05/04/10 SMES Superconducting Magnetic Energy Storage SHRADHA LAMA (B070403EE) TRISHA SINGH (B070309EE)
A Superconducting Magnetic Energy Storage (SMES) system is a device for storing and instantaneously discharging large quantities of power. These systems have been in use for several years to solve voltage stability and power quality problems for large industrial customers. SMES Superconducting conductors
Superconducting Magnetic Energy Storage (SMES) systems store energy in the magnetic field created by the flow of direct current in a superconducting coil which has been cryogenically cooled to a temperature below its superconducting critical temperature. INTRODUCTION 2 MJ SMES
The main characteristics of SMES: High power density but rather low energy density (more a power source than an energy storage device). Very quick response time. Number of charge-discharge cycle very high (infinite). No moving parts / low maintenance. Fast recharge possible. High energy conversion efficiency (> 95 %).
The SMES (Superconducting Magnetic Energy Storage) is one of the very few direct electric energy storage systems. A SMES releases its energy very quickly and with an excellent efficiency of energy transfer conversion (greater than 95 %). The three main applications of SMES are UPS (Uninterruptible Power Supply), FACTS (Flexible AC Transmission System) and pulse power sources for dedicated applications.
The SMES system consists of four main components or subsystems: Superconducting magnetwith its supporting structure. Cryogenic system (cryostat, vacuum pumps, cryocooler, etc.). Power conditioning system (interface between the superconducting magnet and the load or electric grid). - Control system(electronics, cryogenics, magnet protection, etc.) A rectifier/inverter, a power electronic circuit, is typically part of the power conditioning system, as required to convert the direct current (DC) of the superconducting coil to alternating current (AC) and vice versa since the very large majority of the grids operate in AC.
Schematic diagram of SMES connected to electric AC grid Superconducting magnet with shorted input terminals stores energy in the magnetic flux density (B) created by the flow of persistent direct current: the current remains constant due to the absence of resistance in the superconductor. The stored energy (Wmag) is given by the self inductance (L) of the coil and by its current (I): Wmag = ½ LI2
Schematic diagram of SMES connected to electric AC grid When the short is opened, the stored energy is transferred in part or totally to a load by lowering the current of the coil via negative voltage (positive voltage charges the magnet). The Superconducting Magnetic Energy Storage (SMES) is thus a current source. It is the “dual” of a capacitor, which is a voltage source.
D-SMES A Distributed-SMES (D-SMES) system represents an innovative new application of proven SMES technology, enabling utilities to improve system-level reliability and transfer capacity. D-SMES provides cost-effective grid stabilization for entire electric utility systems .
D-SMES systems are employed by electric utilities and are connected to their grids at substations. D-SMES is a shunt connected Flexible AC Transmission (FACTS) device designed to
Rather than protecting one individual customer, a D-SMES system consists of a number of SMES units placed at strategically-selected locations on the utility system. By improving the stability of the entire transmission grid, a D-SMES system can cost-effectively increase system capacity and improve the reliability and quality of electric service to thousands of customers simultaneously.
ADVANTAGES of D-SMES D-SMES systems increase transfer capacity and protect utility grids from the destabilizing effects of short-term events such as voltage dips caused by lightning strikes and downed poles, sudden changes in customer demand levels and switching operations. In many cases, D-SMES is a cost-effective way to reinforce a transmission grid without the costly and environmentally intrusive construction of new lines
CHALLENGES The number of sold SMES units remains very low and does not increase much. The major reason is the high initial cost while in competition with more mature technologies - the dominant cost for SMES is the superconductor, followed by the cooling system and the rest of the mechanical structure. The energy content of current SMES systems is usually quite small. Methods to increase the energy stored in SMES often resort to large-scale storage units.
CHALLENGES A robust mechanical structure is usually required to contain the very large Lorentz forces generated by and on the magnet coils. Critical current - In general power systems look to maximize the current they are able to handle to make any losses due to inefficiencies in the system relatively insignificant. Unfortunately the superconducting properties of most materials break down as current increases, at a level known as the critical current. Current materials struggle, therefore, to carry sufficient current to make a commercial storage facility economically viable. Critical magnetic field - Related to critical current, there is a similar limitation to superconductivity linked to the magnetic field induced in the wire, and this too is a factor at commercial storage levels
POSSIBLE ADVERSE HEALTH EFFECTS The biggest concern with SMES, beyond possible accidents such as a break in the containment of liquid nitrogen, is the very large magnetic fields that would be created by a commercial installation, which would dwarf the magnetic field of the Earth. Little is known about the long term effects of exposure to such fields, so any installation is likely to require a significant buffer zone around and above it to protect humans and wildlife.
CONCLUSION SMES is particularly suitable for power sources of short duration, because the power density is much higher than the stored energy density. It is thus an excellent solution for applications such as pulse power sources, UPS or FACTS for power grids. SMES addresses niche applications having high active power – short time demands. A number of SMES units have been installed and operated successfully during many years demonstrating their very satisfying performance and proving their operational capabilities for short-term (seconds) power at MW scale. They are commercially available and the field test experience is very large in the US and Japan. The obstacle for widespread commercialization of SMES remains the high capital cost. The deregulation of the electricity market and the requirements to enhance the power capacities of the present grids bring the opportunity for FACTs using SMES.
IEEE/CSC & ESAS EUROPEAN SUPERCONDUCTIVITY NEWS FORUM, No. 3, January 2008. Superconducting Magnetic Energy Storage: Status and Perspective-Pascal Tixador American Superconductors http://www.amsc.com Transmission & Distribution World http://www.tdworld.com BIBLIOGRAPHY