Andreas Timinger

Optimized reflectors for non-tracking solar collectors with tubular absorbers

Abstract We present an approach to find optimal reflector shapes for non-tracking solar collectors under practical constraints. We focus on cylindrical absorbers and reflectors with translational symmetry. Under idealized circumstances, edge ray reflectors are well known to be optimal. However, it is not clear how optimal reflectors should be shaped in order to obtain maximum utilizable energy for given operating temperatures under practical constraints like reflectivity less than unity, real radiation data, size limits, and gaps between the reflector and the absorber. For a prototype collector with a symmetric edge ray reflector and a tubular absorber, we derive from calorimetric measurements under outdoor conditions the optical efficiency as a function of the incidence angle. Using numerical optimization and raytracing, we compare truncated symmetric edge ray reflectors, truncated asymmetric edge ray reflectors and free forms parametrized by Bezier splines. We find that asymmetric edge ray reflectors are optimal. For reasonable operating conditions, truncated asymmetric edge ray reflectors allow much better land use and easily adapt to a large range of roof tilt angles with marginal changes in collector construction. Except near the equator, they should increase the yearly utilizable energy per absorber tube by several percent as compared to the prototype collector with symmetric reflectors.

Optimized compact secondary reflectors for parabolic troughs with tubular absorbers

Parabolic trough type solar power plants could reach higher efficiencies by using secondary reflectors, which increase the concentration of the solar irradiance onto the absorber tube. Recently, compact secondary concentrators have been proposed which are constructed according to the edge ray principle; they are composed of an involute part and an edge ray reflector with a form close to a straight line. Using these reflector forms as a starting point, we optimize numerically various secondary concentrators and investigate their performance by means of raytracing, taking into account reflectivity losses, shading and the effective solar angular distribution, i.e. the sunshape. We investigate the performance of secondary concentrators with the outer part approximated by a straight line. The entire characteristic curve of collection efficiency versus concentration was investigated, for both a pillbox sunshape and a Gaussian sunshape. It is found that the secondary reflector significantly improves the concentration and is essentially optimal in a wide class of shapes, even at efficiencies close to unity.

Designing tailored free-form surfaces for general illumination

3-D tailoring is a constructive method for the design of free-form optical elements for illumination. The light of a point source is redirected in a controlled manner to cast a prescribed irradiation pattern on a target surface. Free parameters can be used to control the shape of the surface resulting from the tailoring process. Every change in the parameters may lead to an entirely different design. Hence the choice of parameters is crucial for the technical feasibility and the visual appearance of the luminaire. Examples of free parameters are the chosen caustics, trimming of the surface, the choice between mirror and lens optics, and the mutual orientation of source and optical elements.

PERFORMANCE OF A RECTANGULAR SECONDARY CONCENTRATOR WITH AN ASYMMETRIC HELIOSTAT FIELD

Secondary concentrators with non-regular cross section have been proposed to permit additional degrees of freedom in heliostat field design, free of the limitations imposed by conventional rotationally symmetric concentrators. A new class of concentrators with a rectangular cross section was found by numerical optimization for heliostat fields having an elliptic contour with high eccentricity. An example of such a rectangular concentrator was constructed and tested at the Weizmann Institute, where the heliostat field has a strong asymmetry and is poorly suited for symmetric (having regular cross section) concentrators. The performance of the new concentrator has been tested using calorimetric and radiometric measurements. The tests were carried out for several heliostats, located in representative positions relative to the predicted acceptance contours of the concentrator. The results of the tests show an agreement with the prediction, and validate the new design for use with highly eccentric fields. A more general conclusion is the validation of the approach of optimizing faceted secondary concentrators with flexible geometry to match heliostat fields having a wide range of possible contours.

Optical assessment of nonimaging concentrators

An optical measurement method for nonimaging radiation concentrators is proposed. A Lambertian light source is placed in the exit aperture of the concentrator. Looking into the concentrators entrance aperture from a remote position, one can photograph the transmission patterns. The patterns show the transmission of radiation through the concentrator with the full resolution of the four-dimensional phase space of geometric optics. By matching ray-tracing simulations to the measurement, one can achieve detailed and accurate information about the geometry of the concentrator. This is a remote, noncontact measurement and can be performed in situ for installed concentrators. Additional information regarding small-scale reflector waviness and surface reflectivity can also be obtained from the same measurement with additional analysis.

