Predicting the Efficiency of Chilled Water for Building Comfort Cooling
Efficient chilled water systems play an important role in providing comfort cooling in many commercial and larger-scale buildings. However, predicting their energy performance involves more than assessing equipment efficiency at a single operating condition. Cooling demand varies throughout the year, making part-load and seasonal performance important considerations when designing an energy-efficient system.
At Varming Consulting Engineers, we consider these changing operating conditions when developing mechanical building services solutions that balance occupant comfort, energy efficiency and overall system performance.
Understanding Chilled Water System Efficiency
Chiller efficiency can be expressed using the Coefficient of Performance (COP) or Energy Efficiency Ratio (EER). These measures relate the cooling output provided to the energy input required under defined operating conditions.
While useful when assessing equipment, a single efficiency figure does not necessarily represent how a chiller will perform throughout the year. In practice, cooling loads fluctuate and chillers frequently operate at part load.
Chilled water temperatures, heat-rejection conditions, compressor performance and controls can all influence chiller efficiency. Energy used by associated equipment, including pumps and fans, also contributes to the performance of the overall cooling system.
Assessing Seasonal Performance
Seasonal performance metrics provide a more representative indication of efficiency by considering operation across a range of loads and conditions.
The Seasonal Energy Efficiency Ratio (SEER) assesses cooling performance over a defined cooling season. Under the European Ecodesign methodology for comfort chillers, seasonal performance takes account of efficiency at different part-load conditions and temperatures rather than relying solely on performance at one full-load rating point.
For air-to-water comfort chillers, the methodology includes part-load rating conditions at outdoor temperatures of 35°C, 30°C, 25°C and 20°C, corresponding to cooling loads of 100%, 74%, 47% and 21% respectively.
This approach provides a more representative indication of seasonal equipment performance.
The Importance of Chilled Water Temperatures
Chilled water temperatures have an important influence on chiller efficiency. The European Ecodesign methodology uses standard rating conditions for different comfort cooling applications. These include chilled water inlet/outlet temperatures of 12°C/7°C for fan-coil applications and 23°C/18°C for cooling-floor applications.
These are reference conditions used to assess equipment performance and should not be interpreted as mandatory design temperatures for individual buildings.
In practice, the selected chilled water temperatures should reflect the cooling system, building requirements and design strategy. The temperature at which chilled water is produced influences the operating conditions of the refrigeration cycle and, consequently, chiller performance.
Optimising Chilled Water Performance
Predicting performance begins with understanding the building’s expected cooling demand and how that demand will vary during operation. Appropriate equipment sizing, suitable temperature regimes, effective controls and good part-load performance can all contribute to reduced energy consumption.
The complete cooling system should also be considered. Pumps, heat-rejection equipment, fans and associated controls consume energy. Therefore, evaluating the system as a whole provides a more meaningful understanding of expected building energy performance.
Delivering Efficient Comfort Cooling
Effective chilled water design can contribute significantly to the energy performance of a building’s comfort cooling system. At Varming Consulting Engineers, our mechanical engineers consider cooling loads, operating temperatures, equipment selection and controls as part of an integrated approach to building services design.
Planning a project with comfort cooling requirements? Explore our Mechanical Engineering Services to discover how our team can support the design of efficient chilled water systems, or contact Varming Consulting Engineers to discuss your project.
