New high-flux two-stage optical designs for parabolic solar concentrators

We present a new two-stage optical design for parabolic dish concentrators that can realistically attain close to 90% of the thermodynamic limit to concentration with practical, compact designs (e.g., at parabola rim half-angles of around 45°). For comparison, the parabolic dish-plus-compound parabolic concentrator secondary design, at this rim angle, achieves no more than 50% of the thermodynamic limit. Our new secondary concentrator is tailored to accept edge rays from the parabolic primary, and incurs less than one reflection on average. It necessitates displacing the absorber from the parabola's focal plane, along the concentrator's optic axis, toward the primary reflector, and constructing the secondary between the absorber and the primary. The secondary tailored edge-ray concentrators described here create new possibilities for building compact, extremely high flux solar furnaces and/or commercial parabolic dish systems.     

Compact high-flux two-stage solar collectors based on tailored edge-ray concentrators

The recently-invented tailored edge-ray concentrator (TERC) approach permits the design of compact two-stage high-flux solar collectors, with a focusing primary reflector and a non-imaging TERC secondary reflector. We present a new primary reflector shape based on the TERC approach and a secondary TERC tailored to its particular flux map, such that more compact concentrators emerge at flux concentration levels in excess of 90% of the thermodynamic limit. Calculations and raytrace simulation results are also offered which demonstrate that V-cone approximations to a wide variety of TERCs attain the concentration of the TERC to within a few percent. These V-cones represent practical secondary concentrators that may be superior to corresponding compound parabolic concentrator or trumpet secondaries.     

A novel procedure for the optical characterization of solar concentrators

A novel procedure for the optical characterization of solar concentrators is presented. The method is based on recording at night the light of a star reflected by the mirror. Images of the mirror taken from its focal region allow the reconstruction of the slope map. The application of this technique for the in situ characterization of heliostats is particularly simple and at very low cost. Results on first tests carried out with a heliostat of the CESA-I field at the Plataforma Solar de Almeria have shown the feasibility of this technique. Uncertainties in the reconstructed slopes of about 1.0 mrad have been estimated.     

High-flux photovoltaic solar concentrators with kaleidoscope-based optical designs

We propose, analyze and offer sample designs and results for a high-flux photovoltaic concentrator comprised of a large-aperture paraboloidal-dish primary concentrator, and a second-stage kaleidoscope flux homogenizer. The following key design aims are all satisfied: (1) highly uniform irradiance on the solar cell absorber; (2) maximum collection efficiency; and (3) not exceeding the prescribed target flux level (for illustrative purposes here taken to be 500 suns), despite the dish being capable of much higher concentration. As a result of recent advances in the low cost and ease of production of large dish concentrators, the kaleidoscope-based design offers an intriguing alternative to other high-concentration optical designs developed to date. Admissible kaleidoscope geometries are identified. We generate quantitative results for a compact practical design that incurs low optical losses, and produces a highly homogeneous flux map.     

Optimum power from a solar thermal power plant using solar concentrators

In this paper, the optimum temperature of operation of a solar concentrator and thus the maximum power obtained from a solar thermal power plant has been calculated. Results are plotted graphically and discussed.     

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.     

An optical profilometer for the characterisation of parabolic trough solar concentrators

An optical profilometer has been developed based on the idea that the large panels that compose solar-power concentrators and a tolerance threshold for slope-errors to some milliradians, can allow the use of a geometric-optics framework and the investigation of ray-paths by means of a laser beam. The instrument scheme is discussed in detail together with the related problems and the solutions adopted. Data analysis also makes use of an innovative method, based on the iterative application of a criterion about the intersection between the tangents of adjacent points, which allows a realistic profile of the reflector-surface to be drawn. The methodology adopted allows us to call our instrument a profilometer rather than an instrument that gives only information about the tangent of the profile. The instrument accuracy on profile and arctangent deviations from the ideal parabola is 20 μm and 15 μrad, respectively. The results obtained for a reflecting panel are also reported.     

Design factors of sensors for the optical tracking systems of solar concentrators

Basic diagrams for the sensors of the optical tracking systems of solar concentrators are considered, the design factors that determine their accuracy are analyzed, a new sensor design is suggested, and its optimal parameters are determined.     

