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Cooling Load Calculations for Radiant Systems

©2013 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 55, no. 12, December 2013.

By Fred Bauman, P.E., Member ASHRAE; Jingjuan (Dove) Feng, Student member ASHRAE; and Stefano Schiavon, Ph.D., Associate Member ASHRAE

About the Authors
Fred Bauman, P.E., is a project scientist, Jingjuan (Dove) Feng is a Ph.D. candidate, and Stefano Schiavon, Ph.D., is an assistant professor at the Center for the Built Environment (CBE), University of California, Berkeley.

Interest and growth in radiant cooling and heating systems have increased in recent years because they have been shown to be energy efficient in comparison to all-air distribution systems. Olesen and others have discussed the principles of designing radiant slab cooling systems, including load shifting, the use of operative temperature for comfort control, and cooling capacity. Several case study examples with design information have been reported for an airport, large retail store with floor cooling, and other thermally active floor systems. A database of representative buildings with radiant systems can be found at http://bit.ly/RadiantBuildingsCBE. However, it is difficult to find detailed standardized guidelines for calculating cooling loads for radiant cooling systems, which is the subject of this article.

A radiant system is a sensible cooling and heating system that provides more than 50% of the total heat flux by thermal radiation. There are three primary types of water-based radiant systems: (1) for new construction: plastic tubing (e.g., PEX) embedded in the structural slabs, often referred to as thermally activated building system (TABS); (2) for retrofit or new construction: suspended metal ceiling panels with copper tubing attached to the top surface (radiant ceiling panel, RCP); and (3) for retrofit or new construction: prefabricated or installed-in-place systems consisting of embedded tubing (e.g., PEX, or small, closely spaced plastic tubing “mats”) in thinner layers (e.g., topping slab, gypsum board, or plaster) that are isolated (insulated) from the building structure (embedded surface system, ESS).

In this article, we present recent research evidence that sensible zone cooling loads for radiant systems are different (in fact, are often higher) than cooling loads for traditional air systems. This finding has important implications for the proper design and sizing of radiant systems along with the required reduced-sized air distribution system (for ventilation, control of latent loads, and supplemental cooling). Higher peak design cooling loads, however, is not the same as higher overall energy consumption. Hydronic-based radiant systems have verified advantages over air systems, such as the improved transport efficiency of using water instead of air as the thermal distribution fluid, improved plant side equipment efficiency with warmer cold water temperatures, and, particularly with TABS, the possibility of night pre-cooling using cooling towers.

We begin by reviewing current cooling load calculation methods and then describe the results of simulation and experimental studies addressing the sensible zone cooling load differences between radiant and air systems.

 

Review of Cooling Load Calculations

Compared to air systems, the presence of an actively cooled surface changes the heat transfer dynamics in a zone of a building (Figure 1). The chilled surface is able to instantaneously remove radiant heat (long and short wave) from any external (solar) or internal heat source, as well as interior surface (almost all will be warmer than the active surface), within its line-of-sight view.

This means that radiant cooling systems may impact zone cooling loads in several ways: (1) heat is removed from the zone through an additional heat transfer pathway (radiant heat transfer) compared to air systems, which rely on convective heat transfer only; (2) by cooling the inside surface temperatures of non-active exterior building walls, higher heat gain through the building envelope may result; and (3) radiant heat exchange with non-active surfaces also reduces heat accumulation in building mass, thereby affecting peak cooling loads. Additional details of these differences are discussed later.

 

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