The effect of plane booster reflectors on the performance of a solar air heater with solar cells suitable for a solar dryer

We present a theoretical study of a solar photovoltaic-thermal (hybrid) system consisting of a flat-plate solar air heater mounted with solar cells and a plane booster. A conventional flat-plate collector is converted into a hybrid system by mounting solar cells directly on the absorber plate. A hybrid system is self-sufficient in the sense that the electrical energy required by the pump is supplied by the panel. Such systems are well suited to applications such as solar drying. The combined system is analysed for the case when the radiative and absorptive properties of the cell surface and the absorber plate are nearly the same. The solar cell efficiency is a linearly-decreasing function of the absorber plate temperature. The performance of the system has been evaluated for various combinations of boosters. The minimum area of the solar cells required to run the pump at a given flow rate has been calculated as a function of time, with and without boosters. The minimum cell area required decreases with the use of boosters. High cost cells may be replaced by low cost reflectors. The solar air heaters presently available on the market are not suitable for direct conversion to hybrid systems.     

Design and performance studies of a solar dryer suitable for rural applications

This research paper is aimed at investigating means for food preservation using processes suitable for implementation in rural areas, where energy resources are scarce. Bearing in mind the low cost of capital investment and utilization possibilities, mainly agricultural drying, we have fabricated different types of solar dryers. Air flow in the drying system is by natural convection. The performance of the various types of solar dryers, along with a preliminary heat transfer analysis, is presented.     

太阳能电池残片再利用的优化排样算法

为了避免太阳能电池片因破损导致的浪费,提高太阳能电池残片的利用率,提出了一种基于图像的太阳能电池残片排样算法。首先通过边缘检测,从原始图像中提取出电池片有效区域。再在电池片有效区域上依据所设计的算法,依次按给定的小矩形尺寸进行排样。最后生成激光切割线,并对切割路径进行优化。实验结果显示,对于各种形态的破损电池片都能够得到较好的排样结果和切割路径,表明所设计的方法具有较好的实用和参考价值。     

Determination of the optimal solar investment decision criterion

This paper deals with the validity of the solar investment decision criteria employed in various studies. We begin by examining the life-cycle cost criterion (or positive net present value criterion) commonly used by solar analysts, and subsequently show that, given the theoretical hypotheses of dynamic investment planning and decision making, this criterion is suboptimal for evaluating the economic viability of fuel saver solar systems. The optimal “present cost competitive” criterion is then established and analyzed. The effect of uncertainty is introduced into the analysis by an examination of the payback period criterion. To highlight the differences between these criteria, a comparison of the timing of and net benefits derived from investments in a residential solar space and water heating system made under each criterion is presented.     

Uniqueness verification of direct solar spectral index for estimating outdoor performance of concentrator photovoltaic systems

To analyze the impact of a direct spectral distribution of the solar spectrum on the outdoor performance of concentrator photovoltaic (CPV) systems, an index for the direct spectral distribution is needed. Average photon energy (APE), the average energy of a photon in the direct solar spectrum, is one of these indexes. In this contribution, the uniqueness of APE to the direct solar spectral distribution is statistically analyzed to assure that an APE value uniquely yields the shape of a direct solar radiation spectrum. The results have exhibited the uniqueness of the direct normal solar spectrum with each APE value, in which the standard deviations are quite small. Short-circuit current density of the InGaP/InGaAs/Ge triple-junction solar cell in the CPV system is additionally calculated using the direct spectral irradiance with different APE values. It is revealed that APE is a useful index to describe the direct spectral distribution to evaluate the outdoor performance of the CPV systems.     

Investigation for the optimal thermohydraulic performance of artificially roughened solar air heaters

Artificially roughened solar air heaters perform better than the plane ones under the same operating conditions. However, artificial roughness leads to even more fluid pressure thereby increasing the pumping power. Roughness and flow parameters viz. relative roughness pitch p/e, relative roughness height e/D and flow Reynolds number Re have a combined effect on the heat transfer as well as fluid pressure (friction factor). Investigation for the optimal thermohydraulic performance (i.e. maximum heat transfer for minimum friction loss) of artificially roughened solar air heaters has been carried out. An optimisation parameter known as roughness Reynolds number which combines the roughness and flow effect and is expressed as has been considered. Thermohydraulic performance has been defined by the equation η_thermo=(St_r/St_s)³/(f_r/f_s). It has been found that e⁺_opt24 gives the optimal thermohydraulic performance in such collectors and therefore the optimal thermohydraulic performance curves [3], for designing such collectors for practical applications are suitable. The value of optimal thermohydraulic performance has been found to be about 71% corresponding to e⁺_opt=24.     

