Many storage technologies have been considered in the context of utility-scale energy storage systems. These include:
- Pumped Hydro
- Batteries (including conventional and advanced technologies)
- Superconducting magnetic energy storage (SMES)
- Fuel Cell/Electrolyser Systems
- Conventional Capacitors
Each technology has its own particular strengths and operational characteristics. For example, pumped hydro is best suited for large-scale bulk electrical energy storage (if suitable geographic topology, geology and environmental conditions exist). Pumped hydro generating stations have been built capable of supplying 1800MW of electricity for four to six hours.
This CTW description focuses on Superconducting Magnetic Energy Storage (SMES). This technology is based on three concepts that do not apply to other energy storage technologies (EPRI, 2002). First, some materials carry current with no resistive losses. Second, electric currents produce magnetic fields. Third, magnetic fields are a form of pure energy which can be stored.
SMES combines these three fundamental principles to efficiently store energy in a superconducting coil. SMES was originally proposed for large-scale, load levelling, but, because of its rapid discharge capabilities, it has been implemented on electric power systems for pulsed-power and systemstability applications (EPRI, 2002).
Figure 1 is an illustration of a commercially produced SMES product. The individual, trailer-mounted Distributed-SMES units consist of a magnet that contains 3 MJ of stored energy (EPRI, 2002).
The combination of the three fundamental principles (current with no restrictive losses; magnetic fields; and energy storage in a magnetic field) provides the potential for the highly efficient storage of electrical energy in a superconducting coil. Operationally, SMES is different from other storage technologies in that a continuously circulating current within the superconducting coil produces the stored energy. In addition, the only conversion process in the SMES system is from AC to DC. As a result, there are none of the inherent thermodynamic losses associated with conversion of one type of energy to another (EPRI, 2002).
The original development of SMES systems was for load levelling as an alternative to pumped hydroelectric storage. Thus, large energy storage systems were considered initially. Research and then significant development were carried out over a quarter century, beginning in the early 1970s. In the U.S., this effort was mainly supported by the Department of Defense, the Department of Energy, and Electric Power Research Institute (EPRI). Internationally, Japan had a significant program for about 20 years, and several European countries participated at a modest level.
At several points during the SMES development process, researchers recognized that the rapid discharge potential of SMES, together with the relatively high energy related (coil) costs for bulk storage, made smaller systems more attractive and that significantly reducing the storage time would increase the economic viability of the technology. Thus, there has also been considerable development on SMES for pulsed power systems.
Duue to its rapid discharge capabilities the technology has been implemented on electric power systems for pulsed power and system stability applications. The discharge capabilities of SMES compared to several other energy storage technologies is illustrated in Figure 2.
While SMES currently is only applied in small scale system stability applications, there are several design and development programs for large-scale SMES plants. This description also briefly covers these design and development programs, even though they mostly stem from the 1970s and early 1980s.
Independent of capacity and size a SMES system always includes a superconducting coil, a refrigerator, a power conversion system (PCS), and a control system as shown in Figure 3. Each of these components is discussed in this section. This section also covers the technical attributes of SMES.
The Coil and the Superconductor
The superconducting coil, the heart of the SMES system, stores energy in the magnetic fieldgenerated by a circulating current (EPRI, 2002). The maximum stored energy is determined by two factors: a) the size and geometry of the coil, which determines the inductance of the coil. The larger the coil, the greater the stored energy; and b) the characteristics of the conductor, which determines the maximum current. Superconductors carry substantial currents in high magnetic fields (EPRI, 2002).
All practical SMES systems installed to date use a superconducting alloy of niobium and titanium (Nb-Ti), which requires operation at temperatures near the boiling point of liquid helium, about 4.2 K (-269°C or -452°F) – 4.2 centigrade degrees above absolute zero. Some research-based SMES coils use high-temperature superconductors (HTS).However, the state of development of these materials today is such that they are not cost effective for SMES.
