Solare Energietechnik, Deutsches Zentrum für Luft- und Raumfahrt e.V., 51170 Köln, Germany

Tel.: (49) 2203 601 3213, Fax: (49) 2203 66900, e-mail: klaus.hennecke@dlr.de

Solar-Institut Juelich, Fachhochschule Aachen (University of Applied Science), Ginsterweg 1, 52428 Jülich, Germany

Tel.: + 49 2461 993237, Fax.: + 49 2461 993235, e-mail: hafner@fh-aachen.de

While in the 70s and 80s parabolic troughs for domestic or process heat applications were built by several producers worldwide, only the company Industrial Solar Technology (IST) survived the standstill since the end of the 80s. Meanwhile even in climates with less radiation, like in central or northern europe, flat plate collector fields of several hundreds square meters are built. They are operated at temperatures of more than 50°C, which raises the question wether parabolic troughs could be an alternative in this market even in climates like central europe. The energy yield at temperatures between 50 and 100°C for differing climates was therefore investigated for a parabolic trough.

The performance of the IST parabolic trough (Figure 1) in different climates was simulated in TRNSYS with a new module for north-south oriented parabolic troughs. The simulations, which involve complete years in one-hour steps, calculate the annually accumulated energy assuming a steady mean transfer fluid temperature in the collector. The theoretical approach assumes an all-year availability and includes an offset for degradation and soiling of the reflector. Because the rows of parabolic trough fields stand close together the shading of the rows onto each other has to be accounted for.

Results of equal simulations with a flat plate collector (no shading or soiling assumed) permit a comparison of the trough collectors. This will be presented as a function of yearly energy yield per square meter over a temperature range of 50 to 100°C. The approach allows a consideration independent of applications to find out the crosspoint temperature above which the trough collector delivers more energy over the year.

Even more decisive are the costs per kilowatt-hour for the solar heat. Included in the costs are the price of the installed collector field and the operation and maintenance costs. The resulting annual costs are divided through the annual energy delivery to the price per kilowatt-hour. For the price of the collector is a constant, but the energy delivery depends on the mean fluid temperature in the collector, the price per kilowatt-hour is a function of the mean fluid temperature. A crosspoint can be found above which thermal energy from trough collectors costs less than from flat plate collectors.

In german climates (represented in Test Reference Years) the global horizontal radiation varies between 900 and 1200 kWh / a. The simulation results show that the energy delivery of a parabolic trough exceeds that of a flat plate collector in the temperature range between 50 and 80°C, depending on the share of direct radiation. For two climates the results are presented in diagrams.

Since flat plate collector prices for greater fields have dropped rapidly in the last years the crosspoint for the energy price lies higher than 50 to 80°C, depending on the collector costs assumed. Functions for various price assumptions show this.

Seasonal storage of solar thermal heat in water basins is an application that operates at collector temperatures between 40 and 100°C. The former findings indicate that parabolic troughs could serve for heating at lower prices than flat plate collectors. The use of parabolic troughs offers the opportunity to operate the storage at higher temperatures without essentially increasing the thermal losses of the collector. This leads to a reduction of the specific investment costs of the storage system. In diagrams for differing climates and storage loading strategies the energetic and economic results of simulations are presented.

The theoretical results will be compared with measurements taken on a test loop currently under construction at the DLR in Cologne / Germany.

Figure 1: Parabolic Trough Collector of Industrial Solar Technology

Keywords: solar thermal heat, parabolic trough collector, seasonal storage, flat plate collector, applications