•Cost reductions were enabled by time-flexible operation and load sizing for PV-powered systems.•Examined a theoretical reference system and an actual PV-powered EDR system in Chelluru, India.•39% ...cost reduction for the power system achieved due to time-flexible load operation.•Additional 5% cost reduction demonstrated due to optimizing the period of operation.
With their autonomous operation and low environmental impact, solar photovoltaics (PV) are an attractive power source for off-grid systems. However, the variable nature of solar energy is not well-suited to power conventional loads. Without careful consideration of the time-dependent power generation of PV, this discrepancy results in systems that are either over-designed and expensive, or compromise reliability. To accelerate the adoption of PV into new areas, it is essential to design PV-powered systems that are persistent, predictable, and affordable. In this paper, we analyze the cost reductions enabled by design optimization through time-flexible operation and improved load sizing. We consider two cases: (i) an idealized reference system, operating 8 h per day at 1 kW, generating an unspecified accumulable output, and (ii) a village-scale PV-powered electrodialysis desalination system, designed to generate 10 m3 of drinking water per day. We found that time-flexible load operation reduced the power system cost of the idealized reference system by 39%, from $2662 to $1628, and designing its electrical load to operate for an optimal period of time enabled an additional cost reduction of 5% (to $1503). For the village-based desalination system, we found that flexible operation paired with expected large decreases in membrane cost (from $150 to $20 per unit) reduced the associated power system cost by 57.6% from $8935 to $3788.
Flat-panel tandem solar cells have demonstrated the potential to exceed the efficiencies of their single-junction constituents. However, robust design rules for tandem solar cells are currently ...lacking, slowing the development of cost-effective implementations of this technology. A double-junction solar cell with four-terminal (4T) architecture stacks two electrically independent subcells and avoids current-matching losses, resulting in two main advantages over the conventional integrated two-terminal (2T) architecture: a higher energy yield and a loosened constraint on material bandgap combinations. Because both subcells are contacted independently in a 4T tandem, multiple stacked semitransparent contacts are needed, causing optical and series resistance losses. Moreover, for stationary flat-panel tandems, contacts need to be optimized for a varying direction of incident sunlight. In this study, we develop a framework for optimizing metal grid contacts for 4T tandem solar cells and quantify the electrical and optical loss associated with these contacts. We also explore the range of conditions for which it is beneficial to align metal grid contact fingers. We find that, for most applications, the front and back contacts of the top cell should be aligned, resulting in a decrease in energy yield loss between 5% and 15%, while aligning the bottom cell contacts is not beneficial and may even reduce total energy yield. Finally, we show that for nonideally matched subcell pairings, the contacts loss in a 4T tandem is not enough to counter the yield benefit of 4T over 2T tandems, while the contact loss may make the yield for more ideally matched subcells be comparable for 2T and 4T devices.
In this work a highly scattering rear Si surface texture (RST) is realized by plasma etching of polycrystalline silicon (poly-Si) thin-film solar cells on glass. The resulting RST shows reflection ...haze values of more than 95% at the Si–air interface. The average feature size of the texture is around 200nm. We use a model based on the scalar scattering theory to calculate the scattering properties of the textured surface. We also use a commercial thin-film solar cell simulator to evaluate the light trapping and current enhancement induced by the texture. Combining this submicron RST with a micrometer-scale glass texture can produce a multi-scale rear Si surface texture. Assuming a 1900nm thick poly-Si solar cell on glass with a high-quality back surface reflector (silicon dioxide/silver stack), the calculated photon density absorbed in the poly-Si solar cell with the multi-scale rear Si surface texture corresponds to a 1-sun short-circuit current density (jsc) of 31.1mA/cm2, which is 1mA/cm2 more than the calculated jsc of a poly-Si solar cell with the same thickness on textured glass but without RST. The calculated current densities do not fu lly take current loss due to parasitic absorption into consideration, hence are slightly overestimated.
•Plasma etching of poly-Si thin-film produces a submicron rear Si surface texture.•Combining the submicron texture with a glass texture produces a multi-scale texture.•The multi-scale silicon texture shows very good scattering properties.•A current density of 31.1mA/cm2 is estimated for a multi-scale textured solar cell.
