When to use cooling plant optimization controllers

Cooling systems account for 15% of the electricity use in U.S. commercial buildings, according to the U.S. Energy Information Administration. Improving cooling efficiency in both new construction and retrofit applications can reduce operational expenditure while striving to exceed environmental stewardship targets.

Addressing this challenge has attracted the attention of industry professionals across the spectrum of equipment manufacturers, system design engineers, facility operators, commissioning agents and controls professional. A cooling plant optimization controller, known as CPOC, is a class of dedicated plant controllers intended to achieve peak cooling plant performance.

Consider a retrofit chiller plant using the below sequence of operations for controlling two 800-ton chillers with variable-flow primary pumping and variable-speed cooling tower fans. What is the best setpoint for staging on the second chiller?

A review of the chiller performance map shows that the best efficiency point shifts based on both the condenser water temperature and the load. Below is an illustrative sequence of operations:

1. Cooling plant is enabled, lead equipment activates:

• Lead condenser water pump activates to 100%.
• Lead cooling tower fans modulate to maintain a condenser water temperature setpoint of 7°F over the ambient wet bulb temperature, but no more than 85°F.
• Lead chilled water pump enables and modulates to maintain a differential pressure setpoint.
• Lead chiller enables and maintains a chilled water setpoint on an outdoor air reset schedule between 44°F and 49°F.

2. If the lead chiller exceeds 85% load for 10 minutes:

• Lag condenser water pump activates to 100%.
• Lag chilled water pump activates and synchronizes with lead pump.
• Lag cooling tower is enabled and the fan modulates similarly to the lead tower fans.
• Lag chiller enables and maintains a chilled water setpoint.

The illustrative sequence of operations offers several features familiar to operators and designers. It is relatively easy to look at the system and to know if it is operating as intended with pumps paired with chillers. The sequence limits run-hours and chiller starts, promoting longer equipment service life. The variable speed drives on the cooling tower fans, chilled water pumps and chillers are all able to modulate with building load.

In a nutshell, the sequence of operations checks many of the customary high-efficiency boxes (or literal boxes if utility rebates are involved). However, significant performance opportunity has been left on the table:

• Chillers staged on capacity rather than best efficiency operating points significantly impacts the power needed to meet the load.
• Pumps also have peak efficiencies in certain ranges on their curves. Running two pumps at part load is often more efficient than one pump at full load when meeting the same head and flow requirements. In some cases, parallel part load operation also can improve reliability and maintenance.
• No automation is present analyze and prioritize resets between the condenser water flow, temperature and chiller compressor lift.
• Maintaining boundaries for stable chiller operation at part load values.

All of these points beg the question: How can smarter plant controls, capable of integrating high-performance equipment, be systematically implemented? A strong solution is a cooling plant optimization controller.

When and why to consider a CPOC

A CPOC is a dedicated controller for the plant, housing the logic for staging equipment to meet the load while minimizing energy consumption. It comprises specialized software package, typically housed on building automation controller hardware. Vendors typically offer several CPOC packages tailored to support plants of varying complexity and size, ranging from two chillers up to large plants with thermal storage.

The major building automation system companies each has its own CPOC systems and there are several third-party companies competing in the marketplace. It should be noted that CPOC systems tend to be proprietary, often referencing patented algorithms for each manufacturer’s nuanced approach to optimization (see Figure 2).

There are many strategies for optimization, the technical details of which are beyond the scope of this article. At its core, the CPOC endeavors to:

• Provide a means for customizing the sequence of operations to meet the needs of specific projects. In other words, perform the traditional plant control function.
• Integrate chiller performance curves, including kilowatts/ton data for both varying loads and varying condenser water temperatures.
• Integrate chilled and condenser water pump curves and/or pump power draw.
• Integrate cooling tower fan energy.
• Manage reset schedules for chilled and condenser water temperature setpoints and pressure setpoints.
• Include an open protocol (BACnet or other) means such that the plant can be integrated into a supervisory BAS.

While the optimization logic and aspects of the staging control functions are prepackaged, it is still something that requires project by project configuration. Additionally, the CPOC may require additional inputs beyond what a traditional plant sequence of operations would need, including current transducers, flow meters, temperature sensors and differential pressure sensors.

In fact, the CPOC hardware is a portion of the overall cost of the system. Time spent adapting the packaged logic to the site, adding in sensors and integrating with the existing automation system are influential on the project cost.

The following list provides considerations to guide whether a CPOC upgrade is applicable to a facility.

• Confirm the plant is suitably complex that it will benefit from advanced controls. A CPOC will typically benefit multichiller, multipump plants that have varying loads and flows.
• The return on investment for CPOC upgrade depends on significantly improving plant performance. If a plant is already performing well, the cost of the upgrade may not be justified. Plants where investments in modern pump controls, good chilled water delta T and automatic temperature and pressure reset strategies should be reviewed carefully before allocating funds for an upgrade.
• For retrofit applications, review the existing chiller equipment. If one chiller operates at 0.4 kilowatts/ton while an older chiller operates at 0.9 kilowatts/ton, preferential efficiency order may be obvious and a sophisticated controller will not tell the operator anything they don’t already know. In such circumstances, a CPOC can be a source of frustration and will end up overridden. Reviewing the chiller performance maps and pump curves in detail is required to determine the opportunity.
• Variable speed drives on all equipment are not a prerequisite for a CPOC project. Depending on the pumping configuration, a plant with multiple single speed pumps and one variable frequency drive-driven pump could achieve the necessary modulation without the cost of multiple VFDs.
• A CPOC project may have nontrivial upfront costs and should be considered in the context of payback and energy efficiency goals for the facility.

