Effect of absorption of solar radiation in glass-cover(s) on heat transfer coefficients in upward heat flow in single and double glazed flat-plate collectors

Absorption of solar radiation in the glass cover(s) of a flat plate solar collector increases the temperature of cover(s) and hence changes the values of convective and radiative heat transfer coefficients. The governing equations for the case of single as well as double glazed collector have been solved for inner and outer surface temperatures of glass cover(s) with/without including the effect of absorption of solar radiation in the glass cover(s), with appropriate boundary conditions. The effects of absorption of solar radiation on inner and outer surface temperatures and consequently on convective and radiative heat transfer coefficients have been studied over a wide range of the independent variables. The values of glass cover temperatures obtained from numerical solutions of heat balance equations with and without including the effect of absorption of solar radiation in the glass cover(s) are compared. For a single glazed collector the increase in glass cover temperature due to absorption of solar radiation could be as high as 6°. The increase in temperatures of first and second glass covers of a double glazed collector could be as high as 14° and 11°, respectively. The effect on the convective heat transfer coefficient between the absorber plate and the first glass cover is substantial. The difference in the values of the convective heat transfer coefficients between the absorber plate and the first glass cover (h_cp1) of a double glazed collector for the two cases: (i) including the effect of absorption and (ii) neglecting the effect of absorption in glass cover, could be as high as 49%. Correlations for computing the temperatures of inner and outer surfaces of the glass cover(s) of single and double glazed flat plate collectors are developed. The relations developed enable incorporation of the effect of absorption of solar radiation in glass cover(s) in the relations for inner and outer surface temperatures in a simple manner. By making use of the relations developed for inner and outer surface temperatures of glass cover(s) the convective and radiative heat transfer coefficients can be calculated so close to those obtained by making use of surface temperatures of glass cover(s) obtained by numerical solutions of heat balance equations that numerical solutions of heat balance equations are not required.     

Combined natural convection and surface radiation in solar collector equipped with partitions

A comprehensive numerical study was carried out to study the thermal performances of a solar collector equipped with partitions attached to its glass cover. This technique is used to favorise and increase the heat transfer coefficient between the working fluid, (air) and the absorber in the solar collector. A specifically developed numerical model based on the finite-volume method and the SIMPLER algorithm is used for the solution of the mass, momentum and energy transfer governing equations. The discretized equations are solved iteratively by an algorithm based on a preconditioned conjugate gradient method. Concerning the radiation exchange, we consider that the working fluid is transparent, so only the solid surfaces contribute to the radiation exchange and assumed to be diffuse-gray. A complete parametric study is made for various partitions length Lp, aspect ratio A and Rayleigh numbers Ra for Prandtl number Pr = 0.71. Results are reported in terms of isotherms, streamlines and average Nusselt-numbers along the absorber plate.     

Radiative heat transfer in a solar air heater covered with a plastic film

This article presents a new set of equations for the radiative balances of the absorber plate and the transparent cover of a solar air heater covered with a plastic film. Air flow is supposed to pass between the absorber plate and the bottom of the collector, while the transparent cover and the absorber plate are separated by an immobile air layer. This configuration is shown to be the best suited for a plastic covered solar air heater used in tropical countries, from a practical point of view.     

Natural convection from an inclined plate embedded in a variable porosity porous medium due to solar radiation

Natural convection boundary-layer flow of an absorbing and electrically-conducting fluid over a semi-infinite, ideally transparent, inclined flat plate embedded in a porous medium with variable porosity due to solar radiation is considered. The governing equations are derived using the usual boundary-layer and Boussinesq approximations and accounting for the presence of an applied magnetic field and an applied incident radiation flux. To account for the heat loss from the plate surface, a convective-type boundary condition is employed there. These equations and boundary conditions are non-dimensionalized and transformed using a non-similarity transformation. The resulting non-linear partial differential equations are then solved numerically subject to the transformed boundary conditions by an implicit iterative finite-difference scheme. Graphical results for the velocity and temperature fields as well as the boundary friction and Nusselt number are presented and discussed for various parametric conditions.     