Yearly average performance of the principal solar collector types

The results of hour-by-hour simulations for 26 meteorological stations are used to derive universal correlations for the yearly total energy that can be delivered by the principal solar collector types: flat plate, evacuated tubes, CPC, collectors that track about one axis, collectors that track about two axes, and central receiver.The correlations are polynomials of first and second order in yearly average insolation, latitute, and threshold (= ratio of heat loss and optical efficiency). With these correlations the yearly collectible energy can be found by reading a single graph and multiplying the coordinates by the collector parameters. This simple method reproduces the results of hour-by-hour computer calculations with an accuracy (rms error) of 2 per cent for flat plates and 2–4 per cent for concentrators.This method can be applied to any system where the collectors operate year-round in such a way that no collected energy is discarded. This includes photovoltaic systems; solar-augmented industrial process heat systems; and solar thermal power systems. In addition, the method is recommended for rating collectors of different types or different manufacturers on the basis of yearly average performance. The method is also useful for evaluating the effects of collector degradation, the benefits of collector cleaning, and the gains from collector improvements (due to enhanced optical efficiency or decreased heat loss per absorber surface). For most of these applications, the method is accurate enough to replace a system simulation.     

Development of a suitable model for characterizing photovoltaic arrays with shaded solar cells

The aim of this study is to investigate the effects of non-uniform solar irradiation distribution on energy output of different interconnected configurations in photovoltaic (PV) arrays. In order to find which configuration is less susceptible to mismatch effects, a PV module model is developed. This model can take into consideration the effects of bypass diodes and the variation of the equivalent circuit parameters with respect to operating conditions. The proposed model can provide sufficient degree of precision as well as solar cell-based analysis in analyzing large scale PV arrays without increasing the computational effort. In order to produce more reliable and robust simulations, improved and extended algorithms are presented. Some results are discussed in detail and some recommendations are extracted by testing several shading scenarios.     

Determination of the optimal tilt angle and orientation for solar photovoltaic arrays

This paper deals with the determination of optimum tilt angle and orientation for solar photovoltaic arrays in order to maximize incident solar irradiance exposed on the array, for a specific period of time. The method is extended, by introducing a second objective, i.e. minimization of variance of the produced power, in terms of hourly power generation throughout the given period of time. The proposed method uses both well-established models and data collected from the particular area where the photovoltaic panels will be installed and is built upon four steps. In the first step, the recorded data are used in order to select the most accurate, among several isotropic and anisotropic models that can be found in the literature, for predicting diffuse solar irradiance on inclined surfaces. In the second step, the recorded data and the selected model are used to construct a database that contains the averages and the variances of the hourly global solar irradiance on tilted surfaces over specific periods of time, for various tilt angles and orientations. In the third step, the database of the previous step is utilized to produce meta-models that correlate tilt angle and orientation with mean global irradiance and its variance on tilted surfaces. Finally, an optimization problem is formulated, aiming to determining the optimum values of tilt angle and orientation, taking into account the constraints and limitations of the system.     

A presentation of solar irradiation data suitable for solar energy application

Temporal global solar radiation patterns are discussed for two climatically different U.S. stations (Phoenix, Arizona and Geneseo, New York). A concept of the solar utilization season is developed along with methodology to assess the frequency of periods of consecutive days when solar irradiation values do not attain specified threshold values. This analysis is considered more appropriate in applications of solar energy than the common use of means and assumed Gaussian data properties that can be obtained from several compilations and summaries of national solar irradiation data.     

