Finding ways to enhance and improve the efficacy of photovoltaic (PV) systems is critical as the use of green power solutions continues to grow around the world. The concept behind one such solution to improve the performance of photovoltaic solar modules came to Ignacio Valiente-Blanco, a mechanical engineering professor in the Department of Signal Theory and Communications at the University of Alcalá (UAH) in Madrid, when he was hiking in the mountains near his home in Spain.
“I had been thinking for some time about how to improve this,” Valiente-Blanco said. During that particularly warm day, when temperatures climbed to about 97 °F (36 °C), “We found a small lake supplied by an underground aquifer. I was surprised about how cold the water was compared to the very warm environment. Then, it hit me. Why not use the cold temperatures underground to cool down solar panels in a sustainable and stable way?”
The epiphany resulted in a novel cooling system for photovoltaics that uses the underground as a heat sink that was both theoretically described and experimentally validated in “Efficiency Improvement of Photovoltaic Solar Modules by Cooling Using an Underground Heat Exchanger,” published in ASME’s Journal of Solar Energy Engineering in December 2022. Valiente-Blanco led a team of six researchers and two technicians on this two-year effort.
Playing It Cool
“Commercial solar panels present very low-efficiency levels, typically in the range of 15 percent to 25 percent,” explained Valiente-Blanco. “Moreover, they dramatically lose efficiency when their temperature increases during operation, especially in the summer season.”
PV cells are dark so they can absorb as much solar energy as possible. But since only a fraction of the sun’s radiation is converted to electricity, most of it is transformed into heat.
“Solar cells typically show a reduction in efficiency as they get hotter, thus the importance of cooling the solar panels, which can reach temperatures above 70 °C (~160 °F) given the right conditions,” explained doctoral student Diego López-Pascual. The ideal operating temperature for solar photovoltaics is around 77 °F (25 °C). Such overheating can also result in “potential power losses in the panels between 12 percent and 27 percent, limiting the productivity of photovoltaic facilities all around the world,” according to the report.
It also leads to problems related to thermal cycling and performance degradation, the researchers wrote. These fluctuations affect the life and amortization costs of solar panels, with typical degradation rates falling around 0.7% per year.
Cooling solutions are nothing new either. In the report, the team points to two main technology buckets, passive and active. Passive technologies, such as finned heat dissipators and heat pipe systems, don’t consume energy during operation and are more widely researched and deployed. Active tech, ranging from forced air ventilation and single-phase water cooling to and water evaporation, often can do a better job at reducing temperatures, but at a cost.
“During our initial research, we found that most of the active cooling alternatives explored showed at least one of three main issues,” López-Pascual said. “They required a lot of additional power to actuate the cooling system, they used large amounts of water, or they presented very heavy modifications to the solar panel.”
Closing the Circuit
Limited research has been done on geothermal cooling for solar panels, according to the research team, but given the stability of low temperatures at a certain depth underground, its promise is significant.
“The underground typically sits at a constant temperature through the year past a certain depth. This temperature is similar to the yearly average ambient temperature in the region,” said López-Pascual. Depending on location, underground temperatures are usually between 50 °F (10 °C) and 68 °F (20 °C) between 5 meters and 15 meters underground, according to the report.
The UAH team’s cooling system takes advantage of this by using the ground as a heat sink. It’s a relatively simple working concept, López-Pascual said.
“By circulating a water-based coolant through the solar panel’s heat exchanger, the panel is cooled. This warm water is then pumped to another heat exchanger installed underground where the extracted heat is dissipated given the colder and more stable temperature of the ground,” he explained. “Water then flows back towards the solar panel, completing the circuit and ensuring that the solar panel stays significantly colder,” while also improving its efficiency.
The second heat exchanger is in an underground borehole that stays at a constant temperature of 15.7 ± 0.5 °C (or about 60 °F) at a 15-meter depth, the researchers noted.
Part of the additionally generated power is used to pump the coolant through the heat exchange system. However, since it’s closed-circuit and relatively close in height to the solar module itself, the amount of energy consumed is “relatively low” compared to the additional energy the solar module creates due to the cooling system, still resulting in a net power gain, the researchers state.
“Since our system presents a closed circuit using the stable underground temperature as a heat sink, pumping power is significantly reduced and there is no water consumption required after filling the circuit,” López-Pascual said. “We also achieved a modular heat exchanger design that allowed us to extract great amounts of heat from the panel without heavy and complex adaptations.”
Plus, the closed-loop nature of the system limits the chances of contaminating nearby soil and water with coolant.
Put to the Test
To further prove the technology’s viability, Valiente-Blanco’s team built a prototype that was integrated into an isolated photovoltaic facility in Alcalá de Henares, north of Madrid, making use of two ATERSA model Ultra 270 P polycrystalline silicon solar panels.
Peak power production of these panels is 270 W, with a power temperature coefficient of 0.43 percent per degree Celsius change in temperature. One of the panels remained unaltered for a control, while the other was outfitted with the prototype cooling system.
For the heat exchanger installed on the back of the panel, six flattened U-shaped copper tubes were molded into a quasi-rectangular shape to increase the contact surface with the panel, then all the copper tubing was covered with polyethylene foam. A mix of 80 percent water and 20 percent ethylene glycol carried the heat from the panels to another copper-tube heat exchanger in a borehole.
Testing occurred from July to September 2021 and showed promising results. The prototype improved net power generation of the outfitted solar panel by a max of 12.4 percent, with daily averages of about 9.8 percent. A coolant flow rate at 1.84 liters per minute per square meter of the solar module kept its temperature down by up to 14 °C as well. Testing also showed that the increase in power generation depended on the pump’s efficiency, the amount of solar radiation, and ambient temperature. The system was more effective at boosting power gains on days where solar radiation and ambient temperature were higher.
Valiente-Blanco’s team is already working on next-generation prototypes as well. “One of our active research lines is focused on applying this concept to concentration photovoltaics, where the thermal management problem is even more critical. We are also thinking about new ways to take advantage of the excess heat,” he said.
Although the team has done some initial estimates on cost-effectiveness, a full in-depth study hasn’t happened yet, but will once testing is completed.
“As part of my PhD thesis, we developed a thermo-electrical calculation model that allows us to predict with good accuracy the system’s behavior under different environmental conditions,” López-Pascual said. “Even though the year-long test campaign is not yet completed, we had enough data to confirm the accuracy of the calculation model. We then used it to estimate a 5.9 percent increase in net energy output through the year compared to the same solar panel without the cooling system.”
And considering manufacturing costs, installation, and maintenance, this increase in energy production would make the system profitable for electricity production installations of just a few megawatts, and even more as the size of the installation increases, he added.
As the development phase continues, the UAH team is working on optimizing system performance and reducing costs, Valiente-Blanco said.
“A couple of companies have already contacted us interested in the development. We are looking forward to seeing what the future brings for us,” he added.
Louise Poirier is senior editor at Mechanical Engineering magazine.
