The use of planar reflectors for increasing the energy yield of flat-plate collectors

A mathematical model to simulate the performance of flat-plate collector-reflector systems is presented. First the collector energy balance is modified to account for the reflected energy. Then the exchange area for a diffuse reflector is obtained by integrating over both reflector and collector surfaces. For the specular reflector, the collector area exposed to reflected radiation is calculated from geometrical relations. Shading effects are also found from the system geometry. Fair agreement is obtained between the model and some experiments on a water heating collector in Brisbane, Australia. Finally, the model is used to predict the annual performance of a water heating system with several values of the reflector angle.     

Optical collection efficiencies of tubular solar collectors with specular reflectors

The optical collection efficiencies of arrays of evacuated tubular collectors with specular involute reflectors have been studied using computer ray tracing techniques incorporating all the major physical features. The optical collection efficiencies under various representative assumptions concerning the angular distribution of incident radiation are presented as functions of the tube to tube spacing. The hemispherical collection efficiency obtained with collector aperture equal to absorber surface is in accordance with the expected loss resulting from the truncation of the reflector, and the losses associated with the mirror, with the envelope and with the absorber. At smaller apertures, the collection efficiency for hemispherically incident radiation agrees with a recent theory of O'Gallagher et al.     

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.     

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.     

Computation of glass-cover temperatures and top heat loss coefficient of flat-plate solar collectors with double glazing

A set of correlations for computing the glass-cover temperatures of flat-plate solar collectors with double glazing is developed. A semi-analytical correlation for the factor f₂—the ratio of outer to inner thermal resistance of a double-glazed collector—as a function of collector parameters and atmospheric variables is obtained by regression analysis. This relation readily provides the temperature of the second (outer) glass cover (T₂). For estimating the temperature of first (inner) glass cover (T₁), another relation for the factor f₁—the ratio of thermal resistance between the two glass covers to the thermal resistance between the absorber plate and first glass cover—is developed. A wide range of variables is covered in the present analysis. The results are compared with those obtained by numerical solutions of heat-balance equations. Using the proposed relations of glass-cover temperatures, the values of top heat loss coefficient (U_t) can be computed and are found to be very close to those obtained by numerical solutions of heat-balance equations. The maximum absolute error in the calculation of U_t by the proposed method is only 1.0%, so numerical solutions of heat-balance equations for the computation of U_t are not required.     

Detailed comparison of the performance of flat-plate and vacuum tube solar collectors for domestic hot water heating

The performance of a DHW system fitted with flat-plate and vacuum collectors has been analysed. Simulations were carried out on the basis of equal aperture or gross areas of basis using TMY data for hour-by-hour dynamic simulation generated for 1036 sites located in different regions of the world. The performance indicators of solar fractions and number of days were determined for specific water mean temperatures in the storage tank of 37 °C, 45 °C and 55 °C. It has been shown that vacuum tube collectors provide slightly better DHW performance indicators than typical flat-plate collectors having the same aperture area. However, since the space that needs to be provided and directly or indirectly paid for is what matters to the user of solar heating systems, the analysis was also carried out on equal gross area basis; on this basis, the advantages of using vacuum collectors are not obvious.     

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.     

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.     

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.     

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 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.     

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.     

Results and analysis of a round robin test program for liquid-heating flat-plate solar collectors

A round robin test program was conducted to determine the intercompatibility of thermal performance data on 2 liquid-heating flat-plate solar collectors. Efficiency tests were performed at 21 test facilities, distributed across the United States, using a common test procedure. The results were statistically analyzed and showed a relatively large spread in the measured values of collector efficiency. Data from approximately half the facilities were selected for detailed analysis. A collector analytical model was used to show that less than of the chi-square for a second order polynomial could be attributed to different environmental conditions from facility to facility. In general, data from a single facility were consistent and the majority of scatter was attributed to systematic uncertainties from facility to facility. When the data from six participants reportedly adhering to the requirements of ASHRAE Standard 93-77 were analyzed, the scatter was found to be within normal limits expected for the test procedure.     

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.     

Calorimetric determinations of bond conductances in pipe and fin type flat-plate solar collectors

A simple indoor calorimetric test set up for measuring the bond conductance in pipe and fin type flat-plate solar collectors at different tube and fin temperatures under steady-state conditions has been developed. With this apparatus several new clamping arrangements can be developed. Tests on the wrapped and wired aluminium fin showed a very significant improvement, with bond conductance being about 22 kcal/m/h °C. The wrapped and wired aluminium fin with heat conducting cement showed maximum promise.     

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^′.     

Thermal performance enhancement of flat-plate and evacuated tube solar collectors using nanofluid: A review

During the past decades, the technology to make particles in nanometer dimensions has been improved and a new kind of solid–liquid mixture, which is called a nanofluid, has appeared. Nanofluids are an advanced kind of fluid containing a small quantity of nanoparticles (usually less than 100 nm) that are uniformly and stably suspended in the liquid. The dispersion of a small amount of solid nanoparticles in conventional fluids such as water or ethylene glycol changes their thermal conductivity remarkably. Since then, nanofluids have been applied to enhance the thermal performance of many engineering systems. Recently, nanofluids have been used as heat transfer fluids to enhance the performance of solar collector devices. This paper reviews the recent progress and applications of nanofluids in flat-plate and evacuated tube solar collectors. Other than to review the efficiency of solar collectors with nanofluids, the paper also discusses the impact of nanofluids in solar collectors on economic and environmental viewpoints. Finally, the challenges and future trends in the application of nanofluids in thermal solar collectors are discussed.