As businesses embrace environmental initiatives to combat climate change, facility managers and building operators are feeling pressure to increase building efficiency, reduce energy consumption and operating costs. Many are looking to ‘free cooling’ solutions to achieve those goals. Free cooling can take two different forms: air-side economizers that directly exchange cool outdoor air with the building or water-side economizers that use outdoor air to cool the chilled water used to cool the building. This article focuses on water-side economization in particular using two different methods: stand-alone dry coolers and air-cooled chillers with integrated free cooling coils.
Determining the most effective free cooling strategies requires taking a myriad of factors into consideration. No one scenario may be perfect, but simulations can help reveal which types of buildings and applications are the most likely to benefit.
There’s a catch, however: These aren’t one-size-fits-all solutions. Free cooling chillers perform differently depending on facility size and location, and effectively applying them into a facility requires understanding the key optimization strategies to make integrated free cooling a viable solution.
The Rise of the Integrated Water-Side Economizer
Before integral mounting of economizer coils on air-cooled chillers was commonly available, some systems were designed to use separate dry coolers to subject the building heat to the cool outdoor air. Both solutions offer different advantages but making the best choice requires an understanding of the nuances between the two competing designs.
The Old Way: Stand-Alone Dry Coolers and Chillers
A dry cooler is a stand-alone, fluid-to-air heat exchanger that receives the building cooling loop glycol directly, then exchanges the heat with the outdoor air to cool the loop glycol before returning it to the building or routing through the chiller. In warmer ambient conditions where the dry cooler is unable to cool the fluid completely, the fluid flows through the chiller to receive supplemental mechanical cooling. To control the fluid flow path, the three-way valves connecting the building cooling water loop to the chiller and fluid cooler open or close, allowing the flow to bypass the dry cooler or chiller when appropriate, which helps manage pressure drop.
The New Way: Integrated Free Cooling
Similar in concept to a dry cooler, integrated free cooling uses glycol to air coils, but unlike dry coolers, these coils are typically attached to the chiller on the outside of the primary condenser coil to cool the process fluid using low temperature ambient air. Integrated free cooling chillers typically operate in one of three “modes.”
In mechanical cooling mode, the unit functions just like a normal air-cooled chiller, cooling the glycol using the refrigeration cycle. This is done when the ambient temperature is above the leaving glycol temperature.
In hybrid mode, glycol is diverted first through the air coils where it is partially cooled, and then diverted into the evaporator where it is further cooled to meet the design fluid temperature setpoint. Hybrid mode is used when the ambient temperature is below the entering fluid temperature, but not low enough to achieve 100 percent free cooling.
Because hybrid mode operates in mild ambient temperatures, it can often represent the greatest number of run hours. This means optimizing operation during hybrid mode is crucial for maximizing system efficiency and achieving the best return on investment.
Free cooling mode takes place when ambient temperatures are well below the leaving fluid temperature setpoint, often 20-30°F (11.1 – 16.7°C) lower, depending on the particular system design. In this mode, all the cooling of the glycol is achieved through the use of the coils and the compressors are turned off. As a result, this mode draws very low levels of power since only the condenser fans and fluid pumps require electricity.
The greatest benefit to integrated free cooling is a repeatable and optimized control sequence, developed by factory engineers who understand chiller efficiency curves and who can customize the operating sequence to improve the efficiency of the combined system. This is different than applying separate dry coolers and chillers, where the controls to make the equipment work together typically have to be installed and programmed by a third party.
However, it is important to understand that integrating free cooling coils onto the chiller does come with some drawbacks. In addition to heavier weight and higher initial cost, there is an efficiency penalty to the mechanical cooling mode. Adding free cooling coils over the existing condensing coils can increase the air pressure drop, causing the fans to work harder and raise the condensing temperature, which can lead to lower mechanical cooling efficiency when compared to a similar chiller without those coils. For free cooling to make sense, there must be enough load in the winter, where free cooling saves tremendous kilowatt usage, to more than offset the summertime efficiency penalty and still provide a reasonable payback period. Evaluating this balance is where building energy simulations can provide tremendous value.
Evaluating the Benefits of Free Cooling: The Scenarios
To assess the potential effectiveness of integrated water-side economizer coils for different climates and building types, 365-day building energy simulations are a useful comparative tool. The following simulations covered three prototype buildings across seven climate zones, and used the U.S. Department of Energy’s EnergyPlus (version 8.5) total building energy simulation tool to model a chiller plant consisting of commercially available chillers with and without integrated free cooling. The model generated hourly building loads based on typical meteorological year weather data and standard ASHRAE 90.1 inputs for building envelope and lighting power densities.
It is important to remember that studies like this are comparative so they can serve as good indicators for which applications may be better or worse than other applications. However, actual energy usage and dollar savings values may vary significantly in any given building. These simulations serve as a good starting comparison, rather than a conclusive rule.
The first scenario applied integrated free cooling to a hospital application. In this hospital, the chiller runs predominantly in mechanical cooling mode in the summer months and shoulder seasons due to a sizable cooling load and because temperatures are too warm for free cooling. Despite the presence of a wintertime cooling demand, free cooling operation sees limited run hours in this application due to the impact of fresh air ventilation.
