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About the role of energy storage and synthetic fuels in defossilized energy systems

By Dr. Diana Böttger et al.

In this article we analyze three different scenarios for a complete defossilization of the German energy supply system. The focus is hereby on flexible technologies and synthetic fuels in the year 2050. These technologies are necessary to balance supply and demand in a system based on weather-dependent renewable energy sources. The power sector is able to cover a considerable share of the energy demand in the heat and transport sector. This is made possible by flexible sector coupling technologies such as heat pumps and electric mobility. All considered models manage to find solutions for a deep defossilization if flexibility and storage options are available.

In the 2015 Paris Agreement, the international community of states agreed to balance anthropogenic greenhouse gas emissions and sinks in the second half of the 21st century to counter the consequences of climate change [1]. Based on that, the European Commission reconfirmed the objective to become greenhouse gas neutral by 2050 in the proposal of the first European Climate Law as part of the European Green Deal [2].

Key strategies to reduce greenhouse gas emissions that are already agreed upon include energy efficiency measures and substituting fossil fuels with renewable energy sources [3,4]. Solar and wind energy are expected to become the most important generation technologies in the electricity sector due to their potential and relatively low costs [5]. The weather dependency of these energy sources increases the need for flexibility options to balance electricity supply and demand at all times [6,7]. There is a common understanding that through electrification of appliances, a decarbonized electricity system can be the basis for emission reductions in many sectors. However, some energy services such as aviation, long-distance transport or steel production remain difficult to decarbonize [8].

Electricity-based fuels (synthetic or e-fuels) have the potential to address both issues. On the one hand synthetic fuels can serve as long-term energy storages and are suitable to provide upward and downward flexibility. On the other hand synthetic fuels can be greenhouse gas neutral substitutes for fossil fuels in energy services that are difficult to decarbonize.

Nevertheless, synthetic fuels compete with a wide range of flexibility options, not only from the electricity sector itself (battery storage, pumped hydro storage, electricity grids) but also from the heat sector (heat pumps, electric boilers, heat storage, combined heat and power plants) as well as from the transport sector (electric vehicles, eHighway trucks).

We are going to focus the analysis on the use of these technologies in three recent studies for the German energy system. Therefore, we compare three scenarios for the year 2050 with a carbon neutral energy system in Germany. These scenarios are developed by three different Fraunhofer Institutes with three different energy system models. These models are all large-scale energy system optimization models for investment and dispatch decisions. Nevertheless, they differ concerning the regional or temporal scope and in the selection of considered technologies. Furthermore, each model uses own assumptions for future investment costs of renewable energy sources as well as synthetic fuels. By comparing these three scenarios we get general findings for such systems that turn out independent from methodology and scenario assumptions.

The electricity system has to be constantly in balance. This aspect is captured by all models used in this research at least on an hourly level. In the electricity system of previous decades flexibility was provided mostly by thermal power plants and (pumped) hydro power. In an electricity system dominated by fluctuating generation from wind and solar energy, additional flexibility options have to be utilized.

All considered models manage to find solutions for a deep defossilization if flexibility and storage options are available. Depending on the assumptions in the models, the scenarios have some general trends in common. On the other hand, some results concerning the use of flexibility options and synthetic fuels vary substantially.

In all scenarios, electricity and fuels derived from electricity become the central pillar of the energy system. Other energy sources (like biomass and geothermal or solar thermal energy) play only a secondary role. For cars and space heating, direct electrification through e-mobility and heat pumps becomes dominant in all scenarios. The power consumption rises by 50 to 75 % due to additional direct and indirect electrification.

Key flexibility options in all scenarios are:

  • Interregional and international balancing of renewable energy fluctuations: The interconnector capacity between Germany and its neighbors is expected to increase by the factor 1.3 to 3.3. In two scenarios Germany is going to become a net importer of power.
  • Flexible and constantly available power plants: The resulting capacity as well as the fuels used (hydrogen or synthetic methane) varies between the scenarios. These power plants are only used as back-up units with 1,000 full load hours without heat usage and about 2,000 full load hours as combined heat and power plants.
  • Flexible power demand: Demand side management from charging of electric vehicles as well as from power-to-heat applications enables the integration of weather-dependent renewables.

