Lesson 8 – Stratified Tanks and the ΔT Trap
Lesson Purpose
This lesson explains how chilled water storage tanks actually work internally, and why many TES systems fail even when the tank size is theoretically correct.
The focus here is not tank geometry, but thermal behavior and its direct impact on usable storage capacity.
What Is a Stratified Tank? (Simple Definition)
A stratified chilled water tank relies on a simple principle:
” Cold water stays at the bottom,
warm water stays at the top,
with a relatively thin transition zone in between.“
This vertical temperature separation is called stratification.
When stratification is maintained:
- The tank delivers close to its full designed ΔT
- The stored energy is usable
When stratification collapses:
- ΔT drops
- Usable storage drops
- TES performance collapses
Why Stratification Matters More Than Tank Size
A chilled water tank does not store energy simply because it is large.
It stores energy only if:
- Cold and warm layers remain separated
- Mixing is minimized
- The designed ΔT is preserved
A perfectly sized tank with poor stratification will underperform.
The ΔT Trap (Common but Dangerous)
The ΔT trap occurs when:
- The tank is sized assuming a certain ΔT
- But the system operates at a lower effective ΔT
This gap between design ΔT and operating ΔT is the root cause of many TES failures.
Numerical Example: Same Tank, Different ΔT
Design Intent
- Required storage: 3,000 ton-hours
- Design ΔT: 14°F
- Tank volume selected accordingly (from Lesson 7 logic)
At design conditions:
- Tank delivers ≈ 3,000 ton-hours
- Peak period is fully covered
What Happens in Real Operation
Now assume that due to:
- Poor flow control
- Improper diffuser design
- High return temperature instability
The effective ΔT drops to 10°F.
Impact on Usable Storage
From basic TES relationships:
- Storage capacity is directly proportional to ΔT
- Reducing ΔT reduces usable energy even if volume stays the same
So:
- Designed ΔT = 14°F → 100% capacity
- Actual ΔT = 10°F → ~71% capacity
Usable storage becomes:
- 3,000 ton-hours × (10 / 14) ≈ 2,140 ton-hours
What This Means Practically
Instead of covering:
- 1,000 TR for 3 hours
The tank can now only cover:
- 1,000 TR for just over 2 hours
The remaining peak hour must be met by:
- Running chillers during peak
- Or exceeding electrical demand targets
The TES system is still “working” — but it is failing at its primary objective.
Why This Failure Often Goes Unnoticed
This type of failure is subtle because:
- The tank is physically present
- The system appears to operate normally
- Comfort is still maintained
But:
- Peak demand reductions are smaller than expected
- Economic benefits do not materialize
- TES is blamed instead of ΔT control
Key Insight for Engineers
TES tanks do not fail because they are too small.
They fail because:
- ΔT collapses
- Stratification degrades
- Usable energy shrinks silently
This is why experienced engineers treat ΔT as a design constraint, not an assumption.
Key Takeaways from This Lesson
- Stratification is the backbone of chilled water storage
- Tank volume alone does not guarantee storage capacity
- ΔT directly controls usable ton-hours
- Small ΔT losses create large performance losses
- Most TES failures are operational, not conceptual
Important Reflection
Before moving on, ask yourself:
“If your TES tank delivers only 70% of its expected capacity,
would you still call it a successful design?”
That question defines whether TES was truly understood.