REP Storage and Grids 4 — Questions and Answers
Question 1: Which storage duration category is most appropriate for seasonal energy storage needed to balance renewable generation across months?
- Short-duration (< 1 hour)
- Medium-duration (1–10 hours)
- Long-duration (10–100 hours)
- Ultra-long-duration (weeks to months) (Correct answer)
Correct answer: Ultra-long-duration (weeks to months)
Seasonal storage addresses multi-week to multi-month imbalances between renewable generation and load, requiring technologies like hydrogen, synthetic fuels, or large pumped hydro.
Question 2: In a vanadium redox flow battery, what component physically separates the two electrolyte half-cells while allowing ion exchange?
- Current collector plate
- Proton exchange membrane (Correct answer)
- Carbon felt electrode
- Bipolar plate
Correct answer: Proton exchange membrane
The proton exchange membrane in a vanadium flow battery allows proton transport between the positive and negative vanadium electrolytes while keeping them from mixing.
Question 3: What does the term 'capacity factor firming' mean in the context of pairing renewable energy with storage?
- Increasing the nameplate capacity of the renewable generator
- Using storage to smooth intermittent renewable output into a more consistent power delivery profile (Correct answer)
- Reducing the levelized cost of storage by cycling more frequently
- Synchronizing renewable output with transmission congestion schedules
Correct answer: Using storage to smooth intermittent renewable output into a more consistent power delivery profile
Capacity firming uses storage to fill in generation gaps from variable renewables, delivering a steadier, more predictable output that better resembles a firm power resource.
Question 4: Which electric vehicle charging strategy helps utilities avoid distribution transformer overloads by delaying or slowing charging until off-peak hours?
- Vehicle-to-grid (V2G) discharge
- Smart (managed) charging (Correct answer)
- DC fast charging
- Bidirectional metering
Correct answer: Smart (managed) charging
Smart charging controls the rate and timing of EV charging via utility signals or time-of-use pricing to prevent coincident peak demand that could overload local distribution infrastructure.
Question 5: What is the primary purpose of spinning reserve in a power grid with high renewable energy penetration?
- Providing base-load power during overnight low-demand periods
- Maintaining instantly available generation capacity to respond to sudden generation shortfalls (Correct answer)
- Storing excess solar and wind energy for later dispatch
- Regulating long-term voltage profiles on transmission lines
Correct answer: Maintaining instantly available generation capacity to respond to sudden generation shortfalls
Spinning reserve consists of online generation running below full output that can ramp up within 10 minutes to cover unexpected loss of generation or load spikes.
Question 6: Which grid interconnection configuration allows a solar-plus-storage project to share a single point of interconnection, reducing interconnection costs and queue position requirements?
- AC-coupled standalone configuration
- Co-located DC-coupled configuration (Correct answer)
- Virtual power plant aggregation
- Distributed network protocol (DNP3) integration
Correct answer: Co-located DC-coupled configuration
DC-coupled co-location connects the storage system directly to the solar array's DC bus before the shared inverter, allowing both resources to share one grid interconnection point.
Question 7: What does the term 'state of health' (SOH) measure for a battery energy storage system?
- Current energy stored relative to full capacity
- Remaining usable capacity relative to original rated capacity as the battery ages (Correct answer)
- Temperature differential between cell core and ambient environment
- Ratio of charge cycles completed to rated cycle life
Correct answer: Remaining usable capacity relative to original rated capacity as the battery ages
SOH expresses how much of the battery's original capacity remains available, typically declining from 100% to an end-of-life threshold (often 80%) through aging and cycling.
REP Storage and Grids 4
Which storage duration category is most appropriate for seasonal energy storage needed to balance renewable generation across months?