Computations of the optical properties of metal/insulator-multilayers for solar thermal applications

A computer program has been developed to calculate the optical properties of metal/ insulator-multilayer systems from complex refractive indices of corresponding bulk materials. In addition, interdiffusion in multilayer structures could be detected by fitting experimental reflectance spectra. Among M/Al₂O₃-systems, M = (Ta, Mo, W, orPt), optimum multilayer structures were evaluated as selective absorbers for solar thermal applications at various operating temperatures. The influence of the kind of metal, the modulation number, and the layer thicknesses on the solar thermal performance were studied. At temperatures of T_B ≈ 1100 K and 100-fold sunlight concentration Mo/Al₂0₃- and W/Al₂O₃-multilayers with a modulation number of 6.5 and metal layer thicknesses of a few nm exhibited an absorptance of merit up to A_m ≈ 0.8. Lower temperatures favoured Pt-systems (A_m ≈ 0.94 at 400 K) and higher temperatures Ta-systems (A_m ≈ 0.72 at 1400 K).     

Nontracking solar concentrators

We derive the theoretical upper limit for concentration of direct solar radiation at low latitudes with nontracking concentrators from the projected solid angle sampled by the apparent motion of the sun, for the case where the energy efficiency is referred to the energy incident on the entrance aperture. Based on the fact that the solar radiation is not uniformly distributed within this projected solid angle and that the apparent solar motion is known, we derive the optimal acceptance as a function of direction and time, which means rejecting the lower density radiation and switching off the device when losses would be higher than gains. Just as a device may gain concentration by rejecting radiation from certain directions, it can also gain by not operating at all, thus avoiding losses at certain times. Trough-type systems, which have translational symmetry, cannot be ideal nontracking concentrators, but for low losses they perform only slightly worse than general three-dimensional concentrators.     

Performance comparison of wedge-shaped and planar luminescent solar concentrators

A wedge-shaped luminescent solar concentrator (LSC) with the capability of high collection efficiency and flux gains is described. Monte Carlo simulations of direct insolation conditions for a mid-latitude location are used to form a comparison of a realistically sized wedge LSC and conventional LSC with the same area footprint. The results show that when the sun is high in the sky, such as during early summer, the planar LSC outperforms the wedge LSC in terms of efficiently concentrating light. Under these conditions, the average wedge LSC concentrator efficiency is 3.5%, while the planar LSC achieves an efficiency of 6.3%. However, when the sun stays low in the sky, such as during early winter, the wedge LSC concentrates light with a maximum efficiency of 32.8%; more than four times greater than that of the planar LSC at 7.6%. Moreover, on a seasonal basis, the wedge LSC is estimated to produce more electrical energy per square meter of PV cells than a planar LSC or conventional solar panel placed parallel to the horizon.     

Reverse ray-tracing model for the performance evaluation of stationary solar concentrators

The yearly energy collection efficiency of stationary solar concentrators can be evaluated using reverse ray-tracing, and a solar radiation model. In reverse ray-tracing, rays originating at the receiver of the concentrator are traced towards the surrounding hemisphere. The method allows for the evaluation of the absolute energy collection: new concentrators may be optimized for location and tilt, requiring one-time ray-tracing. The tilt of existing concentrators is optimized. Only possible solar incidence is considered by our model. The method is fast and realistic; it can be modified for concentrators in tilt operation.     

Planar concentrators for flat-plate solar collectors

A systematic study has been made of the effectiveness of planar specular reflectors for solar energy collectors. Two daily averaged indices of performance were used. One, the area ratio, indicates the amount by which the reflector extends the effective receiver area. The other is the enhancement factor, which is used to compare the energy received by an augmented collector with that by a reference collector at optimum tilt.

A reflector can be mounted either above or below a flat-plate collector. Both combinations are evaluated fully, by varying separately the angular position and dimensions of the reflector and of the collector. The principal parameters are identified and the main characteristics summarised as a series of performance curves. These curves provide an easy method for determining optimum reflector geometries.

Use of the performance curves may be extended to obtain the configuration of the two reflectors in a trough concentrator. This also allows the single-reflector system to be compared directly with the trough concentrator. Evidence is presented which shows the advantages of an asymmetrical trough configuration over a symmetrical concentrator.     

Perspectives for solar thermal applications in Taiwan

Taiwan has long depended on imported fossil energy. The government is thus actively promoting the use of renewable energy. Since 2000, domestic installations of solar water heaters have increased substantially because of the long-term subsidies provided for such systems. However, data on the annual installation area of solar collectors in recent years indicated that the solar thermal industry in Taiwan has reached a bottleneck. The long-term policy providing subsidies must thus be revised. It is proposed that future thermal applications in Taiwan should focus on building-integrated solar thermal, photovoltaic/thermal, and industrial heating processes. Regarding building-integrated solar thermal systems, the current subsidy model can be continued (according to area of solar collectors); nevertheless, the application of photovoltaic/thermal and industrial heating systems must be determined according to the thermal output of such systems.     

