The role of flexibility in the light of the COVID-19 pandemic and beyond: Contributing to a sustainable and resilient energy future in Europe

The energy sector provides fuel for much of everyday life, particularly economically and socially. Fighting against the COVID-19 pandemic, a well-functioning and resilient energy sector is vital for maintaining the operation of critical infrastructures, including, most importantly, the health sector, and timely economic recovery. Notwithstanding its importance in everyday life and crises, the energy sector itself is currently in a complex and far-reaching transformation to combat climate change whilst supporting the transition to a low-carbon economy and society, mainly through the development of variable renewable energy sources (RES) such as wind and solar photovoltaics. This paper highlights the need for energy resilience as countries face the triple challenge of the COVID-19 health crisis, the consequent economic crisis, and the climate crisis. Focusing on Europe, it is advanced here that with the ability to balance fluctuating electricity generation and demand, flexibility allows the energy sector to utilise low-carbon RES reliably, ensuring a more resilient and sustainable energy future. This paper derives five urgent policy recommendations for Europe that address possible impacts of COVID-19 on the economic and societal prerequisites for flexibility in energy systems.


Introduction
In everyday life, energy forms the basis of economic welfare and the satisfaction of societal needs, including education, nutrition, and leisure activities. The COVID-19 pandemic and the consequent economic recession stress the crucial role of a well-functioning energy system in times of crises [1]. While energy access has proved pivotal for healthcare and households during the current crisis [2], it will also be a precondition for a smooth post-COVID-19 economic recovery.
With the global economy shutting down almost completely within only a couple of weeks of the declaration of the pandemic, electricity systems around the world faced severe shocks both on the supply and demand side. For example, countries experienced an overall decrease in electricity consumption, altered consumption profiles, and a temporally strong decrease of fuel prices that affected the dispatch of power plants [3][4][5][6][7][8]. Despite these shocks, initial evidence indicates that in many countries, electricity systems operated without far-reaching blackouts [9]. Hence, if one were to see the COVID-19 pandemic as a stress test on the resilience of global electricity systems, one could draw the preliminary conclusion that electricity systems worked quite well. While COVID-19 underlines the importance of reliable electricity supply in crises situations, this paper argues that resilience cannot be taken for granted in future electricity systems.
With the ongoing complex energy transition from high-carbon towards low-carbon systems and an increasing share of renewable energy sources (RES), a stable operation of the electricity system is at risk [10][11][12]. While RES such as wind and solar photovoltaics (PV) allow for a more sustainable generation of electricity and are therefore a means to combat climate change [13], they are, however, also highly variable [14]. The limited controllability of RES threatens the stable operation of future electricity systems, potentially leading to an increase in the costs of system operation when a high level of energy security is desired [11]. For instance, costs for grid interventions and in-feed management reached 1.2 billion Euros in 2019 in Germany [15]. As such costs are often directly passed on to end consumers [16], they threaten the general affordability of energy. Above all, such costs may contribute to energy poverty, which is a severe worldwide challenge to be solved [17].
This paper considers the current COVID-19 pandemic as an instigator for change. Given the coincidence of three different crises at the same timethe COVID-19 health crisis, the consequent economic crisis, and the climate crisisthis paper aims to advance the role of flexibility in the electricity system. In particular, it highlights how the COVID-19 pandemic can contribute to successfully integrating RES into the energy system and delivering increased resilience, preparing the electricity system for future shocks that may, for instance, also stem from natural disasters [18,19].
Flexibility is defined as the ability of the electricity system to react to changes in supply and demand to ensure a corresponding spatiotemporal balance in the underlying electricity grid [20]. Against this background, flexibility is a precondition for a stable and resilient electricity system. To successfully integrate RES, and to maintain or even increase the resilience of future electricity (and, ultimately, energy) systems, post-COVID-19 energy policy formulation needs to ensure that the flexibility of future electricity systems is further strengthened through adequate policy measures and corresponding flexibility investments. The development of sufficient flexibility has a two-fold benefit of meeting societies' energy and climate targets and ensuring that the electricity system plays its role in securing sustainable, long-term economic growth while contributing to the United Nations' Sustainable Development Goals (SDGs). In this context, a stable and resilient electricity system that activates and integrates the relevant flexibility suppliers (including, for example, industrial companies, households, owners of electric vehicles, or storage facility operators) may lead to a more inclusive and just energy system benefitting all relevant stakeholders, that is, citizens, society, industrial companies, the environment, etc. While this paper focuses on European countries, its implications may be relevant for all countries facing the challenges described above.