Since only a few SMES coils have been constructed and installed, there is little experience with ageneric design. This is true even for the small or micro-SMES units for power-quality applications, where several different coil designs have been used. A primary consideration in the design of a SMES coil is the maximum allowable current in the conductor. It depends on: conductor size, the superconducting materials used, the resulting magnetic field, and the operating temperature. The magnetic forces can be significant in large coils and must be reacted by a structural material. The mechanical strength of the containmentstructure within or around the coil must withstand these forces. Another factor in coil design is the withstand voltage, which can rangefrom 10 kV to 100 kV (EPRI, 2002).
The superconducting SMES coil must be maintained at a temperature sufficiently low to maintain a superconducting state in the wires. As mentioned, for commercial SMES today this temperature isabout 4.5 K (-269°C, or -452°F).
Reaching and maintaining this temperature is accomplished by a special cryogenic refrigerator that uses helium as the coolant. Helium must be used as the so called"working fluid" in such a refrigerator because it is the only material that is not a solid at these temperatures. Just as a conventional refrigerator requires power to operate, electricity isused to power the cryogenic refrigerator (EPRI, 2002). As a result, there is a tremendous effort in the design of SMES and other cryogenic systems to lower losses within the superconducting coils and to minimize heat flow into the cold environment from all sources.
The refrigerator consists of one or more compressors for gaseous helium and a vacuum enclosure called a “cold-box”, which receives the compressed, ambient-temperature helium gas and produces liquid helium for cooling the coil (EPRI, 2002).
Power Conversion System
Charging and discharging a SMES coil is different from that of other storage technologies. The coil carries a current at any state of charge. Since the current always flows in one direction, the power conversion system (PCS) must produce a positive voltage across the coil when energy is to be stored, which causes the current to increase. Similarly, for discharge, the electronics in the PCS are adjusted to make it appear as a load across the coil. This produces a negative voltage causing the coil to discharge. The product of this applied voltage and the instantaneous current determine the power.
SMES manufacturers design their systems so that both the coil current and the allowable voltage include safety and performance margins. Thus, the PCS power capacity typically determines the rated capacity of the SMES unit (EPRI, 2002). The PCS provides an interface between the stored energy (related to the direct current in the coil) and the AC in the power grid.
The control system establishes a link between power demands from the grid and power flow to and from the SMES coil. It receives dispatch signals from the power grid and status information from the SMES coil. The integration of the dispatch request and charge level determines the response of the SMES unit. The control system also measures the condition of the SMES coil, the refrigerator, and other equipment. It maintains system safety and sends system status information to the operator. Modern SMES systems are tied to the Internet to provide remote observation and control.
Figure 3 illustrates how these components are connected in a SMES system.
Technical attributes of SMES
The following four technical attributes of SMES are discussed in this section: a) capacity of a SMES system; b) the energy storage rating; c) the physical dimensions; and d) the efficiency of a SMES system. Information derived from the 2002 EPRI study.
The power capacity for a SMES system is dictated by the application, e.g., power quality, power system stability, or load leveling. In general, the maximum power capacity is the smaller of two quantities: either the PCS power rating or the product of the peak coil current and the maximum coil withstand voltage. The capacities of existing individual micro-SMES installations range from 1 MW to about 3 MW.
b) the energy storage rating
The stored energy in the SMES plant depends on the requirements of the application. It is the product of the power capacity and the length of time the installation is to deliver this power.
c) physical dimensions
The physical size of a SMES system is the combined sizes of the coil, the refrigerator and the PCS. Each of these depends on a variety of factors. The coil mounted in a cryostat is often one of the smaller elements. A 3 MJ micro-SMES system (coil, PCS, refrigerator and all auxiliary equipment) is completely contained in a 40-ft trailer.
d) efficiency of the system
The overall efficiency of a SMES plant depends on many factors. In principle, it can be as high as 95 % in very large systems. For small power quality systems, on the other hand, the overall system efficiency is less. Fortunately, in these applications, efficiency is usually not asignificant economic driver. The SMES coil stores energy with absolutely no loss while the current is constant. There are, however, some losses associated with changing current during charging and discharging, and the resulting change in magnetic field. In general, these losses, which are referred to as eddy current and hysteresis losses, are also small.
