Measuring and evaluating solar radiative properties of plastic shading nets

Plastic shading nets are being used extensively in hot and sunny regions to protect plants from intense solar radiation. Different varieties of these nets are commercially available. However, the choice of net to fulfill specific shading requirements often depends on empirical or economic criteria and not on scientific considerations because essential information used to characterize the different types of nets is not available.In order to replace expensive measuring facilities that use artificial lighting to investigate the radiative properties of plastic shading nets, a simple method was presented to investigate these properties under global, diffuse and direct beam solar radiation conditions. Measurements were performed on clear sunny days, (December 28, 2008-February 10, 2009). Nets with colors and shading factors that are most commonly used in hot regions were selected for the study. The results showed that the behaviors of the plastic nets under solar radiation conditions were similar to those of translucent materials. Accordingly, it is possible to (i) treat plastic nets as translucent materials and (ii) investigate the equivalent optical constants (i.e., refractive indexes and extinction coefficients) for plastic nets as functions of the net solidity, texture type and color. Solidity and color of the net had significant effects on the radiative properties and the effect of color was much more than the effect of solidity. The shading factor of a plastic net, is a function of the daytime, depends on several design and meteorological parameters, so it cannot be used to describe a net. However, the net solidity together with color, and the daily integrals of the shading factor and of the radiative properties are appropriate parameters for describing a net.     

Monthly optimum inclination of glass cover and external reflector of a basin type solar still with internal and external reflector

In this report, we present a theoretical analysis of a basin type solar still with internal and external reflectors. The external reflector is a flat plate that extends from the back wall of the still, and can presumably be inclined forwards or backwards according to the month. We have theoretically predicted the daily amount of distillate produced by the still throughout the year, which varies according to the inclination angle of both the glass cover and the external reflector, at 30°N latitude. We found the optimum external reflector inclination for each month for a still with a glass cover inclination of 10-50°. The increase in the average daily amount of distillate throughout the year of a still with inclined external reflector with optimum inclination in addition to an internal reflector, compared to a conventional basin type still was predicted to be 29%, 43% or 67% when the glass cover inclination is 10°, 30° or 50° and the length of external reflector is half the still’s length.     

New contact frame design for minimizing losses due to edge recombination and grid-induced shading

Edge recombination and grid shading are two loss mechanisms which decrease solar cell efficiency. We introduce a new way for decreasing both significantly by a novel contact frame design which runs along the edge on the surface of a solar cell. No additional processing is necessary for preparing the contact frame. For a 100 cm² commercial c-Silicon (Si) solar cell the efficiency increased from 16.18% to 16.83% at 1 Sun (AM 1.5) as estimated by careful device simulation.     

Daylighting and energy implications due to shading effects from nearby buildings

Daylighting has long been recognized as a potential energy-efficient design strategy for buildings. Natural light can help reduce the electrical demand and the associated sensible cooling load due to artificial lighting. In Hong Kong, however, many buildings are constructed close to each other and hence the external environment plays a significant role in daylighting designs. This paper investigates the shading effects due to nearby obstructions when daylighting schemes are being employed. We used the computer simulation tool, EnergyPlus, to illustrate the energy performance of a generic commercial building with daylighting controls obstructed by neighbouring buildings of various heights. Analysis of electricity savings was carried out for the perimeter zones of the whole building and individual floors. Regression techniques were conducted to correlate the building energy savings and the angles of obstructions. It was found that the shading effects due to nearby obstructions strongly affect the building energy budget when daylighting designs are used. Building designers should critically consider the external environment in order to achieve energy-efficient building designs.     

The impact of shading design and control on building cooling and lighting demand

Shading should be considered as an integral part of fenestration system design for commercial and office buildings, in order to balance daylighting requirements versus the need to reduce solar gains. In this paper, the simultaneous impact of glazing area, shading device properties and shading control on building cooling and lighting demand was calculated using a coupled lighting and thermal simulation module. The interactions between cooling and lighting energy use in perimeter spaces were evaluated as a function of window-to-wall ratio and shading parameters. An exterior roller shade was used as an example. The impact of shading device type, properties and control on building cooling and lighting energy demand was quantified and analyzed. The simulation results indicate that, if an integrated approach for automatic control of motorized shading is used in conjunction with controllable electric lighting systems, substantial reduction of energy demand for cooling and lighting could be achieved in perimeter spaces, depending on climatic conditions and orientation.     

