Shinichi Nomura

Wind farms linked by SMES systems

The objective of this paper is to introduce the concept of wind farms linked by SMES systems. In this work, the SMES system is applied to a wind farm that is interconnected with a grid through a back-to-back DC link for the variable speed operation of the wind turbines. This system enables the output power leveling of the wind farm depending on the power demand and can reduce the capacity of the converter system by selecting an optimal discharge/charge rate of the SMES. By using the stored energy of the SMES, this system can also compensate the inertia of the blades so that the wind turbine speed can be rapidly controlled depending on the wind condition. This paper describes the design condition of the SMES for the output power leveling of the wind farm and discusses the SMES configuration for a 100-MW class wind farm.

Technical and Cost Evaluation on SMES for Electric Power Compensation

RASMES (Research Association of Superconducting Magnetic Energy Storage) in Japan developed a road map of SMES for fluctuating electric power compensation of renewable energy systems. Based on the progress of large superconducting coils, the technical status is already established to develop the several MWh class SMES for frequency control, load fluctuation compensation, and generation fluctuation compensation. With integrated operations of several dispersed SMES systems, it is expected that the 100 MWh class SMES for load fluctuation leveling (peak cut) can be introduced in the period of 2020-30, and the first 1 GWh class SMES for daily load leveling can be installed in the period of 2030-40. From the results of Japanese national projects, experimental device developments and SMES design studies, if the output power of SMES is 100 MW, the target cost of SMES can be evaluated with 2000 USD/kW of the unit cost per output power (the unit cost per kW).

Optimization of SMES coil by using virial theorem

The coil for the superconducting magnetic energy storage (SMES) is optimized by use of the virial theorem with stored energy and stress. In this work, we show the theoretical limit of stored energy with the maximum stress. To achieve the ideal limit, we propose the toroidal coil with helical winding. It is a hybrid coil of a toroidal field (TF) coil and a solenoidal coil helically wound on a torus. The winding is modulated in such a way that the toroidal field is created in the torus whereas the poloidal field is only out of the torus. In this case, the electromagnetic force is represented by the difference in the poloidal and the toroidal magnetic pressure. The virial theorem in the magnet is the relation of the magnetic energy and the averaged stress, and shows that the best coil to store the magnetic energy under the weakest averaged stress requires equal averaged principal stresses in all directions, which determines the ratio of the poloidal and toroidal current of our toroidal coil. The coil increases the magnetic energy to 4 times the conventional TF coil with the same maximum stress.

Flexible Power Interconnection With SMES

Electric power networks are usually interconnected with each other through a back-to-back direct-current (DC) link to increase reliability of electric power networks and to improve system operations. The objective of this work is to discuss the concept of a superconducting magnetic energy storage (SMES) incorporated into a back-to-back interconnection. In this case, the back-to-back system is used as a power conditioning system for the SMES coils. Since the AC/DC converter can be designed independently of the frequency of the power system, a two-way switch is connected to the AC side of each converter. This two-way switch can select the interconnected power system. By using the two-way switches, this system can increase the availability factor of the back-to-back interconnection during the SMES operations and also enables the flexible power interchange between interconnected power networks with an optimal time interval for the power demand of each interconnected power network. This work discusses the design considerations of the back-to-back interconnection with the SMES that enables the replacement of a pumped hydro storage. In this case, the SMES system is composed of a number of superconducting coils in order to decrease the risk of the superconducting coil constructions by the effect of mass production. In this work, the SMES coils are optimized from the required mass of the structure and the leakage magnetic field

