It is well known that some solar cells experience significant efficiency degradation—and possibly decreased lifetime—with elevated operating temperatures. This can happen in concentrated photovoltaic (CPV) applications if appropriate cooling is not provided. We have evaluated PV-cell immersion cooling for power generation in a highly concentrated solar beam.1 We control cell temperature by flowing pure water through a transparent chamber containing an array of cells.
Both passive and active cooling techniques have been applied to control cell temperatures in CPV systems. Passive systems are the simplest and most reliable but may not be very effective when using a densely packed array. For these systems, active cooling is typically applied. Heat is usually removed by water or air, which is often circulated behind the cell modules. Power parasitics are typically associated with such systems.
By placing bare solar cells directly in a dielectric liquid, we aimed to achieve higher electrical performance, based on liquid light-trapping ability and reduced solar-cell surface-recombination velocity. Aside from optical and surface-wetting advantages of liquid immersion, direct contact between cells and their surrounding liquid enables a new cell-cooling method, especially for cells at high concentrations. By eliminating thermal resistance of the contact wall between the solar cell and fluid, as seen in conventional active-cooling approaches that control heat-dissipation rates, cell cooling could be effected for desirable sunlight-to-electricity conversion efficiency.
Solar-cell immersion liquid must meet numerous strict requirements, such as good electrical-insulation properties, nontoxicity, good thermal conductivity, low viscosity, long-term chemical and physical stability, low optical absorptivity, and good optical stability. We chose deionized (DI) water as the cooling agent and tested water-immersion cooling using a large-dish concentrator system. The concentrator has 16 mirrored facets, each possessing 76 mirror tiles, for a total of 113m2 of system mirror area. The resulting focus area of 60 × 60cm2 has a concentration ratio of approximately 250 suns (see Figure 1). The PV module was constructed from Amonix back-point-contact silicon cells, and one module contains 88 cells connected in series. The module was inserted into a glass tube to form a liquid-immersion receiver (see Figure 2). DI water was pumped through the tube for cell cooling, and heat was then rejected. We measured both the cell-module real-time temperature distribution and current-voltage curves. We also investigated effects of climate conditions, flow rate, fin addition, and water resistivity.
Figure 1. Photograph of the system on-sun.
Figure 2. Concentrated photovoltaic receiver. (left) Drawing of side view. (right) Photograph of front.
Module temperature peaks at 49°C, and the temperature-distribution variation is less than 4°C at 250 suns. Here, direct normal irradiance is above 900W/m2, the cooling-medium inlet temperature is approximately 31°C, and the ambient temperature is approximately 17°C. Fin addition decreases the module's average temperature by approximately 10°C and also renders the temperature distribution more uniform. The module temperature decreases with increasing flow rate, while the temperature distribution is more uniform with turbulent flow.
In summary, we verified that direct DI water immersion can cool the cell module effectively and efficiently. However, module electrical performance badly degrades in the coolant resistivity range of 0.5–6.0MΩcm. We deduce that ion concentration is not the only causative agent. Electrolytic reaction at the electrodes or solar-cell connections might cause problems. We further conclude that resistivity is still too low to prevent degradation. Further conceptual investigations continue.
Robert F. Boehm
University of Nevada
Las Vegas, NV
Robert Boehm received his PhD at the University of California at Berkeley. He is distinguished professor of mechanical engineering and director of the Energy Research Center. He is a life fellow of the American Society of Mechanical Engineers and received its John Yellott award. He is also a technical editor for Energy.
1. Li Zhu, Robert F. Boehm, Yiping Wang, Christopher Halford, Yong Sun, Water immersion cooling of PV cells in a high concentration system, Sol. Energy Mater. Sol. C
95, pp. 538-545, 2011. doi:10.1016/j.solmat.2010.08.037