Performance assessment of a solar still using blackened surface and thermocol insulation

Fluoride contaminated drinking water is a severe problem in many parts of the world because of fluoride-related health hazards, which are considered to be a major environmental problem today. The present work is aimed at utilizing solar energy for removal of fluoride from drinking water by using a “solar still”. Also tests have been conducted with the “solar still” to find out hourly output rate and “still efficiencies” with various test matrixes. It is observed that the distillate from “solar still” showed a fluoride reduction of 92–96%. Further, the efficiency of “solar still” got increased by 11% when capacity of water in the solar basin was raised from 10 to 20 L. Upon suitable modification of the solar basin with appropriate base liner and insulation, this efficiency of the “solar still” is found to be further increased by 6% with a 20 L basin capacity.     

Theoretical and numerical investigation of flow stability in porous materials applied as volumetric solar receivers

This article reports results of a theoretical analysis as well as a numerical study investigating the occurrence of flow instabilities in porous materials applied as volumetric solar receivers. After a short introduction into the technology of volumetric solar receivers, which are aimed to supply heat for a steam turbine process to generate electricity, the general requirements of materials applied as solar volumetric receivers are reviewed. Finally, the main methods and results of the two studies are reported. In the theoretical analysis it is shown that heat conductivity as well as permeability properties of the porous materials have significant influence on the probability of the occurrence of flow instabilities. The numerical study has been performed to investigate the occurrence of unstable flow in heated ceramic foam materials. In the simulations a constant heat flow of radiation, that is absorbed in a defined volume, and constant permeability coefficients are assumed. Boundary conditions similar to those of the 10 MW Solucar Solar project have been chosen. In a three dimensional, heterogeneous two phase heat transfer model it was possible to simulate local overheating of the porous structure. The parameters heat conductivity, turbulent permeability coefficient and radial dispersion coefficient have been varied systematically. Consequently, for a heat flux density of 1 MW/m² a parameter chart could be generated, showing the possible occurrence of “unstable” or “stable” thermal and fluid mechanical behaviour. These numerical results are beneficial for the design of optimized materials for volumetric receivers.     

Determining typical weather for use in solar energy simulations

Predicting the performance of a solar energy system by using simulation methods requires weather data input for the locality involved. The present paper describes a method of analyzing an optional number of years of weather data for a chosen month resulting in a “typical week” which is characterized in terms of solar radiation, ambient dry bulb temperature and wind speed. The “typical week” is allowed to vary in length between 5 and 10 days in the analysis in order to enable selection of a period that best represents a given month according to specified criteria.Verification of the method by comparative computer analysis was performed using two forms of weather data as inputs to the solar energy program “TRYNSYS”. The averaging method when compared to the “typical” weather method resulted in differences of less than 7 per cent.The use of “typical” weather appears to give results at least comparable with more established methods while at the same time providing a broad spectrum of the weather typical of an area. The use of “typical” weather can result in savings in computer time.     

Decision making in energy planning: the ELECTRE multicriteria analysis approach compared to a FUZZY-SETS methodology

Every planning activity generally requires to make some choices. After a preliminary analysis of the sector under examination, a forecast of trends of input–output items, the planner must define an action plan voted to arrange all the strategies and specific interventions able to fit demand and supply during the planned time. The redaction of an action plan implies a strong effort in order to synthesize either suggestions coming from the analysis phases either all the constraints linked to technical choices. In the same time, a large number of “external” variables plays a role in orienting decision making. Some of these can be handled by numerical models (economic cost-benefit analyses, market penetration strategies, environmental impacts). Other aspects, concerning social and cultural impact, political drawbacks, aesthetic aspects, etc., can be analysed only in a qualitative way, or are subjected to a not-objective judgement. This paper aims of introducing a methodological tool able to “organize” and “synthesize” the large set of variables coming from several specific judgements (or assessments) helping the “decision maker” to read the complex problem, understand it and make choices. Previous works of the authors have presented possible applications of “multicriteria-analysis”. In particular, the ELECTRE methods family, (Roy, B., Me´thodologie multicrite`re d'aide a` la de´cision, Economica, coll. ‘‘Gestion', Paris, 1985) will be presented under the point of view of energy planning application. It consists in a “flexible” ranking method which takes into account the uncertainties of all the specific assessments, the qualitative nature of some indexes, the weight of the preferences or willingness systems of the decision maker. On the other hand, similar results can be obtained through the application of fuzzy sets theory (Zimmermann, H. J., Fuzzy set Theory and its Application. Kluver Academic Publisher, 1987). A decision making support method, based on fuzzy logic is here tested and compared to the previous one. A case study developed by authors will show differences among these two different approachs. It is focused on the development of a renewable energy diffusion strategies plan. Advantages and drawbacks of both methods will be explored and some suggestions will be proposed.     

