P. Di Lauro*, S. HeB, S. Rose, M. Rommel

Fraunhofer Institute for Solar Energy Systems (ISE), Department Thermal Systems and Buildings,
Heidenhofstrasse 2, 79110 Freiburg, Germany

* Corresponding Author, paolo. di. lauro@ise. fraunhofer. de
Abstract

Purpose

The optical efficiency n0 and the Incidence Angle Modifier (IAM) are very important parameters in terms of the characterisation of the thermal performance of solar collectors. These two parameters give the significant distinguishing information about the optical performance of a solar collector.

Methods

The research group “Thermal Collectors and Applications” at the Fraunhofer ISE uses the simulation program OptiCAD for investigations on the optical properties of collectors. OptiCAD is a so called “forward ray-tracer”. Flat-plate collectors, evacuated tubular collectors and also collectors with reflectors may be modelled in OptiCAD. After parameterisation of different components and materials of the collector (cover glass, absorber, reflector etc.) the optical efficiency and the 3D-IAM (Incidence Angle Modifier in three dimensions) can be calculated by ray-tracing. The principle followed is that every simulated ray starting from the light source is calculated, followed during all processes (transmission, reflection, absorption) until it is finally absorbed.

The properties of the collector with respect to diffuse radiation can be calculated from the simulated 3D-IAM assuming an isotropic distribution of diffuse radiation.

Results and Conclusion

Different collectors which were tested at the testing laboratory of the Fraunhofer ISE have been modelled in OptiCAD and their optical parameters have been determined by simulation. In the poster presentation it is shown that the measured and the calculated parameters fit very well. It is discussed how the ray-tracing investigations are a valuable and helpful tool for the development and improvement of solar thermal collectors.

Keywords: solar thermal collector, 3D-IAM, diffuse radiation, ray-tracing

1. Introduction

The optical efficiency no[11] and the Incidence Angle Modifier (IAM) play an important role for the efficient utilisation of solar radiation by means of thermal collectors. Those parameters often are determined at an outdoor test facility, in most cases by using a tracker, or at an indoor solar simulator. But these characteristics now can also be calculated by modelling the collector in a simulation software. For this process the optical properties of the different component’s materials that the collector is composed of are to be displayed. The correct description of the actual optical and geometrical parameters of all of the collector components is important for the quality of the results of such a ray-tracing simulation. Therefore, it makes sense to measure the optical properties like transmission, reflection and absorption of each individual element and accordingly implement them in the ray-tracing program.

The result of a simulation of an emulated collector is the effective transmittance-absorptance products (та)е^ under a particular direction and incidence angle of sun rays. This characteristic

can be simulated for all directions (3D) of incident rays. However, it has to be considered that this factor is not equal to the mathematical product of the two optical properties (transmission and absorption). It additionally takes into account general geometrical effects of the complete collector and the interaction of the components like multiple reflections between them.

The IAM then presents the relation of (та)during solar radiation of a certain direction to the one

under perpendicular solar incidence. In order to finally validate those simulated data by comparing with the measured “optical efficiencies” an assumption for the collector efficiency factor F’ has to be made which is assumed to be constant.

Moreover, it is possible to give a proposition about the acceptance for diffuse radiation of the collector by using the simulated IAM-values for direct radiation and supposition of an isotropic distribution of diffuse irradiation on the collector from the virtual hemisphere.

Ray-tracing simulations of the optical parameters of solar thermal collectors offer the following advantages:

• Determination of the optical properties of a collector that is not yet build

• Variation and improvement of the design of a collector during the construction phase

• Predictions of solar gains by simulation of the IAM for direct solar incidence under any incidence angle

• Identification of the acceptance for diffuse radiation of the modelled collector by deriving a diffuse IAM from the IAM-values for direct radiation (assumption of isotropic distributed diffuse radiation)