CPC Solar Collectors With Flat Bifacial Absorbers

The design, construction and test results of non-evacuated stationary CPC solar collectors with flat absorbers are presented and discussed. The proposed collector design is based on a truncated asymmetric CPC reflector, consisting of a parabolic and a circular part. A flat bifacial absorber is installed at the upper part of the collector, parallel to the glazing to form a thermal trap space between the reverse absorber surface and the circular part of the mirror. Two prototypes based on the same collector geometry were constructed and tested. The first model consists of one mirror–absorber unit and the second of three smaller units integrated in one collector device. The truncated CPC mirror and the installation of the absorber parallel to the glazing keep the optical efficiency at a satisfactory level. The reduction of radiative thermal losses by using selective absorbers and the suppression of convection thermal losses from the reverse absorber surface to the collector cover result to a significant decrease of the total collector thermal losses. The experimental results showed that the proposed CPC collector could achieve a maximum efficiency of 0.71 and a stagnation temperature of about 180°C, with the multiunit collector device being more efficient and practical.     

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.     

Measurement of radiation distribution on the absorber in an asymmetric CPC collector

A method to estimate the annual collected energy and the annual average optical efficiency factor is suggested. The radiation distribution on the absorber of an asymmetric CPC collector with a flat bi-facial absorber is measured for three different absorber mounting angles using a photo diode. The annual optical efficiency factors and a relative measure of the annual collected energy are determined for collectors with the absorber fin thickness 0.5 and 1 mm, and for a collector with a teflon convection suppression film mounted around the absorber. With the local optical efficiency factors and the annual incident solar energy distribution considered, the analysis indicates that the energy gain for a mounting angle of 20° is higher than for a collector with 65° absorber mounting angle. The annual collected energy is increased with 6–8% if the absorber fin thickness is increased from 0.5 to 1 mm. The annual average optical efficiency factor is relatively independent of the absorber mounting angle. It was found to be 0.87–0.88 for a collector with a 0.5 mm thick absorber fin and 0.92 for a collector with a 1 mm thick absorber fin or for a collector with 0.5 mm thick absorber fin with a teflon convection suppression film added. The low annual average optical efficiency factor is not caused by the uneven irradiance distribution but by the relatively high U_L-values.     

Performance of an aqua-ammonia absorption solar refrigerator at sub-freezing evaporator conditions

Ice making by means of a solar-assisted aqua-ammonia absorption refrigeration unit at subfreezing evaporator conditions is considered in terms of its technical and economic feasibility. A computer-aided thermodynamic analysis is performed for various ranges of operation parameters, three climatic locations varying from 15°N to 43°N latitude and four solar collector types, i.e. flat plate, compound parabolic collector, and east-west and north-south axis tracking concentrators. In order to use an air-cooled condenser, the simultation is predicted on an absorber temperature of 35°C and a condenser temperature of 38°C. The results indicate that for generator pressures of 1.02 to 2.07 MPa, generator temperatures greater than 120°C are required. At these conditions, the COP is on the order of 0.5 and a conventional flat plate collector is not satisfactory. For the three climates, the compound parabolic collector (CPC) has a higher output than an east-west axis tracking concentrator but less than the north-south tracker. The shape of the insolation curves at the lower latitudes causes difficulties in obtaining optimal collector sizes. The cost comparison between the CPC and north-south tracker indicates that overall system costs will range between $85 and $155,000 (1981) for a one ton of ice per day system. The projected costs per ton for a 25-year life are in the range of $10 to $20, which are favorable compared to the cost of domestic ice.     

Correlations for natural convective heat exchange in CPC solar collector cavities determined from experimental measurements

A detailed experimental study was undertaken to analyse the natural convective heat transfer in CPC cavities, a complex function of collector orientation, geometrical aspect ratios and thermal boundary conditions at the enclosure walls. Results are reported for CPC solar collectors with full-, three quarter- and half-height reflectors, C_R = 2 and a 100 mm wide flat plate absorber. Experiments were conducted using a purpose built solar simulator under controlled lab environment employing realistic boundary and thermal conditions. The effects of simultaneous tilting of the solar collectors about both transverse and longitudinal axes, truncation of the reflector walls and inlet water (collector heat removal fluid) temperature on the natural convective heat flow characteristics inside the CPC cavity have been determined. It is concluded that the correlations developed for prediction of natural convection characteristics in rectangular, annuli and V-trough enclosures are not appropriate for application to CPC solar collectors with divergence ranging from 150% to 300%. Based on the experimental data a correlation is presented to predict the natural convection heat loss from the absorber plate of solar collectors for a range of water inlet temperatures.     

