Natural disasters, such as hurricanes, blizzards, thunderstorms, wildfires, and earthquakes can cause widespread and costly power outages that adversely impact society and the economy. Severe weather is the leading cause of widespread power outages, costing billions of dollars per year due to the dependence of modern society on the uninterrupted supply of electricity. The impact of a power outage increases as more industries move from manual to automated. Many critical infrastructures, such as communication, water, food, defense, transportation, and healthcare rely directly or indirectly on the power grid. A 2012 Congressional Research Service study estimates the inflation-adjusted cost of weather-related outages at $25 to $70 billion annually [1]. The cost of power outages includes lost output and wages, spoiled inventory, delayed production, inconvenience, and damages to the electric grid. Sustained power loss can also affect the provision of health and emergency services during and in the aftermath of the disaster, leading to preventable injury and death. Recent examples of power outages caused by natural disasters include the Hokkaido blackout of 2018 that was due to an earthquake, the South Australian blackout in 2016 caused by a mix of storms, the rolling blackouts in California in 2019 due to wildfires, and the outages in Texas in 2021 due to a winter storm. The number of severe weather events and subsequent power outages is expected to rise as climate change increases their frequency, intensity, and duration. In 2020, the direct economic losses and damage from natural disasters was estimated at $268 billion, stemming from 53-billion-dollar economic loss events around the world, the second highest on record [2].

Beyond weather-related events, distribution systems are increasingly at risk from cyberattacks. The introduction of monitoring and control technologies and the use of advanced communication networks has made the grid more interconnected and hence, more vulnerable to these threats. In 2015, a coordinated cyberattack in Ukraine led to a power outage affecting approximately 225,000 customers and causing a 6-h blackout in and around Kyiv ^[3][4][3,4]. This was the first documented case of a cyberattack bringing down the power grid, and the attack strategy is employable to infrastructures around the world [4].

Improving power grid resilience can help mitigate the damages caused by these events. Power grid resilience has been defined as “the ability to anticipate, resist, absorb, respond to, adapt to, and recover from a disturbance” [5]. According to a 2013 report from the Executive Office of the President, investment in grid resilience will reduce the consequences of a power outage, “saving the economy billions of dollars and reducing the hardship experienced by millions of Americans when extreme weather strikes” [1]. Grid resilience investments include system hardening strategies such as undergrounding wires and upgrading substation components and operational strategies such as deploying microgrids or utilizing distributed energy resources.

Mobile energy storage systems (MESSs) have recently been considered as an operational resilience enhancement strategy to provide localized emergency power during an outage. A MESS is classified as a truck-mounted or towable battery storage system, typically with utility-scale capacity. Referred to as transportable energy storage systems, MESSs are generally vehicle-mounted container battery systems equipped with standardized physical interfaces to allow for plug-and-play operation. Their transportation could be powered by a diesel engine or the energy from the batteries themselves. MESS containers typically hold batteries in addition to systems for thermal management, power conversion, and power control. They may also contain balance-of-system equipment such as transformers [6]. The design, operation, and maintenance of a MESS are governed by IEEE Standard 2030.2.1-2019, which stresses the importance of safety measures including anti-vibration, anti-collision, and waterproof capabilities [7].

Unlike conventional emergency response equipment such as diesel generators, MESSs can operate both during normal conditions and during emergency events. During normal operation, they can provide valuable grid services and capabilities including load leveling, peak shaving, spatiotemporal energy arbitrage, reactive power support, renewable energy integration, and transmission deferral. This ability to provide ancillary services on typical days enables a return-on-investment, which is not common for emergency response equipment. Mobile energy storage does not rely on the availability of fuel supplies, which offers an advantage over portable diesel generators, as fuel supplies may be interrupted or restricted by a disaster. MESSs also do not produce greenhouse gas emissions or create air pollution during operation and can be deployed to help meet clean energy targets. MESSs are typically owned and controlled by utility companies, which offers advantages over other mobile energy resources such as electric vehicle fleets and other resilience enhancement techniques such as demand response. MESSs are not subject to the stochastic behavior and demand of electric vehicle drivers and do not require advanced communication infrastructure, smart meters, or interaction with electricity consumers.

The primary advantage that mobile energy storage offers over stationary energy storage is flexibility. MESSs can be re-located to respond to changing grid conditions, serving different applications as the needs of the power system evolve. For example, during normal operation, a MESS could support an overloaded substation in the summer months, and then move to provide ancillary services in another location once demand drops. This avoids creating stranded assets and saves money compared to multiple stationary energy storage systems [8]. MESSs can also provide energy during emergency conditions and their mobility allows for fast deployment at the location where they are most necessary.

