The use of chilled or hot water allows for both more efficient distribution of energy throughout the building as well as centralized facilities for heating and cooling. While water-based systems can gain efficiencies from the use of more efficient boilers and chillers, they can also be further optimized through the use of effective control sequences. Optimization is possible both within the plants (introduction or rejection of heat) and in the distribution system. Let’s look further at each of these.
Waterside distribution.Typically, chilled or hot water is distributed throughout a building to provide the necessary supply for air handlers, reheat coils, and perimeter radiation. The goal is to provide the necessary amount of BTU’s with the minimum amount of energy. There are several ways that this can be optimized.
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Static
pressure reset. Distribution of chilled or hot water
for HVAC can be done using constant or variable flow pumping. The use
of variable flow pumping has many benefits including reduced energy
use and cost reductions in use of two-way control valves in place of
more expensive three-way valves. Variable flow pumping typically uses
the pressure at the furthest point in the pumping loop as an input
for control of the drive serving the primary pump.
Typically, this is controlled to a constant setpoint, which is set in design and validated during commissioning. It is possible, however, to reset the pumping static pressure setpoint based on valve position. Just like on the airside, the concept is to have the pump provide only enough pressure to satisfy the demand of the most extreme zone. In this strategy, the position of the control valves are monitored, and when the most extreme valve is open greater than 95%, the static pressure is increased; when it is less than 90%, the pressure can be reduced. The benefits to this algorithm include not only reduced energy but also improved control and reduced wear on valves and pumps. An added benefit of utilizing valve position feedback as part of pressure control is that the location of the pressure sensor becomes less critical for proper system operation.
- Temperature reset. In addition to resetting the pressure of the pumping loop, it is often advantageous to reset the water temperature as well. Resetting water temperature has many benefits including reduced energy usage for the plant, improved controllability, and reduced heat loss (or gain) from piping. When implementing a temperature reset algorithm, use caution to ensure that key parameters including temperature for cooling and dehumidification are maintained. The designer also needs to weigh any energy savings against additional energy required for pumping.
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Chiller
sequencing.
Chiller sequencing algorithms look at when to start and stop
machines. Often, this will attempt to match the load of the building
against the capacity of the plant and choose not only the most
efficient machines but allow the machines to run at maximum
efficiency. Sequencing algorithms typically not only look at
efficiency but also provide other functions including rotation, run
time leveling, and failure recovery.
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Tower
optimization.
Cooling towers are used for heat rejection from the chiller
plant. Often the towers themselves can be optimized using strategies
that include the use of variable-speed or multiple-speed fans. As
part of an overall chiller plant optimization, the condenser water
setpoint can be varied to maximize efficiency. While there are
several algorithms that are typically used, they often seek the
optimum point of operation where the chillers, towers, and pumps in
combination use the lowest total energy.
-
Free
(or tower) cooling.
Under the right climate conditions, it may be possible to use the
cooling towers alone to cool the building. This is done through the
use of a “free cooling” cycle within the chiller or with the use
of heat exchangers.
- Variable primary pumping. Chilled water pumping within the chiller plant (primary pumping) can also be done using variable flow and can be optimized and scheduled. As with many strategies, caution is required to ensure that proper flow is provided for the machines in the plant.
