Chiller Optimization — Targeting the Biggest Energy Consumer in Large Commercial Buildings
In large commercial buildings — office towers, hospitals, hotels, universities — the chiller plant is typically the single largest energy consumer on site. Chillers can account for 40–60% of a building’s total cooling energy use. Optimizing chiller operation is therefore among the highest-leverage strategies for reducing commercial AC costs.
How Chillers Work
A chiller is a large refrigeration machine that cools water (or a water/glycol mixture), which is then circulated through the building to cool air at air handling units. Most large buildings have multiple chillers that can be staged — run in various combinations — to meet the current cooling load.
The Part-Load Efficiency Problem
Chillers are most efficient at or near their design capacity. Running a large chiller at 20–30% of its capacity to meet a light load is significantly less efficient than running a smaller chiller at 70–80% capacity. Many buildings default to running the same chiller(s) regardless of load, wasting energy during mild weather when cooling demand is low.
Key Chiller Optimization Strategies
Chiller sequencing: Implement controls logic that selects the right combination of chillers to operate at their most efficient load point given current demand. In a plant with multiple chillers of different sizes, this means matching load to equipment capacity.
Chilled water supply temperature reset: Chillers work harder (use more energy) to produce colder water. When cooling loads are moderate, raising the chilled water supply temperature from the standard 44°F to 48–52°F can reduce chiller energy use by 2–3% per degree. Modern BAS systems can automate this reset based on actual load conditions.
Condenser water temperature optimization: Lowering condenser water temperature improves chiller efficiency. Cooling tower controls should be optimized to produce the coldest condenser water that conditions allow, particularly during mild weather and at night.
Variable speed drives on pumps: Chilled water and condenser water pumps sized for peak load run at full speed even when load is low. Variable frequency drives (VFDs) allow pump speed to vary with load, reducing pump energy by up to 50%.
Monitoring and Targeting
Establishing a chiller plant efficiency benchmark — measured in kilowatts per ton of cooling produced — allows facility managers to track performance over time and identify degradation before it becomes costly. A chiller plant delivering 0.6 kW/ton that degrades to 0.9 kW/ton without anyone noticing represents a 50% increase in cooling energy cost.