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Thermal Mass Control for High Performance Building Designs

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It is no surprise that as more jurisdictions and building owners place increased emphasis on sustainable and responsible building strategies, design engineers are looking beyond traditional HVAC solutions to maximize energy efficiency while maintaining occupant comfort and safety. A number of innovative systems have been incorporated on high-profile projects. One such system is in-slab radiant cooling.

In-slab radiant floor heating has enjoyed popularity for many years, though its use has primarily been in residential applications. In these systems, warm water circulates through a series of cross-linked polyethylene (PEX) piping loops embedded in the concrete floor. The flow rate and temperature of the water is controlled to regulate the temperature of the thermal mass. The warmed surface radiates heat to the objects and occupants in the space, creating a comfortable environment. This same principle is used in radiant cooling. In fact, in most cases, the same series of piping loops used for heating can be used for cooling. By controlling the slab temperature using chilled water, a radiant cooling system can effectively manage all or a portion of the building’s sensible load, thereby reducing the total demand placed on a forced-air system.

More Efficiency

Over the past decade, the number of radiant cooling systems designed, installed, and commissioned in North America has increased dramatically. Radiant cooling systems are gaining exposure and popularity for a variety of reasons. But, the main driving factor in the increase in radiant cooling systems design and specifications, however, is the potential for improved energy efficiency.

Radiant cooling systems can reduce overall building energy usage in a number of different ways. First of all, because the heat transfer capacity of water is much higher than that of air, a radiant system that uses a circulator to move water can achieve the same heat transfer using significantly less energy. Secondly, because of the way the human body exchanges heat with its surrounding environment, a radiant system can achieve comparable levels of comfort at higher room temperatures (i.e., 78 degrees F/26.6 degrees C). Also, the use of higher water temperatures for cooling may allow for more optimum operation of the system’s chilled water plant. Other sources for chilled water, such as geothermal systems, may also be used. Therefore, a radiant cooling system that manages the bulk of the building’s sensible loads, coupled with a smaller forced-air system (for ventilation, latent loads, and supplemental sensible loads) can significantly reduce a building’s total energy usage. Studies have shown total energy savings for typical office buildings is 17 to 53 percent (Stetiu 1999, Thornton et al. 2009).

A notable example of the use of radiant cooling to dramatically reduce overall energy usage is the recently completed Pier 15 Exploratorium project in San Francisco, CA. This 330,000-square-foot project renovated an existing, unused pier building to serve as the new home for the Exploratorium, an interactive children’s science museum. Radiant cooling was used throughout a majority of the exhibit spaces and the multipurpose event space. Because radiant cooling systems only require moderate temperatures, the design engineers were able to utilize the San Francisco Bay as a heat sink and source for chilled water. Bay water, piped through heat exchangers, provides cooling water for a significant part of the year. Not only does this innovative use of bay water reduce the overall energy consumption, it also significantly reduces the building’s water demand by eliminating the need for cooling towers. The se of bay water to serve the radiant cooling system, along with a number of other energy conservation measures helped reduce the building’s electrical energy usage for cooling by a staggering 94%.

A Sustainable Strategy

In addition to the potential for dramatic energy savings, radiant cooling can be seen as a sustainable strategy for a number of other reasons. The embedded tubing within the concrete slab requires no maintenance. The radiant cooling system, including the chilled water source and distribution, requires no greater maintenance than typical fluid-based systems. Therefore, a radiant system that can reduce the size of the airside system can reduce the need for disposable filters and belts. Additionally, PEX waste tubing can be readily processed and re-purposed into a variety of consumer goods, such as landscape timbers, asphalt filler, concrete filler, and even other pipe products as filler.

As building owners and jurisdictions continue to demand high-performance buildings, system designers are looking for sustainable solutions to reduce energy usage, while maintaining function. By taking advantage of a building’s thermal mass, an embedded-tube radiant cooling system can be an effective and energy-efficient alternative to a conventional forced-air-only system.

Corina Stetiu, “Energy and Peak Power Savings Potential of Radiant Cooling Systems in US Commercial Buildings,” Energy and Buildings, 127-138.
B.A. Th ornton, W. Wang, M.D. Lane, M.I. Rosenberg, B. Liu, “Technical Support Document: 50% Energy Savings Design Technology Packages for Medium Offi ce Buildings,” September 2009.

Devin Abellon
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Posted In: ACCA Now, Hydronics

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