Sizing and Dispatching a 6,000 Ton-Hour Thermal Storage Tank
A university chilled-water plant has two 1,000-ton constant-speed centrifugal chillers at 0.60 kW/ton including auxiliaries. The design-day load is 2,000 tons between noon and 6 p.m. on weekdays; night load is 400 tons. The utility charges $0.21/kWh from noon to 6 p.m. weekdays, $0.058/kWh all other hours, plus $22/kW-month demand. A vendor proposes a 6,000 ton-hour stratified TES tank at $2.1M, with a $300/kW incentive based on verified on-peak demand reduction. Chillers can recharge from 10 p.m. to 7 a.m. (9 hours) at about 0.65 kW/ton. Chilled-water supply must remain at 42°F; the tank returns at 56°F. The campus energy manager must justify the project to the board on a 10-year simple-payback test and asks whether the tank can actually be recharged and how it should be dispatched.
1. Can the tank be fully recharged overnight? What actually limits the recharge, and what operating problem appears once charging is complete?
Read the worked answer
Recharge capacity available = (2,000 - 400) tons x 9 h = 14,400 ton-hours Required = 6,000 ton-hours Utilization = 42%. Yes, easily - with roughly 2.4x margin.
The tank fills in 6,000 / 1,600 = 3.75 hours, i.e., by about 1:45 a.m., not 7 a.m.
What actually limits recharge (not chiller tonnage):
- Tank charge rate. A stratified tank discharges and charges at a design flow for a 12–16°F spread. 1,600 tons at a 14°F spread is about 2,740 gpm; if the diffuser or the piping was sized for less, the tank sets the schedule, not the chillers. Get the tank's design gpm in writing - this is where vendor proposals usually hide a mismatch.
- Chiller turndown after charging. Once the tank is full at 1:45 a.m., the load drops to 400 tons - 20% of a single 1,000-ton centrifugal, close to or below its surge/turndown limit. The sequence must drop to one machine and verify it holds; two machines at 20% each will cycle and surge.
- Colder supply. Making 42°F water at night costs about 8% more per ton (0.65 vs 0.60 kW/ton), which is already in the energy math below - do not let anyone 'improve' the case by assuming equal efficiency.
Net read: the recharge is feasible with margin, so the project cannot be killed on recharge grounds. Its feasibility rests entirely on dispatching the cubic tons well and on the building's delta-T holding up.
2. Compare a flat 1,000-ton discharge against a 'maximum chiller off' strategy. Which do you specify, and why does the building's delta-T determine whether either one actually saves money?
Read the worked answer
Load to be served on peak: 2,000 tons x 6 h = 12,000 ton-hours. Tank holds 6,000 → 1,000 tons/hour for 6 hours.
| Strategy | Chillers on peak | kW at 0.60 kW/ton | Reduction vs baseline |
|---|---|---|---|
| No TES (baseline) | 2,000 tons | 1,200 kW | - |
| Flat 1,000-ton discharge | 1,000 tons (one machine at full load) | ~600 kW | 600 kW |
| Max chiller off (1,300 t/h discharge) | 700 tons for ~4.6 h, then 2,000 | ~420 kW, then 1,200 | 780 kW for 4.6 h only |
Specify the flat 1,000-ton strategy. Reasons:
- Billing demand is set by the monthly 15-minute peak, not by the average. The max-off strategy produces a deeper but shorter reduction and the plant is back at 1,200 kW for the last 1.4 hours - so it captures essentially the same demand dollars while adding control complexity.
- One centrifugal running near full load is materially more efficient and more stable than two machines at 50%, which is where constant-speed centrifugals surge and hunt.
- The flat strategy is predictable enough to be verified with 15-minute interval data, which is what the utility incentive will be paid on.
Why delta-T decides everything: the tank is charged and discharged at a 14°F spread (42°F supply, 56°F return).
- At design delta-T: 6,000 ton-hours lasts the full 6 hours and shaves 600 kW all afternoon.
- If the building's delta-T collapses to 8°F (low-delta-T syndrome from leaking valves and fouled coils), the tank is drawn down ~43% faster - it empties in about 3.4 hours, the chillers pick up the full 2,000-ton peak for the last 2.6 hours, and the billing peak is unchanged while you have paid $2.1M for the tank.
Therefore: make a verified delta-T of at least 12°F a condition precedent in the contract. Require a spring delta-T survey at each coil, a valve-leakage test, and a low-delta-T alarm in the TES control sequence before the tank is accepted.
3. Run the 10-year simple-payback test. What has to be true for the project to pass, and what non-energy benefit would you put in front of the board?
Read the worked answer
Energy (per full-discharge day):
- On-peak kWh displaced: 6,000 ton-h x 0.60 kW/ton = 3,600 kWh x $0.21 = $756
- Off-peak kWh added: 6,000 ton-h x 0.65 kW/ton = 3,900 kWh x $0.058 = $226
- Net = $530/day; at ~150 full-load days/yr = $79,500/yr
Demand: 600 kW x $22/kW-mo = $13,200/month
- If the 12–6 p.m. cooling sets the campus billing peak in 12 months: $158,400/yr
- If only 7 months (typical cooling-driven peak): $92,400/yr
Payback:
| Case | Annual savings | Net cost ($2.1M - $180k incentive) | Simple payback |
|---|---|---|---|
| 12 peak months | ~$238,000 | $1,920,000 | 8.1 yr - passes |
| 7 peak months | ~$172,000 | $1,920,000 | 11.2 yr - fails |
The project sits right on the 10-year line, so the answer is: not yet - prove the demand coincidence first.
What must be true to pass:
- The 12–6 p.m. cooling load demonstrably coincides with the campus billing peak in at least 10 of 12 months. Pull 24 months of billing history and check the demand interval timestamps - this single fact swings the payback by three years.
- The distribution delta-T is restored to ≥ 12°F before the tank is sized and accepted.
- The utility incentive is paid on measured kW at the meter over a defined M&V period, not on modeled savings.
- The 150 full-discharge days per year is realistic - verify with 15-minute load data, not a rule of thumb.
Non-energy benefit to put in front of the board:
- Capacity deferral. The tank covers 1,000 tons of peak; without it, the next plant addition is a 1,000-ton chiller plus switchgear and service upgrade - realistically $1.2M–$1.8M, and it may require a new electrical service the campus cannot easily get. TES delivers the same capacity with no new electric service and no new refrigerant charge.
- N+1 resilience. With the tank charged, one chiller can be taken down for maintenance on a design day without shedding load - valuable for labs and animal facilities.
- Emissions timing, if the campus reports scope-2 at a grid-average or time-of-use factor.
Recommendation: proceed to a detailed design and a firm M&V plan conditional on the 10-of-12-month peak coincidence test and the delta-T remediation. Do not sign the tank contract on the vendor's modeled number.