The long and short of it18 May 2001
Energy storage is attracting increasing interest around the world, with a wide range of potential uses.
The potential applications of energy storage technology include:
• Load management
Load management includes the traditional load-leveling application of energy storage, in which energy is stored during off-peak hours (typically at night) and then discharged during peak hours. This not only saves money on the basis of the difference between peak and off-peak rates, but also provides a more uniform load factor for the generation, transmission and distribution systems. Other types of load management are ramping and load-following.
• Remote power
In some remote locations it is not practical to bring power to a site from an established utility grid. Power may be generated from diesel or gas generators, fuel cells or renewable sources. For local load management, it may be useful to include energy storage to minimise the generation capacity.
• Spinning reserve
Most electric utilities operate with a requirement for spinning reserve. This generation is ready, or in “hot stand-by”, should an electric generating unit somewhere on the system fail. The available reserve power is determined by the configuration and mix of unit capacities on the system. Typically, the reserve power must equal the power output of the largest generating unit in operation.
• Renewables matching
Renewables are by their very nature intermittent; often the profile of energy generation does not coincide with the demand cycle. Energy storage can be used to match the output of renewable sources with any load profile.
• Transmission enhancement
Energy storage can improve transmission capacity by providing line stability, voltage regulation, frequency regulation, and VAR or phase angle control. Specialised power electronic equipment must be placed in suitable locations along transmission lines. The amount of energy injected is often small, but at relatively high power.
• Distributed resources
Distribution systems in many growing urban and suburban areas are subject to dramatic day-time peaking. It is often more cost-effective to add distributed energy resources in critical locations than to upgrade distribution wires. Energy storage can be ideal for this application because recharging can take place during off-peak periods.
• Power quality
Utility power sometimes suffers disturbances such as momentary voltage sags or even outages. These events, along with harmonic distortions, and other imperfections can affect sensitive processing equipment that needs extremely clean power to operate properly. Energy storage systems are being successfully installed to provide reliable and high quality power to sensitive loads. Sometimes the systems are coupled directly to the critical equipment, and sometimes to the bus feeding a facility or even on a feeder line.
Although the primary end-use application for energy storage is for power quality, there are other customer uses. These include local peak shaving (to avoid time-of-day charges) and process enhancements (eg, in pulsed power processes, or other specialised industrial applications).
Many electric transit systems could benefit from energy storage because of the highly variable load they create during braking and start-up. Many types of system can store energy during regenerative braking, accepting energy from the propulsion system during deceleration; and then providing a boost during acceleration.
These diverse applications of storage can also be characterised by their technical requirements, ie power level, energy storage capacity, and response time. The energy storage capacity is specifically determined by the time duration required for delivery or discharge. Applications tend to fall into time categories of very short, short, long, and very long.
A recent study* under the auspices of the USDOE Energy Storage Systems Program at Sandia National Laboratories has analysed a range of storage technologies according to performance capabilities, cost projections, availability and advantages v disadvantages. For this study the technologies considered were: batteries (lead–acid and advanced), flywheels (low speed and high speed), supercapacitors, compressed air energy storage, superconducting magnetic energy storage, pumped hydro electric storage, and hydrogen storage.
Batteries (lead-acid and advanced)
Conventional batteries are a well known type of energy storage, most systems being based on lead–acid technology. The market for industrial batteries is estimated at around $5 billion per year.
Advanced technologies have been developed which may have advantages over lead–acid, in terms of performance, handling characteristics, cost, or life time. Two types considered in the Sandia study were zinc/bromine (Zn/Br) and sodium/sulphur (Na/S).
Flywheels (low-speed and high-speed)
Flywheel energy storage systems available today are usually categorised as either low-speed or high-speed. High-speed wheels are made of high strength, low-density composite materials; these systems are considerably more compact than those employing lower-speed metallic wheels. However, the low-speed systems are still considerably less expensive (on a per-kWh basis). Both types were considered in the Sandia study, as both are currently being successfully applied to a variety of stationary applications.
