New research by LUT University, Finland, presents the role of flexibility options in highly renewable energy systems research. A database of 1067 scientific articles was used for the analyses. The publication is entitled “Role and trends of flexibility options in 100% renewable energy system analyses towards the Power-to-X Economy” and published in Renewable and Sustainable Energy Reviews.
22 flexibility options have been identified in the 100% renewable energy systems literature across five groups: Power-to-X (PtX), energy storage, demand response, transmission and distribution grids, and curtailment. The groups with the most diverse flexibility options are PtX and energy storage.
PtX processes serve as the technological solution for sector coupling as the central element of the Power-to-X Economy. The PtX concept includes processes that use electricity across various sectors, such as heating, electromobility, industry, and the production of electricity-based fuels (e-fuels), but also e-chemicals, and e-materials. PtX processes can use power from any electricity source, but using non-renewable energy goes against the main goal of cutting costs and moving away from fossil fuels.
To manage the variability in renewable electricity generation and maintain a balance between energy supply and demand, additional measures, such as energy storage in various forms, transmission and distribution grids, and curtailment, are utilized alongside PtX. 1067 scientific articles assessing 100% renewable energy system studies for specific regions were analyzed to identify the emergence, growth, and trends of each flexibility option. The analyzed database for 100% renewable energy system studies has been developed over the years at LUT and has been published earlier with 550 articles for bibliometric analyses.
Out of the 1067 identified articles, 1008 (94.5%) included at least one of the five fundamental flexibility options. The distribution of flexibility option diversity revealed that 75% of the articles included at least two options, 47% included three, 20% included four, and only 4.3% incorporated all five categories of flexibility options. Between 1975 and the early 2010s, PtX technologies attracted relatively little attention in 100% renewable energy systems research. However, since 2014, a significant shift in research trends has emerged, with an increasing focus on integrating various forms of PtX technologies. Storage types appeared in 100% renewable energy studies in a growing variety from 2009 onwards.
The transmission and distribution grid has received more attention, especially as its role in 100% renewable energy systems research has grown over the past two decades. Curtailment and demand response have also been studied since around the same time, with curtailment gaining traction in 2010 and demand response in 2009, both appearing at similar rates.
e-Hydrogen stands out as the most utilized e-fuel among the various PtX technologies, followed by e-methane. Direct air carbon capture and utilization (DACCU) appears eight times more than direct air carbon capture and sequestration (DACCS) in PtX routes. This suggests that studies prioritize using CO2 for end-products rather than for sequestration. 100% renewable energy systems research prioritizes the power sector, followed by the sectors heat, transport, industry, and the carbon dioxide removal sector.
When comparing energy storage technologies, batteries are the most extensively discussed, appearing in about 60% of all studies. Notably, batteries are not only the most mentioned energy storage solution but also stand out among all flexibility options. Stationary batteries are increasingly complemented by vehicle-to-grid storage, indicating intensified research on the coupling of power and transport sectors. Batteries are fast at charging and discharging, hence they can quickly balance solar power fluctuations. This very high battery penetration in studies allows further solar PV ramping. Unlike other storage options, they do not need extra infrastructure like pumped hydro energy storage does. Batteries also work at smaller scales, making them easy to add to homes and grids. Plus, they are getting cheaper at high rates and more efficient, boosting solar PV adoption practically everywhere.
Following batteries, e-hydrogen is the second most utilized energy storage technology, closely followed by pumped hydro energy storage. Thermal energy storage is used as a valuable storage for heat but also to couple the power and the heat sectors. e-Methane storage has complemented e-hydrogen storage since the early 2010s for a broader diversity of gas balancing options.
Quantitative assessment of individual articles across different flexibility options: PtX technologies (top), energy storage (bottom left), and demand response, power grids, and curtailment (bottom right). The values represent how many specific flexibility options were utilized across the entire database. Image: LUT.
Europe significantly outperforms other regions in terms of flexibility options research, since a greater number of studies choose either Europe as a whole or specific European countries as case studies for their analyses. On a global scale, the PtX route for e-hydrogen has proven to be the most widely used technology. In Eurasia, e-methane is the most frequently applied. In the Middle East, seawater reverse osmosis (SWRO) desalination is a commonly employed technology, used in the same frequency as DACCU. The results show that batteries have the highest share of energy storage across all regions.
Sector-wise, the power sector is the only one to incorporate all forms of flexibility options, due to its crucial role in managing the high variability and balancing of renewable energy sources. Following the power sector, the heat and industry sectors gain the most from various flexibility options. The heat sector benefits from nearly all e-fuels, with heat pumps and electric heating playing especially crucial roles.
Utilization of flexibility options across various energy sectors in the database. The values indicate the proportion of each flexibility option distributed across the energy sectors. Image: LUT.
Flexibility is a central concept to reaching 100% renewable energy systems in terms of high efficiency, lower costs, and low curtailment. Electricity emerges as the central energy carrier that couples the different energy sectors.
Authors: Siavash Khalili, Dominik Keiner, and Christian Breyer
This article is part of a monthly column by LUT University.
Research at LUT University encompasses various analyses related to power, heat, transport, desalination, and negative CO2 emission options. Power-to-X research is a core topic at the university, integrated into the focus areas of Energy, Air, Water, and Business and Society. Solar energy plays a key role in all research aspects.
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