Yong Sheng Khoo

Potential-induced degradation in photovoltaic modules: a critical review

Potential-induced degradation (PID) has received considerable attention in recent years due to its detrimental impact on photovoltaic (PV) module performance under field conditions. Both crystalline silicon (c-Si) and thin-film PV modules are susceptible to PID. While extensive studies have already been conducted in this area, the understanding of the PID phenomena is still incomplete and it remains a major problem in the PV industry. Herein, a critical review of the available literature is given to serve as a one-stop source for understanding the current status of PID research. This paper also aims to provide an overview of future research paths to address PID-related issues. This paper consists of three parts. In the first part, the modelling of leakage current paths in the module package is discussed. The PID mechanisms in both c-Si and thin-film PV modules are also comprehensively reviewed. The second part summarizes various test methods to evaluate PV modules for PID. The last part focuses on studies related to PID in the omnipresent p-type c-Si PV modules. The dependence of temperature, humidity and voltage on the progression of PID is examined. Preventive measures against PID at the cell, module and system levels are illustrated. Moreover, PID recovery in standard p-type c-Si PV modules is also studied. Most of the findings from p-type c-Si PV modules are also applicable to other PV module technologies.

Optimal Orientation and Tilt Angle for Maximizing in-Plane Solar Irradiation for PV Applications in Singapore

The performance of photovoltaic (PV) modules and systems is affected by the orientation and tilt angle, as these parameters determine the amount of solar radiation received by the surface of a PV module in a specific region. In this study, three sky models (Liu and Jordan, Klucher, and Perez et al .) are used to estimate the tilted irradiance, which would be received by a PV module at different orientations and tilt angles from the measured global horizontal irradiance (GHI) and diffuse horizontal irradiance (DHI) in Singapore (1.37°N, 103.75°E). Modeled results are compared with measured values from irradiance sensors facing 60° NE, tilted at 10°, 20°, 30°, 40°, and vertically tilted irradiance sensors facing north, south, east, and west in Singapore. Using the Perez model, it is found that a module facing east gives the maximum annual tilted irradiation for Singapores climatic conditions. These findings are further validated by one-year comprehensive monitoring of four PV systems (tilted at 10° facing north, south, east, and west) deployed in Singapore. The PV system tilted 10° facing east demonstrated the highest specific yield, with the performance ratio close to those of other orientations.

Detailed Current Loss Analysis for a PV Module Made With Textured Multicrystalline Silicon Wafer Solar Cells

We present a top-down method to quantify optical losses due to encapsulation of textured multicrystalline silicon wafer solar cells in a photovoltaic module. The approach is based on a combination of measurements and mathematical procedures. Seven different loss mechanisms are considered: 1) reflection at the glass front surface, 2) reflection at the metal fingers, 3) reflection at the textured solar cell surface, 4) absorption in the antireflection coating, 5) absorption in the glass pane and the encapsulation layer, 6) front surface escape, and 7) losses due to a non-perfect solar cell internal quantum efficiency. Losses for each of these mechanisms are obtained as a function of wavelength, and the corresponding current loss for each loss mechanism is calculated. Comparing simulated and measured results, the method predicts the module quantum efficiency with an error of less than 2% and the collected current with an error of less than 1%. In the presented example, the biggest loss (7.4 mA/cm 2) is due to the nonperfect quantum efficiency, followed by reflection losses at the glass front (2.2 mA/cm 2) and absorption in the glass and encapsulation layer (1.1 mA/cm 2).

In-Situ Characterization of Potential-Induced Degradation in Crystalline Silicon Photovoltaic Modules Through Dark I–V Measurements

A temperature correction methodology for in-situ darkI-V(DIV) characterization of conventional p-type crystalline silicon photovoltaic (PV) modules undergoing potential-induced degradation (PID) is proposed. We observe that the DIV-derived module power temperature coefficient (γ_dark) varies as a function of the extent of PID. To investigate the relationship between γ_dark and DIV-derived module power (P_dark (T_s), measured in situ and at the stress temperature) two parameters are defined: change in the DIV-derived module temperature coefficient (Δγ_dark) and DIV-derived module power degradation at the PID stress temperature (ΔP_dark (T_s)). It is determined that there is a linear relationship between Δγ_dark and ΔP_dark (T_s). Based on this finding, we can easily determine the module γdark at various stages of PID by monitoring P_dark (T_s) in situ. We then further develop a mathematical model to translate P_dark (T_s) to that at 25 °C (Pdark (25 °C)), which is correlated with the module power measured at the standard testing conditions (PST C). Our experiments demonstrate that, for various degrees of PID, the temperature correction methodology offers a relative accuracy of ±3% for predicting PST C. Furthermore, it reduces the root-mean-square error (RMSE) by around 70%, compared with the PSTC estimation without the temperature correction.

