The time for new and improved infrastructure development for the prevention of, and fast recovery from, natural disasters is now. Recent natural disasters have caused massive infrastructure damage, cut off critical services, and led to prolonged outages. Examples of these include: The 9.0 magnitude Great East Japan Earthquake and the ensuing tsunami, and severe weather events such as Superstorm Sandy and hurricanes Irene and Katrina. Most recently, Hurricane Maria caused extensive damage in Puerto Rico and the island still has not recovered from the total outage 10 months ago. While outages and grid damage such as these cannot be prevented, their impact needs to be minimized through proper preparation.
Islanding capability with solar photovoltaic (PV) plus battery energy storage (BES) can help critical facilities, such as airports, hospitals, shelters, water treatment facilities, police stations, and emergency assembly zones operate during disasters. Microgrid infrastructure and energy storage have helped such places stay resilient when nature strikes. Developing and implementing these technologies, instead of relying on a single power source, is a very promising path for improving recovery times after natural disasters. For example, renewable energy, such as PV+BES can provide power and grid stabilization for residential, commercial, and industrial loads during a major disaster. Protecting this critical infrastructure during extreme disasters is important and PV+BES is a proven technology that can make resiliency a reality.
This white paper focuses on the benefits of solar PV systems, supplemented by energy storage, to provide for disaster relief and system resiliency. It includes a review of actual case studies detailing the performance of solar PV during natural disasters. We predict that solar plus storage hybrid systems will play a vital role in the creation of resilient power systems to prevent the significant outages which often accompany natural disasters.
The electric grid is a vital component of our everyday life as it generates, transmits, and distributes electric power to millions of homes, schools, offices, factories, and more. The electric grid must be resilient, secure, efficient, and reliable. The leading cause of power outages in the United States is severe weather, which is also when emergency services need power the most. While outages and grid damage from natural disasters cannot be prevented, their impact can be minimized through proper preparation and the establishment of more resilient systems. Solar photovoltaic power generation with battery energy storage (PV+BES) can be employed to provide added system resiliency in the wake of natural disasters.
PV is an established reliable power producing technology, but it is an inherently intermittent power source and non-dispatchable. Modern grid-rated inverters help mitigate the effects of power fluctuation, but are not enough for PV to act alone as a critical power source. Hence the need for the second half of the equation for improved system resiliency. Battery energy storage (BES) is an established technology that provides the power quality and grid services which complement the generating capabilities of PV and together form the basis of a reliable and resilient power source and microgrid.
Recent natural disasters have caused massive infrastructure damage, cut off critical services, and led to prolonged outages. Examples of these include: The 9.0 magnitude Great East Japan Earthquake and the ensuing tsunami, and severe weather events such as Superstorm Sandy and Hurricanes Irene and Katrina. Most recently, Hurricane Maria caused extensive damage in Puerto Rico and the island still has not recovered from the total outage 10 months ago. The National Oceanic and Atmospheric Administration tracks the frequency and cost of large-scale natural disasters, known as billion-dollar disaster events based on the total estimated losses from the disaster. The following graph illustrates the number of large-scale US disaster events over the past 38 years. Based on this data, the deadliest and most damaging natural disasters have been tropical cyclones . The most frequent billion-dollar disasters have been severe storms . Both of these types of natural disasters frequently cause power outages which severely obstructs emergency response and recovery.
Figure 1: Billion-dollar weather/climate disaster events [1} 1
The leading cause of power outages in the United States is severe weather, including blizzards, thunderstorms, and hurricanes . Severe weather accounted for 87 percent of outages affecting 50,000 or more customers from 2002–2012, and within the past decade the number of weather-related power disruptions has grown significantly .
The following figure shows the average electric power service interruptions per customer in the United States based on frequency and duration as recorded by the US Energy Information Administration. Although the average frequency of power outages per customer does not significantly increase when considering major events, the duration almost doubles on average . For such emergency situations, a more resilient grid could extensively improve disaster relief and recovery, especially when considering critical facilities and communications.
Figure 2: Average power outages per US customer 
In its simplest form, grid resiliency is the ability of the system to reduce the magnitude and/or duration of disruptive events on the power grid . Accordingly, grid resiliency has four elements 
- Hardening: The ability to absorb major grid events and continue operating through hardening of the system components
- Ride-through: The ability to manage a crisis as it unfolds and maintain some basic level of electrical functionality without total collapse
- Rapid recovery: The ability to get systems and services back as quickly as possible
- Adaptability: The ability to learn and incorporate lessons from past events to improve resilience for future events
PV+BES microgrid systems can be developed and employed to contribute to the first three of the above elements of system resiliency.
This paper discusses the benefits of PV+BES systems for the resiliency of both a local grid and a microgrid. A microgrid consists of a small number of loads and power sources which can operate connected to the local grid or disconnected, also known as islanded. In order for a microgrid to operate as an emergency power source during a power outage, it needs to be designed to stand alone, “isolate itself from the grid, continue power production, and store excess generation for later use.”  Microgrids can incorporate a number of different power sources (such as microturbines, reciprocating engines, and fuel cells) which are regulated by a controller behind an isolation switch. An example schematic of a PV+BES microgrid is shown in the following figure. This schematic shows the PV and BES systems sharing an inverter which can directly feed the backup loads in the case of islanding under the direction of the controller. The separation/isolation from the grid and the reconnection/ resynchronization to the grid must be seamless to minimize disruptions to the microgrid loads in the case of critical facility emergency power .
