Comparison of Two Different (Quasi-) Dynamic Testing Methods for the Performance Evaluation of a Linear Fresnel Process Heat Collector

Abstract

A small-scale Linear Fresnel Collector (LFC) for the generation of process heat has been tested by Fraunhofer ISE; its performance was evaluated by means of two different methods. The first is a quasi-dynamic testing method performed according to the testing standard ISO 9806:2013, with modifications in the model to accurately describe LFCs. Due to the two-dimensional Incidence Angle Modifier (IAM) of an LFC, an iterative multi-linear regression (MLR) approach has been developed to be able to comprehensively evaluate the optical performance. The second method is a dynamic testing method based on a parameter identification incorporating a multi-node/plug-flow collector model without strict restraints on mass flow and inlet temperature stability. Both methods are briefly described in their conceptual design and their basic requirements, revealing their similarities and differences. Each method is then applied to real measurement data from an LFC, assessing practicability and identification accuracy. For both methods, the mean absolute difference between identified IAM values and results from ray tracing fell in a range of 0.013-0.017, leading to a similar accuracy in LFC performance evaluation. Differences in optical efficiency between the two methods are smaller, with an average absolute difference below 0.0098, even when using different measurement data and simulation models. Thus the dynamic method represents a good starting point for the further development of an alternative dynamic testing and evaluation method with more flexibility than the current testing standard. This will be significant when evaluating large-scale concentrating collectors and collectors with direct steam generation.

IEA SHC Task 49/IV - Deliverable A3.1 - Guideline on testing procedures for collectors used in solar process heat

“…The parameter identification tool used in the DT method is generally approved by the standard and has shown equivalent results in both cases of same and similar/comparative measurement data basis (Hofer et al 2015a). Combined with the vaster degrees of freedom, it must be considered to be the most promising and powerful tool for the characterization of LFC and in situ measurements in general.…”

“…This is also the reason for the limitations in variation of inlet temperature and mass flow. The dynamic testing (DT) method is free of these restraints, and its functionality has been described and compared to QDT results in (Hofer et al 2015a): (Hofer et al 2015a) In the guideline "Dynamic in situ Performance and Acceptance Testing of Line-Concentrating Collectors and Solar Fields" currently under publication and written within the German project StaMeP, the differences and advantages in comparison with QDT have been described (see Hofer and Janotte (2015/16)…”

“…As a counterpart to the QDT-procedure the dynamic testing method has firstly been introduced by Muschaweck and Spirkl (1993), containing a more sophisticated collector simulation tool, but less restriction in measurement data (Muschaweck and Spirkl 1993) Fischer et al (2003) showed that they are equivalent in their results, NLS minimization only being more flexible (Fischer et al 2003). Fischer et al (2006) and Janotte et al (2009) (Platzer et al 2009;Hofer et al 2015a), or modifications to the QDT-methods are inevitable (compare with (Hofer et al 2015a Hofer and Janotte (2015/16)). "…”

“…28 for concentrating collectors and the standard gives specific instructions only for the usage of water as HTF and only up to a maximum temperature of 185°C and a maximum pressure of 12bar. Nevertheless, the QDT method has been successfully applied to PTC and other concentrating collectors (Janotte et al 2009;Fischer et al 2006;Fahr et al 2012) and developed beyond the standard procedure to even be applicable to LFC (Hofer et al 2015a). This chapter shortly describes the standard methods and, additionally, a non-standard fully dynamic testing procedure, which has been developed and successfully applied by Fraunhofer ISE.…”

State of the art of performance evaluation methods for concentrating solar collectors

“…For a global characterization of large-scale collectors (marked with an L), either parabolic trough or linear Fresnel, mainly the dynamic testing method is applied, as with higher working temperatures, energy loads to be cooled to meet stationary inlet conditions cannot easily be fulfilled. In particular for the characterization of linear Fresnel collectors due to their special optical characteristics in terms of a two-dimensional IAM, new approaches by dynamic parameter identification 12,13 , or modifications to the QDT-methods are inevitable (compare with Hofer et al (2015) 13 ).…”