Off-Peak Chilled Water Storage Tank: How Time-of-Use Tariffs Drive Savings
Created on 10.10
Off-Peak Chilled Water Storage Tank: How Time-of-Use Tariffs Drive Savings
An off-peak chilled water storage tank lets a facility run chillers at full output during cheap nighttime electricity and serve the afternoon peak from stored cold. Saving comes from the gap between valley and peak rates, and from shrinking connected chiller capacity where the tariff rewards it. Size the store so charging finishes before the rate rises, and treat usable volume as 70 to 90 percent of the theoretical figure. Payback follows the rate spread, not the vessel size alone.
Cooling sits inside the largest slice of an electricity bill after the IT load, and it is the part that can be moved. A chiller does not have to run exactly when the heat appears if the cold is made earlier and held. That is the basis of a thermal battery data center arrangement built on chilled water storage, and it pays only where the utility prices power by the time of day.
The tariff decides whether the case exists. A flat commercial rate gives little reason to store, because three in the morning and six in the evening cost the same. A time-of-use tariff with a wide gap between valley and peak turns storage into a straightforward arbitrage, and where a demand charge applies the peak demand matters more than the energy. Either way the answer is the same: produce cooling in the cheap window and let the tank carry the expensive one. What follows covers reading a tariff, the charge and discharge schedule, a worked calculation, and the limits on how much load can move.
Reading a tariff before sizing anything
Three charges appear on most supply agreements, and each reacts to storage differently.
Charge | What it measures | Effect of storage |
Energy charge | kilowatt-hours in each period | Falls if load moves to the valley window |
Demand charge | Highest sustained kilowatt in a billing interval | Falls if the chiller peak is clipped by the store |
Reactive or power factor charge | kVA or power factor at peak | Unaffected by cooling storage |
A valley window is usually overnight and at weekends; peak usually covers late afternoon in summer. The spread is what pays for the tank, and it should come from the supply agreement rather than a published average. A ratchet clause may bill a standby demand at a fraction of the highest monthly peak, which changes the value of clipping, and some tariffs apply the higher rate only on selected days, so a summer-week analysis is safer than an annual average.
Building the charge and discharge schedule
The operating pattern is the same everywhere, only the numbers differ.
1. Charging window. Chillers run at or near full load while the rate is low, making 5 to 7 °C chilled water into the store. Plant output exceeds the load because part of it goes into thermal mass.
2. Storage phase. After the cheap window closes, chillers throttle back or stop; standing losses over a few hours are a fraction of a degree.
3. Discharge phase. Cold is withdrawn to the load, return temperature climbs to 12 °C or higher, and the store feeds the room until it reaches its residue level.
4. Refill. The next valley window refills the store before the peak, which sets the daily charge requirement.
Two numbers drive sizing. Usable energy is about 1.163 kWh per cubic metre per kelvin, so a wider dT from 6 °C to 13 °C holds more cold than a tighter loop without a larger vessel. Usable capacity is only 70 to 90 percent of the theoretical figure once stratification and diffuser mixing are counted.
A worked saving calculation
Take a 2,000 kW average cooling load, a valley rate of 40 and a peak rate of 120 currency units per megawatt-hour, with a demand charge on the highest 15-minute kilowatt.
Period | Hours | Plant action | Rate band |
Valley, 23:00 to 07:00 | 8 | Chillers at full output, store filling | Low |
Morning shoulder | 4 | Chillers on load-following | Mid |
Peak, 11:00 to 17:00 | 6 | Chillers off or derated, store discharging | High |
Evening shoulder | 5 | Plant reverses from store to load | Mid |
5. Daily cooling energy is 2,000 kW for 23 hours, or 46,000 kWh.
6. Charging part of that into the store during the valley adds roughly 3,500 to 4,500 kWh of electrical input, depending on chiller efficiency.
7. The shifted volume moves from the peak rate to the valley rate. At a spread of 80 units per megawatt-hour, 10,000 kWh shifted saves 800 units per day.
8. If the store clips the plant peak by 400 kW, the demand charge drops by that amount for the month, often comparable to the energy saving.
9. Required volume for 10,000 kWh at 7 K usable dT and 0.8 usable fraction is 10,000 / (1.163 x 7 x 0.8) = 1,529 m3.
Those daily figures accumulate over a year, and the same calculation on a summer week gives the number to show a finance committee. The result tracks the rate spread and peak hours, not the tank cost.
What limits an off-peak chilled water storage tank
Storage changes plant size rather than removing it, and five ceilings apply.
10. Space and structural load. A store of that size occupies a footprint several times its water content once insulation, access, and slab loading are included.
