Journal Articles (Peer-Reviewed)
(2024): A network design problem for upgrading decentrally produced biogas into biomethane, Journal of Cleaner Production, 452 (5): 1-15.
Abstract: In this paper, we explore the possibility of connecting decentralized biogas plants via a pipeline network to terminals that upgrade biogas into biomethane. We present a mixed-integer linear program that forms subnetworks of such plants, decides on suitable terminal locations, and establishes pipeline connections to maximize profit. We apply this model to a real-world scenario in Northern Germany. The results show a much higher total profit for the optimized network compared to the benchmark solutions where each plant upgrades biogas into biomethane on its own. Therefore, plants can increase their profitability by collaborating with other (neighboring) plants. However, the collaboration requires a fair profit-sharing model as network participation is not individually profitable for all plants, especially small ones.
(2023): A tutorial on value function approximation for stochastic and dynamic transportation, 4OR, 22: 145-173.
Abstract: This paper provides an introductory tutorial on Value Function Approximation (VFA), a solution class from Approximate Dynamic Programming. VFA describes a heuristic way for solving sequential decision processes like a Markov Decision Process. Real-world problems in supply chain management (and beyond) containing dynamic and stochastic elements might be modeled as such processes, but large-scale instances are intractable to be solved to optimality by enumeration due to the curses of dimensionality. VFA can be a proper method for these cases and this tutorial is designed to ease its use in research, practice, and education. For this, the tutorial describes VFA in the context of stochastic and dynamic transportation and makes three main contributions. First, it gives a concise theoretical overview of VFA’s fundamental concepts, outlines a generic VFA algorithm, and briefly discusses advanced topics of VFA. Second, the VFA algorithm is applied to the taxicab problem that describes an easy-to-understand transportation planning task. Detailed step-by-step results are presented for a small-scale instance, allowing readers to gain an intuition about VFA’s main principles. Third, larger instances are solved by enhancing the basic VFA algorithm demonstrating its general capability to approach more complex problems. The experiments are done with artificial instances and the respective Python scripts are part of an electronic appendix. Overall, the tutorial provides the necessary knowledge to apply VFA to a wide range of stochastic and dynamic settings and addresses likewise researchers, lecturers, tutors, students, and practitioners.
(2022): Primal-Dual Value Function Approximation for Stochastic Dynamic Intermodal Transportation with Eco-Labels, Transportation Science, 57 (6): 1403-1719.
Abstract: Eco-labels are a way to benchmark transportation shipments with respect to their environmental impact. In contrast to an eco-labeling of consumer products, emissions in transportation depend on several operational factors like the mode of transportation (e.g., train or truck) or a vehicle’s current and potential future capacity utilization when new orders are added for consolidation. Thus, satisfying eco-labels and doing this cost efficiently is a challenging task when dynamically routing orders in an intermodal network. In this paper, we model the problem as a multiobjective sequential decision process and propose a reinforcement learning method: value function approximation (VFA). VFAs frequently simulate trajectories of the problem and store observed values (violated eco-labels and costs) for states aggregated to a set of features. The observations are used for improved decision making in the next trajectory. For our problem, we face two additional challenges when applying a VFA, the multiple objectives and the “delayed” realization of eco-label satisfaction due to future consolidation. For the first, we propose different feature sets dependent on the objective function’s focus: costs or eco-labels. For the latter, we propose enhancing the suboptimal decision making and observed pessimistic primal values within the VFA trajectories with optimistic dual decision making when all information of a trajectory is known ex post. This enhancement is a general methodological contribution to the literature of approximate dynamic programming and will likely improve learning for other problems as well. We show the advantages of both components in a comprehensive study for intermodal transport via trains and trucks in Europe.
(2022): Eco‐labeling of freight transport services: Design, evaluation, and research directions, Journal of Industrial Ecology, 26 (3): 801-814.
Abstract: The idea of eco-labeling is to provide customers with an easy-to-understand signal regarding the ecological impact of using a product or service. With this paper, we propose an eco-labeling system for freight transportation. We discuss design options based on a common emission reporting standard and a related communication protocol. We further explain a procedure for deriving labels for shipments of goods and provide examples illustrating and evaluating the labeling process at selected land-based freight transport services. Results indicate that eco-labels can grade the environmental impact of a transport service reliably, even if heterogeneous goods are moved together. Finally, we outline challenges for future research associated with eco-labeling in freight transportation markets.
(2022): Collaborative planning for intermodal transport with eco-label preferences, Transportation Research Part D: Transport and Environment, 112 (11): 1-26.