The remainder of this paper is structured as follows. Section 2 briefly reflects on the need for resilience in future electricity systems. Section 3 reviews the concept of flexibility as a basis for a resilient electricity system. Section 4 draws on possible challenges concerning the flexibility transition and derives specific recommendations for policymakers during and after COVID-19. Finally, Section 5 presents the conclusions.

Need for resilience in the electricity system
Given external short-run shocks in various supply networks and shortages of certain goods during the COVID-19 pandemic [21][22][23], the term resilience has gained increasing attention as a design principle for future critical infrastructure, including food systems [21,24], health systems [25,26], and electricity systems [27][28][29]. Resilience as the "ability to absorb shocks and still retain function" [12] has been discussed in the electricity system literature for several years already. In the United States, the National Research Council [30] defines resilience as "the ability to prepare, plan for, store, recover from, and more successfully adapt to adverse events". Notwithstanding its long tradition, the importance of electricity system resilience should not be underestimated, as it helps to counter short-run shocks that would otherwise destabilise the entire electricity system [18]. In turn, a collapsed electricity system may directly have detrimental effects on economic and social life. Regardless of whether it is the agricultural, the educational, or the industrial sector, none can function without electricity [31]. In this context, even short electricity supply interruptions may have adverse effects on safety, product quality, or production waste [31].
There are many examples of disastrous power outages in the past. In 2012, two major blackouts in India left around 600 million people without electricity [32]. More recently, in 2019, Argentina, Paraguay, and Uruguay were affected by a massive blackout, demonstrating the detrimental effects of cascading failures in coupled electricity systems [33]. Globally, lack of resilience is typically associated with increased costs for operating an electricity system [34]. Such costs comprise not only necessary grid interventions and supply re-storage, but also insurance needed by companies, necessary repair work at production plants and home appliances, or increased production waste. Additionally, lack of resilience can incur social costs and lead to social unrest due to blackouts [35]. In many countries, the above costs are directly passed on to society in its role as consumers [36], which then may also challenge energy affordability and contribute to energy poverty [37]. This issue is also stressed by current governmental measures to protect energy consumers in the COVID-19 crisis from the consequences of energy poverty [38].
With respect to energy law and policy which build the governance framework of an electricity system, literature calls for seven core principles for law and policy formulation, where resilience is among these guiding principles [39]. Against this background, modern law, regulations, and rules must set the right incentives to strengthen resilience in electricity systems [40]. One key component lies in new system flexibility, as will be further elaborated in the next section.

What is electricity system flexibility?
Endeavours to decarbonise the electricity system have been initiated globally to counteract and possibly reverse the detrimental effects of climate change. Generally, electricity system transformations build on an increasing share of RES, such as solar PV and wind, which are highly variable [14]. Such variability is due to the fact that the sun does not always shine, and the wind does not always blow. As the laws of physics (e.g., Kirchhoff's Laws) require electricity supply and demand to be balanced, such variability particularly challenges a reliable electricity grid operation [41]. With the storage capacity of traditional technologies being limited in several European countries due to social, economic, technical, or environmental constraints (e.g., pumped hydropower [42]) and with new storage technologies not yet being available at reasonable costs in large scales (e.g., batteries and power-to-gas), other options for electricity system flexibility must additionally compensate for the growing variability on the supply side stemming from the increasing feed-in from RES [20,31,43,44,89]. In this way, flexibility may directly contribute to the resilience of future low-carbon electricity systems.
