Generalization of the two-dimensional optical analysis of cylindrical concentrators

A two-dimensional optical analysis of cylindrical concentrators valid for any incidence angle of the solar rays is described. Unlike previous two-dimensional studies, it takes into account: (a) the angle κ defined by the solar rays and a plane perpendicular to the focal line and (b) the variations of the image width as a function of κ.An equation relating κ to solar coordinates has been obtained. The curves of κ as a function of time for several dates and three orientations of the concentrator are presented.The analysis is applied in detail to the cylindrical-parabolic concentrator and to the fixed-mirror solar concentrator, both with flat receivers. The local concentration factor and its mean value for different values of κ are obtained. Using these results and taking into account the useful range of κ, criteria are given to select the concentrator orientation and the receiver width.     

On the use of ‘solar volume’ for determining the urban fabric

An exhaustive analysis for the determination of the urban fabric is carried out by implementing a computerized model SustArc. The model evaluates the maximum allowed ‘solar volume’ to be built without preventing solar access from each building and from the open spaces during a predefined period of the year. For every urban configuration, the model calculates the obtained urban density. The results show that it is possible to achieve high density urban quarters without violating any solar rights. Optimized urban fabric solutions are presented.     

Development and applications of a two-dimensional optical analysis of non-perfect cylindrical concentrators

The two-dimensional optical analysis of cylindrical concentrators valid for any incidence angle of the solar rays described in [1] is extended to non-perfect concentrators. To this end, it is assumed that the normal to each differential element of the specular surface departs from its correct position by an angle , the possible values of which follow a gaussian distribution of mean value and standard deviation σ.The main purpose of the present paper is to determine the intensity distribution at the receiver plane of the mentioned concentrators. The mathematical treatment is given explicitly and applied to the cylindrical-parabolic and the cylindrical fixed faceted-mirror solar concentrators. Both are considered to have flat receivers and to be located in the E-W direction. Concentration factors and geometrical losses are calculated for typical examples of both concentrator types, for several values of the receiver width and different times of the year and hours of the day. This information is required for a subsequent appropriate selection of the receiver width. Results for = 0 and values of σ within 0–6 mrad range are shown.Changes in the mean concentration factors and in the total geometrical losses due to parallel displacements of the receiver plane as referred to its correct position, are analysed as well. Furthermore, losses due to the shadow projected by the receiver onto the concentrator are given; for the fixed faceted-mirror concentrator this is done as a function of the angle of incidence of the radiation.The mentioned analyses are of special importance when deciding the precision to which a given concentrator will be constructed. In this context, it is convenient to take into account that the final precision requirements will come from a compromise between cost and solar energy conversion efficiency.     

Laboratory evaluation and system sizing charts for a ‘second generation’ direct PV-powered, low cost submersible solar pump

A ‘new generation’ solar operated low-power and low capital cost submersible diaphragm pump designed for medium head applications is evaluated in this paper. The pump is designed and made by SHURflo Ltd. and is the 9325 type. The primary use of this pump is in providing water for remote homes and clinics, for human consumption and for agricultural use. In all tests, the pump was connected to a dedicated controller that allows either 12 V or 24 V operation. The experiments were undertaken by using the pumping test rig at CRES, and the evaluation methodology was ‘simulated field conditions’. The instantaneous water flow versus head characteristics were functions of the global irradiance on the array plane. The PV array power varied between 55 Wp and 220 Wp and both voltage modes were examined. The hydraulic efficiency was also calculated with respect to equivalent head. The daily operation charts were obtained by using the instantaneous pump performance in combination with typical daily irradiation profiles and the pump starting and stopping characteristics. These charts are useful for system sizing, taking into account the solar resource at the site of application, the required daily water delivery at a particular head and the available PV array. The results show that with this ‘new generation’ of direct solar-powered pumping systems, the PV array has been minimised, and so has been the capital investment cost, the need for battery storage has been eliminated and adequate water is delivered at an affordable price.     