An experimental and analytical evaluation of the insulative properties of double glazing and thick curtains

Theoretical and experimental evaluations are made of thermal curtains applied to double-glazed greenhouse structures. The reflectance and emittance of the curtain is permitted to be different on the two surfaces: It may be partially transparent or opaque to thermal radiation, and it may have thick blanket insulative properties. The effect of cloud cover upon the night-sky radiation temperature is included in the model. Convection coefficients are prescribed for the curtain and glazed surfaces, and the effect of air leakage into the space between the curtain and inner layer of glazing is included as an empirically defined parameter. The thermal balance for the system is obtained in terms of the radiative, convective and air-leakage parameters relevant to each component. The resulting set of thermal balance equations are solved iteratively using existing matrix-solution programs at the University of Waterloo. An experimental rig was constructed to determine nighttime heat loss from a conventional double-walled polycarbonate glazing material which is partially transparent to thermal radiation. The rig was operated with and without an insulating curtain. An opaque multilayer curtain and a semitransparent bubble film curtain were tested. The agreement between the thermal model and the test rig was within ±5% for all cases. Cloud cover was found to be a significant parameter that alters heat loss upwards by 11% from fully cloudy to clear-sky conditions. Intermediate cloud cover conditions were investigated as well and were found to be well predicted by an existing correlation for sky temperature vs cloud cover. The analysis is extended to provide a simple resistance coefficient for heat loss from the glazed structure based upon inside and outside temperatures for a specified cloud cover. It is found that the semiopaque curtain and double-glazed combination has 3.74 times the thermal resistance of a single glass structure as compared to a value of 3.81 for the opaque curtain and double-glazed arrangement. In addition the semiopaque curtain can be used for solar shading during the summer time, thereby serving a dual purpose that significantly increases its economic advantage.     

Transmittance-absorptance product of solar glazing with transparent insulation materials

The transmittance-absorptance product of solar glazing containing the transparent insulation material (TIM) of square celled honeycomb is investigated. A method is developed for the determination of transmittance-absorptance product of beam, sky and ground diffuse solar radiations using the individual transmittances of cellular array and encapsulating covers; the internal reflections are taken into account. Three practical cases; cellular array, cellular array with top cover, and cellular array with top and bottom covers are considered. The results are presented for beam radiation as a function of angle of incidence and sky and ground diffuse radiation as a function of tilt angle. The predicted results are tested by measuring the global radiation transmittance of commercial TIM; the predicted results deviate from the measurements by an average of 2.0%.     

Effect of dirt on transparent covers in flat-plate solar energy collectors

The effect of dirt on the transmittance of solar radiation through various inclined glass plates and plastic films, which are used as a transparent cover for flat-plate collectors, has been studied. The dirt correction factor for glass plate inclined at an angle of 45 deg from the horizontal is 0·92, which is significantly different from the value of 0·99 given by Hottel and Woertz[1]. The correction factor is greater for plastic film than it is for glass plate for any inclination.     

Radiation heat losses in solar collectors with plastic covers

The use of plastic covers instead of glass covers in solar collectors requires a modification of the calculation of radiation heat loss, because plastics are, in contrast to glass, partly transparent for thermal radiation. This article describes a simple method which enables calculation of radiation heat loss in collectors equipped with plastic covers. Comparison of this method with an incorrect method used by some authors shows that the latter leads to large errors in some cases. The results of the correct method indicate that in some situations the glass covers can be replaced by much cheaper plastic covers with a negligible increase in collector heat loss.     

Theoretical and experimental studies on collector/storage type solar water heater

Extensive theoretical and experimental studies on a built-in-storage solar water heater which was developed earlier by Garg in India, are carried out. In this water heater the absorber plate performs the dual function of absorbing the solar energy and storing the heated water. In the theoretical study, the transient performance of the system is predicted by solving the mathematical models consisting of energy balance equations which are written on different collector nodes by considering their capacity effects and various heat loss effects. These equations are converted in the finite difference form and then solved by digital computer. Solar radiation and ambient temperature are represented by Fourier series in the theoretical analysis. Its night cooling drawback is somehow checked by covering the collector system by an insulation cover during cooling hours and also by using a insulated baffle plate inside the tank adjacent to the absorber plate. It is observed that by using the insulation cover, the collector performance can be improved by 70 per cent. Use of baffle plate improves the performance during day as well as night time.     