Unfortunately, other parts of the SMES system may not be as efficient as the coil itself. In particular, there are two potentially significant, continuous energy losses, which are application specific:
1) The first is associated with the way SMES systems store the energy. The current in the coil must flow continuously, and it circulates through the PCS. Both the interconnecting conductors and the silicon-based components of the PCS are resistive. Thus, there are continuous resistive losses in the PCS. This is different from batteries, for example, where there is current in the PCS only during charge and discharge.
2) The energy that is needed to operate the refrigerator that removes the heat that flows to the coil from room temperature via: a) conduction along the mechanical supports, b) radiation through the vacuum containment vessel, and c) along the current leads that extend from ambient temperature to the coil operating temperature.
The overall efficiency of a SMES plant depends on many factors. Diurnal (load-leveling) SMES plants designed 20 years ago were estimated to have efficiencies of 90 to 92%. Power quality and system stability applications do not require high efficiency because the cost of maintenance power is much less than the potential losses to the user due to a power outage. Developers rarely quote efficiencies for such systems, although refrigeration requirements are usually specified. A3 MJ/3 MW micro-SMES system, for example, requires about 40 kW of continues refrigeration power.
In Table 1 an illustration of the development status of several key energy storage technologies is given. As can be seen, SMES is a technology that is still largely in the developmental stage. Especially for grid applications, SMES technology is a long-term technology.
|Commercial||Pre-Commercial Prototype||Demonstration stage||Developmental stage|
Flywheels for power quality
applications at the
Flywheel (as load device)
micro- SMES (as load device)
Zinc- bromide battery
Flywheel (as grid device)
Lithium-ion battery for grid applications
Other advanced batteries
The technology status of SMES can be differentiated between three versions of the technology. As can be seen in Table 1, a difference occurs between the application of SMES between a grid device or a load device. In Table 2, this differentiation is illustrated between micro-SMS, Distributed-SMES and SMES for load levelling which is a large scale application. Micro-SMES have been installed around the world in mostly industrial settings to control voltage sag problems on the electrical grid. Distributed SMES is a somewhat larger scale application. Large scale applications for load levelling are currently only in the design phase.
|Application||Micro SMES for power quality||Distributed SMES for system stability||SMES for load levelling|
|Status||Commercial: several units installed||Demonstration||Theoretical|
|Lessons learned||Critical issues in terms of the power output and response time.||Additional information is required.||Long-term development and societal commitment is required for large systems that cost over a billion dollars |
and take more than ten years to complete.
|Major development trends||American Superconductor has several units in the field at this time.||American Superconductor is prepared to deliver additional units and is actively |
searching for customers
|Unresolved issues||Costs of SMES units||Cost effectiveness of this application compared to other solutions.||Costs, and costs compared to other load leveling technologies|
Potential of SMES
SMES has the potential to provide electrical storage to a majority of the applications. However, this technology is still emerging, and more R&D will be needed to make SMES competitive in a wide variety of utility storage markets (Butler, Miller and Taylor 2002).
Three energy market support contributions of this technology are identified in the 2002 EPRI study:
a) System stability and damping
Large power systems may experience instabilities associated with the delivery of power over long distances when there are abrupt changes in operating conditions. Such instabilities can cause high economic damage to installations. An example of such a instability is the north-south power corridor on the West Coast of the United States. A great deal of power is generated in the Pacific Northwest and is delivered to middle and Southern California via multiple transmission lines. Oscillations in power through this corridor are generally insignificant but can be, under certain conditions, damaging. The conventional method to address this issue is to install additional transmission capacity in order to reduce the sensitivity. Therefore, stability can be achieved by adding additional transmission lines, but this is at the expense of a variety of other issues such as environmental degradation.
Renewable energy technologies are often located far from the location where the electricity is required. This is for instance the case with large scale solar and wind farms. To maintain system stability without energy storage with a high discharge rate, implementing additional transmission lines would be necessary.
