Depending on the chiller type and operating conditions, many commercial and industrial chillers achieve their highest operating efficiency at intermediate loads rather than at full load. During a heatwave, however, they often operate close to full load, while higher outdoor temperatures increase the condensing temperature of the refrigeration cycle, further reducing COP. The result is significantly higher electricity consumption and operating costs.
How can HVAC engineers keep chillers operating closer to their most efficient range during the hottest days of the year?
There are several engineering strategies to improve chiller efficiency and reduce operating costs during hot summer conditions. The right solution depends on the application, load profile and project constraints.
One common plant design strategy is staging multiple chillers. Another approach is thermal energy storage, which improves efficiency and load management by optimizing when cooling is produced rather than by increasing installed cooling capacity. This can be particularly attractive when reducing operating costs without investing in additional chiller capacity.
1. Stage multiple chillers
Instead of relying on one large chiller, several smaller chillers are installed and staged so that the operating units remain closer to their most efficient operating range.
Advantages
✔ Better load management
✔ Higher system redundancy and reliability
✔ Maintenance can be performed while the remaining chillers continue operating, provided sufficient capacity remains available
✔ Reduced compressor cycling
✔ Potentially higher average plant efficiency through optimized sequencing
Disadvantages
✖ Higher capital investment
✖ Additional installed cooling capacity that may only be required during peak demand
✖ More equipment to maintain
2. Shift the cooling load with thermal energy storage
Instead of producing all cooling during the hottest hours of the day, part of the cooling energy is generated at night, when outdoor temperatures are lower and chillers operate more efficiently.
The stored cooling energy is then used during the daytime peak, reducing the load on the chiller exactly when its efficiency would otherwise be at its lowest.
This improves efficiency in four ways:
✔ Higher COP: Cooling is generated during cooler night-time conditions, when more favourable condenser-side conditions typically allow the chiller to operate at a higher COP.
✔ Optimal operating range: The thermal battery helps keep the chiller closer to its most efficient operating range, often around 60–80% load depending on the equipment and operating conditions.
✔ Load levelling: The thermal battery smooths cooling demand throughout the day, helping the chiller operate more steadily instead of frequently ramping up, down or cycling on and off.
✔ Peak shaving: During periods of high cooling demand, the thermal battery supplies part of the required cooling, reducing the load on the chiller and the electrical power it requires.
Many applications achieve 20–30% cooling energy savings without replacing the existing chiller. Depending on the application, additional benefits include lower peak electricity demand, fewer compressor starts, longer equipment lifetime and reduced maintenance.
Sometimes the most efficient cooling system isn’t a bigger chiller.
It’s a smarter way of operating the one you already have.
Thermal energy storage can also improve systems that already use staged chillers. By covering short-duration demand peaks, the thermal battery may delay or even prevent the next chiller from starting. This reduces compressor starts, auxiliary energy consumption and peak electrical power demand, while keeping additional chiller capacity available as true redundancy.
The actual benefit depends on the chiller characteristics, staging strategy and load profile, so every system should be evaluated individually.
