Sustainable Contributions

09/30/2003 |

Green Ideas For Energy-efficient HVAC Systems

New trends have had a more pervasive influence on the buildings industry than green design. As more projects stress sustainability and energy conservation, owners are rethinking the basics of building design to balance long-term functional and operational efficiency with sensitivity to the environment and resource use.

Fortunately, energy efficiency is hardly a new consideration for HVAC systems. Concerns about rising energy costs have yielded approaches such as pre-conditioning outside air, variable air volume (VAV) technology, and cleanable filters. These and other proven technologies offer relatively brief payback periods that more than offset any first-cost premium, while also providing long-term reliability and ease of maintenance. Properly designed, these systems can also help owners easily meet the energy conservation and indoor air quality criteria necessary for basic LEED  certification.

Interestingly, what may be considered “new” green HVAC technologies are actually variations of existing ideas. Energy recovery using heat wheels, once derided for their inefficiency and maintenance problems, have staged a comeback of sorts – thanks to improvements in both materials and design. Coupled with a VAV ventilation system, a heat wheel can effectively recirculate ambient air to maintain environmental comfort and reduce overall energy demand.

Raised-floor ventilation systems, a staple of computer rooms for decades, have also been adapted successfully for various building types. Instead of trying to control a room’s entire air volume from the ceiling, a raised-floor system needs to provide circulation only at the level occupied by people. With a smaller volume of air to displace, the system requires less energy to achieve the same circulation rate. And with new air distribution/diffusion devices, these systems are far less obtrusive than their largely utilitarian computer room forebears.

The availability of these approaches alone is not always enough to achieve a project’s energy conversation goals. If a Silver, Gold, or Platinum LEED rating is the goal, energy conservation design becomes more of an art than a science, requiring a higher degree of both creativity and collaboration. But it can be done, and with some remarkable results.

The new 194,000-square-foot School of Nursing and Student Community Center at the University of Texas-Houston illustrates the value of this approach. To help the medical education and research facility achieve LEED Platinum Certification, the M/E/P consultant needed to size the mechanical system for a 20,000-square-foot floor to draw no more than five hp of energy – a third of the power normally required by conventional HVAC systems.

Modeling a raised-floor ventilation system with precisely calculated air quantity and pressure drops, the engineers lowered the projected energy load to 7.5 hp. The M/E/P engineer worked with the owner, architect, structural engineer, and interior designer to reconfigure the floor so that corridors and other transient spaces were placed along the perimeter, where slightly wider temperature variances would go virtually unnoticed. This approach met the budgeted five-hp energy requirement and, more importantly, the owner’s operational needs.

Not every project type lends itself to a green HVAC system, nor are such approaches always cost-effective. But through education, experience, and a willingness of owners, design teams, and engineers to look beyond “cookbook” concepts, the range of economical, energy-efficient building system solutions will continue to grow. And getting more from less is an equation that benefits everyone.

Phil Sheridan, PE is a vice president at Fort Worth, TX-based Carter & Burgess (www.c-b.com), a national full-service A/E firm.


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