Additionally to the standard procedures the validity limit of the functions has to be determined.
The examples shown in the chapter “The limit is dependent on the absorber temperature and on the ambient temperature” are related to an idealized relatively sharp limiting behavior. In reality such limiting measures may not have this sharp onset in the drop-down of the efficiency.
18.6.3.1 Determination of the Validity Limit for the Standard Procedures
The test procedure (6.1.4.4, 6.1.5.5) may be expanded to higher fluid inlet temperatures (in steps of, for example, 5 °C). In steady-state tests the onset of the limiting
|
0 10 20 30 40 50 60 70 80 90 100 absorber temperature [°C] Figure 18.2 Efficiency graph of a solar collector with twin-wall sheet glazing, black absorber (a = 0.95, e = 0.90), and solar irradiation of 1000 W m-2 at ambient air temperature of 20°C (transmittance of thermotropic layer: 85% in clear state). |
behavior can clearly be seen if the range of the statistical least-squares analysis is expanded step by step. Some development work on statistical methods in the case of the quasi-dynamic method may be necessary to determine the limit.
Generally, this method will work if the limiting temperature is near to the normal operating temperature range and the higher temperatures can be tested with the same fluid as for the lower.
If the expected limit is much higher than the normal operating range, then this method can be used to check to see if there is an influence in the upper region of the efficiency curve:
The limit is dependent mainly on the absorber temperature: for example, the thermotropic layer on the absorber or absorber temperature controlled ventilation. This test can be done without variation of the ambient temperature.
The limit is dependent on the absorber temperature and on the ambient temperature: this test should be performed with variation of the ambient temperature (±10 °C).
18.6.3.2 Determination of Stagnation Temperature
The stagnation temperature could be measured directly if the drop-down of the efficiency curve is sharp and the solar irradiance is high enough to reach the limit, but
care should be taken if there is a dependency on the ambient temperature or if there are two discontinuities.
No general rules can be given up to now.
[1] Meteonorm is a global meteorological database for applied climatology, developed by Meteotest in Bern, Switzerland (http://meteonorm. com/).
[2] Circulation caused by gravity (thermosiphon effect) can only be achieved in cases where the store or the consumer is located above the collector. Today, for most technical applications in Central or Northern Europe the opposite is the case.
[3] CombiSol project co-financed by Intelligent Energy Europe program (http://www. combisol. eu).
[4] A simplified design tool is available for free at http://www. combisol. eu.
[5] More details on the calculation of the thermal balance of a collector can be found in Reference [6].
[6] See the regulation guidelines: DIN 4753, DIN1988, EN 1717, and EN 12977.
[7] ISO (1995) ISO/TR 12596:1995. Solar heating – Swimming-pool heating systems – Dimensions, design and installation guidelines. International Organization for Standardization, Geneva.
2 Weiss, W. (2003) Solar Heating Systems for Houses: A Design Handbook for Solar Combisystems, James & James (Science Publishers) Ltd., p. 344.
3 Henning, H.-M. (2007) Solar Assisted Air Conditioning in Buildings: A Handbook for Planners, Springer, 150 pp.
[8] Elias, H.-G. (2003) An Introduction to Plastics, Wiley-VCH Verlag GmbH, Weinheim.
2 Ebewele, R. O. (2000) Polymer Science and Technology, CRC Press, Boca Raton, FL.
3 Young, R. J. and Lovell, P. A. (1991) Introduction to Polymers, Chapman & Hall, London.
4 The Open University (2012) Introduction to polymers. http://openlearn. open. ac .uk/course/view. php? id=2937.
(accessed on 04th June 2012).
5 Wikipedia (2012) Polymer. http:// en. wikipedia. org/wiki/Polymer.
(accessed on 04th June 2012).
6 Polymer Science Learning Center, Universityof Southern Mississippi (2005) Macrogalleria. http://pslc. ws/macrog/
[9] Oswald, T. A. and Menges, G. (1995) Material Science of Polymer, Carl Hanser Publishers, Munich.
2 Wittfoth, A. M. (1978) Plastic Technical Dictionary, Part 3, Carl Hanser Verlag, Munich.
3 Hensen, F., Knappe, W., and Potente, H. (1986) Handbuch der Kunststoff – Extrusionstechnik, Carl Hanser Verlag, Munich.
[10] While the literature indicates that fillers can increase the material yield strength, there have been no studies in whichthe hydrostatic design stress (i. e., burst strength after 100000h) has been evaluated for these materials.
[11] The survey was conducted in late 2010 and published by Banse in 2011 [4].
[12] — = not available, 2 = direct integration possible, = heat exchanger required.
[13] Combisystem: combined system for DHW preparation and space heating.
[14] Weiss, W. and Mauthner, F. (2011) Solar heat worldwide. Presented at Markets and Contribution to the Energy Supply 2009. IEA Solar Heating and Cooling Programme, May 2011. AEE INTEC, Gleisdorf, Austria.
2 Fawer, M. (2006) Sustainability Report – Solar Energy 2006, Bank Sarasin
& Co. Ltd.
3 Kicker, H. (2009) Vergleichende okobilanzierung von solarthermischen
[15] For example, EN 12086, EN 12572, ISO 15106, ASTM E398, and JIS K7129.
[16] Henning, A. (2000) Ambiguous artefacts: solar collectors in Swedish contexts; on
[17] Probst, M. et at (2004) Impact of new developments on the integration into facades of solar thermal collectors, in EuroSun 2004 Proceedings, EuroSun 2004: June 20-24, 2004, Freiburg, Germany, Deutsche Gesellschaft fiir Sonnenenergie e. V. DGS, Munich, PSE GmbH, Freiburg. ISBN 3-9809656-4-3 (CD ROM).
[18] Note values given are minimum values for testing.
[19] One may also think about altering the test conditions (irradiance level, exposure time, wind speed, etc.) to account for proper testing. In addition, it could be possible to define a set of classes (class 1: “high” irradiance, “long” exposure, class n: “low” irradiance, “short-time” exposure) that the manufacturer can choose from and which will then be noted in the test report.
