Bifacial Photovoltaic Research Articles

S-doped CQDs tailored transparent counter electrodes for high-efficiency bifacial dye-sensitized solar cells

Bifacial photovoltaics are promising devices for significantly harvesting solar energy and generating power from either front or rear side. We present here the fabrication of new category of high-performance bifacial dye-sensitized solar cells (DSSCs) tailored with transparent S-doped carbon quantum dots (S-CQDs)/metal selenide counter electrodes. Arising from photoexcitation of S-CQDs and therefore increased electron density at electrode surfaces, the catalytic ability of S-CQDs tailored counter electrodes is markedly enhanced under solar irradiation. The bifacial DSSCs yield maximized front and rear efficiencies of 9.15% and 6.26%, respectively.

Assessment of Bifacial Photovoltaic Module Power Rating Methodologies—Inside and Out

One-sun power ratings for bifacial modules are currently undefined. This is partly because there is no standard definition of rear irradiance given 1000 Wcm−2 on the front. Using field measurements and simulations, we evaluate multiple deployment scenarios for bifacial modules and provide details on the amount of irradiance that could be expected. A simplified case that represents a single module deployed under conditions consistent with existing one-sun irradiance standards lead to a bifacial reference condition of 1000 Wcm −2 G front and 130–140 Wcm−2 G rear. For fielded systems of bifacial modules, G rear magnitude and spatial uniformity will be affected by self-shade from adjacent modules, varied ground cover, and ground-clearance height. A standard measurement procedure for bifacial modules is also currently undefined. A proposed international standard is under development, which provides the motivation for this paper. Here, we compare field measurements of bifacial modules under natural illumination with proposed indoor test methods, where irradiance is only applied to one side at a time. The indoor method has multiple advantages, including controlled and repeatable irradiance and thermal environment, along with allowing the use of conventional single-sided flash test equipment. The comparison results are promising, showing that indoor and outdoor methods agree within 1%–2% for multiple rear-irradiance conditions and bifacial module construction. A comparison with single-diode theory also shows good agreement to indoor measurements, within 1%–2% for power and other current–voltage curve parameters.

Two-dimensional current flow in stringed PV cells and its influence on the cell-to-module resistive losses

In a c-Si wafer based photovoltaic (PV) module, the current generated in each solar cell must flow to the ribbons soldered onto its front and rear surfaces. Conventionally, a simplified current flow pattern is assumed in calculating the resistive power losses in the cell plane and the ribbons. However, it is found in this work that this approach leads to more than 30% overestimation for the resistive loss on the rear-side ribbons for a typical monofacial solar cell. This work uses a detailed two-dimensional network simulation, which accurately solves the current flow patterns in both cell surface planes. It is found from this work that the conventional approach is sufficiently accurate in the case of an H-pattern finger-busbar metallization scheme, but not with a full-area metallization pattern, as commonly used on the rear of silicon wafer solar cells. For a c-Si PV module using 3-busbar monofacial solar cells, the actual resistive loss on the rear metallization plane is only 67% of the value calculated conventionally. Also, 17% of the cell interconnection resistive loss comes from the rear metal sheet, which is often ignored. To correct the results of the conventional approach, we introduce correction factors for c-Si PV modules using monofacial cells. Using the correction factors we achieve a significantly better prediction of the module’s fill factor. We apply this approach to explain why the fill factor gain from replacing full-size solar cells by halved cells in a c-Si PV module is higher for bifacial than for monofacial solar cells. Experimentally, we find a 1.7% relative fill factor increase for monofacial halved-cell PV module, and a 2.0% relative increase for bifacial halved-cell PV module. The simulation results agree quite well with the experimental results. Since the influence from the cell metallization pattern on the cell-to-module resistive loss has not been investigated before, this work can be very useful correction and compensation for established methods analysing the cell-to-module losses. It also offers the guideline for PV module optimization in terms of further reducing the resistive loss.

Simulation study of parameter estimation and measurement planning on photovoltaics degradation

Photovoltaics degradation is one of the key parameters in PV performance evaluation. Units under a degradation study can be either modules or systems. As a single set of degradation measurements based on one unit cannot represent the population nor be used to estimate true degradation of a particular PV technology, repeated measures through multiple units are essential. Linear mixed effects model is a suitable tool for analyzing longitudinal data. In this paper, I use LME model to explain the degradations in PV modules/systems which are installed at a shared location with modules of same technology. The degradation parameters including degradation rate can then be estimated using maximum likelihood estimation. Beside the degradation rate, other parameters of interest, e.g., the degradation distribution quantiles, are also derived to provide valuable information for PV manufacturers and system owners. Two types of measurements describe PV degradation, namely, a regression-based low-accuracy measurement through monitoring data (such as solar irradiance, module temperatures and various electrical parameters) and the flash test which can be considered as a high-accuracy measurement. Given the underlying true degradation of a set of units, the two methods differ mainly in measurement accuracy. The error differences between the low- and high-accuracy experiments are analyzed through simulation.

