CPC Collector Research Articles

Development and performance analysis of compound parabolic solar concentrators with reduced gap losses—‘V’ groove reflector

A system has been developed to use compound parabolic concentrators to collect solar energy and to generate steam. A CPC reflector profile with a V groove at the bottom of the reflector to reduce the gap losses was designed with a half acceptance angle of 23.5° for a tubular absorber of OD 30 mm. Five troughs fabricated with fiberglass substrate pasted over with UV stabilized self-adhesive aluminized polyester foil having high specular reflectivity joined together side by side comprise the CPC module with an aperture area of 2.04 m 2. Copper tubes coated with NALSUN selective coatings and enclosed by borosilicate glass envelope act as absorbers. The reflector absorber assembly housed in a single glass wool insulated wooden box forms the CPC collector. Using water as the heat transfer fluid efficiency tests were carried out with different inlet temperatures. In situ steam generation testing and possible application to steam cooking were also carried out. A theoretical modeling was developed by setting up different heat balancing equations and a reasonable agreement between theoretical computed values and the experimental values was observed.

Concentrating versus non-concentrating reactors for solar photocatalytic degradation of p-nitrotoluene-o-sulfonic acid

The photocatalytic oxidation of the non-biodegradable p-nitrotoluene-o-sulfonic acid (p-NTS) in homogeneous (photo-Fenton reactions) and heterogeneous (with TiO2) solutions has been studied at a pilot-scale under solar irradiation at the Plataforma Solar de Almeria (PSA). In this study two different reactors were tested: a medium concentrating radiation system (Heliomans, HM) and a non-concentrating radiation system (CPC). Their advantages and disadvantages for p-NTS degradation have been compared and discussed. The degradation rates obtained in the CPC collector are around three times more efficient than in the HM collectors. However, in both systems, 100% of the initial concentration of p-NTS was removed. Kinetic experiments were performed in both systems using TiO2 suspensions. During the photodegradation, the disappearance of p-NTS was followed by HPLC, the mineralization of the solution by the TOC technique, the evolution of NO3-, NO2-, and SO4(2-) concentration by ionic chromatography, the toxicity by the standard Microtox test, and the biodegradability by BOD5 and COD measurements. The obtained results demonstrated the utility of the heterogeneous catalysis (using TiO2 as catalyst) as a pretreatment method that can be followed by a biological process.

The use of CPC collectors for detoxification of contaminated water: Design, construction and preliminary results

Photocatalytic processes in the presence of titanium dioxide provide an interesting way to destroy hazardous organic contaminants. Part of CIEMAT’s efforts to commercialize solar photocatalytic detoxification technology has involved the development of photoreactor designs. Although present treatment costs for the solar water detoxification system are higher than for conventional technologies, potentially greater cost reductions are being investigated through the use of one-sun systems. Aspects of these systems that might realize cost reductions include novel collector materials, the manufacturing process and the economics of scale. These issues are being examined extensively in the one-sun photoreactor research effort. A one-sun compound parabolic concentrator designed by CIEMAT and fabricated by the Instituto de Maquina Herramienta may be constructed at a cost under $160/m 2 with an annual efficiency of 71.3%. Studies reported in this paper also evaluate the performance of the low cost CPC prototype built at a previous stage using a model compound, and explore the feasibility of this concept as the basis for solar photocatalytic oxidation facilities. Our preliminary findings show that the overall treatment cost can be reduced mainly by reducing the unit catalyst cost instead of the unit reactor cost.

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.

99/03276 Simulation model of a CPC collector with temperature-dependent heat loss coefficient

99/03276 Simulation model of a CPC collector with temperature-dependent heat loss coefficient

Simulation model of a CPC collector with temperature-dependent heat loss coefficient

We describe a mathematical model for the optical and thermal performance of non-evacuated CPC solar collectors with a cylindrical absorber, when the heat loss coefficient is temperature-dependent. Detailed energy balance at the absorber, reflector and cover of the CPC cavity yields heat losses as a function of absorber temperature and solar radiation level. Using a polynomial approximation of those heat losses, we calculate the thermal efficiency of the CPC collector. Numerical results show that the performance of the solar collector ( η vs. Δ T f(0)/ I coll) is given by a set of curves, one for each radiation level. Based on the solution obtained to express the collector performance, we propose to plot efficiency against the relation of heat transfer coefficients at absorber input and under stagnation conditions. The set of characteristic curves merge, then, into a single curve that is not dependent on the solar radiation level. More conveniently, linearized single plots are obtained by expressing efficiency against the square of the difference between the inlet fluid temperature and the ambient temperature divided by the solar radiation level. The new way of plotting solar thermal collector efficiency, such that measurements for a broad range of solar radiation levels can be unified into a single curve, enables us to represent the performance of a large class of solar collectors, e.g. flat plate, CPC and parabolic troughs, whose heat loss functions are well represented by second degree polynomials.

