SMES design and use, Part-2

SMES design and use, Part-2

Applied Superconductivity Printed in Great Britain. Vol.1, Nos 7-9, pp. 1417 - 1423, All rights reserved SMES DESIGN AND USE, M.Masuda, Shonan ...

429KB Sizes 0 Downloads 51 Views

Applied

Superconductivity

Printed

in Great Britain.

Vol.1, Nos 7-9, pp. 1417 - 1423, All rights reserved

SMES DESIGN

AND USE,

M.Masuda, Shonan Institute R.W.Boom, University

1993 Copyright

0964-1807193 56.00 + 0.00 @ 1993 Pergamon Press Ltd

PART-2

of

of Technology,JAPAN Wisconsin, USA

ABSTRACT The uses for small storage units by industrial and commercial customers will be given in contrast to Part-l, where large coils are discussed. In one specific application for business building, the economical efficiency of small SMES (
INTRODUCTION Large SMES for utility use that can replace the pumped hydra has been extensively discussed in the past decade. On the other hand, the effective for stabilization of a long distance power use of a tiny SMES unit, 30MJ. transmission network has also been demonstrated by BPA’). All are utility applications. However the applications of small or medium size SMES, l-50 MWh, are widely from the pulse load{ fusion reactor-IMWh, railway-1OMWh to steel diverse: mi 1 l-2OMWh) 2, and to space application-lo-100MWh3). very few efforts have been directed to evaluate However, small or medium size unit for commercial use4) except in space application. This probably arises from the fixed idea that such units have two intrinsic difficulties, poor efficiency and poor economic benefits. Commercial use has therefore been mostly ignored. It is an aim of the present paper to re-estimate the storage efficiencies of small and medium-size SMES and to look at the economical aspects of such units for commercial use.

THE EFFICIENCY The efficiency

OF SMALL AND MEDIUM SIZE SMES of SMES is given by the following _________

D =l-W/Es

equation,

(1)

where W and Es are the loss per cycle and total stored energy respectively. One is the cryogenic loss, inSMES loss is divided into two categories. maintaining the cryogenic environment, and the other is the curred while loss in energy conversion during charging and discharging the SMES. for the loss estimation on the duty We begin by making some assumptions the magnet current and so the terminal voltage, cycle of SMES operation, 5,000 MWh that has been we go to the smaller size than After that, on’). well studied by many groups and now is considered as a standard unit. The scaling law to apply to the reduced size loss estimation is assumed to be as follows: Conduction Twist

: Es/t’,

through

strut bonetic

: Es~‘~,

Ed_&! current

: Es/t

hvsteresis 1417

: Es/t2

1418

World Congress hysteresis

where tively.

Es, t and B is

: Ws,

R are the the aspect

stored ratio

on Superconductivity

: Es/R

B.adiatipn energy, of coil.

decay

(@=constant)

time

and

coil

radius

respec-

CRYOGENIC LOSS As mentioned above, some parts of the cryogenic losses have a strong dependence on the operation parameters - the duty cycle and so the terminal voltage and the magnet current. Previously the superconducting magnet were designed with large current; the magnet current of 5,000 MWh unit was designed by 700 kA. for instance, The reasons for this is to avoid the generation of dangerous higher terminal voltages when energy should be quickly dumped to the outside resistor in quenching. However, recent studies consider current diffusion in stabilizer and point out some difficulties of a large current cable6). This favors a lower current and higher terminal voltage design. Recent design for 5,000 MWh has therefore amended the magnet current and magnet voltage from 700 kA to 200 kA and 2.5 kV to 7 kV respectively. another recent study’l) On the other hand, suggests abnormal electric breakdown in a liquid helium. This forms a channel that bridges two electrodes with evaporated helium bubbles in a strong electric field. This carries a high breakdown risk. It could therefore limit designs of SMES with higher terminal voltage. Table 1 shows the result of optimized cryogenic losses for various size units under consideration. Persistent switch losses have not been cited in most studies. Therefore a small experiment was carried out and the resistance of mechanical contact in liquid helium environment has been estimated. It is 10-60hm-ca2 under Meanwhile a magnetic or the contact pressure of 66Kg/cm2. thermal superconducting switch can not be used because its open resistance is too low. The optimum loss in power lead wire has long been estimated on the basis of recent work6) 1 watt loss per 1 KA current. However has greatly improved using high Tc materials that lose approximately the power lead loss by In table 1, the power lead loss with high Tc is listed in O.lW/kA. brackets.

LOSS OF POWER CONDITIONING UNIT Loss in the power conditioning unit lead wire loss and thyristor transformer loss, The basic equations to estimate such losses Wt(L)=0.5+

(0.5

+

Wt (NL) =O. 2+24-2t1 WLw-- ‘thy where lead

5 3

0.2

I = 8 Est=

-

P ws

t

% of

W,(L) ,Wt(NL) ,WLw and wire loss and thyristor

divided loss. are given

% of

rated

power)

(0.2

% of

rated

power)

P

into as

three

types

-

follows’).

power)

(0.2

I 8-

rated

is

( 1 Volt forwa d drop and connection 0 F thyrlstor) transformer Wthy are loss respectively.

