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
]. 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
]. 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 ~$
]. 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.
Over the past five years, there has been an increasing interest in using MESSs for resilience enhancement. Researchers have focused on resource allocation to determine the optimal scheduling of a MESS fleet following a resilience event, considering the interactions between MESSs, microgrids, repair crews, and the transportation network. There have been numerous studies that consider the use of MESSs for distribution system resilience enhancement, demonstrating the need for a collective review on the current practices and challenges that face this topic. Review papers on energy storage systems have mentioned MESSs, but to the best of the authors’ knowledge, no comprehensive review exists [14
]. The remainder of this paper consists of the following sections. Section 2
introduces the concept of power grid resilience and Section 3
describes how MESSs can be used for resilience enhancement. Section 4
presents a review of the current state of the art. Section 5
discusses the gaps in the existing literature and outlines areas of future work, and Section 6
presents the conclusions drawn from this work.
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 [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 [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 [16
]. Bhusal et al. [17
] and Raoufi et al. [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 [19
]. 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) [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 [23
]. 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. [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
, 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 [15
]. Cyberattacks or other man-made threats may not give any warning.
Following a disruption at
, the event progresses
, during which the system function is degraded as damage to the distribution system forces loads to be shed. At
, 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
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
, where fallen lines or damaged equipment are repaired. At
, 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
), 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.
5.1. Research Gaps
While considering the energy dispatch problem of MESSs, the current literature has examined active and reactive power to ensure frequency and voltage stability, respectively. However, if the system has rotating masses in the form of large motors or synchronous generators, dynamic stability needs to be studied as well. Currently, small-signal and transient stability are not considered in the literature as the analysis occurs on the order of every hour and thus allows for the assumption that dynamic stability can be disregarded. However, as the scale of the power system studied decreases, these assumptions may no longer hold; so, it may be necessary to consider dynamic stability for a MESS serving a microgrid or a network of microgrids. Similarly, with MESSs supplying smaller microgrids, voltage and power quality events may become more critical because the system is likely to be less stiff. Additionally, while balanced and unbalanced three-phase systems have been evaluated, no researchers have discussed the potential presence of asymmetry in the distribution system and how that may affect MESS service restoration.
Although MESSs are believed to enhance power grid resilience, the current literature lacks a unified approach to evaluate the effectiveness of their proposed strategies to achieve this goal. It is not enough to state that a measure will enhance resilience, a qualitative analysis is necessary. The absence of agreed-upon resilience metrics makes it challenging to prove resilience enhancement. Additionally, it is difficult to compare existing MESS strategies and to compare MESSs against similar technologies that also increase resilience. Therefore, future work must include a well-defined resilience metric. The same resilience metric can then be used to compare existing strategies such as those presented in this review, or to compare MESSs against stationary energy storage, network reconfiguration, demand response, or distributed energy generators. Often, these technologies may be deployed simultaneously, and a strong resilience metric can help optimize the system of resilience technologies for a specific application. This will create quantifiable results to point towards the best application for MESSs for a given situation. Even when a metric is employed, the method of evaluating the effectiveness of a strategy is often limited to case studies that contain single pre-defined outages, which does not produce a robust measure of the expected resilience enhancement. A stochastic, risk-based approach could improve the measurement of the resilience enhancement for a specific system.
While existing literature has acknowledged the potential costs and benefits of MESSs for resilience enhancement through critical load support, the evaluation is simplistic and unrealistic. The benefits have been evaluated as a reduction in customer interruption costs, which has been modeled using the value of lost load. However, the use of a constant value of lost load does not represent how outage costs evolve and compound throughout a long-duration outage. Additionally, the costs are not representative of the diverse set of customers that could be affected by the outage. Future work should employ duration-dependent outage cost functions that vary based on the customer they represent to better estimate the benefit of increasing resilience and avoiding outage costs. Furthermore, current benefits are calculated based entirely on the monetary costs of an outage. The consequences of a disaster can be exacerbated by social, economic, or political conditions, which are not accounted for in these monetary costs. To properly address issues of equity within MESS power system restoration, it may be necessary to include a social vulnerability index to ensure that non-monetary costs are accounted for. An accurate representation of outage costs can help better estimate the resilience benefit of MESSs, which can help entities make informed investment decisions. Future work can use these benefits in a full cost–benefit analysis of MESSs, considering the stacked benefits achieved from both normal operation and emergency response and the operation cost of a MESS in both situations. Since MESS restoration is limited by the battery capacity and the availability of an energy resource to recharge from, their benefits may only be realized in a small time interval. A cost–benefit analysis is necessary to determine if that time interval is sufficient to offset the costs of deployment.
