Lesson 11- Night Charging and Day Discharging: The Operational Reality of TES
Lesson Purpose
This lesson explains how TES systems actually operate over a 24-hour cycle, and why many TES concepts that look perfect on paper fail or underperform in real operation.
The focus here is operation, not design.
Why Operation Is Where TES Is Won or Lost
Most TES systems are:
- Correctly sized
- Properly selected
- Technically sound
Yet many of them fail to deliver expected benefits.
The reason is simple:
“TES success depends more on how the system is operated than on how it is designed.”
The Basic TES Operating Cycle
A TES system typically operates in two main modes:
- Night Charging Mode
- Day Discharging Mode
Each mode has a different objective and different constraints.
Night Charging Mode
What Happens at Night
During off-peak hours:
- Electricity is cheaper
- Grid demand is lower
- Ambient temperatures are often lower
This is when:
- Chillers operate to produce cooling energy
- Storage tanks are charged
- TES prepares for the next day’s peak
Why Night Charging Is Not “Free”
A common misconception is:
“Night charging is cheap, so efficiency does not matter.”
In reality:
- Chillers may operate at lower temperatures (especially with ice storage)
- Efficiency during charging can be lower
- Longer run hours increase mechanical wear
Night charging must be controlled and optimized, not abused.
Day Discharging Mode
What Happens During the Day
During peak hours:
- Cooling demand is high
- Electricity tariffs are highest
- Grid stress is maximum
This is when:
- Stored cooling energy is released
- Chiller operation is reduced or stopped
- TES delivers its primary value
The Most Common Operational Mistake
The most frequent TES operating error is:
“Running chillers and TES at the same time during peak hours.”
This happens due to:
- Poor sequencing
- Operator override
- Conservative control logic
When this occurs:
- Demand reduction is lost
- TES economic value collapses
- The system becomes unnecessarily complex
A Simple Operational Example
Design Intent
- TES designed to cover 3 peak hours
- Target: avoid chiller operation during peak
Good Operation
- Chillers:
- ON at night (charging)
- OFF during peak
- TES:
- Charged fully before peak
- Discharged during peak
Result:
- Peak demand minimized
- TES objective achieved
Poor Operation
- Chillers:
- ON at night
- ALSO ON during peak “just in case”
- TES:
- Partially discharged
- Underutilized
Result:
- Peak demand still high
- TES investment wasted
Why Operators Do This
Operators often prioritize:
- Comfort
- Safety
- Risk avoidance
Without clear operational philosophy, TES becomes:
- A backup system
- Instead of a primary strategy
This is not an operator failure — it is a design communication failure.
TES Requires Operational Discipline
Successful TES systems have:
- Clear mode definitions
- Simple, transparent sequencing
- Minimal manual overrides
- Operator training aligned with design intent
TES does not tolerate ambiguity.
Key Takeaways from This Lesson
- TES value is delivered during operation, not design
- Night charging must be controlled, not excessive
- Day discharging must be protected from chiller interference
- Running chillers during peak defeats TES purpose
- Operational clarity is more important than control sophistication
Important Reflection
Before moving on, ask yourself:
“If the operator does not fully trust the TES system,
will it ever be allowed to do its job?”
That question determines TES success.