A pilot solar water disinfecting system: performance analysis and testing

In most countries, contaminated water is the major cause of most water-borne diseases. Disinfection of water may be accomplished by a number of different physical–chemical treatments including direct application of thermal energy, chemical and filtration techniques. Solar energy also can be used effectively in this field because inactivation of microorganisms is done either by heating water to a disinfecting temperature or by exposing to ultraviolet solar radiation. A pilot solar system for disinfecting contaminated water is designed, constructed and tested. Investigations are carried out to evaluate the performance of a wooden hot box solar facility as a solar disinfectant. Experimental data show that solar energy is viable for the disinfection process. A solar radiation model is presented and compared with the experimental data. A mathematical model of the solar disinfectant is also presented. The governing equations are solved numerically via the fourth-order Runge–Kutta method. The effects of environmental conditions (ambient temperature, wind speed, solar radiation, etc.) on the performance of the solar disinfectant are examined. Results showed that the system is affected by ambient temperature, wind speed, ultraviolet solar radiation intensity, the turbidity of the water, the quantity of water exposed, the contact area between the transparent water container in the solar disinfectant and the absorber plate as well as the geometrical parameters of the system. It is pointed out that for partially cloudy conditions with a low ambient temperature and high wind speeds, the thermal efficiency of the solar disinfectant is at a minimum. The use of solar energy for the disinfection process will increase the productivity of the system while completely eliminating the coliform group bacteria at the same time.     

Impact of coefficient of attenuation of solar radiation on thermal losses in translucent covers

The technique for calculating the impact of the attenuation coefficient of the passing total solar radiation of the translucent cover material and the intensity of the total (direct and diffuse) solar radiation incident on their frontal surface on their thermal losses is proposed. The multiple inner reflection of the radiation between the inner (x = δ) and outer (x = 0) interfaces and the mutual opposition of the directions of the solar radiation fluxes reflected at these interfaces, as well as the exponential distribution of the power of absorbed radiation over the thickness of the wall of the considered translucent covers are taken into account. The calculations performed based on the practical implementation of this technique are exemplified.     

A hybrid high efficiency single-basin solar still

In this work, an attempt has been made to enhance the distillate output of a single-basin solar still by coupling it with a flat plate solar collector and by coating a thin layer of SnO₂ on one side of the transparent cover plate. The heat transfer fluid was circulated between the still and the collector through a heat exchanger and storage tank by thermosyphonically induced flow. It was observed that good insulation around the storage tank considerably increased the yield at night due to the decrease of ambient temperature. Thermosyphonically induced flow eliminated the need for pumps and control units. A layer of SnO₂ on the transparent cover lowered the thermal radiation loss, one of the major sources of heat energy loss in a solar still. Values for solar radiation, ambient air temperature, salt-water temperature in the basin, temperature of air-vapour mixture within the still, input and output temperatures of the heat exchanger heat transfer fluid, and the distillate yield were collected by a data acquisition system. After making the above modifications, the distillate yield was measured to be 6·745 litres per square metre per day for a September solar radiation of 17820 KJ at Istanbul-Gebze. To compare the distillate yield, a conventional solar still with similar dimensions to those of the improved still was constructed. The overall efficiency of the improved still was found to be 3·26 times the efficiency of the conventional still.     

Comparative analysis of active and passive solar heating systems with transparent insulation

The objective of this research is to achieve a high solar fraction in social housing, for which investment costs are strictly limited. Six houses have been built in the east of France: two with active (solar collectors) and four with passive (Trombe walls) solar heating systems. Two types of transparent covers are compared: a capillary structure and a simpler polycarbonate plate. The project was monitored during 1 year. Experimental measurements as well as simulation results are presented in this article. The use of simulation allowed a comparison of the various systems on a common basis, i.e., minimizing the effect of different occupants behaviour. Compared to the less expensive cover, transparent insulation increased the productivity of the air collectors 25% and doubled the gain of the Trombe wall. Thanks to passive or active controls, the thermal comfort was not reduced by the solar systems in summer nor in mid-season.     

Mathematical modeling of a box-type solar cooker employing an asymmetric compound parabolic concentrator

A novel design of solar cooker is introduced. The cooker is of box-type equipped with an asymmetric compound parabolic concentrator (CPC) as booster-reflector. It consists of an insulated box equipped with a vertical double glazing cover on a side, and a vertical absorber plate laid out just behind the transparent cover. The booster-reflector is fixed on the glazed side of the box. The absorber plate and the glazing form a vertical channel, open at the top and bottom, and enclosed at the sides. The two openings allow the inside air circulation. A mathematical model of the heat transfer processes involved with this solar cooker, containing a cooking pot loaded with water and deposited on the box floor; was developed and the effects of various parameters, such as solar radiation, load of water and clouds on the dynamic behavior of the cooker are studied.     

