Optics-Based Approach to Thermal Management of Photovoltaics: Selective-Spectral and Radiative Cooling

Abstract

-For commercial one-sun solar modules, up to 80% of the incoming sunlight may be dissipated as heat, potentially raising the temperature 20°C-30°C higher than the ambient. In the long term, extreme self-heating erodes efficiency and shortens lifetime, thereby dramatically reducing the total energy output. Therefore, it is critically important to develop effective and practical (and preferably passive) cooling methods to reduce operating temperature of PV modules. In this paper, we explore two fundamental (but often overlooked) origins of PV self-heating, namely, sub-bandgap absorption and imperfect thermal radiation. The analysis suggests that we redesign the optical properties of the solar module to eliminate parasitic absorption (selective-spectral cooling) and enhance thermal emission (radiative cooling). Comprehensive opto-electro-thermal simulation shows that the proposed techniques would cool one-sun terrestrial solar modules up to 10°C. This self-cooling would substantially extend the lifetime for solar modules, with corresponding increase in energy yields and reduced levelised cost of electricity (LCOE).

Optimisation Principles for Photovoltaic/Thermal Hybrid Solar Systems, a Meta Study

“…It is this electron liberation process that is used as electrical energy in all solar powered systems, by creating metal contacts between negative (electron donor) and positive (electron acceptor) type semiconductors to create a circuit. [1][2][3][4][5][6][7][8][9][10][11] Conversely, light that does not have sufficient energy does not interact with the PV component, passing it and thus being collected by the thermal backplate. The backplate also collects radiative heat from the thermal relaxation of promoted electrons that absorbed photons of light larger than Eg that are preparing to recombine in the PV component of the system.…”

“…The backplate also collects radiative heat from the thermal relaxation of promoted electrons that absorbed photons of light larger than Eg that are preparing to recombine in the PV component of the system. [3,4,7] The spectral percentage of sunlight that cannot be collected by Silicon (typically Eg=1.1eV) due to insufficient photon energy (that is to say, the energy of the photon is smaller than Eg) has been widely known to be 19%. [23] Photons absorbed that are above the bandgap also do not fully contribute their entire energy to the production of electricity in a photovoltaic cell, as predicted by the Schockley-Quessier paper detailing efficiency limits.…”

“…Not only does this collection method increase the spectral efficiency of the collection system as a whole [1][2][3][4][5][6][7][8][9][10][11], but the thermal component by nature acts as a heatsink for the photovoltaic component. This allows PVT systems to double down on efficiency gains, as higher thermal output means lower PV operation temperatures, which increase PV collection efficiency as well.…”

“…This allows PVT systems to double down on efficiency gains, as higher thermal output means lower PV operation temperatures, which increase PV collection efficiency as well. [3] Typically solar cells are not only fully thermalised in real world application, but are actually operating at temperatures that are hot enough to decrease electric collection efficiency, typically operating at 40℃ above ambient temperature Ta. This is past the optimal thermalisation temperature and when PV cells tend to display more intrinsic semiconductor properties and lower electrical efficiency due to thermal resistance.…”

“…[3,8] This truth in application has led to some research looking into hydrogenated amorphous Silicon (aSi:H) being used for the PV components as opposed to traditional mono or polycrystalline Silicon (c-Si), as the temperature coefficient for a-Si:H is typically 0.01% per Kelvin [5] compared to the c-Si value of 0.04% per Kelvin. [1][2][3][4][5][6][7][8][9][10] This is also due to the fact that a-Si:H can operate at a higher temperature, allowing the thermal working fluid to be raised to a higher temperature as well since the Second law of Thermodynamics dictates that heat can only go from a hotter body to a colder one. [5] High temperature conditions also cause accelerated degradation of the photovoltaic system through the means of thermally activated degradation processes, which include polymer degradation and collector contact corrosion.…”