When the decision has been made to undertake a cooling plant modernization project that could include a CPOC component, the following items may be considered as part of the design and implementation process:

• Review the condition of equipment and capital plans associated with replacement, repair and/or maintenance on equipment associated with the control upgrades. For example, communication card upgrades for vintage chillers can have a significant cost and lead-time.
• The operations team is recommended to be part of the decision process. Operating a fully automated plant means that the operations team may need support from the design and construction team to receive comprehensive training. For retrofit applications, the historical knowledge of the facility can be invaluable in setting design parameters.
• Involving the controls contractor and CPOC vendor in the project budgeting phase can be beneficial. Particularly in central plant modernization projects, the cost for integration can vary greatly based on what systems are already on an existing BAS. Different vintages and features for VFDs may include points for real-time power use, reducing integration cost.
• Assessment of pumping opportunities and challenges including load side valves, low delta T syndrome, primary versus primary-secondary or tertiary flow configurations.
• Cooling tower fan staging and minimum condenser water flow requirements.
• Obtain and analyze chiller performance maps across multiple loads and varying water temperature.
• Consider how the users will interact with the CPOC. If there is an existing supervisory BAS, it could be preferred to have all controls accessible through a unified “single-pane-of-glass” interface. Alternatively, the CPOC plant graphics accessed on a standalone interface for the plant. Integration into the existing BAS may mean sacrificing some specialized graphics on the CPOC, but integration onto a unified interface will help make the system easily accessible for the operations team. Depending on the vintage of an existing BAS, a hybrid approach could also be taken, where key performance indicators are passed through to a BAS plant graphic, but a native CPOC interface is also made available.
• It is recommended that the manufacturer/controls integrator be a part of the commissioning team, providing support for developing functional performance tests, participating in testing the CPOC and integration with a supervisory BAS.

Table 2: Estimated energy use where chillers stage on in parallel, based the best efficiency points from the equipment performance map. Courtesy: Raths, Raths & Johnson Inc.[/caption]

Monitoring cooling plant performance

Knowing the efficiency of a cooling plant allows teams to make informed decisions around plant performance operation and capital improvement project prioritization. In older systems, it is rare to find real-time wire-to-water plant power use, which is a key tool in enabling operators to know if their adjustments are working. The 2016 and 2019 versions of ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings have added section 6.4.3.11, requiring chilled water plant energy use and efficiency monitoring in new buildings and for new plants in existing buildings under the following conditions:

• Water-cooled chilled water plants larger than 1,500 tons peak cooling capacity for Climate Zones 5 through 8, 3C and 4C and larger than 1,000 tons peak cooling capacity for all other zones.
• Air-cooled chilled water plants larger than 860 tons peak cooling capacity for Climate Zones 5 through 8, 3C and 4C and larger than 570 tons peak cooling capacity for all other zones.

The efficiency must be calculated in kilowatts/ton and configured to trend and graphically display three years of 15-minute, hourly, daily, monthly and annual data. The 2018 and 2021 versions of the International Energy Conservation Code have requirements for energy consumption data recording by source under section C405.12.4: Meters with requirements for data retention and graphical energy reports in the following sections.

Cooling plant performance for electrically driven plants is most often expressed in terms of kilowatts/ton or the unitless coefficient of performance. The former is the amount of electricity required to induce 1 ton of cooling while the COP is the ratio of the work completed over the work applied. Each metric can be calculated as follows:

ASHRAE Guideline 22-2012: Instrumentation for Monitoring Central Chilled-Water Plant Efficiency provides technical guidance for methods and devices for measurements, as well as procedures for data collection and calculations. The guideline “allows the user to monitor chilled-water plant efficiency and make modifications to the setpoints of the system such that the overall efficiency of the chilled-water plant is improved.” Guidance for instrumentation accuracy to achieve a recommended total plant performance calculation within 5% of the true efficiency value is included.

In a three-chiller cooling plant, each motor is equipped with a means for monitoring power draw, which could be accomplished through specification of a VFD that includes a point for real-time power draw or with a standalone power meter. The delivered cooling capacity is established by monitoring leaving water temperature, return water temperature and the chilled water flow.

A CPOC offers an opportunity to unlock cooling plant equipment efficiency, offering long-term performance improvements. A challenge to upgrading the controls is that it requires additional investment beyond the minimum scope required for controlling a plant to the current code.

Design teams and operations teams can work together to assess the opportunity for improvement and the associated ROI. In the future, advances in machine learning and artificial intelligence are expected support additional innovations in plant operation.

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