11. Chiller running hours. Continuous overnight operation shortens the interval between overhauls.
12. Partial-load efficiency. A plant sized for a large daily swing spends part of its time at low load, so peak reduction is not free.
13. Charging window length. The fill must finish before the rate rises; if the window is shorter than the fill time, the case does not close.
14. Failure reserve. A tank used only for tariff shifting has no cold left when a chiller trips, so most designs keep a residue layer for that case.
That last item is often the negotiating point: a tank sized purely for the tariff may be smaller than one that also covers a chiller failure.
Technical Specification
Item | Typical value or option | Note |
Usable dT | 6 to 13 °C, or 5 to 7 °C on a tighter loop | A wider dT cuts required volume |
Usable fraction | 70 to 90 percent | Accounts for stratification and mixing |
Charging setpoint | Chilled water 5 to 7 °C | Matches normal plant output |
Insulation | Mineral wool with aluminium or colour-steel cladding | Limits standing loss during storage |
Tank pressure class | Atmospheric or pressurised | Sets wall thickness and nozzle design |
Project Case
While our delivered reference projects in the water and wastewater sector include large potable water storage, the data-center TES scope is engineered to the same standards. One relevant reference is a potable water installation in Namibia.
Item | Detail |
Project | Namibia — potable water storage |
Product | 01 GFS, glass-fused-to-steel bolted tank |
Capacity | 44,900 m3 total across 4 tanks |
Dimensions | phi 41.26 m x 8.4 m, 4 units |
Completion | 2022-09 |
That vessel type gives the plan area and cold layer geometry a thermal store needs, where an off-peak chilled water storage tank holds cold for most of its life rather than moving it.
Center Enamel Engineering Capability
Center Enamel (Shijiazhuang Zhengzhong Technology Co., Ltd) has designed and fabricated bolted storage tanks since 2008. As the first glass-fused-to-steel (GFS) tank manufacturer in China, the company holds close to 200 enamel-related patents, produces roughly 300,000 enamel-coated steel plates a year, has completed more than 30,000 installed projects and supplies its tanks to over 100 countries. The new 150,000 m² production base was added to raise output capacity, and single tanks are supplied up to 60,000 m³. Manufacturing runs under ISO 9001 and ISO 45001, with product certification including NSF/ANSI 61, WRAS, FDA, LFGB, CE (EN 1090), ISO 28765, FM, BSCI and EUROCODE, and design referenced to AWWA D103-09, AWWA C550 and NFPA where the application requires it.
Charge window capacity
· Off-peak charging only pays if the tariff gap is large enough to cover the extra chiller runtime and the standing loss, so the tank volume follows from the tariff shape as much as from the cooling load.
· A bigger store lets the chiller run flat out during the cheap window instead of cycling, which is the operating pattern that produces the saving.
· Our capacity envelope of single tanks up to 60,000 m³, and deliveries such as the Namibia 44,900 m³ scheme, is the headroom for charging an entire shift in one go.
Frequently Asked Questions
Q1: Is storage worth building on a flat tariff?
A1: Rarely on energy charges alone. The case usually needs a demand charge or a peak that can be clipped, so read the supply agreement before the headline unit rate.
Q2: How many hours of storage are typical?
A2: Two to eight hours of full cooling load is the common range. The figure follows from the peak length and from how much of the peak the plant leaves to the store.
Q3: Does the tank reduce the chiller size needed?
A3: It can, where demand is metered per facility and the charging window supports it. Where demand is billed across the whole site, the chiller may still need full capacity.
Q4: What happens to efficiency when chillers run overnight?
A4: Chillers at steady full load are efficient, but part-load hours and pumping energy belong in the calculation, or the saving gets overstated.
Q5: Can one tank serve tariff shifting and failure reserve?
A5: Yes. Most designs keep a residue layer above the low-level alarm for a chiller trip and fill the remainder on the tariff schedule.
Q6: How long does a store hold cold without use?
A6: With mineral wool insulation and intact cladding a standing store loses a fraction of a degree per hour, which makes a daily cycle practical.
An off-peak chilled water storage tank pays for itself where the gap between valley and peak prices is wide, and it pays back faster when a demand charge rewards a lower plant peak. The method is short: shift energy into the cheap window, size the volume from usable dT and the derating for stratification, and confirm that charging finishes before the rate changes. Re-run the numbers seasonally, because the spread moves between winter and summer. The result worth carrying into a design meeting is three figures: cubic metres required, hours of shifting the rate structure supports, and the annual saving at current prices.
Talk to an Engineer
Send your tariff schedule with the valley, shoulder and peak windows, your cooling load curve, and whether a demand charge applies. We will return the required volume, hours of shifting, the annual saving calculation, and a specification sized to the charging window.
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