K. Roth, V. Scherer, T. Pockrandt, Ruhr-University of Bochum M. Eck; German Aerospace Center (DLR), Stuttgart

Introduction and purpose

In some European countries a specific percentage of the electricity has to be generated by renewable energy sources in order to reduce the emissions of greenhouse gases like CO2 and to promote renewable energy. The legislation in Italy, for example, requires 2 % nowadays, rising to 4.4 % in 2012. A promis­ing technique to meet these goals is to employ a hybrid power plant, where a renewable energy source is added to a conventional and fossil fueled power plant cycle. The great advantage of this concept is that it can be realized as a retrofit to existing power plants. Mainly solar thermic plants are favourable to be located in southern European countries, that are offering a high insolation. Such kind of power plants, for example with parabolic trough collectors heat­ing a synthetic oil which transfers the heat via heat exchangers to a conven­tional steam turbine cycle, are operating since many years and have proven their technological reliability. Direct solar steam generation is a possible im­provement of the parabolic trough power plants vaporizing water directly in the absorber pipelines and eliminating costly equipment like the heat exchanger and the oil pump. This steam generated by solar energy can be integrated into the water steam cycle of a fossil fueled power plant [1].

A schematic process diagram is shown in figure 1. By this means less steam is consumed by the high pressure and low pressure preheaters. One or more steam extraction lines can be closed and the steam can be used for a very fast and dynamic additional power generation in the steam turbine. The power plant’s efficiency is improved simultaneously.

Since the integration of the transient heat sources will lead to substantial dy­namic energy shifts within the power plant’s water-steam-cycle, the occurring time dependent processes have to be studied on a numerical basis using a dy­namic simulation software. The description and analysis of these shifts to­gether with a cycle evaluation using an unsteady process simulation tool are the content of the current paper.