Numerical investigation of natural convection heat loss in modified cavity receiver for fuzzy focal solar dish concentrator

In the present work, a 2-D-model is used to investigate the approximate estimation of the natural convection heat loss from an actual geometry of the modified cavity receiver (hemisphere with aperture plate) of fuzzy focal solar dish concentrator. The analysis of the receiver has been carried out based on the assumption of the uniform and maximum solar flux distribution in the central plane of the receiver. The total heat loss from the receiver has been estimated for both the configurations “with insulation” (WI) and “without insulation” (WOI) at the protecting aperture plane of the receiver. The convection heat loss of the modified cavity receiver was estimated by varying the inclinations of the receiver from 0° (cavity aperture facing sideways) to 90° (cavity aperture facing down). The convection heat loss is maximum at 0° and decreases monotonically with increase in angle upto 90°. The effect of operating temperature on convection heat loss for different orientations of the receiver was studied. The results of the numerical analysis are presented for a modified cavity receiver “with insulation” (WI) and “without insulation” (WOI) in the form of Nusselt number correlation: and . The maximum convection heat loss occurs at 0° inclination for both cases of the receiver, which is 63.0% (WI) and 42.8% (WOI) of the total heat loss, though the heat loss in WI configuration is lower than that of WOI configuration. Upon increasing the inclination of the receiver, the convection heat loss reduces to a minimum of 12.5% (WI) and 24.9% (WOI) of the total heat loss at 90°. The result of the present numerical model of standard receiver configuration (modified cavity receiver with insulation at bottom) is comparable with other well-known models.     

A comparison between one year of daily global irradiation from ground-based measurements versus METEOSAT images from seven locations in Tunisia

Three numerical images from METEOSAT B2 per day have been processed over a period of 12 months, from October 1985 to September 1986, to estimate the daily values of available solar radiation in Tunisia. The methodology used, GISTEL, on the images of the “visible” channel of METEOSAT, is described. Results are compared with measured radiation values from seven stations of the “Institut de la Météorologie de Tunisie.” Among more than 2200 measured-estimated daily pairs, a high percentage, 89%, show a relative error of + or −10%. Many figures concerning Sidi-Bou-Saïd, Kairouan, Thala, and Gafsa are presented to show the capability of GISTEL to map the daily available solar radiation with a sufficient spatial resolution in countries where radiation measurements are too scarce.     

Cost predictions for photovoltaic energy sources

A preliminary assessment of the solar cell cost prediction art has been made, both in terms of a review and analysis of the prior literature and through an extension of these prior studies. The primary purpose of this study is to establish a basis for judging the relative credibility of the existing cost estimates.The initial step in this evaluation is to develop a self-consistent system of “reaonable” assumptions concerning the future market environment of solar cell energy sources. The second step is to establish the key assumptions which dominate the cost estimates obtained in prior studies. The results from these first two steps form the basis for judging the relative credibility of the existing cost predictions.Finally, an alternative set of cost estimates is generated. These alternative cost predictions are made for single-crystal Si cells (Czochralski vs “ribbon” growth), current technology Cu₂S---CdS cells, and a “generalized” thin-film photovoltaic device.     

