What Is a Thermal Energy Storage System?
Thermal energy storage (TES) makes ice or chilled water at night (cheap off-peak electricity) and melts it for cooling during the day (expensive peak hours) — shifting the building's cooling load to off-peak rates. The MEP design covers the storage tanks, the chiller's ice-making mode, and the controls orchestrating the charge/discharge cycles. TES cuts demand charges (often the largest line on commercial electric bills) by 20-40%.
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How TES works
At night, the chiller runs in ice-making mode (colder evaporator temperatures — less efficient per ton-hour, but electricity costs a third of peak rates), freezing water in storage tanks (internal-melt ice coils or ice-on-coil systems). During the day, building return water circulates through the melting ice — 32°F water cooling the building while the chiller stays off or runs reduced. The controls optimize: charge fully on cheap nights, discharge strategically through the peak window.
Design considerations
The MEP engineer sizes: the storage capacity (ton-hours — from the building's peak-day cooling profile and the utility's rate structure), the chiller (sized for the ice-making duty — larger than a conventional chiller for the same building, because it must make ice in the overnight window), tank footprint (the space requirement — often the constraint in retrofits), and the controls (the rate-structure-aware sequencing that captures the savings). Partial storage (shaving the peak) vs. full storage (the chiller never runs on-peak) — the economic optimization.
Economics
TES pays where demand charges dominate (utilities with $15-$30+/kW-month demand rates — the storage erases the peak), where time-of-use spreads are wide, and where utility incentives reward load-shifting. The chiller and tank premium ($100,000-$1M+ depending on scale) pays back in 3-8 years on strong rate structures — and provides resilience (the stored cooling rides through daytime outages). Evaluate against battery storage — thermal storage is far cheaper per kWh for cooling loads.
The bottom line
Thermal energy storage shifts cooling to off-peak rates via ice/chilled-water tanks — cutting demand charges 20-40%. Size from the cooling profile and rate structure; evaluate against batteries.
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