting research direction that could potentially be extended to other systems (e.g., batteries, electronics) and could spur further research into passive cooling materials for renewable-energy systems.
© Raphael Low. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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