Simulation of the Start-Up Procedure of a Parabolic Trough Collector Field with Direct Solar Steam Generation

Abstract

The market for solar thermal power plants especially in Spain becomes more and more interesting since primaries allow profitable operation of such plants. Besides the efficiency numbers for solar field and power block components the operational strategy offers potential for reducing the costs of electricity production. This paper concentrates on the start-up procedure for a solar field in order to identify an ap-propriate strategy and to estimate the duration of the process. As a reference plant a 5 MWel parabolic trough plant with direct steam generation is chosen. It consists of seven parallel loops with a length of 1000 m each. The transition from subcooled water to saturated wa-ter-steam and finally to superheated steam flow along the collector row makes the system complex and highly transient. The Modelica language was chosen for the simula-tion of the system since it offers enough flexibility to combine fluiddynamic components together with control systems in one simulation tool. The models for the parabolic trough collectors and other fluid system components are taken from a proven library at the institute. For the analysis of the start-up procedure the whole model was split into two layers. The plant layer represents the physical components like collectors, tanks, junctions, and pipes. The con-trol layer is responsible for the generation of all in-put-signals for the plant layer. The layers communi-cate via a bus system. Since the start-up procedure can be described as a sequence of system states the Modelica StateGraph library is used to model the different states of the system and the transitions be-tween them. Depending on the actual state, control-lers, switches, set-points and parameters can be de-fined. After a short introduction of the power plant system the paper presents the idea for the structure of the control system and the way the coupling between the two layers is implemented. Using one set of bound-ary conditions the results of several simulation runs are presented and discussed. It is shown that the de-sign of the process is of vital importance for its en-ergy consumption. Especially the pre-heating of the field with water stored in isolated tanks over night has positive effect on the duration of the start-up and therefore on the economics of the plant.