Flexibility may come in different forms, serve specific purposes in an electricity system, and be characterised by different time frames [45]. In particular, there are five main technical flexibility options to be developed in future electricity systems [20,31] including (1) supply-side flexibility (provided, e.g., by highly efficient gas power plants), (2) transmission flexibility (i.e., by an extension of the electricity grid), (3) demand-side flexibility (e.g., demand-side management of industry or households), (4) inter-sectoral flexibility (i.e., connecting the energy sector with other sectors such as mobility or heat), and (5) storage flexibility (provided, e.g., by electric batteries) [20,[46][47][48][49][50][51][52]. Depending on the technical flexibility option chosen, flexibility may allow for balancing local and temporal mismatches between supply and demand [43,53].
In many countries, the RES-driven demand for new flexibility is additionally fuelled by the phaseout of conventional power plants such as coal power plants [43]. In Germany, coal-fired power plants currently Sustainable Development Goals provide a large share of the necessary energy system flexibility from the supply side to solve grid bottlenecksas can be seen from recent data on redispatch in Germany, see, for example, [54]. Generally, these power plants are expected to run with higher ramping rates in the future [43,45,54,55]. In addition, the ongoing electrification of sectors traditionally based on fossil fuels, for example, the mobility sector, results in growing electricity demand, which, if met by RES, will require even more system flexibility to counteract volatility in electricity generation [56]. Against this background, research has identified an increasing demand for flexibility in electricity systems where the energy transition is based on variable RES, such as wind and solar PV, that replace conventional power plants for electricity supply [14,[55][56][57]. For example, [58] provide an overview of the flexibility needs of the European energy system in 2030. Applying a similar time frame, [59] discuss the flexibility requirements in Europe for a system in which RES make up a 50% share. Additionally, the European transmission system operators make projections of a strongly increasing flexibility demand for 2025, 2030, and 2040 [60]. For an even longer time frame, [61,62] project the increasing flexibility needs in Europe in 2050.
Research refers to these flexibility needs as the "flexibility gap" that is expected to appear in the future [45,63,64] and needs to be addressed using specific policy targets for flexibility expansion in analogy to the targets for RES expansion set in countries such as Germany [45]. Otherwise, lack of flexibility may directly lead to a decrease in supply security and pose a threat to the resilience of future electricity systems [31]. Such a decrease would also result in high societal costs, as highlighted in the previous section. In Europe, the COVID-19 pandemic has highlighted the importance of electricity supply securitywhich, to a large degree, builds on electricity system flexibility in the future [65][66][67]. However, the pathway towards a successful expansion of flexibility is still non-existing or highly vague in many countries [31] and may even be further obscured by the COVID-19 pandemic. As such, this paper emphasises the mid-to-long-term perspective on flexibility. In contrast, the short-term mechanisms to deploy flexibility such as balancing power markets, for example, seem to work correctly, which is also reflected by the so far maintained stability of the electricity system during the COVID-19 pandemic [68]. However, such short-term stability cannot be generalised for the upcoming decades and, therefore, flexibility needs to be a crucial component of the future energy system and must be considered from a holistic context [43].
It is only very recently that research has explored the wider societal implications of energy system flexibility [69][70][71][72]. Overall, there is a growing consensus over the negatives resulting from electricity generation, transmission, and consumption, all of which may affect the environment, the wider society, or the economy (which directly relates to the three dimensions of the energy trilemma) [73]. Recently, the 2015 Paris climate agreement and the United Nations' SDGs have stressed the importance of the energy transition happening in a way that is "just". Taking up this call for a just transition from a high-carbon towards a low-carbon economy and society [74], researchers started to analyse how the development of and investment in electricity system flexibility may affect justice in the electricity system and the wider society [31,69,75]. There are multiple goals to which flexibility may actually contribute tofrom SDG7 (affordable and clean energy) to the other 16 SDGs [76].
In this context, the term "flexibility justice" has been introduced by [69], which is further advanced in [31] for the flexibility option of industrial demand-side flexibility. Being an applied form of justice [69], flexibility justice is a key for the overall energy system transformation of countries and calls for an active involvement of all the relevant stakeholders concerning the investment in new flexibility, the actual trade and supply of flexibility, as well as the fair distribution of overall flexibility costs and benefits (e.g., reduced electricity bills) in a future electricity system. Stakeholders shall be involved in the formulation of appropriate energy laws and policies already from the beginning, which builds the basis for an inclusive flexibility transition towards a low-carbon, resilient, and just electricity system (see also the previous section). In this context, COVID-19 has significantly affected the economic and societal prerequisites for flexibility [18,77] (including a possible change in public attitudes and, therefore, societal acceptance) and these effects must be considered when designing the actual pathway.