Design and test of non-evacuated solar collectors with compound parabolic concentrators

The intermediate range of concentration ratios (1.5X–10X) which can be achieved with CPCs without diurnal tracking provides both economic and thermal advantages for solar collector design even when used with non-evacuated absorbers. The present paper summarizes more than 3 yr of research on non-evacuated CPCs and reviews measured performance data and critical design considerations. Concentrations in the upper portions of the practical range (e.g. 6X) can provide good efficiency (40–50 per cent) in the 100–160°C temperature range with relatively frequent tilt adjustments (12–20 times per year). At lower concentrations (e.g. 3X) performance will still be substantially better than that for a double glazed flat plate collector above about 70°C and competitive below, while requiring only semi-annual adjustments for year round operation. In both cases the cost savings associated with inexpensive reflectors, and the optimal coupling to smaller, simple inexpensive absorbers (e.g. tubes, fins, etc.) can be as important an advantage as the improved thermal performance.The design problems for non-evacuated CPC collectors are entirely different from those for CPC collectors with evacuated receivers. For example, heat loss through the reflector can become critical, since ideal CPC optics demands that the reflector extend all the way to the absorber. Recent improvements in reflector surfaces and low cost antireflection coatings have made practical a double-glazed non-evacuated CPC design. It is calculated that a 1.5X version of such a collector would have an optical efficiency η_o = 0.71, a heat loss coefficient U = 2.2 W/m²°C and a heat extraction effciency factor F′ ≥ 0.98, while requiring no tilt adjustments.     

An evaluation of a transverse slatted flat plate collector

It is generally accepted that the insertion of a type of honeycomb structure into the air gap between the absorber plate and the transparent cover of a flat plate solar collector will suppress convection if the honeycomb dimensions are matched to the particular dimensions and operating temperatures of the collector. However relatively little research has been carried out to characterise the effectiveness of a convection suppression device under actual operating conditions.This paper surveys the experimental work carried out at the University of Melbourne, Mechanical Engineering Department, and its relationship to other experimental and theoretical research, reported in the literature. The experimental program involved the comparative testing of two collectors, identical except that one was fitted with a convection suppression device made of parallel glass slats placed laterally across the collector between the absorber plate and the cover glass. Testing was carried out in a laboratory situation with five convection suppression devices of differing aspect ratio ( ), and the most effective of these devices (aspect ratio ), was tested in the Melbourne University Solar Testing Area under a range of actual operating conditions.In the laboratory tests, the ability of the honeycomb to suppress convection was tested, whilst in the outdoor tests, the influence of the honeycomb on the transmission of solar radiation to the absorber plate was also evaluated. It was found that at high operating temperatures the convection suppression device gave rise to considerable improvement in performance. A forty percent improvement in instantaneous thermal efficiency was produced for fluid temperatures of approx. 100°C. However, if the collector is not oriented correctly the overall improvement in thermal performance will not be as large, due to the decrease in solar transmittance caused by the honeycomb. This indicates the probable need for some form of collector tilt adjustment during the year if the long-term thermal performance is to be optimised.     

Experimental comparison of alternative convection suppression arrangements for concentrating integral collector storage solar water heaters

An experimental investigation of an inverted absorber integrated collector storage solar water heater mounted in the tertiary cavity of a compound parabolic concentrator with a secondary cylindrical reflector has been performed under simulated solar conditions. The solar water heaters performance was determined with the aperture parallel to the simulator for a range of transparent baffles positioned at different locations within the collector cavity. Results indicate that glass baffles located at the upper portion of the exit aperture of the CPC can reduce thermal losses through convection suppression without significantly increasing optical losses.     

Performance of the Sacramento demonstration ICPC collector and double effect chiller

In 1998 two new technologies were demonstrated for the first time in a commercial building: (1) a new integrated CPC reflector evacuated solar collector (ICPC) and (2) the solar operation of a double effect absorption (2E) chiller. The 106.5 m² ICPC collector array consisted of 336 evacuated tubes. A commercial 2E gas-fired absorption chiller was modified to operate with 150 °C hot water from the solar collector array. Daily collection efficiencies of nearly 50% and instantaneous collection efficiencies of about 60% were achieved throughout the first two years of operation. Daily chiller COPs of about 1.1 were also achieved.     

