M. Eck8, E. Zarzab

aDLR, Institute of Technical Thermodynamics, Pfaffenwaldring 38-40, 70569 Stuttgart bCIEMAT, Plataforma Solar de Almeria (PSA), P. O. Box 22, 04200 Tabernas (Almeria),

Spain

Abstract — The direct steam generation (DSG) in parabolic trough collectors is an attractive option regarding the economic improvement of parabolic trough technology for solar thermal electricity generation in the multi Megawatt range. The European DISS project has proven the feasibility of the direct steam generation under real solar conditions in more than 4000 operation hours [1]. Within the European R&D project INDITEP the detailed engineering for a pre-commercial DSG solar thermal power plant with an electrical power of 5 MW is being performed. This small capacity is chosen to minimise the risk for potential investors.

Regarding DSG solar thermal power plants only steam cycles using superheated steam have been investigated so far. In this paper a steam cycle operated with saturated steam is investigated for the first time. For near term applications this might be an interesting alternative in the chosen small capacity range. This choice would offer some specific advantages:

• Lower complexity of power block and thus lower investment but also lower efficiency of the power block

• Simple set up of the collector field

• Proven safe operation and higher thermal efficiency of the collector field

Introduction

During operation of the DISS facility, the life size DSG test facility at the Plataforma Solar de Almena (PSA), it turns out that the so called recirculation mode is advantageous for near term application mainly because of its safe operation of the collector field [1]. A recirculation mode driven collector row is subdivided into an evaporating and a superheating section by a separator. At the end of the evaporating section the excess water is separated from the steam flow and recirculated to the collector inlet. The saturated steam thus produced is fed into the superheating section and is superheated up to 400°C or more. In the end the superheated steam is expanded in a turbine that runs a generator.

the detailed engineering for a first DSG solar thermal power plant is performed. A size of 5 MW was chosen for this plant. This size might be too small to guarantee a cost — effective operation of the plant but it minimises the financial risk of potential investors. For the detailed design a site close to the Spanish city of Seville was selected (latitude: 37°24’ N; longitude: 5°58’ W). A dry cooling condenser is considered. Due to the high ambient temperature at the site in summertime a condensation pressure of 0.1 bar has been considered for the design point (June 21st), which is equivalent to a condensation

temperature of 45.8°C. A minimum steam quality at the turbine outlet of 0.85 is allowed. The basic boundary conditions are summarised in Table 1.

Due to the small size of the turbine, the number of steam extractions at the turbine is limited. In case of a 5 MWe turbine there is only a single extraction line. The according bleed stream is used for dearation in the feed water tank. The dearator pressure is set to

5.6 bar. The schematic diagram of such a simple steam cycle operated with superheated steam is shown in figure 1. The collector field is operated in recirculation mode. In this paper this basic operation mode is compared to a DSG plant operated with saturated steam. The according schematic diagram of this option is displayed in figure 2.

In figure 2 the collector field is also operated in recirculation mode. The saturated steam leaving the field separator is fed directly to the saturated steam turbine. Again there is a single extraction line for dearation. In this case the steam re-entering the second stage of the turbine has to be dried in a second separator to guarantee the minimum steam quality at the outlet of the second stage. The saturated water at the outlet of the second separator is fed to the feed-water-tank.