Trends in Green (Sustainable Innovations on Campus)

Thermal 'Ravioli'

As we progress toward an increasingly knowledge-based economy, technology within the workplace advances and occupant density increases. The combination of the building and workplace paradigm shifts result in buildings that use a tremendous amount of energy, collectively accounting for 41 percent of the U.S. annual energy demand, and 75 percent of U.S. electrical demand.

Historically, massive construction helped to keep occupants comfortable through leveraging climate for low-energy heating and cooling strategies. We know energy flows from hot to cold, and that mass needs to be cooler than the surrounding space to absorb the heat from occupants, equipment and sunshine. The high density of mass coupled with the difference of temperature creates a reservoir of “coolth” that absorbs interior heat gain with minimal temperature increases to surrounding spaces.

The absorbed heat can then be released at night to help warm the space, or rejected into cool night air, to recharge the mass and begin the cycle anew the following day. In climates with nighttime temperatures that dip well below daytime highs, mass can be recharged by simply opening the windows during cooler evening hours and closing the windows when temperatures begin to climb. A bio-based phasechange material, or PCM, created by Phase Change Energy Solutions, uses the concept of mass thermal storage and release by leveraging the energy associated with a change of phase.

Like Ice in a Chest

A material changes phase when it transitions from a solid to a liquid, and a liquid to vapor. Much more energy can be absorbed or released from the change of phase than from heating or cooling a material alone, requiring much less volume to achieve similar thermal capacity. For example, one inch of phase-change material is as effective as 12 inches of concrete.

Bio-PCM uses palm and soy oils rather than water as the medium, with a melting point near 72 degrees Fahrenheit. The temperature within the room will stay near 72 degrees, the melting point of the oils, until the capacity of the phase change is exceeded, and all of the oils have melted. By using phase-change material for cooling, the effectiveness of natural ventilation strategies can be expanded. Due to its relatively high melting/freezing point, PCM can be recharged for the following day by exposing it to cool night air. Bio-PCM is produced in large sheets with the oils encapsulated in pockets similar to a sheet of uncut ravioli. The sheets can be stapled to studs in wall assemblies or laid on top of a drop-ceiling assembly.

PCM at UW

The University of Washington Molecular Engineering and Sciences Building, designed by ZGF Architects LLP, is composed of research laboratories, open-office graduate workspace and faculty offices. Due to the very different usage and space conditioning strategies, and desire to provide visual access and daylight across the building, the laboratory was separated from the offices by a glass wall. The laboratories are conditioned by standard means, with frequent ventilation air changes to remove airborne contaminants and maintain safety.

The office area is naturally ventilated by operable windows and exhaust stacks. Phase-change material was used to expand the capacity of the natural ventilation strategy as the majority of concrete in the building was covered with finishes suited for a world-class research facility.

The combined reduction in energy use from the natural ventilation strategy resulted in a nearly 98 percent decrease in fan and cooling energy in the offices when compared with a conventional cooling system.

Because Bio-PCM is new to the market, a monitoring scheme was developed to measure the temperature of the PCM over time to verify that the PCM performed as expected. Due to a lack of true control and experimental conditions, the ability to understand the full benefit of the PCM is limited. ZGF did not want to create a sacrificial space completely devoid of PCM for control purposes and run the risk of compromising occupant comfort.

Instead, PCM was excluded from one stud bay cavity in an office and one section in the ceiling. The intent is to use data collected from the areas without PCM as a comparison to areas with, and extrapolate to understand the effects on the larger environment.

Data-loggers produced by Onset Corp. were placed within adjacent wall cavities with and without PCM and similarly in the ceiling. Temperature measurements are recorded at 15-minute intervals. Initial data has illustrated that bio-PCM solution has been effective in moderating temperature swings within the space.

In locations where energy is more costly than in Washington State, the use of phase-change material in conjunction with complementary strategies such as natural ventilation has the potential to deliver significant operational cost savings over the life of the building without compromising occupant comfort.

This article originally appeared in the November 2014 issue of College Planning & Management.

About the Author

Ed Clark, LEED-AP, BD+C, is a sustainable strategist at ZGF Architects LLP (www.zgf.com).

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