An improved dynamic solar collector test method for determination of non-linear optical and thermal characteristics with multiple regression

The objective is to characterise the solar collector during a relatively short testing period with no requirement for steady state climatic conditions. This information is then used for predicting annual performance of the collector. A standard collector model that is compatible with the ISO 9806-1 test standard is used with correction terms for beam and diffuse incidence angle modifiers, thermal capacitance, wind speed and sky temperature. This results in a more complete characterisation of the collector. The collector parameters are identified by multiple linear regression, MLR. The method has been tested for characterisation of unglazed collectors, glazed flat plate collectors, evacuated tubular collectors, CPC collectors and concentrating collectors with satisfying results. Typically the correlation coefficient R 2 is better than 0.99 and the standard deviation of the difference between model and measurement is in the range 3–10 W/m 2. In the original method the angular dependence of the optical efficiency and the temperature dependence of the heat losses are supposed to be adjusted to a predetermined function. The most recent development is a routine that makes it possible to accurately identify non-linear optical and thermal performance. This extended MLR method can identify the zero loss efficiency for every angle of incidence interval and the temperature dependent heat losses for every temperature interval. This opens the application of the method to collectors with special incidence angle and heat loss effects that cannot be described easily with a combination of elementary functions. Instead a table of parameter values is determined, which is used directly in standard simulation programmes. This method will further increase the accuracy when comparing different collector designs. It has been used for comparing different glazings and for comparison with spectrophotometric measurements. It has also been used for analysing the heat loss factors for Teflon and honeycomb glazings. Since the total power output of the collector is less dependent on the heat loss coefficient than on the optical efficiency the scattering in this data is larger than for the incidence angle curves. The reflectance of booster mirrors cannot be derived with the MLR-method with acceptable accuracy. The correlation between direct irradiance and irradiance from the reflector exhibit a very strong correlation. Instead the effective reflectance of the mirror can be estimated by comparison of the measured output with calculation by the complete collector and reflector model. This effective reflectance is not compatible with the specular reflectance obtained from spectrophotometric measurements caused by large differences in acceptance angles. Standard multiple linear regression available in most spread sheet and statistical programs can be used for the parameter identification in the extended MLR-procedure. The identification takes only a few seconds. At the Älvkarleby Laboratory the test method is now used as a routine tool for the evaluation of new collector materials and designs. The Swedish National testing institute has evaluated the methods with the conclusion that they have a potential for being used in standardised collector testing.

On the factorisation of incidence angle modifiers for CPC collectors

It has been suggested earlier that the incidence angle modifier K τα for low concentrating collectors with tubular absorbers could be factorised according to K τα ( θ t, θ l) ∝ K τα ( θ t,0) K τα (0, θ l, where θ tand θ l are the projected incidence angles in the transversal and longtitudinal projection planes, respectively. Ray-tracing calculations on low-concentrating CPC collectors with flat absorbers parallel to the cover show that a K τα factorisation overestimates the annual delivered energy from the collector by about 4–5%, when compared to calculations using the full incidence angle modifier. Data from outdoor testing has been used for characterization of incidence angle behaviour for a truncated CPC with a concentration of C = 1.56. Multiple linear regression analysis was used. This analysis technique makes feasible the determination of angular dependent incident angle modifiers and is an efficient tool to use for all collectors which cannot be characterised by standard equations of the incidence angle dependence.

Low-concentrating CPC collectors for photocatalytic water detoxification: comparison with a medium concentrating solar collector

The photocatalytic oxidation of 2,4-Dichlorophenol (DCP), using TiO2 suspensions under solar radiation, has been studied at pilot-plant scale at the Plataforma Solar de Almería (PSA). Two different reactor designs were tested: a medium concentrating radiation system called a Parabolic-Trough-Collector Reactor, PTCR, equipped with two motors (azimuth and elevation) to adjust the position of the module perpendicular to the sun, and a low-concentrating radiation system, the Compound-Parabolic-Concentrator Reactor, CPCR, facing south and inclined 37 degrees. Substrates were dissolved in water to required mg L−1 levels in a reservoir tank. In both cases, 0.2 g L−1 of the suspended TiO2 catalyst was used in a 250 L solution of the contaminant, which was recirculated through the photoreactors using a centrifugal pump and an intermediate reservoir tank. The advantages and disadvantages of the two types of photoreactors in DCP oxidation are compared and discussed. The strong potential of photocatalytic peroxydisulphate-assisted degradation in high DCP concentrations was demonstrated in both systems, and chemical actinometry (the decomposition reaction of oxalic acid by radiated uranyl salts) in the CPC reactor is compared with the results obtained in the PTC.

