Selected Publications

2022

• Unnewehr JF, Weidlich A, Gfüllner L, Schäfer M
  Open-data based carbon emission intensity signals for electricity generation in European countries - top down vs. bottom up approach
  2022 Cleaner Energy Systems, volume: 3, issue: 100018, pages: 1 - 13
• Unnewehr J, Schäfer M, Weidlich A
  The value of network resolution – A validation study of the European energy system model PyPSA-Eur
  2022 Proceedings of the 1st International Workshop on Open Source Modelling and Simulation of Energy Systems, pages: 1 - 7
  
   
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2021

• Weidlich A
  Infrastrukturen für ein klimaneutrales Energiesystem: Szenarienstudien weisen den Weg
  Stiftung Energie und Klimaschutz, 2021

2020

• Unnewehr J
  Tracking CO2 emissions from power generation in high spatial and temporal resolution – Case study for the German electricity system
  2020 Energy Informatics, volume: 3
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  Abstract

  Until today it is not possible to allocate the CO2 emissions in the German electricity system to a specific region and its corresponding electricity demand that caused them. This paper presents a new energy system model and uses established methods for answering this question. A detailed bottom-up model of the German electricity system is built to represent a highly spatial and temporal representation of the Germany electricity system in 2019. In combination with a customized input-output analysis, the individual emissions from the producer to the caused consumer can be traced. The analysis demonstrates the importance of considering spatial and temporal effects as well as electricity exchanges between regions in estimating emissions footprints.
• Sato S, Weidlich A
  Analysis of Avoided Transmission Through Decentralized Photovoltaic and Battery Storage Systems
  2020 IEEE T Sustain Energ, volume: 11, issue: 3, pages: 1922 - 1929
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  Abstract

  Decentralized renewable energy systems can be low-carbon power sources, and promoters of local economies. It is often argued that decentralized generation also helps reducing transmission costs, as generation is closer to the load, thus utilizing the transmission system less. The research presented here addresses the question whether or not, or under what circumstances this effect of avoided transmission can actually be seen for a community-operated cluster of photovoltaic (PV) power plants in two sample locations, one in Germany and one in Japan. For the analysis, the newly developed instrument of MPI-MPE diagrams is used, which plot the maximum power import (MPI) and maximum power export (MPE) in relation to the reference case of no local generation. Results reveal that for moderately sized PV systems without battery storage, avoided transmission can be seen in the Japanese model location, but not in Germany. It was also found that an additional battery storage can lead to avoided transmission in both locations, even for large sizes of installed PV capacity.
  
  
   
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• Schäfer M, Tranberg B, Greiner M
  Power flows in complex renewable energy networks
  In: Discoveries at the Frontiers of Science
  2020, Springer, pages: 239 - 251,
• Unnewehr J F, Waldl H-P, Pahlke T, Weidlich A
  Reducing Operational Costs of Offshore HVDC Energy Export Systems Through Optimized Maintenance
  2020 Energies, volume: 13, issue: 1146, pages: 1 - 20
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  Abstract

  For the grid connection of offshore wind farms today, in many cases a high-voltage direct current (HVDC) connection to the shore is implemented. The scheduled maintenance of the offshore and onshore HVDC stations makes up a significant part of the operational costs of the connected wind farms. The main factor for the maintenance cost is the lost income from the missing energy yield (indirect maintenance costs). In this study, we show an in-depth analysis of the used components, maintenance cycles, maintenance work for the on- and offshore station, and the risks assigned in prolonging the maintenance cycle of the modular multilevel converter (MMC). In addition, we investigate the potential to shift the start date of the maintenance work, based on a forecast of the energy generation. Our findings indicate that an optimized maintenance design with respect to the maintenance behavior of an HVDC energy export system can decrease the maintenance-related energy losses (indirect maintenance costs) for an offshore wind farm to almost one half. It was also shown that direct maintenance costs for the MMC (staff costs) have small effect on the total maintenance costs. This can be explained by the fact that the additional costs for maintenance staff are two orders of magnitude lower than the revenue losses during maintenance.
  
  
   
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• Schäfer M, Tranberg B, Jones D, Weidlich A
  Tracing carbon dioxide emissions in the European electricity markets
  2020 Proceedings of the 17th European Energy Market Conference, pages: 1 - 6
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  Abstract

  Consumption-based carbon emission measures aim to account for emissions associated with power transmission from distant regions, as opposed to measures which only consider local power generation. Outlining key differences between two different methodological variants of this approach, we report results on consumption-based emission intensities of power generation for European countries from 2016 to 2019. We find that in particular for well connected smaller countries, the consideration of imports has a significant impact on the attributed emissions. For these countries, implicit methodological choices in the input-output model are reflected in both hourly and average yearly emission measures.
  
