Comparison between Steady State and Quasi-dynamic test method. according to EN 12975 — application to flat plate collectors
Jose Afonso1, Nuno Mexa1, Maria Joao Carvalho ^
1 INETI, Department of Renewable Energies, Campus do Lumiar do INETI, 1649-038 Lisbon, Portugal
* Corresponding Author, mioaoa. carvalho@ineti. pt
Abstract
Presently two test methodologies are available for characterization of the efficiency (thermal performance) of glazed collectors: i) steady state test methodology according to EN 129752: section 6.1 and ii) quasi-dynamic test methodology according to EN 12975-2: section 6.3.
The most commonly used methodology is steady-state according to EN 12975-2: section
6.1, considered applicable to all stationary collector, e. g., flat plate and evacuated tubular collectors.
But quasi-dynamic test methodology can have an important influence on the performance of a test laboratory since it allows for a quicker response in characterization of thermal performance of collectors as shown by Rojas, D. et al. (2008).
As preparatory work for accreditation of this test methodology at the Solar Collector Testing Laboratory, tests according to the methodology were preformed.
The necessary tools for testing, such as data acquisition programme and a MLR (Multi Linear Regression) tool were developed. First results of the test of a flat plate collector are analysed. In the case of evacuated tubular collectors, test sequences were obtained and analysed showing the need for further development of the MLR tool.
Keywords: Solar thermal collectors, Test methods, Steady-state, Quasi-dynamic
Presently two test methodologies are available for characterization of the efficiency (thermal performance) of glazed collectors: i) Steady State (SS) test methodology according to EN 12975-2: section 6.1 [1] and ii) Quasi-Dynamic (QD) test methodology according to EN 12975-2: section
6.3 [1].
The most commonly used methodology is SS according to EN 12975-2: section 6.1, considered applicable to all stationary collector, e. g., flat plate and evacuated tubular collectors. But QD test methodology can have an important influence on the performance of a test laboratory since it allows for a quicker response in characterization of thermal performance of collectors as shown in reference [2].
The Solar Collector Testing Laboratory (LECS) of INETI is an accredited Laboratory for test of solar thermal collectors and the determination of thermal performance is made according to the SS test method (EN 12975-2:section 6.1 [1]). Two main reasons impose the need to develop the knowledge and the necessary practical conditions for use of the QD test methodology at LECS:
a) one is the improvement of use of time the collectors stay at the laboratory, allowing for a quicker response to the test requests [2];
b) the other is the fact that for some types of collectors, the QD test methodology allows for a better characterization of the collector behaviour [3, 4].
The SS test method requires steady conditions that significantly condition validation of the obtained data and therefore the time needed to conduct a test. The measured values for globlal irradiance, ambient temperature, fluid flow rate and fluid inlet temperature must not have significant variations (described in the standard [1]) for the set of data to be validated. When a steady state is not verified the set of obtained data is rejected.
The QD test method is much less restrictive. For example, the collector may continually be kept in the same position (facing south), regardless of the angle of incidence. There is also a greater variability of the measured values, which allows for more variable weather conditions. For this reason, use of the QD method to test solar collectors enables data to be obtained with less operator intervention. Because steady weather conditions are not required, solar collector tests may also be conducted over a greater annual period and in a more simplified manner. On the other hand, the calculation methodology, to determine the parameters that characterise the thermal performance of the collectors, is more complex.
In this work a short description of the test conditions for SS and QD test methods is presented in section 2. In this section, also the relevant equations and characteristic parameters for each test method are highlighted. Section 3. describes the different steps necessary or implementation of QD test method at the Laboratory. Section 4. presents and discusses the first test results obtained. In section 5. conclusions on the future development for implementation of QD test methodology, are presented.