Flexible polymer photovoltaic modules with incorporated organic bypass diodes to address module shading effects

We present experimental results on large-area low-cost processed flexible organic photovoltaic (OPV) modules incorporating organic bypass diodes to eliminate the negative effects of shading on the module power output. A fully organic-based structure (organic solar module combined with an organic bypass diode) is essential to allow monolithic interconnection of the bypass diode during the solar module production within the same printing steps. The origin of shading losses in organic photovoltaic modules is analyzed in detail, and guidelines for the design and architecture of flexible OPV modules are derived. Inorganic and organic diodes were tested on their functionality as bypass diodes, and a set of diode specifications to minimize shading losses is summarized. Organic bypass diodes were found to efficiently reduce the adverse shading effects in OPV modules.     

A model for managing and evaluating solar radiation for indoor thermal comfort

Thermal comfort of people occupying indoor spaces depends, to a large extent, on the direct component of solar radiation incident on the human body. In turn, even the diffuse component of the solar radiation could affect the thermal sensations of people. Despite this evidence, at the present there is a lack in the availability of simple and reliable methods capable of taking into account the influence of the solar radiation on thermal balance in the human body. In this work a comprehensive method is presented for the computation of the mean radiant temperature of people in thermal moderate indoor environments in the presence of solar radiation. The effects produced on the amount of solar radiation entering rooms in the presence of shadowing devices are also analysed. Finally, an application of the method is provided for a non-parallelepiped room equipped with a south window: results are shown in terms of the mean radiant temperature. A simple evaluation of thermal comfort conditions, referring to the present international standards, is also provided.The model can be easily linked to the computerized methods for analyzing the thermal behaviour of buildings, and is intended as a support for the thermal comfort evaluation methods.     

A study on optimum insulation thicknesses of external walls in hot summer and cold winter zone of China

The employ of thermal insulation is one of the most effective ways of building energy conservation for cooling and heating. Therefore, the selection of a proper insulation material and the determination of optimum insulation thickness are particularly vital. Four typical cities of Shanghai, Changsha, Shaoguan and Chengdu are selected to represent A, B, C and D subzone of hot summer and cold winter zone in China, respectively. The optimum thicknesses of five insulation materials including expanded polystyrene, extruded polystyrene, foamed polyurethane, perlite and foamed polyvinyl chloride are calculated with a typical residential wall using solar-air cooling and heating degree-days analysis and P₁–P₂ economic model. And then, life cycle total costs, life cycle savings and payback periods are calculated based on life cycle cost analysis. Considering different orientations, surface colors, insulation materials and climates, optimum thicknesses of the five insulations vary from 0.053 to 0.236 m, and the payback periods vary from 1.9 to 4.7 years over a lifetime of 20 years. The maximum life cycle savings are 54.4 $/m² in Shanghai, 54.8 $/m² in Changsha and 41.5 $/m² in Shaoguan (with a deep-colored northeast wall), and 39.0 $/m² in Chengdu (with a light-colored northwest wall). Finally, an approach to analyze economical efficiency of insulation materials is developed, result shows that expanded polystyrene is the most economic insulation material of the five because of the highest life cycle saving and lowest payback period.     

Nonlinear predictive control with dead-time compensator: Application to a solar power plant

This paper presents the application of a Nonlinear Model Predictive Controller (NMPC) extended with a dead-time compensator (DTC) to the control of a distributed solar collector field. This nonlinear controller uses the Nonlinear Extended Prediction Self-Adaptive Controller (NEPSAC) algorithm. A nonlinear grey-box model of the plant, based on first principles and tuned according to real measurements, is used in the simulation tests. The resulting controller is compared to other architectures based on DTC, showing very good performance for reference tracking and for disturbance rejection.     

Explicit model of photovoltaic panels to determine voltages and currents at the maximum power point

A simple explicit photovoltaic formulation for characterizing and dimensioning cell-arrays is presented. The method permits the short-circuit current, the open-circuit voltage, the maximum cell power and the optimum cell-operation conditions to be determined. Further, the model also allows quantifying the effects of panel temperature and solar irradiance on key cell parameters. Based on several datasheets, the methodology is validated by covering a wide range of operation conditions. The proposed approach can thus, be very useful for design engineers to quickly and easily determine the performance of any photovoltaic array without performing tedious numerical calculations.     

