Passive Hybrid Downdraught Cooling
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Learning Objectives Explain the differences between passive downdraught cooling, active downdraught cooling and hybrid downdraught cooling Name three technologies that can form effective hybrid systems with passive downdraught cooling Discuss how passive cooling might affect a building s form Explain the LEED credits associated with passive hybrid downdraught cooling
PDEC PHDC Uses evaporative cooling and natural ventilation exclusively. Uses both evaporative downdraught cooling and a mechanical backup system.
Air Conditioning 12% U.S. electricity supply 40% of Peak Load 50% Efficiency of fossil fuel electric plant
Evaporative Cooling
Air Circulation
What is Passive Downdraught Cooling?
Passive Appropriate for hot, dry conditions Depends on evaporating water in the airstream Uses natural ventilation Active Appropriate for hot, humid conditions Depends on fans, chilled water coils or a/c Uses mechanical equipment for cooling and/or to speed ventilation Hybrid Combines techniques
Applying Passive Cooling
Active vs. Passive Cooling Chart
Established Related Technologies
Evaporative (Swamp) Coolers
Adding a Heat Exchanger
Qanat
Two Systems Working Together Cooling systems located at the top of the building or in a tower Vertical air circulation space or spaces Provision for horizontal air circulation Properly sized warm air vents Complementary strategies
Passive Cooling Methods Sprayed mist or water droplets Shower (shower towers) Misting apparatus (misting towers) Drip pipe or board Wet pads, filters or porous surfaces Porous ceramics Wetted cellulous pads An air stream flowing past a stream of water Qanat Waterfall or water feature
Central atrium Central shaft Perimeter tower Detached tower
Air Intake Geometry: Central Atriums and Shafts
Air Intake Geometry: Perimeter Towers
Air Exhaust Geometry
Ask Yourself Where will the cool air be used? Just in the central atrium? In the spaces around the cooling area? In the entire building? How can this best be accomplished? By a central open area? By a closed tower with vents? By shafts or light wells with vents? Where will the air intakes be located? Wind direction? Height? Architectural form? Where will the intake and/or exhaust vents be located, and how will the air reach them?
Cool Towers
A Cellulous Pad Distribution pad Cooling pad Water drains to recirculation system
Porous Ceramic Evaporators (EvapCool)
Porous Ceramic Design Concepts
Shower and Misting Towers
Misting Towers
Designing a Passive Cooling Tower Calculate internal heat gain Develop thermal comfort model Calculate flow rate required to remove heat gains Relate to air flow rate and cooling efficiency Calculate air intake opening sizes and exhaust vent size
Designing a Cool Tower
Thermal Modeling
Sample Design Process
Active Cooling Systems for Hybrid Use Fans Chilled water cooling coils Conventional air conditioning Ice storage air conditioning
Chilled Water Cooling Coils
Ice Storage Air Conditioning Photo: Trane
Conventional Air Conditioning vs. Chilled Water Cooling Coils
Measuring Efficiency Air Conditioning Systems EER Energy Efficiency Ratio BTU/watt At 95 F, outdoor dry bulb temperature Reflects summer peak loads Best for peak load reduction SEER Seasonal Energy Efficiency Ratio BTU/watt On average for the whole cooling season across the U.S. Used to compare systems
Complementary Strategies Green Roofs and Landscaping White and Blue Roofs Wind Catchers Night Ventilation Night Sky Radiation Ground Source Cooling Controlling Solar Gain
Wind Catchers
Night Ventilation
Night Sky Radiation
Ground-Source Cooling
Controlling Solar Gain Building orientation Window and door design Overhangs Shading devices Thermal mass Insulation
Scottsdale Fire Station No. 2 LEED Platinum 14,000 SF 40% Less power $1,000 energy savings / year
Communications antenna Fan Misting ring 84 F @ 19% RH 64 F @ 85% RH
Zion National Park Visitor Center 7,600 SF Cool tower, cellulous pads Natural ventilation Nighttime ventilation Overhangs
Comfort Center
Source: NREL and NPS
Zion Visitor Center Complex Actual Energy Use, 2001-2002
Torrent Research Centre Gujarat, India Misting system in atriums, chimney stacks Complimentary strategies: Thermal mass Nightime ventilation Shading and recessed windows Vegetation and landscaping
Torrent Centre User Perceptions Too Hot or Too Cold
Torrent Centre User Perceptions Too Dry or Too Humid
Sandra Day O Conner Courthouse Phoenix, Arizona Misting tower Double facade
Project Specific Considerations Local Climate Cost and suitability Level of control Heating coordination
NO PASSIVE HYBRID Check microclimate PASSIVE Or PHDC Check microclimate (maybe no)
Cost / Benefits 2-15 years 4-12% ROI for retrofits depending on climate of new build cost for PHDC 19-23% of new build costs for A/C
LEED Credit Synergies EA Prerequisite/ Credit Energy Performance IEQ Credit Outdoor Air Delivery Monitoring IEQ Credit Increased Ventilation SS Credit Stormwater Design
Resources PHDC (design software available) www.phdc.eu The Architecture and Engineering of Downdraught Cooling, a Design Sourcebook, by Brian Ford, et.al. Whole Building Design Guide: Natural Ventilation www.wbdg.org/resources/naturalventilation.php Sustainable Federal Buildings Database femp.buildinggreen.com LEED Projects Directory www.usgbc.org/projects
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Poor Fair Average Good Excellent
Passive Hybrid Downdraught Cooling Thank you for listening!