EuroSun2008-2

Test of a Mini-Mirror Array for Solar Concentrating Systems

J. Gottsche1, B. Hoffschmidt1, S. Schmitz1, M. Sauerborn1*, Ch. Rebholz2, D. Ifland2, K.

Badstubner2, R. Buck3 and E. Teufel3

1 Solar-Institute Julich (SIJ), Aachen University of Applied Sciences (AcUAS), D-52428 Julich, Germany

2 Fraunhofer Institute for Reliability and Microintegration (Fh-IZM), D-82234 Oberpfaffenhofen, Germany 3 German Aerospace Centre (DLR), Institute of Technical Thermodynamics, D-70569 Stuttgart, Germany

Corresponding Author, sauerborn@sii. fh-aachen. de

Abstract

To reduce the construction costs of solar tower power plants an innovative new mirror system with mini mirrors is on the way as an uncommon alternative to the currently used heliostats with huge surface (actual up to 120m2). An about (0.6 x 0.6) m miniature test unit has now been con­structed. This first demonstration unit at first consists of a boxed array of 25 small parallel ar­ranged mirrors with a size of (10 x 10) cm each. All mirrors are linked by an agile, crossed stick stage with an integrated drive system to move them simultaneously into all necessary positions. This specimen, developed in cooperation with all partners, will be tested in the specially pre­pared artificial sun of the SIJ in different ways under changing conditions.

In a simulation an optimum field layout was calculated. With this data a theoretical comparison was made between a typical existing huge heliostat and the developed mini-mirror array unit with respect to the efficiency and cost-performance relations for a solar tower power plant. Keywords: mini mirror array, solar thermal tower power plant, heliostat field layout, raytracing

1. Introduction

The heliostat field of a solar tower power plant is with up to 50% a significant part of the investment cost. During the last 30 years of developing heliostats, it has been found that the key to essential de­crease of expenses lies mostly in innovative design modifications which strongly reduce the material input. The main cost driver of all low cost versions is still the steel structure, securing the mirror and the movement system that has to guarantee high accuracies even under high wind loads and extreme thermal stress situations.

The typical cost figure of heliostats depends on the stiffness requirements of the steel and is currently in the area of 130-150€/m2 (Solucar 120 m2). Because of the foreseeable increasing of the raw materi­als price the cost problem will in future gain even more relevance. Figure 1 shows a rough estimate of the maximal cost reduction potential. To simplify the calculation, the cost of the heliostat construction is reduced to the weight of the steel used (steel price actual > 2€/kg) and ignores the production ex­pense for further treatment (pillar, framework), higher techniques (actuator, controller, sensors) and the mirrors. When a wind load must be taken into account, the decrease of the expense is limited to max. 46% of the current mirror price.

In this way, only a heliostat design with a slight or even without sensitivity to wind load will offer the chance to scale down the mechanical layout to reduce the material input substantially. The easiest way to guarantee very small wind loads on the mirror system is to install the system in a box with a plane surface design. To solve this restriction and to keep the heliostat at a suitable size, small mirrors (such as (10 x 10) cm) could be used. The principle design of a Mini-Mirror Array proposed in this work is shown in figure 2.

example: Solucar (Spain)

An additional effect of the reduction of weight and the decrease of the associated friction losses will be the option to use a less robust actuator system for the mirror movement. So in the end the material in­put and the energy requirement of the system can be reduced effectively.