The starting behaviour of a small horizontal-axis wind turbine

This paper presents selected results taken from an extensive investigation of the starting performance of a small horizontal-axis wind turbine. Starting was observed for blade pitch angles varying between 0 and 35° in 5° increments. At 0°, the angle for maximum power, the turbine’s 5 m diameter blades produce 5 kW at a wind speed of 10 m/s. At this pitch, starting is characterised by a long “idling period” in which the blade’s angular velocity increased only slowly because of the very high angles of attack. As the pitch angle increased, the idling period decreased. At all pitch angles, the measurements of angular velocity are compared with those obtained from a numerical integration of the equation for angular acceleration. The aerodynamic torque was obtained from a quasi-steady blade element analysis and the resistive torque of the drive train and generator was subtracted to determine the net torque accelerating the blades. The agreement between predicted and measured angular velocity was generally good and improved as the pitch angle increased.     

On the climatic optimization of the tilt and azimuth of flat-plate solar collectors

A numerical climatic model for computing total solar irradiance on the surface of a flat-plate collector, positioned at any tilt and azimuth, is described. Owing to a small time-step (one hour), and a quasi-realistic characterization of a collector's environment, the algorithm is able to produce credible estimates of both the climatically “optimal” position and the amount of energy lost to a collector when it is non-optimally positioned. Exemplary computations for Sterling, Virginia and Sunnyvale, California are presented and they suggest that the non-optimal positioning of a collector, e.g. as a simple function of latitude and a few highly summarized climatic-environmental variables, will not, in many cases, produce significant losses of available solar irradiance. In other situations, however, where a collector's horizon is significantly obstructed and/or the climatic environment of the area creates large diurnal or seasonal asymmetries in available irradiance, non-optimal positioning may cause sizeable energy losses. It is also apparent that even moderately sized horizonal obstructions, which are “seen” by a collector, can substantially reduce the amount of available irradiance, relative to an unobstructed horizon.     

Hybrid photovoltaic-thermosyphon water heating system for residential application

In order to improve the energy performance of the photovoltaic system, much effort has been spent on the research and development of hybrid PVT (photovoltaic-thermal) technology using water as the coolant. The fin performance of the thermal absorber is known to be one crucial factor in achieving a high overall energy yield of the collector. Accordingly, an aluminum-alloy flat-box type PVT collector was constructed, with its fin efficiency approaching unity. Its design is primarily for natural circulation and for domestic water heating purpose. Our test results showed that a high final hot water temperature in the collector system can be achieved after a one-day exposure. A numerical model of this photovoltaic-thermosyphon collector system was also developed and the model accuracy was verified by comparison with measured data. The energy performance of the collector system was then examined first, through reduced-temperature analysis, and second, as applying in the “hot summer and cold winter” climate zone of China. The numerical results are found very encouraging, and the equipment is capable of extending the PV application potential in the domestic sector.     

The performance results of Trombe-Wall passive systems under Aegean Sea climatic conditions

Since 1977, at the “Solar Energy Thermal Applications Laboratory of M.T.A.” in Marmaris, Turkey, an experimental study of a “Trombe Thermal Storage Wall” passive system continues in operation; two analogous houses have been built, one of which is serving as a reference system. The incident solar radiation on a vertical surface, the temperature changes of the internal and external surfaces of the wall, at interval of thirty minutes, and the convective flow through the “Trombe-Wall” have been measured in order to determine the solar fractions transmitted by the processes of radiation, convection and thermocirculation. In the present study, the variation of the “Trombe-Wall” efficiency has been calculated during the period of a year using a computer evaluation model in which the half hour measurement data have been introduced. The experimental results show that, according to the months, 15–35 per cent of the incident solar radiation over the south facade is transmitted to the interior through the “Trombe-Wall”, while the reference house transmits from the south facade 8–19 per cent of solar radiation received. Finally, it can now be confirmed that the results of the “Trombe-Wall” system without any special type insulation, contributes to practically the total heating load of the village houses under the sunny Aegean Sea Climatic conditions.     

