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List of Chilled Water Temperature Reset Strategies | System Optimization

List of Chilled Water Temperature Reset Strategies | System Optimization

Chilled-water temperature reset adjusts the leaving chilled water set point to improve the efficiency of the chiller and thereby reducing the energy consumption. Generally, a chilled-water-reset raises the set-point temperature when the building load is at less-than-design conditions. Producing warmer chilled water lessens the burden on the compressor, which means that the chiller consumes less energy. 

Points to Consider Before Employing Reset 

  • Raising the chilled-water temperature in variable-volume systems increases the energy consumption of pumps, and may more than offset the savings in chiller efficiency. 
    • Below figure explains how the total energy including chiller compressor and chilled water pumps vary with chiller reset in a variable volume systems. Hence it is most suitable to reset till to the optimum point where net savings are always positive.
    • In case of complex systems where secondary pumping loops and many AHU / FCU’s are in line then this optimum point further shifts towards left. 

Chiller System Energy Consumption with Temperature Reset Strategy

  • If humidity control is critical, ensure that the warmer chilled water does not cause the loss of space temperature or humidity control
  • Maintain the minimum pressure differential (specified by the manufacturer) between the condenser and evaporator, to ensure adequate refrigerant flow and maintain proper oil movement within the chiller.
  • Also, ensure changing chiller sequencing algorithms accordingly before resetting chilled water outlet temperature

Savings Estimate

For every 1 degree F rise in CHW temperature 

  • 0.5 to 0.75% savings in kW/TR in fixed speed chiller
  • 2 – 3 % in VFD chillers

Different Chilled Water Temperature Reset Strategies Possible

  • Reset based on outdoor air conditions
    • To investigate reset opportunities for the chilled water supply temperature setpoint, review the plot of chilled water supply temperature and outdoor-air temperature versus time. Below figure gives an example of an office building for a 2-week period in November in which the outdoor-air temperature varies between 40F and 80F, and the chilled water supply temperature is pretty stable around 45F. This is an example of bad operation because the chilled water supply temperature setpoint is never changed to take advantage of cooler outdoor conditions (i.e., when the Outdoor Air Temperature is less than 60F).
    • In this case, suppose the chilled water supply temperature setpoint is reset by 5F when the OAT is less than 60F. This reduces the load on the chillers and saves energy during advantageous outdoor conditions.
Chilled Water Reset with Outdoor Air Temperature
  • Rest based on Average AHU coil Position
    • A variable to consider when implementing reset on the chilled water supply temperature setpoint is based on load as indicated by the cooling coil valve positions for all air handlers served by the chiller. If the maximum open valve is less than 90% to 95% open, the chilled water supply temperature setpoint can be increased and if there is more than one valve open 100%, the chilled water supply temperature setpoint should be decreased accordingly.

These strategies provide a very basic understanding about implementing chiller water temperature reset, but anyhow the performance values change widely with building occupancy, seasonal variations, chilled water pumping & distribution scheme etcetera and hence a careful study on case to case basis for different types of applications is recommended.

*Image & Little Experimental Data Credits to Pacific Northwest National Laboratory

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