Optimal geometries of plane mirror solar collectors

This paper presents an analytical study of a stationary plane mirror solar concentrator. It is composed of an array of East-West oriented trapezoidal channels with two sided reflecting walls and a tubular absorber as a receiver at the base. We have analysed and identified the most practical design parameters for a trough like concentrator. The one- and two-faceted plane side wall configurations with tubular receiver at the base of the trough have been studied in detail. It has been concluded that large savings in reflecting surfaces are possible while sacrificing some reduction in concentration. A theoretical prediction for the dependance of absorber efficiency on temperature has been obtained.     

Methods for calculating the concentration fields of mirror solar collectors

Two methods for calculating the solar collector concentration fields, i.e., the tomographic and vector methods, are considered. They make it possible to calculate the energy distribution over the surface of the receivers of arbitrary shape. Three calculation examples are presented. The methods are shown to be equivalent.     

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.     

Computational evaluation of solar heating systems using concrete solar collectors

This paper uses the F-chart technique to evaluate three types of solar heating systems, namely; space solar heating and domestic hot water system (SHDHW), domestic hot water system (DHW) and solar swimming pool heating system (SPHS), using three types of concrete solar collectors, models A, B, and C, and one conventional metallic solar collector.

The economical analysis of SHDHW system revealed that the concrete collectors provided about 49 and 63% of the annual load when the collecting area of the solar panel increased from 55 to 88 M² (25 to 40% of the building roof area). The corresponding solar contributions when conventional metallic collectors were used are 41 and 53%, respectively. This represents an improvement of the annual solar fraction of about 19% when concrete collectors are used instead of the metallic collectors.

It was found that solar heating systems with concrete solar collector models gave higher solar fractions and total life cycle savings than the conventional solar metallic collector.     

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.     

Design considerations for residential solar heating and cooling systems utilizing evacuated tube solar collectors

As solar heating systems become a commercial reality, greater efforts are now being employed to incorporate solar cooling components in order to obtain a complete solar heating and cooling system and thus take advantage of the cost-effectiveness of year-round use of the solar equipment. Because of the exceptional performance and high efficiency of evacuated tube solar collectors, these advanced collectors are receiving considerable attention for use in solar heating and cooling systems. While improved performance is readily obtained with these sophisticated solar collectors, there are also numerous difficulties and problems associated with their use in a solar system. This paper addresses many of the design considerations which must be included in any realistic solar system design. Most of the considerations presented here are based on the experience gained in the design and performance of the solar heating and cooling systems for CSU Solar Houses I-IV.     

A general chart for sizing the collectors of solar heating systems

A general chart is developed for calculating the collector area required to provide a prescribed value of the annual load fraction for solar heating systems. The relationship between collector area and annual load fraction can then be easily obtained for specified collector design parameters, load and location. The construction of the chart is based on correlating data generated by the f-chart method. Data of 13 locations in the U.S.A. are considered. The good agreement of the results obtained by the present simple method and the f-chart for both space and combined space/domestic water heating proves that the two methods are almost of the same accuracy. Since the present chart is not location dependent and allows direct comparison of different collectors, it is a very valuable design aid for sizing and selecting solar collectors.     

Mirror enclosures for double-exposure solar collectors

A conventional flat-plate collector panel can be employed in a double-exposure configuration when the panel is glazed on both sides and when mirrors are provided to reflect solar radiation onto the back side of the panel. In this paper, several flat-mirror configurations are evaluated and optimal configurations are determined for different solar energy applications at lat. 35, 40 and 45°. The various mirror configurations are evaluated theoretically by calculating direct-beam and diffuse solar radiation enhancement factors. The enhancement factors are defined as the ratio of the solar flux absorbed by both sides of a double-exposure panel to that absorbed by an identical single-exposure panel tilted at the latitude angle from the horizontal. The enhancement factors are calculated using the method of images and take account of the variation of glazing transmittance with incident angle. Optimal mirror configurations were determined for direct-beam solar radiation for both fixed-mirror configurations and adjustable-mirror configurations with semi-annual mirror rotations. Optimal fixed-mirror configurations were obtained for both winter space-heating and year-round applications. An adjustable-mirror configuration, however, was determined to be optimal for year-round solar collection and overall to be most adaptable to a variety of solar energy applications. The same adjustable-mirror configuration was determined to be optimal at all three latitudes and therefore a single design can be employed at diverse locations.     