Performance analysis of high-concentrated multi-junction solar cells in hot climate

Multi-junction concentrator solar cells are a promising technology as they can fulfill the increasing energy demand with renewable sources. Focusing sunlight upon the aperture of multi-junction photovoltaic (PV) cells can generate much greater power densities than conventional PV cells. So, concentrated PV multi-junction solar cells offer a promising way towards achieving minimum cost per kilowatt-hour. However, these cells have many aspects that must be fixed to be feasible for large-scale energy generation. In this work, a model is developed to analyze the impact of various atmospheric factors on concentrator PV performance. A single-diode equivalent circuit model is developed to examine multi-junction cells performance in hot weather conditions, considering the impacts of both temperature and concentration ratio. The impacts of spectral variations of irradiance on annual performance of various high-concentrated photovoltaic (HCPV) panels are examined, adapting spectra simulations using the SMARTS model. Also, the diode shunt resistance neglected in the existing models is considered in the present model. The present results are efficiently validated against measurements from published data to within 2% accuracy. Present predictions show that the single-diode model considering the shunt resistance gives accurate and reliable results. Also, aerosol optical depth (AOD) and air mass are most important atmospheric parameters having a significant impact on HCPV cell performance. In addition, the electrical efficiency (η) is noticed to increase with concentration to a certain concentration degree after which it decreases. Finally, based on the model predictions, let us conclude that the present model could be adapted properly to examine HCPV cells' performance over a broad range of operating conditions.     

A computational study on optimal design for organic tandem solar cells

A new model is developed by combining the optical model and the drift-diffusion model to optimize the thicknesses of active layers of individual sub-cells for high performance of organic tandem solar cell. When the photovoltaic properties of tandem organic solar cells based on poly(2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta (2,1-b;3,4-b′) dithiophene)-alt-4,7-(2,1,3-benzo-thiadiazole))/(6,6)-phenyl C71-butyric acid methyl ester and poly(3-hexylthiophene)/(6,6)-phenyl C61-butyric acid methyl ester are calculated as functions of thicknesses of individual sub-cells using the new model, it is found that the optimum thickness pair of active layers is 150 and 120 nm for the front and back sub-cell, respectively. Comparison of simulation with experiment reveals that the simulated results are very consistent with experimental ones.     

Uniqueness verification of solar spectrum index of average photon energy for evaluating outdoor performance of photovoltaic modules

To analyze the effect of a spectral irradiance distribution of solar spectra on the outdoor performance of photovoltaic (PV) modules, an index for the spectral distribution is needed. Average photon energy (APE) which represents the average energy per photons included in a spectrum is one of these. In this study, the uniqueness of APE to the spectral irradiance distribution was statistically analyzed to assure that an APE value uniquely yields the shape of a solar spectrum. The similar methodology adopted in International Electrotechnical Commission to rate the spectral matching of a solar simulator was used for the analysis. The results showed that an APE value yielded a spectral irradiance distribution with quite small standard deviation. The analysis using APE showed that the outdoor performance of crystalline Si PV modules depended almost only on a module temperature, while that of amorphous Si ones mainly depended on APE. The behaviors were reasonable considering from the operation mechanisms of the PV modules. These results demonstrate that APE is a reasonable and useful index to describe the spectral irradiance distribution for evaluating the outdoor performance of PV modules.     

Full-building radiation shielding for climate control in desert regions

The efficacy of full-building radiation barriers for use in climate control has been analyzed both analytically and experimentally. The mutually reinforcing results suggest that heat transfer through corrugated metal roofs can be dramatically reduced through the use of inexpensive, highly reflective sheets. The experiments were performed on two types of structures, one with a single metal roof and the other fabricated with a dual-roof system. In both cases, it was found that the radiative barriers provide excellent insulation. The reduction of heat transfer from the roof to the building proper was determined both analytically and through experiment. The excellent agreement between the two solution methods suggest that either technique is sufficient for future use.     

Matching of solar cells and performance of a solar battery

The paper seeks to extend the results reported by the authors at the Paris Conference of the ISES, 1973 and the Hamburg Conference, September 1974. Starting with the deduction that matching with the external load ensures operation at the maximum power point of the I–V characteristics of a solar cell and that the internal resistance and efficiency of a cell are dependent on intensity of the incident radiation, the effect of variation of insolation on the performance of a solar battery-motor system is examined in general terms. The cases of cells with series resistance dependent on and independent of insolation are separately considered and the desirability of introducing new criterion in design of motor, when driven by a solar battery, is stressed. The role of a concentrator, in preserving matching and uniformity of output, is pointed out and its optimum values for minimum variation, arising out of the diurnal variation of the AM-value, are obtained for typical Si and CdS cells. The more important parameters, determining the effective efficiency of solar cell in a battery using concentrator and storage system, are introduced and the expression for cost/W, as reported in Paris Conference, is further refined. Methods for estimation of these parameters, keeping in view the concentrator used, are also indicated. It is found that shadowing and non-uniform illumination of the solar cell panel can affect both the efficiency and matching of the system.     

