On the optimum orientation of solar collectors

A simple model of atmospheric absorption and scattering, incorporating only three parameters, has been used to fit meteorological data for direct and diffuse radiation and hours of sunshine at latitude 55°N, longitude 6°W. Using this fit for insolation on a horizontal plane, annual and monthly figures for insolation on an inclined plane have been computed. It is found that the annual insolation is relatively insensitive to orientation for angles of tilt between zero and 60°, but that the optimum tilt is 30.5°. When ground reflection is included, the optimum tilt angle rises steadily to 65' as the ground albedo increases from zero to unity. Optimization for summer and winter operation is also considered, as is the effect of various characteristics of practical collectors.     

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.     

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.     

Different strategies for operation of flat-plate solar collectors

Three strategies for solar collector operation are defined. These involve keeping one of the following constant during the day:

1. (i) the average working fluid temperature,

2. (ii) the outlet temperature and

3. (iii) the inlet temperature.

A graphical and analytical method previously developed by the authors was generalized to analyse and compare these strategies. For a constant flow rate, the best strategy is to maintain a constant inlet temperature. A constant outlet temperature is recommended when flow-rate control is possible.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

The effect of inclination on the heat loss from flat-plate solar collectors

One of the many design variables that affects the heat losses from flat-plate solar collectors is the angle of inclination of the collectors to the horizontal. This is due to the variation in natural convection conductances in spaces between flat plates, with their angle to the horizontal. The top loss heat transfer coefficient is calculated for a series of plate temperatures, ambient temperatures, external convective heat transfer coefficients and plate emittances for angles of inclination from 0 to 90° using the natural convection correlation developed by Hollands et al.[4]. A sky temperature 12°C below ambient temperature is used as the radiant sink temperature and an effective sink temperature for the top losses is defined. Curves are presented showing the variation of the top loss coefficient with temperature and wind speed for two plate emittances at an angle of inclination of 45°. It is shown that the value of the top loss coefficient is insensitive to the effective sink temperature (as found by Duffie and Beckman [5]) and that the effective temperature is determined solely by the wind speed, for a given collector inclination.The top loss coefficient at any angle of inclination is expressed as a ratio of the top loss coefficient at 45°. The results indicate that there is a continual reduction in the top loss coefficient up to an inclination of 90°. The effect this has on the overall collector loss coefficient is illustrated and the change in collector instantaneous efficiency is estimated.     

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.     

The effect of off-south orientation on the performance of flat-plate solar collectors

There are many instances in which an off-south installation of a flat-plate solar collector is more compatible with a building's orientation than a due-south installation. In these cases it is important to determine the magnitude of the performance sacrificed by conforming to the building architecture. This study investigates the collector performance and optimum tilt as functions of the off-south angle, collection temperature, number of glass covers and the relative amounts of direct and diffuse radiation. It was found that the yearly energy collection for a given collector tilt is insensitive to the off-south angle and that in some cases it actually improves with increasing azimuthal angle. It was also found that for a given azimuthal angle an optimum collector tilt exists which is between 3 and 10° less than the latitude. Calculations were based on New York City weather.     

On the correlations between collector efficiency factor and material content of parallel flow flat-plate solar collectors

The collector efficiency factor F^′, besides the collector heat loss coefficient U_L, characterizes the thermal quality of a solar collector. As F^′ is strongly influenced by the tube distance w and the absorber plate thickness δ, F^′ is also correlated with the material content of absorber plus tubing. Due to the future mass production of collectors and to the restricted copper resources (in the literature, a range until 2026 is given), the role of material savings can be expected to become more and more important. This paper focuses on the correlations between F^′ and the material content of absorber and tubing for flat-plate collectors with the fin-and-tube geometry. The correlations between w, δ, F^′ and material content are presented in a new type of nomograph. This nomograph indicates the values of w and δ that minimize the material content (for a given F^′). For a typical absorber with F^′=0.90, material savings of 25% can theoretically be achieved without any deterioration of F^′, by reducing the absorber plate thickness and the tube distance. The resulting plate thickness is below 0.1 mm; the respective tube distance will be about 7 cm. Practical restrictions are discussed. In a sensitivity analysis, the influence of different parameters on F^′ is investigated. The most important parameters are w, U_L,δ and the Reynolds number. The technique chosen for contacting tube and absorber has only a minor influence on F^′.     

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.     

Exergy efficiency and optimum operation of solar collectors

Efficiency and optimum operation of flat-plate solar collectors are investigated in terms of exergy delivery of the collector. Various exergy efficiencies are defined and output exergy efficiency is used to determine the optimum flow rate of a typical collector allowing for the pressure drop in tubes. The operation of a collector is investigated for optimum flow rate and various constant inlet temperatures using the output exergy efficiency and thermal efficiency together.