Energy Use and Performance of a New Variable Primary-Flow Chilled Water System

The traditional chilled water loop has been designed as a primary/secondary (P/S) system for several years. The primary loop maintains constant chilled water flow through the evaporator, and the secondary loop is designed as a variable flow system in response to variations in building cooling load. The primary/secondary design separates the chiller operation from the building load requirements. This design principle has been working adequately for old chillers that require constant chilled water flow to achieve stable chiller performance. However, these chillers operate inefficiently and consume unnecessary pump power. In recent years, the chiller industry has begun the use of variable water flow through evaporators. Significant energy savings can be achieved by using the variable flow principle. This paper compares the P/S system with variable chilled water system and also proposes an innovative solution: chilled water supply temperature reset to maintain the minimum chilled water flow and eliminate by-pass water flow. The energy consumption models including pump power and chiller compressor power improvement are presented to simulate the energy performance of two systems. ASHRAE detailed cooling coil models are also adopted to simulate the cooling coil discharge air humidity in order to verify the impacts of the chilled water supply temperature reset. Finally, a case study building demonstrates experimental results. Up to 10% pump and compressor power saving is demonstrated by simulations and the case study.