High performance anti-reflection coatings for broadband multi-junction solar cells

The success of bandgap engineering has made high-efficiency broadband multi-junction solar cells possible with photo-response out to the band edge of germanium. Modeling has been conducted which suggests that current double-layer antireflection coating technology is not adequate for these devices in certain cases. Approaches for the development of higher performance antireflection coatings are examined. A new antireflection coating structure based on the use of Herpin equivalent layers is presented. Optical modeling of the new AR coating suggests a decrease in the solar weighted reflectance of 2.5% absolute over typically used double-layer antireflection coatings. This structure requires no additional optical material development and characterization because no new optical materials are necessary. Experimental results and a sensitivity analysis are presented.     

Novel solar cookers: suitable for small families

This paper presents the fabrication details and on-field experimental studies of two novel solar cookers, suitable for cooking requirements of small families; these are named as small family solar cookers (SFSC-1 and SFSC-2). Small size, good thermal performance, light weight, low-cost and short payback periods are some important features of these cookers. The values of some essential thermal performance parameters, first figure of merit (F₁), second figure of merit (F₂) and standard cooking power suggested by Bureau of Indian Standards and International Standard for box-type solar cookers, have been evaluated by experimental studies and found to be 0.116°C m²/W, 0.466, 30 W and 0.118°C m²/W, 0.488, 50 W for SFSC-1 and SFSC-2, respectively. A comparative analysis of the thermal performances of SFSCs with the solar cookers, developed by many authors, has also been presented here. The payback periods with respect to different cooking fuels for SFSCs have been found to be reasonably short.     

Improvement in long-term stability of dye-sensitized solar cell for outdoor use

To improve the intrinsic stability of the component of dye-sensitized solar cells (DSCs), we have fabricated the unit cell using solvent-free ionic liquid electrolyte. The degradation in the continuous 1 sun light soaking test at 60 °C over 15,000 h was effectively suppressed, compared with the cell using γ-butyrolactone electrolyte. The lifetime for outdoor use was estimated over 15 years from acceleration factor based on the outdoor exposure test. To confirm the stability of the DSC under practical outdoor use, we fabricated the solar light using the DSC modules, rechargeable batteries and bright light emitting diode (LED). The solar lights have been emitting a bright white light at night using the electricity from batteries charged by the DSC modules during the daytime in any weather condition for a half year.     

The equivalence of outdoor and mixed indoor/outdoor solar collector testing

An extension of the heat transfer analysis for a flat plate collector is presented in order to provide a single equation incorporating the effects of solar heating in the collector cover, variations in longwave atmospheric radiation and variations in the individual heat transfer coefficients in the collector.

By considering this equation it is shown that variations in longwave atmospheric radiation can influence the collection efficiency by up to 3 per cent, and that solar absorption in typical collector cover glasses also has an influence of 2–3 per cent.

The magnitude of the plate efficiency factor (F′) is shown to decrease as the collector temperature increases because it is a function of collector heat transfer coefficients. In poor collectors this effect can alter the collection efficiency by as much as 10–15 per cent.

The sum of these variations in collector efficiency is small for “good” collectors and it follows that the mixed indoor/outdoor test will produce quite representative results for these. However, when the variations are large, such as is the case for poor collector designs, then the mixed indoor/outdoor test will over-estimate the collection efficiency.     

High performance light trapping textures for monocrystalline silicon solar cells

Two novel texture schemes for the front of a c-Si silicon wafer solar cell are presented. The “bipyramid” texture is of two inverted pyramids of similar sizes laid out in alternating order. The “patch” texture uses a checkerboard layout of blocks of parallel grooves, with the grooves of alternating blocks perpendicularly oriented to each other. We estimate that these textures, which almost fully trap light for the first six passes through the substrate, can deliver better optical performance than the standard inverted pyramid texture, especially in narrow-band applications.