I. How a building banks cold overnight
An ice-storage system runs the building's chillers (the machines that make chilled water for air conditioning) mostly at night. Instead of sending that cold straight to the air handlers, it freezes tanks of water, or chills a large insulated tank close to freezing. By morning the building holds a bank of stored cooling, measured in ton-hours (one ton of cooling is, by the old definition, roughly the rate it takes to melt about a ton of ice over a day).
During the afternoon peak the chillers throttle back or shut off, and the building draws its cooling from the tank rather than from the grid. The electricity to run the compressors was already spent hours earlier, off peak. What the utility meter sees in the hot part of the day is a smaller, flatter draw, which is the whole point.
II. Where Houston already runs the trick
The clearest local example sits in the Texas Medical Center. Thermal Energy Corporation, the nonprofit that supplies chilled water and power to the medical campus, runs a central plant with a large chilled-water thermal storage tank, on the order of several million gallons at last public description. It chills water when demand is low and pulls from the tank when the campus load climbs through the day.
Downtown office towers and some college campuses use smaller versions of the same idea, often ice tanks tucked onto a mechanical floor or into a basement. The economics work best for buildings with a sharp afternoon peak and a long overnight lull, which describes a lot of Houston: offices that empty by evening, schools, and houses of worship that fill for a few hours and sit quiet the rest of the week.
III. The demand-charge and 4CP math
Two numbers decide whether storage pays. The first is the demand charge, the fee a commercial customer pays on the single highest fifteen-minute burst of power it pulled all month, billed in dollars per kilowatt. Shave a few hundred kilowatts off a July afternoon, and at the several-dollars-per-kilowatt range common on commercial rates, the saving runs into the thousands every summer month.
The second, and often larger, is a mechanism ERCOT calls Four Coincident Peak, or 4CP. For large commercial and industrial accounts, a big share of next year's transmission cost is set by the building's average demand during the four fifteen-minute intervals when the whole ERCOT grid peaked, once each in June, July, August, and September. Ride through those four moments on stored cold rather than live chiller power, and the transmission bill for the following year drops. Nobody knows in advance which afternoons will set the 4CP, which is why load-shifting that runs on every hot day tends to win over guessing the forecast.
IV. Worth watching this month
1. ERCOT's seasonal assessment and any new system peak-demand record, since how high and how often the grid peaks shapes every 4CP bet (routine reports, but the numbers matter).
2. The monthly ERCOT system peaks for the summer, published after the fact, which together set next year's 4CP transmission charges for large loads.
3. Any CenterPoint Energy rate case at the Public Utility Commission of Texas, since a change to delivery or demand charges changes the storage math (these move slowly and matter a lot).
4. Utility or federal efficiency incentives for thermal storage or peak-shifting equipment, which open and close on their own schedules (mostly routine, but worth a check before you buy).
5. Your own interval data from your retail provider, which shows exactly when your peak lands and whether a tank would have caught it.
Questions readers ask
Is ice storage the same as a battery?
No. A battery stores electricity; thermal storage stores cold itself, as ice or chilled water, and only helps with cooling load. It is usually cheaper per unit of shifted load, but it does just the one job.
Does it lower my total energy use?
Not really, and sometimes it uses a little more, because chillers are slightly less efficient making ice at night. The savings come from when you use power, not how much.
Would it help a single-family home?
Rarely worth it. Homes do not pay demand charges or 4CP the way large commercial accounts do, so the math that makes storage pay mostly is not there.
How long do these systems last?
The tanks are long-lived, often decades, while the chillers and controls follow normal equipment lifecycles. Plan on the mechanical parts, not the tank, driving any replacement.
Sources of record
- ERCOT (Texas grid operator), Grid conditions, seasonal assessments, and system peak demand data for the market that sets 4CP., www.ercot.com
- Public Utility Commission of Texas (state regulator), Rate cases and rules governing CenterPoint delivery and demand charges., www.puc.texas.gov
- U.S. Department of Energy, Building Technologies Office (federal), Background on how thermal energy storage works and where it fits in a building., www.energy.gov/eere/buildings
- CenterPoint Energy (Houston delivery utility), The poles-and-wires company whose delivery and demand charges show up on commercial bills., www.centerpointenergy.com
- Thermal Energy Corporation (Texas Medical Center district energy), Operator of the medical center's chilled-water plant and large thermal storage tank., www.thermalenergycorp.com