Dirk Kruger, Dirk Mangold*, Klaus Hennecke, Ralf Christmann, Jurgen Dersch, Eckhard
Lupfert and Klaus-Jurgen Riffelmann,

Solar Research, Institute of Technical Thermodynamics
Deutsches Zentrum fur Luft — und Raumfahrt e. V., 51170 Koln
Tel: (49) 02203 601 2661, Fax: (49) 02203 66900, E-mail: dirk. krueger@dlr. de
* Solar — und Warmetechnik Stuttgart (SWT)
ein Forschungsinstitut der Steinbeis-Stiftung, 70550 Stuttgart
Tel: (49) 0711 685 3279, Fax: (49) 0711 685 3242, E-mail: mangold@swt-stuttgart. de

The high exergy of the solar radiation allows to produce heat in thermal collectors, and also to generate electricity as in large solar thermal power plants or with photovoltaics in smaller applications. Solar district heating systems can be enhanced by small engines converting the valuable part of the energy at high temperature into electricity, while the remaining fraction at lower temperature is still used for heat production. This will improve the benefit for solar district heating. In this paper a solution for producing electricity and heat from one solar system, a parabolic trough collector field in conjunction with a steam engine is presented (Figure 1).

Applications

In the municipal sector various solar installations for district heating, sometimes with seasonal storage, have been erected for domestic hot water and heating purposes. Combining such kind of systems with a heat engine to produce electricity is the principle of a solar combined heat and power solution. Process heat applications needing heat up to 100°C in conjunction with electrical power are also appropriate. The concept is interesting for small heat and power applications in residential homes as well.

Collector

Temperatures of 200°C to 400°C are desired in order to reach appropriate engine efficiency. Large parabolic trough collectors for solar power plants as the EuroTrough collector (Geyer et al (1), Geyer et al (2), Lupfert et al) can deliver heat at these temperatures efficiently, but they are not cost effective for smaller solar fields of up to several thousand square meters of aperture area. Existing medium sized parabolic trough collectors for process heat can deliver heat at 200°C and more, but their efficiency is fairly low at elevated temperature (Kruger et al). Assuming a small scale parabolic trough
collector would be enhanced by performance-improved features typical for solar power plant collectors as eg vacuum receivers and sufficient concentration quality, an annual yield described in Figure 2 could be reached according to simulations in TRNSYS. Especially a small collector, with a tilted north-south axis, could reach energy yields around 600 kWh/m2*a, when enhanced to an efficiency of the EuroTrough collector. Tracking from east to west provides a high output level for several hours on sunny days (Figure 3), as the collector is close to perpendicular irradiation all day.

Typical property of the solar heat is its discontinuous power and temperature level due to the day-night cycle and irradiance variations with weather conditions. This affects the selection of the appropriate heat engine.

Upper curve: High performance collector, tilt 35° to south

Centre curve: High performance collector, horizontal axis

Lower curve: Process heat collector of Industrial Solar Technology, horizontal axis

900 800 700 600 500 400 300 200 100 0

Engine

In principle, various types of engines can be used for conversion of solar heat to electricity: Steam turbines, ORC turbines, Stirling engines and steam engines. Steam turbines are
nowadays hardly available for the range below 500 kW electrical power. ORC turbines also start in the range of 500 kW electrical power. Stirling engines have been developed for small domestic CHP (Combined Heat and Power Production) and tested in combination with high temperature heat from parabolic dishes. The temperatures necessary exceed the temperature provided by parabolic trough collectors. New low temperature Stirling engines may be developed though. Steam engines are today only commercially available from the company Spilling, starting from 60 kW nominal electrical power with good part-load behaviour.

As the solar output of the collector varies with radiation and incident angle, a steam engine with its high part load efficiency is chosen for this study. Spilling produces a 120 kW machine, which can be operated by 210°C saturated steam.

Nominal thermal input power is 960 kW. According to supplier information, part load is possible down to 30%. Between 100% power down to 30% part load the gross electrical efficiency is almost constant at 12.5%. Parasitic power for pumps and assemblies amount to about 3 kW over full and part load. Outlet steam quality is wet steam at 110°C and 1.5 bar.