Mass Transport and Chemical Reactions

R&D Services for Mass Transport and Chemical Reactions

In the research topic Silicon Photovoltaics, we offer the following services in the area of mass transfer and chemical reactions.

• Mapping of arbitrary components in a FEM model and calculation on a High Performance Computing Cluster (HPC cluster)
• Surface treatment of solar cells, heat pumps and heat and cold storage systems
• Optimization of reaction chambers including flow and heat distribution
• Virtual product development and optimization
• Spatially resolved modeling of surface reactions
• Experimental validation

Rounding of Micropyramids

In the photovoltaic industry, wet chemical baths are used for surface structuring, conditioning and cleaning. In addition to cleaning the wafers, surface etch removal  also occurs during ozone-based wet chemical cleaning processes, resulting in two effects: The etching back of the emitter as well as the rounding of textured surfaces. To simulate and predict these roundings of micropyramids during the cleaning process, a two-dimensional process simulation model was built to describe the diffusive and convective mass transport of ozone along the pyramids. Important parameters for the evaluation, which are derived from experimental results, are the etch rate and the resulting roundness (R) of the micropyramid tips. A parameter study was performed to fit the model. The simulation represents the experimental data very well, after 2 min exposure R = 67 nm for the experimental and R = 66 nm for the simulation data.

Adsorption of Water Vapor in Microporous Adsorbents

Building air conditioning and the need for domestic hot water account for a significant share of energy consumption in private households. Likewise, there is a large demand for cooling in data centers, for example. Thermally driven heat pumps can help reduce energy consumption here. Adsorption heat pumps and chillers use microporous adsorbents that release heat while absorbing water vapor. To achieve a high power density of the adsorption heat pump or chiller, it is crucial that the heat and mass transfer processes in the components take place as quickly as possible. At the same time, it is important for high efficiency that the thermal mass of the components is as low as possible. A promising design for adsorption heat exchangers are fiber structures with a large surface area (8000-12000 m²/m³) coated with a microporous adsorbent. An understanding of the coupled heat and mass transfer processes is required to design this type of heat exchanger.

Simulation of the Microclimate

All parts and components (such as PV modules, cables, solar thermal collectors, etc.) that are used outdoors are exposed to influences that usually result in aging. The decisive factor for the aging processes is the microclimate in the component or components, which can sometimes differ significantly from the measurable macroclimate (ambient temperature, relative humidity). Especially for special geometries, the microclimate is difficult to measure, but can be reliably described by FEM simulations. The input variables are temperature, relative humidity and material-specific parameters such as permeability or mass transfer coefficients, thermal conductivity and density, which can be measured in our Center for Material Characterization and Durability Analysis. In the example shown here, the moisture balance within a PV module consisting of backsheet, EVA, silicon cell and cover glass (not shown) was simulated during a steam-heat test. The determined water concentrations can then be used in the development of accelerated aging tests.