WIND ENERGY SYSTEMS - Cd3wd

Chapter 7—Asynchronous Loads 7–33
1. How many cells are required?
2. What is the total land area required?
3. What is the battery current during charge and during discharge?
4. What should be the power rating of the turbine generator if it is to be operated at rated power
for four hours during the day?
For part (a), if the total energy storage is 100 MWh = 100 × 10 6 Wh, and each cell contains 200
Wh, the number of cells is 100 × 10 6 /200 = 500,000. This is obviously a significant manufacturing
endeavor.
For part (b), if we stack the cells two high, we need only the area for 250,000 cells taking up space
5cmonaside.
Area = 250, 000(0.0025 m 2 ) = 625 m 2
This is an area 25 m on a side, which may be unacceptably large at some locations. The area can
be reduced to one third of this value by stacking the cells six high rather than two high. This would
probably have some benefits in terms of lowered losses to the atmosphere and easier recovery of heat
for the turbine generator.
For part (c), the total energy required during charge is 100/0.7 = 143 MWh. The average power
during charge is then 143/5 = 28.6 MW. Supplying this power at 2500 V dc requires a current of
28.6 × 10 6 /2500 = 11,440 A. The average power during discharge is 100/3 = 33.3 MW. The current
during discharge would be 33.3×10 6 /2500 = 13,330 A. These currents are approaching a practical limit
for conductors and protective devices, so it may be worthwhile to consider the economics of raising the
voltage to 5000 V dc or more and lowering the current a proportional amount.
For part (d), the losses during a 24 hour period are 100/0.7 -100 = 43 MWh. Half of this amount
or 21.5 MWh is available to our turbine generator in the form of 350 o C heat. The power output over
a four hour period would be 21.5(0.25)/4 = 1.34 MW. If this can be used during the discharge cycle,
the effective power rating during discharge would increase from 33.33 MW for the batteries to 34.67
MW for batteries plus waste heat. It would be desirable, therefore, to enter the peak load period of
the day with the batteries as hot as possible and leave the peak load period with the batteries as cool
as possible. If the batteries would tolerate a 40 or 50 o C temperature swing over a three hour period,
both the stored heat and the losses could be used to power the waste heat generator.
5 HYDROGEN ECONOMY
The concept of the hydrogen economy has received considerable attention in recent years,
especially since 1973[3, 1]. This concept basically describes an energy economy in which
hydrogen is manufactured from water by adding electrical energy, is stored until it is needed,
is transmitted to its point of use and there is burned as a fuel to produce heat, electricity,
or mechanical power. This concept has some disadvantages, primarily economic in nature,
but also has some major advantages. One advantage is that the basic raw material, water,
is abundant and inexpensive. Another advantage is the minimal pollution obtained from
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001