Modeling methods for building-integrated and mixed-mode ...
Transcript of Modeling methods for building-integrated and mixed-mode ...
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Timothy Moore ApacheHVAC Product Manager
Building Energy
Simulation Forum Portland, OR
October 2013
Modeling methods for building-integrated and mixed-mode HVAC systems
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Building-integrated and mixed-mode HVAC topics
• Natural ventilation for fresh air and cooling in mixed-mode systems • Thermal displacement and stack-effect ventilation • Vented double-skin facades • Hydronic radiant cooling slabs • Ground-coupled thermal labyrinths • Underfloor Air Distribution (UFAD) • Thermally stratified high-solar-gain spaces • Passive downdraft cool towers
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
General approach to illustrate methods for presentation
• Examples given from mix of fictitious and actual building models.
• Single geometry model or subset it used to perform all analyses within the IES Virtual Environment for each particular topic or project.
• Thermal, solar pre-sim data, bulk-airflow, HVAC, controls at each time step
• CFD using boundary conditions from single time step of thermal model
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Bulk-airflow modeling
• Cross-ventilation
• Single-sided ventilation
• Stack-effect ventilation
• Controlled openings
• Mixed-mode operation
• Interaction with mechanical HVAC
• Displacement ventilation
• Vented double-skin facades
• Solar stack pre-heat of intake air
• Buoyancy-driven earth tubes
• Passive downdraft cool towers
Augment with CFD as needed (but no more)
• Initial boundary conditions for CFD module
Modeling natural ventilation and mixed-mode operation
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
• Opening types assigned to fenestration — 1st-floor awning windows + atrium
Natural Ventilation: Opening selection and assignment
Open-plan offices
Atrium stairwell open to 1st floor
WC
First floor plan
Upper floors have laboratory spaces that cannot have operable windows, and use active chilled beams to address high loads
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Natural ventilation: visualization of wind-driven potential
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Natural ventilation: visualization of wind-driven potential
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Natural ventilation: visualization of wind-driven potential
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
• Operable opening types are selected or defined by user • Exposure • Operable area • Aerodynamics of opening type • Crack flow (when closed) • Control of openings
• schedules, sensors, and formulae
Natural ventilation: opening selection and assignment
Window/door: side hung Window: Center-hung Window: Top-hung Window: Bottom-hung Window: Parallel Window: Sash Louvre Grille Duct (smooth) Acoustic Duct
Special opening types needed for intake grills, diffusers, and openings between segments in ducted flow, double-skin facades, earth tubes, etc.
• Algorithms differ from those for punched openings between larger zones.
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
• Wind pressure coefficients
Natural ventilation: pre-defined exposure types
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
• Opening types assigned to fenestration — 1st-floor awning windows + atrium
Natural ventilation: opening selection and assignment
Open-plan offices
Atrium stairwell
WC
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
• Formula profile for control of window opening • Describe BAS control sequence • Mimic occupant behavior for manually operated windows
Open if and to the extent…
gt(ta,72,4) & gt(to,56,8) & (to<(ta-2))
Natural ventilation: control of opening operation
Open if room air temperature is greater than 72°F. Ramp opening from closed to fully open over 4°F control band (from 70—74°F) .
AND
Open if outside air temperature is greater than 56°F. Ramp opening from closed to fully open over 8°F control band (from 52—60°F)
AND
Open if outside air temperature is at least 2°F below room air temperature (BAS version)
Occupant version: Open if outside air is not more than 3°F warmer than room air (subject to fuzzy > ramp)
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Natural ventilation: mixed-mode HVAC system operation
Enabling control
• Mixed-mode controls within a prototype VAV system
• Set up controls to mirror BAS version of operable opening formulae
Differential temperature sensor
Atrium
CO2 and High-temperature overrides re-engage VAV flow
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
VAV Controls — mixed-mode operation
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
• On cooler days, natural ventilation maintains indoor temperatures quite well.
• As outdoor temperature exceeds indoor cooling setpoint, mechanical HVAC takes over.
Natural ventilation vs. mechanical HVAC
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
• Stack flow up through and out of the atrium under ideal conditions is significant.
