When Do Variable Flow Fume Hoods Save Energy? Implications for lab design and behavior modification Michael Gevelber, Robert Choate, Kevin Sheehan, Brian Lo Boston University, Mechanical Engineering Outline Why all the fuss about fume hoods & energy? What is the conventional wisdom to save energy? When won’t a VAV fume hood save energy? Analysis of several BU lab buildings Pay back analysis Monitoring & behavior modification

How much energy/$ does a fume hood use? For $3.38/cfm, a 1000cfm hood costs $3,380/year Analysis by Mills and Sartor, LBL 2005 paper LBL Fume Hood Energy Model: Provides basis to use local climate, energy rates, efficiency to calc $/cfm & load costs

National Impact of Fume Hoods Mills and Sartor 2006 paper Significant energy use in US: ~750,000 hoods $4.2 billion/yr, 5100 MW of electric demand 26 TWh of electricity & 193 X BTU/yr Solutions –New designs –What to do about existing hoods? Variable  close the sash Constant volume  should they be changed to variable? Question: When will these solutions for existing hoods really pay off? Should we use variable for new hoods?

What Does University Websites Say to Do? Johns Hopkins A fume hood consumes 3.5 times the amount of energy consumed by the average house One simple action can make the difference between a wasteful lab and a responsible lab: CLOSE THE SASH! Cal Tech Shut your sash! - A variable volume fume hood is 60% more energy effective when the sash is down when not in use One fume hood uses as much energy as 3 typical American homes Stanford University Fume hoods are big energy hogs because they use so much conditioned air. When the fume hoods are not in use but left open, a tremendous amount of energy is wasted by the conditioned air flowing through the hoods and out of the building. University of Notre Dame Keeping just one variable air volume hood closed when not in use rather than leaving it open all the time saves $1,000 a year and is equivalent to taking 3 cars off the road. Pennsylvania State University With several hundred fume hoods at Penn State, we could save $250,000 to $500,000 in energy costs if the sashes are closed when the fume hoods are not being used. Is it as simple as it seems? No! Need to view the hood as part of the room’s HVAC system.

Variable Volume Fume Hoods System maintains constant face velocity, varies flow rate High capital cost but lower operating costs –IF the sash is closed Easy, inexpensive modification with large return –Reduce face velocity to 100 ft/min –Education and monitoring program Photo Credit: oregonstate.edu/vent/bypass

Constant Volume Fume Hoods “Constant” air flow regardless of sash height Lower capital cost, higher operating cost More challenging and expensive retrofit: need a variable speed [ventilation fans/motors] –Rebalance air system –Reduce flow rate when closed using a low position sensor –Minimize face velocity to 100 ft/min Photo Credit:

BU Fume Hood Background 385 fume hoods on Charles River campus –$1.5 million (~$4K per hood) in fume hood energy use costs –34% variable 66% constant –Several new lab buildings and rehab, done with phoenix valves, and all variable fume hoods. What should we do to reduce waste? - launched study to investigate cost of converting constant to variable

Hood/Room Exhaust Savings Analysis Total exhaust includes: + Min total exhaust based on room ACH Lab safety requirements: 6<ACH<12 If, No Savings Hood Exhaust

Hood/Room Exhaust Savings Savings Depend on: No. of Hoods in room ACH spec: 6 – 10 –UnOcc ACH reduction (4) Hood min/ max flow User behavior ACH 6 ACH 8 ACH 10 Assumptions:1000 sq ft room 1000 cfm hoods 63%-88% of “expected savings” 30%-70% of “expected savings” Need to redo analysis for different size hoods

What Are Payback Implications? What is relative cost savings if switch/design continuous to variable hoods? # Hoods/Room2 Hoods3 Hoods “Expected” Yearly Savings $4,000$6,000 Actual Savings $1,600$3,600 ∆Cost of VAV $20,000$30,000 “Expected” Payback 5 years Actual Payback 12.5 years8.3 years Assumes: 1000cfm Hood, $4/cfm, $10,000 per hood Closed 50% of time

Spreadsheet calculator for actual fume hood savings Can extend to actual savings by using $/cfm

A Look at Fume Hoods in 3 Lab Buildings at BU Photonics 8 St. Mary’s St 290,000 ft 2 32 Fume Hoods Life Sciences 24 Cummington 148,000 ft 2 79 VAV Hoods 10 CV Hoods Science Center 590 Comm 284,000 ft Fume Hoods

Hood Density Per Room No Savings Implications In many cases, if just 1 fume hood in room, should be constant volume & used as part of room exhaust. –what ACH is required for safety? –how implement safe unoccupied mode for room  down to 4 ACH

Fume Hoods Size (cfm) Distribution Red indicates one hood per room

Fume Hood Position Monitoring System For Behavior Modification Use Building Automation System to track which hoods are left open –Report on monthly basis to lab managers, staff, & professors –Need a champion in building, track only hoods that matter Simple iterative algorithm Lots of people will take your $ to provide a fancy system, but the above is easy to implement

Trial Monitoring Results 1 Day Dynamic ResultsAverage over 70 days , never close their hoods seems to be open at 70% 24/ , 848.5, doing a decent Job Modulates, but why not close more? Fume Hood Open Percent Fume Hood AVG Percent Open 50%

Summary Hoods are energy hogs, BUT we need to look how they function as part of room and building HVAC systems to figure out potential savings Savings for VAV Hoods likely to be less for rooms with only 2-3 hoods in them, maybe none for 1. For those rooms where there’s an impact, consider Fume Hood monitoring, large variation of behavior & changes term by term

Comparative Analysis of Two Size Hoods Q_hex_max = 800cfm, Q_hex_min = 200cfm Q_hex_max = 1200cfm, Q_hex_min = 200cfm