Timothy M. Walsh

Photocatalytic superhydrophilic TiO2 coating on glass by electrospinning

A photocatalytic, superhydrophilic, transparent, porous TiO2 film consisting of rice-shaped nano/mesostructures deposited on glass substrates by electrospinning has been developed. Optical properties such as transmittance and absorbance, as well as the superhydrophilicity and self-cleaning properties (photocatalysis) of the deposited TiO2 film are studied. The water contact angle decreases with increase in the thickness of the TiO2 resulting in superhydrophilic transparent coatings which may be used for applications in window coatings and photovoltaic modules. The self-cleaning property of the TiO2 film consisting of rice-shaped nano/mesostructures is compared with that of commercially available Degussa P-25 in the photodegradation of Alizarin red dye and found to be more effective.

Electrospun SiO2 nanofibers as a template to fabricate a robust and transparent superamphiphobic coating

A one-dimensional morphology of electrospun nanofibers has been used as a template to fabricate a robust and transparent superamphiphobic coating. The template is created by the deposition of a thick layer of SiO2 nanofibers on glass. The developed template (SiO2 nanofibers) is coated with an ultrathin (25 nm) porous silica membrane by the vapor deposition technique. After heat treatment (600 °C), a transparent, superhydrophilic coating consisting of a hybrid silica network (SiO2 nanofibers enclosed by the silica membrane) is obtained. It is observed that during the heat treatment process, the coated silica membrane reinforces the SiO2 nanofibers and prevents the fibers from disintegrating into nanoparticles, resulting in the formation of a hybrid silica network. The fiber morphology assisted the hybrid silica network to keep its roughness and surface texture. After silanization, the coating exhibited superamphiphobic property with surface contact angles achieved using water and hexadecane are 161° and 146.5°, respectively.

Performance Degradation of Various PV Module Technologies in Tropical Singapore

The performance of ten photovoltaic (PV) modules with nine different solar cell technologies (and one different module construction) is monitored in the tropical climate of Singapore. The types of modules included in this study are monocrystalline Si (glass-backsheet with frame and glass-glass without frame), heterojunction crystalline Si, monocrystalline Si back-contact, multicrystalline Si, double-junction “micromorph” Si, single-junction/double-junction amorphous Si, CdTe, and CIGS. Three years of outdoor monitoring data are used to extract degradation trends of the performance of the various modules. Statistical decomposition methods are used to extract trends for performance ratio (PR), short-circuit current (ISC), open-circuit voltage (VOC), and fill factor (FF). The degradation rates of the monocrystalline Si modules are found to be equal to or less than -0.8% per year, mainly contributed by the decrease in ISC. The multicrystalline Si module shows a slightly higher degradation rate of -1.0% per year. The amorphous Si, micromorph Si, and CdTe modules show degradation rates of around -2% per year. The CIGS module showed an exceptionally high degradation rate of -6% per year. The decrease in FF and VOC is found to be significant for all the thin-film modules but not for the crystalline silicon modules.

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).

Effect of Solar Spectrum on the Performance of Various Thin-Film PV Module Technologies in Tropical Singapore

The spectral influence on the performance of four different thin-film photovoltaic (PV) modules (single-junction amorphous Si, CdTe, CIGS, and double-junction “micromorph” Si) is studied. Two methods are used to quantify the effective irradiance intensity for the investigated four modules. The first is based on spectral mismatch factors calculated from the measured outdoor spectrum and the spectral responses of the PV modules. The second is based on the measured short-circuit currents of the modules. The effective irradiance ratio (EIR) of a PV module technology, for a given time period, is defined as the ratio between the cumulative effective irradiance intensity received by the module and the cumulative irradiance intensity measured by a c-Si reference cell. We find that the average photon energy of the spectrum in Singapore is higher than that of the AM1.5G reference spectrum, indicating that the spectrum is “blue-rich.” Compared with the AM1.5G spectrum, this blue-rich spectral irradiance results in an annual EIR of 1.07 and 1.03 for the single-junction a-Si module and the CdTe module, respectively. An EIR larger than 1 indicates energetic irradiance gain. CIGS is not significantly affected by the blue-rich spectrum, while micromorph Si shows an annual EIR of less than 1.

