Lesson 12 – Controls, Valves, and Why TES Systems Fail in Practice
Lesson Purpose
This lesson explains why many TES systems fail not because of wrong sizing or wrong concepts, but because of control logic, valve behavior, and real-world operation.
TES is highly sensitive to control quality.
Small mistakes here can destroy the value of the entire system.
The Illusion of “Correct Design”
Many TES projects fail after commissioning, even though:
- The tank volume is correct
- The strategy is correct
- The equipment selection is reasonable
The failure usually happens between design intent and actual operation.
That gap is almost always caused by:
- Controls
- Valves
- Sequencing
Why TES Is Control-Sensitive
TES systems operate in multiple modes, such as:
- Charging
- Discharging
- Normal cooling
- Hybrid operation
Each mode requires:
- Clear enable/disable logic
- Strict separation from other modes
- Predictable transitions
If modes overlap, TES loses its purpose.
The Most Common Control Failure
The most frequent TES control failure is simple:
“The system allows chillers and TES to operate together during peak hours.”
This happens due to:
- Poorly defined priorities
- Conservative safety logic
- Manual overrides that never get removed
Once this happens:
- Peak demand reduction disappears
- TES becomes an expensive buffer tank
- Operators lose confidence in the system
Valves: The Silent TES Killers
Control valves are often treated as minor components.
In TES systems, they are critical elements.
Common valve-related issues include:
- Leakage through isolation valves
- Improper valve authority
- Slow or unstable response
- Valves not fully closing
Even small leakage can:
- Mix hot and cold layers
- Collapse ΔT
- Reduce usable storage capacity
A Simple Failure Example
Design Intent
- TES designed to cover 3 peak hours
- Chillers off during peak
- Storage supplies cooling
What Happens in Practice
- One bypass valve leaks slightly
- Return temperature rises
- ΔT drops from 14°F to 10°F
- Stratification degrades
Result:
- Storage capacity drops by ~30%
- Chillers start during peak to “help”
- Demand reduction disappears
The system still cools the building — but TES has failed.
Why These Failures Go Undetected
TES failures are often invisible because:
- Comfort is maintained
- Equipment is running
- No alarms are triggered
What is lost is:
- Economic value
- Demand reduction
- Strategic benefit
TES failures are silent failures.
Controls Should Be Simple, Not Clever
A common mistake is:
- Overengineering the control logic
- Adding unnecessary modes and conditions
Successful TES systems use:
- Clear priorities
- Minimal overrides
- Simple, enforceable rules
Complex control logic often increases risk rather than reducing it.
The Role of Commissioning
TES systems cannot be commissioned like conventional HVAC systems.
They require:
- Mode-by-mode testing
- Verification of valve closure
- Confirmation of ΔT under each mode
- Operator training aligned with design intent
Skipping this step guarantees underperformance.
Engineering Judgment Perspective
Experienced engineers know:
TES does not forgive small mistakes. It amplifies them.
This is why TES success depends more on discipline than creativity.
Key Takeaways from This Lesson
- TES failures are usually control-related
- Valves are as important as tanks
- Mode overlap destroys TES value
- ΔT collapse is often a control symptom
- Simple control logic outperforms clever logic
Important Reflection
Before moving on, ask yourself:
If a single leaking valve can destroy 30% of storage capacity,
how much attention should controls receive during design?
That answer defines TES success.