Determination of optimum insulation thicknesses of the external walls and roof (ceiling) for Turkey's different degree-day regions

The external walls and roof of a building are the interface between its interior and the outdoor environment. Insulation of the external walls and roof is the most cost-effective way of controlling the outside elements to make homes more comfortable. Although insulation is generally accepted as a factor increasing the building costs, with the calculations we have shown that this is not the case. Fuel consumption and operational costs are reduced by increasing the thickness of the external walls and roof (ceiling), despite an increase in the investment costs. According to Turkish Standard Number 825 (TS 825), there are four different degree-day (DD) regions, and the required heat loads for the buildings in these regions exhibit large differences. Therefore, a method based on costs is needed for the determination of optimum insulation thicknesses of different DD regions. In this study, optimum insulation thicknesses for different DD regions of Turkey, namely, Izmir (DD: 1450), Bursa (DD: 2203), Eski?ehir (DD: 3215) and Erzurum (DD: 4856), have been determined for a lifetime of N years, maximizing the present worth value of annual energy savings for insulated external walls.     

Analysis of a solarimetric database for Mexico and comparison with the CSR model

An analysis of the solar radiation database from the network of meteorological stations of the Mexican National Weather Service was carried out. The database includes global irradiance measurements from the oldest 136 stations distributed in the Mexican territory. The consistency of data acquisition from the launch of the stations until 2010 was checked, and visual inspection of graphs of daily irradiance data was carried out, for the first three years operation, to ensure quality and reliability of the data. The results indicate that less than half of the stations have an adequate regularity for data records. With a limited number of selected stations that passed the applied quality criteria, evaluation of hourly and daily global irradiations was carried out. These results were compared to satellite derived data for Mexico, based on NREL's CSR model. The results of the comparison show a good agreement between measured and modeled daily global solar irradiation with an average RMSE of 6.6%. Based on the selected stations, a daily irradiation mean of 5.5 kWh/m²/day is estimated for the country.     

The interaction of hydrogen jet releases with walls and barriers

It has been suggested that separation or safety distances for pressurised hydrogen storage can be reduced by the inclusion of walls or barriers between the hydrogen storage and vulnerable plant or other items. Various NFPA codes [1] suggest the use of 60° inclined fire barriers for protection against jet flames in preference to vertical ones. Work by Sandia National Laboratories [2] included experiments and modeling aimed at characterisation of the effectiveness of barrier walls at reducing hazards.This paper describes a series of experiments performed in order to compare the performance of 60° barriers with that of 90° barriers. Their relative efficiency at giving protection from thermal radiation and blast overpressure was measured together with the propensity for the thermal radiation and blast overpressure to be reflected back to the source of the leak.The work was primarily focused on compressed H₂ storage for stationary fuel cell systems, which may be physically separated from a fuel cell system or could be on board such a system. Different orifice sizes were used to simulate different size leaks; all releases were made from storage at 200 bar.Overall conclusions on barrier performance were made based on the recorded measurements.     

Performance model for parabolic trough solar thermal power plants with thermal storage: Comparison to operating plant data

This paper describes a simulation model that reproduces the performance of parabolic trough solar thermal power plants with a thermal storage system. The aim of this model is to facilitate the prediction of the electricity output of these plants during the various stages of their planning, design, construction and operation. Model results for a 50 MW_e power plant are presented and compared to real data from an equivalent power plant currently operated by the ACS Industrial Group in Spain.     

Analytical and quasi-explicit four arbitrary point method for extraction of solar cell single-diode model parameters

In this paper, the five parameters of the solar cell single-diode model are analytical and quasi-explicitly extracted for the first time, just using the coordinates of four arbitrary points of the characteristic I–V curve and the slopes of the curve in these points. The new method presented, called Analytical and Quasi-Explicit (AQE) method, is exact because no simplifications of the model nor a priori approximations of the parameters are used and, it is quasi-explicit in the sense that all the parameters except one are explicitly given. The unique parameter not explicitly computed is easily obtained by solving a five-degree polynomial equation. Accurate and practical conditions are provided to select which solution of the previous equation is the desired parameter.It is also introduced a very easy method to obtain, directly from real data measurements, the needed four points of the I–V curve as well as the slopes in these points, without using any kind of sophisticated techniques.Finally, some experimental results are presented to demonstrate the high accuracy and simplicity of the new method. The results are compared with the well-known analytical five-point method and the recent oblique asymptote method.     

