Thermal energy storage (TES) lets a cooling plant make chilled water or ice at one time and use it later. Most commonly, chillers run at night when electricity is cheaper and demand on the grid is lower, and the stored cooling is used during the afternoon peak. Storage can reduce peak electrical demand, shrink chiller plant size, and provide backup cooling when chillers are offline.
Storage is not new; many large campuses and district systems have used it for decades. Changes in utility rate structures, peak-demand programs and electrification are renewing interest in Canada. This guide explains the main technologies, operating strategies and the factors that determine whether storage makes sense.
How storage creates value
| Benefit | How it works |
|---|---|
| Lower demand charges | Chillers run less during peak hours, cutting the building's peak kW |
| Lower energy cost | More cooling energy is bought at off-peak rates where time-of-use pricing applies |
| Smaller chiller plant | Chillers are sized for average rather than peak load in partial storage designs |
| Capacity for growth | Adding storage can meet new load without new chillers |
| Resilience | Stored cooling can carry critical loads during chiller failure or power interruption |
| Grid support | Load shifting helps utilities manage peaks, sometimes with incentives |
Value depends heavily on the utility rate structure. Large demand charges, time-of-use energy rates or programs that charge customers based on demand during system peak hours make storage more attractive. In Ontario, for example, large customers' share of certain electricity costs is based on their demand during a small number of province-wide peak hours, which gives strong incentive to shift load. Rate structures vary by province and utility and change over time, so analysis must use current tariffs. See utility incentives for HVAC upgrades in Canada.
Chilled water storage
Chilled water storage uses a large insulated tank, usually stratified. Cold water, being denser, stays at the bottom while warmer return water sits at the top, separated by a thin transition layer called the thermocline. Diffusers at the top and bottom introduce water slowly enough to preserve stratification.
Storage capacity depends on tank volume and the temperature difference between cold and warm water. Each cubic metre of water stores about 4.19 MJ per kelvin, or about 11.6 kWh of cooling at a 10 K (18 F) delta-T. Large delta-Ts are therefore critical: a plant suffering from low delta-T loses usable storage capacity in direct proportion.
Characteristics
- Chillers operate at normal chilled water temperatures, so efficiency is not penalized.
- Tanks are large and are often built as above-ground steel tanks or below-grade concrete tanks on campuses.
- Economies of scale are strong: large tanks cost much less per unit of storage than small ones.
- A chilled water tank can double as a fire protection water reservoir in some cases, subject to the authority having jurisdiction.
Ice storage
Ice storage takes advantage of water's latent heat of fusion, about 334 kJ/kg. Ice stores far more cooling per unit volume than chilled water, typically needing several times less space.
Common ice storage types
- Internal melt ice-on-coil: glycol circulates through coils inside a tank of water. During charging, ice builds on the coils; during discharge, warm glycol melts the ice from the inside. Common in packaged modular tanks for commercial buildings.
- External melt ice-on-coil: ice forms on coils, and water circulates around the outside of the ice during discharge. Provides very cold water quickly, used in district cooling and process applications.
- Encapsulated ice: water-filled containers in a tank, frozen and melted by glycol flowing around them.
- Ice harvesters: ice forms on evaporator plates and is periodically released into a storage bin.
Characteristics
- Charging requires chillers to produce glycol at sub-freezing temperatures, typically several degrees below 0 C. This reduces chiller capacity and efficiency compared with normal chilled water operation.
- Discharge can deliver colder water than conventional plants, enabling low-temperature air distribution with smaller ducts and fans, though this needs careful design to avoid condensation and comfort problems.
- Smaller footprint than chilled water storage for the same capacity.
Full vs partial storage
Full storage shifts the entire peak-period cooling load to storage, so chillers are off during peak hours. It maximizes demand savings but needs large storage and chiller capacity sufficient to recharge overnight.
Partial storage runs chillers during peak hours as well, with storage covering the difference between chiller capacity and load. Load leveling, where chillers run at constant capacity around the clock, minimizes chiller size. Demand-limiting partial storage runs chillers at reduced capacity during peak periods. Partial storage is more common in commercial buildings because it balances capital cost and savings.
Design and operation
- Develop an hourly load profile for the design day and for typical days through the cooling season.
- Analyze the utility tariff to identify what peak demand and energy are worth.
- Choose the storage strategy (full, load leveling or demand limiting) and the technology.
- Size chillers and storage together, including recharge time and chiller capacity at charging conditions.
- Design hydronics, including heat exchangers to separate glycol from building water where needed.
- Write control sequences for charging, discharging and chiller priority, using weather and load forecasts where possible.
- Commission with full charge and discharge tests, measuring delivered capacity.
Controls matter a great deal. A storage system that discharges too early in the day and runs out before the peak, or that recharges during peak hours, can increase costs. See chiller plant sequencing and optimization.
Storage and electrification
As buildings electrify heating, winter electrical peaks grow. Hot water storage, and storage paired with heat pumps and heat recovery chillers, applies the same principle to heating. Chilled water storage is also used in data centres to provide thermal ride-through while chillers restart after a power interruption.
Frequently asked questions
Does thermal storage save energy?
Not necessarily. It usually saves money by shifting when energy is used, and can reduce total energy if chillers run more efficiently at night with cooler condenser conditions. Ice making typically uses more energy per unit of cooling, which must be offset by rate savings.
Is storage practical for a small building?
Packaged ice tanks make storage feasible at modest scales, but economics depend on tariffs and plant size. A detailed analysis is needed.
Key takeaways
- Thermal storage shifts cooling production to off-peak hours, cutting peak demand and sometimes plant size.
- Chilled water storage needs large tanks but keeps chiller efficiency high; ice is compact but penalizes chiller efficiency during charging.
- Large delta-T is essential for chilled water storage capacity.
- Utility rate structures drive the economics, and they vary by province and utility.
- Storage strategy and controls should be tested with hourly energy modeling using current tariffs.
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