Fab support by simulation and scenario comparison

Simple short-cut calculations are difficult for capacity check of facilities in the event of changes in production (e.g., capacity increase, new technologies, new chemicals), due to the mutual influence of several facility systems. Only a full software-based calculation model provides enough stability for thorough case studies.

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by Gabriele Fetzer, Birgit Schluter, Martin Schlotter; M+W Zander, Stuttgart, Germany

Simple short-cut calculations are difficult for capacity check of facilities in the event of changes in production (e.g., capacity increase, new technologies, new chemicals), due to the mutual influence of several facility systems. Only a full software-based calculation model provides enough stability for thorough case studies.

While many photovoltaics (PV) fabs have been built over the past several years, the optimum running conditions of these facilities with respect to environmental aspects often could not be achieved quickly. But now, a new simulation model has been developed to find optimum solutions.

The software tool Umberto has been used for modeling different solar cell fabrication and supply processes. This tool is designed to model, calculate, and visualize material and energy flow systems, and is used to analyze the process systems—either in a plant or a company, or along a product life cycle. Results can be assessed using economic and environmental performance indicators. Cost data for materials and processes can be entered to support managerial decision-making. Data can also be used for material flow analysis studies or for product life cycle assessment (LCA) studies.

This simulation program can be used in an early stage of fab design or for analysis of existing systems. The program can also be used for a whole production line or only selected processes. A possible result could be an energy balance or a mass balance sheet. For example, it can answer the question, “what kind of abatement system can guarantee emissions will be in accordance with the law in force, with less investment costs?”

Examples of system optimizations
Rather than an in-depth case study, the following examples serve to highlight potential applications. Entire fab studies can provide, as an example, CO2 footprint calculations, because complete mass balances are produced. A technical scenario comparison can provide exhaust or waste water treatment optimization. Often, capacity adaptation (increase/optimization) is supported by scenario calculations prior to decision-making.

Exhaust treatment. Exhaust treatment is often necessary if defined emission concentrations must be guaranteed. Clients desiring green fabrication can ask questions about perfluorochemical (PFC) reduction rates, or the CO2 footprint of their respective productions. The cited tool has been used to assess the CO2 footprints of different solar cell production technologies.

For the treatment of volatile organic compounds (VOC), M+W Zander and the University of Pforzheim investigated the treatment of air loaded with organic materials and its influence on the ecosystem, climate, consumption of raw materials, and human health. [2] The study sought to show the impact of untreated air in comparison to treated air. Different concentrations of organic material in the air were calculated. The result was astonishing. The treatment of air with low concentrations of organic material has a positive effect on human health. But the influence on all other parameters—ecosystem, climate, and the consumption of raw materials—is negative. This will change with higher concentrations. Figure 1 shows the calculation result for air loaded with 800mg/m3 organics

Figure 1. Environmental impact of VOC effluent treatment.


The model can also be used to investigate other problems. The market of air treatment systems is big; many different systems with different functions are offered. A simulation program can be helpful for finding the optimum system at minimal cost. From a theoretical standpoint, this could be regarded as a more trivial matter, but can be of high practical importance.

Tools for exhaust air treatment are often working at higher temperatures. Additionally, energy sources and, therefore, achievable temperatures, may differ as well. Investment and/or operational costs may also differ. The idea that a system operating at high temperatures would give the best results could be misleading. With high temperatures, a good cleaning result could be expected, but unwanted reactions with the air are also possible. Higher temperatures are also the reason for higher energy consumption and operational costs.

In a simulation, it is possible to visualize different operation conditions. A mass balance sheet will show the input/output concentrations. The optimum temperature and operating conditions can be found with this program, which will help save money by investment or during operation. The customer will be sure that the treatment system will operate in an efficient stable phase, and the allowed emission concentrations will be safely achieved.

Modeling will help in choosing the best system—one that may be simple and operating at lower, energy-saving temperatures. Stable abatement systems are good for the environment and also save operational costs.

Wastewater treatment. Another facility area where modeling may be helpful is the planning or expansion of wastewater treatment. Wastewater is generated in different processes. Depending on the needs, there might be only a neutralization, but precipitation of fluoride or even removal of heavy metals might be necessary. Often, purification can be achieved by different technologies. All the scenarios can be calculated and compared to each other on a technical and cost basis, as well as for robustness, before decisions and investments are made. This not only allows the definition of the wastewater network prior to finalizing the design, but also the best retrofit option in solving a problem (e.g., unwanted precipitations in pipes) can be found.

Capacity increase. As the worldwide global recession lifts, the question about whether to increase capacity in existing installations will be posed again. It is extremely important to have the model available that gives the full impact of a capacity increase to all facility systems, including their mutual interdependence. Late detection of a bottleneck will end up in delayed ramp-up of new installations, increased investment compared to initial estimates and, in the worst case, a project failure.

Figure 2. Interdependence of cooling power generation.

The model containing the know-how of many system experts in “crystallized” form (as software code) enables the user to make scenario comparisons without repeatedly interviewing all system experts for the facility systems involved. For example, the climatization of a cleanroom requires several types of coolant, which are partly interdependent (Fig. 2). Only a specialist, or the user of the software, can make a prediction regarding how an increase in capacity will affect the rest of the network.

Putting together the modules. So far, these have been examples for single systems and applications. Whenever a holistic picture of an intended project, investment, or change is required, it is preferably solved with a software model containing all the elements and their interdependence. So, on a per project basis, information collected within the frame of a single project is always less complete than a database that is based on a software tool developed over the course of several subsequent projects.

Various applications accrue from a calculation model that includes the mass and energy flows in production and facilities. Not only can environmental parameters (such as emissions or CO2 footprints) be produced, but technical scenario comparisons are accessible to support decisions with respect to the investment required for such calculations as capacity increases in production. Maintaining a model allows collection of multiple information from a variety of sources over time that are then made useful for engineering calculations.

1. M. de Wild-Scholten and M. Schottler, Thin Film Forum Berlin, 2008.
2. H. Hottenroth, M. Schmidt and M. Schottler, ACHEMA 2009, Frankfurt.

Gabriele Fetzer received her masters and PhD at the U. of Stuttgart, Germany, and is project manager for permitting processes at M+W Zander FE GmbH, Environmental Applications, Lotterbergstrasse 30, 70499 Stuttgart, Germany; +49 711 8804-2637; gabriele.fetzer@mw-zander.com

Birgit Schluter received her masters at the U. of Stuttgart, Germany, and is project engineer for industrial engineering at M+W Zander FE GmbH.

Martin Schottler received his masters and PhD at the U. of Darmstadt, Germany, and is project manager for industrial engineering at M+W Zander FE GmbH.


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