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.     

An internal cusp reflector for an evacuated tubular heat pipe solar thermal collector

This study involves the optical analysis of a slightly concentrating, symmetric cusp reflector inside a tubular glass envelope with a cylindrical heat pipe as the solar absorber. The basic design features of this non-tracking, evacuated, modular collector and the principles of heat removal are shown in Figs. 1 and 2. Differential equations of the cusp reflector optics, given the geometrical restrictions in Figs. 1 and 2, are derived, and solutions for the largest possible aperture inside a given diameter envelope and acceptance angle are presented.As an extension of the same study, the optical efficiency of a single collector tube has been simulated by means of a Monte Carlo Ray-Tracing Program. For a concentration ratio of 1.15, the flux distribution around the heat pipe is computed as a function of incidence angle. In addition, the impact of mirror defects and absorber misalignment on the optical performance are analyzed.     

Relative cost-effectiveness of CPC reflector designs suitable for evacuated absorber tube solar collectors

Specular reflectors of the fixed CPC type are compared in terms of yearly energy collection and relative cost-effectiveness. The reflector designs used are designed for use with a circular-cylindrical evacuated tubular absorber, and a gap is allowed between reflector and absorber to accommodate the tubular glass envelope and evacuated space. Stainless steel, aluminum, and thin, back-silvered glass mirrors were modelled.The results show that the choice of acceptance angle of a reflector for use with a moderately priced evacuated tube at water heating temperatures is not critical; almost any reflector acceptance angle will do so long as the aperture is carefully chosen, and both North-South or East-West orientations have approximately similar performance. Under such conditions, other factors such as mirror self-cleaning and manufacturing ease may be decisive in the choice of design.At temperatures above 100°C or for high tube costs, an East-West reflector design of concentration >1.4 is strongly indicated.At the time of writing, polished stainless steel is as cost-effective a choice as any other for a mirror material, and is probably more durable and amenable to mass production.     

Optical evaluation and analysis of an internal low-concentrated evacuated tube heat pipe solar collector for powering solar air-conditioning systems

An optical evaluation and analysis of an internal low-concentrating evacuated tube heat pipe solar collector designed to enhance the collection of solar radiation for medium temperature applications is presented in this paper. The internal low-concentrating evacuated tube heat pipe solar collector was designed with an acceptance angle of 20° given a geometrical concentration ratio of 2.92. The truncation of the upper part of the reflector giving a geometrical concentration ratio of 1.95 was carried and enabled the internal low-concentrating evacuated tube heat pipe collector to be enclosed by a borosilicate glass tube with 100 mm and 93 mm outer and inner diameters, respectively. Ray trace analysis at different transverse angles determines optical efficiencies, related optical losses and flux distribution on the absorber of the internal low-concentrating evacuated tube heat pipe solar collector. A detailed two dimensional ray trace techniques considering only the direct insolation component predicated overall ray’s acceptance of 93.72% and optical efficiency of 79.13% from transverse angles of 0° to 20°.     

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.     

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.     

Calorimetric measurement of absorptance and emittance of the Sydney University evacuated collector

Simple calorimetric techniques have been developed for determining the absorptance and emittance of individual evacuated tubular collectors incorporating a selective surface, and the efficiency, η_o, of evacuated collectors in various mirror systems. The absorptance and efficiency measurements are made in natural sunlight without the use of a solarimeter by establishing an absorptance standard based on Nextel black paint. Calibration of solarimeters using the established absorptance standard is discussed. Emittance measurements are made by measuring radiative heat losses from the absorber tube of a collector.Absorptance and emittance measurements for a number of Sydney University evacuated collectors gave values of absorptance α = (0.92 ± 0.01) and emittance = 0.05 at 120°C for the selective surface utilized. Efficiency (η_o) measurements for Sydney University collectors in two simple mirror systems are also reported.     

A comparison of optical performance between evacuated collector tubes with flat and semicylindric absorbers

In this article, concepts of solar irradiance ratio and absorbed energy factor on the surface of the evacuated collector tube absorbers were presented respectively. For evacuated collector tubes with flat and semicylindric absorbers, we used a solar simulator as a light source, measured separately distribution of the solar irradiance ratio that varies with incident angles on various points on the absorber surface in a glass-covered tube, and gave their three-dimensional regressive equations correspondingly. Experimental measurement of solar irradiance ratio and solar absorptance of coatings on absorber surfaces was carried out. On this basis, rules of absorbed energy factors on absorbers in two shapes that vary with incident angles were analyzed and studied. According to clear-day model, the daily absorbed energy and its annual changes of single evacuated collector tubes with absorbers in two shapes placed under 40° northern latitude, 40° inclined angle and south orientation were calculated and compared. The results show that the annual absorbed energy of evacuated collector tube with a semicylindric absorber is 15.9% higher than that with a flat absorber. In addition, optimized incident angles for the absorber in two shapes of evacuated collector tubes operated in a whole year were tentatively investigated.     