4 series

loss of on-load, The P,Es,t,I and

no-load, V are the

World Congress on Superconductivity

1419

DC current and DC voltage respecrating power, storage energy, decay time, tively. It is clear that the electrical losses have a strong dependence on t and therefore on the pattern of the duty cycle. The duty cycle for various size SHES is assumed as t=31ogEs-2, where t is the charging and discharging time. The estimated losses of the power conditioning units for different size unit are shown in table 2.

OVERALL EFFICIENCY OF SMES Thus the overall efficiency of various sizes of SMES can be estimated from equation 1 as given in figure 1. The curve of efficiency starts at 90% for standard 5,000 MWh unit and descends as the size of SMES decreases. For smaller units, lo-20 MWh, it is acceptable figure because conventional 10_15%. This is a commercially battery storage has the same value. It has been generally believed that the efficiency of small SMES might be For this reason, very poor, something like (50 %. its commercial use has On the contrary, however, the present study gives strongly doubted. been reasonable results. Still we should take the figures listed in table 1 and 2 as ideal one, we might lose sight of other factors or rely to optimistisince otherwise cally on an estimation of various losses. Other considerations that we should include, for example, 1) loss in an extended length of the piping for liquid helium specifically for a toroidal magnet configuration: 2) loss in a helium dump system, however, whether such system is necessary or not is still in doubt; 3) under-estimating the power conditioning unit loss specifically for a The fraction of the thyristor loss will increase in smaller small unit. units: 4) the need to consider loss in the persistent switch. The table gives an this does not consider the life time ideal formula of such losses, however, of mechanical parts. How to suppress such loss should be a basic consideration for a practical design.

The USE OF SMALL UNITS FOR INTELLIGENT BUILDINGS Business buildings which work mainly during day time are one of the causes of the heavy burdens on the utility’s load in a big city. Therefore it is clear that the use of energy storage for peak shaving could be effective. In addition, fully computerized modern building that has a power failure is serious, even for a very short time. The quick response from effective energy storage is very welcome. A 30-stories business building is picked for a reference study of small size SMES applications. The appropriate size of SMES has been investigated. As a result, a 6.5 MWh and 1.4 MW unit is considered. Table 3 shows the comparison of the annual depreciation cost of equipment and the cost of electricity in two systems, one conventional and the other using a 6.5 WWh SMES. Thus the economical break-even capital cost (BECC) is derived. The estimation is 1.6 billion yen for 30 years depreciation of the 6.5 MWh unit. For economical discussions the BECC should be compared with the construction costs of such a unit. Construction costs of such a unit are not yet completely understood. One reason is that such big superconducting unit has not been constructed.

1420

World Congress

on Superconductivity

Although we refer to the SMES unit as “medium-sized” or “small”, the scale is 100 times bigger than the largest superconducting magnet ever built. We use the conventional method to estimate the cost for various sizes, using the law of C~CES”~ where C and Es are the cost and the energy storIn a cost estimation, we have two methods of extrapolaage respectively. one of which is to work down from the theoretically estimated cost of tion, a 5,000 MWh unit and the other is to work UP from the cost of GJ class magnets made for the fusion research. We did not take this latter method because its cost includes large indirect cost arising from the long-term researches in that field. We therefore estimated down from the 5,000 MWh, construction cost which is estimated as 290 billion yen. SMES for intelligent buildings will be constructed in a densely Meanwhile populated area where leakage magnetic fields are strongly prohibited. This magnet . The cost of the toroid is twice as expensive leads to a toroidal as the conventional solenoid types. Thus the construction cost of a 6.5 MWh unit is estimated as 3 and 6 times more expensive than BECC for solenoidal and toroidal schemes respectively.

CONCLUSION The storage efficiency of a small SMES using a metal superconductor is estimated as 75 % for a 10 MWh unit. If we need higher efficiency, we will need to further cut the cryogenic loss (the main part of the total Still it is hard to greatly reduce cryogenic losses loss). even using all in the case of the metal superconductors. available techniques In addithe toroidal schemes or other specified schemes tion, seems to involve more losses than estimated in the present work. One possibility to achieve a drastic loss reduction is the employment of a high Tc superconductor that enables the use of higher temperature environment for superconductivity. For instance, it might be 20 or 70 K depending on advances in future research in high Tc materials. However, 75 % efficiency, if it could be achieved with a metal superconductor, is sufficiently high for some SMES applications. Low Tc SMES, therehas capability for some commercial use. fore, From the stand point of economics for a small SMES, the construction cost is compared with the break-even point, BECC, derived from the reference study. The toroidal and low Tc SMES shows construction costs six times more expensive than the BECC. This seems to be a fatal picture for commercial applications of small SMES. However, we should consider the additional benefit of small SMES depending on various applications, even in the construction industry. As already pointed out, the ability to protect from power failure (which puts all computer activity at serious risk) could be one additional benefit of SMES for the intelligent building. On the other hand, we can suggest another in the case of the skyscraper. It is customary to design such tall building with a soft structure to protect against damage from the frequent seismic tremors in Japan. As a result, the characteristic frequency of these building is very low and they are easily swung by strong winds or 1 ight earthquakes. This is unpleasant for people working inside and some active methods to dismiss unpleasant vibrations have been introduced. The its quick response, SMES, with can effectively supply power for such devices and provide large additional benefits. If such additional benefits can be derived, we could bridge the gap between the construction cost and the BECC. Such dynamic benefits of SMES have been evaluated only for a big unit and thus the economical discussion of such a unit has become more precise and the additional capability of small SMES more realistic”). In this sense,

