Predicting Early EVA Degradation in Photovoltaic Modules From Short Circuit Current Measurements

Abstract

One of the common failures in photovoltaic modules is the degradation of the ethylene-vinyl acetate (EVA) encapsulant due to prolonged ultraviolet exposure and other environmental stress factors, such as temperature and humidity. Experimental studies have shown that significant reduction in the optical transmission due to EVA degradation leads to loss in the available power by more than 50%. In this article, a novel approach to predict the early degradation of EVA encapsulant is proposed by correlating EVA degradation with short-circuit current (I SC ). An electrical circuit simulator, simulation program with integrated circuit emphasis (SPICE), is used to evaluate the short-circuit current obtained under varying optical transmission caused by EVA discoloration. The simulation follows three steps: simulation of the transmitted solar spectrum; simulation of the spectral short-circuit current density; and simulation of the current-voltage (I-V) curve to obtain short-circuit current (I SC ), maximum power output (P max ), opencircuit voltage (V OC ) and fill factor. Results show that the reduction in short-circuit current due to EVA degradation differs from the reductions expected due to a spectrally-uniform reduction of intensity of the solar irradiance. Both types of variation are linear, however, the slope due to EVA degradation is larger than the slope obtained for normal intensity variations in the solar irradiance. This model, when applied in conjunction with solar irradiance measurements, can predict early onset of EVA encapsulant failure, thereby enabling preventative measures to be taken.

Temperature Profiles of Field-Aged Photovoltaic Modules Affected by Optical Degradation

“…A change in the colour of encapsulant towards yellow and dark brown indicates optical degradation [1,7,12]. However, the effect of optical degradation could precede the change in the colour of the encapsulant [13].…”

“…The MID species can lead to delamination and discolouration of encapsulants, snail trails, potential induced degradation (PID), loss of adhesion, corrosion of metal grids and other components of the PV module [1,6]. These degradation mechanisms have also been found to serve as a precursor for other degradation mechanisms in PV plants [1,4,7,8].…”

“…Over time, the issue of optical degradation becomes more pronounced and can in the most severe cases constitute more than 50% degradation in the rated power output of the PV module [7]. The loss of power output is due to increased optical reflection due to "light decoupling" with reduced photon absorption in the active PV material [1,7,10]. This has dire implications for PV module efficiency and costs for operating PV plants over their guaranteed lifetime.…”

“…Hypothetically, variations in the module temperature coefficients can be traced to optical and/or electrical degradation [19]. According to earlier reports, optical degradation affects the electrical performance characteristics: especially the short circuit current (I sc ) and the fill factor (FF) of the PV modules, which lead to subsequent power degradation [1,7]. Optical degradation influences the electronic charge transport properties in the PV module bulk [20,21].…”

“…These chromophores increase the UV absorption efficiency of the encapsulant with its attendant increased module operating temperature [24]. Moreover, the chromophores can also absorb/block visible light, hence, reduce the amount of useable photons reaching the active solar cell materials [7,25]. This leads to accumulation of current in the affected areas accompanied with high localized inhomogeneous cell temperatures known as hotspots in defective modules [14,18].…”