Test of a “trumpet” secondary concentrator with a paraboloidal dish primary

Secondary concentrators properly designed according to the principles of nonimaging optics can increase the achievable concentration ratio of a point focus solar concentrator substantially above the level for a focussing primary alone. One such secondary is the so called “trumpet” concentrator whose hyperbolic shape is derived from the theory of the geometrical vector flux. A practical version was designed, fabricated and tested recently on the large 11-meter Paraboloidal Test Bed Concentrator operated by the Jet Propulsion Laboratory. The secondary concentration ratio was 2.1x. That is, it reconcentrated the energy in a focal spot of 20.3 cm diameter into a circle of 14.0 cm diameter corresponding to an area reduction of 2.1 to 1. The trumpet performed as predicted by optical models. When operating with a receiver corresponding to a gross geometrical concentration ratio of 4800:1, addition of the trumpet increased the intercepted energy by 30%. Characterized in another way the trumpet increased the geometric concentration ratio from 2200:1 for the primary alone to nearly 4800:1 with an efficiency of >96%. The results of the test demonstrate that with a properly cooled secondary one can either improve the achievable concentration ratio or relax the primary tolerance requirements for essentially negligible increase in system cost or complexity.     

Procedure for thermal performance evaluation of steam generating point-focus solar concentrators

Solar energy can be used for substitution of the depleting fossil fuels in thermal applications and electricity generation through thermal route. For medium and high temperature applications, solar concentrators are required. Proper sizing and selection of concentrator for any thermal application calls for characterization of the concentrator at the required operating temperature. There are few procedures reported in literature for testing and evaluating solar concentrator performance which are based on sensible heating of the working fluid. One of the limitations of these procedures is requirement of precise operating conditions during testing. A test procedure for characterization of point-focus steam generating solar concentrators based on latent heating at different operating temperatures is proposed. The proposed procedure uses the phase change characteristic of water at constant temperature to measure the thermal performance. This procedure can be used to estimate thermal efficiency of solar concentrator at different operating temperatures above 100 °C. This procedure was used to estimate the efficiency of a point-focus solar concentrator having 25 m² aperture area at 161 °C (equivalent to 5.4 bar (g)). The efficiency was estimated as 47 ± 3.5%. The test procedure can be used for field evaluation of existing systems also with minimum amount of instrumentation and controls.     

Conical receiver for a paraboloidal concentrator with large rim angle

The design and optimization of novel type of receiver for a paraboloidal concentrator with 90° rim angle is carried out by means of detailed ray tracing simulations. Cylindrical, conical, and spherical geometries are compared and their dimensions optimized. The chosen design is based on a conical cavity, which differs from similar receivers developed for concentrators with smaller rim angles. In particular, the receiver is able to catch concentrated solar energy both on its outer side and on the inner walls. Water flows inside the receiver along the conical geometry, in a double layer configuration. The receiver was built and implemented in a 90° rim angle paraboloidal concentrator. Thermal efficiency of the system is evaluated for different flow rates and inlet temperatures, both in stationary and in transient regimes, and results for fluid temperatures are compared with the results predicted by a thermal model. The time constant is evaluated.     

Alternative designs of parabolic trough solar collectors

Parabolic trough collector (PTC) is the most established solar concentrating technology worldwide. The conventional parabolic trough collectors are used in various applications of medium and high-temperature levels. However, there are numerous studies which investigate alternative designs. The reasons for examining different PTC configurations regard the thermal efficiency increase, the reduction of the manufacturing cost and the development of more compact designs. The objective of this review paper is to summarize the existing alternative designs of PTC and to suggest the future trends in this area. Optical and thermal modifications are examined, as well as the use of concentrating thermal photovoltaic collectors. The optical modifications include designs with secondary concentrators, stationary concentrators and strategies for achieving uniform heat flux. The thermal modifications regard the use of nanofluids, turbulators and the use of thermally modified receivers with insulation, double-coating and radiation shields. The concentrating thermal photovoltaics are systems with flat or triangular receivers which can operate in low or in medium temperature levels with the proper alternative designs. It has been found that there are many promising choices for designing PTC with higher thermal performance and lower cost. The conclusions of this work can be used as guidelines for future trends in linear parabolic concentrating technologies.     

Amorphous and ‘micromorph’ silicon tandem cells with high open-circuit voltage

For amorphous and ‘micromorph’ silicon multi-junction solar cells, we have developed tunnel recombination junctions consisting of two microcrystalline doped layers with a defect-rich interface. While the solar cells performed reasonably well under AM 1.5 light, we found in spectral response measurements that the first deposited cell of tandem structures in nip and pin configuration was apparently leaking under low light conditions. Insertion of a thin protection layer of n-type amorphous silicon solved this issue, and led to an increase in open-circuit voltage. Voltages as high as 1.76 V have been obtained for a-Si/a-Si pinpin tandem cells.     