Comparison of the optics of non-tracking and novel types of tracking solar thermal collectors for process heat applications up to 300 °C

Evacuated CPC (compound parabolic concentrator) collectors with non-tracking reflectors are compared with two novel tracking collectors: a parabolic trough and an evacuated tube collector with integrated tracking reflector. Non-tracking low concentrating CPC collectors are mostly mounted in east–west direction with a latitude dependent slope angle. They are suitable at most for working temperatures up to 200–250 °C. We present a tracking evacuated tube-collector with a trough-like concentrating mirror. Single-axis tracking of the mirror is realized with a magnetic mechanism. The mirror is mounted inside the evacuated tube and hence protected from environmental influences. One axis tracking in combination with a small acceptance angle allows for higher concentration as compared to non-tracking concentrating collectors. Ray-tracing analysis shows a half acceptance angle of about 5.7° at geometrical concentration ratio of 3.2. Losses of well constructed evacuated tube collectors (heat conductivity through the manifolds inside the thermally insulated terminating housing are low) are dominated by radiation losses of the absorber. Hence, reducing the absorber size can lead to higher efficiencies at high operating temperature levels. With the presented collector we aim for operating temperatures up to 350 °C. At temperatures of 300 °C we expect with anti-reflective coating of the glass tube and a selective absorber coating efficiencies of 0.65. This allows for application in industrial process heat generation, high efficient solar cooling and power generation. A first prototype, equipped with a standard glass tube and a black paint absorber coating, was tested at ZAE Bayern. The optical efficiency was measured to be 0.71. This tube-collector is compared by ray-tracing with non-tracking market available tube-collectors with geometrical concentration ratios up to 1.1 and with a low cost parabolic trough collector of Industrial Solar Technology (IST) with an acceptance half angle about 1.5°, a geometrical concentration ratio of 14.4 and a measured optical efficiency of 0.69.     

An exergy analysis of a solar-driven ejector refrigeration system

Exergy analysis is used as a tool to analyse the performance of an ejector refrigeration cycle driven by solar energy. The analysis is based on the following conditions: a solar radiation of 700 W/m², an evaporator temperature of 10 °C, a cooling capacity of 5 kW, butane as the refrigerant in the refrigeration cycle and ambient temperature of 30 °C as the reference temperature. Irreversibilities occur among components and depend on the operating temperatures. The most significant losses in the system are in the solar collector and the ejector. The latter decreases inversely proportional to the evaporation temperature and dominates the total losses within the system. The optimum generating temperature for a specific evaporation temperature is obtained when the total losses in the system are minimized. For the above operating conditions, the optimum generating temperature is about 80 °C.     

Experimental investigation of heat losses from low-concentrating non-imaging concentrators

Heat loss measurements have been performed on a V-trough collector model with concentration ratio 1.56 and with flat absorbers consisting of five parallel reflector troughs aligned east-west. The collector was tilted 45°. Depending on the similarity in geometry between V-troughs and compound parabolic concentrators, the results should in general be valid also for low-concentrating CPCs. The absorbers were heated electrically and the heat losses were calculated from the input power to the absorber surface. Several geometrical and material properties that affect the heat losses from the collector were investigated. It is concluded that the use of transparent insulation, such as Teflon® films, in low-concentrating solar collectors can reduce the heat losses substantially. The reflector emittance in the infrared has an impact on the heat losses. Use of highly emitting reflectors instead of low-emitting reflectors increases the overall heat losses by about 5–8%. The conversion of experimentally measured heat losses into heat losses for real collectors and practical material considerations is discussed.     

