Engineering practice · Air conditioning
Article 09 / 10
The chiller became more efficient, but the system did not: why the entire cooling system must be assessed
The chiller settings were changed, and its electrical power input fell. At first glance, the optimisation was successful. But at the same time, water flow increased and the operation of pumps and terminal unit fans changed.
A facility is not buying low consumption from an individual compressor. It is buying the required room cooling at the lowest justifiable cost across the entire system.
First, define the boundaries
The chiller plant assessment includes the chillers and their auxiliary equipment. For water-cooled condensers, it also includes the associated pumps and cooling towers. If an intervention changes the operation of fan coils or air handling units, this effect must be included in a wider assessment. An electrical load must not be counted twice if it is already included in the unit’s measured power input. 1
The main condition for comparison is the same duty delivered: load, temperatures and humidity within agreed limits.
How local savings become an overall loss
Take an illustrative load of 400 kW. Before the operating conditions change, the chiller draws 100 kW and the auxiliary equipment included in the assessment draws 20 kW. The total is 120 kW.
After the change, assume 92 kW for the chiller and 31 kW for the auxiliary equipment in this calculation example. The total is 123 kW.
The chiller alone uses 8% less power. But the assessed system as a whole uses 2,5% more:
These power inputs are assumed to demonstrate the balance, not taken from tests of specific equipment. The example shows why an impressive graph for a single unit is not enough.
Water temperature affects several components
Raising the chilled water temperature can reduce compressor work. At the same time, terminal units may require higher water flow, and dehumidification capability may decline. Trane explicitly discusses this trade-off in its materials on chiller system control. 2
The practical approach is therefore to check the setpoint against the capabilities of all critical terminal units. Water should not be kept colder than necessary. But its temperature must not be raised solely to improve the chiller’s performance figure.
One unit or two at part load
There is no universal answer. The specific chiller performance characteristics, pump operation, minimum loads and standby requirements must be compared.
We would assess typical operating conditions separately: low morning load, the working day, peak load and transitions between stages. Chiller assessment tools also account for installation location, loads and system parameters, rather than just the rated operating point. 3
How to move from kilowatts to seasonal performance
If total power input falls from 120 to 108 kW in a particular scenario, the reduction is 12 kW. Over 1000 hours at precisely these operating conditions, that amounts to 12 000 kWh.
But this 10% cannot automatically be applied to the whole season. Other loads and operating conditions must be calculated for the remaining hours. The seasonal percentage is determined against baseline seasonal consumption.
AI can help select operating modes using forecasts and the system’s actual response. Its objective must remain system-wide, rather than being limited to a single COP.
Optimising a cooling system means coordinating chillers, pumps and terminal units. The best operating conditions for one component are not necessarily the best for the building.