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

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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-C1-585.

�10.1051/jphyscol:19841117�. �jpa-00223586�

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. ~ a k a ~ a m a * ~ ,

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 sole- noid 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, com- parable 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 http://dx.doi.org/10.1051/jphyscol:19841117

JOURNAL DE PHYSIQUE

Table 1 Main Parameters of SMES System Availability of stored energy

Charge and discharge time

8

hours each Waiting and holding time 4 hours each

Operation mode 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

~ / m m '

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

_{s % ,. ', m n a a "}

", ..

^{N ,.}

" ..

⁴⁰¹

Shape parameter

^{nos a1}

B

^a3

^I Fig. 1 Variation of main parameters

Fig.

2 Variation of main param-

of solenoid coil as a eters of toroidal coil

function of B as

a

function of 6

o The tensile stress is minimum near R of 0.05.

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

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.

Toroidal C o ~ l Major Radius 272

m

Inner Minor 14.8 m Radius

1.5 x lo6 tons Open Trench Structure

Depth 45 m

2.19 380 m 800 m

The coil is fabricat- ed in a factory near the site

Each coil can be re- placed for repair Relatively easy Proven techniques are available

94%

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

If all the electromagnetic force O F 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.

Solenoid Coil Inner Radius 373.4 m Outer Radius 385.9 m Coil Height 18 -9 m 8.5 x lo5 tons(2) Tunnel structure

Depth 500 m

1.71 (2) 900 m (2)

( 2 )

2,900 m

The coil is fabricated within the tunnel

~ o t easy

~ o t easy

NO

experience in excavat- ing such large-scale, deep tunnels

94%

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.

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 con- ventional 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 earth- quakes, 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 pro- portional 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 mini- mizes 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.

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.

340-343

(2) R.W. Boom, et al,

IEEE

Transactions on Magnetics, Vol. Mag-17, No. 5, September

1981, pp. 2178-2181