The Solarbacks researched and designed a variety of cooling methods that could be used to improve the efficiency of photovoltaics. These cooling methods can be separated into two categories: active and passive methods. The active cooling method of hydraulic cooling and the passive cooling methods of heat sinks (fins), optical coatings, thermosyphons, phase change materials, and thermoelectric generators were all taken into consideration as potential cooling methods. Passive cooling methods were preferred because the use of electricity required for the cooling mechanism would reduce the net electricity and subsequent profit from the panels.
Two variations of hydraulic cooling were researched: water spraying and the use of closed channels along the back of the panel. Both water spraying and closed channels along the back of the panel could effectively cool down photovoltaics, but the energy required to pump the necessary amount of water would exceed the additional power generated from cooling. Both variations would also require significant capital cost and would be difficult to scale up.
Two passive methods – thermosyphons and phase change materials – were researched but not tested as a final design. Thermosyphons use heat from the panel to boil a working fluid, increased buoyancy moves the fluid upwards where excess heat is released into the environment, condensing the fluid back into a liquid. This starts the process over again. Thermosyphons have been proven to work effectively for concentrated photovoltaic systems; however, the layout of typical solar farms is not conducive for thermosyphons if they utilize a solar tracking system. Chosen phase change materials would have a melting point that is within the operating range of the heated solar panel, and would cool the panel through conductive heat transfer from the back of the panel to the phase change material. When put in thermal contact with the panel, the panel’s temperature would not exceed the melting temperature of the material until all of it had melted. This method was disregarded because once the material had melted, the panel would no longer be cooled.
Additional passive methods were researched and tested. Ideal optical coatings reflect any solar irradiance that is not used by the panel to produce electricity, however, the coatings researched and tested produced minimal cooling. The coating Solarbacks tested was a thin sheet of mylar (saran wrap). The average cooling produced by the saran wrap was about 2.4oC. However, most of this cooling is thought to be a result of a thermosyphon effect because the saran wrap was elevated off the surface of the panel rather than being directly attached. This elevation likely induced forced convection with the outside air to cool the panel. Fins as a heat sink work by increasing the surface area that heat can be dissipated from.
One of the biggest disadvantages to fins is that their efficacy is strongly dependent on ambient conditions. The fins tested by Solarbacks were 1” tall, spaced 1” from each other, and placed on a 1/8” aluminum sheet and attached to the photovoltaic panel using a thermal mastic. The approximate cost of materials per panel would be around 28whenmaterialsarepurchasedinbulkfora1/32”thicknessextrudedfin.Testingshowedthatfinscouldcoolthepanel14oCduringpeaktemperaturesandincreasepoweroutputbyabout5.52Thermalelectricgenerators(TEGs)useelectricallydissimilarsemiconductorstoproduceanelectriccurrent.Whenputinthermalcontactwiththebackofthepanel,thegeneratorwoulduseanyexcessheattoproduceelectricity.TheheatTEGsusetoproduceelectricitycouldhelpcoolthepaneltosomedegree,buttheirmaincontributionistheadditionalelectricitytheygenerate.Thisadditionalelectricitywouldoutweighthelossesduetoheatingandincreasetheprofitabilityofeachsolarpanel.IfthebackofapanelwascoveredwithTEGsanda20oCtemperaturedifferencewasmaintainsfor5.5hr.adayinNewMexico,theTEGswouldproduceanadditional0.778kWh/day.ThebiggestdisadvantagetousingTEGsisthecapitalcost.UsingtypicalTEGdimensions(40mm∗40mm),536ofthemwouldneedtobeboughtperpanelwitheachTEGcostingabout2.92. Larger TEGs could be produced to fit to back of each panel and could reduce this capital cost significantly.
Overall, TEGs with fins provides the greatest amount of panel cooling and additional power production. There is an average of a 12.1°C temperature difference along a panel with this solution installed. Using manufacturer data, an estimated 135W can be produced from the TEGs at a 20°C temperature differential along the TEGs. However, when payout for this method is considered, it would take nearly 45 years. Purchasing additional panels that produce the same amount of power as the TEGs would have a payout period of less than 2 years. TEGs with fins at their current cost is not an economic alternative to purchasing more panels despite its cooling and power production capabilities
