Solar thermal collector yield — experimental validation of calculations. based on steady-state and quasi-dynamic test methodologies
P. Horta[13]* , M. J. Carvalho1 and S. Fischer[14]
1 INETI, Department of Renewable Energies, Campus do Lumiar do INETI, 1649-038 Lisbon, Portugal
2 Institute for Thermodynamics and Thermal Engineering (ITW) - University of Stuttgart,
Pfaffenwaldring 6, 70550 Stuttgart, Germany
Corresponding Author, pedro. horta@lneti. pt
Abstract
The characterization of collector efficiency is the fundamental tool for long term calculation of collector yield. It is, thus, one of the most important inputs in software tools aiming the design of solar thermal systems.
Presently two test methodologies are available for characterization of the efficiency of glazed collectors: i) steady state test and ii) quasi-dynamic test, methodologies based in different model approaches to a solar collector, providing different collector efficiency curve parameters and, consequently, imposing different power calculation algorithms.
Moreover, Horta et al (2008) demonstrated that the use of the collector efficiency curve derived from steady state test method is not enough for a thorough characterization of the long term performance of a collector.
The present work takes into account the introduction of the above referred test methodologies in the European Test Standard for Solar Thermal Collectors, and aims at clarifying how each test results should be used in long term thermal performance calculations.
The paper presents a synthesis of the different efficiency parameters provided by each test methodology and corresponding algorithms, applicable in the calculation of delivered power. Application of these algorithms to two days of measured data allows for a comparison of the results obtained with these different methodologies.
For validation purposes, results of tests performed on a CPC type collector with a concentration ratio C=1.72 are used. Measurement sequences are used to validate the calculation of power delivered by the collector using both algorithms based on steady-state methodology (with and without correction) and quasi-dynamic methodology.
Keywords: solar thermal energy; efficiency curve parameters; solar system simulation; long term performance assessment
Presently two test methodologies are available for characterization of the efficiency of glazed collectors: i) steady state test methodology according to EN 12975-2: section 6.1 and ii) quasi-dynamic test methodology according to EN 12975-2: section 6.3.
It should be stressed that these methodologies, based on different model approaches for a solar collector, provide different collector efficiency curve parameters and, consequently, impose different algorithms for calculation of the power (and energy) delivered by solar thermal collectors.
In recent studies, Horta et al. (2008) demonstrated that the use of the collector efficiency curve derived from steady state test method is not enough for a thorough characterization of the long term performance of a collector, especially if its optical characteristics differ from the simplest flat plate collector.
Considering that, at present, steady-state tests are more commonly used and the majority of available collectors are characterized by steady state based efficiency curve parameters, a methodology for correction of power/energy results obtained with those parameters was proposed by Horta et al. (2008).
Recently, in project NEGST (Carvalho et al., 2006) it was also highlighted that for a correct characterization of stationary collectors with special optical characteristics or for tracking collectors, the quasi dynamic test method is the most appropriate test methodology.
The paper presents a synthesis of the different efficiency parameters provided by each test methodology and corresponding algorithms, applicable in the calculation of delivered power (see section 2). A validation of the methodology proposed by Horta et al. (2008), for the correction of long term performance calculations based on steady-state parameters, is also presented, after the results of tests performed on a CPC type collector with a concentration ratio C = 1.72.