Performance analysis of new-design solar air collectors for drying applications

This paper presents a performance analysis of four types of air heating flat plate solar collectors: a finned collector with an angle of 75°, a finned collector with an angle of 70°, a collector with tubes, and a base collector. In this study, the first and second laws of efficiencies were determined for the collectors and comparisons were made among them. The results showed that the efficiency depends on the solar radiation and the construction of the solar air collectors. The temperature rise varied almost linearly with the incident radiation. The first law of efficiency changed between 26% and 80% for collector-I, between 26% and 42% for collector-II, between 70% and 60% for collector-III, and between 26% and 64% for collector-IV. The values of second law efficiency varied from 0.27 to 0.64 for all collectors? The highest collector efficiency and air temperature rise were achieved by the finned collector with angle of 75°, whereas the lowest values were obtained for the base collector. The effectiveness order of the collectors was determined as the finned collector with angle of 75°, the finned collector with angle of 70°, the collector with tubes, and the base collector.     

Impact of different internal convection control strategies in a non-evacuated CPC collector performance

Over the last decade the technological advances observed in solar collector materials, namely better spectrally selective absorber coatings and ultra clear glass covers, contribute to performance improvements and translate into higher operational temperature ranges with higher efficiency values.While the use of Evacuated Tube Collectors (ETCs) is becoming widespread in the thermal conversion of solar energy, non-evacuated solar collectors still hold advantages at manufacturing, reliability and/or cost levels, making them interesting and competitive for a large range of applications, in particularly, in temperature ranges up to 80 °C. However, these advantages have not prevented the major drawback of these collectors when compared to ETCs: thermal losses due to internal convection which prevent their general use in the range of operating temperatures up to 150 °C.Insulation, double glazing or selective coatings can be used in non-evacuated collectors to reduce heat losses. To prevent internal convection losses in these solar collectors, different control strategies have been studied, such as the adoption of different inert gases within the collector cavity, physical barriers reducing air flow velocities over the absorber or cover surfaces or the use of concentration.In the present article, an assessment of adopting such internal convection control strategies in a CPC collector is presented. Each of the presented strategies is assessed in terms of the resulting collector optical and thermal characterization parameters and yearly collector yield. For this purpose, an integrated tool allowing the design, optical and thermal characterization of CPC collectors was developed. The results obtained provide valuable guidelines for anyone wishing to implement any of these strategies in a new collector design.     

Analysis of wind flow around a parabolic collector (1) fluid flow

Applications of parabolic collectors for solar heating and solar thermal power plant increased in the recent years. Most of the solar power plants installed with parabolic collectors are on flat terrain and they may be subjected to some environmental problems. One of problems for large parabolic collector is their stability to track the sun with respect to time very accurately. Any small off tracking as well as the collector structure stability will be affected by strong wind blowing for the regions where the wind velocity is high.In the present study, a two-dimensional numerical simulation of turbulent flow around a parabolic trough collector of the 250 kW solar power plants in Shiraz, Iran is performed taking into account the effects of variation of collector angle of attack, wind velocity and its distribution with respect to height from the ground.Computation is carried for wind velocity of 2.5, 5, 10, and 15 m/s and collector angles of 90°, 60°, 30°, 0°, −30°, −60°, and −90° with respect to wind directions. Various recirculation regions on the leeward and forward sides of the collector are observed, and both pressure field around the collector and total force on the collector are determined for each condition. The effect of absorber tube on the flow field was found negligible, while the effect of the gap between the two sections of parabola at midsection and the gap between the collector and ground were found considerable on both flow field and pressure distribution around the collector.     

The use of moderate vacuum environments as a means of increasing the collection efficiencies and operating temperatures of flat-plate solar collectors

It is shown that evacuating a flat-plate solar collector to a pressure 1–25 torr results in elimination of the natural convection heat loss from the absorber for absorber-to-cover spacings up to 15 cm. This mode of heat transfer then reduces to pure conduction through the air space between the absorber and the cover. The effect of this reduction on the total upward heat loss from the collector is considered for a variety of collector operating conditions and is shown to be especially pronounced for collectors employing wavelength-selective surfaces (high absorptance for solar radiation, but low emittance for the energy re-radiated by the absorber). Computer simulations of collector performance for the Dallas, Texas area indicate that the combination of a moderate vacuum and a selective surface (α = 0·90, = 0·15) can increase daily energy collection as much as 278 per cent over that obtained with a non-vacuum collector using a flat-black (α = = 0·95) surface and can make it possible to operate at a temperature of 150°C with a daily energy collection efficiency of more than 40 per cent. The theoretical predictions are supported by the results of twelve experiments with a no-load solar tester. At an absorber-to-cover spacing of 7·5 cm, the steady-state temperature of a moderately selective absorber (α = 0·75, = 0·3) was increased from 115°C at atmospheric pressure to 179°C at a pressure of 25 torr.     