Solar process heat generation using an ETC collector field with external parabolic circle concentrator (PCC) to operate an adsorption refrigeration system

The design and construction of a collector system for process heat generation up to 150°C is described. The thermal energy is used to operate an ammonia/carbon ice maker generating block-ice for the fishing industry in developing countries.Various analytical and experimental investigations have shown that with operational temperatures beyond 100°C concentrators like CPCs (Winston, 1974; Winston and Hinterberger, 1976; Rabl, 1976; Winston and Welford, 1978) or similar ones improve substantially the efficiencies of evacuated tubular collectors (ETC). We developed and investigated a shape consisting of segments of parabolas and circles, respectively, because of the far easier manufacturing (Schreitmüller et al., 1991; Niemann, 1992; Niemann and Schreitmüller, 1993). The envisaged collector held consists of evacuated tubular collectors with external parabolic circle concentrators (PCC). The non-tracking PCC-collector with a concentration ratio of 5 is mounted in an east-west direction.The ammonia carbon adsorption ice maker was developed by the University of Warwick, Department of Engineering, Coventry, U.K. (Critoph and Gong, 1992). This unit is a rapid cycling refrigerator, producing 500 kg ice per day, in which two complete adsorption systems operate out of phase. This means one is heated whilst the other is cooled, resulting in a semi-continuous refrigeration process.     

An evaluation on thermal performance of CPC solar collector

The main objective of this work is the investigation and improvement of thermal performance of evacuated CPC (Compound Parabolic Concentrator) solar collector with a cylindrical absorber. Modified types of this solar collector are always combined with the evacuated glass envelop or tracking system. The conventional stationary CPC solar collector has been compared with the single axis tracking CPC solar collector in outlet temperature, net heat flux onto the absorber and thermal efficiency. Numerical model has been analyzed based on the irradiation determined actually and the results have been calculated to predict the thermal efficiency. Based on the comparison of the measured and calculated results, it is concluded that the numerical model can accurately estimate the performance of solar collectors. The result shows the thermal efficiency of the tracking CPC solar collector is more stable and about 14.9% higher than that of the stationary CPC solar collector.     

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.     

Optimization of stationary nonimaging reflectors for tubular evacuated receivers aligned north-south

Several manufacturers are developing solar collectors with tubular evacuated receivers aligned north-south. Adding low-concentration, wide-acceptance-angle reflectors to such tubes allows greater tube spacings, reducing the number of tubes per area of collector. It also improves collector efficiency, particularly for conditions of high , such as high temperatures or low light levels. This detailed study optimizes the reflector design for maximum daily energy collection and includes the effects of reflection losses, reflector-receiver alignment errors, variation of selective surface absorptance with angle of incidence on the receiver, and losses through the gap between the receiver and the reflector.Three general conclusions have been reached: The use of optimized nonimaging reflectors with tubular evacuated receivers will increase the energy collection efficiency—particularly for high-temperature and harsh environment conditions.Wide-acceptance-angle reflectors are forgiving to receiver-reflector alignment errors. It is neither necessary nor desirable to design reflectors for undersize receivers in order to compensate for misalignments that result from manufacturing tolerances.The daily energy collection of collectors using these reflectors having acceptance half-angles in a range. near 60° is not a sensitive function of the acceptance angle. Manufacturers' final reflector design decisions will probably be based on technical considerations related to fabrication and assembly techniques and possibly on market-related considerations such as collector appearance.     

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.     

Bonding solar-selective absorber foils to glass receiver tubes for use in evacuated tubular collectors: Preliminary studies

The glass receiver tubes used in evacuated tubular collectors are currently vacuum coated with a selective surface in a batch process. Solar-selective absorber foils can be produced successfully by a number of methods and offer economic advantages over batch collector coatings in many instances. Bonding of these foils to a variety of surfaces is done at present with adhesives whose upper temperature limit is about 250°C. However, many absorber coatings are stable to much higher temperatures. Thus, suitable techniques for bonding solar-selective absorber foils to high-temperature (up to 400°C) glass receivers would be desirable. This paper describes possible foil-glass bonding techniques and the results of experiments on selected techniques, principally electrical field-assisted bonding and the use of glass sealing frits. Foil-bonded samples were thermally cycled in a vacuum to temperatures typical of stagnation in evacuated collectors, and changes in bond strength and foil optical properties were observed. Finally, the economic viability of foil bonding processes is discussed.     

