Microgrids and energy storage are highly promising and frequently discussed topics in the energy community. Growing cybersecurity threats and frequent natural disasters that pose risk to the electric system have made microgrid solutions a desirable infrastructure improvement for customers and utilities.
At times, however, the terms “microgrid” and “energy storage” are used interchangeably – implying energy storage systems naturally provide energy security. It is important to recognize that microgrids and energy storage are not the same thing.
The answer to whether energy storage is essential to a functioning microgrid is: well, it depends. Energy storage is a flexible, versatile distributed energy resource that can provide significant benefit to a microgrid.
However, implementing an energy storage system alone does not constitute a microgrid, and there are many scenarios where microgrids can be designed and implemented without storage.
Since adding storage resources carries significant additional capital investment to a project, it’s important for customers to identify their resiliency and energy security goals and work with qualified energy solutions partners to achieve those goals. Qualified partners should be independent to provide the greatest portfolio of solutions, have experience in energy efficiency to maximize cost effectiveness, and have demonstrated expertise in generation, controls and storage to effectively develop, design and implement successful energy security solutions.
The Spectrum of Resiliency
It is important to define what a microgrid is to begin to answer the question of how critical an energy storage system is to its design.
Microgrids provide users of electricity the ability to safely disconnect from the primary utility service connection and to independently serve on-site electric loads in a safe and reliable manner. This disconnected state is commonly referred to as “islanding”, because it’s effectively a small powered system that serves its own requirements, without transferring power in or out of the island. If the entirety of a site’s load can’t be served by on-site generation, priority needs to be placed on those loads that are deemed critical to a site’s operations.
The core functions of Ameresco’s approach to an effective microgrid design include: energy conservation; distributed generation; microgrid controls; and, to some degree, energy storage. This efficiency focused approach ensures the systems are first optimized for efficiency to minimize wasted load and most cost effectively invest in new generation, storage and control equipment.
Three primary questions help illustrate a spectrum of resiliency that begins to define to what extent each of these core functions are required. First, how quickly does the customer need to make that transition from grid-tied to islanded condition? Second, how much load of the site’s total load is deemed “critical”? And, third, through what outage duration is the customer expecting to serve this critical load (e.g. minutes or weeks)?
The spectrum depicted in Figure 1 ranges from customers with no resiliency to utility disturbances to customers who are fully resilient and can make a virtually seamless transition from utility connected to utility independent (“islanded”) operation. A customer with no microgrid capability, for example most readers’ homes, will experience full loss of power during a loss of utility service. Power returns once the utility has diagnosed and resolved the fault or failure on their grid.
Next, customers who have on-site emergency backup generation and a transfer switch are partially to fully resilient, in that some or all their load can be served depending on the size of the generator(s). Power will be temporarily interrupted until the backup generator can synchronize to serve load. In this scenario, it is likely that a small amount of especially critical load (e.g. computers or data servers) will have integrated uninterrupted power supplies (UPS) that sustain continuity of power supply to especially critical loads for a limited duration. Many office buildings are examples of this scenario.
For some customers, this interruption of power supply poses a significant risk to critical operations or creates a notable economic liability in the form of decreased productivity, scrapped manufacturing throughput, compromised research in a university or laboratory environment, or other lost value. This introduces the need for an uninterrupted transition to independent islanded operation during utility disturbances, meaning that that certain loads within a customer’s facility would experience no interruption at all.
While an uninterrupted transition (depicted on the right half of the figure above) may be desirable for many customers, this capability increases the complexity and cost of a microgrid. To attain an uninterrupted transition to independent islanded operation and without disturbing continuity of power supply, protective electrical controls must safely and immediately isolate the site from an unstable utility connection and on-site generation must immediately serve load. This can be achieved if on-site spinning generation (e.g. a combined heat and power plant (CHP)) is available to serve load at the time of the utility disturbance, and microgrid controls are in place to recognize and promptly react to disturbance.
A critical determination for successful uninterrupted transitioning to island operation is if sufficient generation is available to serve the site’s critical load, or if less critical loads need to be shed (turned off) to match the generation that is available. This microgrid control action may need to take place quickly to maintain customer generator stability and sustain continuity of service.
To achieve a fully resilient microgrid with uninterrupted transitions between utility and on-site sourcing of power, as shown on the far-right end of the spectrum, sufficient generation needs to be available that can meet all site load requirements. This must be true at the time when the outage occurs, as well as continuously for hours or days until a healthy grid returns. Even in this circumstance, microgrid controls may be considered to allow the system to respond to contingencies, such as the failure of a customer generator unit, or an unexpected change in load levels.
If sufficient generation exists or is planned to be implemented, and the proper microgrid controls and appropriate microgrid control infrastructure is in place, an uninterrupted transition can be achieved without storage. However, as further described below, adding storage to the mix can significantly enhance the microgrid scenario described above. Or, for customers that don’t have continuous spinning generation in place, storage becomes a very important aspect to the microgrid architecture.
The Role(s) of Energy Storage in a Microgrid Solution
Modern storage systems are unique in that they are very fast responding resources that can both generate and absorb power and, in some cases, regulate real and reactive power quality in an electric distribution system. These capabilities allow storage to serve a variety of roles within a microgrid for instances where customers have a need for uninterrupted islanding, have no on-site generation, or need to supplement the on-site generation that exists in their distribution system. Let’s look at the role storage plays in two distinctly different operation phases of a microgrid: while making the transition from grid-tied to islanded operation; and during continued islanded operation.