In this study, we propose a geometric optical model to represent alkaline saw-damage-etched (SDE) surfaces of monocrystalline silicon wafers. An experimental study is carried out to characterize the ...optical properties of alkaline SDE surfaces on monocrystalline silicon wafers. Based on the surface characteristics measured by goniometry and height profiling, a geometric optical model is developed to describe the SDE surface with two parameters: characteristic angle and planar fraction. Using the path-tracing method, spectral reflectance simulations are carried out for four different types of samples. With the measured characteristic angle of 22° and planar fraction of 0.25 or 0.36, we find that this representation of SDE surface can predict the reflection and transmission with a root-mean-square error (RMSE) of the equivalent current density from 0.19 to 0.57 mA/cm 2 . The developed model is also applied to the optical loss analysis of aluminum local back surface field (Al-LBSF) solar cells with an SDE rear surface. We find that SDE rear surfaces provide better light trapping than planar surfaces. As a consequence, Al-LBSF solar cells with pyramids on the front and an SDE rear are predicted to produce 0.6 mA/cm 2 more photocurrent than similar cells with a planar rear surface.
Content Identifying Methodology or (CIM) was developed to measure public preferences in order to reveal the common characteristics of landscapes and aspects of underlying perceptions including the ...individual's reactions to content and spatial configuration, therefore, it can assist with the identification of factors that influenced preference. Regarding the analysis of landscape photographs through CIM, there are several studies utilizing image analysis software, such as Adobe Photoshop, in order to identify the physical contents in the scenes. This study attempts to evaluate public’s ‘preferences for aesthetic qualities of pedestrian bridges in urban areas through a photo-questionnaire survey, in which respondents evaluated images of pedestrian bridges in urban areas. Two groups of images were evaluated as the most and least preferred scenes that concern the highest and lowest mean scores respectively. These two groups were analyzed by CIM and also evaluated based on the respondent’s description of each group to reveal the pattern of preferences and the factors that may affect them. Digimizer Software was employed to triangulate the two approaches and to determine the role of these factors on people’s preferences. This study attempts to introduce the useful software for image analysis which can measure the physical contents and also their spatial organization in the scenes. According to the findings, it is revealed that Digimizer could be a useful tool in CIM approaches through preference studies that utilizes photographs in place of the actual landscape in order to determine the most important factors in public preferences for pedestrian bridges in urban areas.
One of the key issues of thin-film silicon solar cells is their limited optical absorptance due to the thin absorber layer and the low absorption coefficient for near-infrared wavelengths. Texturing ...of one or more interfaces in the layered structure of these cells is an important technique to scatter light and enhance the optical pathlength. This in turn enhances the optical absorption of the solar radiation in the absorber layer and improves the solar cell efficiency. In this paper we investigate the effects of textured glass superstrate surfaces on the optical absorptance of intrinsic a-Si:H films and a-Si:H p-i-n thin-film solar cell precursors deposited onto them. The silicon-facing surface of the glass sheets was textured with the aluminium-induced glass texturing method (AIT method). Absorption in both intrinsic silicon films and solar cell precursor structures is found to increase strongly due to the textured glass superstrate. The increased absorption due to the AIT glass opens up the possibility to reduce the absorber layer thickness of a-Si:H solar cells.
Microcrystalline silicon (μc-Si:H) thin-film solar cells are processed on glass superstrates having both micro- and nanoscale surface textures. The microscale texture is realised at the glass ...surface, using the aluminium-induced texturing (AIT) method, which is an industrially feasible process enabling a wide range of surface feature sizes (i.e., 700 nm–3 μm) of the textured glass. The nanoscale texture is made by conventional acid etching of the sputter-deposited transparent conductive oxide (TCO). The influence of the resulting “double texture” on the optical scattering is investigated by means of atomic force microscopy (AFM) (studying the surface topology), haze measurements (studying scattering into air), and short-circuit current enhancement measurements (studying scattering into silicon). A predicted enhanced optical scattering efficiency is experimentally proven by a short-circuit current enhancement Δ I sc of up to 1.6 mA/cm2 (7.7% relative increase) compared to solar cells fabricated on a standard superstrate, that is, planar glass covered with nanotextured TCO. Enhancing the autocorrelation length (or feature size) of the AIT superstrates might have the large potential to improve the μc-Si:H thin-film solar cell efficiency, by reducing the shunting probability of the device while maintaining a high optical scattering performance.
