©2012 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 54, no. 2, February 2012
By Bjarne W. Olesen, Ph.D., Fellow ASHRAE
About the Author
Bjarne W. Olesen, Ph.D., is director, professor, International Centre for Indoor Environment and Energy, Technical University of Denmark in Lyngby, Denmark.
Using the thermal storage capacity of the concrete slabs between each floor in multistory buildings to heat or cool is a trend that began in the early 1990s in Switzerland.1,2 Pipes carrying water for heating and cooling are embedded in the center of the concrete slab. In central Europe (Germany, Austria, Netherlands, etc.), this type of system has been installed in a significant number of new office buildings since the late 1990s. The trend is spreading to other parts of the world (the rest of Europe, North America and Asia).
Thermo active building systems (TABS) are primarily used for cooling multistory buildings. By activating the building mass, there is a direct heating-cooling effect. Also, because of the thermal mass, the peak load will be reduced and some of the cooling load will be transferred beyond the time of occupancy. Because these systems for cooling operate at water temperatures close to room temperature, they increase the efficiency of heat pumps, ground heat exchangers and other systems using renewable energy sources.
TABS are an embedded water-based surface heating and cooling system, where the pipe is embedded in the central concrete core of a building’s construction (Figure 1).
The great feature of this type of radiant surface system is the thermal coupling of the emitting element (e.g., pipe coil) with the main building structure (concrete ceiling or wall).
Thermo active building systems exploit the high thermal inertia of the slab to perform peak shaving, which consists of reducing the peak-required cooling power (Figure 2) so that it is possible to cool the structures of the building during a period in which the occupants are absent (during nighttime as in offices. This way, the energy costs can be reduced using the lower nighttime electricity rate. At the same time, a reduction in the size of heating/cooling system components (including the chiller) is possible. As the water temperature used is near room temperature, the COP of chillers and heat pumps will increase and the energy consumption reduced.
During daytime, heat is extracted from the occupied space by the ventilation system when the supply air temperature is lower than the exhaust air temperature. Much of it is stored in the concrete slabs (Figure 3). Then, during the night, the level of ventilation is reduced, and the circulation of cool water in the slabs removes the stored heat.
TABS may be used with natural and mechanical day and night ventilation, with or without dehumidification, depending on external climate and indoor humidity production. In the example in Figure 2, the peak cooling power needed to dehumidify the ventilation air during daytime is sufficient for cooling the slab during nighttime.
This approach to radiant heating/cooling of the building system that consists of pipes embedded in concrete slabs between stories began in the 1930s. In Switzerland in 1937, a radiant system was installed (called “Crittall”) that was constructed with embedded steel-welded pipes in a concrete slab.
Most of the early systems failed because of the condensation that often occurred during operation in cooling mode. This problem was further studied and results showed that condensation could be avoided if the radiant system was used in combination with a control of the water supply temperature or a ventilation system for keeping a low absolute humidity of the indoor air. Another problem was the use of steel pipes and risk of leakages. In the early 1990s, the popularity of TABS began to increase. In Switzerland in 1993, R. Meierhans constructed a system using embedded plastic PEX pipes.
TABS can be used for both heating and cooling. However, the main reason to use TABS is the need for cooling as most of the heat exchange is over the ceiling where the internal heat exchange coefficient is highest compared to other surfaces (ISO 11855-2).3
TABS often are used in multistory buildings and partly substitute for a full air-conditioning system. The air system can be downsized (only ventilation), and suspended ceilings avoided. This may reduce the height of each floor by ~0.6 m (2 ft) and save on building materials, etc. As the piping is embedded in the building structure, there is basically no maintenance.
Citation: ASHRAE Journal, vol. 54, no. 3, March 2012
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