Steffen Jack1, Michael Kohl1, Axel Mkller2, Karl-Anders Weiss1*

1 Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstrasse 2, 79110 Freiburg, Germany
2 Dr. Axel Muller — HTCO, Postfach 700203, 79056 Freiburg, Germany
* Corresponding Author, Karl-Anders. Weiss@ise. fraunhofer. de

Abstract

In order to develop a preferably efficient and durable polymer collector the use of computational fluid dynamics (CFD) and FEM-Simulations opens up a fast and efficient way to determine key parameters and problems. The low intrinsic thermal conductivity of polymeric materials and their limited temperature stability can be partly compensated by the optimization of the design of the collectors. Here numerical simulation tools are utilised to analyse and compare different geometries of absorbers or absorber tubes.

Computational fluid dynamics is used to optimize the collector geometries by Dr. Axel Muller — HTCO. The aim is to develop a layout which assures a homogeneous flow, a maximized contact area between the absorber and the heat transfer fluid and an optimised heat transfer into the fluid. The simulation of different collector and absorber layer geometries discussed in this paper point out the quantifying parameters which allow an efficiency optimisation.

Mechanical stresses in the collectors and channels which are caused by temperature gradients or external loads are calculated with FEM-tools. This topic is important in particular if low-cost polymers like polyolefins shall be used. The material stresses are in acceptable range for the analyzed steady state situations. But the combination of different polymers in an integrated collector with bonded absorber and frame can lead to high deformations due to thermal expansion.

Keywords: Solar Thermal Collectors, Polymers, Fluid Dynamics, FEM-Simulation

1. Introduction

The scarcity of fossil fuels is beyond question — one way to save these resources is to make use of solar thermal energy for domestic hot water. So far, solar thermal collectors mainly consist of glass and metal parts. Not simply substituting materials in existing systems but developing a funda­mentally new design for a polymeric collector is the objective of the research at the Fraunhofer ISE and Dr. Axel Muller — HTCO in the framework of Task 39 of the Solar Heating and Cooling Programme of the IEA.

Since the economic viability of solar collectors is strongly linked to the costs of the system, a decrease of the system costs would lead to a higher market penetration. Key advantages of polymers are abundance, weight reduction and more freedom in design, along with the benefits and cost savings associated with well established manufacturing processes and improved fastening, reduced number of parts, and overall assembly refinements. However, also the probably changed system performance is an important element and may not be forgotten.

The Fraunhofer ISE is currently working on the concept of a fully polymeric collector, in order to consider these elements in an integrated way, as only then the full potential of polymeric materials can be used. Important parameters are of course the absorption of solar radiation and — in comparison to metals — the usually lower thermal conductivity and heat capacity. On the other hand one has to consider the intrinsic stress factors like UV-radiation, high temperatures and mechanical loads because the systems have to reach a service life-time of more than 20 years.

In this early development phase numerical simulation tools are used to analyse these topics. The aim is to develop a collector layout considering production and material boundary conditions with a best possible degree of efficiency. With the help of fluid dynamic simulations, one can calculate e. g. the heat transfer from the absorber layer into the heat transfer fluid and optimize it. So one can realise cost and time savings for engineering and prototype production. Parameter sensitivity studies for absorber channels designs are presented as well as efficiency calculations of entire collectors. Another important topic is the simulation of the behaviour of collectors or parts of them during stagnation conditions. Here the temperature distribution and the maximum temperatures are most interesting in reference to thermo-mechanical stresses which occur due to thermal expansion.