The demand among building owners and operators for increased efficiency is driven by a number of factors, including increasing utility savings, reducing greenhouse-gas emissions, and enhancing public image. Furthermore, government mandates continue to demand increased levels of energy efficiency, and the industry standard for green-building certification is becoming increasingly stringent. Local utility rebates make energy-saving initiatives even more attractive.

Although the drivers for energy-efficient investments are many, one of the largest barriers to building owners making these investments is limited capital availability. Financial limitations underscore the importance of finding ways to do more to enhance building efficiency with fewer dollars. To meet the triple bottom line of sustainability—fiscal, environmental, and social—organizations are taking a holistic look at their building operations.

Approaching the Limit for Efficiency
Over the past 25 years, the efficiency of HVAC equipment has increased steadily. In some cases, the efficiency of these components, such as chillers, has improved by as much as 40 percent. However, HVAC equipment alone will not achieve optimal energy savings. The industry is quickly approaching the limit of how much efficiency can be expected from individual components. Similar gains cannot reasonably be expected in the future.

Central chilled-water plants often fail to maintain their anticipated efficiency level over time. This is because traditional methods of plant operation and maintenance treat the plant as a collection of disparate pieces of mechanical equipment. Engineers must look beyond the component level to reach today’s aggressive efficiency goals. Taking a holistic view of the central plant reveals many opportunities for savings that previously were unattainable.

The central chilled-water plant is the largest consumer of energy within a building, consuming as much as 30 percent of a facility’s power. As such, the central plant represents the biggest opportunity for saving energy, reducing environmental impact, and achieving a faster payback on upgrade investments.

Today, a whole-building philosophy to central-plant optimization is generating industry attention. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) is developing new energy targets based on the performance of a building as a whole. According to a recently released committee report, one of the society’s goals is to develop standards for the calculation of buildingwide energy use so that ANSI/ASHRAE/IES Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings, can include system-level efficiency targets beginning in 2016.

A Checklist for Achieving Central-Plant Optimization
Even as new energy targets are being defined, consulting engineers and building owners can take advantage of the opportunities presented by central-plant optimization. Consider the following as a checklist for optimizing your central plant:

1. Design or retrofit the central plant with a flexible infrastructure, including headered piping and variable-speed pumping.
2. Choose HVAC components (particularly chillers) based on real-world operating conditions, rather than full-load design conditions.
3. Apply HVAC components in a way that maximizes the specific operating and efficiency strengths of those components.
4. Automate the chiller plant with a modern building-automation system.
5. Integrate networked optimization software to optimize the plant.
6. Identify operational issues with a comprehensive preventive- and predictive-maintenance program.
7. Measure, verify, and manage energy performance.

STEP 1: Create a FlexibleInfrastructure
The foundation of an optimized central plant is a well-designed system infrastructure. The infrastructure should allow for flexibility across the life cycle of the system. Greater control flexibility can result in significant energy savings when leveraged correctly.

Although it may be more expensive to design or retrofit a plant with flexible infrastructure, the payback can be demonstrated quickly. A well-designed plant will run at a higher level over its life cycle, leading to improved return on investment.

In the design of a new chilled-water system, the most flexible, efficient system infrastructure combines a headered pumping system with variable-primary-flow pumping. Variable-speed drives (VSDs) increase efficiency potential, and headered piping allows the most flexible range of control.

In existing buildings, addressing design deficiencies can help achieve better results. These changes may include upgrading piping configurations; adding VSDs to chillers, pumps, and cooling-tower fans; and automating the plant.

STEP 2: Select Components Based on Real-World Operating Conditions
A building's HVAC components often are chosen based on their efficiency at full-load design operating conditions. Instead, the best practice is to select plant components that will operate most efficiently at the conditions at which they will run most often. A chiller with a more favorable part-load efficiency profile will demonstrate superior performance in a real-world environment.

STEP 3: Properly Apply theComponents
After the right components are selected, they must be applied and operated appropriately. Some best practices for equipment application include:

• Run the plant at its designed chilled-water temperature. If the plant was designed to run at 44°F, run it at 44°F. Running the plant at 42°F will reduce its efficiency.
• Do not push too much or too little water through the chiller. Too much water may decrease the efficiency of the overall system, while too little water may diminish the efficiency of the chiller itself.
• Take advantage of the environment. Install equipment that is capable of taking advantage of colder condenser-water temperatures available during the majority of operating hours. Improper component application diminishes system efficiency, although the impact can go unnoticed if central-plant performance is not being monitored effectively.

STEP 4: Maximize Efficiency With Building Automation
Building owners who have a building-automation system (BAS) in place have more opportunities for enhanced efficiencies. A building with a BAS is positioned to take advantage of today’s optimization software. Even the most skilled human facility operators will struggle to match the efficiency and effectiveness of a modern BAS.

A BAS will start the proper equipment at the right time to maximize efficiency based on the run history and efficiency profile. With VSDs, a BAS can select the right speed at which to operate pumps and tower fans. These systems enhance plant efficiency further with tuning algorithms that adjust control routines continually based on system dynamics and seasonal changes.

A modern BAS offers monitoring and reporting tools to help sustain central-plant performance over time.

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