Solar photovoltaics (PV) are the main solar energy technology used in distributed solar generation. Photovoltaic (PV) materials and devices convert sunlight into electrical energy. A single PV device is known as a cell, which typically produces about 1-2 watts of power. PV cells are typically connected in chains to form larger units known as modules or panels, which can increase system capacity and power output of PV cells . Modules can be used individually, or several can be connected to form arrays. One or more arrays can then be used as a standealone system or connected to the electrical grid as part of a complete PV system. PV systems can be configured to meet a variety of power needs due to their modular design. Systems also include mounting structures that direct panels toward the sun, and components such inverters that convert the direct (DC) produced by a PV system into alternating current (AC) that can be used at an individual site or supplied directly onto the electric grid. The DOE Solar Energy Technologies Office Solar Energy Technology Basics website can provide additional detail into solar energy technologies and configurations.
Solar PV can offer benefits to critical infrastructure facilities and increase resilience for local communities by providing a backup power supply in the case of a utility outage or natural disaster event. Distributed solar installations have also allowed utilities to defer costly capital investments for distribution-level equipment, and can be useful in shifting peaks to increase overall grid reliability. Given the variable nature of solar energy generation, standealone solar PV systems may not be able to provide critical backup power and resilience capabilities at all times for communities and critical infrastructure facilities. However, when combined with energy storage, these types of distributed energy systems can provide backup power to a wide variety of facilities and communities that require a reliable source of energy.
One of the distinctive characteristics of the electric power sector is that the amount of electricity that can be generated is relatively fixed over short periods of time, although demand for electricity fluctuates throughout the day. Energy storage technologies can manage the amount of power required to supply customers at peak times when demand is highest. At the distribution level, energy storage can assist is smoothing the variable output of renewable energy and other DERs, making them more dispatchable. They can also help balance microgrids to achieve a good match between generation and load, and can provide a number of ancillary services for the grid, such as frequency regulation and voltage control.
Distribution level energy storage includes technologies such as batteries, fuel cells, compressed air energy storage, and flywheel storage systems. Battery storage systems are the most common technology combined with solar PV to create distributed systems capable of providing countinous reliable power to critical facilities or communities. There are a wide variety of different battery types and design configurations that can be utilized at the distribution level or combined with solar PV. The Energy Storage Association's (ESA) page on Energy Storage Technologies can provide additional detail on the different types of energy storage technologies and use cases. A Sandia National Laboratory report: Energy Storage Procurement Guidance Documents for Municipalities in 2016 that was aimed at supporting the Massachusetts Department of Energy’s Community Clean Energy Resilience Initiative can also be a useful too for any municipality looking to incorporate energy storage into resilience planning.
In order to provide resilient power to critical facilities or a community microgrid, distributed solar + storage resources must be capable of islanding from the grid and operating independently during outages and storm events. If solar + storage resources are carefully designed and equipped with the appropriate transfer or disconnect switches, a critical load panel, smart inverters, and sufficient control systems, they can together act as a uniform and reliable distributed resource with islanding capabilities. During normal operations, the solar PV can provide power to the facility or community and charge the battery with the excess energy generated, and the battery can discharge during times of low solar irradiance or when grid power prices are high. In the event of a grid outage, the system can disconnect from the grid and can continue to operate in island mode, utilizing the solar when available, and discharging the battery when necessary. For extended outages, the system may be limited to the amount of solar resource available over that time, but the power output of the system can be configured to only serve critical loads when islanded.
A report from the Clean Energy Group, Solar+Storage 101: An Introductor Guide to Resilient Power Systems provides a general overview of the benefits that distributed solar + storage systems can offer, along with basic technical details of system configurations. In addition to the resilience benefits that solar + storage systems can provide, there are a number of other factors that can make a distributed solar + storage system beneficial compared to traditional backup generation when serving facility or community critical loads:
| Reliability: Solar + storage systems operate continuously and are not just used in emergency conditions, reducing potential startup failures similar to that of diesel generators | Operations & Maintenance: Solar + storage systems have limited ongoing O&M costs, and typically only require visual inspections or light maintenance work periodically |
| Financial & Economic: Solar + storage systems can capitalizing on arbitrage opportunities in elctricity markets, limit peak charges for customers, and storage can also participate in ancillary services markets | Safety & Environmental: Solar + storage systems can significantly reduce GHG emissions, and do not require onsite fuel storage, which can pose safety risks |
Many of current standalone and combined solar + storage systems are designed for providing economic benefits to the end-user or utility, or providing a solution to electric grid challenges or constraints. These systems are specifically configured to operate in a way that maximizes these economic or technical benefits, but designing a solar + storage system for resilience may require an entirely different approach. A recent analysis from NREL and the Clean Energy Group has identified considerations for sizing a solar + storage system specifically for resilient operations, which are highlighted below:
| Current Electricity Costs | Time of Day When Outages Occur |
| Building Load Profile | Time of Year When Outages Occur |
| Average Duration of Outages | Critical Loads |
| Average Cost of Outages | Other Uses for Battery |
When pursuing a solar + storage project for resilient onsite power, understanding not only the resiliency value that these system can provide, but also the additional technical and economic benefits that they offer is important to overall project implementation and ultimate performance. There are a number of solar and energy storage resources highlighted below that can provide additional details on technical specifications for solar and energy storage, solar + storage programs, and other resources that may help decision makers or utilities pursue solar + storage opportunities.
The U.S. Department of Energy Solar Energy Technologies Office supports early-stage research and development to improve the affordability, reliability, and performance of solar technologies on the grid.
DOE's Office of Electricity (OE) Energy Storage Program performs research and development on a wide variety of storage technologies, collaborates with utilities and state energy organizations on storage research and programs, and supports analytical studies on the technical and economic performance of storage technologies.
The National Renewable Energy Laboratory (NREL) offers a number of tools, maps, and calculators specific to distributed solar resources and applications, such as the PVWatts Calculator and Solar Maps highlighting solar potential throughout the U.S.
A report from NREL, Valuing the Resilience Provided by Solar and Battery Storage Systems provides an assessment of the potential resilience values that can be attributed to solar + storage systems, and also highlights a number of considerations when designing a solar + storage system for resiliency.
The Federal Energy Management Program (FEMP) through the DOE recently published two reports: one providing Procurement Specifications Templates for Onsite Solar PV and the other an ESPC Energy Sales Agreement Toolkit. Both can be useful for municipalities or state governments seeking to implement renewable energy projects.
San Francisco's Solar Resilient project is aimed at creating a pathway for deploying solar + storage systems for resilience by minimizing the regulatory, financial, and technical barriers that currently exist. Funded by a grant from the DOE's Solar Market Pathways Program, they are looking to solar + storage systems to increase overall community resiliency by integrating them into the City's emergency response plans.