Feasibility of Hydrogen Cooled Superconducting Magnets

It looks feasible to realize hydrogen cooled superconducting magnets with High Tc Superconductors (HTS) and newly discovered magnesium di-boride (MgB2). As is well known, liquid and slush hydrogen between 15~20 K, could be not only an excellent refrigerant for HTS and MgB2, but also a clean energy transporter without exhaust of carbon di-oxide. HTS cooled by liquid hydrogen at 22 K can carry as large as several times current at 77 K cooled by liquid nitrogen. These are reasons why liquid hydrogen is preferable for SMES refrigerant to liquid nitrogen. One of such possibilities, an emergent power supply composed with SMES, Liquid Hydrogen Storage, Fuel Cell and Lithium Battery is designed and proposed. As a practical example, the conceptual design of 200 MW, 10 hour emergency power supply is described

Helically wound coils for high field magnets

The winding current density of a superconducting coil is one of the key parameters to realize high field magnet systems with smaller sized superconducting coil. Force-balanced coil (FBC) which is a helically wound toroidal coil can control the distribution of working stresses and minimize the structure requirements by selecting an optimal number of poloidal turns. The winding current density of a superconducting coil is estimated from the relationship between ampere-meters of conductor and structure requirements based on the virial theorem. In this case, the FBC can obtain the stored energy for the same winding current density 20 times larger than that in the toroidal field coils case and about 120 times larger than that in the solenoid case. By applying the FBC concept, superconducting magnets will be realized in smaller size.

Force-balanced coil for large scale SMES

In large scale SMES, huge electromagnetic force caused by high magnetic fields and coil currents is a serious problem. In order to solve this problem, we propose a concept of force-balanced coil (FBC) applied to the SMES. The FBC balances the centering force with the hoop force, both of which are exerted in the major radius direction. Moreover, for the optimization of large aspect ratio superconducting coils, we propose a stress-balanced coil (SEC) concept, improving the concept of FBC, which balances magnetic pressures at the coil nose part where the produced magnetic field reaches its maximum value. Comparing the toroidal magnetic field coil (TFC) and FBC with SEC, the last can reduce coil stresses and obtain large stored energy with shorter length conductors and/or lower magnetic fields.

Variations of force-balanced coils for SMES

Strong electromagnetic force caused by high magnetic fields and coil current is a serious problem in superconducting magnetic energy storage (SMES) systems. In facing this problem, we proposed the concept of the force-balanced coil (FBC) which is a helically wound toroidal coil applied to SMES. This paper shows the variations of the helical windings in order to reduce structure requirements. We discuss the relationship between the shape of SMES coils and the structure requirements based on the virial theorem. From this result, the FBC can minimize the working stresses by selecting the optimal number of poloidal turns.

Design considerations for force-balanced coil applied to SMES

Strong electromagnetic force caused by high magnetic field and large coil current is a serious problem in superconducting magnetic energy storage (SMES) systems. In facing this problem, we propose the concept of force-balanced coil (FBC) which is a helical-winding toroidal coil and can reduce the centering force of toroidal field coil (TFC). The helical-winding of the FBC is modulated in order to reduce the torsional force. This paper describes the cost-related parameters of the FBC in terms of the dimension of the coil, the ampere-meters of superconductors and the surface area of the coil compared with the TFC and the solenoid. Moreover, we discuss the structure requirements by the virial theorem and improve the concept of FBC in terms of the further optimization of SMES through the structural analysis.

Design of SMES System With Liquid Hydrogen for Emergency Purpose

The emergency power supply system, which is composed of a SMES system cooled by liquid hydrogen and a fuel cell (FC), is proposed as a backup for supplying electricity to hospitals, high-rise office buildings, and intelligent equipment. The capacity of the system is 2 MW for 10 hours. At a power failure, the SMES immediately functions to supply electric power at start-up for 60 seconds, and then after the FC generates electricity for 10 hours. Since the system is designed to be driven with liquid hydrogen and oxygen, it can be installed to any isolated spaces like spaceships, tunnels, and so on. The development scenario, that an 1-kW prototype, an 100-kW module as a basic unit and then a practical system will be developed step by step, is proposed. The system can be rather flexible for load requirements by adjusting the composition of the basic modules.