•Adaptive power management scheme results in more reliable PV-powered IoT devices.•The scheme can be based on either the available energy storage or solar irradiance.•It ensures persistence of data ...transmission despite unpredictability of sunlight.•It reduces the optimum cost by >10% and battery volume by 30%, for <3% less data.•The scheme is suitable for IoT applications with flexible data transmission rate.
Photovoltaics (PV) can provide power autonomy to sensors and communication devices comprising the Internet of Things (IoT). An outstanding challenge is to create design rules that transform intermittent and non-dispatchable solar energy into persistent, stable, and low-cost power for sensors. These design rules govern optimal system architecture design and optimal power consumption patterns. In previous work, we considered (i) the variability of solar insolation across different time scales (hourly, daily, and seasonal); (ii) the solar cell device characteristics; and (iii) the power consumption of the sensing and communication modules. From these constraints, we described a methodology to calculate the necessary energy storage capacity to reliably power the device over several years. The system size can then be optimized to minimize cost or volume. In this paper, we describe a methodology to improve the reliability of solar-powered IoT devices in the event of outlier conditions (i.e., periods of anomalously low insolation that reduce battery state-of-charge). This theoretical study, based on actual sensor and insolation data, describes how an adaptive power consumption scheme decreases the dimensions of solar cell and battery as well as system cost, assuming flexible operating scheme is allowed. The adaptive power consumption scheme means the data acquisition and transmission rate — which governs the power consumption profile — changes based on availability of stored energy in the battery and recent solar insolation. The results show for 3% reduction in average data transmission, the cost of the power components (i.e. battery and solar panel) can be reduced by 10% and the volume of the battery by 30%. The design is constrained by the available power as well as the minimum expected system performance, using the rate of acquired data from the sensor. We find that implementing an adaptive power consumption scheme based on available battery capacity for solar powered sensor can reduce battery capacity by 90%, while ensuring that data transmission rate occasionally varies but never extinguishes completely.
Light scattering at randomly textured interfaces is essential to improve the absorption of thin-film silicon solar cells. Aluminium-induced texture (AIT) glass provides suitable scattering for ...amorphous silicon (a-Si:H) solar cells. The scattering properties of textured surfaces are usually characterised by two properties: the angularly resolved intensity distribution and the haze. However, we find that the commonly used haze equations cannot accurately describe the experimentally observed spectral dependence of the haze of AIT glass. This is particularly the case for surface morphologies with a large rms roughness and small lateral feature sizes. In this paper we present an improved method for haze calculation, based on the power spectral density (PSD) function of the randomly textured surface. To better reproduce the measured haze characteristics, we suggest two improvements: i) inclusion of the average lateral feature size of the textured surface into the haze calculation, and ii) considering the opening angle of the haze measurement. We show that with these two improvements an accurate prediction of the haze of AIT glass is possible. Furthermore, we use the new equation to define optimum morphology parameters for AIT glass to be used for a-Si:H solar cell applications. The autocorrelation length is identified as the critical parameter. For the investigated a-Si:H solar cells, the optimum autocorrelation length is shown to be 320 nm.
We present a modeling framework to quantify current density losses and determine optimization steps for four-terminal tandem solar cells, which consist of a high-bandgap thin-film top cell and an ...industrial-type Si wafer bottom cell. We demonstrate the framework on the example of a 21.3% efficient stacked four-terminal GaAs/Si tandem solar cell. In a first step, we develop an optical model for stacked tandem configuration and carry out the current loss analysis for this tandem device. Photon absorption in the tandem is broken down into different channels. All parasitic absorptions that do not contribute to current generation are considered part of a loss channel. In a second step, we quantify the potential current gains from single-parameter optimizations, the redistribution of photon losses into other channels, and the synergistic gains that can be achieved by co-optimization. Finally, using the simulation model, we are able to determine the best sequence of optimizing the short-circuit current density in the GaAs/Si tandem solar cell to possibly achieve 37.8 mA/cm2 with experimentally feasible parameters.
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