Optimizing the Front Electrode of Silicon-Wafer-Based Solar Cells and Modules

Front electrode optimization is one of the important design considerations that affects the efficiency of a silicon wafer solar cell. The optimization of the front electrode is usually done to maximize cell efficiency at standard test conditions (STC). However, with increasing prices in silver, optimization of the front electrode should be done by taking into account the cost of the silver paste. In this study, optimization of the front electrode is done at the cell level at STC (dollars per watt peak), module level at STC (dollars per watt peak), and under real-world module conditions (dollars per kilowatthour), taking into account the cost of the silver paste. For commercial screen-printed multicrystalline silicon wafer solar cells, it is found that to achieve the most cost-effective cell design at the outdoor module level (dollars per kilowatthour), the number of front metal fingers can be strongly reduced (by more than 20) compared with a conventional cell design, which is maximized for STC cell efficiency. For silver price of

Comparison of Angular Reflectance Losses Between PV Modules With Planar and Textured Glass Under Singapore Outdoor Conditions

1286/kg, optimization at the cell and module level for lowest cost will yield up to 1% cost savings compared with optimization for maximum efficiency. Optimization for the lowest levelized cost of electricity (LCOE) will yield on average 0.6% lower LCOE compared with optimization for maximum annual energy output.

Quantitative analysis of relationship between leakage current and power loss of multi-crystalline silicon photovoltaic module during potential-induced degradation test

Photovoltaic (PV) modules are rated under standard test conditions with normally incident light, whereas under outdoor conditions photons arrive on a PV module surface at all angles. In this study, the angular losses of PV modules with a planar and textured front glass are investigated for the tropical conditions of Singapore. Angular reflectance for both modules is first measured using a goniophotometer. From the measurements, the angular loss factors due to the different radiation components are calculated. Then, the angular losses under Singapore outdoor conditions are determined using three transposition models: Liu and Jordan, Hay and Davies, and Perez et al. Two 60-cell PV modules with a planar and textured glass are fabricated and measured outdoors for validation of the modeling results. Outdoor measurement results show that the PV module with textured glass captures 1.4% extra light compared with the PV module with planar glass for the 6-mo period studied. Next, the annual angular loss, monthly angular loss, and typical meteorological day angular loss for both module structures are studied. For all cases studied, the PV module with textured glass has consistently lower angular losses compared with the PV module with a planar glass.

Bifacial solar cell measurements under standard test conditions and the impact on cell-to-module loss analysis

The relationship between the leakage current and the power loss of a multi-crystalline silicon photovoltaic module during potential-induced degradation (PID) tests was analyzed. Since the current flowing into cells through a cover glass and an ethylene–vinyl acetate encapsulant is highly related to Na+ ion migration, which is presumed to be the main cause of PID, a setup for accurately measuring the current was designed. PID tests were also conducted under different temperature and voltage conditions following the same setup. From the current measured during PID tests, the charge transferred onto the active cell area was estimated. It was found that there is a one-to-one relationship between the charge transferred onto the active cell area and the power loss of a module. This result suggests that the PID power loss is due to the amount of Na+ ions accumulated on the active cell area.

Evaluation of transposition and decomposition models for converting global solar irradiance from tilted surface to horizontal in tropical regions

Bifacial cells are conventionally measured using gold-plated chuck, which is conductive and reflective. This measurement setup does not portray the actual operating conditions of the bifacial cells in a module. The reflective chuck causes an overestimation of the current due to the cell transmittance for the infrared light. The conductive chuck creates a shorter current flow path in the rear side of the cell and causes an over inflation of the fill factor measurement. In this study, we characterize and quantitatively analyze the difference between the bifacial cell measurements on different mounting chucks and calculate the cell-to-module (CTM) loss. To characterize the optical behavior of the bifacial cell and module, we perform external quantum efficiency, reflectance and transmittance measurements. The electrical behavior of the bifacial cell is studied using in-house developed software Griddler. Using Griddler, we calculate the difference in the fill factor of the bifacial cell due to the measurement using a conductive and non-conductive chuck, and estimate the corresponding CTM resistive losses.