Planar concentrators for flat-plate solar collectors

A systematic study has been made of the effectiveness of planar specular reflectors for solar energy collectors. Two daily averaged indices of performance were used. One, the area ratio, indicates the amount by which the reflector extends the effective receiver area. The other is the enhancement factor, which is used to compare the energy received by an augmented collector with that by a reference collector at optimum tilt.

A reflector can be mounted either above or below a flat-plate collector. Both combinations are evaluated fully, by varying separately the angular position and dimensions of the reflector and of the collector. The principal parameters are identified and the main characteristics summarised as a series of performance curves. These curves provide an easy method for determining optimum reflector geometries.

Use of the performance curves may be extended to obtain the configuration of the two reflectors in a trough concentrator. This also allows the single-reflector system to be compared directly with the trough concentrator. Evidence is presented which shows the advantages of an asymmetrical trough configuration over a symmetrical concentrator.     

Cover systems for high temperature flat-plate solar collectors

A simulation study has been conducted of the influence of cover design on the thermal performance of flat-plate solar collectors for use at temperatures of 150°C. Detailed results are presented of the effects of changes in cover materials, cover surface treatments, cover system configuration and absorber plate surface treatments on both the instantaneous efficiency and the long term solar contribution of flat-plate collectors. For the simulation conditions, it is shown that flat-plate collectors consisting of a single high transmittance cover, a convection suppressing device such as a honeycomb and a selective absorber surface yield long term solar contributions comparable to those of evacuated tubular collectors, whereas other configurations simulated (single and multiple cover systems) are significantly inferior.     

Evaluation of a transient test procedure for solar flat-plate collectors

A dynamic method for testing solar flat-plate collectors under unsteady weather conditions has been validated through detailed experiments and compared with two established standards: the ASHRAE 93–86 standard for steady state testing and the British standard BS 6757 for transient testing. The new method is based on a lumped capacity model derived from a general energy balance of the collector under actual conditions. The characteristic parameters are estimated using the standard methods for unconditional non-linear optimisation. Extensive experiments have been carried out under a wide range of operating and environmental conditions. Four different collectors commercially available in the market have been tested at the same location and using the same experimental rig. The results on the basis of the new method are very close to those obtained from the ASHRAE standard. The average values of F_R(τα)_e and F_RU_L by the new method are within ±3% of the steady state values. The results of the BS 6757 method are within ±2% for F_R(τα)_e but those of F_RU_L are about 12% lower than the ASHRAE values. On average, the difference between the theoretical predictions for the outlet temperature by the new method and the corresponding experimental measurements are about ±0.3°C, while the predictions by the British standard under the same conditions are about 2°C lower than measured values. The percentage deviations of predictions for the temperature rise based on the two methods, averaged over a day, are about ±8% and ±36% respectively. The new dynamic method requires less time for experimentation, one day's test is enough to give accurate estimation of the collector parameters. The method does not impose any restriction on the variation of weather or operating parameters and, therefore, has a quite general applicability.     

Free-convection effects inside tubes of flat-plate solar collectors

An experimental and theoretical study of the effects of free convection superimposed on forced flow in inclined tubes is made. The theoretical analysis considered fully developed laminar flow under uniform heat flux and constant pressure gradient. Solutions for velocity and temperature equations have been obtained by perturbation analysis, in terms of power series of Rayleigh number. Nusselt number has been evaluated on bulk temperature difference basis and a maximum value of it appears to lie between 20 deg and 60 deg of tube inclination. An experimental study was done for 45 deg tube inclination and also for vertical position.     

Thermodynamic method for designing dryers operated by flat-plate solar collectors

In this work a thermal performance analysis of the solar drying process is presented. It describes semi-empirical models for the thermal characterization of an experimental indirect solar dryer device. A procedure for designing drying equipment, which takes into account the varying operating conditions given by the variations of the environmental conditions is also presented. On the other hand, a simplified method to design solar collectors based on the determination of minimum entropy generation during the thermal conversion of solar energy, is described. Finally, the results of the preliminary design of the solar dryer are presented using the thermal analysis procedure and the method derived from the Second law of thermodynamics.     

A new method for testing the performance of flat-plate solar collectors

In this study, a new method is proposed for determining the thermal performance parameters of a flat-plate water heating collector. A parametric identification principle is applied to a mathematical model of the collector operating in a real system. The advantages of the method are that it does not need a sophisticated testing system, it avoids the problem of regulating the inlet fluid temperature (which must be held constant during the tests in the standard procedures) and a cloudless sky is not essential. Solar radiation of 600 W m⁻² was chosen as a threshold to start experiments. Applied to a flat-plate collector with a single-glazed cover, the proposed method gave satisfactory results for the determination of its thermal performance coefficients. To verify the adequacy of the present method, it was used to predict the outlet fluid temperature. The results indicate a satisfactory agreement between predicted and measured values with a correlation coefficient of 0.99 and an error of 0.4%. We propose that performance results will be presented as a technical sheet showing collector features and accompanied with a curve giving the correlation between estimated and measured output temperature of the collector fluid.     

