S. Alexopoulos1*, J. Gottsche1, B. Hoffschmidt1, C. Rau1, P. Schwarzbozl2

1 Aachen University of Applied Sciences (AcUAS)/ Solar-Institut Julich (SIJ); Heinrich-Mufimann-Strafie 5;

D-52428 Julich

2 German Aerospace Center (DLR), Institute of Technical Thermodynamics, Linder Hohe, Porz-Wahnheide,

D-51147 Koln

* Corresponding Author, alexopoulos@sij. fh-aachen. de

Abstract

Solar thermal power is an emerging technology that provides clean electricity for the growing energy market. To the group of solar thermal power systems belongs the solar tower where a field of two-axis tracking heliostats reflect the solar radiation to the receiver aperture, where concentration ratios up to 1000 are reached. This offers the potential to provide high steam temperatures for a rankine cycle or ultimately even high enough temperature heat for the gas turbine system of a combined cycle power plant.

One major option for the accelerated market introduction of solar thermal power technology is the concept of solar-fossil hybrid power plants. Their advantage, compared to solar-only systems, lies in low additional investment costs due to an adaptable solar share and reduced technical and economical risks.

In this paper a simulation model for the calculation of a hybrid power plant is described. Also the design and performance assessment of solar hybrid tower plants in the power level of 1-2 MW, is presented. An advanced software tool library is developed for the modelling of such small hybrid power plants. First validation and simulation results for parts of a small solar tower power plant are described and discussed.

Keywords: solar thermal power, central receiver system, solar tower, hybridization, simulation

1. Introduction

Solar tower power plants are an innovative type of power production facility based on solar radiation as primary energy resource instead of conventional combustion processes. A 1.5 MWel demonstration plant with volumetric air receiver is being built in Juelich, Germany ([5], [7]).

In the primary cycle of the Juelich concept, air is heated up to about 700°C and used to generate steam in a heat recovery steam generator (HRSG) (see Fig.1 without consideration of the burner). Steam parameters in the secondary cycle may be up to 30 bar and 500°C. The steam drives a turbine/generator system and returns in form of condensate to the steam generator. A thermal storage system is connected parallel to the boiler. When charging, hot air passes through the vessel filled with ceramic material which develops a temperature profile between the hot and the cold end of the storage. The cold air exiting the steam generator and/or storage is returned to the receiver. To discharge the

storage, cold air from the steam generator is circulated in reverse flow through the storage vessel and back to the steam generator. The air, which delivers almost all its energy in the heat transfer process, leaves the boiler component with a temperature of about 120°C. After the boiler, a blower forces the air to mix with the portion of air that has delivered its energy to the thermal storage. A second blower located after the thermal storage forces the total combined flow of air to the receiver [1].

This system concept offers several benefits. Air as heat transfer medium is available for free, non-toxic and does not require freeze protection during times of non-operation. Operation with steam parameters as customary in conventional power plant engineering ensures high efficiency and optimum use of the available solar energy. Major parts of the plant can be built by using standard components from conventional power plant construction.

In Juelich (North Rhine-Westphalia, Germany) the first solar tower plant with gas as heat transfer medium is under construction. The concept of the solar tower power plant Juelich (STJ) is the result of the development of the solar power plant technology with an open volumetric receiver. An open volumetric receiver enables the better exchange of energy between the inserting solar power and the transport medium. The incident radiation is absorbed from the receiver modules and is connected by convective heat transfer with the flow of ambient air which is moved trough the structure [2]. The STJ will finish construction by the end of 2008 and begin feeding electricity into the public grid for the first time.

In order to enable the plant to produce electricity in periods where the solar radiation is not available a hybridization concept is been taken into consideration. The concepts of hybridization consist of the use of a gas burner or a gas turbine as an auxiliary system.

This is an important step towards cost reduction of solar thermal power. The advantages of hybrid power plants are a variable solar share, a fully dispatchable power and a 24 h operation without storage.

The hybridization of a solar thermal power plant provides an increase of the efficiency and availability. In regions with very high radiation, solar thermal power plants with heat storage facilities can reach a maximum of 3.000 to 4.000 nominal load hours per year. Through hybridization, e. g. by combustion of biofuels, the production of electricity can be increased to almost 8.600 hours. It is expected that such hybrid power plants will have a high potential for market introduction in the next decade.