Development of a c-Si Photovoltaic Module for Desert Climates

Abstract

For thirty years the c-Si photovoltaic module industry has not incorporated larger changes in the module design and production process. The c-Si based photovoltaic modules still consist of solar cells connected in series by means of soldering and laminating in between sheets of ethylene-vinyl acetate with glass as front cover and white Tedlar®-Polyester-Tedlar based backsheet as rear cover. Moreover, it is not only that traditional modules look almost identical to the ones from thirty years ago, they are also mostly constructed and adjusted for European environmental conditions.Desert climate zones have high solar irradiation which is desirable for solar power generation, but they also have harsh surrounding conditions such as high environmental temperatures, drastic temperature variations between day and night, and dusty environments, among other conditions. To allow a photovoltaic module to last at least thirty years, a proper set of packing materials and module design must be chosen according to the environmental characteristics of the actual climatic conditions where the module will be installed.In this work, the prominent materials to fabricate commercial photovoltaic modules are investigated in terms of reliability and electrical performance at field conditions. Any further improvement in optical transmittance of glass and encapsulant does not directly imply an enhancement of power generation. This is not only due to low efficiencies caused by operating temperatures compared to standard conditions, but to the large ohmic loss produced at high irradiance levels, effects that would not have been seen in measurements under standard test conditions. Making efforts to adjust and improve the optical transmittance of the glass-encapsulant interface makes little sense if the module ohmic losses are not reduced. For this reason, the tabbing ribbons must be carefully evaluated according to the targeted geographical location and to the current output of the solar cells.The properties of three solar cell types and several encapsulants have been evaluated to withstand ultraviolet doses, demonstrating that n-type solar cells are the suitable alternative for desert applications. In addition, the use of silicone as encapsulant is recommended to permit photons of higher energy to reach the solar cell, which allows an enhancement in module efficiency. Silicone also prevents the premature oxidation of the cell metallization and tabbing ribbons caused by the creation of acetic acid, as it could be the case of using ethylene-vinyl acetate.For desert applications, it has been demonstrated, up to a certain extent, that due to the significant difference of coefficient of thermal expansion between the glass, encapsulant and backsheet, the proper solution to withstand shear stress caused by the heating of the module packing materials is by using double glass packing design. This also places the solar cells to be in the neutral plane of the module hence the tension, compression and stress applied over the solar cells is lower than in a glass-foil design while the module is bent.Furthermore, the effect of soiling for two different desert locations is also observed and quantified. Modules made of flat glass coated with anti-reflection layers suffer larger optical losses caused by soiling compared to those with non-coated flat glass. If the rear side of a monofacial module is transparent, the effect of soiling is then slightly mitigated. Nevertheless, the power loss due to soiling is further reduced by using bifacial modules. In addition, it is shown that systems based on vertically mounted bifacial modules allow not only to complement the power generation profile during the day (single peak versus double peak curves), but it can also harvest higher annual energy compared to conventionally mounted monofacial modules due to the lower (or almost zero) dust accumulation.