Low-concentrating CPC collectors for photocatalytic water detoxification: Comparison with a medium concentrating solar collector

The photocatalytic oxidation of 2,4-DicWorophenol (DCP), using TiO2 suspensions under solar radiation, has been studied at pilot-plant scale at the Platafonna Solar de Almeria (PSA). Two different reactor designs were tested: a medium concentrating radiation system called a Parabolic-Trough-Collector Reactor, PTCR, equipped with two motors (azimuth and elevation) to adjust the position of the module perpendicular to the sun, and a low-concentrating radiation system, the Compound-ParaboLic-Concentrator Reactor, CPCR, facing south and inclined 37 degrees. Substrates were dissolved in water to required mg L-1 levels in a reservoir tank. In both cases, 0.2 g L-1 of the suspended TiO2 catalyst was used in a 250 L solution of the contaminant, which was recirculated through the photoreactors using a centrifugal pump and an intennediate reservoir tank. The advantages and disadvantages of the two types of photoreactors in DCP oxidation are compared and discussed. The strong potential of photocatalytic peroxydisulphate-assisted degradation in high DCP concentrations was demonstrated in both systems, and chemical actinometry (the decomposition reaction of oxalic acid by radiated uranyl salts) in the CPC reactor is compared with the results obtained in the PTC.

Optical and thermal testing of a new 1.12X CPC solar collector

A new CPC collector is described and tested, both optically and thermally, in different configurations. This CPC has a receiver shaped as an inverted “V”, which was conceived to accommodate a large gap between itself and the reflector, without optical losses, a novelty which distinguishes this device from others previously proposed. Its acceptance angle was chosen to be large (56.4°, before truncation, and 76° after truncation) in order to allow for both fully stationary E.W. and N.S. possible orientations for the collector to be used. Collector height was chosen to allow for the inclusion of convection suppression mechanisms, to enhance the thermal performance of the collector at high temperature. Measurements of instantaneous efficiency are presented, for the collector in E.W. and N.S. orientation. Measurements are also presented for the instantaneous efficiency when a double cover configuration achieved with a Teflon film and a transparent insulation material of the honeycomb type are included. It is shown that the collector's low heat losses are reduced by roughly 30% trough the addition of such convection suppression devices. Based on these results, the average yearly performance is calculated and compared with the energy delivered by other collector types (flat pates, evacuated tubular collectors). It is shown that, up to 100°C, the present collector outperforms the others, and has a cost which is comparable, if not potentially lower than conventional flat plates.

A spray evaporation type solar still

The single effect basin type solar still has been for long the cheapest way to produce drinkable water from sea water using solar equipment (Howe and Tleimat, 1977). Nevertheless the operating efficiency is low, around 35 %, due mainly to the rejection to the atmosphere of the latent heat of condensation and consequently the production is also low, less than 5 1/m 2 day for a good insolation climate. Furthermore the device presents some operation problems like condensation on the inner side of the glass and algae growth both decreasing the radiation absorption of the basin, and others. Several attempts have been made to increase production, based mainly on the utilization of several evaporation stages using in each stage the latent heat of condensation rejected by the preceding stage (Férnandez and Chargoy, 1990, Joyce et al., 1993). Together with the recover of the latent heat one can increase the evaporation area either by utilizing a fabric where the previously heated salt solution circulates (Baumgartner et al., 1991) or by using some kind of high contact surface material like sponges or thorn bushes (AQUASOLAR, 1988). Another possibility of increasing the evaporation area is just by spraying the salt solution in air and then condensate the moisten air. The present paper describes the details of construction of a device based on spray evaporation and presents the results obtained in laboratory using water previously heated by electric resistances. The experimental results where then used to tune a computational model of the chamber [6] to obtain the performance (daily production and energy consumption) of the system when connected to a CPC solar collector. Testing of the device with CPC collectors will be carried on in the summer of 94.

