FEASIBILITY STUDY OF A 10-GWh TOROIDAL SUPERCONDUCTIVE MAGNETIC ENERGY STORAGE SYSTEM1. SYSTEM DESIGN

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1 FEASIBILITY STUDY OF A 10-GWh TOROIDAL SUPERCONDUCTIVE MAGNETIC ENERGY STORAGE SYSTEM1. SYSTEM DESIGN M. Shimizu, Y. Tanabe, T. Yoshioka, K. Takeda, T. Hamajima, N. Miki, Y. Nakayama, M. Udo, N. Takeda, H. Miyazaki, et al. To cite this version: M. Shimizu, Y. Tanabe, T. Yoshioka, K. Takeda, T. Hamajima, et al.. FEASIBILITY STUDY OF A 10-GWh TOROIDAL SUPERCONDUCTIVE MAGNETIC ENERGY STORAGE SYS- TEM1. SYSTEM DESIGN. Journal de Physique Colloques, 1984, 45 (C1), pp.c1-581-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1984 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque CI, suppl6ment au no 1, Tome 45, janvier 1984 page CI-581 FEASIBILITY STUDY OF A 10-GWh TOROIDAL SUPERCONDUCTIVE MAGNETIC ENERGY STORAGE SYSTEM 1, SYSTEM DESIGN M. Shimizu, Y. Tanaber7 T. ~oshioka*, K. Takeda*, T. ~amajima*', N. ~iki**, Y. ~aka~ama*~, M. IJdor*, N. Takedarrr, H. ~i~azaki"*, K. ~amemura*** and M. ~amamoto*** The Kansai Electric Power Company, I%., Japan *The Institute of Applied Energy, Japan "Toskiba Corporation, Japan ***Taisei Corporation, Japan Resume - Un systeme Z aimant torotdal est probablement preferable celui d'un aimant solenoxda1 du point de vue des champs de fuite, de la fabrication, de l'entretien et des r6parations. Au vu des propri- Btes non connues des roches et du niveau technologique actuel pour faire des excavations, il semble raisonnable d'adopter une structure en tranchee ouverte qui ne soutienne que la force Blectromagnetique centripete agissant sur l'aimant toroldal. Abstract - A toroidal coil system is probably preferable to a solenoid coil system in terms of stray magnetic field, fabrication, maintenance, and repair. Judging from the uncertain properties of rock masses and present level of excavation technology, it seems reasonable to choose an open trench structure that supports only the centering electromagnetic force of the toroidal coil. 1. INTRODUCTION A superconductive magnetic energy storage (SMES) system has recently been considered as an attractive measure for load leveling. As the result of our study of various energy storage systems, we concluded that advanced batteries may be available for a small-scale on-site system, and the SMES is suitable for a large-scale system, comparable to a conventional pumped hydro storage station. Solenold coil systems have been studied for a large-scale SMES system because their weight is lighter than that of toroidal coil systems (I),(2). The rock mass in the solenold coil system, however, has to support an enormous electromagnetic force. It is probably not easy for a rock mass to sustain a large electromagnetic force because of uncertain properties such as cracks and inhomogeneity. When factors such as fabrication, maintenance, repair, and influence of stray magnetic field are also taken into account, it seems that the solenoid coil system is not necessarily advantageous. This paper compares the two coil types based on the main specifications summarized in Table 1. A toroidal coil system was selected for our feasibility study. In order to study the possibility of a rock mass as a support structure, a conceptual site survey was performed withln the service area of the Kansai Electric Power Company, Inc., which is the largest power company in the southern part of Japan, considering environmental conditions as well as the properties of the rock mass. Detailed designs for the coil system and rock mass structure are presented in our following papers. Article published online by EDP Sciences and available at

3 JOURNAL DE PHYSIQUE Table 1 Main Parameters of SMES System Availability of stored energy Charge and discharge time Waiting and holding time Operation mode 8 hours each 4 hours each 940 MW constant power operation 2. COMPARISON OF SOLENOID COIL AND TOROIDAL COIL SYSTEMS In comparison of the two types, the shape parameter, B, is introduced. For a solenoid coil, R is defined as: B = H/~R where H: coil height, R: inner radius, and for a toroidal coil, which is the reciprocal of aspect ratio, where a: minor radius, R: major radius. For this comparison, the following conditions were considered reasonable for the present state of superconductive technology: Maximum magnetic field (Bmax) 10 Tesla Operating current 100 ka Overall current density of superconductor 10 ~/mm' Major parameters of the solenoid coil are shown in Fig. 1 as a function of D. From Fig. 1, it is known that the larger R is, the smaller the volume and ampere-meters become. On the contrary, the electromagnetic pressures become severer with increase in 8. These two opposing changes make a 8 of 0.05 approximately optimum. In the same way, the result of a toroidal coil is represented in Fig. 2, revealing the following features: Shape parameter 0..,...,. ', " ", N " s% mnaa 401 nos a1 a3 I Shape parameter B Fig. 1 Variation of main parameters Fig. 2 Variation of main paramof solenoid coil as a eters of toroidal coil function of B as a function of 6

