What motivated us to conduct the study?

The COP28 Agreement 2023 declares that energy production needs to transition away from fossil-based resources in an unprecedented swift, just and equitable manner. Despite the expectation that nuclear power will play an important role in the transition of electricity production in the near future, this technology is also associated with reactor safety issues, risks from radiation accidents and the production of high-level radioactive spent nuclear fuel that needs to be managed for thousands of years in the future. Therefore, we found it highly relevant to assess the potential for a fully long-term renewable electricity system. While it is well known that globally, there is enough renewable energy available to fulfil any foreseeable future human need (IEA, 2016), the significant spatial and temporal climate variations across planet Earth generate great electricity transmission problems and a significant need for energy storage to overcome energy drought periods. Will the available hydropower reservoirs be enough for storing energy, or is there a need to provide great additional energy storage capacity? Hydropower represents a relatively small share of the current electricity production (12% in Europe), and its estimated global potential is only 9% of the total energy demand (IEA, 2021). To assess whether a renewable electricity system is possible without nuclear power and fossil fuel, we decided to disregard these resources and leave out bioenergy. There is a large global potential for bioenergy; however, it is associated with complex interactions in the water-energy-food nexus, and as an example, it competes for land and water resources for food production and the preservation of wildlife environments.

Figure 1 Energy storage shares for the scenario with 2:4:1 shares of solar-wind-hydro power. In this scenario, the power sources provide a electricity production capacity of 4,494 TWh/y over continental Europe. The storage demand is 131 TWh and the storage capacity available in existing hydropower reservoirs is 183 TWh. Spatiotemporal sharing and complementarity stand for nearly 80% of the total storage demand.

Theoretical approach

Climate fluctuations occur across a wide spectrum of temporal and spatial scales. To match the demand for energy with the highly fluctuating renewable energy availability, it is important to account for the magnitude of the energy fluctuations as well as their durations. In our research, we developed a spectral theory that directly relates all climate and demand fluctuations to an energy storage need given a certain period during which management is considered.

What we found

We found that the existing hydropower reservoirs in Europe provide sufficient energy storage to overcome energy drought periods, but only if the renewable electricity production is complemented with appropriate shares of wind and solar power and if the production-demand system is managed at the continental scale. Spatiotemporal management of solar, wind and hydropower can induce a virtual energy storage gain (VESG) that is several times greater than that available in existing hydropower reservoirs (Figure 1). In this analysis, we assumed that the long-term average production capacity exactly matched the long-term demand, implying that no potential renewable energy production is curtailed. In practice, in renewable energy production, a common approach is to install a higher power capacity than needed from a long-term average point of view; hence, energy curtailment is a common phenomenon. The finding that solar wind and hydropower can, in principle, provide enough energy for all at all times indicates a possible scenario for the energy transition towards a fully renewable electricity system.

Is it realistic to envision a fully renewable energy system?

Our study indicates that a fully renewable electricity system is possible in Europe if suitable shares of solar, wind and hydropower (2:4:1) are managed across the whole continent and given the current electricity production. This would require several new continental ultrahigh-voltage transmission lines and stable international collaboration and trade. Transition to a fully renewable energy system would be even more challenging. The current global energy production is estimated to be 168,000 TWh/y, where electricity represents only 26,936 TWh/y and hydropower represents only 4,329 TWh/y (IEA, 2021). The limited share of hydropower limits the energy storage capacity required to balance climate fluctuations. The global potential for hydropower has been estimated to be approximately 14,500 TWh/y (Kumar et al., 2011), possibly much more depending on what sites are acceptable from environmental viewpoints (Hoes et al., 2017). If the suitable shares of solar, wind and hydropower (2:4:1) are generalized, the potential global hydropower capacity could provide energy storage support for renewable electricity production of 101,500 TWh/y. This does not consider that only part (71.6%) of the energy storage capacity available for hydropower was utilized in the scenario presented in our study, suggesting that it might be possible that a renewable energy system relying entirely on hydropower storage could have a total power capacity of up to 140,000 TWh/y. Furthermore, it is plausible that other storage alternatives will develop with time, such as hydrogen storage, other chemical storage methods, biofuel and geothermal energy. Another option for reducing the need for energy storage is to install a surplus of installed power capacity. Hence, we believe that a fully renewable energy system should be possible in the future from climatic and technological viewpoints. However, a major problem is the currently large share of fossil fuel-based energy production, which, for economic and practical reasons, may require a long transition period based on a parallel build-up of both renewable production and nonrenewable, nonfossil-based production, such as nuclear power.

References

Hoes OAC, Meijer LJJ, van der Ent RJ, van de Giesen NC. Systematic high-resolution assessment of global hydropower potential. PLOS One; 2017. doi: 10.1371/journal.pone, 0171844.

IEA, 2016. World Energy Outlook 2016.

IEA, 2021. Key World Energy Statistics 2021. Statistics Report of IEA.

Kumar A, Schei T, Ahenkorah A, Caceres Rodriguez R, Devernay J-M, Freitas M et al. Hydropower. In: Edenhofer O, Pichs-Madruga R, Sokona Y, Seyboth K, Matschoss P, Kadner S et al., editors, IPCC special report on renewable energy sources and climate change mitigation; 2011.