Reduction of reflection losses of PV-modules by structured surfaces

Structuring the transparent cover of solar cell modules reduces reflection losses, particularly at large angles of incidence. Relevant aspects are good transmission efficiency independent of wavelength and a low sensitivity to pollution. The macroscopic, linearly grooved structure proposed in this article shows good performance only in combination with a textured cell because large angles are likely to occur inside the structured cover. A classification is made with the concept of annual averaged transmission efficiency for the climatic zone of Freiburg. Calculations and measurements for different combinations of smooth and structured covers and solar cells are presented. From the calculated 97.8% entering the structured glass cover, a measured 93.2% can be coupled into a pyramidal textured monocrystalline solar cell. This is an absolute improvement of 17% compared to a smooth, incoated solar cell with a smooth glass cover. Outdoor measurements showed that a textured solar cell with a structured cover has between 5 and 10% higher values of short-circuit current than a textured cell with a smooth cover.     

Influence of multiple internal reflection of transmitted solar radiation between interfaces on temperature regime of transparent flat solar collector casing covers

Analytical expressions are proposed to calculate the temperature regime of the transparent covers of flat solar collector casings. These expressions account for the influence of multiple internal reflection of transmitted solar radiation between the interfaces of the transparent covers. An example of the calculation of a practical implementation of the results is presented.     

Radiation Exchange Calculations in Enclosures Composed of Boundary Walls Partly Transparent at Long Wavelengths†

In the present work an effort will be made to analyze the radiation exchange in enclosures with boundary walls partly transparent to long wave thermal radiation. The objective of the analysis will be to provide a general set of equations describing the radiation exchange between the boundary of the enclosure, the sky and other distant objects when the boundary walls of the enclosure have asymmetric radiative properties and an arbitrary transparence at long wavelengths. The governing equations are sufficiently general to account for surrounding surfaces of an arbitrary number and separately for the radiant heat fluxes exchanged between the boundary of the enclosure and the surfaces surrounding it within and outside the 8-14 μm atmospheric window band. The proposed method is shown to be in good agreement with experiments using a radiating surface covered with polyethylene under solar radiation incidence, as well.     

Experimental thermodynamic cycles and performance analysis of a solar-powered adsorptive icemaker in hot humid climate

This paper analyzes and presents the thermodynamic cycles and the experimental performance parameters of a solar adsorptive icemaker that uses activated-carbon methanol pair. The solar energy technology employed is far less sophisticated than that of collectors using selective surface or evacuated tubes. The collector-adsorber is multi-tubular with an opaque black radiation-absorbing surface, and thermal insulated by means of transparent covers, the so-called transparent insulation material (TIM). The solar radiation hits on both faces of the tubes by means of semi-cylindrical reflectors. It is shown the results of tests carried out in a region of Brazil close to the Equator, on days characterized by the predominant cloud cover degree. Three cycles have been analyzed: one with clear sky, another with partially cloudy sky, and a third under entirely cloudy sky. The maximum regenerating temperatures were 100.1, 87.3 and 92.7 °C, with an ice production of 6.05, 2.10 and 0 kg by square meter of projected area, for cycles of clear sky, partially cloudy and overcast nights, respectively.     

Comparative performance investigation of integrated collector‐storage solar water heaters with various heat loss reduction strategies

A detailed comparative assessment is reported on the thermal performance of integrated collector‐storage (ICS) solar water heaters with various strategies for reducing top heat losses. The objective of this investigation is to assess and compare heat loss reduction strategies. The shape of ICS solar water heater considered in present investigation is rectangular. The thermal performance of the solar water heater is evaluated and analyzed for the following cases: (1) single glass cover without night insulation; (2) single glass cover with night insulation; (3) double glass cover without night insulation; (4) transparent insulation with single glass cover; and (5) insulating baffle plate with single glass cover. Energy balances are developed for each case and solved using a finite difference technique. The numerical assessment of the system performance is performed for a typical July day in Toronto. Each strategy is observed to be beneficial, reducing top heat losses, and improving system performance. The greatest performance enhancements are observed for the water heater with a single glass cover and night insulation and for the system with a double glass cover and without night insulation. Copyright © 2010 John Wiley & Sons, Ltd.     

Solar transparent insulation materials: a review

This paper presents a status report on solar transparent insulation materials (TIM). It covers a survey of the literature, various physical and other properties of TIM devices, their classifications, applications, fabrication procedures, availability and cost trends. The global resurgence of research is clarified. Subsequently, the development of TIM cover systems (often referred to as advanced glazing) from such products as polymer sheets, capillaries and cellular profiles, is discussed. Their design and performance characteristics are investigated; results corresponding to experimental measurements, as well as computational models, are presented. An explicit comparative study of absorber parallel and absorber perpendicular configurations of TIM cover systems is presented. The TIM covers with black end cover plates, and cellular walls of high emissivity, as well as those with selective cover plates and cellular walls fully transparent to IR radiations, have relatively lower heat loss coefficients.