In the July issue of Engineered Systems we discussed the importance of optimizing the design and operations of a the Building Operation System (BAS) to achieve the maximum efficiency. Last month’s column focused on the control of the air side with the use of strategies including demand controlled ventilation and static pressure reset. This month we would like to explore the control of the water side.

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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 systems that are water based 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:

Water Side 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.

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 set-point which is set in design and validated during commissioning. It is possible, however, to reset the pumping static pressure set-point 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 and 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 caution needs to be used to assure that key parameters including temperature for cooling and dehumidification are maintained. The designer also needs to balance the energy savings gained through reset against the additional energy required for pumping.

Central Plant Optimization:

In addition to optimizing energy usage for the distribution of hot and chilled water the central plant can also utilize controls to greatly optimize operations.

[an error occurred while processing this directive] Chiller Plant:

Chiller plants tend to be fairly complex due to the interaction between pumping, the chillers and heat rejection (towers). Many strategies exist to optimize the operation of a chiller plant and designers should carefully evaluate which ones to utilize based on project type and function as well as equipment selections. Some of the major strategies to consider include:

• 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.

• 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 condenser water set-point 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. Like many strategies caution is required to assure that proper flow is provided for the machines in the plant.

Boiler Plant Optimization:

Like the chiller plant the boiler plant can also be both sequenced and scheduled for maximum efficiency. The goal is again to pick the right capacity of boilers and to be using them at their maximum efficiency.

About the Authors

Paul and Ira first worked together on a series of ASHRAE projects including the BACnet committee and Guideline 13 – Specifying DDC Controls. The formation of Building Intelligence Group provided them the ability to work together professionally providing assistance to owners with the planning, design and development of Intelligent Building Systems. Building Intelligence Group provides services for clients worldwide including leading Universities, Corporations, and Developers. More information can be found at www.buildingintelligencegroup.com  We also invite you to contact us directly at Paul@buildingintelligencegroup.com or ira@buildingintelligencegroup.com
 

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