Water-in-glass manifolds for heat extraction from evacuated solar collector tubes

Measurements are reported on three novel manifolds of the water-in-glass type for evacuated all-glasssingle-ended tubular collectors. The manifolds provide for series connection of tubes, but because there is virtually no partitioning of the inner volume of the collector tubes, the manifolds are extremely simple and exhibit low impedance to fluid flow. The efficiency of heat extraction from the tubes has been determined by measuring temperatures at various points on the surface of glass tubes in a panel of area 1.2 m² while heating the tubes electrically to simulate solar energy input. Measurements have been made for a range of tube inclinations (0–80°), water flow rates (0.5–5 lmin⁻¹, water inlet temperatures (13–70°C), and effective solar fluxes (100–1000 W/m²) for two absorber tube diameters. The results show that for a wide range of operating conditions buoyancy effects alone result in efficient heat transfer to the tops of the tubes. The manifold designs described offer a possible low cost solution to the problem of manifolding evacuated collectors for sub-100°C heat extraction for domestic and industrial applications.     

Thermal conversion with fluorescent concentrators

Fluorescent planar collector-concentrators are a new possibility for the conversion of solar energy into thermal energy. The collection and concentration of direct and diffuse radiation is feasible, using a transparent sheet of material doped with a fluorescent dye. The collector offers the advantage of separating the global irradiation into different spectral regions. This geometrical and spectral concentrated light can be converted with adapted highly selective absorbers into high temperature heat. Intensity and spectral region of the sunlight and the selectivity of the absorber determine the thermodynamically possible maximum absorber temperature. A test collector with a fluorescent concentrator area of 0.8 m² with an absorber pipe of 3 mm diameter in an evacuated glass tube was built. At a total irradiation of 850 W/m² on the fluorescent collector surface, a maximum stagnation temperature of 555°C (828 K) was reached. Under diffuse light conditions (150 W/m²), stagnation temperatures above 250°C (523 K) were measured. Thermodynamic calculation, experimental setup and results are given.     

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.     

Technical note: Comparison between inverted absorber asymmetric and symmetric tubular-absorber compound parabolic concentrating solar collectors

The Hottel–Whiller–Bliss characteristics of particular comparable examples of asymmetric and symmetric compound parabolic concentrating solar energy collectors are presented. It is shown that, over the relevant range of operating conditions at an optimum absorber configuration, an asymmetric inverted flat absorber compound parabolic concentrating collector has a solar conversion efficiency 10–15% greater than a comparable tubular absorber symmetric compound parabolic concentrating collector.     

Analytic modeling of a solar power plant with parabolic linear collectors

An analytic model for a solar thermal electric generating system with parabolic trough collectors was developed. The energy conversion of solar radiation into thermal power along the absorber tube of the parabolic collector is studied, taking into consideration the non-linearity of heat losses and its dependence on the local temperature. The coupling between the collector and the thermodynamic cycle is made up of three heat exchangers, yielding the characteristic temperatures of the cycle. The conventional Rankine cycle is treated as an endo-reversible Carnot cycle, whereby the mechanical and electric power is calculated. For comparison, we refer to the Solar Electric Generating System VI (SEGS VI), installed in the Mojave desert-CA, whose solar field is composed by LS2 parabolic trough collectors. We simulated the efficiency curves of collectors LS2 with evacuated and non-evacuated absorbers and compared with experimental results. A second simulation was carried out to estimate the optimum quantity of non-evacuated LS2 collectors in series in a collectors’ row, when friction losses along the absorber tubes are considered. Also, the performance of a 30 (MWe) power plant, composed of 50 rows with 16 LS2 collectors in series (total 800 collectors) was simulated. Three fields of different collectors were considered, the first field with evacuated absorbers, the second with non-evacuated absorbers and the third with bare absorbers. Finally, the output power of the plant is analyzed as a function of the evaporation temperature of the water-vapor fluid. A large maximum of the overall cycle efficiency is found for evaporation temperatures around 320 °C. Good agreement is obtained when comparing the results of this model with experimental data belonging to the Solar Electric Generating Systems (SEGS) installed in the Mojave desert. The analytic model developed combines simplicity, precision and flexibility, making it an attractive tool for simulation and design of solar power stations.     

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.