Radiant Exchange in Stratified Environments: Considerations and Challenges for Hourly Energy Simulation of UFAD, DV, and Radiant Cooling Systems CBE Timothy Moore CENTER FOR THE BUILT ENVIRONMENT SimBuild August 2006
Published methodologies for UFAD and DV
Load splits or Plume Factors UFAD and TDV differences in stratification height and gradient based upon Loads and T Airflow rate Coupling with radiant systems Cooling vs. Dead-band vs. Heating modes Radiant and conductive heat transfer to UFAD plenum Radiant exchange (with all view factors accounted for) plus convective heat transfer in radiant cooling applications
UFAD and TDV differences in stratification per T
Cooling Load splits for UFAD and TDV Apportioning internal loads to return plenum is a good idea, but methods proposed in EDR document need refinement or replacement Assigning occupants to plenum zone w/o fixing OA per zone results in reduction of ventilation rate and related loads One overall split may be adequate for component loads, but needs to vary with system mode (cooling, heating, or dead-band) Differentiation based on system type & configuration (table ) Evolving UFAD version of equest/doe-2 comes closer Cooling, heating, and dead-band plume factors for UFAD or DV Ability to model UFAD supply plenum (including thermal decay) Plume factors can be used for some radiant cooling applications to apportion load to chiller/cooling tower and not airside systems
Cooling Load splits for UFAD and TDV Perimeter loads not adequately addressed in EDR guidelines are included in equest-ufad development Existing method would need additional glazing in RA plenum zone Proportionate U-Factors and SHGC adjustment in both added and actual to maintain same loads (but would need to leave T vis in actual glazing unchanged for daylighting) equest-ufad single plume factor for each zone effectively incl. this Need guidance on splits for different diffuser types, blinds, etc. Need guidance re: TDV limitations and variable load splits Not typically recommended for perimeter zones with high loads Maintaining comfort avoiding excessive stratification - requires high airflow or added sensible load removal (e.g., radiant cooling) High airflows = high fan energy & reduced load split (plume factor) Diffuser size limits capacity for high airflows
Stratification change with airflow rate Test results for airflow rate with constant load, swirl diffusers, interior zone Height, ft 11 10 9 8 7 6 5 4 3 2 1.0 cfm/sq. ft 0.6 cfm/sq. ft 0.3 cfm/sq. ft ASHRAE Std.55-2004 5 F T 1 0 Still satisfies vertical temperature difference (5 F) with 40% less air 69 70 71 72 73 74 75 76 77 78 79 80 81 82 Room Temperature, F
Cooling Fan energy Fan power reduction for lower static Essential to provide for reduced static pressure set point: e.g., 0.5-1.0 iwc less than OH for some UFAD designs Depends on reduction of ductwork plus air-side load fraction Allow for static pressure reset with load in VAV systems 80% % Fan Power Savings (Relative to Overhead VAV System) 60% 40% 20% Underfloor VAV Low Volume: 90% CFM, 75% FSP Underfloor VAV High Volume: 120% CFM 75% FSP 0% -20% 50% 60% 70% 80% 90% 100% % of Design Load
Cooling - Zone equipment Zone equipment needs to be adequately represented Series fan-powered-boxes Perimeter heating by FPB drawing from occupied space vs. plenum Cooling-only boxes for modulating diffuser systems Fan coils or radiant systems for TDV Return Air Plenum No U/A diffusers in perimeter zones Raised Access Floor Return Air Grille T Heating Coil Glazing Fan Coil w/ ECM motor Flex Duct
Cooling Economizer Not accounting for thermal decay in UFAD plenum will lead to overestimate of economizer savings Savings for Oakland with AHU supply air temperature (SAT) : 65 F = 83% 60 F = 31% Must account for thermal decay in plenum affect on SAT Decay depends on airflow and plenum configuration Economizer operation also limited in some radiant applications High-capacity panel system +DOAS dehumidification may limit economizer hours in off-design conditions Hours 300 250 200 150 100 50 0 33 37 39 41 43 45 47 San Francisco Outdoor Temperature Distribution (Dry Bulb temperatures betweeen 8am and 8pm) 49 51 53 55 100% Economizer Load Reduction? 2217 Hours 54 Hours 57 59 61 63 65 67 69 71 73 75 Outdoor Dry Bulb Temperature [F]?? 77 79 81 83 85 87 89 91 93 95
Heating TDV Separate heating system typically required Baseboards, radiant, or fan coils in perimeter Fan coils ok for interior, but switch to mixed Becomes well-mixed space in heating operation UFAD Need heating options for heating-only fan coils or baseboards Becomes well-mixed space in heating operation
