National Renewable Energy Laboratory's Energy System Integrated Facility—Exhaust Fan Energy Versus Fume Reentry
Ron Petersen, Ph.D., CCM, CPP, Inc.
The country is focused on methods to save energy with particular focus on turning off lights, riding a bicycle, etc., yet there is a huge amount of unrealized energy savings available in our nation's research and teaching laboratories. A typical laboratory consumes up to 10 times the energy per square foot of an office building. The ventilation system uses a high percentage of this energy, often up to 80 percent. Energy-saving strategies often overlook the exhaust system, even though it accounts for approximately 30 percent of the laboratory building's total energy consumption.
This talk will discuss the methods used to help develop an energy efficient laboratory exhaust system design for the National Renewable Energy Laboratory's (NREL) Energy System Integrated Facility (ESIF). The ESIF is designed to have six laboratory exhaust stacks with volume flows of approximately 14,000 cubic feet per minute (cfm) each. The ESIF will also have some smaller exhaust systems. Past studies by the author have shown that by using a variable air volume (VAV) exhaust system tied to a weather station, significant fan energy savings can be achieved (at least $0.50 per cfm per year). Traditionally, laboratories have been designed such that the exhaust system must operate at full load conditions 24 hours a day, 365 days a year (i.e., a constant volume [CV] exhaust system). Full load often meant the minimum flows to avoid adverse fume reentry, which in some cases could be significantly greater than the ventilation requirement. In addition, this fume reentry set-point was based on the worst-case wind condition, which typically occurs only for a small fraction of total hours each year. To overcome this limitation, the ESIF is planning to utilize an energy-efficient VAV exhaust system tied to a weather station.
The methodology employed to develop the energy-efficient and safe (from a fume reentry perspective) exhaust system, consisted of first constructing a 1:240 scale model of the ESIF, surrounding buildings, and nearby terrain. The scale model was then positioned in a boundary-layer wind tunnel, and fume reentry measurements were obtained due to the laboratory exhaust stacks. Methods outlined in a recent Labs21 Best Practice Guide (772 KB, 12 pp) were utilized. During the testing, measurements of wind speed and wind direction at a weather tower that would be positioned on the roof of the ESIF were also obtained. A correlation between the wind speed/direction, exhaust volume flow, and acceptable fume reentry was then developed. This correlation function was then supplied to the design team for use in the building automation system. The detailed evaluation showed that, under most weather conditions, the fan flows could be turned down by at least 50 percent. The design team is expecting that once the building is operational, significant fan energy savings will be achieved.
As a principal at CPP, Inc., Ron Petersen has led the exhaust dispersion group for more than 30 years, during which time the group has specified exhaust parameters for more than 500 laboratories, such as: NREL's ESIF, the Lawrence Berkeley National Laboratory Solar Energy Research Center, the University of California, San Diego, Clinical & Translational Research Institute, the University of California, Los Angeles, Western Institute of Nanotechnology on Green Engineering and Metrology, the University of California, Berkeley, Energy Biosciences Building, the Georgia Tech Biotechnology Campus Quad, and numerous others. During those 30 years, laboratory exhaust design has undergone several evolutions. Early laboratory exhaust design used rules-of-thumb and past experience with little attention given to health and safety or energy concerns. More recently, Dr. Petersen has led the way in developing methods for minimizing fan energy costs while at the same time maintaining health and safety. Much of this development was carried out through ASHRAE- and CPP-funded research studies related to exhaust design. The current state-of-the-art has been documented in a Labs21 Best Practice Guide (772 KB, 12 pp) (primary author: Dr. Petersen) and ASHRAE Journal articles.