Effectively making the transition to island operation requires a great deal of coordination and very fast control action – on the scale of milliseconds. During this time, the high power and fast response capabilities of commercially proven battery systems (e.g. lithium-ion) can be used to provide effectively instant power to the microgrid for a limited duration of time to bridge an outage period until on-site generation can serve the majority of load for during a prolonged utility disturbance. If no backup generation exists, the microgrid would only be sustained until the storage capacity is exhausted, typically fifteen minutes to a few hours. During a transition from utility supply to an independent island, some storage inverters have the ability to regulate voltage and frequency within the islanded system, maintaining power quality of the islanded electric system. This function is typically satisfied by the utility grid when healthy, or spinning generation when available, but can be achieved with storage if the system is designed appropriately.
Once the transition to islanded operation is complete, the role of the storage system shifts from an immediate short duration response to maximizing the longevity of the microgrid’s operating duration. The ability of the storage system to charge and discharge creates a dispatchable resource that can follow commands from a microgrid control system to contribute additional generation, or balance supply of on-site generation with electric demand. Again, since today’s commercially available storage systems can usually be sized to provide rated power for up to four hours, the storage system’s ability to continuously support the microgrid is finite unless re-charged by on-site generation. Storage without generation therefore poses a risk to continuity of critical power supply for very long durations.
Perspectives from the Field
Example Ameresco microgrid and energy storage projects serve to illustrate the varying roles of energy storage in a microgrid. Ameresco’s approach to energy security is founded upon developing, designing and implementing solutions that are appropriate a customer’s site-specific needs and conditions. This is critical to providing customers with cost-effective and technically competent solutions because microgrids and energy storage are not one-size-fits-all concepts.
Ameresco demonstrated the ability to implement a microgrid solution without storage at Portsmouth Naval Shipyard in Kittery, ME. Funded through the Department of Defense’s ESTCP program, this project provided the site uninterrupted transition from utility power to island operation by complementing existing on-site CHP and diesel generation (DG) assets with new microgrid controls.
Before implementation, the site’s existing generation assets included two combustion turbines and two emergency diesel generators. However, the site’s electric demand often reached values well above the capacity of these units. During normal operation, the site purchases the power from the utility to serve loads it support with on-site generation. Before the project, utility loss or disruption at times of high site electric load, caused the site to experience a full blackout since the on-site generation could not carry the site’s load and the backup diesel generators were typically offline and not immediately available to avoid an outage.
The project focused on implementing an advanced microgrid controls system. This system senses when an impending utility outage is coming and very quickly (on the order of tens of milliseconds) shuts down non-critical loads throughout the base to prevent a broader site power outage. Though a large storage system would certainly enhance the solution, this project shows how energy storage is not always a firm requirement for an effective microgrid.
On the other end of the spectrum, Ameresco is planning to implement a microgrid solution at a US Government site in Central America. The site’s connection to the local utility grid has historically been very unreliable, forcing them to serve 100% of their electric load with expensive diesel generated power.
Ameresco plans to implement a large solar PV generation array, a large battery storage system, an advanced microgrid control system, thereby reducing the site’s traditional diesel backup capacity.
The inclusion of these assets allows the site to opt back in to a traditional utility service as the proposed solution mitigates the original utility reliability concerns. When the local utility connection is lost, there is no continuous spinning generation at this site to maintain power quality on-site. Thus, the storage system will hold an important responsibility to control voltage and frequency of the system and stabilize power flow alongside available PV generation until the backup DGs can be brought online. Without a battery as part of this solution, the site could not achieve an uninterrupted microgrid transition.
Finally, Ameresco has implemented and is developing numerous projects where battery storage systems provide significant grid-tied value to a customer as a primary driver. One example of this is at a federal courthouse in California. Ameresco recently brought online a 750kW – 1,425 kWh Li-ion battery system, alongside rooftop solar PV and building energy efficiency measures. The battery system is significantly reducing the site’s electric demand charges and on-peak energy consumption from the utility in a location where electric demand rates are amongst the highest in the country. Utility cost drivers spurred the development of this solution, rather than energy security.
Implementing an energy storage system by itself doesn’t constitute a microgrid as considerations need to be made for generation and controls to satisfy the customer’s transition from utility service to islanded operation and to sustain the site for the expected outage duration.
An energy storage system isn’t an explicit requirement of every microgrid. A site with appropriately sized on-site generation, microgrid controls could achieve its energy security objectives without the added cost of a storage system.
Energy storage should be viewed as a valuable asset that can provide significant added benefits to a microgrid. By providing instantaneous power generation, introducing the ability to absorb and discharge power, and helping to maintain power quality of an islanded system, storage systems can serve as the glue that holds the microgrid together while generating value during normal grid-tied operation. The speed of response allows storage systems to provide benefits some customer generation equipment cannot offer. In conclusion, one solution or component doesn’t suit all circumstances. Facilities considering energy resiliency solutions should look for independent and experienced partners to help assess, design, implement, and operate a successful solution that integrates their existing assets, risk tolerance, mission, and cost objectives.