An essential element of any power system is the energy storage component. Requirements may include providing power for solar or wind-driven applications during times of low sunlight or wind, peak-demand buffering for electrical grids, pulsed load averaging, peak load shaving for consumers, and uninterruptable power for energy surety.
Specific requirements are driven by the application. For example, requirements for transportable systems that might be used at forward operating bases are driven by the need for small size, low weight, moderate energy storage capacities and low deployment costs. Fixed-site and substation installations, on the other hand, may have requirements driven by the need for very large storage capacities, where size and weight are less important.
Although a number of battery and other storage technologies have been in use for decades, storage technologies that can deliver large amounts of energy and high power at reasonable cost have matured to the point where they are commercially available in small quantities. The accompanying figure puts some of these into perspective in terms of power capacity and available run time. These technologies are used in applications such as:
- Power quality: Typically in the range of milliseconds to seconds of discharge time and power levels of 50 kilowatts to 50 megawatts and greater. Stored energy in these applications is required for only seconds or less to assure continuity of quality power and frequency regulation. Technologies that meet this need include flywheels, superconducting magnetic energy storage (SMES), lead acid batteries, lithium ion batteries, flow batteries and ultracapacitors.
- Uninterruptable power supply (UPS) bridging: Typically in the range of seconds to minutes of discharge time and power levels of 5 to 500 kilowatts. Stored energy, in these applications, is used to assure continuity of service when switching from one source of power generation to another. These demands are traditionally met by battery technologies such as lithium ion batteries, lead acid batteries, nickel metal hydride (NiMH) batteries and nickel cadmium (NiCd) batteries.
- Energy management: Typically in the range of hours to days of discharge time and power levels of greater than 1 megawatt. Stored energy in these applications is used to accommodate periodic variation in power-generating capacity, avoid peak demand charges, provide backup during outages, and maintain optimal loading of generators. Technologies to be considered are compressed air energy storage (CAES), pumped-storage hydroelectricity, sodium sulfur (NaS) batteries, advanced absorbent glass mat lead acid batteries, and flow batteries for larger energy systems. Various battery technologies may be considered for smaller applications, especially where mobility is a requirement.
The U.S. Department of Energy has invested substantially in research and development of new storage technologies. This section highlights a few of these and several commercial off-the-shelf (COTS) technologies that are potentially of greatest value to meeting the requirements of the U.S. Department of Defense.
Advanced Absorbent Glass Mat Lead Acid
Lead acid battery storage is one of the oldest and most developed technologies. Its low cost, fast response time, and good round-trip efficiency (75 to 90 percent) make it a popular choice for power quality and UPS applications. Until recently, its utility as an energy storage medium has been limited due to its low cycle life (500 to 700 cycles). However, recent developments are increasing cycle life to more than 4,000 cycles, making these "long life" lead acid batteries good candidates for energy storage applications. Advanced lead acid batteries are being used for power quality in multiple wind farms in Japan, as well as in utility applications in the United States and elsewhere.
Flow batteries consist of electrolyte storage reservoirs that are pumped into and out of cell stacks that consist of two compartments separated by a membrane. The potential between the two different electrolytes generates current. Flow batteries (such as zinc bromine and vanadium redox) are attractive for their low cost (the membranes, cells and electrolytes are composed of plentiful and cheap materials); excellent energy storage capacity; and available power. This makes the flow battery a strong choice for energy management as well as some power quality applications. Round-trip efficiencies vary from 65 to 80 percent.
Lithium ion batteries consist of a lithiated metal oxide (such as LiCoO2 and LiMnO2) cathode, a carbon graphite anode, and a lithium salt plus organic carbonate electrolyte. During charging, the lithium atoms in the cathode are ionized and migrate through the electrolyte toward the carbon anode. The lithium ions combine with external electrons and are deposited between carbon layers as lithium atoms. This process is reversed during discharge. Lithium ion batteries are popular for their high volumetric and gravimetric energy density, relative to other batteries, and have high round-trip efficiencies (85 to 90 percent or more). The drawbacks to lithium ion batteries are their high cost and their inability to store large amounts of energy for stressful operational scenarios with extended durations and many deep cycles. Research efforts are underway to extend the cycle life past the approximately 3,000 deep cycles that currently characterize the technology. These batteries are used to store energy on the DC bus of a hybrid energy storage system. The stored energy can be tapped and converted to either DC or AC, or can be combined with other storage systems in an "islanded" mode, where a portion of the grid-tied load is operating in isolation from the normal power source.
Superconducting Magnetic Energy Storage
SMES operates by storing energy in the magnetic field of a superconducting wire inductor configured into a torus or a solenoid. This technology has high efficiency (greater than 95 percent round trip) and high reliability, and can repeat the charge–discharge sequence hundreds of thousands of times without degrading the inductor. Unlike most other storage technologies, SMES is capable of both fast discharging and charging, which makes it attractive for applications requiring high repetition-rate power delivery. SMES is costly and currently used only for power quality and frequency regulation applications at utilities servicing manufacturing plants that require ultra-clean power. However, the U.S. Department of Energy is funding development of this technology to make a less costly system that is capable of greater energy storage.