The term “supercapacitor” reflects orders of magnitude of improvement in the energy density of DC capacitors through state-of-the-art selection and processing of electrode materials. They differ from common dielectric capacitors because they store energy in a polarised liquid layer at the interface between a conducting ionic electrolyte and a conducting electrode. Because the capacitance is proportional to the surface area of the electrode, surface area enhancements are provided by using highly porous material. A wide variety of electrolytic solutions and surface treatments are currently being advanced. Many of these products are targeting electric vehicle applications, but are becoming available for higher power stationary applications. Cycle life for supercapacitors is also many times that of conventional capacitors.
Compressed air energy storage (CAES)
CAES systems store energy by compressing air within an air reservoir using a compressor powered by off-peak/low cost electric energy. During charging, the plant’s generator operates in reverse – as a motor – to send compressed air into the reservoir. When the plant discharges, it uses the compressed air to operate the combustion turbine generator. Natural gas is burned during plant discharge, in the same fashion as a conventional turbine plant. However, during discharge, the combustion turbine in a CAES plant uses all of its mechanical energy to generate electricity; thus the system is more efficient.
Compressed air can be stored in several different types of reservoirs: in naturally occurring aquifers (similar to conventional natural gas storage), in solution-mined salt caverns, or in constructed rock caverns. Aquifer storage is by far the least expensive and occurs in the most locations. Both aquifer and salt cavern storage systems are currently being operated at Huntorf, Germany, and at the McIntosh plant for the Alabama Electric Co-op, respectively. CAES is an attractive energy storage technology for large, bulk storage. A 2700 MWe compressed air storage system is at the planning stage in Ohio (see panel, left).
Another approach to compressed air storage is also under development. Referred to as CAS (compressed air storage in vessels) in the Sandia report, fabricated high-pressure tanks are used as the reservoir. Because of the expense of such tanks, only several hours worth of storage has been proposed for this concept.
Superconducting magnetic energy storage (SMES)
In SMES, energy is stored in the magnetic field produced by current circulating through a superconducting coil. The system is efficient because there are no resistive losses in the superconducting coil and losses in the solid state power conditioning are minimal. Like a battery, a SMES provides rapid response for either charge or discharge. Unlike a battery, the energy available is independent of the discharge rate. The interaction of the circulating current with the magnetic field produces large forces on the conductor. In a small magnet, these forces are easily carried by the conductor itself. In a large magnet, a support structure must be provided either within the coil windings or external to the coil to carry these loads.
Today’s SMES units use conventional metallic superconductor material (Nb-Ti or Nb3Sn) cooled by liquid helium for the coil windings. High temperature ceramic superconductors (HTS) cooled by liquid nitrogen are now being used in the power leads that connect the coil to the ambient temperature power conditioning system. Complete coil and lead designs based on HTS materials are in development because the refrigeration requirement is significantly reduced.
In the Sandia study, SMES systems are considered in three sizes: micro SMES (4 MJ or less), mid-SMES (up to 20 MWh), and SMES (up to 5000 MWh). Each size category differs in both design and cost from the others because of significant non-linearities in stored energy scaling over orders of magnitude.
Pumped hydro electric storage
In pumped storage, generation and pumping can be accomplished either by single-unit, reversible pump-turbines, or by separate pumps and turbines. Mode changes between pumping and generating can occur within a period of minutes, and up to 40+ times daily. Pumped storage facilities have operated in the USA since the late 1920s. Within the last ten years, advanced pumped storage (APS) technology has been developed to improve speed, reliability and efficiency. These plants are designed hydraulically and mechanically for ultrafast loading and ramping, allowing frequent and rapid (under 15 sec) changes between the pumping, generating and stand-by spinning modes.
Hydrogen is not a primary energy source. Like electricity, it is an energy carrier between various sources and end uses. When used for energy storage, hydrogen is a fuel, storing energy in its chemical potential. Power is generated from hydrogen either by conversion in a fuel cell, or by combustion in an internal combustion or turbine engine. In this report, a hydrogen fuel cell system at both “high” and “low” cost projections is compared with all the other energy storage technologies. The hydrogen-fueled combustion engine is compared only with the fuel cell. Hydrogen can be stored in many configurations: as compressed gas in tanks, in underground reservoirs, or in tiny microspheres; as a (cryogenic) liquid; in hydride compounds; or in other chemical forms. The various storage types have different characteristics, some of the most important ones being energy density and cost. For the purposes of the Sandia report, the primary storage form considered was as compressed gas in high pressure tanks, although hydride storage should eventually be comparable in cost. Only for very long duration applications (requiring large storage volume) is underground storage considered.
