CEA Energy Storage & Grid Integration 3 — Questions and Answers
Question 1: A 100 MWh battery discharges at 50 MW for 2 hours, then must recharge. If round-trip efficiency is 85%, how much energy (MWh) must be input to fully restore the battery?
- 85 MWh
- 100 MWh
- 117.6 MWh (Correct answer)
- 150 MWh
Correct answer: 117.6 MWh
Energy in = energy out / RTE = 100 MWh / 0.85 ≈ 117.6 MWh must be charged to recover 100 MWh of usable capacity.
Question 2: What is 'time-of-use (TOU) arbitrage' in the context of energy storage?
- Charging storage during peak-price hours and discharging during off-peak hours
- Selling ancillary services during high-volatility pricing periods
- Shifting grid imports to overnight hours when renewable supply is lowest
- Charging storage during low-price periods and discharging during high-price periods (Correct answer)
Correct answer: Charging storage during low-price periods and discharging during high-price periods
TOU arbitrage captures the price spread by charging when electricity rates are low (off-peak) and discharging when rates are high (on-peak).
Question 3: Which type of energy storage technology is most suitable for seasonal (multi-month) storage due to negligible self-discharge?
- Lithium-ion batteries
- Pumped hydro storage
- Compressed hydrogen (Correct answer)
- Flywheel energy storage
Correct answer: Compressed hydrogen
Hydrogen produced via electrolysis can be stored for months in tanks or geological formations with minimal energy loss, unlike batteries or flywheels.
Question 4: In the context of grid services, what does 'spinning reserve' require of a storage resource?
- The resource must be fully discharged and ready to absorb excess generation
- The resource must be online and able to increase output within 10 minutes (Correct answer)
- The resource must be pre-positioned to provide reactive power only
- The resource must be capable of providing sustained output for 8 hours
Correct answer: The resource must be online and able to increase output within 10 minutes
Spinning reserve requires resources that are already synchronized to the grid and can ramp up to full capacity within 10 minutes to cover sudden generation loss.
Question 5: What is the primary function of a Battery Management System (BMS) in a grid-scale BESS?
- Convert DC battery output to AC grid-compatible power
- Monitor cell voltage/temperature and balance cells to protect battery health (Correct answer)
- Schedule dispatch based on real-time electricity prices
- Manage grid interconnection protection relays
Correct answer: Monitor cell voltage/temperature and balance cells to protect battery health
The BMS monitors individual cell conditions and performs balancing to prevent overvoltage, undervoltage, and thermal runaway, extending battery life.
Question 6: How does FERC Order 2222 differ from FERC Order 841 regarding energy storage?
- Order 2222 addresses retail markets while Order 841 addresses wholesale markets
- Order 2222 enables aggregations of distributed resources (including storage) to participate in wholesale markets (Correct answer)
- Order 2222 mandates minimum storage procurement by utilities
- Order 2222 establishes interconnection fees for storage projects under 20 MW
Correct answer: Order 2222 enables aggregations of distributed resources (including storage) to participate in wholesale markets
FERC Order 2222 extended market access to aggregations of distributed energy resources, allowing small behind-the-meter storage to participate in wholesale markets as a fleet.
Question 7: What is 'capacity factor' for a pumped hydro storage facility, and why is it typically lower than for conventional generators?
- It measures the ratio of actual output to maximum possible output; storage assets operate only during peak demand windows, not continuously (Correct answer)
- It measures water reservoir fill rate relative to annual precipitation averages
- It measures the ratio of stored energy to generating capacity in MW
- It is always above 90% because pumped hydro uses water continuously
Correct answer: It measures the ratio of actual output to maximum possible output; storage assets operate only during peak demand windows, not continuously
Capacity factor = actual MWh generated / (rated MW × 8,760 hrs); storage is dispatch-driven and may sit idle for many hours, reducing its annual capacity factor.
CEA Energy Storage & Grid Integration 3
A 100 MWh battery discharges at 50 MW for 2 hours, then must recharge.
If round-trip efficiency is 85%, how much energy (MWh) must be input to fully restore the battery?