# Wind Energy Questions and Answers – Wind Energy Storage – 2

This set of Wind Energy Question Paper focuses on “Wind Energy Storage – 2″.

1. What is depth of discharge?
a) Percentage of the battery that is discharged during a cycle
b) Percentage of the battery that is discharged
c) Amount of charge discharged from a battery during a cycle
d) Amount of charge discharged from a battery

Explanation: Depth of discharge (DoD) is the percentage of the battery that is discharged during a cycle. It has a wider use because it indicates a percentage instead of an absolute value. Indicating the absolute value of the charge discharged is not practical as it is unique for every battery and cannot be generalized.

2. What is charge-to-discharge ratio?
a) Time taken to charge the device
b) Ratio of time taken to charge the device relative to the time taken to discharge the device
c) Ratio of the amount of charge during charging cycle to the amount of charge during discharge cycle
d) Time taken to discharge the device

Explanation: Charge-to-discharge ratio is the ratio of time taken to charge the device relative to the time taken to discharge the device. For instance, if a device takes 2 times longer to charge than to discharge, it has a charge-to-discharge ratio of 2:1.

3. What is memory effect?
a) Electro-hydrogen generation
b) Pumped storage
c) Remembering a charging level
d) Electric grid

Explanation: Memory effect is mainly used to describe rechargeable batteries. If they are not fully charged for long periods of time then they ‘remember’ that charging level. Thus, they lose some of their capacity. The other options are storage techniques of wind energy.

4. Which of the following have large power (>50MW) and storage capacities (>100MWh)?
a) Memory effect
b) Battery energy storage (BES) and flow battery energy storage
c) Super capacitor energy storage and flywheel energy storage
d) Pumped hydro-electric storage and underground pumped hydro-electric storage

Explanation: Pumped hydro-electric storage (PHES) and underground pumped hydro-electric storage (UPHES) are grouped under the category of large power (>50MW) and storage capacity (>100MWh). Compressed air energy storage (CAES) is another example of this category. Memory effect is a parameter used to describe a rechargeable battery.

5. Efficiency of pumped hydro-electric storage is in the range of _________
a) 50% — 80%
b) 80% — 120%
c) 10% — 20%
d) 45% — 50%

Explanation: Efficiency of pumped hydro-electric storage is in the range of 50% — 80%. It has large power (>50MW) and storage capacity (>100MWh). The efficiency is further being improved using variable speed machines.
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6. How does pumped hydro-electric storage work?
a) Pumps water from higher level to a lower level
b) Pumps water from lower level to higher level
c) Uses electrical energy present in the water
d) Uses nuclear energy present in the water molecules

Explanation: PHES uses the gravitational potential energy of water to generate electricity. During off-peak hours, water is pumped from a reservoir at a lower level to a reservoir at a higher level and stored. When required, the water falls from the higher level and rotates turbines connected to an electrical generator.

7. Which of the following is an application of pumped hydro-electric storage?
a) Energy source
b) Frequency regulation only in pumping mode
c) Frequency regulation in both pumping and generation modes
d) Absorbs power to increase the net expenses

Explanation: Pumped hydro-electric storage is used for frequency regulation in both pumping and generation modes. In fact, it absorbs power in a more cost-effective manner and boosts efficiency upto 3%.

8. What is a major disadvantage with pumped hydro-electric system?
a) No dependence on geographical locations
b) Small scale construction for effective functioning
c) Low initial costs of construction
d) Depends on specific geographical locations for the construction of the two reservoirs

Explanation: A major disadvantage with PHES is its dependence on specific geographical locations for the construction of the two reservoirs. It also demands that the two reservoirs be located in close proximity to reduce costs. This increases the initial costs of construction considerably.

9. Which of the following best indicates the flow diagram of storage mode in compressed air energy storage?
a) Power from grid → motor → compressor → storage
b) Power from grid → motor → storage → compressor
c) Power from grid → compressor → motor → storage
d) Motor → power from grid → compressor → storage

Explanation: In CAES, power is drawn from the grid during off-peak hours to run a motor which in turn drives a compressor. Air passed into the compressor is then compressed and stored.

10. Which of the following best indicates the flow diagram of power generation mode in compressed air energy storage?
a) Storage → LPT → HPT → generator
b) Storage → HPT → LPT → generator
c) Storage → LPT → generator → HPT
d) HPT → Storage →LPT → generator

Explanation: In CAES, the compressed air is first sent from the storage tank to a high pressure turbine (HPT) which is in turn connected to a lower pressure turbine (LPT). Finally, the LPT is connected to an electric generator.

11. Which of the following are used for large scale battery energy storage?
a) Li-ion batteries
b) Capacitors
d) Reservoirs

Explanation: Lead acid (LA) is used for large scale BES. Nickel-cadmium (NiCd) and sodium-sulphur (NaS) are also commonly used BES. Capacitors are not batteries. Li-ion is an upcoming technology and hasn’t been established as the industry standard yet. However, it is likely to take over the industry in the next decade.

12. How does a flow battery energy storage work?
a) Electrolyte tanks → pumps → electrodes → current
b) Electrolyte tanks → pumps → current → electrodes
c) Electrolyte tanks → current → electrodes → pumps
d) Electrolyte tanks → electrodes → current → pumps

Explanation: FBES operate in a similar way. Two charged electrolytes are pumped to the cell stack where a chemical reaction occurs, allowing current to be obtained from the device when required. This current is then harnessed by sending it to a load.

13. The central shaft in a flywheel energy storage device rotates on two magnetic bearings.
a) False
b) True

Explanation: A flywheel energy storage (FES) consists of a central shaft. The shaft holds a rotor and a flywheel. The central shaft rotates on two magnetic bearings which considerably reduces friction losses. The entire setup is placed in vacuum to prevent drag losses.

14. Flywheels are used in uninterrupted power supply (UPS).
a) True
b) False

Explanation: The entire setup is mechanical, it can be repeatedly charged and discharged without much damage to the setup itself. Thus, flywheels are used in uninterrupted power supplies (UPS).

15. Capacitors do not suffer from ________
a) dielectric breakdown
b) material dependence
c) excess electric potential
d) memory effect

Explanation: Capacitors store energy in the form of electric potential. They do not suffer memory effect as the energy is stored between two conducting plates. However, they do suffer from dielectric breakdown when excess electric potential is applied.

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