By Karl-Anders Weiss, Fraunhofer Institute for Solar Energy Systems, Freiburg, Germany
Solar collector designs based on polymers replace metal with plastics and offer potential of far lower costs. To analyze these designs, researchers must model factors like heat transfer, structural deformation, and stress.
About half of fossil fuels are used in heating, so there is a huge potential in replacing them with renewable sources such as solar power. But standard solar collectors use copper or aluminum as the energy-absorbing material, so if we were to supply just 1% of the world’s heating energy with conventional solar energy, collectors would require 22 million tons of copper – more than the worldwide output in 2006. And these metals are costly, so there is a clear impetus to examine polymers as an alternative.
However, polymers don’t have the same ability to withstand high temperatures as metals do, so we need completely new designs for solar collectors using them. Thus, it is necessary to analyze durability of future solar energy systems, using modeling with Multiphysics software to analyze complex relationships between stress, strain, heat, and flow.
Design optimization for collectors
Polymers in solar energy applications offer many advantages. First, of course, is its price compared to today’s collector materials. Next, polymers offer great freedom in terms of design – we can develop new collector layouts that would be impossible with conventional materials. For instance, with an extrusion process it might be possible to mass-produce complex geometries in lengths of kilometers and thus bring economies of scale. Further, polymers allow the manufacture of collectors that are lighter in weight.
One possible geometry for a solar absorber made of polymer materials.
Polymeric materials have a low intrinsic thermal conductivity. This, however, can be compensated by optimized collector geometries with the goal being a layout that assures homogenous flow and maximal contact area between the absorber and the heat-transfer fluid. With solar collectors, heat transfer is certainly dependent on a material’s thickness and heat conductivity. But an even more predominant effect can be the heat-transfer coefficient between the fluid and the wall, which is determined by the fluid dynamics in the vicinity of the surface, and they depend on the surface shape. Because polymeric materials can have almost any form, the goal is to optimize a polymeric absorber’s shape so that heat transfer by convection overcomes the lack of heat conductivity.
Advantages of design optimizations are best described by the results of adding an additional plate as absorber into the design, which could increase the internal conductance from 95 W/m2K to 540 W/m2K. The illustration above demonstrates a possible layout for a thermal absorber based on multi-wall sheets where the heat-transfer fluid passes through channels that are surrounded by channels filled with air to provide heat insulation from the environment.
The Von-Mises stress
The Von-Mises stresses within a polymer-based solar collector at a normal inlet temperature of 350 K can vary widely depending on the material; here a comparison of the stresses and deformation between polymethyl methacrylate(left) and polypropylene (right) is shown.
Stress level analysis
Because collectors deform when heated, stress distribution and deformation represent potential risks for their stability and durability, especially at mechanical connection points. We want to estimate a product’s useful lifetime due to mechanical stresses that arise not only during normal operation but also during stagnation, the worst-case situation when the energy storage is no longer able to take heat from the collector. We created a COMSOL model that accounts not only for the temperature distribution that varies with the position of the absorber layer but also other factors that affect the temperature level including the amount of irradiance, inlet temperature and the collector’s thermal losses. This temperature data, seen on the preceding page, enables the determination of the collector’s deformation and mechanical failures shortening the service lifetime.
Humidity transport in PV modules
Polymers can also improve cost efficiency of photovoltaic (PV) solar modules. These consist of a front cover of glass, encapsulated solar cells and a back sheet, which is usually made of polymers. These polymeric back-sheets and encapsulants provide a barrier to keep humidity, atmospheric gases and pollutants away from the silicon solar cells and protect them mechanically. The ingress of humidity is a serious reason for their degradation, which can hardly be measured without physically destroying the module. Therefore, we work on developing measurement technologies and the mathematical modeling of the humidity transport.
Through COMSOL modeling, we can compare different polymeric collector geometries and materials for various energy carriers to reach an optimal collector design in terms of efficiency and price. We have also confirmed that our design is as efficient as conventional collectors and that the mechanical stability is sufficient if the collector is constructed properly. The next step is to model longer time periods to guarantee sufficient durability.
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