5.2. MESS Challenges and Opportunities
A primary challenge when deploying MESSs for service restoration is that they are finite energy resources if not recharged. Thus, MESSs may act as short-term solutions to long-term problems in the case of extensive infrastructure damages. Researchers have considered the charging and discharging patterns of a MESS combined with microgrids in specific case studies, but that work could be extended to evaluate where additional reinforcement is necessary to create areas where a MESS can charge during an unexpected outage. Alternatively, MESSs themselves could include integrated renewable generation such as photovoltaic (PV) panels or micro wind turbines to allow for on-site charging.
In either case, the capacity of one MESS may not be sufficient to restore power to the entire outage area. In that case, MESS deployment may need to be coordinated with demand response or load shedding as suggested in [44
]. Further research could examine how to perform load shedding for residential, non-controllable loads or how to implement programs to reduce power consumption once a customer’s power has been restored by a MESS. Alternatively, a load area could be broken into multiple microgrids so that the MESS capacity can support the entire smaller system. While microgrid formation and MESS deployment have been well studied, the MESS allocation problem could be combined with mobile switching and separation devices to implement network reconfiguration and adaptive microgrid strategies on systems with fixed separation points.
5.3. MESSs vs. Electric Vehicles for Resilience Enhancement
In addition to the mobile energy resources (MESSs, mobile emergency generators, and electric buses) discussed in this review, electric vehicles can act as an energy resource to aid in service restoration [60
]. The use of electric vehicles for grid support, or vehicle-to-grid (V2G) technologies, has been well studied, and while V2G can serve the same applications as MESSs, there are several key differences to the deployment strategies. One advantage of V2G over MESSs is that electric vehicles are more dispersed and distributed, and are therefore able to support loads across a wider geographic area. While EVs have much smaller batteries than MESSs, on aggregate, they can represent a resource of a similar scale. However, because EVs are not utility-owned or controlled like MESSs, V2G requires advanced communications and economic incentives to coordinate them for power injection and grid support. In addition, the point of entry to the grid would differ between EVs and MESSs, as MESSs typically connect to a substation, whereas EVs would have to plug in at a dedicated charging station.
In the face of natural disasters that are exacerbated by climate change, it has become increasingly important to increase power grid resilience. More resilient power systems can better prepare for, withstand, and recover from disasters, avoiding the social and economic costs of a power outage. Mobile energy resources, specifically MESSs, can increase power grid resilience by restoring power to critical loads following a contingency. Their mobility allows for increased flexibility compared to stationary DERs. MESSs can also provide ancillary services during normal operation, recouping investment decisions, a rare ability for emergency response equipment. As the cost of batteries continues to decrease, commercial deployment of MESSs will likely grow.
This paper provides a comprehensive review of the use of mobile energy resources (including MESSs, EBs, and MEGs) for resilience enhancement. The routing and scheduling optimization problem formulation is discussed along with the constraints imposed by both the power system and the transportation system. Major gaps in the literature stem from a lack of consensus surrounding qualitative metrics with which to measure power grid resilience and subsequent improvements, as well as simplistic and unrealistic models of the customer interruption costs for a long-duration power outage. Risk-based and stochastic analyses are necessary to quantitatively measure resilience enhancement and compare MESSs against other similar technologies. To support informed MESS investment decisions, the customer interruption cost during an outage should be modeled to represent different customers and how costs can accrue throughout a long-duration outage.
This study only considers the use of MESSs in emergency conditions to restore power following an outage, but other applications during normal operation exist. These include load leveling, peak shaving, reactive power support, voltage regulation, the support of dispersed renewable energy integration, and transmission upgrade deferral. MESSs can generate value in both scenarios and implement value stacking to increase their cost-effectiveness. In addition to their use for emergency response, a comprehensive review of the use of MESSs during normal operations for applications is needed for a thorough understanding of the role of MESSs in the future power system.