  
   
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2019

• Abdel-Khalek H, Schäfer M, Vásquez Torres RA, Unnewehr J, Weidlich A
  Forecasting Cross-Border Power Transmission Capacities in Central Western Europe Using Artificial Neural Networks
  2019 Energy Informatics, volume: 2, supplement: 1, pages: 1 - 13
  
   
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• Schäfer M, Hofmann F, Abdel-Khalek H, Weidlich A
  Principal Cross-Border Flow Patterns in the European Electricity Markets
  2019 16th International Conference on the European Energy Market
  
   
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2018

• Unnewehr J, Waldl H-P, Pahlke P, Herráez I
  Reducing operational costs of offshore HVDC energy export systems through optimized maintenance
  2018 17th Wind Integration Workshop
• Hörsch J, Schäfer M, Becker S, Schramm S, Greiner M
  Flow tracing as a tool set for the analysis of networked large-scale renewable electricity systems
  2018 Int J Elec Power, volume: 96, pages: 390 - 397
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  Abstract

  The method of flow tracing follows the power flow from net-generating sources through the network to the net-consuming sinks, which allows to assign the usage of the underlying transmission infrastructure to the system participants. This article presents a reformulation that is applicable to arbitrary compositions of inflow appearing naturally in models of large-scale electricity systems with a high share of renewable power generation. We propose an application which allows to associate power flows on the grid to specific regions or generation technologies, and emphasizes the capability of this technique to disentangle the spatio-temporal patterns of physical imports and exports occurring in such systems. The analytical potential of this method is showcased for a scenario based on the IEEE 118 bus network.
• Tranberg B, Schwenk-Nebbe LJ, Schäfer M, Hörsch J, Greiner M
  Flow-based nodal cost allocation in a heterogeneous highly renewable European electricity network
  2018 Energy, volume: 150, pages: 122 - 133
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  Abstract

  For a cost efficient design of a future renewable European electricity system, the placement of renewable generation capacity will seek to exploit locations with good resource quality, that is for instance onshore wind in countries bordering the North Sea and solar PV in South European countries. Regions with less favorable renewable generation conditions benefit from this remote capacity by importing the respective electricity as power flows through the transmission grid. The resulting intricate pattern of imports and exports represents a challenge for the analysis of system costs on the level of individual countries. Using a tracing technique, we introduce flow-based nodal levelized costs of electricity (LCOE) which allow to incorporate capital and operational costs associated with the usage of generation capacity located outside the respective country under consideration. This concept and a complementary allocation of transmission infrastructure costs is applied to a simplified model of an interconnected highly renewable European electricity system. We observe that cooperation between the European countries in a heterogeneous system layout does not only reduce the system-wide LCOE, but also the flow-based nodal LCOEs for every country individually.
• Hofmann F, Schäfer M, Brown T, Hörsch J, Schramm S, Greiner M
  Principal Flow Patterns across renewable electricity networks
  Epl-europhys Lett, volume: 124, page: 18005, 2018

2017

• Schäfer M, Schwenk-Nebbe LJ, Hörsch J, Tranberg B, Greiner M
  Allocation of nodal costs in heterogeneous highly renewable European electricity networks
  2017 14th International Conference on the European Energy Market (EEM), 2017, pages: 1 - 6
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  Abstract

  We consider a simplified model of a future European electricity network with a high share of renewable generation. In a cost optimal design of such a system, most of the renewable generation capacity is placed at locations with favorable weather conditions, that is for instance onshore wind in countries bordering the North Sea and solar PV in South European countries. Countries with less favorable renewable generation conditions benefit from this capacity by importing the respective electricity as power flows through the transmission grid. Using flow tracing techniques, we disentangle the emerging pattern of imports and exports and assign shares of the distributed generation capacity in the European system to the countries which actually make use of them. This procedure yields nodal levelized costs, which incorporate both internal and external generation costs associated with the electricity consumption in a country.
• Schäfer M, Tranberg B, Hempel S, Schramm S, Greiner M
  Decompositions of injection patterns for nodal flow allocation in renewable electricity networks
  2017 Eur Phys J B, volume: 90, page: 144
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  Abstract

  The large-scale integration of fluctuating renewable power generation represents a challenge to the technical and economical design of a sustainable future electricity system. In this context, the increasing significance of long-range power transmission calls for innovative methods to understand the emerging complex flow patterns and to integrate price signals about the respective infrastructure needs into the energy market design. We introduce a decomposition method of injection patterns. Contrary to standard flow tracing approaches, it provides nodal allocations of link flows and costs in electricity networks by decomposing the network injection pattern into market-inspired elementary import/export building blocks. We apply the new approach to a simplified data-driven model of a European electricity grid with a high share of renewable wind and solar power generation.
• Schäfer M, Hempel S, Tranberg B, Hörsch J, Schramm S, Greiner M
  Power Flow Tracing in Complex Networks
  In: New Horizons in Fundamental Physics
  2017, Springer International Publishing, pages: 357 - 373,
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  Abstract

