Moustafa M. Elsayed

Optimum orientation of absorber plates

Abstract Long-term analyses are presented to predict the optimum tilt angle of an absorber plate at any surface azimuth angle γ. The analyses include the effects of number of glass covers, latitude angle, monthly average clearness index, month, and ground reflectivity. The effects of each of these parameters on the optimum tilt of a south-facing surface are studied. Two numerical correlations, for ground reflectivity equal 0.2 and 0.7, are developed to predict the monthly optimum tilt of a surface. The two correlations are used to predict the optimum tilt of a surface over any specified period of time that extends from one month up to several months or a year. The analyses are also extended to predict the optimum tilt angle and azimuth angles of surfaces exposed to shading by surrounding objects. Illustrative examples are presented to demonstrate the application of the analyses.

Effectiveness of heat and mass transfer in packed beds of liquid desiccant system

A liquid desiccant system (using CaCl2) is presented for air dehumidification using solar energy or any other low grade energy to power the system. The system utilizes two packed beds of counterflow between an air stream and a solution of liquid desiccant for the processes of air dehumidification and solution regeneration. To simplify the prediction of the performance of the system an effectiveness of heat transfer and an effectiveness of mass transfer in the packed beds are defined. A finite difference model is developed to model the heat and mass transfer in packed beds during the air dehumidification mode and the solution regeneration mode. This finite difference model is used to calculate the effectiveness of heat and mass transfer in the packed beds at various bed heights, various air and solution flow rates, various inlet temperatures of air and solution to the bed, and various concentrations of CaCl2 solution at the bed entrance. Charts of the effectiveness of heat and mass transfer are presented in a convenient form. A designer of a liquid desiccant system may use the charts in predicting the performance of these systems without having to use the finite difference model for this purpose.

Performance of multiple effect diffusion stills

Abstract Experimental testing of a three-effect diffusion still is carried out. The experiment is designed to test the effect of the heating fluid temperature input to the still T h and the feed rate of desalinated water to the still F on the performance of the still. The results produced several empirical relations to predict the following in terms of the T h and F : the average temperature of each effect, the rate of distillate produced by each effect, and the performance ratio of the still. In addition, a mathematical simulation is given for the n -effect diffusion still. The mathematical model is verified by comparing its results with the experimental results of the three-effect diffusion still. The model is further used to predict the performance of a ten-effect diffusion still at the following parametric conditions: various values of the dimensionless parameters N 3 and N 4 , various heating medium temperatures input to the still, and various feed rates to the still.

Parametric study of a packed bed dehumidifier/regenerator using CaCl2 liquid desiccant

The processes occurring in a packed bed dehumidifier, which is part of a liquid desiccant solar cooling system, are mathematically simulated. The air flows in a counter flow direction to the liquid desiccant (CaCl2). The effect of varying the air, liquid flow rates and bed geometry are studied in addition to studying the effect due to varying the air and liquid desiccant inlet coditions. The inlet temperature of the liquid desiccant during the air dehumidification process has a strong effect on the other parameters, while the air inlet temperature has a negligible effect. It is also noticed that higher temperatures of air and while the air inlet temperature has a negligible effect. It is also noticed that higher temperatures of air and liquid desiccant enhance the liquid desiccant regeneration processes but by different ratios. The study showed that both the air and liquid desiccant flow rates have negligible effect on the bed exit humidity ratio of air whereas the liquid flow rate has a strong effect on the bed exit moisture content of the liquid. It is also observed that increasing the air flow rate enhances the liquid desiccant regeneration (air humidification) process. Regardless of the inlet moisture content of the liquid desiccant, it is found that as the product (LAs) gets larger, the exit air humidity gets less.

Analysis of air dehumidification using liquid desiccant system

Analyses are presented for air dehumidification using a liquid desiccant system. The expression for the coefficient of performance of the system is derived in terms of the operating and design parameters where the effect of each parameter can be directly identified. Also, the expression for the maximum coefficient of performance expected from the system for a given operating condition is deduced. The operating ranges of flow rates of air and solution are examined. A methodology is presented to determine the optimum air temperature for the reconcentration of liquid desiccant, and the optimum flow rates of air and solution in the system. In addition, a parametric study is carried out to find out the factors that contribute to the improvements of the coefficient of performance.