Commercial deployment of MESSs is limited, but expected to increase as the cost of utility-scale batteries continues to fall ^[6][9][6,9]. In 2016, Consolidated Edison of New York announced their plans to develop an 800 kWh MESS unit with Electrovaya, a lithium-ion battery company [10]. Power Edison has deployed mobile energy storage systems for over five years, offering utility-scale plug-and-play solutions [11]. In 2021, Nomad Transportable Power Systems released three commercially available MESS units with energy capacities ranging from 660 kWh to 2 MWh [12]. However, the adoption of MESSs as a resilience resource is hindered by high capital costs, deployment logistics challenges, concerns about interoperability with existing distribution systems, and insufficient connection infrastructure [6]. The capital cost of a standalone, stationary 1 MW/2 MWh battery typically falls between $377/kWh and $831/kWh, depending on the application [6]. The 1 MW/2 MWh Nomad unit has a capital cost of $1,599,000, or ~$800/kWh [13]. In addition to investment costs, battery storage also incurs ongoing operation and maintenance costs. Compared to an ESS, a MESS will likely introduce a cost premium of 5–10% associated with the labor and fuel for transportation [6]. Additionally, the lack of generation during an outage may mean that MESSs are a short-term solution to a long-term problem if they cannot re-charge.

2. Power Grid Resilience

Power grid resilience has recently attracted much attention from both academia and industry. Compared to reliability, which concerns typical, short-term outages, resilience is focused on large-scale disturbances caused by long-duration, high-impact, low-frequency events, such as natural disasters or man-made threats. While reliability definitions and metrics are mature and broadly accepted, resilience definitions vary. Several approaches have been developed to quantify resilience, however, no widely adopted metric is currently in use ^[14][16]. While resilience metrics attempt to holistically measure system resilience, resilience evaluation criteria can be used to show how certain measures can enhance total system resilience without having to provide a picture of the overall resilience ^[15][17]. Evaluation criteria include performance metrics about the scope and duration of an outage. These include hours of outage, lost load, percentage of customers experiencing an outage, number of critical services without power, and time to recovery ^[14][16]. Bhusal et al. ^[15][17] and Raoufi et al. ^[16][18] provide comprehensive reviews of the current state of the art in power system resilience, detailing potential metrics and evaluation criteria. Whenever applicable, power grid resilience can be viewed in terms of the timeline of the event under study, i.e., before the event starts, during the course of the event, and during its aftermath. The solutions for each phase, known as preventive, corrective, and restorative mitigation strategies, respectively, need to address different objectives subject to varying types and/or levels of uncertainty. No one-size-fits-all solution exists, and the best resilience strategy may very well vary from one system to another, and from one type of disaster to another. Preventive strategies are proactive in nature and focus on grid reinforcement to help prevent or minimize the potential impacts of upcoming disasters. These may include hardening substation equipment, hardening control rooms against water hazards or earthquakes, undergrounding lines, deploying distributed energy resources, and/or reconfiguring the network to enable microgrid islanding. Acknowledging the critical role of the control and communication network in maintaining power grid stability during disturbances, some researchers have instead focused on making the IT infrastructure robust ^[17][18][19][19,20,21]. A downside of these solutions is the normally high costs associated with them, especially given the fact that events of interest are comparatively low frequency (although high impact). Given the constant investments that are needed to maintain utility operations and upkeep, such reinforcement and capacity expansion projects may easily get deprioritized. Corrective and restorative strategies, on the other hand, are reactive in response to an ongoing or recently terminated event. The goal of both strategies is to utilize the existing power grid resources to maintain connectivity and continue supplying the loads to the extent technically possible. Despite the importance of restoring power once a disturbance has run its course, the power system must continue operating reliably and securely during the event. The colossal amount of destructive energy released by a high-intensity natural disaster event makes it impractical, if not infeasible, to guarantee the availability of all grid components. Hence, the system operator can put in place a predictive control strategy that dispatches the system in anticipation that some sections/resources may become affected by the event and hence may become unavailable. This has been extensively addressed in the literature within the context of security-constrained optimal power flow (SCOPF) ^[20][22]. The objective is to ensure that the system remains secure with respect to credible contingencies, and the system constraints are maintained should one of these contingencies happen. While this was traditionally done through performing deterministic contingency analysis and security assessment, utilities are migrating towards more advanced risk-based approaches ^[21][22][23,24]. As opposed to traditional SCOPF-based approaches, risk-based SCOPF attempts to provide a secure solution with less likelihood of exposure to failure by taking into consideration the severity as well as the likelihood of contingency events. No matter how strong a power system is and how efficiently it is operated once exposed to a natural disaster, it is still possible to be left with large-scale outages due to component failure or damage. This calls for the third category of mitigation strategies: restorative solutions whose goal is to find alternative sources and alternative routes to provide power to as many customers in the outage area as possible while the faulty sections of the grid are being repaired. Grid capabilities, such as microgrid islanding, localized load shedding, and localized power supply through distributed energy resources (DERs), especially units with black start capability, can significantly enhance the chances of a successful restoration of the outage area, whereas the duration of the outage, the expected repair time, the availability of fuel for distributed generators, and the availability of charge in energy storage systems can hinder them.