Transparent bifacial dye-sensitized solar cells based on an electrochemically polymerized organic counter electrode and an iodine-free polymer gel electrolyte

Bifacial transparent dye-sensitized solar cells (DSSCs) assembled with a highly transparent poly(3,4-ethylenedioxythiophene) (PEDOT) counter electrode (CE) and a non-absorptive iodine-free polymer gel electrolyte were developed. The PEDOT CEs were obtained via a facile in situ electrochemical polymerization method. Different charge capacities were applied in the polymerization process, and the PEDOT CEs were optimized with high transparency (>90 %) and favorable activity for catalyzing the reaction between the redox couples in the electrolyte. The gel electrolyte was prepared using an ionic liquid (1,2-dimethyl-3-propylinmidazolium iodide, DMPII) as the charge transfer intermediate and a polymer composite (PEO/PVDF-HFP) as the gelator without the addition of iodine. Besides high conductivity, the iodine-free electrolyte presented non-absorption in visible light region. The assembled bifacial DSSC device showed a power conversion efficiency (PCE) of 6.35 % under front-side irradiation, and 4.98 % under rear-side irradiation with intensity of 100 mW cm−2, which approached almost 80 % of that of the front-side irradiation. These promising results demonstrated the potential of these transparent organic CE and non-absorptive polymer gel electrolyte for further applications such as developing transparent bifacial photovoltaics.

The Application of Novel Platinum-Reinforced Tin-Silver-Copper Solder to Bifacial Photovoltaic Module for Improvement of Yield and Reliability

The characteristics of solder joints between the busbars of solar cells and copper ribbons can affect the performance of a photovoltaic (PV) module significantly. The resistivity of the joints and the intermetallic compound structures within the joints are the two main characteristics that impose a substantial impact on the yield and the reliability of the PV module. In this paper, we aim to present and analyze a novel platinum-reinforced tin-silver-copper (Sn-3.8Ag-0.7Cu-0.2Pt) as the lead-free solder material to connect copper ribbons to the metallization of bifacial solar cells. The performance of the PV module using platinum-reinforced solder is investigated by constructing two bifacial PV modules using the popular lead-free Sn-3.8Ag-0.7Cu solder and Sn-3.8Ag-0.7Cu-0.2Pt solder, respectively. Micrographs of the joints are obtained to show that the platinum-reinforced solder joint has fewer voids and a more evenly distributed and thinner intermetallic layer than that of a conventional SnAgCu solder joint. As a result, the physical attachment between the busbars and the ribbon using SnAgCuPt solder is stronger than that using SnAgCu solder. The power outputs of both PV modules are measured together with two commercial PV modules under the sun using an IV plotter. The results show that the total energy yield of the bifacial PV module with the new solder is about 6–10% higher than that with the conventional SnAgCu solder. The energy yield of the bifacial PV module using SnAgCuPt solder is 35.8% and 0.2% higher than that of the commercially available monofacial polycrystalline and monocrystalline PV modules, respectively.

Analysis of output power and capacity reduction in electrical storage facilities by peak shift control of PV system with bifacial modules

Bifacial photovoltaics are widely investigated with the aim of reducing the amount of silicon used and increasing conversion efficiencies. The output power of bifacial photovoltaics depends on the quantity of solar radiation incident on the reverse face. Furthermore, controlling the orientation can distribute the times of peak power output in the morning and afternoon to better match the demand. In this study, the demand patterns of individual houses or the whole Hokkaido region were analyzed assuming the substitution of a conventional large-scale electric power system with one using bifacial photovoltaics. The supply–demand balances and electrical storage capacities were investigated. When comparing a large scale solar power plant (mega-solar power plant) using monofacial photovoltaics or vertical bifacial photovoltaics (in which the orientation could be adjusted), the supply–demand could be better balanced for individual houses in the latter case, thereby allowing the storage capacity to be reduced. A bifacial solar module was modeled by 3D-CAD (three dimensional computer aided design) and thermal fluid analysis. The module temperature distribution of bifacial photovoltaics was calculated with respect to the environmental conditions (wind flow, direct and diffuse solar radiation, etc.) and internal heat generation, as well as the orientation of the solar panels. Furthermore, the output power of bifacial photovoltaics can be easily obtained from the analysis result of modular temperature distribution and the relation between temperature and output power.

Electrical characterization method for bifacial photovoltaic modules

We present a method to characterize bifacial photovoltaic (PV) modules for simultaneous front and rear side illumination (i.e., bifacial illumination). The method uses standard monofacial (i.e., single-sided) indoor current–voltage measurements on the front and rear sides of the bifacial module, while covering the other side with a non-reflecting black cover. To define the performance under bifacial illumination, new bifacial parameters (short-circuit current, open-circuit voltage, fill factor and efficiency) are calculated using a one-diode model of the PV module. Assuming linear current response of PV modules for different illuminations, the method numerically simulates the performance of bifacial PV modules for various combinations of front and rear side irradiances. The method is useful in predicting the behavior of the bifacial PV module under bifacial illumination, without actually measuring the module under these conditions. Using a silicon wafer based bifacial PV module, we demonstrate experimentally that the method can predict the performance to within 1% variation with actual measurements.

