Yifan Wang

During my doctoral degree I looked into the application of mathematical optimisation on energy systems, in particular with regard to seasonal energy supply and storage. The research project I worked on during my research stay is part of my doctoral thesis:

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The University of Minnesota

Complex processes, such as those of direct air capture (DAC) plants, must be integrated into future energy systems, to allow for the incorporation of renewable energy sources. DAC plants rely on physical or chemical methods for extracting CO2 from the air. They play a key role when it comes to achieving sub-zero emissions, and they can be integrated directly into future energy systems such as power-to-gas energy systems [1]. However, the goal to integrate DAC plants into energy systems gives rise to new challenges concerning the development of optimised operation strategies:

1) A DAC plant is a complex process system whose operation typically includes the following phases: adsorption, blowdown, heating, desorption and cooling.

2) Different durations and operating conditions apply for each phase [2]. The adsorption phase is electricity-intensive, the heating phase requires high heat consumption and the separated CO2 is only available as output during the desorption phase [2]. In the blowdown phase, for example, the DAC plant relies exclusively on electricity for about 30 seconds, but the separated CO₂ is not delivered until the desorption phase, which can last several hours.

3) The operating output of a phase is always linked to the condition of the previous phase [2].

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Yifan Wang at Foundations of Computer Aided Process Design conference (FOCAPD 2024)

In order to ensure optimised operation of a DAC plant within an energy system at minimised costs (comprising electricity and heat procurement), it is essential to develop a model that integrates the output of the DAC plant into the optimisation of the energy system, taking the different operating conditions of the individual phases into account. At the same time, the computing effort required to solve the resulting optimisation problem should be reasonable.

The Decision Discovery and Optimization (DDO) Lab run by Professor Qi Zhang [3] specialises in decision-making processes for complex systems and the development of computational and data-powered methods. I benefited greatly from the lab’s expertise and experience, in particular with regard to innovative approaches to complex process optimisation and the application of advanced analytics tools, which contributed directly to the progress I made in my research work. During my research stay I also had a weekly meeting with Professor Zhang and the opportunity to discuss technical matters with other doctoral candidates and Postdocs throughout my stay.

The result of the research project is a new approach to modelling complex process systems for energy system optimisation. The resulting optimisation issues can be solved efficiently and at low computational effort by means of a solution approach that comprises three different variants. Following my research stay, I am currently working on a publication aiming to publish the results in a peer review journal.

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Yifan Wang alongside the Goldy Gopher Sculpture and at the Library of the University of Minnesota

Alongside working on my own project, I had the opportunity to meet the other researchers and learn about the exciting research topics addressed at the lab. I got along so well with some of my colleagues that we stayed in touch personally. I was particularly fascinated by the research topic “inverse optimisation”, where the underlying parameters of an optimisation issue are derived from observed decisions or solutions, rather than directly addressing a problem using known parameters.

The lab and the department also organised regular discipline-specific seminars and meetings in which renowned researchers, leaders from industry and aspiring junior researchers presented their work. I always perceived the different perspectives that are shaped by the wide range of academic backgrounds as an asset.

I also had the opportunity to take part in three local specialist conferences during my stay. Two of these were organised by the University of Minnesota, while attending the FOCAPD 2024 conference [4] had been scheduled as part of my research stay. I had the opportunity to present my own work on a poster there. The title of the project is RiNSES4: Rigorous Nonlinear Synthesis of Energy Systems for Seasonal Energy Supply and Storage.

In addition to traditional networking, I was very impressed by the format of the conferences: throughout the day there were presentations held by the invited professors, decision-makers, politicians and industry leaders and experts, which gave me access to a wide range of insights and new perspectives. The poster sessions took place on a smaller scale, so that I was able to have some profound discussions about my research with academics from various universities and disciplines. I was able to gather some valuable feedback here that has enriched my work.

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The Golden Gate Bridge

I was born and grew up in China and then lived in Germany for over ten years, so it was my first visit to the USA. It was a great academic and personal opportunity for me. I am seeking a career in academia, and was therefore impressed by the intensity and the scope of the research activities. The great number of opportunities and challenges available to researchers is remarkable. Students at American higher education institutions are, for example, encouraged to obtain their bachelor's, master's and doctoral degrees from different universities and to exchange ideas, which is why there is a very strong expert network in the USA.

I had a good time, also beyond my academic activities. My mentor and his doctoral candidates were very supportive when it came to finding a room, arriving and getting to know the university and the city. I was able to take over a studio apartment from another visiting scholar at the lab. The rent was rather high (about USD 1,500 per month), but the apartment was near the campus and very well equipped. This allowed me to save money on public transport and made everyday jobs such as grocery shopping easier, too.

My colleagues at the lab received me warmly and showed me many different sides of Minneapolis. Seeing the Minnehaha waterfall and the aurora borealis, attending the Minnesota State Fair and trying culinary specialities from various cultures, such as Ethiopia, India and Hong Kong, were among my personal highlights. I particularly enjoyed the atmosphere of Minneapolis in summer. When I needed a break, I would go for a walk or a run along the Mississippi River.

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Minniapolis from another perspective

Compared to my everyday life in Germany, I found people to be much more open-minded in the USA. When ordering a coffee in the US, you often start by asking “How are you today?”, for example, while in Germany just saying “Hello” will do. Activities in the US often focus on seeing impressive nature or attending large-scale events, such as trips to national parks or music festivals. In Germany, people are more likely to choose cultural or historic activities, such as visiting museums, castles or traditional fairs. I did, however, miss Germany’s convenient public transport system very much.

I was able to go on two holidays, too: first I travelled to Boston and New York and later from San Diego to San Francisco, via Las Vegas and Los Angeles. On these trips I got to see various other sides of the USA and I gathered many new impressions. I was able to visit the world’s best universities in Boston and was impressed by the magnificent and luxurious design and decorations in Las Vegas. However, there appear to be some bad sides to America, too, such as the story of the so-called mole people in Las Vegas that I heard for the first time. San Francisco impressed me with its beautiful Golden Gate Bridge and the advanced technology of driverless taxis that are travelling around the city.

My research stay was a big success on the whole. Those six months were full of an incredible amount of learning, experiences and new impressions. I received a lot of support and help to make this successful research stay possible.

I would like to thank my academic advisor at the Chair of Technical Thermodynamics, Professor Niklas von der Aßen, my local mentor in Minneapolis, Professor Qi Zhang, and Professor Kai Leonhard from the Chair of Technical Thermodynamics. It was their comprehensive support that has made this research stay possible. The funding from the DAAD-Stiftung's ‘KSB Stiftung Scholarship’ played a key role for me, because it gave me additional freedom, and allowed me to discover and explore a new world, not only academically but also personally and culturally.