3. Mobile Energy Storage for Resilience Enhancement

Mobile energy storage increases distribution system resilience by mitigating outages that would likely follow a severe weather event or a natural disaster. This decreases the amount of customer demand that is not met during the outage and shortens the duration of the outage for supported customers. MESSs can be physically dispatched to prioritized locations and critical loads to support emergency response surrounding a natural disaster, providing backup power and black-start services. Mobile energy storage can be used to form a microgrid at a facility or set of facilities with proper connection infrastructure, reducing the amount of lost load during an outage. MESSs can be pre-positioned to vulnerable areas before disaster strikes, be allocated to support outages as the disaster unfolds, or coordinate with repair crews to aid in power system restoration. MESSs can respond quickly to the evolving needs of a community experiencing an outage, providing enhanced resilience and flexibility over stationary technologies [6]. In addition to microgrid support, mobile energy storage can be used to transport energy from an available energy resource to the outage area if the outage is not widespread. A MESS can move outside the affected area, charge, and then travel back to deliver energy to a microgrid. The available resource could be a nearby feeder that is still connected to the transmission system, or a generation resource (such as a utility-scale wind farm or photovoltaic system) that has been stranded due to downed wires or damaged utility infrastructure. This ability to utilize stranded assets could help avoid the economic losses of unused generation. However, if a generation asset or nearby feeder is not available, a MESS is a limited resource, and can only provide backup power with the charge left in its batteries. This may cause customers to lose power once the batteries are depleted, as disaster-related power outages can last days to even weeks. Thus, without the ability to recharge, MESSs are a short-term solution to what may end up being a long-term problem. Additionally, the state of charge of the batteries at the onset of the outage is hard to predict. If the disaster strikes without warning, the batteries may not be fully charged, or worst case may be depleted, rendering the MESS less useful than intended. Inspired by Bie et al. [5], Mishra et al. [3], and Lei et al. ^[23][25], Figure 1 depicts conceptually how MESSs can improve distribution system resilience as an event unfolds. The system function with and without MESSs during the event is shown. For a distribution system, system function in normal operation is the amount of load served, with the highest system function achieved when all demand is met. During an outage, the loads may be weighted by their criticality to give priority to critical infrastructure. The period associated with the event is divided into multiple event stages. These stages begin with normal operation [𝑡0,𝑡1], when planning and preventative measures, such as MESS pre-positioning and charging, can take place depending on the advance notice of the disaster. For events, such as hurricanes, blizzards, or wildfires, there may be advance notice of over a day, but for earthquakes or tornadoes there may be less than an hour to prepare ^[24][15]. Cyberattacks or other man-made threats may not give any warning. Following a disruption at 𝑡1, the event progresses [𝑡1,𝑡2], during which the system function is degraded as damage to the distribution system forces loads to be shed. At 𝑡2, the system reaches the post-event degraded state where all system damages have occurred, and no loads are yet restored. In reality, service restoration may begin before all damages have occurred, so the system function after 𝑡1 may not be monotonically decreasing. If MESSs are pre-positioned at locations that would otherwise experience an outage, the system function during the event progression and post-event degraded state is improved. Following the degraded state, the response and recovery stage begins where service is restored. With the help of MESSs, service restoration can begin while utility infrastructure is still damaged. Infrastructure recovery begins at 𝑡4, where fallen lines or damaged equipment are repaired. At 𝑡5, the system has recovered fully and is functioning in its final operating mode. MESSs can improve the system function compared to conventional restoration by energizing loads that would otherwise experience an outage, shown by the dotted line in Figure 1. Additionally, the service restoration time begins earlier (represented by 𝑡′3), and the final operating mode is reached sooner. Overall, the system resilience is improved by reducing the lost load and improving the system function from the solid line to the dotted line. This corresponds to a shorter and less severe outage with MESSs than without.