Dynamic model of a solar thermochemical water-splitting reactor with integrated energy collection and storage

Water-splitting solar thermochemical cycles are important in meeting the challenges of global climate change and limited fossil fuels. However, solar radiation varies in availability, leading to unsteady state operation. We propose a solar receiver-reactor with integrated energy collection and storage. The reactor consists of a double-pipe heat exchanger placed at the focal line of a parabolic trough solar concentrator. Molten salt passes through the jacket, absorbing energy from the irradiated outer surface while driving the endothermic oxygen production step of the copper-chlorine water-splitting cycle in the reactor bore. Excess energy is stored in a thermal storage tank to buffer the reactor from changes in insolation. Dynamic simulation indicates that the reactor can sustain steady 100% conversion during 24/7 operation with a reasonable plant layout. The technology employed is extant and mature. This is important in view of the urgency to reduce dependency upon fossil fuels as primary energy sources.     

Prediction of hourly solar radiation using a novel hybrid model of ARMA and TDNN

In this work, a new approach that contains two phases is used to predict the hourly solar radiation series. In the detrending phase, several models are applied to remove the non-stationary trend lying in the solar radiation series. To judge the goodness of different detrending models, the Augmented Dickey-Fuller method is applied to test the stationarity of the residual. The optimal model is used to detrend the solar radiation series. In the prediction phase, the Autoregressive and Moving Average (ARMA) model is used to predict the stationary residual series. Furthermore, the controversial Time Delay Neural Network (TDNN) is applied to do the prediction. Because ARMA and TDNN have their own strength respectively, a novel hybrid model that combines both the ARMA and TDNN, is applied to produce better prediction. The simulation result shows that this hybrid model can take the advantages of both ARMA and TDNN and give excellent result.     

Twin cylinder alpha stirling engine combined model and prototype redesign

This article shows the development of a twin cylinder alpha type Stirling engine model, as well as the redesign of a prototype, its manufacture and preliminary results. The mechanical model is coupled with a thermodynamic model which allows an analysis and comparison of the theoretical and experimental behavior of the machine and a better understanding of the whole process. On the other side, a prototype redesign is presented. Thermal circuit was improved in order to eliminate losses, reduce dead volumes and improve heat transfer to the working fluid. Modifications of the seal system and the pistons design are included as well. An equation for the sizing of the flywheel is developed and finally the results obtained in tests between the new and the previous design are compared.     

Analytical model to relate DMFC material properties to optimum fuel efficiency and system size

In the design of the direct methanol fuel cell and the evaluation of new materials and their appropriateness for inclusion, it is helpful to consider the impact of material properties on the performance of a complete system: to some degree, methanol crossover losses can be mitigated by proper system design. As such, an analytical model is developed to evaluate the methanol concentration profile across the anode backing layer and membrane of the direct methanol fuel cell. The model is integrated down the anode flow channel to determine fuel utilization as a function of the feed concentration, backing layer properties, and membrane properties. A minimum stoichiometric ratio is determined based on maintaining zero-order methanol kinetics, which allows the fuel efficiency to be optimized by controlling these physical properties. This analysis is then used to estimate the required flow rates and the size of system components such as the methanol storage tank, based on the minimum methanol flow rate that those components must produce to deliver a specified current; in this way, the system-level benefits of reduced membrane crossover can be evaluated.     

Equivalent model of monocrystalline, polycrystalline and amorphous silicon solar cells

The current–voltage (I–V) characteristics of monocrystalline, polycrystalline and amorphous silicon solar cells are measured in the dark. A two diodes equivalent model is used to describe the electronic properties of solar cells. The non-linear curve fitting of the dark I–V curves obtain besides the diode ideality factors and the reverse saturation currents, the series and shunt resistance of the solar cells. These parameters determine the fill factor and the efficiency of the solar cells.