For buildings district heating also plays a significant role. The supply side of district heating grids is going to get more hybrid. This means that depending on the weather situation, either combined heat and power plants accompanied by fuel boilers, or heat pumps fulfil the demand. Heat storages further enable a flexible operation of these hybrid heat supply systems.

The role and fields of application of hydrogen varies between the scenarios, as does its origin. In one scenario it is only used in the industry sector, in the second scenario it is used in the transport sector and in the third scenario it plays a major role in the conversion sector. The generation of the hydrogen through electrolysis also varies substantially, not least in the utilization of the electrolyzers. In some models the electrolysis is used as a flexibility option, while in others a high utilization of the facilities predominates. Other synthetic fuels are mainly used in the transport sector. Only in one scenario a considerable share is also consumed in the power and heating sector. These fuels are mainly imported.

Additional sources of flexibility are batteries, which play a substantial role in two scenarios, while they are not cost optimal in the third scenario. Market-based curtailment of renewables can be kept low if enough flexibility options are available.

Acknowledgment

This paper was financed by Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. within its “Cluster of Excellence Integrated Energy Systems” (CINES[2]).

References
  • United Nations, “Paris Agreement”, 2015.
  • European Commission “Proposal for a REGULATION OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL establishing the framework for achieving climate neutrality and amending Regulation (EU) 2018/1999 (European Climate Law)” 2020.
  • European Parliament, Council of the European Union, “DIRECTIVE (EU) 2018/ 2001 OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL - of 11 December 2018 - on the promotion of the use of energy from renewable sources” 2018.
  • European Parliament, Council of the European Union “DIRECTIVE (EU) 2018/ 2002 OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL - of 11 December 2018 - amending Directive 2012/ 27/ EU on energy efficiency”, 2018.
  • Pfluger, “Assessment of least-cost pathways for decarbonising Europe's power supply: A model-based long-term scenario analysis accounting for the characteristics of renewable energies”, Karlsruhe, KIT Scientific Publishing, 2014.
  • Huber, D. Dimkova, T. Hamacher “Integration of wind and solar power in Europe: Assessment of flexibility requirements”, Energy 2014, 69:236–46. doi:10.1016/j.en-ergy.2014.02.109.
  • Kondziella, T. Bruckner, “Flexibility requirements of renewable energy based electricity systems – a review of research results and methodologies”, Renewable and Sustainable Energy Reviews 2016, 53:10–22. doi:10.1016/j.rser.2015.07.199.
  • J. Davis, N. S. Lewis, M. Shaner, S. Aggarwal, D. Arent, I. L. Azevedo et al., “Net-zero emissions energy systems”. Science (New York, N.Y.) 2018;360 (6396).

 

[1] This article is a summary of the conference paper Böttger et al. “Interaction Of Energy Storage Technologies And Synthetic Fuels In Long-Term Decarbonization Scenarios” presented at IRES 2020 Web Summit on May 25, 2020.

Diana Böttger
Fraunhofer Institute for Energy Economics and Energy System Technology IEE
Kassel, Germany
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Christoph Kost
Fraunhofer Institute for Solar Energy Systems ISE
Freiburg, Germany
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Daniel Wrede
Fraunhofer Institute for Solar Energy Systems ISE
Freiburg, Germany
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Benjamin Lux
Fraunhofer Institute for Systems and Innovation Research ISI
Karlsruhe, Germany
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Tobias Fleiter
Fraunhofer Institute for Systems and Innovation Research ISI
Karlsruhe, Germany
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Benjamin Pfluger
Fraunhofer Institute for Systems and Innovation Research ISI
Karlsruhe, Germany
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Judith Heilig
Fraunhofer Institute for Solar Energy Systems ISE
Freiburg, Germany
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Norman Gerhardt
Fraunhofer Institute for Energy Economics and Energy System Technology IEE
Kassel, Germany
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Michael Haendel
Fraunhofer Institute for Systems and Innovation Research ISI
Karlsruhe, Germany
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