Performance of an array of compound parabolic concentrations with plain tubular receivers

Operational aspects for an array of four 1.27X-CPC troughs, connected in series, were evaluated using solar radiation to establish the characteristic performance curve. To produce the limbs of each concentrator, a carefully prepared master mold was fabricated from which castings of polyester resin/fiber glass were produced and made reflective with a film of aluminized acrylic laminate. Four different cases investigated included the array with and without an acrylic cover plate and provided with copper receiver tubes which were either oxidized or made black using a special treatment which increased the optical efficiency 7–10%. The experimental results obtained were used to generate efficiency vs Δt/ G relationships. For each case, these relationships could be represented linearly with intercepts to yield optical efficiencies that ranged from 0.545 up to 0.695. The introduction of the cover plate, although decreasing the optical efficiency by 15%, served to suppress heat losses by a factor greater than two. This type of solar collector is suited for applications such as hot water generation, space conditioning, and moderate temperature process heat. Even though CPC collectors have been studied for more than 10 years, many new potential applications remain.

CPCs with segmented absorbers

One of the most promising means of improving the performance of solar thermal collectors is to reduce the energy lost by the hot absorber. One way to do this, not currently part of the technology, is to recognize that since the absorber is usually not irradiated uniformly, it is therefore possible to construct an absorber of thermally isolated segments, circulate the fluid in sequence from low to high irradiance segments, and reduce loss by improving effective concentration. This procedure works even for ideal concentrators, without violating Winston's theorem. Two equivalent CPC collectors with single and segmented absorber were constructed and compared under actual operating conditions. The results showed that the daily thermal efficiency of the collector with segmented absorber is higher (about 13%) than that of the collector with nonsegmented absorber.

A model for the directional distribution of the diffuse sky radiance with an application to a CPC collector

The development of a new, semi-empirical model for the directional distribution of the diffuse radiance is reported. The proposed regression-type model, with the form of its base functions obtained from physical principles, is based on a combination of the purely physical reasoning approach and the purely empirical approach. Direct multiple-scattering calculations are circumvented through the use of the method of successive orders of scattering. The model is calibrated for the mean diffuse radiance estimated under all sky conditions reported in a large and comprehensive diffuse radiance data set. It is found that only a small increase in accuracy is gained by including higher orders of scattering, and this increase does not justify the complexity of the resulting model. Therefore, the single scattering approximation is recommended. The use of the model is illustrated in a typical application, in which the fraction of diffuse radiation intercepted by the receiver of a compound parabolic concentrator is computed.

Flow distributions of the heat transfer fluid for E-W and N-S alignments of CPC solar energy collectors

Abstract The flow distributions through the absorber tubes of a CPC collector are analysed for both E- W and N-S alignments of the collector. It is shown that the flow distribution is non-uniform in an E-W alignment, compared with a close approximation to a uniform distribution for the N-S alignment. However, both flow distributions are shown to be beneficial with respect to the collector's thermal performance.

A generalized thermal analysis of tubular solar collectors

A generalized thermal analysis of tubular solar collectors such as the CPC, CPC with bare absorber and flat-plate (all having U-tube fluid carriers) is presented. The analysis is applicable for smaller as well as larger size troughs. It has been found that earlier published results of thermal analyses of the CPC collectors are not applicable to all CPC collectors encountered in the literature. The present analysis can, however, be successfully applied to all such collectors.

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 4–6 X 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.

Description and Testing of a Non-Evacuated 1.5×CPC Collector Thermal Performance Comparison With Other Collector Types

A 1.5 × non-evacuated CPC type concentrator is described and tested. The results obtained can be summarized by F′ηo = 0.673 ± 0.001 and F′U = (2.64 ± 0.041) W/m2 ° C. The early average performance of the concentrator is calculated and compared with the performance of two other collector types at constant operating temperature: a selectively coated regular flat plate and an evacuated tube type collector. It is shown that the concentrator performs better than both the flat plate and the evacuated tube collector for constant operating temperatures for 35° C to 100° C in a climate like the one in Lisbon. The three collectors are also compared operating in two systems: (1) DHW in which they all deliver comparable yearly average amounts of energy, and (2) IPH at 95° C (process return temperature = 65° C) in which the flat plate delivers ∼30 percent less yearly energy on the average in comparison with the other two which behave very much in the same way. The 1.5 × low cost is discussed in comparison with the other two collector types, establishing the concentrator as an excellent choice for hot water heating applications.

Empirical equations for calculation of CPC collector loss coefficients

Empirical equations for calculation of CPC collector loss coefficients