Dipl.-Ing. Max Mertins, University of Karlsruhe, Englerstr. 7, 76128 Karlsruhe Dipl.-Phys. Hansjorg Lerchenmuller, Fraunhofer ISE, Heidenhofstr. 2, 79110 Freiburg Dr. Andreas Haberle, PSE GmbH, Solar Info Center, 79072 Freiburg Dr. Ing. habil. Volker Heinzel, University of Karlsruhe, FZK, 76131 Karlsruhe

Abstract

The Fresnel solar collector is a promising concept to reduce the electricity cost price in solar thermal power plants. The optical performance of a Fresnel collec­tor depends on material properties, on its geometric layout and on the level of op­tical accuracy that can be obtained. A variety of geometric parameters, e. g. the height of the absorber, the number, size and distance of primary mirrors influence the shading and blocking of rays and the amount of rays missing the absorber. To evaluate the influence of the parameter variation regarding the electricity cost price and to yield an optimization, the optical performance is assessed with an annual simulation based on hourly weather-data. To permit a consideration of changes in collector cost according to different geometric layouts, cost factors where allocated to geometric parameters. The paper presents the method and the simulation re­sults of the optimization under different boundary conditions and shows how the developed simulation tool can lead to an optimum collector design with respect to cost price of electricity. The sensitivity of the results will be discussed.

Introduction

Similar to the parabolic trough system the linear Fresnel-collector, which is a piecewise approximation to the parabola, is suitable to produce steam for use in solar thermal power plants. The collector comprises of slightly elastic curved mirror-stripes, which reflect the sunlight to a fully stationary receiver (see figure 1).

The receiver consists of a secondary CPC — type concentrator and a selective coated tubular absorber with no need of a vacuum insulation. Principally the collector is not lim­ited in aperture width[6], therefore a wide range primary mirrors glass plane —

of free geometry parameters is possible. The width of the primary mirrors B has to be

TOC o "1-5" h z coordinated with their gaps D, their number :

N and, the hight H of the receiver. Several studies ([1], [2]) have certified promising cost

perspectives of the linear Fresnel-concept ——- —

but at less specific energy yield. Hence an W ■

optimization of the geometry and the field Figure 1: Principle °f the linear Fresnel size is only meaningful with an economic collector

assessment.

The collector considered in this paper is intended to produce superheated steam at 440 °C

and 50 bar, therefore the collector-field is divided into sections for preheating, evaporating and superheating.

The main geometric losses of the Fresnel-concept are shown in figure 2. These effects can be decreased by heightening the absorber and by widening the gap between primary mirrors. On the other hand geometric changes cause losses due to inaccuracy of assembly and tracking of the primary mirrors.

Approach

For analyzing and optimizing the geometry, the optical efficiency is not of primary interest. The optimization for maximum performance of the collector at noon would differ from the one for lowest LEC[7]. The difference would be up to 10% in the LEC between the different optimization targets. Therefore an integral view on the whole system is essential. A link between the optical, thermodynamic and cost models is necessary to take the main influences into account. The electricity yield of the power-block is calculated via annual simulation based on hourly weather-data of a certain site under specific boundary conditions. After consideration of the specific cost the LEC is evaluated and chosen to assess the configuration. Hence arbitrary geometries and receiver-concepts can be investigated.