Non-linear model for absorption in SiO2 optical fibres: Transport of concentrated solar energy

In order to determine the maximum solar energy than can be transported using SiO₂ optical fibres, analysis of non-linear absorption is required. We propose a model based on Maxwell's equations and the Drude–Lorentz theory to determine the non-linear absorption for the maximum possible concentration ratio for circular concentrators. The relation between the electric susceptibility and the refractive index with microscopic parameters is provided. To solve the non-linear model for absorption experimental parameters are used. Our results estimate that the average value over the solar spectrum for the non-linear extinction coefficient for SiO₂ is k₂=10⁻²⁹ m² V⁻². With this result we conclude that the non-linear part of the absorption coefficient of SiO₂ optical fibres during the transport of concentrated solar energy achieved by a circular concentrator is negligible.     

A new calorimetric facility to investigate radiative‐convective heat exchangers for concentrated solar power applications

A new calorimetric facility for the aerothermal assessment of radiative‐convective heat exchangers in concentrating solar power applications has been developed and is described in this paper. The configuration of volumetric solar receivers enables concentrated sunlight to be absorbed and conducted within their solid volume, from where it is gradually transferred by forced convection to a heat transfer fluid flowing through their structure. Current design trends towards higher thermal conversion efficiencies have led to the use of complex intricate geometries to maximise temperatures deep inside the structure. The work presented aims to aid these objectives by commissioning a new experimental facility for the fully integrated evaluation of such components. The facility is composed of a high‐flux solar simulator that provides 1.2 kW of radiative power, a radiation homogeniser, inlet and outlet collector modules, and a working section that can accommodate volumetric receivers up to 80 mm × 80 mm in aperture. Irradiance levels and flow field nondimensional governing parameters are highly representative of on‐sun experiments at larger scales. Results from experiments with a siliconised silicon carbide monolithic honeycomb are presented, conducted at realistic conditions of incident radiative power per unit mass flow rate to validate its design point operation. Measurements conducted include absorber solid temperature distributions, air inlet and outlet temperatures, pressure drop, incident heat flux, and overall thermal efficiency. The relative influence of different sources of thermal loss is analysed and discussed.     

Optical simulation model for flat mirror concentrators

Different simulation models for solar concentrators were developed to obtain the irradiance distribution on the absorber. Usually these optical simulations were valid only for a particular concentrator. Other simulations adequate for different concentrator shapes are based on ray tracing algorithms requiring high computing resources. An optimized reverse ray tracing model for flat mirror concentrators that allows to reduce the noise and the computing time necessary for such simulations is described.     

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 m². 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.     

Asymmetric CPC solar collectors with tubular receiver: Geometric characteristics and optimal configurations

We derive analytic expressions for the geometric characteristics of extremely asymmetric nonimaging solar concentrators (“asymmetric CPCs”) with tubular receivers, of fixed maximum concentration ratio C_max. We show that, at fixed C_max, there are nonideal concentrator configurations that minimize reflector area and average number of reflections, and determine configurations that are approximately optimal (maximizing yearly energy delivery at minimal material requirements), independent of climate and economics.     

Linear Dielectric Non-Imaging Concentrating Covers For PV Integrated Building Facades

A three-dimensional optical analysis of two dielectric, non-imaging concentrating covers for building integrated photovoltaics shows that an asymmetric concentrator is more suitable for use at building facades. For a wide range of solar incidence angles, optical efficiencies are over 90% for both concentrators. The optimum collection tilt angle for two different latitudes and the monthly and annual collected solar energy for both concentrators are predicted and compared to flat photovoltaic covers of the same photovoltaic and aperture area. Employing high transmittance materials for dielectric concentrating covers enables such refractive systems to achieve high solar energy acceptance thus requiring less photovoltaic material thereby reducing initial capital cost.     

Design and fabrication of low concentrating second generation PRIDE concentrator

Prototype first generation Photovoltaic Facades of Reduced Costs Incorporating Devices with Optically Concentrating Elements (PRIDE) technology incorporating 3 and 9 mm wide single crystal silicon solar cells showed excellent power output compared to a similar non-concentrating system when it was characterized both indoors using a flash and continuous solar simulator. However, durability and instability of the dielectric material occurred in long-term characterisation when the concentrator was made by using casting technology. For large scale manufacturing process, durability, and to reduce the weight of the concentrator, second generation PRIDE design incorporated 6 mm wide “Saturn” solar cells at the absorber of dielectric concentrators. Injection moulding was used to manufacture 3 kWp of such PV concentrator module for building façade integration in Europe. Special design techniques and cost implications are implemented in this paper. A randomly selected PV concentrator was characterised at outdoors from twenty-four (≈3 kWp) 2nd-G PRIDE manufactured concentrators. The initial PV concentrators achieved a power ratio of 2.01 when compared to a similar non-concentrating system. The solar to electrical conversion efficiency achieved for the PV panel was 10.2% when characterised outdoors. In large scale manufacturing process, cost reduction of 40% is achievable using this concentrator manufacturing technology.     