World Congress for building forward to

must evaluating

be

counted as a other additional

on Superconductivity kind of benefits

1421

dynamic in the

benefit future.

and

REFERENCE l)Rogers,S.D. et al., IEEE, m-19. 1078(1983) 2)Eastham A.R., Pringle D.M. and Austin P.R., Proc. of Workshop on SMES, 506(1981) 3)Eyssa Y.M. et al., Submitted to “European Space Power Conf.,” (1991) T., IEEE, U-19, 1074(1983) 4)Masuda M. and Shintomi in Superconductivity”, 1289, Spring-Verlag 5)Masuda M., “Advances on Fusion Technology (1983) 6)Hilal M.A. et al., Proc. 9-th Symp. 7)Hara M., Suehiro J. and Matumoto H., Cryogenics, 3-Q. 787(1990) alprivate communication T., Cryogenics, U, 607(1977) 9)Masuda M. and Shintomi 10)New Energy Development Organization Report, NEDO-P-8408 (1984)

,’ v

LL

E

we

US-Japan Florence

(1991)

I

sot

I

I

40-

I

I

I

10

100

1lUoo

I

10900

STORED ENERGY FIG 1 OVERALL EFFICIENCIES

OF VARIOUS SIZES OF SMES

Mwli)

look

World Congress

1422 Table

1.

Size

Cryogenic

on Superconductivity

Losses

(MWh)

of

different

size

SMES (MW)

5000

100

10

Conduction

2.99

0.22

0.05

Radiation

0.73

0.015

0.0015

Hysteresis (conductor)

0.50

0.023

0.01

Hysteresis (mechanical)

0.31

0.006

0.000

Miscellaneous

0.91

0.053

0.012

Power

0.76(0.08) (252KA)

Lead

Persistent

SW

Total Daily % of

Loss

(MWh)

Es

Table 2. Conditioning

Electrical Unit

0.01

0.001

7.Oc6.3)

0.35CO.33)

0.083(0.075)

168.0(151.2)

8.40(7.92)

1.98(1.80)

3.4l3.1)

8.4c7.9)

19.8(18.0)

in

Parameter and Losses % of Stored Energy(Es) 5,000

Power

Terminal

100 9

Time (hr)

Output

555

(MW)

Voltage

0.008(0.000) (4.5KA)

0.8

Size(MWh) Cycle

0.02(0.002) (11.3KA)

(KV)

of

Power

10

4

1

25

10

2.2

2.2

2.2

Maximum

Voltage

(KV)

7.0

7.0

7.0

Maximum

Current

(KA)

252

11.3

4.5

of Transformer On- load (%I Off-load(%)

0.33 0.00

0.33 0.80

0.33 4.4

Wire

0.33

0.33

0.33

0.36

0.36

0.36

1.02

1.82

5.42

Loss

Lead Thristor

% of

Loss(%) Loss

Es

(%)

World Congress Table for 30

3. Comparison Story Business

of Cost Building

on Superconductivity

of

Electricity

1423

between

Both

Systems

6 5MWh. 1.4MW(SMES)

conventional (demand charge) daytime night deduction night sub-total

11.808Myen(704.5MWh) 1.114Myen(66.5MWh) 1.200Myen(204.5MWh) 14.122Myen(975.5MWh)

(569. OMWh) 0 (0 MWh) 2 385Myen(406.5MWh) 11 923Myen(975.5MWh)

(electricity cost) daytime night deduction night sub-total

10.571Myen(694.6MWh) 0.984Myen(58.6MWh) 0.847Myen(142.6MWh) 12.312Myen(894.9MWh)

7 955Myen (522. OMWh) 0 (0 MWh) 2 215Myenc372.9MWh) 10 170Myen(894.9MWh)

annual

cost

207.702

Myen

20.000 22.000 0

Myen Myen

diesel generator CVCF (CDF) SMES

annual

cost A:unit

BECC of

SMES

cost

billion

156

387

Myen

4 (MW)xA Myen

207.702xBt42.000 of SMES,

: 1.58

9 537Myen

156 387xBt1.4xA Myen B:years of depreciation

yen

for

30

years

depreciation

Myen