Description of task
Chilled water for space cooling is often produced at a constant supply temperature (e.g. 5°C). This temperature is often set to satisfy the worst case cooling requirements for a maximum cooling day.
But chillers operate most of the time at part load, not full load. Under partial cooling loads, either the chilled water bypasses cooling coils or the chiller is cycled on and off to avoid an over-cooling situation. If instead the chilled water supply temperature is raised, less energy is required to chill the water.
How to deliver
A number of different types of reset controls can be used. In buildings with an energy management system (EMS), signals from space thermostats can be monitored.
- If the space temperature moves upwards indicating a need for additional cooling, the chilled water temperature resets downward;
- If the space temperature moves downward indicating less cooling is required, the chilled water temperature resets upward.
A variation of this method is to monitor the position of the chilled water coil admission valve.
- If any valves are fully open, the chilled water temperature resets downward;
- If all valves are either closed or not fully open, the chilled water temperature resets upward;
- Chilled water temperatures are typically reset upward or downward in this manner by small increments (0.25–0.5°C) until acceptable temperatures are met. This method is very accurate but requires an EMS and fairly complicated programming.
Simpler methods include:
- Holding the chilled water return temperature constant while allowing the chilled water supply temperature to float up or down in response to changing return water temperatures;
- Varying the chilled water supply temperature based on the outdoor air temperature;
- Varying the chilled water supply temperature to maintain a constant differential between the supply and the return water temperatures;
- Manually resetting chilled water temperature based on the building operator’s idea of what the cooling requirements will be throughout the day. The energy savings from this method are difficult to predict as they depend on the diligence of the building operator and consistency between operators on different shifts.
These simpler methods provide less accurate control over space temperatures, but only require a local controller rather than a more expensive full building EMS. They also require a certain amount of trial and error by the building operator to find:
- Optimum chilled water temperature setting based on the parameters measured (return chilled water or outside air temperatures);
- Acceptable comfort levels of the building’s occupants.
Carry out a survey of your chilled water system to identify:
- Operation trends;
- Existing control capabilities.
Record by location (building, floor, room number, aisle):
- Chiller compressor type (e.g., reciprocating or screw);
- Chiller efficiency rating (kW/ton);
- Operating supply and return temperatures;
- Compressor cooling capacity;
- Average operating hours and cooling load.
Points to note/troubleshooting
You may need to employ an HVAC expert to undertake a complete analysis of the benefits of installing chilled water temperature reset controls.
Accurate calculation of energy savings requires an experienced HVAC engineer taking a systems level approach and considering all variable speed motors and chilled water requirements for all spaces served.
A single space or a small number of spaces may require colder chilled water than other spaces, thus eliminating many of the benefits of chilled water reset controls. In such cases, determine any limiting factors causing this condition (e.g. inadequate airflow across the cooling coils).
Any chilled water supply pumps or any supply air fans controlled by a variable speed drive may speed up when chilled water temperatures are raised. This will cause these motors to use more energy. But the energy savings from operating a chiller at a higher chilled water temperature should more than offset any increase in energy use from operating pumps or supply fans at higher speeds.
Maintenance personnel may tend to manually override the system to satisfy complaints from building occupants. It is therefore important that the system is designed with adequate airflow and controls to preclude a need to manually override.
Typical payback time
Typical payback times are less than 1 year with larger chiller systems and up to several years for smaller systems.
Typical reduction in chilled water compressor energy use is about 4% per °C of temperature reduced.
Annual chiller energy consumption can typically be reduced by 5–15%. Actual savings will depend on the compressor’s efficiency at part-load operation.
A rough estimate of savings based on 4% savings per °C of temperature increase can be calculated from the following equation:
Annual kWh Savings = Chiller Efficiency * (1 – ∆T * .04/°C) * Cooling Load * Annual Hours of Operation