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.     

Shadow effect of adjacent solar collectors in large scale systems

In large scale solar systems and in other cases with limited field area (such as on tops of buildings), shadowing of collectors, (thermal or photovoltaic) by their neighbours might occur during the day. This situation calls for an optimal solution of collector deployment in a given field area for maximum or desired energy.

The paper deals firstly with the shadowing analysis of vertical and inclined poles and collectors (the shadow components, height and area). This useful information is used in an example of optimal deployment of collectors in a given area (which includes the tilt angle, collector size, spacing between collectors and the number of collector rows).     

Chemical vapour deposited SnO2:Sb heat mirror coatings for cylindrical solar collectors

A chemical vapour deposition technique for growth of SnO₂:Sb heat mirror coatings on the inner walls of long cover glass tubes for cylidrical solar collectors is reported. The best performance of the tin-oxide film is obtained for those films grown from a source of SnCl₂ + 1 mol% Sb on Corning glass tubes at 520°C. These films, supported on 2 mm glass substrates, have a solar transmittance of 0.85 and an infrared reflectance of 0.8. The heat mirror coatings are observed to increase the stagnation temperature of the absorber in anevacuated tubular collector from 142 to 161°C under incident optical flux of 1150 W/m²     

Testing of solar collectors

In order that tests on different solar collectors conducted at different times shall be comparable, it is necessary that the test define the governing equation of the collector. This involves measurements at different solar intensities, different operating temperatures and different flow rates.The paper describes the theory and the experimental procedure which involves connecting, say, four similar collectors in series and measuring the temperature rise across each collector. Since, at any instant of time, the flow rates and the solar intensity are identical for all the collectors, a number of points on the efficiency-temperature curve are obtained from which the characteristic equation of the collectors is readily established. Rules are established to limit the temperature rise permitted and a “standard” day is suggested to provide a quick comparison basis.     

Orientational relationships for optically non-symmetric solar collectors

More details of collector orientations and incidence angle modifiers are required to specify the instantaneous optical efficiencies of optically non-symmetric solar collectors, such as those using nonimaging concentrators, than to specify the efficiencies of symmetric collectors such as flat plates. In this paper, expressions are presented for the projected incidence angles necessary for evaluating the instantaneous optical efficiencies of non-symmetric collectors. These projected angles are also used when a nearly biaxial incidence angle modifier is approximated by the product of two single-axis incidence angle modifiers. In addition, relationships for incidence angles onto an absorber tube are developed. These differ from the angle onto a cover glazing, and they effect the absorptance of the tube's coating and the transmittance of a cover tube as used in tubular evacuated collectors.     

Performance evaluation for solar collectors in Taiwan

In this paper, the global irradiation observed in Taiwan from 1990 to 1999 was used to estimate the optimal tilt angle for solar collectors. The observed data are resolved into diffusion and beam components, and transformed into instantaneous time frames using mathematical models. The energy gain on installing a single-axis tracked panel as compared to a traditional fixed panel is originally analyzed theoretically. In addition to the observation data, both types of radiation will be taken into account for comparison, i.e. both extraterrestrial radiation and global radiation predicted using empirical models. The results show that the yearly optimal angles for six selected stations are about 0.95 and 0.88 times their latitudes for extraterrestrial and predicted radiation, respectively. All of the observed irradiations are less than the predicted values for all times and stations, consequently resulting in a flatter tilt angle, with a few exceptions in summer. Since Taipei has the lowest clearness index, its yearly optimal angle calculated from observed data shows the greatest discrepancy when compared to its latitude. By employing a tracked panel, the yearly gains calculated from the observed data lie between 14.3% and 25.3%, which is significantly less than those from the extraterrestrial and predicted radiations.     

Maximally concentrating collectors for solar energy applications

The performance characteristics of a solar energy collector can be described by a function relating the concentration to the direction of incident radiation. A strict bound on the integral of this function is established. This is used to define a realistic class of collectors with maximal concentration which can serve as a basis for design and optimality studies.