Bernhard Hoffschmidt

Performance Evaluation of the 200-kWth HiTRec-II Open Volumetric Air Receiver

The High Temperature Receiver (HitTRec) consists of a modular ceramic absorber, a supporting structure and an air-return system. It has been designed to prevent possible flow instability at 700-800°C average outlet air temperature with atmospheric pressure. The HiTRec-II prototype was developed to solve the structural problems of the first prototype (HiTRec-I). Testing in the Plataforma Solar de Almeria (PSA) test bed lasted from November 2000 through May 2001, accumulating 150 test hours under concentrated sun. Results demonstrated the durability of the modified stainless-steel structure. Inlet aperture flux was up to 900 kW/m 2 and average outlet air temperatures of up to 840°C with peak outlet air temperatures of up to 950°C. Thermal efficiency under steady-state conditions was (76±7)% at 700°C, nominal conditions for a PHOEBUS-type volumetric receiver Other performance characteristics were also evaluated (e.g., Air Return Ratio of 46% and characteristic receiver response time of 70 s).

Air-Sand Heat Exchanger for High-Temperature Storage

In view of rising energy prices and an increasing share of power generated by renewable energy sources, the importance of energy storage is growing. In the framework of this project, a thermal energy storage concept for solar power towers is being developed, in which quartz sand serves as a storage medium. Sand is suitable due to its properties such as high thermal stability, specific heat capacity, and low-cost availability. Compared with storages based on ceramic bodies, the use of sand promises to reduce costs of energy storage and thus to reduce the costs of electricity generation. In addition, the storage concept could be applicable in the steel industry. The central element of the storage concept is an air-sand heat exchanger, which is presently under development. This paper describes simulation results and measurements of the heat exchanger prototype. It includes sand flow behavior and experience with different porous walls as well as up-scaling options.

3.06 – High Concentration Solar Collectors

Solar thermal concentrated power is an emerging technology that provides clean electricity for the growing energy market. To the solar thermal concentrated power plant systems belong the parabolic trough, the Fresnel collector, the solar dish, and the central receiver system. n nFor high-concentration solar collector systems, optical and thermal analysis is essential. There exist a number of measurement techniques and systems for the optical and thermal characterization of the efficiency of solar thermal concentrated systems. n nFor each system, structure, components, and specific characteristics types are described. The chapter presents additionally an outline for the calculation of system performance and operation and maintenance topics. One main focus is set to the models of components and their construction details as well as different types on the market. In the later part of this chapter, different criteria for the choice of technology are analyzed in detail.

Test Facility for Absorber Specimens of Solar Tower Power Plants

The Solar-Institute Jülich (SIJ) has initiated the construction of the first and only German solar tower power plant and is now involved in the accompanying research. The power plant for experimental and demonstration purposes in the town of Jülich started supplying electric energy in the beginning of 2008. The central receiver plant features as central innovation an open volumetric receiver, consisting of porous ceramic elements that simultaneously absorb the concentrated sunlight and transfer the heat to ambient air passing through the pores so that an average temperature of 680°C is reached. The subsequent steam cycle generates up to 1.5 MWe. A main field of research at the SIJ is the optimization of the absorber structures. To analyze the capability of new absorber specimens a special test facility was developed and set up in the laboratory. A high-performance near-infrared radiator offers for single test samples a variable and repeatable beam with a power of up to 320 kW/m² peak. The temperatures achieved on the absorber surface can reach more than 1000°C. To suck ambient air through the open absorber - like on the tower - it is mounted on a special blower system. An overview about the test facility and some recent results will be presented.

Solar Concentrating Systems Using Small Mirror Arrays

The cost of solar tower power plants is dominated by the heliostat field making up roughly 50% of investment costs. Classical heliostat design is dominated by mirrors brought into position by steel structures and drives that guarantee high accuracies under wind loads and thermal stress situations. A large fraction of costs is caused by the stiffness requirements of the steel structure, typically resulting in ∼20 kg/m2 steel per mirror area. The typical cost figure of heliostats is currently in the area of 150 €/m2 caused by the increasing price of the necessary raw materials. An interesting option to reduce costs lies in a heliostat design where all moving parts are protected from wind loads. In this way, drives and mechanical layout may be kept less robust thereby reducing material input and costs. In order to keep the heliostat at an appropriate size, small mirrors (around 10 cm × 10 cm) have to be used which are placed in a box with transparent cover. Innovative drive systems are developed in order to obtain a cost-effective design. A 0.5 m × 0.5 m demonstration unit will be constructed. Tests of the unit are carried out with a high-precision artificial sun unit that imitates the sun’s path with an accuracy of less than 0.5 mrad and creates a beam of parallel light with divergence less than 4 mrad.Copyright