On the clear sky model of the ESRA — European Solar Radiation Atlas — with respect to the heliosat method

This paper presents a clear-sky model, which has been developed in the framework of the new digital European Solar Radiation Atlas (ESRA). This ESRA model is described and analysed with the main objective of being used to estimate solar radiation at ground level from satellite images with the Heliosat method. Therefore it is compared to clear-sky models that have already been used in the Heliosat method. The diffuse clear-sky irradiation estimated by this ESRA model and by other models has been also checked against ground measurements, for different ranges of the Linke turbidity factor and solar elevation. The results show that the ESRA model is the best one with respect to robustness and accuracy. The r.m.s. error in the estimation of the hourly diffuse irradiation ranges from 11 Wh m⁻² to 35 Wh m⁻² for diffuse irradiation up to 250 Wh m⁻². The good results obtained with such a model are due to the fact that it takes into account the Linke turbidity factor and the elevation of the site, two factors that influence the incoming solar radiation. In return, it implies the knowledge of these factors at each pixel of the satellite image for the application of the Heliosat method.     

Assessment of the potential improvement due to multiple apertures in central receiver systems with secondary concentrators

When striving for maximum efficiencies in solar thermal central receiver systems (CRS) the use of gas turbines with bottoming cycles is inevitable. Pressurized volumetric receivers have proven their feasibility and good performance, and their integration into gas turbine cycles has been demonstrated. One disadvantage of this system is the necessity to use secondary concentrators. The sunlight has to be concentrated into the relatively small glass windows of the receiver, which leads to a limited view cone. This means that of all the possible heliostat positions around the tower, only those within the ellipse, resulting from the section boundary of the view cone with the ground plane, are usable.For small systems, for which tower costs are small, the resulting heliostat field layout is similar, with or without secondary concentrator. For large systems, which are more cost-effective, tower costs become significant, and the losses due to atmospheric attenuation and spillage dominate over the cosine losses. Thus, the purely North-oriented fields become increasingly sub-optimal.This article shall demonstrate at what power levels this problem can be alleviated by not using a single, North-oriented aperture, but up to six apertures—each of them associated with a separate heliostat field.     

Comparison of Fresnel concentrators for building integrated photovoltaics

To develop concentrating photovoltaic systems for building integration applications, two optical devices are proposed. The concentrators are based in stationary linear Fresnel lenses and secondary CPC. The moving focal area is ten times smaller than the Fresnel lens aperture. Concentrator characteristics are studied in detail: shadowing effect, placement of the focal area and optical concentration efficiency. The main contribution of this paper is the three-dimensional optical analysis of the non-imaging concentrating systems. In terms of solar radiation, photovoltaic moving modules placed in the focal area of stationary concentrators are compared with simply fixed photovoltaic modules. In favourable weather locations, the beam radiation incident on the concentrating modules would be a large percentage, more than 50%, of the global radiation received by the fixed photovoltaic devices.     

Performance model for two-stage optical concentrators for solar thermal applications

A performance model has been developed for evaluating benefits associated with the addition of a nonimaging secondary concentrator to a conventional paraboloidal solar dish. The model uses a Monte Carlo ray-trace procedure to determine the focal plane distribution as a function of optical parameters and, by evaluating the trade-off between thermal losses and optical gain, calculates the corresponding optimized concentration and thermal efficiency as a function of temperature, both with and without the secondary. These comparative optimizations, carried out over a wide range of design parameters, show that the efficiency of a two-stage concentrator is always greater than that of a single stage if all other design parameters are the same. For example, for a reference design corresponding to a dish with a focal length to diameter ratio of 0.6 and a characteristic slope error of 5 milliradians operated at a receiver temperature of 1000°C, the optimized efficiency with a secondary is 0.70 compared to 0.59 for the primary alone. At fixed focal ratio, the relative performance advantage with a secondary increases, if either the temperature or the primary slope error or both, are increased, whereas it decreases if they are decreased. However, the advantage remains significant at temperatures above 400°C, even in the “high performance limit” of slope errors <2 milliradians.