Four years ago, Carol Anderson, director of facilities for Humber College Institute of Technology & Advanced Learning in Toronto, began planning an upgrade of the school's 33-year-old chiller plant, which was operating with R-11, a chlorofluorocarbon-based refrigerant no longer in production. Beyond meeting the challenges presented by any chiller-plant upgrade, Anderson had to follow Humber's commitment to reducing electricity demand, greenhouse-gas emissions, and carbon-dioxide emissions. Additionally, Anderson had to allow for future expansion of campus facilities, or a 40-percent increase in total cooling load.
Following a feasibility study, in which current and future loads, technology, energy consumption, operating flexibility, and load shedding were analyzed, a design calling for an all-variable-speed chiller plant, a dual-fuel generator sized to service one chiller for electrical-demand management, and an all-variable-speed plant-automation solution was chosen. To help address space constraints, site-access issues, and a tight schedule targeting the 2007 cooling season, the design specification called for prefabricated pump stations (automated pump packages), with controls and electrical components integrated into the assembly, for both the chilled- and condenser-water circuits. The plant layout went to a full variable-primary configuration.
The plant's three 550-ton chillers were purchased prior to the project's tendering to safeguard against one of the existing machines failing before the new system could be made operable and, considering the tight retrofit window, to ensure they would be on hand when needed.
After the contractor and suppliers were selected, all of the key stakeholders met to review the scope of work and coordinate execution of the project. Because of the existing plant's relatively small footprint (approximately 2,500 sq ft), there was little space for multiple work crews to operate. The prefabricated pump stations resulted in a net reduction of critical-path time of approximately three weeks.
With Hartman Loop technology as the basis of design, the integrated plant-control (IPC) system enables Web-accessible or local plant touchscreen control with a user-friendly icon-driven system schematic and downloadable data log. Ease of use and monitoring were important to both the college and the local utility, which contributed funding for the net reduction in annual kilowatt-hours. To further centralize data access, the IPC system was tied into the existing building-automation system (BAS) through a serial communication link. This communication link read-enabled more than 40 key plant parameters and write-enabled the BAS with four key operating instructions.
To enhance plant availability for part-load scenarios, the design called for the three chillers, three chilled-water pumps, three condenser pumps, and three cooling towers to be piped in parallel, which meant major piping modifications.
Most efficient chiller plants of this size operate at an annualized average of 0.75 to 1.1 kw per ton, employing a variable-secondary pumping system, variable-speed chillers, and variable-speed cooling-tower fans. An “ultraefficient” plant incorporates variable-speed primary pumping, variable-speed chillers, variable-speed condenser pumps, and variable-speed cooling-tower fans and operates at an annualized average of less than 0.45 kw per ton. All electrically driven rotating mechanical equipment in the plant is variable-speed, enabling a more precise match of equipment operating speed/load to building load, while using the least amount of energy possible.
Aside from two-speed condenser fans, the original plant system was constant-speed, with the components selected for peak-load operation. The chilled-water-distribution system was constant-flow, utilizing three-way control valves, which were good for temperature control, but used more energy than necessary when in bypass position, which was 95 percent of the time. The original system operated at an annualized average of 1.2 to 1.4 kw per ton.
With a typical North American plant operating at part load 95 percent of the time, the new plant-system components were selected for high part-load efficiency. In a typical part-load daytime scenario, the new variable-speed chillers operate at approximately 0.35 kw per ton, whereas the former chillers operated at around 0.85 kw per ton. At full load, the new chillers operate at close to 0.6 kw per ton, compared with the former chillers, which operated at approximately 0.78 kw per ton.
The plant-automation system seamlessly trades off capacity between rotating components to achieve the most efficient operation. Enabled by integrated network systems, the Hartman Loop employs three patented control methods in place of traditional capacity-based sequencing, proportional-integral-derivative feedback loop control, and chilled-water reset. These control methods are:
Natural-curve sequencing. A natural curve connects the most efficient points on a piece of equipment's performance curve for multiple scenarios. Parallel equipment is staged so that it remains as close to the natural curve as possible.
Equal Marginal Performance Principle (EMPP). The EMPP allows a controller to select the most efficient loading point for a combination of components. In other words, the system trades off inefficient energy for efficient energy by shifting more or less load on a component, all the while meeting demand. For example, operating cooling towers at 90 percent on a 75-percent-demand day will lower chiller condensing temperature, providing more lift in the chiller, considerably improving operating efficiency with very little energy input. However, reducing the power to the tower fans by 1 kw might cause only a 0.5-kw increase in power draw at the chiller. When the net reduction of tower-fan speed for 1 kw leads to a net increase in power draw at the chiller of 1 kw, the two devices are balanced to the EMPP. For a chilled-water plant, condenser-water-pump speed, chilled-water-pump speed, chiller speed, and tower-fan speed need to be coordinated to minimize net system kilowatt power draw.
Demand-based control. Modern information-technology networks and microprocessor capacity are utilized to plot system-efficiency curves for plant load as a function of power input for each device. By knowing plant load at a specific point in time, one can reference those performance curves and determine the EMPP. The performance curves are specific to manufacturer data for variable-speed devices and generated separately for each plant design.
Benefits of the new plant design and all-variable-speed IPC system include:
Reduced demand charges from the utility.
A reduction in electricity consumption (kilowatt-hours) of approximately 50 percent.
Reduced wear on equipment.
Lower maintenance costs.
Reduced greenhouse-gas emissions and use of ozone-depleting refrigerants.
The site work, which included the removal of existing equipment, began in late November 2006; by April 2007, a new ultraefficient chiller plant was ready for use.
During a plant-startup ceremony on June 4, Humber College was presented a plaque by the minister of energy in recognition of its leadership in the sustainable and energy-efficient design of its new chiller plant.
Robert W. Clements, B.Sc.Eng., is sales manager for the province of Ontario, and Peter Thomsen, P.Eng., MBA, is the director of the Systems Customer Solution Group (CSG), for S.A. Armstrong Ltd. With more than 20 years of HVAC-industry experience, Clements has a strong interest in energy conservation, space optimization, and life-cycle costing for both new-construction and retrofit projects. He can be contacted at [email protected]. Thomsen has held a number of technical positions within the HVAC, rail, power-generation, and industrial automation markets. With the Systems CSG, he is focused on quality value-added services and solutions for cooling and heating applications.
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