Potential of flexibility during and after COVID-19
This section derives five policy recommendations that address the possible impacts of COVID-19 on the economic and societal prerequisites for flexibility. While these policy recommendations mainly focus on Europe, they may also be highly relevant for all countries coping with the issues mentioned above.
i. Ensuring flexibility is at the heart of the energy transition Adverse effects and shocks will always produce winners and losers, tending to polarise society [17]. COVID-19 might inhibit similar effects. Policy faces the challenging tasks of balancing corresponding gains and costs that may intertwine with the effects of the energy transition itself. Countries are generally on a critical path to a successful energy transition without really knowing how the path will look. With COVID-19, it seems that additional fog has emerged that reduces the policy vision in many countries [78]. However, policymakers and the broader public must know flexibility is indispensable. Therefore, flexibility must play an important role in a future low-carbon electricity system and a targeted pathway towards such a futurecomplementary to other objectives. As the European Union has discussed the first European Climate Law during COVID-19, policymakers need to immediately understand the role of flexibility and ascribe it the required role in law. Here, it is necessary to define long-term goals for the flexibility expansion in an electricity system, which complement the goals for the expansion of RES feed-in capacities [79]. These goals must then be translated into specific measures and corresponding investments [80] (for more details see also the next subsections).
ii. Ensuring immediate policy action to increase flexibility investments in the electricity system during COVID-19 Keeping the energy and flexibility transition going requires active policy endeavours to support flexibility investments, which are already needed during the COVID-19 pandemic. With huge rescue and economic recovery plans (e.g., there is a 130 billion Euros recovery package in Germany) being planned and implemented, governments must act now and not lose time in the flexibility transition. Here, it is important that the COVID-19 pandemic does not push the energy and flexibility transition off the political agenda. Instead, the momentum of governmental action, which has proven to be effective in the past wave of COVID-19, should also be used to integrate incentives for flexibility investments in economic recovery programs. The challenge is to align these incentives with national or supranational goals and principles, such as the avoidance of discrimination against specific flexibility technologies. If some mechanisms are, for instance, only open for conventional supply flexibility technologies, this may lead to a lock-in effect. Here, actual fuel prices must generally account for externalities, such as environmental costs associated with CO 2 emissions. Gas-fired power plants, for example, will undoubtedly play a decisive role in the future energy system due to the technological ability to ramp up quickly and the much lower greenhouse gas (GHG) emissions compared to coal power plants. Policymakers should, however, provide the incentives for, for instance, "green" gas being used in the energy system [81]. Overall, policymakers must consider early on possible trade-offs between the flexibility that different technologies can provide and the energy transition's overall goal of decarbonising the energy system. Against this background, what policymakers should ultimately avoid are path dependencies, in which some technologies are preferred over others, although the flexibility potential of the former might be outpaced by the potential of the latter in the future, making the electricity system inflexible.
iii. Creating long-run investor certainty for flexibility options Probably, the most direct economic impact of COVID-19 on flexibility was the temporary drop in fossil fuel prices due to a demand shock [82]. Low fuel prices generally increase the competitiveness of flexibility assets that are based on GHG-emitting fossil fuels. As a result, the economic prospects for non-fossil flexibility options such as storage declined temporarily and therefore created planning uncertainty, while the prospects for fossil fuels temporarily increased. Such uncertainty and the corresponding (dis-)incentives may impede the transition towards GHG neutral flexibility options that require high investments to realise the necessary new system flexibility. Indeed, during COVID-19, some electricity utilities made massive orders of oil and gas when corresponding fuel prices were low, even building new tanks to store them. Against this background, it is now vital to create certainty for flexibility investments and clear pathways for stakeholders to invest in flexibility, accounting for the fact that investments are typically associated with long-term rather than immediate rewards. The investment plans and programs mentioned must therefore be in line with reliable long-term climate goals of national governments based on a broad public consensus on these goals (see also the next paragraph on the role of stakeholders). These ambitions should also rely on a principle-based approach to legal formulation to ensure investor certainty and public consensus [39]. This will create a coherent, stable, and credible investment climate. Here, it is equally important that politicians acting as policymakers do not arbitrarily adjust or misuse corresponding programs to maximise their short-run chances of re-election. Otherwise, a non-credible investment environment will make private investors wait and postpone their flexibility investments.
