Energy storage plays a key role in microgrids

Microgrids continue to gain popularity in the built environment due to improved power reliability and quality, increasing system energy efficiency, resiliency from grid anomalies and financial benefits. Energy storage systems play an integral part of most microgrids. An energy storage system combined with a solar array or wind turbine managed by a specialized control system form the most common arrangement of a renewable microgrid.

The Department of Energy defines the microgrid as ‘‘a group of interconnected loads and distributed energy resources within clearly defined electrical boundaries that acts as a single controllable entity with respect to the grid. A microgrid can connect and disconnect from the grid to enable it to operate in both grid-connected or island mode.”

Energy storage technology

A microgrid using renewable energy such as photovoltaics and wind power as its primary source of energy often includes dispatchable stored energy to balance the difference between the load and the intermittent energy produced by the renewable source. Up until now, most stored energy has been in the form of carbon-based fuels that are converted to electricity on demand via equipment like diesel generators.

However, due to the worldwide push for reduced carbon emissions, battery energy storage systems are becoming more commonplace. The market-leading technology for battery energy storage to date is lithium-ion.

Lithium-ion technology has advanced rapidly due to investment into the product, primarily for electric vehicles and consumer electronics. Lithium-ion batteries have the benefit of being high power density and high efficiency.

However, they suffer from limited lifetime, limited operating temperature ranges for efficient operation and flammability. In addition, end-of-life recycling programs are currently limited, which may result in a flood of undesirable waste in the future.

Up-and-coming challengers to lithium-ion batteries include vanadium and saltwater batteries. These technologies are based on liquid electrolytes. Vanadium and saltwater batteries offer benefits of over lithium-ion technology including unlimited lifetimes (with respect to discharge cycles), a wider range of operating temperatures, use materials which are more common on earth and are less flammable. The vanadium and saltwater technologies are both less power dense than lithium-ion. This is not as big of an issue in stationary applications than for mobile applications such as consumer electronics and automobiles.

Aside from battery storage, there are various other storage mechanisms available or in development. Hydrogen by way of electrolysis is another emerging technology that has potential for microgrids. Hydrogen has seen limited, but increasing, application in the automobile industry. Mechanical-based solutions, such as flywheels and gravity storage, are additional options for energy storage in a microgrid application. These solutions can be difficult to implement because they commonly require specific-site considerations.

Figure 3: The energy curve for this control strategy reduces the need additional energy sources. Courtesy: DLR Group[/caption]

Control system strategy

The applicable control system strategy for an energy storage system as part of a microgrid or as a standalone energy resource depends on the desired performance for the installation. The development of the control system strategy defines how the energy storage system is used.

There are three common control strategies for energy storage systems:

• Power replacement.
• Peak shave.
• Cost transfer.

For each of these strategies, the control system is programmed to deploy the energy storage in lieu of another power resource to achieve the desired outcome.

The power replacement strategy can be defined in at least two operating characteristics. For the first example, assume the microgrid has a solar array and an energy storage system and an operating parameter that requires a minimum level of power production during the day. The microgrid can be grid-coupled or island type. An energy storage system can supplement the solar array during a cloud coverage or other short-term event that may affect the performance of the solar system. Then, the energy storage system can recharge when the solar array is operating at peak performance.

In another example, assume the microgrid has a solar array, an energy storage system and another power generating resource like a fuel cell or diesel generator. In this case, the operating characteristic is to minimize of the operating time of the fossil-fuel powered energy resource. The control strategy would deploy the energy storage resource as a supplement to the solar resource at the beginning and end of the daylight hours when the solar production is reduced. This approach reduces the need for the other energy resources. The energy storage system can recharge when the solar array is operating at peak performance.

The size of the solar array and energy storage system must be made with the control strategy in mind for maximum recharge of the storage system with renewable resources and to avoid use of fossil fueled power generating equipment.

Peak shave strategy is based on the premise that most commercial utility tariffs base their rates on energy consumed (measured in kilowatt-hours) and energy demand (measured in kilowatts). The rate structures of demand charges widely very depending on the utility and type of electrical service. The overall concept is the same — there is an added charge for the peak energy consumed during a defined period. The cost associated with energy demand can be much as 50% of the overall utility bill for a commercial facility. Energy storage systems can be deployed to minimize the peak demand and avoid the associated costs.

The peak shave strategy is most applied to grid-coupled energy storage systems.

Cost transfer strategy has some similarities to the power replacement strategy. For this example, assume a grid-coupled microgrid with a solar array, energy storage system and another power generating resource like a fuel cell or diesel generator. The utility rate tariff has a much higher cost of energy in the overnight hours compared to daytime. The fossil-fueled energy resources are intended to be a backup to the renewable resources. For this strategy, the energy storage system is designed for hours of deployment to meet the needs of the facility during the overnight hours. The energy storage system is recharged during the day using the solar array or the less expensive grid purchased power. The overall impact is avoiding the purchase of the more expensive grid power (or operating the fossil-fueled energy resource) during the overnight hours.

The size of the solar array and energy storage system must be made with the control strategy in mind so maximum recharge of the storage system with renewable resources and avoid use of fossil fueled power generating equipment.

Figure 5: Shown is the energy curve for the cost transfer control strategy. Courtesy: DLR Group[/caption]

Design challenges

Designing microgrids does not come without challenges. A short-term outage between 50 and 100 milliseconds is common for microgrid systems designed to operate both connected to the grid or as an island completely disconnected from the grid. This is caused by the system going from “grid following,” in which the grid is establishing the frequency and voltage, to “grid forming” in which the control system and master inverter establishes the frequency and voltage for the rest of the system to follow. During this switchover, power inverters may be down for as much as five minutes resulting in a need for separate uninterruptible power supplies if critical loads exist.

Battery storage based microgrids also inherently do not have inertia, meaning that they are not very tolerant to in rush currents. This needs to be considered, especially with microgrid systems during power switchover or startup to ensure that all loads do not try to start up simultaneously.

As microgrids are still in their infancy, product limitations must also be considered during design. Most available energy storage systems on the market are currently targeting peak shaving applications and thus the ratio of maximum kilowatt output to kilowatt-hour is typically two and no larger than four hours. A microgrid may want to have more than four hours of energy storage, in which case the inverter will likely be sized much larger than it needs to be. This may trickle down to the rest of the electrical distribution system, requiring upsized equipment and greater cost.

Finally, most energy storage systems, do not carry life safety listings. This results in additional equipment for life safety loads, generator or UL 924: Standard for Emergency Lighting and Power Equipment battery inverter that exist in parallel to the microgrid equipment. Code requirements for operation of emergency systems will also dictate requirements of the system design and sequence of operations. It is likely that energy storage systems will eventually be categorized as life safety systems, which will simplify the designs in the future.

The worldwide electrification of power sources combined with the demand to minimize carbon emissions has created a new set of challenges for engineers to solve. The development of creative solutions like microgrids and advancement in technologies used in batteries, photovoltaic modules and other power generating sources are providing the tools to meet these challenges.

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