ESCO Energy Storage Systems 1 — Questions and Answers
Question 1: What is the primary purpose of a battery energy storage system (BESS) in a commercial ESCO project?
- To replace all grid electricity with stored power
- To store excess energy and dispatch it when needed to reduce costs or improve reliability (Correct answer)
- To convert DC electricity to AC for building use
- To provide backup power during scheduled maintenance only
Correct answer: To store excess energy and dispatch it when needed to reduce costs or improve reliability
A BESS stores surplus or off-peak energy and dispatches it during peak periods to reduce demand charges and improve energy reliability.
Question 2: Which energy storage technology dominates new commercial and utility-scale installations as of the mid-2020s?
- Lead-acid batteries
- Pumped hydroelectric storage
- Lithium-ion batteries (Correct answer)
- Vanadium redox flow batteries
Correct answer: Lithium-ion batteries
Lithium-ion batteries dominate new commercial installations due to their high energy density, falling costs, and long cycle life.
Question 3: What does 'round-trip efficiency' measure in an energy storage system?
- The number of charge/discharge cycles before replacement
- The ratio of energy output to energy input over a complete charge-discharge cycle (Correct answer)
- The speed at which a battery can be fully charged
- The percentage of capacity remaining after one year of operation
Correct answer: The ratio of energy output to energy input over a complete charge-discharge cycle
Round-trip efficiency is the ratio of energy recovered during discharge to energy consumed during charging, expressed as a percentage.
Question 4: In a chilled water thermal energy storage (TES) system, energy is stored as:
- Compressed air pressure
- Chemical potential in a reaction vessel
- Cooling capacity, typically in the form of chilled water or ice (Correct answer)
- Electrical charge in a capacitor bank
Correct answer: Cooling capacity, typically in the form of chilled water or ice
Chilled water TES stores cooling energy by chilling water or making ice during off-peak hours for use during peak cooling periods.
Question 5: What depth of discharge (DOD) is generally recommended for lithium-ion batteries to balance usable capacity and cycle longevity?
- 100% — full discharge on every cycle
- 20-40% to preserve cell chemistry
- 80-90% to balance usable capacity and longevity (Correct answer)
- 50% exactly to prevent degradation
Correct answer: 80-90% to balance usable capacity and longevity
Operating lithium-ion batteries at 80-90% DOD balances maximizing usable capacity while maintaining reasonable cycle life.
Question 6: What is the 'usable capacity' of a battery energy storage system?
- The total nameplate capacity listed by the manufacturer
- The portion of total capacity that can be used without damaging the battery (Correct answer)
- The capacity available only during peak demand periods
- The maximum capacity achievable at freezing temperatures
Correct answer: The portion of total capacity that can be used without damaging the battery
Usable capacity is the portion of total rated capacity accessible within safe operating limits set by the battery management system.
Question 7: In ESCO projects, demand charge management through energy storage involves:
- Negotiating lower rates with utilities through long-term contracts
- Discharging stored energy during peak demand intervals to reduce the building's peak load (Correct answer)
- Installing smart meters to track demand in real time
- Curtailing non-essential loads during high-demand periods without storage
Correct answer: Discharging stored energy during peak demand intervals to reduce the building's peak load
Energy storage discharges during peak demand windows to shave the highest demand spikes, reducing the monthly demand charges on the utility bill.
ESCO Energy Storage Systems 1
What is the primary purpose of a battery energy storage system (BESS) in a commercial ESCO project?