Load-Split Plume Factors Proliferation In addition to Cooling, heating, and deadband Case UFAD Core zone, high-throw vs. low-throw diffuser % load to Space UFAD Perimeter, blinds open, high-throw vs. low-throw diffuser UFAD Perimeter, blinds closed UFAD Atrium (two-or-more-story, fully glazed atrium) UFAD with Radiant chilled ceiling (high- T panels vs. low- T overhead slab) TDV Core zone TDV Perimeter (low loads or loads reduced by radiant cooling, or similar) TDV Atrium (two-story fully glazed atrium with chilled slab) 10 9 3.0 2.7 10 9 Peak solar load Height [ft] 8 7 6 5 4 2.4 2.1 1.8 1.5 1.2 Height [m] Height [ft] 8 7 6 5 4 Blinds open, 8 Linear bar grilles, vanes at 90 1.6 cfm/sf Blinds closed, 8 Linear bar grilles, vanes at 90 1.0 cfm/sf 3 0.9 3 2 0.6 2 1 0.3 1 0 0.0 68 70 72 74 76 78 80 Room Temperature [ F] INT_8-2 INT_8-3 INT_8-4 INT_8-5 INT_8-6 INT_8-7 0 68 70 72 74 76 78 80 82 84 Room Temperature [ F]
Typical Distribution of Room Cooling Load Heat transfer into SA plenum: UFAD with hung ceiling RA plenum Heat gain into space 100% (internal and perimeter loads) Through slab 24% Through raised floor (mostly radiation from ceiling) 11% Airside Extraction 65% Total into supply plenum ~35% (+/- 5% per airflow rate) Removed from airside (still seen as coil load to maintain SAT at diffuser)
Potential for thermal decay in UFAD plenum Initial thermal conditions in UFAD plenum (before implementing spread diffusers on stub ducts) --- Flack+Kurtz CFD modeling
Stacked thermal zones simulation approach A commonly used and often recommended approach to modeling stratified environments with geometrically based thermal model + linked inter-zonal bulk airflow model Atria, UFAD, TDV, radiant cooling (radiant designs where downsized airside system leads to thermal stratification)
Stacked thermal zones simulation approach Separate RA plenum (if included) and SA plenum zone for UFAD is straightforward Zones linked only by holes or air barriers can be used to represent occupied and stratified sub-zones Loads with appropriate radiant and convective splits can be places in relevant sub-zones Added sub-division of stratified zone improves representation of thermal stratification, but exacerbates fundamental limitation
Stacked thermal zones simulation approach 76 76 74 74 Temperature ( F) 72 70 68 Temperature ( F) 72 70 68 Stacked zones do permit approximation of stratified environments using linked bulk airflow models, however 66 66 64 64 62 62 60 00:00 06:00 60 07 08 09 10 Air temperature: 0 Return Air Plenum (ufad - energy pw 3 strat z + occ bot zx - with stub duct.aps) Air temperature: 3 lower Stratified zone (ufad - energy pw 3 strat z + occ bot zx - with stub duct.aps) Air temperature: 6 UFAD Plenum (ufad - energy pw 3 strat z + occ bot zx - with stub duct.aps) Air temperature: 0 Return Air Plenum (ufad - energy pw 3 strat z + occ bot z - with stub duct.aps) Air temperature: 3 lower Stratified zone (ufad - energy pw 3 strat z + occ bot z - with stub duct.aps) Air temperature: 6 UFAD Plenum (ufad - energy pw 3 strat z + occ bot z - with stub duct.aps) Air temperature: 7 Stub Duct to UFAD Pln (ufad - energy pw 3 strat z + occ bot z - with stub duct.aps)
Direct-bean solar and LW IR reaching floor Simple shoe-box model used to test effect of stacked zones in geometrically based simulation tools (such as EnergyPlus, iesve, etc.) While there is full accounting for solar radiation and radiant heat transfer between surfaces in a single zone, stacked zones present view-factor issues
Limitations of stacked thermal zones approach Floor Surface Temperature Floor Surface Temperature and Incident and Incident Solar Solar Flux FluxReaching the Floor relative to single zone model 2 zones, 4 spaces Solar Flux temp Temperature (F) solar flux (Btu/hr-ft^2) 1 zone, 4 spaces 2 zones, 2 spaces 1 zone, 2 spaces 1 zone, 1 space 0% 20% 40% 60% 80% 100% 120%
Possible simulation tool solutions Appropriate dynamic simulation of stratified environments, or those with significant buoyancy-driven thermal convection, where radiant heat transfer is also significant, may be accomplished by: Adding capability for appropriate accounting of solar gains and radiant exchange between surfaces across inter-zonal adjacencies so that inter-zonal airflow models can be employed (i.e., across sub zones) OR Including appropriate multi-node airflow models applicable within a single zone (wherein radiant exchange is properly accounted for by the geometrically based thermal model of that zone) Must either be adaptable to or adjustable for various configurations, or provided in versions for each (e.g., for radiant cooling, DV, UFAD, atria, etc. and various combinations thereof). EnergyPlus UFAD and DV airflow models point to potential paths, but there is much work yet to be done here.