• Ideal conditions, however, are not consistently available.
Natural ventilation vs. mechanical HVAC
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
• Flow rate and air temperature entering and leaving the cooling coil indicates:
• Fair number of hours with reduced or eliminated HVAC operation
• Numerous airside economizer hours
Natural ventilation vs. mechanical HVAC
Entering cooling coil Leaving cooling coil
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
• Nat vent over initial shoulder-season test period for AHU serving first-floor & atrium
• reduced chiller energy ~ 13 %
• reduced fan energy ~ 15%
• Mixed-mode operation in this test describes means of providing operable windows for users while moderately reducing cooling system energy.
Potential improvements
• More complete evaluation
• Refined control strategies
• Climates more conducive than Atlanta, GA !
Natural ventilation vs. mechanical HVAC
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
…
Vented double-skin façade & mixed HVAC systems
• Bulk-airflow network used for vented façade. • Coupled to HVAC only via heat transfer
• DV hospital patient rooms on HVAC network
• Excess air from corridor: • Flows into patient room, then WC • Extracted via WC exhaust.
Patient rooms – VAV thermal displacement (DV) • Air introduced in Occupied zone • Air not transferred to WC flows to Stratified zone
Patient water closets • Transfer and exhaust (no direct supply air)
Corridor – VAV • Fully mixed, positively pressurized
Double-skin façade – vented by thermal stack effect, but no air exchange
Bottom of stratified zone
Adiabatic isolation shell
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
HVAC system network for airflows other than vented facade
Occupied zone
Stratified zone
RA
WC
Corridors
• Fully mixed corridors, stratified patient rooms, transfer air, and exhausted WC.
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Vented double-skin façade
• All surfaces should have thermal properties (absorption, radiation, etc.)
• All opening should have aerodynamic properties (drag, turbulence, etc.)
• Façade openings — controllable “doors”
• Catwalk horizontal shading devices — opaque surfaces with “doors”
• Solar control and thermal properties of interstitial louver as “window” + louver
• Combined solar, thermal, & bulk-airflow simulation of façade interacts with HVAC network via inner façade (wall and fixed solar-control-low-e glass).
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Vented double-skin façade – exploring results
• Solar gain for outer vs. inner façade cavities • Temperatures resulting from gain and convective heat transfer to
each subsequent cavity with vertical airflow
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Vented double-skin façade – exploring results
• Solar gain vs. external gain via air from external and internal vent openings
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Vented double-skin façade – exploring results
• Flow rates driven by solar gain and sequential façade cavity temperatures • Peak buoyancy-driven flow at peak solar gain (below left)
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Vented double-skin façade – exploring results
• Outdoor temperature • Inside glass surface temperatures
• Facing into occupied zone • Facing into façade cavity
• Room temperature
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
• Hydronic radiant cooling slabs can be accurately modeled as a thermal zone, much like a concrete UFAD plenum with negligible air volume, as heat transfer through slab is the main constraint.
• Tube spacing and depth … • THERM or similar 2D heat transfer
model provides combined U-value for all paths between water and slab surface...
Radiant cooling — hydronic thermo-active slabs
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
• U-value from THERM… is converted to adjusted conductivity for slab material (specific to the tubing type, depth, and spacing).
Radiant cooling slabs
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
• Adjusted conductivity is then applied directly to the slab construction. • Values for slab top and bottom will differ when tubes are above or
below the slab center and when surface boundary conditions differ.
Radiant cooling slabs
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
• One surface per zone should be tagged for a surface temperature sensor.
Radiant slab: surface temperature sensors
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Radiant cooling: waterside — plant loop
Consider…
• Cooling plant, such as water-source heat exchanger, matched to warmer SWT used for radiant cooling slab.
• Maximize waterside economizer operation via SWT reset and coupling HX to secondary loop return.
• Cascading loop temp control (not shown)
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Radiant cooling: waterside — zone hydronic loops
• In ApacheHVAC the “chilled ceiling” panel dialog doubles as zone loop.
