Direct Steam Generation in Parabolic Troughs: First Results of the Diss Project

This article presents the latest experimental results of the European DISS (DIrect Solar Steam) project. The experiments are subdivided into steady state and transient tests. The goal of the steady state tests is the investigation of the thermohydraulic phenomena of the occurring two phase flow, whereas the transient tests are needed for the controller design. The experimental results are compared to simulation studies. Implications for the plant operation will be discussed. INTRODUCTION The direct steam generation (DSG) in parabolic trough collectors is a promising option for the improvement of the reliable parabolic trough technology for solar thermal electricity generation. Several open questions concerning the DSG process have to be answered before a commercial plant can be designed. These are among others the: • operation of single and parallel rows under steadystate and transient conditions, • start-up and shut-down procedures, • strain of the absorber tubes due to transients and • definition of the best operation mode To overcome this lack of knowledge the DISS project has been initiated. The DISS test facility at the Plataforma Solar de Almeria (PSA) in Spain (Zarza and Hennecke 2000) is a full-scale solar steam generator with a length of 500 m and a maximum thermal power of approx. 2 MW. The design pressure at the outlet of the collector loop is 100 bar. The collector loop consists of modified LS-3 parabolic trough collectors, used to investigate and compare different process options under real operating conditions, as well as to develop and demonstrate 1 The LS-3 (Luz System 3) is the collector used in the blocks VII to IX of the SEGS power plants. The LS-3 collector has a length of 99 m and an aperture width of 5.76m. appropriate components, control algorithms and operation strategies. Fig. 1: Basic concepts considered for solar steam generation in parabolic troughs. There are three different process options, the oncethrough, recirculation and injection concept (s. Fig. 1). The once-through-concept is the same as in a conventional Benson boiler. This concept is the most simple one but its controllability is to be investigated. The recirculationconcept is equivalent to a conventional recirculation boiler. It is the most secure concept but it is more expensive due to the additional separator and recirculation pump. An injection-mode collector loop is subdivided into different collector units. The series connection of all subgroups build the collector loop, where each subgroup consists of a collector, an injection and gauging equipment. The advantage of this concept is that the controllability of the process is expected to be better. The flexible design of the DISS test facility allows the investigation of the three different operation modes. To investigate the DSG process, models describing the dynamic and steady state behavior of DSG have been