In Figure 1, the blue bars represent a standard chiller without integrated free cooling, modeled in the different locations on the X axis. The red bars represent a chiller with an integrated free cooling coil. Comparing the red and blue bars, free cooling represents an increase in overall energy usage for this application. Click to enlarge.
Building codes typically require high ventilation rates for hospitals so cool outdoor air already provides the bulk of the cooling during the colder ambient conditions when hybrid cooling or free cooling would normally be active. As a result, there is relatively little load left to remove using the free cooling function. Conversely, in the summertime, the free cooling coils reduce the efficiency of the mechanical cooling, so the net effect is a higher overall energy use, even in cold climates like Duluth, Minn. This highlights one key aspect of chiller integrated free cooling: It provides the best benefits when used in conjunction with an application that has higher wintertime run hours, and cannot cost-effectively use air-side economizing to meet the wintertime cooling demand.
Figure 2 depicts the savings associated with integrated free cooling when compared to a non-free cooling chiller for each hospital location. Note that none of the locations experienced net savings throughout the year with the inclusion of integrated free cooling. This is due to very low run hours for free cooling due to smaller internal loads in the wintertime that were almost completely offset by the cooling effect from the ventilation air, leaving almost no run hours for the integrated water-side economizer. Click to enlarge.
Data Center Simulation
The next simulation examined the cooling needs and performance of a relatively small data center, which used chilled glycol to indirectly cool its servers. For this data center, the wintertime load is much higher than the previous hospital scenario because the load is tied to the electrical load of the servers, as opposed to being dependent on ambient temperature. Additionally, data centers generally run warmer chilled water temperatures so more hours of free cooling are available.
Figure 3 depicts energy consumption in a data center simulation featuring a 350-ton air-cooled screw chiller. Click to enlarge. Click to enlarge.
This trend can be seen by looking at the varying heights of the red and blue bars in Figure 3. In a location like Miami, because the number of free cooling hours is small, using integrated free cooling results in higher energy use because of the lower use of mechanical cooling. In all other locations, however, simulations showed that integrated free cooling resulted in energy savings — with significant savings noted in moderate to cold climates. For example, in Duluth, the annual energy usage dropped to almost half when using a chiller with an integrated water-side economizer, resulting in more than $50,000 of annual energy savings.
As previously mentioned, it’s important to recognize that in the case of data centers, the chilled water loop leaving fluid temperature is often designed for higher temperatures than those normally used for comfort cooling. While a typical comfort cooling application may use a 44°F (6.7°C) degrees Fahrenheit leaving fluid temperature, data centers often use 55°F (12.8°C) or higher, with the most common designs in the 60-70°F (15.6 – 21.1°C) fluid temperature range. This means that free cooling can engage at a relatively higher ambient temperature than comfort cooling applications that use lower leaving fluid temperature designs, leading to a higher number of free cooling operating hours and better payback.
Office Building Simulation
The final simulation evaluated free cooling in a medium-sized office building with a 600-ton comfort cooling load and ASHRAE-standard ventilation rates. The results fell somewhere between data centers and hospitals, with less winter load and run hours than a data center, but more than a hospital, due to lower ventilation.
Figure 4 shows how an integrated free cooling chiller can offer energy savings for an office building application when applied in moderate and cold climates. Click to enlarge.
In reviewing the results, Atlanta was just under break-even. So climates cooler than Atlanta could expect to see some energy savings in an office-building-type application.
Figure 5 shows yearly energy savings resulting from integrated free cooling in the Atlanta office building simulation. Click to enlarge.
The yearly spend on energy is shown in Figure 5, with the best result seen in Boston, which achieved $12,000 in annual energy savings. Many older buildings and systems may have ventilation rates less than those modeled here so energy savings in those instances could be even more pronounced.
Tapping into Savings Potential
Overall, these results demonstrate that integrated free cooling can result in significant energy savings when applied to the correct application. For data centers in particular, the energy savings of integrated free cooling during the cooler months far offset the penalty to mechanical cooling efficiency during the warmer months in all but the balmiest climates. It makes sense, then, that other applications with similarly high wintertime loads, such as process cooling applications for plastic manufacturing, pharmaceutical production, or food and beverage production, could similarly benefit from integrated free cooling, especially if those applications also use elevated leaving fluid temperatures.
Even office buildings and similar comfort cooling applications may see some benefit from an integrated free cooling solution. Due to climate and building differences, however, integrated free cooling may result in more varied results and thus requires more careful consideration.
AUTHOR BIO: As a Chiller Product and Applications Engineering Manager for Daikin, Rob Landes helps develop screw and scroll chillers that are not only efficient and reliable, but also cutting edge. For 13 years, Landes has worked for Daikin Applied both in product management and engineering roles. Prior to his current role, he worked with various HVAC products as a technician for a residential service and installation contractor.
ABOUT DAIKIN APPLIED: A member of Daikin Industries, Ltd., Daikin Applied designs and manufactures advanced commercial and industrial HVAC systems for customers around the world. The company's technology and services play a vital role in creating comfortable, efficient and sustainable spaces to work and live — and in delivering quality air to workers, tenants and building owners. Daikin Applied solutions are sold through a global network of dedicated sales, service and parts offices. For more information, visit www.daikinapplied.com.
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