On the optimum exposure of flat-plate fixed solar collectors

With the increasing range and scale of applications of solar energy conversion systems a detailed knowledge of the available solar fluxes on surfaces of various tilt and slope aspect is required. The widely used engineering routines for siting and exposing of solar collector arrays may not always be consistent with the maximum available solar energy income. In the present study a computational algorithm is proposed suitable for the calculation of optimum tilt angles of a solar collector, receiving the maximum insolation for given values of direct beam, global and diffuse radiation, and given surface reflectivity. Formulas are derived also for the calculation of optimum tilt angles yielding maximum daily insolation for each month of the year. The dependence of the optimum tilt angle on the diffuse-to-global radiation ratio, as well as on the surface reflectivity is evaluated.     

Exergy optimization of flow rates in flat-plate solar collectors

The optimal flat-plate collector mass flow rate is determined by maximizing the exergy (available energy) delivery of the collector as the objective function. Collector and storage dynamics are neglected. Although the case where the pumping power loss is ignored results in bang-bang control, the case where this loss is included in the exergy equation results, after some assumptions, in an optimal mass flow rate that is a function of collector parameters, inlet and ambient temperatures and solar heat gain. Daily performance of a typical flat-plate solar collector with optimum mass flow rate is compared with the performance of the same collector using the mass flow rate obtained by maximizing the difference between the collected thermal energy and the required pumping power.     

平板型太阳集热器吸收与透过村料的进展

平板型太阳集热器结构简单、运行可靠、成本低廉,与真空管集热器相比它还具有承压能力强、吸热面积大等特点,是太阳能与建筑结合最佳选择的集热器类型之一.众所周知,由于太阳能技术的进步和市场产品竞争的激烈,平板集热器产品正面临着严峻的考验.只有依靠不断的技术进步,提高产品性能,才能使平板集热器产品在激烈的市场竞争中立于不败之地.     

Optimal operation of solar collectors

Optimal operation of a solar collector is investigated in terms of exergy delivery of the collector. Using the Bliss equation for the collector, an approximate exergy equation is derived and, through this equation, the optimal mass flow rate, and optimal inlet and outlet temperatures are obtained. The approximate equation is also compared with the non-linear exergy delivery equation of the collector.     

Comparison of zero testing with conventional testing for flat-plate air-heating solar collectors

We have compared BSE zero testing with ASHRAE outdoor steady-state testing using a verified computer model of a rear-duct air-heating solar collector. The two tests are likely to yield similar results provided that the air inlet temperature is considerably greater than the ambient temperature.     

Experimental and theoretical evaluation of dynamic test procedures for solar flat-plate collectors

This communication presents a critical evaluation of nine dynamic test methods for solar flat-plate collectors. The theoretical basis, the technique of parameter estimation and the test procedure of each method have been reviewed and compared. Extensive experimental studies have been carried out under a wide range of weather and operating conditions. Two commercially available collectors (from two different manufacturers) have been used in the investigation. The tests were carried out at the same location using a common test-rig, measuring transducers and controlling and data-acquiring facilities. The characteristic parameters of the collectors have been obtained on the basis of each procedure and compared with those based on the steady-state ASHRAE 93-86 standard. Further, for the methods which prescribe similar test sequences, the collector parameters have been extracted from the same data sets according to their procedures for providing a direct and very clear comparison between the methods. A sensitivity study has also been carried out in order to examine the effect of uncertainties in measurements on the values of the estimated parameters from different methods. Also investigated is the error propagation wherever applicable. Among the methods evaluated, the new dynamic method (NDM) seems to be quite reliable. The quick dynamic test (QDT) method is the most simple method and could be adopted by manufacturers as an effective tool for the purpose of quality control of their products. From the point of view of theoretical completeness, Perers’ method accounts for almost all effects.     

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.     

Stationary reflector-augmented flat-plate collectors

A general procedure for determining the optimum geometry of a reflector-augmented solar collector which produces a desired pattern of flux-augmentation is described. The example used for illustration is a stationary collector whose winter performance is to be improved. Consideration both a flat-plate collector with a bottom reflector and one with a top reflector led to distinct differences in their optimum configuration and performance being identified. Since either systems can be used to augment winter flux, a criterion for selecting the appropriate system is given. This criterion is based on the displacement in collector tilt from latitude inclination.     

A design method of flat-plate solar collectors based on minimum entropy generation

A procedure to establish the optimal performance parameters for the minimum entropy generation during the collection of solar energy, is presented. The Entropy Generation Number, N_s, and the criterion for the optimal thermodynamic operation of a collector under nonisothermally, finite-time conditions, are reviewed. The Mass Flow Number, M, corresponding to the optimum flow of working fluid as a function of the solar collection area, is also considered. A general method for the preliminary solar collector design, based on N_s, M and the “Sun–Air” or stagnation temperature, is developed. This last concept is defined as the maximum temperature that the collector reaches at nonflow conditions for given geographic location, geometry and construction materials. The thermodynamic optimization procedure was used to determine the optimal performance parameters of an experimental solar collector.     

On determining the thermotechnical characteristics of the flat-plate solar air heating collectors

The technique and analytic expressions for determining the values of the thermotechnical parameters of the flat-plate solar air heating collectors are presented. These expressions are obtained on the basis of the laboratory and quasistationary full-scale experiments.