There are numerous different types of HVAC systems that building owners and managers can select for their high-performance buildings, including rooftop units, variable refrigerant flow (VRF) systems, geothermal or air-source heat pumps, and, of course, chillers. Chillers have long been the HVAC systems of choice for large commercial buildings and other facilities, but they are facing fierce competition from new technologies, such as VRF systems, which are frequently considered to be easier to install and maintain.
Manufacturers are pushing back, though, noting that chillers offer exceptional energy efficiency as well as greater design flexibility, better comfort, and lower total life-cycle costs — all of which can make them a perfect choice for high-performance buildings.
The primary focus of a high-performance building is energy efficiency, which is where chillers really shine. In fact, ASHRAE Standard 90.1-2013 shows water-cooled centrifugal chillers with capacities of 400 ton or more to be more efficient than other mechanical cooling technology, including air-cooled chillers, rooftop units, and self-contained systems, said Christine Detz, senior product manager of centrifugal chillers, building technologies and solutions, Johnson Controls Inc.
“Water-cooled chillers can also be designed to function over a wide operating envelope, which means warmer chilled water temperatures and colder tower water can be used for greater energy savings,” said Detz. “Other types of chiller designs, such as air-cooled free-cooling chillers or variable-speed drive (VSD) chillers, can minimize compressor runtime, which helps to increase energy savings even more. In other words, chillers deliver efficiencies, capacities, and options that make them advantageous for buildings designed for exceptional performance — not just for energy savings but also for cost-effectiveness, sustainability, functionality, productivity, and other performance-related criteria.”
Since HVAC systems account for a significant portion of a building’s energy use, chillers can be critical components in improving a building’s overall efficiency, noted Mike Patterson, centrifugal chiller product manager, Trane.
“Chillers also have a low total cost of ownership, offering high-performance building owners cost savings over the lifetimes of the systems. Well-designed chillers work with HVAC systems to deliver the right temperatures, humidity levels, and ventilation for the space while also prioritizing low operating cost and energy efficiency and ensuring low sound levels and minimal environmental impact.”
Indeed, sound is a major consideration, especially when mechanical rooms are located near occupied spaces in educational, health care, and performing arts facilities.
“Chillers allow for compressors to be isolated from the occupied space while still providing comfort cooling to each individualized zone,” said Thomas McDermott, director of sales, ClimaCool Corp. “Additionally, sound panels for air-cooled chillers and modular chillers in banks allow for low-sound options since the chiller only operates to satisfy the cooling load requirement. Using scroll compressors, which have inherently lower noise levels than air-cooled screw compressors, can result in further sound reductions.”
Chillers also contribute to the sustainability of high-performance buildings, because they operate at a lower head pressure, which uses much less energy and reduces greenhouse gas emissions, noted McDermott.
“Modular chillers, in particular, can recover and harness the heat produced in one part of the building and use it in another, which helps to increase the building’s overall energy efficiency,” McDermott said.
In addition to great efficiency, chillers offer increased simplicity of installation, said Rob Landes, product manager of positive displacement chillers, Daikin Applied.
“Owners benefit from the fact that it’s easier to distribute water piping for chillers throughout some types of buildings versus distributing extensive ductwork required by other HVAC systems,” he said.
In high-performance buildings, chillers also offer precise control to match the building load, said Landes.
“No two buildings are alike, and no two days present the same conditions,” he said. “Chillers that are engineered to perform best at off-design conditions deliver the most optimized performance and cost of ownership.”
COSTS AND ADVANCEMENTS
Commercial building owners and managers are typically sensitive to the first cost of equipment, but chillers can compare favorably to other HVAC technologies, depending on the size and type of the system.
“Larger HVAC systems will typically lend themselves to a chilled water design,” said Landes. “Configurable air- and water-cooled chillers available today can offer the flexibility to minimize first cost while providing the maximum return on investment by focusing dollars on improving efficiency where the system is most likely to operate, which is usually at off-design conditions. In addition, centralized maintenance of a chiller can be simpler and less expensive than trying to maintain many individually dispersed pieces of equipment.”
Modular chillers operating in heat recovery or heat pump mode can also offer substantial lifetime energy savings for a high-performance building.
“Using a heat recovery mode of operation can provide building owners with the advantage of having both chilled and hot water in one piece of equipment,” said McDermott. “By recovering the heat already in the space, it can be used in other parts of the building, minimizing energy usage. Heat recovery modules of modular chillers can reach combined efficiencies of up to 25 EER.”
Heat recovery is a significant chiller system advancement that can be of great value in a high-performance building.
“Chillers with heat recovery reduce the ancillary power necessary to reject the heat while also reducing the amount of purchased heat required. This reduces operating costs and lowers the emissions output through the reduction of burned fossil fuels, such as natural gas, that would otherwise be needed to generate heat,” said Patterson.
Free cooling and thermal energy storage are two additional chiller system advancements that should be of interest to high-performance building owners. Free cooling is a refrigerant migration feature that can provide up to 45 percent of the nominal chiller capacity without running the compressor, which reduces energy use and annual operating expenses, said Patterson.
“With a thermal energy storage system, the chiller makes ice during nighttime hours — when utility rates are usually at their lowest — which is then used to help cool buildings during peak-rate daytime hours” he said. “Ice-enhanced cooling systems not only use less expensive electricity, they also use less of it in a more environmentally sustainable manner.”
The use of variable-speed drives is another technology that can lead to significant energy savings by allowing optimization of load and lift conditions. In real-world operations, load refers to how many Btu of heat a chiller can remove in an hour while lift is defined as the difference between the evaporator pressure and condenser pressure, explained Detz.
“The cooling load, of course, is a factor that affects chiller energy consumption,” she said. “VSDs can vary the compressor motor speed to more closely match the load, which lowers energy consumption,” she added.
A greater savings, however, can be realized with low-lift conditions — when either tower water temperatures can be reduced or the chilled water-supply temperatures can be increased — if the VSD chiller is designed to operate in these off-design conditions, said Detz.
“Some chillers can run with tower water as low as 36°F, which yields incredible efficiency levels,” she noted. “In many installations, a VSD chiller can take advantage of these off-design conditions to achieve payback in less than a year or two.”
Besides reducing energy usage, manufacturers of water-cooled chillers are continuing to develop technologies to shrink water tower size and water requirements.
“One innovation is an air-cooled radiator that removes heat from water before it reaches the tower, which helps reduce water tower size,” said Kevin Stockton, product manager — applied DX, building technologies and solutions, Johnson Controls Inc. “Air-cooled fluid coolers also reduce the amount of water lost by evaporation, which can yield huge water savings.”
There are several reasons building owners and managers should consider chillers for their high-performance buildings. As chiller technology continues to advance, the opportunities for greater energy efficiency, lower water use, and better reliability will almost assuredly continue to improve.
Publication date: 7/17/2017