Etendue-matched two-stage concentrators with multiple receivers

A possible way to concentrate sun light is by using a Fresnel reflector: a large number of small mirrors (called heliostats) that mimic the behavior of a large concentrator, replacing it. These heliostats can move to track the sun, keeping its light concentrated onto the receiver. Fresnel concentrators, however, may have important losses. If the heliostats are spaced from each other, some light will miss them and be lost. If the heliostats are close to each other, they will block part of each other’s reflected light, also producing losses. One possible way to minimize these losses is to intersect two focusing Fresnel concentrators forming a Compact Linear Fresnel Reflector - CLFR. Although improving on a simple focusing Fresnel concentrator, these optics are still not optimal. Here new geometries for Fresnel reflectors are explored, minimizing their losses and increasing their concentration. This is achieved by changing the overall shape of the primary, making it a wave-shaped trough surface and/or by allowing for a variable size and shape of the heliostats as a function of the position in the heliostat field. These new Fresnel concentrators may also be combined with secondaries significantly improving their total concentration, which now approaches the theoretical maximum.     

Comparison of solar concentrators

Even though most variations of solar concentrators have been studied or built at some time or other, an important class of concentrators has been overlooked until very recently. These novel concentrators have been called ideal because of their optical properties, and an example, the compound parabolic concentrator, is being tested at Argonne National Laboratory. Ideal concentrators differ radically from conventional instruments such as focussing parabolas. They act as radiation funnel and do not have a focus. For a given acceptance angle their concentration surpasses that of other solar concentrators by a factor of two to four, but a rather large reflector area is required. The number of reflections varies with angle of incidence, with an average value around one in most cases of interest. In order to help provide a rational basis for deciding which concentrator type is best suited for a particular application, we have compared a variety of solar concentrators in terms of their most important general characteristics, namely concentration, acceptance angle, sensitivity to mirror errors, size of reflector area and average number of reflections.The connection between concentration, acceptance angle and operating temperature of a solar collector is analysed in simple intuitive terms, leading to a straightforward recipe for designing collectors with maximal concentration (no radiation emitted by the absorber must be allowed to leave the concentrator outside its acceptance angle). We propose some new concentrators, including the use of compound parabolic concentrators as second stage concentrators for conventional parabolic or Fresnel mirrors. Such a combination approaches the performance of an ideal concentrator without demanding a large reflector; it may offer significant advantages for high temperature solar systems.     

Ideal prism solar concentrators

Non-imaging solar concentrators using both symmetrical[1–3] and asymmetrical mirrors have recently been described by several workers[4–6]. In this paper, a completely separate but parallel family of non-imaging concentrators is proposed which utilises the phenomenon of Total Internal Reflection within a material of high refractive index to achieve concentration. In both symmetrical and asymmetrical forms, the new concentrators satisfy the maximum concentration limits for ideal radiation transformers for a given acceptance angle[7]. Light radiating from the exit aperture is restricted in angle, but concentration performance is very good and irradiation of the exit aperture is much more uniform for a distant point source than in any other design. The new forms are easier to construct than CPC concentrators filled with refractive medium because, in most designs, only flat surfaces are required and far less refractive material is used for a given aperture. Stationary concentrators with acceptance angles up to that of a flat plate are possible.     

Two-stage tilting solar concentrators

In this paper a novel type of two-stage non-focussing concentrator is described which makes economic photovoltaic and thermal solar conversion possible at current prices in many locations. The new concentrators use exceptionally little mirror (1.3–1.8 times full aperture), yet achieve concentrations up to 12 × without diurnal tracking. Optical efficiency is high, and no moving parts are required except for a simple pivot and securing device.     

Photovoltaic thermal hybrid solar system for residential applications

Electrical and thermal energy have wide applications for the future of mankind. A solar photovoltaic thermal system is a hybrid system, which can produce both thermal and electrical energy. Chennai has an appropriate climate and is highly suitable for using photovoltaic thermal hybrid systems. This article presents the mathematical analyses of the thermal, electrical, and exergetic performance of a photovoltaic thermal system augmented by a flat plate collector for a typical domestic application. The system is found to have 11% average electrical efficiency, 15% overall exergy efficiency, and 56% overall energy efficiency.     

Design and optical performance analysis of seasonally adjusted discrete mirror solar concentrators

Mirror profiles composed of small plane mirror elements have been developed for four different linear absorbers—flat horizontal, flat vertical, triangular cross-section and tubular—on lines similar to those outlined by Winston and Hinterberger. The concentration characteristics of such systems have been studied. The local concentration ratio distributions over these absorber surfaces have been investigated using the ray-tracing procedure.