Massive storage walls as passive solar heating elements: An analytic model

A simple analytic method for the prediction of the long-term thermal performance of passively-heated solar houses is presented. The treatment includes a new coarse method for “energy bookkeeping” and the use of a distribution function which represents the frequency of occurrence of different values of the solar load ratio. Due to its generality, this formalism is applicable to any passive heating element. As specific examples, the cases of direct gain and water wall houses are treated in detail. Relative to the parameterization of computer simulation results, this method offers the user a design tool that can be used to predict, in closed form, the thermal effect on the house of different building and climatic parameters and is not restricted to a “reference” building. Agreement with published numerical simulation results in satisfactory. The presentation is divided into two separate papers: a users guide for the reader who may not be interested in the details of derivation and validation, and the present paper, in which the theory is presented in detail.     

Oxide electrochromics: Why, how, and whither

Electrochromic oxides form the basis of “smart windows” which are able to provide energy efficiency and indoor comfort simultaneously. This paper attempts to give an introduction to “smart windows” technology, which finally seems to be ready for large-scale applications. The “whys” and “hows” are discussed from the viewpoints of materials, device technology, low-cost manufacturing aspects, and applications to buildings as well as niche products. Furthermore, there are some speculations as to the “whithers” of oxide electrochromics for applications to buildings of the future.     

Optimum performance of thermal trap collectors

The “thermal trap effect” in semitransparent material and the trapping system in the conventional flat-plate collectors with two, three or four glass or plastic covers with air-gaps in between are analysed under a common heading of “thermal trap collectors”. In general, a thermal trap collector consists of one or many slabs of semitransparent material of finite thickness with air-gaps in between, and an ideal withdrawal mechanism at the base of the trapping system to withdraw all available energy. This approach makes a comparative study of the two types of collectors possible, and provides data to design the appropriate withdrawal mechanism and operating conditions. A steady state analysis which neglects internal reflections and body radiation shows the existance of an optimum performance in single-layer thermal trap collectors and its dependence on thickness. A model which includes internal reflections is then analysed and the existance of the optimum performance and its dependence on thickness is demonstrated by taking the example of a single slab of methylmethacrylate plastic. The model is extended to multilayer thermal trap collectors and two examples are considered; a multilayer methylmethacrylate thermal trap collector and a multilayer “glass” thermal trap collector. The results show that the two-layer methyl methacrylate thermal trap collector has, in general, a better performance than the corresponding single or three and four-layer systems. But at high withdrawal efficiencies of about 60 per cent, the single layer methyl methacrylate shows its uniqueness and becomes competitive with the two-layer system. But the three and four-layer “glass” thermal trap collectors perform better than the corresponding single and two-layer ones, with the three-layer system having an overall better performance. These results show that the number of slabs in addition to thickness are important parameters in the study of the performance of thermal trap collectors.     

Health and safety issues for microwave power transmission

A general public perception that microwaves are hazardous has been a key obstacle for acceptance of microwave power transmission (MPT). This perception will eventually dissipate and then attention will focus on a real technical problem, that of interference (RFI). This can range from perceptible through annoying to hazardous. A program of actions is proposed to accelerate the goal of public acceptance of MPT.In this paper, a historical review shows that the solar power satellite (SPS) was reviewed a number of times relative to potential microwave exposure hazards. In all cases, no “show-stopper” was found but often the shibboleth “more research is needed” was aired. It is shown that standards for safe exposure to microwaves are the most important asset in convincing an audience that microwave exposure associated with MPT or SPS is safe. Standard-setting, world-wide, is shown to converge towards rational limits that are supportive of the MPT/SPS concepts. In recent times there has been the proposed substitute of “risk communication” (“prudent avoidance”). This is an unwise substitute for standards.Other aspects of microwave exposure standards are the new interface with RFI—hence the need for a rational division of responsibility between the radiators and the victim devices, like medical electronics—using both radiation limits and susceptibility limits. Beneficial applications of microwave exposure are being developed.Several studies are recommended which could put into perspective the likelihood of improbable events that represent “catastrophe”—e.g. the inadvertent focusing of a great amount of energy into inhabited areas.     