Figure 3: Generic schematic of PV+BES microgrid 
PV+BES systems can be added into microgrids to provide additional power generation and stabilization capabilities, or they can be used throughout a local grid with the same benefits on a larger scale. These resiliency benefits to a grid or microgrid are discussed in the following sections.
The first element of system resiliency is to prevent a power outage in the first place or to harden the grid to disturbances. PV+BES systems have high availability and robust physical design, making it a resource capable of reducing or mitigating the loss of power where it is applied.
Dependable fuel source
PV power production is entirely dependent on the available solar resource which is why it is often typified by intermittency due to variances in cloud cover. PV panels do not require direct sunlight or irradiation (as is needed for the operation of concentrating solar plants). PV panels can generate power under diffuse irradiation, albeit with lower outputs. This is fundamental to PV reliability because the system can produce power as long as there is daylight even under cloudy skies, which is not the case for conventional power generation, which would not be available because of any extreme weather or wind turbines without wind. According to an NREL study on the operational reliability of 50,000 PV systems in the United States, the loss in PV system production due to extreme weather (such as lightning strikes or hurricanes) was more likely due to grid outages than PV system damage or underperformance . So, even in a severe weather event, a PV system designed with standalone operations can be relied upon to produce valuable power on a daily basis even during a system outage. When coupled with a dispatchable BES system, this reliable source of generation can be used as needed to prevent disruption of power during extreme weather events. Figure 4 shows the performance of a 1 MW utility-owned, ground-mount PV system during and after Superstorm Sandy.
Figure 4 shows that there was very low irradiation the day before and during the storm, and sunny to partially cloudy days immediately afterwards . However, most PV systems in the region affected by Superstorm Sandy were inoperable due to the grid outage since they were not designed to operate in island mode . If these systems had dynamic controls and islanding capabilities, they would have been able to maintain power during the critical days and weeks following the storm when disaster relief faltered due to the widespread grid outages. Hence the need for the second half of the equation for improved system resiliency: the BES component of the PV+BES system.
Power control and stabilization
BES systems provide numerous dynamic control capabilities and grid services such as energy time-shift, power quality, frequency regulation, spinning reserves, and voltage support. These services are essential for microgrid control/stabilization with integration of PV power production. In summary, BES systems provide every complementary service to PV systems for improving system resiliency.
Both PV and BES systems are renowned for high availability rates due to the inherent simplicity of the system designs. The conservative assumption for plant operation is 98% availability based on historical operating trends, but a rate of 99% is often guaranteed by operators. In 2013, BEW Engineering performed a third-party analysis of the availability over two to eight years of 16 utility-scale PV power plants operated by SunPower. This analysis resulted in a 10-year P95 availability of 99.2% including external outages, which strongly supports the validity of the high availability of PV systems . Based on a cost study performed by DNV GL in 2016, the operator guaranteed availability of typical Li-ion BES systems was over 97%, suggesting that actual performance is even higher and in-line with the high availability of PV systems . With proper design and maintenance, it is unsurprising that PV+BES plants not only withstand, but operate under the extreme conditions of natural disasters.
If the grid does experience an outage, then the second element of system resiliency takes effect, which is the ride-through capability of the facilities through the extreme weather event. Critical facilities include hospitals, police stations, shelters, and water treatment facilities. During a disaster, these services are just that, critical. If the grid experiences an outage, these facilities must maintain power and are designed with backup power to form a microgrid. Microgrids are technology neutral, but typical backup power for critical facilities is provided by diesel generators with fuel storage that are only utilized for emergency events. However, PV+BES systems are particularly well suited to augment these microgrids for emergency power applications. The applicability of PV+BES systems to be included in these critical microgrids is growing considering (1) the continuing decrease in costs for PV and BES systems individually, (2) the improved commercial viability and general reliability of the systems, (3) the potential for continuous power generation without grid support or fuel supply, and (4) the functionality of the system to produce revenue during normal operations .
After the grid experiences an outage, then the last element of system resiliency comes into play, which is the repair and recovery of the power supply. The keys listed by EPRI to the rapid recovery of electricity supply are: damage assessment, crew deployment, and readily available replacement components . Both PV and BES are typified by their relatively plain design topology. An advantage of this simplicity is the speed and ease of repair. PV+ BES systems are relatively easy to assess damage, which can often be accomplished using the distributed control system data (often remotely accessible) and visual inspections. Spare parts are easily maintained because both systems utilize a repeated block design of PV panels or battery modules and inverters. However, in extreme cases, even the simple and robust PV system design can experience severe damage. While PV systems in areas that were hit by Superstorm Sandy were mostly intact, thousands of PV panels in Puerto Rico and the US Virgin Islands were damaged beyond a simple repair from Hurricane Maria . So, although PV+BES systems are well-suited for improving system resiliency, they are not without limits. As such, they should be adopted as part of the local grid for overall system resiliency improvements or combined with other technologies for microgrid applications of critical loads.