4 o Volume is only a weak function of R, o Small R is favorable for ampere-meters but the pressure due to centering force has a counter characteristic, and o The tensile stress is minimum near R of Judging from these features, I3 of about 0.05 is also considered optimum for the toroidal coil system. Table 2 shows the main dimensions calculated at a constant R of 0.05 together with other items such as the stray magnetic field and construction cost. Table 2 Comparison of Solenoid Coil and Toroidal Coil Main Dimensions Weight Rock Mass Construction Cost (1) Stray Magnetic Field (a) Radius at 20 G (b) Radius at 0.3 G Fabrication, repair, and maintenance (a) Fabrication (b) Repair (c) Maintenance Excavation Efficiency Solenoid Coil Inner Radius Outer Radius Coil Height 8.5 x lo5 tons(2) Tunnel structure Depth 1.71 (2) 900 m (2) (2) 2,900 m m m 18-9 m 500 m The coil is fabricated within the tunnel ~ o easy t ~ o easy t NO experience in excavating such large-scale, deep tunnels 94% Toroidal Co~l Major Radius Inner Minor Radius 1.5 x lo6 tons 272 m 14.8 m Open Trench Structure Depth 45 m m 800 m The coil is fabricated in a factory near the site Each coil can be replaced for repair Relatively easy Proven techniques are available 94% Notes: (1) Superconductor for the toroidal coil corresponds to 1. Fabrication cost is not fully included. (2) The figures include the effect of the cancel coil. If all the electromagnetic force OF the solenoid coil is supported by a rock mass, the rock mass is required to support an inner pressure of 9.2 MPa. The depth satisfying this requirement is over 600 m. However, the inner pressure decreases to 5.7 MPa if the electromagnetic force is partially supported by the coil support system. The rock mass at a depth of about 350 m is able to withstand this pressure, but this depth is not enough to satisfy requirements for stray magnetic field. For the solenoid coil, a depth of 500 rn was chosen in view of the stray magnetic field. In the case of the toroidal coil, a tunnel structure is one option but an open trench structure at the depth of only 45 m was available due to very low stray magnetic field.

5 JOURNAL DE PHYSIQUE The weight of the toroidal coil is about 1.8 times heavier than that of the solenoid coil mainly due to its stainless steel support structure, which is a dominant element in the weight of the SMES system. On the other hand, the superconductor occupies approximately half the construction cost in both coil systems. According to our estimation, the difference in the construction cost between the two coil types is about 30%, throuqh a further study of the cost is necessary. In spite of higher construction cost, the toroidal coil has some excellent advantages concerning the stray magnetic field, fabrication, maintenance, repair and excavation. That is why the toroidal coil was selecte& in our study. 3. POSSIBILITY OF ROCK MASS USE One of the properties of rock mass, vp, the seismic longitudinal velocity which represents such rock mass characteristics as strength, deformation, and composition, is important in view of its practical use. vp of more than 4 km/s was required in our conceptual site survey because this condition is applied to rock masses for conventional dams and tunnels. Typical rocks satisfying this condition are granite, andesite, clay slate, shale and so forth. Other criteria for the site selection were scale of rock mass, history of earthquakes, dislocation, 500-kV power transmission trunk line and density of population. The survey was made to find several suitable places within the service area of the Kansai Electric Power Company, Inc. In the SMES system, inner pressure due to electromagnetic force causes difficult technical problems, as does the lack of experience in large-scale excavation so deep underground. The rock mass cannot support the high inner pressure exerted on the wall of a. tunnel made with present technology,.and the problem is more intricate if the fatigue characteristic of the rock mass is taken into account. Though the exact allowable inner pressure, Pal, cannot be derived at present, it was preliminarily defined as: Pal = Po/SF where Po = yh (y: unit weight of rock mass at 2.5 g/cm3) (h: depth of tunnel (cm)) SF: safety factor which was assumed to be 1.5 in our study. Pal was calculated as 3.2 MPa and 8.1 MPa at depths of 200 m and 500 m, respectively. It is apparent from the above expression that the allowable inner pressure is proportional to the depth. Based on a trade-off between the merits of deepening for effective rock mass use and the difficulty of excavation, the open trench structure was chosen which supports only the centering force. 4. SMES SYSTEM LAYOUT Fig. 3 is a bird's-eye view of the SMES system layout including coils, refrigerators, coil factory, substation, main control building, and converter equipment. The outer part of the open trench is used to withdraw the coils for repair. This Layout minimizes the influence of stray magnetic field on various peripheral facilities, Maximum exposure is 20 Gauss for the refrigerators and 5 Gauss for the control units. Maximum magnetic field outside the site is only 0.3 Gauss, which is similar to the terrestrial magnetism. The overall site area is about 1.3 million square meters, which corresponds to 1.4 m2/kw, approximately equal to conventional pumped hydro storage stations.

6 Fig. 3 Bird's-eye view of the SMES system 5. CONCLUSION Our study resulted in preference for a toroidal coil system rather than a solenoid coil system because of many practical points. The open trench rock mass structure can probably be constructed even by means of existing excavation technology. The technical problems can be solved in parallel with the development of various superconductive machines such as alternators and nuclear fusion devices, but great efforts must be made to improve the economics of the SMES system. References (1) R.W. Boom, IEEE Transactions on Magnetics, Vol. Mag-17, No. 1, January 1981, pp (2) R.W. Boom, et al, IEEE Transactions on Magnetics, Vol. Mag-17, No. 5, September 1981, pp