Outdoor rating conditions for photovoltaic modules and systems

Historically, flat-plate photovoltaic modules have been given a “peak-watt” rating indicating the power generated under 1000 W/m² global irradiance at a standard temperature. However, questions have arisen regarding the direct-normal irradiance, ambient or cell temperature, and wind speed (when it is specified) that should be used for evaluating the performance of flat-plate and concentrator modules. By studying the conditions that are observed when the global irradiance on a 2-axis-tracked surface is 1000 W/m², our analysis provides an objective, quantitative basis for the choice of the “peak-watt” rating conditions for both types of collectors. These observed conditions are consistent with commonly used values of 850 W/m² for direct-normal irradiance and 20°C for ambient temperature. Evidence is given that wind speed should be increased from the commonly used 1 m/s to a more frequently observed 4 m/s.     

Power loss in photovoltaic arrays due to mismatch in cell characteristics

Variations in the current-voltage characteristics of photovoltaic cells can lead to significant power loss “due to mismatch” when the cells are connected together in a network. This study explores how this mismatch loss depends on variations in max-power current and max-power voltage from cell to cell. An analysis of a series string is first performed. Losses in a parallel string are also determined. Estimates of mismatch losses in more complex arrays are then obtained. In addition to generally excellent comparison with several numerical studies, results show that, for a series string, there exists a critical magnitude of deviation in cell max-power current beyond which the power loss due to mismatch is sensitive to both the number of cells placed in series and the shape of the probability density function defining variations in max-power current. This critical level also depends on the cell fill-factor.     

Photoelectrochemical cells: Laboratory determination of solar conversion efficiencies

To define the solar-to-electricity conversion efficiency η of a regenerative photoelectrochemical (PEC) cell is a straightforward matter. The actual determination of η from real laboratory measurements is, however, far from trivial. In this paper the problem is analyzed in detail, based on two types of laboratory measurements: (i) the photocurrent as a function of photoelectrode potential at constant wavelength of illuminating light, and (ii) the photocurrent as a function of wavelength at constant potential. The analysis results in four somewhat different “recipes” for computation of η, based on two choices: (a) whether or not to “correct” for dark current, and (b) how experiments of type (i) and type (ii) are weighed together. The analysis is applied to experiments on III–V semiconductors. The η values computed according to the four equations are in good agreement; a recommendation is given to which one(s) to prefer. The η values depend on the air mass (AM) value assumed for the solar spectrum. Although reported data probably should refer to AM1, other AM values would be more realistic.     

Modelling and experimentation of the “Shell” dryer

The “Shell” dryer is an African solar food dryer using natural convection. Its design has been defined by users, in function of local working conditions and of its easy using. But it is necessary for the best behaviour of this dryer to optimize its air flow section design. We present here a theoretical study of a “rustic” process, through mathematical modelisation tools.     

Development of a tube-type solar still equipped with heat accumulation for irrigation

A tube-type solar still is found to be suitable for use in desert irrigation. The effectiveness of a heat accumulator with regard to distillate productivity is experimentally and numerically verified. The heat accumulator consists of tube bundles immersed in wax in order to utilize the latent heat of wax. The dynamic response to stepwise variation of irradiative intensity verified the contribution of wax to an increase of productivity only when the phase change of wax occurred. The effective distillate productivity was found to be 294.3 g/m² during the cyclic stepwise change of irradiative intensity, from 200 to 600 W/m² and back. Velocity vectors driven by natural convection and temperature contours estimated by numerical simulation verified the effectiveness of the heat accumulator especially after peak solar intensity. The latent heat of wax effectively contributed to a 15% increase in total distillate productivity per day. The still can feasibly meet irrigation water supply demands above an irrigative threshold of 17 MJ/m² d.