Measurements of solar flux density distribution on a plane receiver due to a flat heliostat

Abstract An experimental facility is designed and manufactured to measure the solar flux density distribution on a central flat receiver due to a single flat heliostat. The tracking mechanism of the heliostat is controlled by two stepping motors, one for tilt angle control and the other for azimuth angle control. A x-y traversing mechanism is also designed and mounted on a vertical central receiver plane, where the solar flux density is to be measured. A miniature solar sensor is mounted on the platform of the traversing mechanism, where it is used to measure the solar flux density distribution on the receiver surface. The sensor is connected to a data acquisition card in a host computer. The two stepping motors of the heliostat tracking mechanism and the two stepping motors of the traversing mechanism are all connected to a controller card in the same host computer. A software “TOWER” is prepared to let the heliostat track the sun, move the platform of the traversing mechanism to the points of a preselected grid, and to measure the solar flux density distribution on the receiver plane. Measurements are carried out using rectangular flat mirrors of different dimensions at several distances from the central receiver. Two types of images were identified on the receiver plane—namely, apparent (or visible) and mirror-reflected radiation images. Comparison between measurements and a mathematical model validates the mathematical model.

Yearly-averaged daily usefulness efficiency of heliostat surfaces

Abstract An analytical expression for estimating the instantaneous usefulness efficiency of a heliostat surface is obtained. A systematic procedure is then introduced to calculate the usefulness efficiency even when overlapping of blocking and shadowing on a heliostat surface exist. For possible estimation of the reflected energy from a given field, the local yearly-averaged daily usefulness efficiency is calculated. This efficiency is found to depend on site latitude angle, radial distance from the tower measured in tower heights, heliostat position azimuth angle and the radial spacing between heliostats. Charts for the local yearly-averaged daily usefulness efficiency are presented for Φ = 0, 15, 30, and 45°N. These charts can be used in calculating the reflected radiation from a given cell. Utilization of these charts is demonstrated.

Heliostat minimum radial spacing for no blocking and no shadowing condition

In order to fully utilize the total heliostat reflected area and collect the maximum solar radiation for central receiver plants, one should minimize the heliostat shaded and blocked areas. This condition must be satisfied during the day and around the year. Based on the criterion of no blockings and no shadowing, the heliostat fields is subdivided into zones. Each zone is characterized by a predominant shadowing or blocking effect. The minimum yearly radial spacing between two adjacent heliostats for no blocking and no shadowing condition is calculated for all zones. Results for sites at latitude angles of 101°, 20°, 20° and 40°, and for daily operation periods of 4, 6 and 8 hours are obtained.

Parametric study of a direct solar-operated, multiple-effect, diffusion still

Abstract Mathematical simulation is presented for a direct solar-operated, multiple-effect, diffusion still. The study has been prepared so as to predict the effect of the following parameters on the performance of the still: magnitude of solar radiation, flux received by the cover of the still, number of effects in the still, thickness of the diffusion gap between successive plates, rate of feed (per unit area) to each effect and the kind of diluent gas in the still. The results of the calculations showed that the operational efficiency is improved by: the increase in the magnitude of the solar radiation flux; reducing the diffusion gap thickness; increasing the number of effects; reducing the feed rate to each effect; and/or using helium instead of air as diluent gas. The effect of changing the diffusion gap thickness, feed rate to each effect, and/or the kind of diluent gas on the operational efficiency of the still becomes more pronounced as the number of effects in the still is increased.

CALCULATION OF SHADING FACTOR FOR A COLLECTOR FIELD

A method is presented to calculate the instantaneous shading factor of any element in a collector field caused by the relative position of other collectors located in the successive southern row. The collector field consists of rows of collectors parallel to the east-west direction and oriented facing the equator. The method takes into account the variation in: latitude angle, day of the year, time of day, collector tilt angle, length to width ratio of the collector, spacing distance between rows, and number of collectors in each row. By means of geometrical analysis of a dimensionless field and by separation of variables, the calculation of the shading factor is carried out using a scale coefficient and two sets of basic shading factors. For the special case when the collectors are tilted with an angle equal to the latitude angle, and for a field using collectors with ratio of length to width equal to 2, a chart is prepared to predict the average row shading factor. An illustrative example is finally given to demonstrate the application of the method.