Effects of spectral albedo on solar photovoltaic devices

Although the spectral effects of direct and diffuse radiation on solar photovoltaic (PV) performance are relatively well understood, recent investigations have shown that there can be a spectral bias introduced due to albedo from common ground surfaces that can impact the optimal selection of PV materials for a known location. This paper extends analysis to the effects of spectral bias due to the specular reflectivity of 22 commonly occurring surface materials (both man-made and natural) and analyzes the albedo effects on the performance of seven PV materials covering three common PV system topologies: industrial (solar farms), commercial flat rooftops and residential pitched roof applications. An effective albedo is found for each surface material and PV material combination, which can be used in lieu of broadband albedo values in PV simulations. These results enable PV material selection for specific environments enabling geographic optimization for the micro-environment, while at the same time assisting optimal surface selection in the vicinity of existing or planned PV arrays. This analysis is of particular significance for the modeling of performance of bi-facial PV modules and vertical BIPV.

Output Characteristic Analysis of Partially Shaded Back Surface of Bifacial Photovoltaic Solar Module

Using an equivalent circuit model, the output characteristics of a bifacial photovoltaic solar module under various shaded conditions of the back side of the module were analyzed in detail. From the dark current-voltage (I-V) characteristics of bifacial photovoltaic solar cell, the diode parameters were extracted. The extracted diode parameters were applied to the equivalent circuit model for the bifacial photovoltaic solar module, and the solar-module performance was calculated. There was good agreement between the measured and calculated IV characteristics of the bifacial photovoltaic solar module under various shaded conditions.

Vertically mounted bifacial photovoltaic modules: A global analysis

Bifacial PV (photovoltaic) modules have recently come to increasing attention and various system designs have been investigated. In this paper, a global comparison is made between vertically mounted bifacial modules facing East–West and conventionally mounted mono-facial modules. An analytical method is used to calculate the radiation received by these two module configurations. It is found that the answer to the question which of these two module configurations performs better strongly depends on three factors: (i) the latitude, (ii) the local diffuse fraction and (iii) the albedo. In a subsequent part of the paper, the minimum albedo required to result in a better performance for vertically mounted bifacial modules is calculated for every place in the world. The calculation is based on measured data of the diffuse light fraction and the results are shown in the form of a global map. Finally, the albedo requirements are compared with the measured global albedo distribution. The calculation allows a distinct decision which module configuration is more suitable for a certain place in the world. The result is also shown as a map defining the corresponding areas.

Characterization of a Bifacial Photovoltaic Panel Integrated with External Diffuse and Semimirror Type Reflectors

Silicon wafer accounts for almost one-half the cost of a photovoltaic (PV) panel. A bifacial silicon solar cell is attractive due to its potential of enhancing power generation from the same silicon wafer in comparison with a conventional monofacial solar cell. The bifacial PV cell is able to capture solar radiation by back surface. This ability requires a suitable reflector appropriately oriented and separated from the cell’s rear surface. In order to optimize the bifacial solar cell performance with respect to an external back surface reflector, diffuse and semimirror reflectors were investigated at various angles and separations from the back surface. A simple bifacial solar panel, consisting of four monocrystalline Si solar cells, was designed and built. Reflection from the rear surface was provided by an extended semimirror and a white-painted diffuse reflector. Maximum power generation was observed at 30° with respect to ground for the semimirror reflector and 10° for diffuse reflector at an optimized reflector-panel separation of 115 mm. Output power enhancement of 20% and 15% from semimirror and diffuse reflectors, respectively, were observed. This loss from diffuse reflector is attributed to scattering of light beyond the rear surface capture cross-section of the bifacial solar panel.

Photovoltaic/thermal solar hybrid system with bifacial PV module and transparent plane collector

Electric energy production with photovoltaic (PV)/thermal solar hybrid systems can be enhanced with the employment of a bifacial PV module. Experimental model of a PV/thermal hybrid system with such a module was constructed and studied. To make use of both active surfaces of the bifacial PV module, we designed and made an original water-heating planar collector and a set of reflecting planes. The heat collector was transparent in the visible and near-infrared spectral regions, which makes it compatible with the PV module made of crystalline Si. The estimated overall solar energy utilization efficiency for the system related to the direct radiation flux is of the order of 60%, with an electric efficiency of 16.4%.

Bifacial Photovoltaic Panels with Sun Tracking

This paper analyzes the energy collected by bifacial photovoltaic panels that track the sun. A theoretical model is described that calculates the collection of light by both sides of a bifacial panel installed on a one- or two-axis tracker and placed against a variety of surroundings. The model has been verified experimentally, and then used to predict the annual energy collected at Madrid for a number of cases of practical interest. The results for two-axis tracked bifacial panels show that annual back energies of the order of 25% of the front energies can be obtained. This implies that the total (front plus back) annual energy collected by such panels can be 80% greater than that collected by a stationary monofacial panel, or some 30% greater than that collected by a stationary bifacial one.