N. Benz, F.-D. Doenitz, Th. Kuckelkorn

SCHOTT, Business Unit Solar Thermal Schott Rohrglas GmbH, D-95660 Mitterteich, Germany Phone: +49 9633 80 401, Fax: +49 9633 80 757 e-mail: nikolaus. benz@schott. com

ABSTRACT

SCHOTT developed a new receiver for parabolic trough collectors used in solar thermal power plants. In particular we developed a new absorber coating which is temperature stable up to 500 °C with excellent optical properties. We found an anti reflective coating for the special glass tubing with high solar transmittance, durability and improved abrasion resistance. To reduce breakage of the glass-to — metal sealing, which is the main cause for damages in existing power plants, we developed a new sealing with adapted CTE of metal and glass. To meet the requirements we had to introduce a new glass type suitable for large tubes. A new design approach minimizes the undesirable shading of the absorber by the bellows. Finally we achieved an active length of the receiver of more than 96%.

INTRODUCTION

Solar technology is one of the key business issues of SCHOTT in the future. The activities are bundled in a strategic business unit: the joint venture with RWE for PV together with the solar thermal division producing evacuated tube collectors for residential applications.

End of 2001 SCHOTT started to develop a receiver for parabolic trough collectors to enter the upcoming market of solar thermal power generation.

The development is part of the joint project PARASOL, supported by the German Federal Ministry for the Environment. Project partners are the Flagsol GmbH and the German Aerospace Center (DLR).

Parabolic trough collectors

More than a decade after the last construction of a solar power plant with parabolic troughs, the interest in this technology is rising again. Projects are scheduled or even in ogress in Europe, USA and some other countries in the sun belt [1]. The upcoming new market for solar power is not only driven by the global search for clean energy. Apart from the four countries which anticipate funding from the GEF, the most promising opportunities are opening up in the US and in Europe: In the US projects benefit from the renewable portfolio standard in several states of the south-west which require a certain percentage of electricity supply to come from renewable sources. In Spain a “feed-in-tariff” was introduced to guarantee a premium payment for electricity generated in solar thermal power plants.

The receiver requires the most challenging technology and has a decisive influence on the overall efficiency. It consists of a cylindrical absorber placed along the focal line to capture the energy reflected by parabolic mirrors. To achieve high efficiency the absorber must show high solar absorptance as well as low thermal emittance. To suppress heat conduction losses, the absorber is usually insulated with vacuum enclosed by a
transparent cover. Therefore a glass tube is used which is coated with anti reflective films for high solar transmittance. Metal bellows are used to accommodate for thermal expansion difference between the steel tubing and the glass envelope. To provide high vacuum where gas heat conduction is totally suppressed, a durable and tight glass-to — metal seal (GMS) is required.