Abstract: Sustainability is a common concern in intermodal transport. Collaboration among carriers may help in reducing emissions. In this context, this work establishes a collaborative planning model for intermodal transport and uses eco-labels (a series of different levels of emission ranges) to reflect shippers’ sustainability preferences. A mathematical model and an Adaptive Large Neighborhood Search heuristic are proposed for intermodal transport planning of carriers and fuzzy set theory is used to model the preferences towards eco-labels. For multiple carriers, centralized, auction-based collaborative, and non-collaborative planning approaches are proposed and compared. Real data from barge, train and truck carriers in the European Rhine-Alpine corridor is used for extensive experiments where both unimodal carrier collaboration and intermodal carrier collaboration are analyzed. Compared with non-collaborative planning without eco-labels, the number of served requests increases and emissions decrease significantly in the collaborative planning with eco-labels as transport capacity is better utilized.
(2020): Comparing emission estimation models for rail freight transportation, Transportation Research Part D: Transport and Environment, 86 (9): 1-13.
Abstract: This study reviews emission estimation models that aim at providing realistic estimates of the emitted greenhouse gases from rail freight transportation. Five models are considered: two models from the MEET project, the ARTEMIS model, the EcoTransIT World model, and the Mesoscopic model. For each of the five models, this paper describes the estimation principles, methodology, and procedure, as well as relevant input parameters. An experimental study demonstrates the impact of train and trip specific parameters on each model’s emission estimate. Results are presented for varying values of a train’s number of wagons, the payload per wagon, the average speed, the trip distance, the number of stops, and the altitude profile along the route. In so doing, given a specific transportation scenario, the paper supports decision makers from industry and researchers to find and apply an appropriate emission estimation model for evaluating the eco-friendliness of rail freight transportation.
(2020): Emission limits and emission allocation schemes in intermodal freight transportation, Transportation Research Part E: Logistics and Transportation Review, 141 (9): 1-26.
Abstract: The problem addressed in this paper seeks for an optimal routing of freight orders through an intermodal transportation network. We consider the case of environmentally aware customers that request to ship orders with no more than a specified amount of greenhouse gases, which establishes so-called emission limits. In order to ensure that a routing plan complies with each order’s emission limit, it is necessary to estimate emissions caused by the used transport services and to allocate these emissions to the orders. We model this problem under cost-, emission- and service-objectives and apply it to an intermodal rail/road network in Europe.
(2018): Emission rates of intermodal rail/road and road-only transportation in Europe: A comprehensive simulation study, Transportation Research Part D: Transport and Environment, 65: 421-437.
Abstract: Intermodal rail/road transportation combines advantages of both modes of transport and is often seen as an effective approach for reducing the environmental impact of freight transportation. This is because it is often expected that rail transportation emits less greenhouse gases than road transportation. However, the actual emissions of both modes of transport depend on various factors like vehicle type, traction type, fuel emission factors, payload utilization, slope profile or traffic conditions. Still, comprehensive experimental results for estimating emission rates from heavy and voluminous goods in large-scale transportation systems are hardly available so far. This study describes an intermodal rail/road network model that covers the majority of European countries. Using this network model, we estimate emission rates with a mesoscopic model within and between the considered countries by conducting a large-scale simulation of road-only transports and intermodal transports. We show that there are high variations of emission rates for both road-only transportation and intermodal rail/road transportation over the different transport relations in Europe. We found that intermodal routing is more eco-friendly than road-only routing for more than 90% of the simulated shipments. Again, this value varies strongly among country pairs.
Book Chapters
: Emission Oriented vs. Time Oriented Routing in the European Intermodal Rail/Road Freight Transportation Network, in: Bierwirth, Christian, Thomas Kirschstein and Dirk Sackmann (ed.): Logistics Management, Springer International Publishing: Cham, 188-202.
Abstract: This study compares emissions and transit times from an environmentally oriented and a time oriented routing of large freight shipments in the European rail/road transportation network. We use the terminal-and-service selection problem (TSSP) to find the optimal routings under the different objectives. We show that substantial differences exist between the emission oriented routing and the time oriented routing. A large-scale simulation study reveals that shipments in the emission minimizing routing emit on average almost half as much emissions as if they were routed with the objective to minimize transit time. At the same time, the average transit time of shipments in the emission oriented routing almost triples compared to the transit time in the time optimal routing. This shows by experiment that substantial emission reductions can be achieved in the European freight transport sector by a corresponding routing of shipments but that this comes at the cost of a much lower service quality.