A stationary evacuated collector with integrated concentrator

A comprehensive set of experimental tests and detailed optical and thermal models are presented for a newly developed solar thermal collector. The new collector has an optical efficiency of 65 per cent and achieves thermal efficiencies of better than 50 per cent at fluid temperatures of 200°C without tracking the sun. The simultaneous features of high temperature operation and a fully stationary mount are made possible by combining vacuum insulation, spectrally selective coatings, and nonimaging concentration in a novel way. These 3 design elements are “integrated” together in a self contained unit by shaping the outer glass envelope of a conventional evacuated tube into the profile of a nonimaging CRC-type concentrator. This permits the use of a first surface mirror and eliminates the need for a second cover glazing. The new collector has been given the name “Integrated Stationary Evacuated Concentrator”, or ISEC collector. Not only is the peak thermal efficiency of the ISEC comparable to that of commercial tracking parabolic troughs, but projections of the average yearly energy delivery also show competitive performance with a net gain for temperatures below 200°C. In addition, the ISEC is less subject to exposure induced degradation and could be mass produced with assembly methods similar to those used with fluorescent lamps. Since no tracking or tilt adjustments are ever required and because its sensitive optical surfaces are protected from the environment, the ISEC collector provides a simple, easily maintained solar thermal collector for the range 100–300°C which is suitable for most climates and atmospheric conditions. Potential applications include space heating, air conditioning, and industrial process heat.     

Development and testing of a solar air-heater with conical concentrator

The free convection performance of a solar air heater with a cylindrical absorber centred to a conical concentrator for focusing incident solar radiation was studied. The primary objective was to heat air to higher temperatures than those obtainable in flat-plate collectors.The experiments were carried out and the data recorded in summer daytime, considering collector tilting angle and type of absorbing surface as the investigation parameters.It was found that a tilting angle under local latitude would be appropriate for collector installation. Although the efficiency of the heater at free convection conditions was very much smaller than flat-plate solar air-heaters, exit air temperatures reached up to 150 °C, which could allow utilisation in high temperature applications. A selective absorber surface improved appreciably the performance of the solar air-heater.     

Relative cost-effectiveness of periodically adjusted solar collectors using evacuated absorber tubes

Periodically adjusted parabolic mirror/evacuated tube absorber combinations are evaluated using computer simulation methods. The results show that a 4–6X reflector adjusted 10–15 times per year, operating at 150°C, competes favourably in cost-effective terms with a fixed reflector CPC collector operating at 50°C. Periodically adjusted collectors are advocated for medium temperature industrial applications below 200°C.     

Numerical investigation of natural convection heat loss in modified cavity receiver for fuzzy focal solar dish concentrator

In the present work, a 2-D-model is used to investigate the approximate estimation of the natural convection heat loss from an actual geometry of the modified cavity receiver (hemisphere with aperture plate) of fuzzy focal solar dish concentrator. The analysis of the receiver has been carried out based on the assumption of the uniform and maximum solar flux distribution in the central plane of the receiver. The total heat loss from the receiver has been estimated for both the configurations “with insulation” (WI) and “without insulation” (WOI) at the protecting aperture plane of the receiver. The convection heat loss of the modified cavity receiver was estimated by varying the inclinations of the receiver from 0° (cavity aperture facing sideways) to 90° (cavity aperture facing down). The convection heat loss is maximum at 0° and decreases monotonically with increase in angle upto 90°. The effect of operating temperature on convection heat loss for different orientations of the receiver was studied. The results of the numerical analysis are presented for a modified cavity receiver “with insulation” (WI) and “without insulation” (WOI) in the form of Nusselt number correlation: and . The maximum convection heat loss occurs at 0° inclination for both cases of the receiver, which is 63.0% (WI) and 42.8% (WOI) of the total heat loss, though the heat loss in WI configuration is lower than that of WOI configuration. Upon increasing the inclination of the receiver, the convection heat loss reduces to a minimum of 12.5% (WI) and 24.9% (WOI) of the total heat loss at 90°. The result of the present numerical model of standard receiver configuration (modified cavity receiver with insulation at bottom) is comparable with other well-known models.