Chilled Water Reset
Chilled water reset is a control strategy that will optimize chiller performance and reduce overall energy consumption. This strategy will work for most chillers, both air-cooled and water-cooled. Water-cooled chillers (heat is rejected from the condenser side of the chiller by a water loop connected to a cooling tower) can also utilize a condenser water reset strategy, which generates savings in the inverse way of a chilled water reset.
A chiller will typically be designed to supply water to a building at a predetermined setpoint, such as 44 °F. The reason for this is that it is simple for the manufacturer to program and install and this value is well within the safe limits for the chiller. This setpoint is essentially designed to minimize potential complications for the building engineer. However, a static setpoint is not the optimal way for a chiller to run from an efficiency perspective. When a proper chilled water reset is implemented, we should expect to see the following:
The chilled water setpoint fluctuate within predefined and safe boundaries.
Internal space conditions to be satisfied in the same manner prior to implementation.
Significant energy and cost savings.
So how is this done? There are several ways to program this, but the standard approach is to control the setpoint based on outdoor air temperature. The figure to the left shows one potential reset curve that could be followed. The dotted line shows the current 44 °F setpoint. The setpoint is the temperature of the water leaving the chiller and being supplied to the building. The solid line shows the new setpoint, which is correlated to outdoor air temperature. During colder weather and the building is calling for cooling (return air temperature is rising as opposed to falling), the chiller can get away with supplying 54 °F. As the weather warms, the chiller can gradually reduce the setpoint until finally reaching 44 °F, which is reserved for the hotter days of the year. When generating a curve like this, remember to account for the internal heat of a building. Buildings will typically still need to be cooled even at cold outdoor air temperatures due to lights, people, computers, etc.
And how exactly does this save the chiller energy? The figure above shows the standard refrigeration cycle. The refrigeration cycle is incredibly important to understanding the working principle of most HVAC equipment. A detailed description of the refrigeration cycle deserves its own article. When the leaving water temperature is raised, it raises the pressure in the evaporator. By raising the pressure across the evaporator, the necessary compressor work is reduced. The savings are represented by the area filled in blue.