A Quantitative Analysis of Photovoltaic Modules Using Halved Cells

In a silicon wafer-based photovoltaic (PV) module, significant power is lost due to current transport through the ribbons interconnecting neighbour cells. Using halved cells in PV modules is an effective method to reduce the resistive power loss which has already been applied by some major PV manufacturers (Mitsubishi, BP Solar) in their commercial available PV modules. As a consequence, quantitative analysis of PV modules using halved cells is needed. In this paper we investigate theoretically and experimentally the difference between modules made with halved and full-size solar cells. Theoretically, we find an improvement in fill factor of 1.8% absolute and output power of 90 mW for the halved cell minimodule. Experimentally, we find an improvement in fill factor of 1.3% absolute and output power of 60 mW for the halved cell module. Also, we investigate theoretically how this effect confers to the case of large-size modules. It is found that the performance increment of halved cell PV modules is even higher for high-efficiency solar cells. After that, the resistive loss of large-size modules with different interconnection schemes is analysed. Finally, factors influencing the performance and cost of industrial halved cell PV modules are discussed.

Comparison of Glass/Glass and Glass/Backsheet PV Modules Using Bifacial Silicon Solar Cells

Bifacial solar cells can be encapsulated in modules with either a glass/glass or a glass/backsheet structure. A glass/backsheet structure provides additional module current under standard test conditions (STC), due to the backsheet scattering effects, whereas a glass/glass structure has the potential to generate additional energy under outdoor conditions. In this study, we quantify the current contributions due to various mechanisms in both module structures under STC. The current contributions due to different mechanisms are calculated by measuring the reflectance and transmittance of mini-modules with both structures, together with a MATLAB-based simulation. Our results show that under STC, glass/backsheet modules provide approximately 2.2% more power, as compared with glass/glass modules using the same bifacial solar cells with a standard cell gap of 2.0 mm. Using module optimization, we demonstrate that the maximum possible cost reduction benefit in

On the spectral response of PV modules

/WP of glass/backsheet modules over glass/glass modules under STC is limited to 3.3%. Due to the potential outdoor energy yield advantages of glass/glass modules reported in the literature, we recommend a glass/glass module structure for bifacial solar cells. Furthermore, in order to compensate for the lower performance of glass/glass modules under STC, we propose a methodology to measure and fairly rate bifacial glass/glass photovoltaic (PV) modules.

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

The spectral response of silicon wafer based and thin-film photovoltaic (PV) modules is studied using simulation and experimental methods. Circuit simulations show that the module spectral response (SR) depends on (1) the SR of the cells, (2) the shunt resistance Rshunt of the cells, and (3) the bypass diodes of the module. For realistic Rshunt values, the module SR is significantly higher than the minimal SR of the individual cells (which would be the module SR in the case of infinite Rshunt). Round-robin tests using different experimental methods (partial illumination and full-area illumination) to determine the SR of a wafer-based module and a thin-film silicon module were conducted. Both SR methods are found to agree reasonably well. However, circuit simulations indicate that if only one cell, or a few cells, within the module have significantly different characteristics but not known, the results may differ considerably. The partial illumination method can access the SR of the individual cells within a module, but it possibly requires a long measurement time in order to measure the SR and Rshunt of each cell for confirming the SR of the whole module. In contrast, full-area illumination methods measure the module SR directly, but they cannot access the cell SRs if problematic cells exist. An uncertainty analysis of the full-area illumination method is conducted, which reveals that?if the calibrated reference cell is chosen properly?the calibration uncertainty of the reference cell itself is the main source of uncertainty.

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

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