Lesson 6 – Load Shifting vs Load Leveling: Selecting the Right TES Strategy
Lesson Purpose
This lesson explains the difference between load shifting and load leveling, and why confusing these two concepts leads to incorrect TES decisions.
Before discussing storage size or technology, an engineer must clearly define what TES is expected to achieve.
TES Is Not a Single Strategy
Thermal Energy Storage is often discussed as if it were one solution.
In reality, TES can be applied using different operating strategies, depending on:
- Load profile shape
- Peak duration
- Tariff structure
- Operational priorities
The two most common TES strategies are:
- Load Shifting
- Load Leveling
They serve different engineering objectives.
What Is Load Shifting?
Load shifting means:
“Producing cooling energy during off-peak hours and using it during on-peak hours.”
In this strategy:
- Chillers operate mainly at night
- Thermal storage is charged during off-peak hours
- During peak hours, cooling demand is met partially or fully from storage
The primary objective of load shifting is:
- Reducing peak electrical demand
- Minimizing demand charges
- Shifting energy use away from expensive tariff periods
What Is Load Leveling?
Load leveling means:
“Operating chillers at a relatively constant load while storage absorbs short-term fluctuations.”
In this strategy:
- Chillers run more evenly throughout the day
- Storage handles peaks and valleys
- Electrical demand becomes smoother but not necessarily lower
The primary objective of load leveling is:
- Operational stability
- Improved chiller efficiency
- Reduced cycling and part-load penalties
A Simple Numerical Example (Dubai Context)
To clearly see the difference, consider the same building using two different TES strategies.
Building Assumptions
- Location: Dubai
- Peak cooling load: 1,000 TR
- Peak duration: 3 hours
- Average load during remaining operation: 500 TR
- Total daily operation: 12 hours
Daily Cooling Energy
- Peak period:
1,000 TR × 3 h = 3,000 ton-hours - Remaining period:
500 TR × 9 h = 4,500 ton-hours
Total daily cooling energy = 7,500 ton-hours
This total energy is the same in all cases.
Case 1: Load Shifting Strategy
Objective
Reduce peak electrical demand and demand charges.
System Behavior
- Chillers operate primarily during off-peak (night) hours
- TES is fully charged at night
- During peak daytime hours:
- Chiller operation is minimized or stopped
- Storage supplies cooling demand
Example Operation
- Chillers run at 750 TR for 10 hours at night
→ 7,500 ton-hours produced - Daytime peak cooling is supplied from storage
Result
- Daytime peak electrical demand is significantly reduced
- Demand charges drop
- Chillers can be sized closer to average load, not peak load
TES is used as a demand-management tool
Case 2: Load Leveling Strategy
Objective
Stabilize chiller operation and improve part-load performance.
System Behavior
- Chillers operate continuously
- At a nearly constant load
- TES absorbs short-duration peaks and valleys
Example Operation
- Chillers operate at approximately 625 TR for 12 hours
→ 7,500 ton-hours produced - During peak:
- TES supplements cooling
- During low load:
- TES absorbs excess production
Result
- Electrical demand is smoother
- Peak demand is reduced slightly, but not eliminated
- Chillers operate more efficiently and reliably
TES is used as an operational stabilizer
Key Engineering Insight
Both strategies:
- Deliver the same total cooling energy
- Achieve the same comfort conditions
- Use the same TES concept
The difference lies in:
- Why TES is used
- When chillers operate
- Which cost or performance metric is prioritized
Common Design Mistake
A frequent error is:
- Selecting TES technology first
- Then attempting to justify a strategy afterward
Correct engineering logic follows this order:
- Understand the load profile
- Understand the tariff structure
- Define the objective
- Select the TES strategy
- Then select the storage technology
Why This Matters in the GCC
In the GCC:
- Cooling dominates electrical demand
- Peak penalties are significant
- Off-peak hours are long
As a result:
- Load shifting is often more valuable than load leveling
- Partial storage solutions are more common than full storage
However, this must always be confirmed through load profile analysis.
Key Takeaways from This Lesson
- TES can be applied using different strategies
- Load shifting targets cost and demand
- Load leveling targets stability and efficiency
- Strategy selection must precede technology selection
- Using the wrong strategy can eliminate TES benefits
Important Reflection
Before moving on, ask yourself:
What is the primary goal of TES in this project — reducing peak demand, or stabilizing operation?
The answer defines the strategy.