How to optimize an HVAC system

True optimization comes from a three-pronged methodology that starts with system design and controls the HVAC plant as a holistic system.

08/20/2018


Figure 1: Holistic optimization that focuses on the entire system is key to achieving maximum savings. This chart shows the comparison range of average annual chiller plant efficiency (kilowatt per ton and coefficient of performance) including all chillers, cooling tower fans, condenser pumps, and chilled-water pumps.?Courtesy: Optimum Energy Learning objectives

  • Understand what holistic HVAC optimization is and how it works.
  • Explore HVAC system designs that claim to be optimized.
  • Learn what a truly optimized HVAC system should deliver.

When designing a high-performance building, engineers search for the most energy-saving HVAC components-super-efficient chillers, high-efficiency pumps and cooler towers, and variable frequency drives (VFDs). But even the most efficient components cannot reach aggressive energy targets on their own. Reaching peak performance requires a broader, holistic approach to efficiency that includes central plant optimization.

The trouble is, "optimization" has become a buzzword in the facilities management industry, with competing vendors offering a wide array of solutions. This creates the belief that optimization is a stand-alone bill of goods. Some vendors say the solution is a piece of hardware, some say it's software, while others say it's a cloud-based dashboard. But the truth is central plant optimization is not a single thing. It's an overall methodology that starts with a good design and construction, integrates automation and optimization, and provides maintenance and monitoring throughout a building's lifecycle. When done correctly, optimization maintains central plant operations at peak performance day after day, year after year.

Optimizing central chiller plants, specifically, is critical because they usually consume more energy than any other building system; however, the lessons also apply to airside and boiler systems.

HVAC optimization

For true optimization, the solution must automatically control HVAC equipment as a holistic system 24/7 so that it uses the least amount of energy without sacrificing performance. It's impossible to judge a solution's optimization capabilities without first understanding the three laws of optimization:

1. Measurement comes first. Without an accurate measure of energy use by each piece of equipment, it's impossible to accurately predict and report the impact of varying conditions on the entire system. If you can't measure it, you can't optimize it.

2. Optimize systems, not just individual components. If an optimization plan focuses only on installing the most efficient pieces of equipment or saving energy in one subsystem without considering how to maximize the performance of the whole system, it will fail to capture the total available efficiency potential. (See Figure 1)

3. Optimization must be automatic, dynamic, and continuous. To achieve maximum efficiency, optimization must be a real-time dynamic process, not a static set-and-forget process. Operational control must be automatic and based on real-time inputs and adjustments.

Figure 2: This is an example of a common approach to optimization, using PID control loops to try to simultaneously change supply-air temperature and static-pressure setpoints. Poorly tuned PID control leads to system hunting, which creates instability and eventually leads to operator override of the system and a loss of savings. Courtesy: Optimum Energy When optimization fails to meet expectations, it's often because the solution couldn't deliver closed-loop optimization in real time. Products sold as being able to "optimize" vary widely, from efficient components to component-based efficiency tools to systemwide HVAC optimization. Because no standard definition for "HVAC optimization" is guiding the industry, engineers often don't get the energy and cost savings they expect from the products they specify.


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