Concluding: system stabilization, as provided by the distributed form of SMES, provides benefits primarily through the avoidance or deferral of new transmission requirements. In other words, the higher reliability of the system removes the need to construct additional transmission lines. As such, SMES is a technology that supports the deployment of renewable energy technologies.
b) Power quality
Power quality and back-up power is necessary at industrial installations substations of the electricity grid. The power quality and back-up energy is used for a variety of conditions such as when momentary disturbances require real power injection to avoid power interruptions. In the case of industrial customers, a local source of power may be required when there is an interruption of power from the utility. This power source may need to function until power from the utility is restored, until a reserve generator is started, or until critical loads are shut down in a safe manner. As such, SMES technology can provide this role of power quality and back-up power source. In the case of a substation, a variety of momentary disturbances such as lightning strikes or transmission flash overs cause power trips or low voltages.
Therefore: power quality benefits result primarily from reliable service. Issues such as outages and voltage sag can be avoided which can have high economic benefits.
c) Load levelling
Demands for electric power vary both randomly and with predictable variations. Perhaps the most significant variation of power demand is the diurnal change associated with the functioning of an industrial society. Both commercial and residential demands are greater during the day than at night. On the other hand, many power plants operate most efficiently and have longer lives if they operate continuously near their maximum power output. One method of accommodating users’ power demands and the characteristics of these plants is to install an energy storage system that can accept energy at night and can deliver it back to the grid during periods of high demand. The value of this type of storage is based on the difference in marginal cost of off-peak power and the price paid for power during the peak. An additional impact of diurnal storage is that it can replace or defer the installation of extra generation capacity to accommodate.
Therefore: load- levelling benefits, or arbitrage benefits, result from the differential between the cost of on-peak and off-peak power.
In a Sandia Report of 2003 Schoenung and Hasselzahn (2003) calculate the financial aspects related to SMES technology compared to several other energy storage technologies. However, since SMES on a large scale is not (yet) available, the study focuses on micro-SMES in the power quality application.
Regarding power quality, micro- SMES is one of the cheaper technologies as long as the required discharge time is small. Longer discharge times considerably raises the cost of micro-SMES technology for this application.
The costs spread out over the different components shows that no replacement costs or fuel costs are associated with micro-SMES. The main costs for a micro-SMES installation are capital costs associated with the superconducting coil and the cryogenic refrigerator. Additionally, since the superconductor is one of the major costs of a superconducting coil, one design goal is to store the maximum amount of energy per quantity of superconductor. Many factors contribute to achieving this goal. One fundamental aspect, however, is to select a coil design that most effectively uses the material. This is generally accomplished by a solenoidal configuration.
Over a longer discharge time, it can be seen that the annual costs over the components of a micro-SMES installation raise sharply. This is due to the fact that a battery sized for one-second of discharge at a certain capacity is the same as a battery sized for 20 or 30 seconds of discharge. This is not true for micro-SMES or supercapacitors. The comparison between the technologies therefore changes dramatically over longer discharge times.
Therefore, for power quality applications, the best choice is strongly dependent on the discharge time required. For a one- to two-second discharge, SMES and supercapacitors are attractive, whereas at 20- to 30-seconds, some flywheels or battery systems are less expensive (Schoenung and Hasselzahn, 2003).
APS, 2007. Challenges of Electric Energy Storage Technologies: A Report from the APS Panel on Public Affairs Committee on Energy and Environment. Document can be found online at: www.aps.org/policy/reports/popa-reports/upload/Energy_2007_Report_ElectricityStorageReport.pdf 
EPRI, 2002. Handbook for Energy Storage for Transmission or Distribution Applications. Report No. 1007189. Technical Update December 2002. Document can be found at: www.epri.com 
Schoenung, S., M., & Hassenzahn, W., V., 2002. Long- vs Short-Term Energy Storage Technology Analysis: A life cycle cost study. A study for the Department of Energy (DOE) Energy Storage Systems Program. Document can be found online at: http://www.ntis.gov/help/ordermethods.asp?loc=7-4-0#online 
Butler, P., Miller, J. L., Taylor, P. A., 2002. Energy Storage Opportunities Analysis Phase II Final Report A Study for the DOE Energy Storage Systems Program. Document can be found online at: prod.sandia.gov/techlib/access-control.cgi/2002/021314.pdf