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Free Cooling: An Intelligent Solution for Year-Round Cooling

March 13, 2017
The combination of higher chilled-liquid temperatures and intelligent fan and compressor control increases operation with free or partial free cooling.

Chiller free cooling has been around for decades, with systems historically limited to relatively low chilled-liquid temperatures (approximately 55°F and below) and operation during winter conditions. Recent trends in buildings and increased emphasis on energy conservation, however, have sparked an evolution in chiller design, with expanded operation and a wider range of chilled-liquid temperatures.

Energy Savings From Free Cooling

The combination of higher design liquid temperatures and waterside-economizer free cooling can greatly improve energy efficiency, even in warm climates. Proven solutions for reducing the operating costs of facilities with year-round cooling requirements include field-erected systems consisting of a chiller and a separate dry cooler, waterside economizers packaged on chillers, and evaporative coolers used in combination with chillers.

New free-cooling-chiller technology integrates a waterside economizer with condenser heat exchangers, allowing easy service access to all mechanical components and optimizing the footprint, compared with systems with separate economizers. As a factory-packaged solution, the chiller features a user interface with controls and piping that are the same as a traditional chiller’s. A single temperature setpoint permits easy adjustment as building needs evolve. Temperature changes do not require commissioning, ensuring maximum savings are delivered throughout the chiller’s design life. Additionally, a single liquid inlet and outlet simplify field piping.

Figures 1 and 2 show the results of free cooling with 50°F leaving liquid for a fixed 300-ton load in Hartford, Conn., and Houston, respectively. The figures show annual energy use for different temperature bins for a base air-cooled screw chiller and a free-cooling air-cooled screw chiller. As expected, the cool climate of Hartford resulted in a substantial annual energy savings of 25 percent; no benefit was seen in Houston.

FIGURE 1. Free cooling with 50°F leaving liquid (35 percent propylene glycol), Hartford, Conn.
FIGURE 2. Free cooling with 50°F leaving liquid (20 percent propylene glycol), Houston.

Figures 3 and 4 show the results of free cooling with 70°F leaving-liquid temperature in Hartford and Houston, respectively. In both locations, the benefit from free cooling greatly increased. In Hartford, free cooling provided 60 percent energy savings; in Houston, it provided 25 percent energy savings. The savings are in addition to those achieved with the higher leaving-liquid temperature for the base air-cooled screw chiller.

FIGURE 3. Free cooling with 70°F leaving liquid (35 percent propylene glycol), Hartford, Conn.
FIGURE 4. Free cooling with 70°F leaving liquid (20 percent propylene glycol), Houston.

How Free Cooling Achieves Savings

Not all free-cooling chillers are the same. Waterside-economizer free cooling can significantly reduce operating costs for facilities with year-round cooling requirements. However, not all free-cooling systems deliver the same savings. It is important to understand how recent advances in free-cooling technology deliver greater efficiency throughout the year.

Pioneering technologies for increased savings. The latest free-cooling-chiller technology combines new control strategies with mechanical advances to further enhance performance at design and off-design conditions. The mechanical improvements include variable-volume-ratio compressor control, which prevents the waste of energy on overcompression at reduced ambient temperatures, and variable-speed drives for compressors and fans. Intelligent controls continuously evaluate and optimize compressor and fan speeds to minimize total energy use.

Operating modes. In addition to optimizing compressor and fan speed at all conditions, the controls automatically transition between operating modes, depending on ambient temperature and cooling requirements, as shown in figures 5, 6, and 7.

In Figure 5, the chiller operates in a mechanical mode when it is too warm to use ambient air for free cooling. In this mode, the unit performs as a standard chiller. An automatic liquid control valve, factory-packaged and controlled, bypasses the free-cooling coils to reduce pump head and save energy. When either cooling load or ambient temperature is below design condition, the variable-speed screw compressors and condenser fans modulate to minimize energy use.

FIGURE 5. Mechanical mode.

In Figure 6, the chiller operates in hybrid mode, which optimizes energy use through simultaneous operation of the compressor(s) and free-cooling coils. With variable-speed compressors, power consumption is minimized through reduced compressor speed as the free-cooling coils deliver partial cooling. The controls automatically adjust fan speed to optimize the free-cooling benefit while ensuring reliable compressor operation.

FIGURE 6. Hybrid mode.

Across a wide range of ambient conditions, the power used in variable-speed-optimized hybrid mode is less than the power consumed by simply running the fans at a high speed to meet the cooling demand with free cooling alone. In these cases, hybrid mode outperforms simple waterside-economizer free-cooling systems. The chiller controls automatically ensure the chiller selects the most efficient operating mode. Figure 8 shows the hours the chiller operates at an advantage, compared with a traditional fixed-speed chiller and waterside economizer with fixed shutoff temperature for compressors.

FIGURE 8. Ambient temperature vs. efficiency.

As shown in Figure 7, free-cooling mode operates at lower ambient temperatures, when the required cooling load can be delivered most efficiently by the free-cooling coils. Compressors shut off and variable-speed fans modulate to meet the cooling setpoint. The controls determine the optimum conditions, moving between different operating modes to minimize energy use while ensuring reliable operation. Additionally, starting in free-cooling mode at very cold ambient conditions only requires fan motors to start, reducing the possibility of nuisance trips that can occur at low-ambient and/or high-wind conditions.

FIGURE 7. Free-cooling mode, open loop.

Alternate configurations for different building types. The aforementioned configurations circulate glycol through free-cooling coils directly. This is the most efficient and least expensive chiller solution, but requires glycol in the building loop to prevent freeze damage to the free-cooling coils. For installations that require water in the building loop, a closed-loop configuration is available. As shown in Figure 9, the closed-loop configuration uses a glycol-water heat exchanger and circulating pump to isolate the free-cooling glycol loop from the building water loop. The three-way valve used in the open loop to divert glycol into the free-cooling coils is not required.

FIGURE 9. Free-cooling mode, closed loop.

End Result: Reduction in Annual Energy Use

One easy method of confirming the benefit of a system is to compare the energy consumption using a multipoint rating for a range of operating temperatures. Alternately, total annual energy use can be calculated quickly by summing the kilowatt-hours for each temperature bin in a given location using publicly available weather data. Tools such as energy-consumption calculators provide easy-to-read, comprehensive reports that can be shared with owners and operators to accurately estimate total energy cost.

These advances in technology enable a chiller to deliver low annual operating cost. Using the highest leaving-water temperature provides greater savings and more cooling capacity in the same footprint as traditional systems. The combination of higher chilled-liquid temperatures and intelligent fan and compressor control during shoulder periods provides more operating hours with free or partial free cooling to deliver the lowest operating cost, making it an ideal solution for warmer climates and critical process operations.

William Kopko is engineering manager, technology and innovation, and Christian Rudio is director of product management, air-cooled chillers and heat pumps, for Johnson Controls.

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