9.7 Thermal Energy Storage: Full vs. Partial Storage, Chilled Water & Ice Systems

9.7 Thermal Energy Storage: Full vs. Partial Storage, Chilled Water & Ice Systems

Thermal energy storage (TES) decouples when cooling is produced from when cooling is used. It is listed under NCEES sub-topic 3D alongside chillers and heat pumps because the storage decision changes the chiller selection - sometimes dramatically.

State the economics up front, because exam distractors depend on it: TES almost never reduces annual energy consumption. Making ice at a 22 F suction temperature is less efficient than making 44 F chilled water. What TES buys is a cost shift - cheaper off-peak energy and, usually more valuable, a lower billed peak demand.

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1. Storage Media and the Volume Math

Sensible Storage - Chilled Water

Capacity comes from the temperature swing of the stored water:

$Q = \rho \, V \, c_p \, \Delta T$

Converting to the units engineers actually use, with 8.33 lb/gal and a specific heat of 1 Btu/lb-F, the gallons required per ton-hour are:

$\frac{\text{gal}}{\text{ton-hr}} = \frac{12{,}000}{8.33 \times \Delta T}$

• At a 16 F swing: 12,000 / 133.3 = 90 gallons per ton-hour
• At a 20 F swing: 12,000 / 166.6 = 72 gallons per ton-hour

The temperature swing is limited by what the coils can use. Stratified tanks rely on the density difference between roughly 40 F and 60 F water, and a thermocline of 1 to 3 ft separates the two layers. Excessive inlet velocity destroys stratification, so diffusers are sized to keep the inlet Froude number low.

Latent Storage - Ice

Ice stores the heat of fusion of water, 144 Btu/lb, which is far more than any practical sensible swing:

• Ice required per ton-hour: 12,000 / 144 = 83.3 lb
• As water volume: 83.3 / 8.33 = 10.0 gallons per ton-hour

Ice therefore needs roughly one-sixth the water inventory of a 16 F chilled water system. In practice the installed tank volume is larger than 10 gal/ton-hr - typically 2.4 to 3.3 ft3 (about 18 to 25 gallons) per ton-hour once you account for the heat exchanger coils, the unfrozen water that must remain to carry flow, and the fact that not all water is converted to ice. Even so, ice remains three to five times more compact than chilled water storage.

Ice Storage Configurations

Eutectic salt phase-change materials freeze near 47 F rather than 32 F, which lets a conventional chiller charge them without derating, at the cost of a much lower storage density (about 6 Btu/lb).

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2. Operating Strategies

Cooling load (tons)
  |          FULL STORAGE                     PARTIAL STORAGE
  |     +---------------+                +---------------+
  |     |   storage     |                |   storage     |
  |     |   discharge   |                |   discharge   |
  |  ---+---------------+---          ---+===============+---  <- level chiller output
  |  chiller off on-peak                 chiller runs 24 hours
  +------------------------------      ------------------------
        on-peak window                       on-peak window

Worked Example - Partial Storage, Load Leveling

A building has a peak cooling load of 400 tons, a total daily cooling requirement of 3,400 ton-hours, and a 10-hour on-peak window during which 3,000 ton-hours of cooling is required.

Step 1 - Level chiller size. Running continuously, the chiller only has to produce the daily average:

• 3,400 ton-hr / 24 hr = 142 tons

Step 2 - Storage required. During the on-peak window the chiller supplies 142 x 10 = 1,420 ton-hours directly, and storage must cover the rest:

• 3,000 - 1,420 = 1,580 ton-hours of usable storage

Step 3 - Compare tank volumes.

• Chilled water at 16 F swing: 1,580 x 90 = 142,000 gallons
• Ice, installed basis at roughly 21 gal/ton-hr: 1,580 x 21 = 33,000 gallons

Step 4 - Correct the chiller selection for ice-making. The 142-ton figure is a load requirement. A chiller producing 24 F glycol instead of 44 F water loses roughly 30% to 35% of its nameplate capacity, so the machine must be selected at approximately 142 / 0.68 = 209 nominal tons. Skipping this derate is the single most common error in TES problems.

Even after the derate, the plant is 209 nominal tons instead of 400 tons - and the entire on-peak electrical demand associated with 1,580 ton-hours has been removed from the utility bill.

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3. What Storage Does and Does Not Change

That low-temperature air distribution point is the underrated one. Ice systems can supply 44 F air rather than 55 F air, which nearly doubles the temperature difference the air carries. Supply airflow, duct size, and fan power all drop accordingly - and those fan savings can offset the chiller's ice-making penalty entirely.