Energy Efficiency

Literature Review of Flexible and Adaptive HVAC Distribution Systems for Office Buildings

The literature review was conducted by searching the Internet and electronic engineering databases, and contacting manufacturers for product literature. The located articles are classified as general articles, case studies, experimental studies, computational fluid dynamics (CFD) studies, and manufacturer products. The authors should be contacted for copies of the articles cited in the report. The review indicates that, for forced ventilation systems designed for offices, the conditioned air is transported either in the ceilings or under the floors. The overhead ceiling system is the conventional way to transport conditioned air. The review suggests that there are minimal flexible and adaptive options for the conventional overhead systems (COSs). Therefore, the flexible and adaptive HVAC distribution systems are generally restricted to systems where the conditioned air is supplied from under the floor.

More than 30 projects involving several different types of facilities have installed raised floor systems (RFSs) in the US within the past four years. There are more than 20 articles
available concerning experimental studies related to RFSs. In these articles, some experiments are performed to validate the results of CFD studies. CFD studies that involve numerical simulations of air flow and temperature distributions and contaminant transport within workspaces are receiving increasing attention and are being acceptable as analysis and design tools. Because the concept of RFS has only been accepted in recent years, the review indicates that there are few RFS products available in the US. As discussed in the located articles, a RFS is one kind of displacement ventilation system where the supply air temperature is approximately 10°F higher than that for a COS. Typical underfloor supply air temperatures are above 63ºF (17ºC) and nearly all office installations are carpeted so that cold floors or cool feet should not be a problem. There appears a need for further studies about whether the supply air flow rate for a RFS is greater or less than that of a COS. Individually controlled supply diffusers allow occupants to adjust the local air flow to improve the personal climate. A RFS is more energy efficient because the air temperature near the ceiling is higher than at the comfort value, and the temperature difference between the supply air and return/exhaust air is higher than that of a COS. Experiments also show that floor-mounted diffusers provide a piston-like flow that is more efficient than wallmounted diffusers that provide mixing ventilation type flow.

Although there have been several experiments on RFSs, more experiments are needed to provide additional information to such concerns as energy efficient and thermal comfort to accelerate the acceptance of RFSs. Parallel tests involving RFS and COS would be a way to address some of these concerns. More experiments and monitoring of actual installations should be conducted to quantify the energy efficiency of RFSs. Test parameters should be based on the influence of various heat sources, different heat load densities, nonuniform load distributions; and the effects of the perimeter load. More tests should be done on the evaluation and optimization of comfort control for the system. The quantification of thermal storage in the concrete slab, the heat transfer rate to the underfloor supply air, and the air temperature variations across the plenum should be tested. Additionally, parallel with the experimental tests, CFD studies should be conducted. There appears opportunities for additional products for RFSs.

(complete report available upon request of the Iowa Energy Center)

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