iv. Increasing the role of stakeholders in the flexibility transition The COVID-19 pandemic demonstrates that global crises affect every country and human on earth. Increased awareness of such global interdependencies may offer the opportunity to strengthen the involvement of various stakeholders in the transition to a low-carbon economy and society and the fight against climate change. As a current example, despite the COVID-19 pandemic and its negative economic effects, Germany has drafted legislation for its phaseout of coal by 2038 based on recommendations of a coal exit commission that collected the fears and hopes of the main stakeholders, which is also classed as part of the just transition to a low-carbon economy. In this way, the public (and, e. g., not only industrial companies) could actively influence the chosen pathway for Germany's energy future. However, some parts of the public may not accept the necessary measures to counteract the global challenges of climate change, as the threat of climate change is rather abstract [83] and the success of a (costly) countermeasure can only be observed with a high time delay. Here, it is important to inform and educate the public about the detrimental effects of climate change and the corresponding need for a flexibility transition. Possibly, the public perception about the "value of prevention" might also have changed with COVID-19 and the necessary lockdown measures. Against this background, research may further elaborate on the question of whether public acceptance for other preventative measures (e.g., measures to counteract climate change) has actually changed during the pandemic and whether society as a whole is now more open to the energy and flexibility transition than before. As many European countries currently draw new energy legislation, possibly increased public awareness may then directly translate into increased involvement of stakeholders during COVID-19.
v. Ensuring flexibility justice and contribution to the United Nations' SDGs Electricity supply is pivotal for all countries and their future development, both in terms of economic growth and societal development. The actual pathways to a low-carbon electricity system need to be based on flexibility, the United Nations' SDGs, and energy justice (which incorporates the following key justice perspectives: procedural, distributive, recognition, cosmopolitanism, and restorative) [31,84,85]. In this context, policy measures to counteract energy poverty should continuously be implementedalso in times of no crisis. Here, energy flexibility may help to lower the overall costs of operating a low-carbon energy system [86][87][88], where end consumers may directly benefit from reduced electricity prices. Countries must also ensure that currently drafted short-term recovery programmes for the economy do not risk achieved success concerning justice and SDGs. This includes SDG 7, that is, access to affordable, reliable, and sustainable energy for all, but also other important goals that, for instance, focus on access to education, healthcare services, or food. In this context, current global cooperation (including funding initiatives) to help developing countries realising the benefits that new system flexibility will bring is undoubtedly a key to the worldwide success of the SDGs.

Conclusion
The COVID-19 pandemic underlines the importance of wellfunctioning and resilient electricity systems. System flexibility will be crucial to ensure and strengthen the resilience of future low-carbon electricity systems that rely on an increasingly variable electricity supply from RES. Flexibility is defined as the ability to address imbalances between electricity demand and supply and can be delivered by various technical flexibility options, including electrical storage or sectoral coupling. Unfortunately, in the societal and economic spheres, the COVID-19 pandemic has impacted the pathway towards the necessary expansion of flexibility.
This paper highlights five policy recommendations for system flexibility during and after COVID-19 to keep the direction and accelerate the speed of the flexibility transition. As this paper argues, policymakers must act now and ensure that flexibility is an important component of an ongoing energy transition, which must not be paused but rather strengthened. Investment certainty is important to ensure sufficient flexibility and avoid lock-in into an electricity system that is based on fossil fuels. Finally, this paper calls for an active role and involvement of different stakeholders in the flexibility transition, ensuring (flexibility) justice and a successful contribution to the United Nations' SDGs. Future research must support policymakers in implementing corresponding measures quickly and this requires close and effective collaboration between different research disciplines. In this way, the COVID-19 pandemic can be a catalyst for the needed change in European energy systems in the battle against climate change.

Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.