In order to adopt a PV+BES microgrid, it is important to size the system in order to meet the backup loads under emergency conditions. Many elements affect the sizing of a PV+BES microgrid system including interconnection approach, normal and backup loads, land/rooftop availability, use cases/functionality, other power sources in the microgrid, and existing infrastructure. At a high level, the basic sizing of a PV+BES system for backup power can be estimated using the following four-step approach:
- Define loads desired for critical operation.
- Define historically low daily solar irradiation (assuming PV recharges BES on daily basis).
- Size BES energy capacity to store enough energy to power critical loads for minimum duration.
- Size PV to charge battery for minimum duration during single day of low solar irradiation.
To illustrate this approach, the following outlines the sizing of an example police station PV+BES system located in central Nevada: Critical loads are defined to be continuous 45 kW. Historically low daily solar irradiation (20 year low) for a typical central Nevada location is 600 Wh/m2/day based on NREL 20-year data. High-level BES capacity requirement is calculated as follows:
High-level PV capacity is calculated as follows assuming a fixed tilt, ground-mount PV design:
This high-level estimate approach oversizes the PV to charge the BES in a worse-case scenario, which means the system will typically be able to provide significantly more power to be used by the facility for daily non-critical loads or sold back to the utility, which is a consideration for the interconnection approach.
This high-level calculation may be adequate for community shelters, schools, fire stations, and other public or private facility with a need for resilient power when the grid goes down. Hospitals have stringent backup power requirements, and PV+BES would need to be supplemented with additional infrastructure to meet all the requirements.
As adoption of PV+BES microgrids continues to become more widespread, the performance of these systems under extreme weather events is becoming better substantiated. This section summarizes five case study examples of successful PV+BES system installations which provided improved system resiliency.
Borrego Springs Microgrid Demonstration Project, California
In the Borrego Springs neighborhood of San Diego, the community has installed 700 kW of distributed rooftop PV systems with distributed BES located at substations, distribution circuits, and a few residential locations. The power to the neighborhood is supplied through a single transmission line that often experiences weather- related disruptions. This distributed PV+BES community microgrid is currently improving reliability for this fringe-of-network community. Communication and control technologies were added at the substations and distribution circuits, and residents can participate in a smart grid load management program. This project was sponsored by the US Department of Energy, San Diego Gas and Electric, the California Energy Commission, as well as other contributors, all working together to improve electricity supply dependability. 
The community’s microgrid, which includes distributed generation, storage units, and control technologies, has provided power during foreseen and unforeseen outages on several occasions; moreover, successful islanding of neighborhood distribution circuits was demonstrated during those occasions. This distribution system capability, following power outages, demonstrates the benefits of microgrids and the impactful contribution to community resiliency. 
Midtown Community School, New Jersey
Midtown Community School located in Bayonne, New Jersey, installed a hybrid solar-diesel generating system in 2004. Although BES systems were not a part of this system, the PV in conjunction with the diesel generator provided a steady supply of electricity throughout Hurricane Sandy, which enabled the school to be used as a shelter throughout the disaster. Other backup systems lost power due to flooding or the shortage of diesel fuel as the recovery of the grid took weeks in some regions. The Midtown system idled during daylight hours, which allowed for a decreased consumption of fuel and the conservation of valuable resources. 
Stafford Hill Solar Farm and Microgrid, Vermont
The town of Rutland, Vermont, showcases the country’s first 100% solar-powered microgrid constructed on a repurposed landfill. This community was one of the hardest hit areas of the state during Hurricane Sandy and experiences frequent storm-related power outages. The innovative project is capable of producing 2.5 MW of solar power and 4 MWh of battery storage, enough to supply 2,000 homes with electricity during normal weather conditions . This PV+BES microgrid is designed to supply critical power to Rutland High School, which has been designated as an emergency shelter. When the grid is working properly, the PV+BES system enables services to the grid, regulating and smoothing supply and demand. This project is also recovering cost through the daily storage capability. 
Scripps Ranch Community Recreation Center, California
In San Diego, the Scripps Ranch Community Recreation Center had the problem of periodic blackouts caused by reoccurring wildfires. This center improved reliability by adding lithium-ion battery storage to their already existing solar array. This community center has been transformed into an emergency command center and, with its islanding capability, serves as a communication center for fire and police operations during grid disruptions. This PV+BES project was the first of its kind in Southern California and the system cuts cost by reducing peak demand charges and providing grid support services. 
Severe weather is the leading cause of power outages, and although natural disasters cannot be prevented, the electric grid must offer more resiliency. Prevention of power disruption through hardening and ride-through and recovery of electricity are the main elements necessary for a resilient system which can be afforded by PV+BES systems. PV is an established, reliable power producing technology, which, when complemented by BES, can deliver these key system resiliency qualities either to the local grid or to a microgrid. Thus, PV+BES can be employed to provide added system resiliency in the wake of natural disasters.