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7 Steps to Optimizing a Central Plant

Aug. 1, 2011
Optimization of a central chiller plant can achieve and sustain ongoing energy and cost savings

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 Flexible Infrastructure
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 the Components
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.

STEP 5: Take Central Plants Further With Networked Optimization Software

Networked optimization software is the intelligent logic that holistically operates a plant in the most efficient manner by taking full advantage of the capabilities of a BAS. This type of specialized software once was available only as a custom solution, but today's software offerings are standardized and scalable, decreasing the cost and risk to project stakeholders. Intelligent optimization software understands the efficiencies of system components. The system's sequence is optimized on an energy basis, rather than solely a load basis. This enables central plants to meet the required load with the least amount of energy possible.

The most advanced optimization software has relational-control algorithms that optimize all the equipment so that each component uses the least amount of power required to meet the load and maintain occupant comfort. Control setpoints are calculated automatically based on real-time building-load-data inputs received from the BAS. The optimization software evaluates the building data and makes recommendations for the BAS to execute.

These optimization solutions are scalable for any building or campus. Building stakeholders can test-drive optimization software at one location and then scale it across an entire enterprise or portfolio of buildings, such as a multisite health-care organization or a university with a campus of buildings. These networked solutions also deliver Web-based, real-time measurement, verification, and management of central-plant operating performance, making it possible for building operators to increase and sustain energy savings over both the short- and long-term.

STEP 6: Aim for Predictive, Not Reactive, Maintenance
The role of maintenance has evolved over time and is critical to optimizing a central plant. Historically, maintenance was primarily reactionary. If cold air was not being delivered, it was a signal that something was wrong, and the appropriate fixes were made. That was followed by a focus on maintaining occupant comfort and increasing efficiency, which meant maintenance became more routine and more proactive.

With today's ultraefficient components, maintenance is predictive and essential to maintaining the optimization of central chilled-water plants. The evolution to predictive maintenance has placed an inherently different responsibility on the people who provide service.

STEP 7: Measure, Verify, and Manage Performance Data
To make sure efficiency levels are being maintained over a plant's lifecycle, performance data must be measured, verified, and managed regularly as part of a continuous commissioning process. The availability and visibility of real-time building data enables systems managers to find and address performance drift quickly and easily. Issues with performance can be identified long before degradation results in significant loss of efficiency.

Web-based tools provide operating and performance data around the clock. These tools provide continuous feedback by providing detailed, real-time and historical data. Operators then can detect, diagnose, and resolve system faults quickly. The visibility of data also has improved with easier-to-read graphs and analysis tools to enable a timely diagnosis of underperformance on energy dashboards and kiosks. Alerts and notifications can be sent automatically, including to mobile devices.

The importance of measurement has not gone unnoticed across the industry. ASHRAE is taking notice of the value delivered by real-time measurement, verification, and management data. The industry is considering the development of a building-classification system that would require owners to measure the performance of a central plant continuously and post updated efficiency levels regularly.

Seize the Opportunity
Building owners and operators must seize the opportunity to increase energy and operational efficiencies by taking a holistic view of where their organization consumes the most energy: the central plant. Optimizing a central plant provides the potential to achieve and sustain ongoing energy and cost savings, which ultimately helps improve an organization's bottom line. Central-plant optimization provides the road map for facilities owners or managers looking to improve energy efficiency. Best of all, these measures can be implemented while maintaining day-to-day operations.

Dave Klee is director of Optimized Building Solutions for Johnson Controls. He is in his 15th year with the company. He closely monitors market trends and customer requirements, and drives the direction of Johnson Controls' optimization offerings. He is responsible for the successful implementation of optimized HVAC systems, incuding central-plant optimization. He graduated from the University of Michigan College of Engineering and earned his MBA at Babson College in Wellesley, Mass. He is also a LEED accredited professional.