  The increasing share of decentralized renewable power generation represents a challenge to the current and future energy system. Providing a geographical smoothing effect, long-range power transmission plays a key role for the system integration of these fluctuating resources. However, the build-up and operation of the necessary network infrastructure incur costs which have to be allocated to the users of the system. Flow tracing techniques, which attribute the power flow on a transmission line to the geographical location of its generation and consumption, represent a valuable tool set to design fair usage and thus cost allocation schemes for transmission investments. In this article, we introduce a general formulation of the flow tracing method and apply it to a simplified model of a highly renewable European electricity system. We review a statistical usage measure which allows to integrate network usage information for longer time series, and illustrate this measure using an analytical test case.
• Schäfer M, Bugge Siggaard S, Zhu K, Risager Poulsen C, Greiner M
  Scaling of transmission capacities in coarse-grained renewable electricity networks
  2017 Epl-europhys Lett, volume: 119, page: 38004
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  Abstract

  Network models of large-scale electricity systems feature only a limited spatial resolution, either due to lack of data or in order to reduce the complexity of the problem with respect to numerical calculations. In such cases, both the network topology, the load and the generation patterns below a given spatial scale are aggregated into representative nodes. This coarse-graining affects power flows and thus the resulting transmission needs of the system. We derive analytical scaling laws for measures of network transmission capacity and cost in coarse-grained renewable electricity networks. For the cost measure only a very weak scaling with the spatial resolution of the system is found. The analytical results are shown to describe the scaling of the transmission infrastructure measures for a simplified, but data-driven and spatially detailed model of the European electricity system with a high share of fluctuating renewable generation.

2015

• Tranberg B, Thomsen AB, Rodriguez RA, Andresen GB, Schäfer M, Greiner M
  Power flow tracing in a simplified highly renewable European electricity network
  2015 New J Phys, volume: 17, page: 105002
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  Abstract

  The increasing transmission capacity needs in a future energy system raise the question of how associated costs should be allocated to the users of a strengthened power grid. In contrast to straightforward oversimplified methods, a flow tracing based approach provides a fair and consistent nodal usage and thus cost assignment of transmission investments. This technique follows the power flow through the network and assigns the link capacity usage to the respective sources or sinks using a diffusion-like process, thus taking into account the underlying network structure and injection pattern. As a showcase, we apply power flow tracing to a simplified model of the European electricity grid with a high share of renewable wind and solar power generation, based on long-term weather and load data with an hourly temporal resolution.

2012

• Springer-Verlag
  Future Energy Grid: Migrationspfade ins Internet der Energie
  Dänekas C, König A, Mayer C, Rohjans S, Bischoff S, Breuer A, Drzisga T, Hecht J, Holtermann M, Luhmann T, Maerten M, Stadler M, Terzidis O, Plöger W, Theisen T, Wortmann F, Weidlich A, Weinmann J, Winter R, Wissing C

2009

• Veit D, Weidlich A, Krafft J
  An agent-based analysis of the German electricity market with transmission capacity constraints
  2009 Energy Policy, volume: 37, issue: 10, pages: 4132 - 4144

2008

• Heide D, Schäfer M, Greiner M
  Robustness of networks against fluctuation-induced cascading failures.
  2008 Phys Rev E, volume: 77, issue: 5 Pt 2, pages: 056103 - 056103
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  Abstract

  Fluctuating fluxes on a complex network lead to load fluctuations at the vertices, which may cause them to become overloaded and to induce a cascading failure. A characterization of the one-point load fluctuations is presented, revealing their dependence on the nature of the flux fluctuations and on the underlying network structure. Based on these findings, an alternate robustness layout of the network is proposed. Taking load correlations between the vertices into account, an analytical prediction of the probability for the network to remain fully efficient is confirmed by simulations. Compared to previously proposed mean-flux layouts, the alternate layout comes with significantly less investment costs in the high-confidence limit.

2006

• Schäfer M, Scholz J, Greiner M
  Proactive robustness control of heterogeneously loaded networks.
  2006 Phys Rev Lett, volume: 96, issue: 10, page: 108701
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  Abstract

  A proactive measure to increase the robustness of heterogeneously loaded networks against cascades of overload failures is proposed. It is based on load-dependent weights. Compared to simple hop weights, respective shortest flow paths turn a previously heterogeneous load distribution into a more homogeneous one for the nodes and links of the network. The use of these flow paths increases the networks robustness and at the same time reduces the investment costs into the networks capacity layout. These findings are of relevance for critical infrastructures like communication and transportation networks.