Substation-Scale and Community Energy Storage
- Written by Brad Roberts
Energy storage systems, essential for balancing dynamic sources and loads across electric power grids worldwide, can address renewable energy intermittency and integration, manage larger and less predictable peak demands from electric vehicles and other loads, and enable secure microgrids. Community Energy Storage, in particular, and substation-scale batteries, have a key role to play.
In 2010, Texas deployed the largest sodium-sulfur battery in the United States, in Presidio, a city that sits at the end of a 100-kilometer-long, 1940s transmission system. Due to the aged grid connection and the grid’s vulnerability to frequent electrical storms, outages were common enough to warrant investment in the 4-MW battery substation. The NaS battery, coupled with a four-quadrant inverter power conversion system, is capable of providing the city’s 4,000 residents with power for up to eight hours during an outage. The battery can respond rapidly, managing voltage fluctuations and momentary outages.
The system, installed by S&C Electric Company of Chicago, lets the local Texas utility control battery function, notably to store grid power off-peak and re-dispatch it to the grid as needed.
Such energy storage systems, as generally recognized, can be deployed virtually anywhere to help balance the dynamic sources and loads impacting electric power grids. Storage holds vast potential to improve the grid’s reliability, efficiency, capacity and responsiveness. It can optimize electricity flow, allowing two-way power flow while smoothing and storing wind and solar energy for later dispatch to meet peak load demands.
Less universally appreciated, perhaps, is the fact that pushing energy storage farther out onto the distribution system, in closer proximity to loads, maximizes its benefits. The closer energy storage is located to loads, the better job it will do increasing overall grid reliability, managing the variability associated with widely distributed, small-scale renewable energy generators, and integrating electric and hybrid electric vehicles into community grids. Electric vehicles and plug-in hybrid electric cars have the potential, indeed, to form a big part of local storage infrastructure, feeding power to the grid at times of peak demand and drawing it during the off-peak late evenings and nights.
At the system level, storage provides capacity relief, reactive power compensation and greater grid stability; it can also provide frequency regulation. But at the substation and feeder levels, storage provides peak shaving and load leveling, time-shifting of renewable energy, voltage stabilization, reduced cold loads and load transfers, and reactive power compensation. Locally, energy storage ensures power reliability through islanding, improves voltage control, provides extra energy for electric vehicle charging and much more.
The Presidio, Texas, project marked the first time a state public utilities commission had allowed a transmission company rate-based recovery for battery storage. In fact, it was the first time a state commission allowed recovery for any distributed storage project. The impact on utility customers was dramatic. The battery solved a sizable problem that could previously only be addressed by replacing transmission lines. But even replacement would not provide all the benefits of this landmark solution.
The Presidio project is substation-scale and, as such, is somewhat unusual. Community energy storage (CES) is more typical of present-day developments. CES units distribute discrete amounts of storage at the grid’s edge, closest to customer loads. They provide reliable backup power within communities when grid power is unavailable. CES systems integrate residential-scale renewables and manage their intermittency, thus improving voltage control and providing efficiency gains through power factor correction, virtually eliminating the need for fixed capacitor banks on distribution circuits. Through peak load shaving, CES provides asset relief, helping utilities defer capital expenditures for major substation and distribution system upgrades.
Community storage also enables the grid to perform more effective management of the larger peak loads associated with electric vehicle charging. Fleets of CES units can decrease the strain on distribution grid assets during peak demand periods. As electric vehicle loads increase, demand patterns will also be less predictable and more challenging to control even if most charging occurs at night, as studies by the Electric Power Research Institute and Sandia National Laboratory have found. Distribution transformers may become overloaded when an unusually large number of electric vehicles charge concurrently in the same geographic zone. CES units will be able to swiftly provide demand management: Though they are at the very edge of the grid, they are under the utility’s supervisory control and data acquisition (SCADA) system, which can peak shave or off-load customers to reduce overloads.
For these reasons, CES is being adopted increasingly as an effective smart grid solution. A number of utilities are already using or evaluating CES systems with large-scale deployments in mind. S&C, a leading supplier of distribution automation solutions, offers a radio-based system that controls fleets of up to 1,000 CES units for demand management and other grid services.
Storage is integral to microgrids—“energy islands” that can supply power when grid service is interrupted, unreliable or too expensive. Microgrids increase integration and energy use from renewables such as solar and wind. Military bases, correctional facilities, university campuses and other self-contained or isolated facilities requiring highly secure 24x7 power are prime microgrid candidates.
One of the most sophisticated microgrids, recently deployed by Chevron Energy Solutions, serves Santa Rita Jail in California's Alameda County, the largest correctional facility in the United States. Using onsite renewable generation, a 2-MW/4-MWh lithium-ion battery and a 2-MW power control system, Santa Rita’s microgrid can operate indefinitely without a local utility grid connection. The control system regulates battery charging and discharging for facility power needs, excess renewable energy storage, intermittent output smoothing and reliable power dispatching when demand exceeds generation. The microgrid will yield nearly $100,000 annually in energy savings. More importantly, the system demonstrates the viability of large-scale microgrids.
When strategically deployed, storage can provide balancing energy for intelligent electric grids worldwide, while enabling other critical systems such as microgrids. It is essential to the future of power systems, which already face larger, less predictable peak loads and variable resources. Storage is key to a reliable, efficient, responsive smart grid that can manage rapidly evolving power needs. The concept has been demonstrated technically, more than once. The next step is to update utility regulations for increased deployment of grid-tied community energy storage with fair compensation for the value it provides.