Comparison of different technologies for process heat generation

Greenius has been used to perform a study on the specific output of different solar process heat generating systems. The site chosen for this comparison is Almeria (Spain), with an annual sum of direct normal irradiance of 1918 kWh/m2a and a global horizontal irradiance of 1812 kWh/m2a. The main parameters of all solar fields investigated are shown in table 1. The aperture area is almost the same with small deviations caused by the area of a single collector unit. Solar only operation was assumed which might be interpreted as an additional solar plant operated as fuel saver in parallel to a large fossil process heat generating system.

The first 3 collectors are representing the state-of-the-art for their technologies and the last one (nominated as “advanced parabolic trough”) is a kind of hypothetical collector with the same optical performance as the existing 1ST type but with a vacuum tube receiver. Actually the heat loss characteristics from the Schott PTR70 Receiver have been used, a receiver which is installed in several of the parabolic trough power plants currently built in Spain and USA.

Figure 4 shows the results of the study: At low temperatures of 50 or 60°C the flat plate collector shows the best performance but if temperatures of 100°C or higher are needed, the CPC or the parabolic trough collector show better annual specific outputs due to their improved optics and the reduced heat losses. The difference between the CPC and the 1ST collector at temperatures above 140°C is insignificant. The CPC collector is able to compensate the performance gain of the tracking parabolic trough collector by the inclined installation and the vacuum tube. The curve of
the “advanced parabolic trough” shows the impact of such a vacuum receiver on the performance and as well the potential of parabolic troughs for process heat applications.

Collector type

Flat plate

CPC1}

Parabolic

trough

Advanced parabolic trough

Model

Wagner C20 AR

Paradigma CPC40 Allstar

1ST

IST with vacuum receiver

Number of collectors

110

66

20

20

Aperture area

262.9m2

265.3

264m2

264m2

Peak optical efficiency

85.4%

64%

71.5%

71.5%

Orientation

South, 40°

South, 40°

South-Nord, 0°

South-Nord, 0°

Table 1. Important parameters of the solar collector fields

1) CPC: Compound parabolic collector with vacuum tube

Подпись: flat plate CPC parabolic trough -—advanced parabolic trough Fig. 4. Specific annual output from different solar collector technologies depending on the fluid temperature (calculated with the new Greenius version).

collector temperature in °C

Greenius offers further options to analyze different solar field layouts. Figure 5 shows the daily solar field output of the enhanced parabolic trough system for two different orientations of the collector axes. The left hand side of Figure 5 is based on south-north orientation of the tracking axis whereas the right hand side is based on east-west orientation. South-north orientation gives a maximized annual yield of 911 kWh/m2a, but a strong seasonal variation, and east-west orientation gives somewhat lower annual yield of 849 kWh/m2a with smaller differences between summer and

winter. These plots have been generated using the build-in capabilities of Greenius but the user interface specific frames are not shown to enhance the readability of the figure.

image61

Fig. 5. Daily thermal output of the enhanced parabolic trough collector field for two different orientations of the collector axes: south-north (left) and east-west (right)

Although Greenius offers also an economical evaluation of these projects by calculating a couple of economical and financial figures of merit, this feature has been omitted in this paper since the results are strongly dependent on the input data, particularly the specific system costs in €/m2. Due to the small number of parabolic trough plants for process heat applications, it is hardly possible to identify the costs for these systems. Therefore an economic comparison would have been based on rough assumptions, but the technical comparison shows, that parabolic trough are quite competitive to CPC collectors in the upper temperature range if they can reach the same cost level as the CPC collectors.

4. Conclusion and Outlook

The software tool Greenius is now ready to study process heat generation by concentrating and non-concentrating solar collectors. It is therefore a valuable tool to perform fast case studies for different sites and different renewable energies.

The results of such a study for a site in southern Spain show that parabolic trough collectors may provide process heat with similar specific annual yield like CPC collectors at sufficient direct irradiance and at temperatures above 140°C. They offer the potential of much higher annual yields if the can be equipped with vacuum tube receivers, which CPC’s are already using.

Further tests and validation runs are scheduled, particularly for the parabolic trough models used.

A pilot plant is just under construction at Ennepetal, Germany, and first measurements from this plant will be available soon.

5. Acknowledgement

The work has been financed by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (Grant ID: 0329609A). The authors are responsible for the content.

References

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[2] Quaschning, V.: Understanding Renewable Energy Systems. EarthScan London, 2005.

[3] Theunissen, P.-H.; Beckman, W. A.: Solar Transmittance Characteristics of Evacuated Tubular Collectors with Diffuse Back Reflectors. In: Solar Energy Vol. 35 (1985) Nr. 4, S. 311-320

[4] http://www. itw. uni-stuttgart. de/~www/ITWHomepage/TZS/Berichte. html

[5] Lippke, F.: Simulation of the part load behaviour of a 30MW SEGS Plant, Technical report SAND95- 1293, Sandia National Laboratories, Albuquerque, New Mexico, 1995

[6] Quaschning, V.; Kistner, R.; Ortmanns, W.: The Influence of Direct Normal Irradiance Variation on the Optimal Parabolic Trough Field Size: A Problem Solved with Technical and Economical Simulations. Proceedings of Solar 2002, Sunrise on the Reliable Energy Economy, ASME June 15-20, 2002, Reno, Nevada.

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