Compressed Air Energy Storage
CAES has historically been used by pre-compressing air using low-cost electricity from the grid, and then utilizing that energy plus gas fuel in a surpercharging process that significantly increases the efficiency of the gas-driven turbine engine, resulting in lower overall electrical energy production costs. The compressed air is stored in abandoned underground mines or salt caverns (which take one to two years to create), and the system is capable of storing gigawatt-hours of energy. Renewed interest in CAES is a result of system developments in above-ground compressed air storage (AGCAES), which are being funded by the Department of Energy. These isothermal designs use the compressed air to drive pistons that are coupled to an alternator to generate usable electrical energy. The AGCAES system has the advantage of being able to perform hundreds of thousands of deep cycles, thus making it attractive for long service-life applications.
Flywheel Energy Storage
Flywheel storage systems are kinetic energy reservoirs. Depending on the design, the rotor in a flywheel spins from 5,000 to 50,000 rotations per minute. When power is needed, the rotors release the requested energy by translating their rotational energy via an electric dual function motor-generator into usable electrical energy. Flywheels are similar to SMES due to their: ability to perform rapid charge as well as discharge at high-round-trip efficiencies (85 percent or more); long lifetimes (more than 150,000 full charge and discharge cycles); and favorable power quality and frequency-regulation characteristics.
Other Storage Technologies
Although the majority of applications for Raytheon utilize the technologies outlined above, the following storage technologies are part of the solutions considered when proposing system designs specific to customer applications:
Pumped hydroelectric storage: Over 99 percent of the world's total electrical energy storage capacity is presently in the form of pumped hydroelectric power1; however, this requires specific geographical features and cannot be made portable or installed flexibly, as many customer applications require.
NiCd and NiMH batteries are COTS technologies, but newer storage media are more attractive for the applications considered here. Both battery chemistries have been rendered virtually obsolete by lithium battery technology.
Sodium sulfur (NaS) is a promising near-COTS technology for bulk energy storage that Raytheon is presently investigating for potential applications.
Ultracapacitors have an admirable power capacity and cycle life, but significant advancements are needed to reach energy densities suitable for bulk energy storage.
The following Raytheon applications require energy storage capabilities that span the range of these energy storage technologies for the purposes of maintaining power quality and providing energy surety and continuity.
- Mobile tactical systems: In a tactical environment, power surety is vital to executing the planned missions, where power interruptions could potentially cause catastrophic damage to equipment and personnel, compromising mission success. Flexible systems allow the warfighter to parallel-connect multiple energy storage modules to meet evolving unplanned and emergency demands. Weight, size and portability of the storage modules are significant considerations for systems requiring ease of movement. Storage technologies that can support this in a stand-alone configuration or in a hybrid system coupled to the existing diesel generator include a wide variety of electrochemical batteries such as lithium ion and lead acid, as well as ultracapacitors.
- Small energy grids that employ renewable energy sources: The storage technologies that can meet these needs include lithium ion and lead acid batteries, flow batteries, NaS batteries and, potentially, containerized CAES. For mobile nano and microgrid applications, the power levels are lower and portability becomes a more significant factor. Total ownership cost and utilization of existing inventory are heavily weighted factors in determining the technology solution.
- Naval electric ships, including electrically driven weapons systems, propulsion and distributed zonal power: In electric ships, energy storage will be used in the hybrid electric drive design as backup short-term propulsion. Weapons systems such as the rail gun and free electron laser can benefit from energy storage that effectively averages peak load demands, which reduces the size and number of diesel or turbine generator sets. This results in a significant savings in topside volume, maintenance and fuel. Some of the storage technologies being considered to meet this demanding load averaging requirement include flywheels, batteries and SMES.
- Unmanned vehicles and aircraft that require extended mission durations using a variety of sensor suites: Both anaerobic underwater and in-air unmanned systems are the most constrained systems under consideration, due to gravimetric and volumetric energy density requirements that exceed state-of-the-art capabilities. In order to meet stringent volume and weight requirements of novel power systems architectures for these applications, Raytheon engineers closely monitor emerging energy-storage technologies. Underwater anaerobic systems can potentially use the most advanced batteries, especially those using seawater as the electrolyte to improve weight and volume densities. In-air systems have used fuel cells to increase their mission durations and have optimized their weight and volume densities with the addition of smaller advanced lithium ion batteries to average the peak loads. CAES, SMES, flow batteries, and similar technologies are not presently being considered for these unmanned systems.
There is no single technology that applies universally. The storage selection needs to be made carefully in order to optimize the system it is designed to work within. New energy storage systems are enablers for realizing reduced fuel consumption by capturing surplus renewable energy for synchronized real-time power-combining, and for providing flexible user-configurable energy systems to meet the evolving needs of the military for fixed-base and deployable systems.
1 Source: Energy Storage Systems for Communities, Dan Rastler, Electric Power Institute, Communities for Advanced Distributed Energy Resources (CADER) Conference 2010, April 28–29, 2010, San Diego, Calif.
Peter Morico, Gami Maislin, Ryan Faries