Transmission and distribution (T&D) organizations within electric utilities are transitioning to battery-based grid energy storage solutions. As a result, these groups are reporting that the applications for this “reservoir of electrons” reaches far beyond the grid applications that they are now busy solving. Even regional energy system operators such as independent system operators (ISOs), distribution system operators (DSOs) and transmission system operators (TSOs) are implementing these advanced battery energy storage systems as a means of providing stability and reliability within their respective energy system service territories.
Batteries offer long-term energy storage and are the dominant element in the energy-only needs of the grid. But surprisingly, battery technology – including lithium-ion and other new chemistries – offer very low power density and often do not offer all of the energy and power capacity that is needed for the integrity of T&D grid operation. Moreover, most electrochemical-based batteries lose the ability to cycle and store energy over its lifetime. These batteries are limited to an average lifespan of approximately 5,000 to 10,000 charge-discharge cycles, which is dependent upon the battery depth of discharge, frequency of discharge-recharge cycles and the time occurrence of repetitive and rapidly occurring grid events.
A hybrid approach
To bridge this energy and power density gap, utilities are turning to a single container energy storage hybrid approach to meet both the energy and power grid needs. These systems include both battery and ultracapacitor (also known as supercapacitor) elements. Given ultracapacitors’ electrostatic (vs. electrochemical) mechanism, these electronic devices deliver massive amounts of power measured in hundreds of kilowatts or megawatts. They can be discharged in cycles and recharged in seconds instead of minutes or hours, unlike their battery counterparts. The delivered and available power density of ultracapacitors is a factor of 1,000 W/kg greater than batteries, and can deliver power to the grid when needed as opposed to catching the battery system output at the right time following a discharge-recharge cycle. Hybrid battery-ultracapacitor energy storage systems are now being constructed to deliver specified amounts of energy and power to account for megawatts of power delivered by the ultracapacitors, as opposed to megawatt-hour-centric energy delivered by batteries alone.
As utilities apply the new “how does it measure up” analysis to these hybrid battery-ultracapacitor systems, utility executives and project managers are especially emphasizing the on-call versatility of the energy storage system deployed. To actively and successfully compete in the business eye of the utility, battery energy storage systems are migrating to stacked and often-simultaneous use of utility applications, including specific benefit-to-cost business cases for multi-functional grid storage systems. As utilities and regional operators have an insatiable appetite for storage cost effectiveness, the stacking of energy storage system applications in a grid deployment application may eventually spell a reduction in the single use energy storage systems currently deployed.
Utility Duke Energy’s battery-ultracapacitor system
In North America, Duke Energy commissioned a stacked, dual application and simultaneous use battery-ultracapacitor system earlier this year. This hybrid system leverages ultracapacitors to perform solar smoothing tasks at the distribution level in real time – particularly when the solar power on the grid fluctuates due to cloud cover, atmospheric conditions or as unforeseen PV array and system circumstances may develop. While the ultracapacitors are performing distribution-level real-time solar smoothing, the batteries are simultaneously tasked to perform time-energy shifting of a megawatt-scale solar system on the distribution circuit feeding the substation.
This system is delivering an intelligent combination of high power and fast response with the ultracapacitors. The same system is also combined with energy dense batteries to maximize utility system value. By offering dual and “simultaneously occurring” multiple grid services at a lower system cost than batteries alone, this hybrid system optimizes the benefit-to-cost ratio and delivers results at the utility substation level. The fast-responding ultracapacitors will also significantly extend the battery life by relieving the battery system from repetitive charge-discharge cycling.
Battery - ultracapacitor system electrical architecture overview. Image: Maxwell Technologies.
In Europe, Ireland remains a leading test bed for advanced energy storage systems. A microgrid stabilisation system was installed in a smart grid testbed. Tests have been carried out to demonstrate the capabilities of the system. Based on field-testing and measurements, demonstration of fast (high speed) frequency response and rate of change of frequency response indicate astounding speed. The results demonstrate that ultracapacitors, combined with advanced power electronics, provide “synthetic inertia” well within 20 milliseconds of frequency events and that the active power response can be provided proportional to the severity (steepness of ramp) of the frequency event.
At an unspecified American western utility, an ultracapacitor-only system is currently being commissioned for microgrid stabilisation to provide system reliability to a remote community served by the utility. Within this microgrid, ultracapacitors, power electronic conversion, internal microgrid and utility communications and control, and internal control systems are housed in a standard 20-foot container. With the comparatively high cycle life of ultracapacitors, this system is anticipated to perform well over 500,000 charge-discharge cycles.
Batteries are the dominant energy storage solution used for grid storage applications. But will technical performance limitations and newly developing utility requirements force a change to these hybrid battery and ultracapacitor only systems? To be sure, battery charge and discharge is a comparatively slow process to the actual needs of an operating grid. Newly developing grid applications including renewables smoothing, fast responding frequency regulation, rapid power delivery to stabilize T&D systems, fast ramping and rapidly occurring T&D voltage sags and other power quality issues, are now dominating utility thought processes as opposed to one-way centralized power generation and single action battery storage. Add on the uncharted waters of distributed and renewable generation system anomalies and utilities are revising their short- and long-term T&D planning and integrated resource plan processes to include both power and energy needs of the grid in the utility “new think”.
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