Comparing the technologies
In the Sandia study the various technologies were compared in terms of the following characteristics: capital cost (balance of plant, energy-related, power-related); operating features (efficiency, O&M costs, cycle or shelf life); other technology-specific costs (parasitics, replacement); size; and siting issues (environmental, safety, other features).
The costs and efficiencies used in the Sandia study are listed in Table 1. Most of these were developed through discussions with vendors while others were found in the literature.
The technologies can be matched to applications in a variety of ways. Certainly cost can be a deciding factor; but the performance must also meet the application requirements. The most important characteristics are power, stored energy, and response time. If a technology cannot provide all of these characteristics, it is not suited to the application. Figure 1 shows energy storage technologies plotted by characteristics of power delivered and energy stored. Overlaid on the chart are lines indicating discharge times: 1 sec, 1 min, 1 hr. Take note that the plot is log-log and covers a very wide range of time scales. Some general application areas are indicated: eg, power quality, load management, distributed resources.
Some general conclusions from Figure 1 are:
• Supercapacitors are best suited for smaller applications, ie, end-use.
• Batteries and SMES cover the broadest range of applications, from less than a MW to thousands of MW.
• For very high power and energy applications, only a few technologies are suitable – CAES and pumped hydro.
Figure 2 indicates typical response times for the various technologies. Those with solid state power conversion interfaces can often respond at sub-cycle rates, assuming they are on “stand-by.” Those with mechanical inertia, such as air or water turbines, require longer start-up or response time. Most fuel cell systems also require warm-up or flow time, but recent advances are making quick-response fuel cells available as well.
Using a specially developed methodology, the Sandia study calculates the capital costs of the various technologies for a variety of applications. After considering the performance fit and capital costs, an assessment was made of the technologies that best fit the various applications. Table 2 summarises applications and appropriate technologies.
The Sandia study confirms that the various energy storage technologies serve some applications better than others. Distinctions can be made on the basis of long- or short-term storage (or discharge duration), size (power level), response time, and also on the basis of cost. The Sandia study concludes, among other things, that:
• Flywheels are a good match for a range of short-term applications up to a size of several MW.
• Batteries currently have the broadest overall range of applications.
• Fuel cells should be applicable and cost effective in a very broad range of applications in the future.
• Hydrogen-fueled combustion engines are a currently available technology for short-term applications including distributed peaking, renewables matching, and spinning reserve.
• CAES and pumped hydro are best for load management when geology is available and response time in the order of minutes is acceptable.
• SMES is a niche technology for power quality and especially high power distribution or transmission networks. Projected costs for bulk storage, however, show it to be expensive.
Putting a value on storage
The value placed on storage is very application dependent. It is particularly attractive where there are multiple benefits (including equipment procurement deferrals). Power quality issues become important in the case of high value product and a sensitive manufacturing process. Energy applications depend on generation or transmission shortages and bottlenecks or price volatility. To assign a value to storage requires detailed investigation of each specific case. The wide variations are shown in Table 3, which summarises system costs for some actual energy storage projects.
Included here is an estimate for the Regenesys regenerative fuel cell a promising new technology, which is a form of flow battery. Flow batteries, including Regenesys, are included in Figure 3, which compares them with other technologies in terms of discharge period vs power rating. The first full scale Regenesys plant is currently under construction in the UK, while a second is planned for a TVA site, which will serve as the American reference plant. Regenesys is capable of 1/4 cycle response time and could be available in sizes from 5 to 500 MW. It is an example of a storage technology with multiple potential benefits and a variety of potential value streams for the end-user.
Finally, Figure 4 shows the technologies in terms of commercial maturity.
TablesTable 1. Costs and efficiencies for energy storage technologies included in the Sandia study Table 2. Applications and appropriate storage technology, as determined by the Sandia study Table 3. Summary of some energy storage system costs (adjusted to year 2000 US$) - source J Boyes