Optimized secondary concentrators for a partitioned central receiver system

We present secondary concentrators with non-regular shapes for increasing the concentration of radiation from a given field of heliostats, well suited for partitioning the receiver into several units, arranged side by side. For a general heliostat field with a non-axisymmetric directional distribution of the radiation at the entrance aperture of the secondary concentrator, concentrators with non-regular shape can significantly increase the concentration as compared to their symmetric analogs. Our optimizations indicate best results for concentrators based on rectangular entrance and exit apertures. The concentration may be increased by a factor of 2.3 at an optical efficiency of 90%. If the shape of the exit aperture is required to be close to circular, concentrators based on non-regular hexagonal apertures may reach concentration higher than their symmetric analogs by a factor of 1.3. For the given radiation, concentrators with polygonal apertures perform significantly better than concentrators with smooth elliptic apertures.

Non-axisymmetric reflectors concentrating radiation from an asymmetric heliostat field onto a circular absorber

Abstract In solar tower plants, where a rotationally symmetric field of heliostats surrounds the tower, an axisymmetric secondary concentrator such as a compound parabolic concentrator (CPC) or a tailored concentrator or a cone is the obvious choice. For locations at higher latitudes, however, the reflecting area of the heliostats may be used more efficiently if the field of heliostats is located opposite to the sun as seen from the tower. Then the field is asymmetric with regard to the tower. In the case of an asymmetric field, an axisymmetric concentrator necessarily has a concentration significantly lower than the upper limit. Furthermore, the area on the ground from which a tilted axisymmetric concentrator accepts radiation is an ellipse, including also heliostats very distant to the tower producing a large image of the sun. For these reasons we investigate asymmetric secondaries. From the shape of the edge ray reflectors constructed for rays in the central south–north plane we conclude that a skew cone reflector might be appropriate for the field, and optimize its free parameters by means of ray tracing. Asymmetric concentrators may increase the concentration by up to 25% at the same efficiency compared to optimized axisymmetric CPC or cone reflectors.

Complex 3D-tailored facets for optimal lighting of facades and public places

Due to antiquated technologies (calculation methods, regulations, lighting and luminaire concepts, production techniques) current outdoor lighting causes a lot of problems like light pollution, glare, energy waste etc. New types of luminaires, and in consequence new outdoor lighting concepts, can be created by combining advanced calculation methods for optical surfaces with recent production technologies and novel light sources such as short arc metal halide lamps. Light emitted from this small Etendue light sources can precisely be redirected by 3D-curved surfaces manufactured with injection molding, milling and aluminium metallization. The required optical design may use techniques like complex surface calculations and 3D-Tailoring. An innovative concept based on the latest findings in visual perception research is to focus the light of such short arc light sources onto a facetted secondary mirror which provides the desired illuminance distribution on a facade or a public place. These systems are designed to fulfill lighting requirements as well as providing visual comfort. Thus lamps with improved color rendering, luminous efficacy and increased lifetime are used and glare is minimized by splitting the reflector into many facets (light spot decomposition). A few examples of realized projects will be presented where such complex facetted surfaces are used to reach a special quality of light. Using novel techniques like 3D-Tailoring, each facet can be designed to individually create the desired (e.g. uniform) illuminance distribution on the target surface - in this case, a large facade. For this particular application, we chose to impose a square boundary for each facet, in order to tile the rectangular aperture of the secondary mirror without compromising efficiency.

Street-lighting with LEDs

Applications requiring high lumen packages are traditional the domain of light sources like discharge lamps. Currently, LEDs make their way into such applications. LED street lighting projects, which are regularly covered in the press, provide a case of point. Life time and luminous efficacy are considered as being the main advantages of LEDs. Nonetheless, other current light sources for street lighting have similar performance. Analysing a street-lamp as a complete system, we can show that LED solutions have significant advantages if highly efficient optics are used. We present an example with tailored free-form optics. These make efficient use of the valuable LED light by exactly redistributing it into the desired illuminance pattern.

Optical In-Situ Assessment of a Nonimaging Secondary Concentrator in a Solar Tower

A method for remote optical measurement of the geometry of nonimaging concentrators is presented. A concentrator installed in a solar tower was measured by observation of transmission patterns from the heliostat field, and comparison of the measured patterns to a ray tracing simulation. The actual geometry of the concentrator was derived from optimization of the match between real and simulated patterns. The measurement was sensitive and accurate enough to detect small errors in the concentrator geometry, such as 1 millimeter in linear dimension and 0.1° in concentrator tilt angle. The measurement procedure is simple and can be easily adapted to a wide range of nonimaging optical systems. @DOI: 10.1115/1.1488668#