• So long as ample capacity at a very small reference temperature difference is provided in this dialog, the “chilled ceiling” model stays out of the way and allows the hydronic slab model to be constrained only by water flow rate, temperature, and heat transfer through the slab.
The “number of units” can also be multiplied in the zone-level controller. VE users, see Appendix F of the ApacheHVAC for details. EnergyPlus offers a dedicated hydronic slab model, but has limitations related to application and control in combination with airside systems.
• The concrete slab properties plus loop temperature and flow control are
the critical elements of the model…
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Radiant cooling: airside and assignment of hydronic loops
• Add chilled slab to pre-defined DOAS + heated & cooled panel system
• Radiant panels or heated/cooled slabs can serve a space also conditioned by any airside system.
Radiant slab zone on each multiplex layer (one for each occupied zone)
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Radiant cooling: controls
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
• Zone loop temperature • Track secondary CHWL supply temperature, including reset. • Use temperature control to represent zone-level mixing value.
• Zone loop flow rate, along with temperature, is the critical determination of capacity at the core of the slab. • Set this value with some care, and beware that autosizing this can be
sketchy, as the thermal capacitance of the slab plays a significant role.
Radiant cooling: controls
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
• Time switch controls and midbands for sensor-based on/off control, proportional flow control, and temperature resets can use formula profiles for timing.
Radiant cooling: controls
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
• Proportional flow control can use separate sensor—e.g., slab surface or in occupied space above/below the slab—sensed variable, radiant fraction, etc.
• Modulating flow rate in the slab according to surface temperature has been shown to work particularly well in spaces with high solar gain, leaving the room temperature sensor to determine when the cooling loop should be off.
• Logical AND/OR connections in loop controls can reference any other controller.
Radiant cooling: controls
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
• Radiant slabs results include surface temperatures, influence on MRT, thermal comfort analyses, boundary conditions for CFD,…
Slab core and surface temperatures
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
• Thermal comfort analysis — Predicted mean vote (PMV)
Radiant cooling: thermal comfort
Room dry resultant temperature – operative temperature with still air (°F) Predicted mean vote (PMV) Slab surface temperature – chilled slab (°F) Slab core temperature – chilled slab (°F)
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
• Thermal comfort analysis — Percent People Dissatisfied (PPD)
Radiant cooling: thermal comfort
PPD (%) 10 8 6 4 >2
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
• Dry resultant space temperature as simple basis for matching performance.
Radiant cooling: thermal performance
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
• Comparing plant equipment and energy performance • Important to account for difference in pre-cooling potential
Radiant cooling: system-level results
6.41
4.51
3.64
1.930.26
3.85
4.13
4.19
0.64
0.36
0.36
0.49
0.21
VAV Baseline
VAV with WSFC
VAV+WSFC-precool
Radiant+DOAS
Cooling Season HVAC System Energy
Chillers (VAV only), cooling towers, and chilled water pumps (MWh)
Hydronic system pumps and evaporative cooling spray pump (MWh)
Fans (including cooling tower fan for waterside free cooling) (MWh)
Boilers, natual gas (MWh)
Estimated savings = 62 to 71%
May – September Denver, Colorado (TMY climate data)
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
• Labyrinth geometry should be explicitly modeled. • Segments provide progressive delta-T. • Soil (12—36”) and then monthly ground temperatures according to depth.
• Convective heat transfer coefficients are set as appropriate if fan-forced flow.
• Bulk-airflow model is needed if buoyancy-driven flow, but only HVAC network and thermal model otherwise needed for dynamic simulations.
• CFD takes boundary conditions from initial dynamic simulation and provides feedback for adjusting heat transfer coefficients until agreement is strong.
• Labyrinth is then ready for use in thermal comfort and energy modeling.
Ground-coupled thermal labyrinth
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
• Labyrinth model provides both tempering and capacitance
Temperatures at segments or zones within the labyrinth • OA 80.0 °F • Zone 9 71.7 °F exhibits temporal shift relative to OAT profile. • Zone 18 66.2 °F shifts further relative to OAT profile. • Zone 37 62.0 °F profile is shifted ~11 hours relative to OAT. • Outlet 63.5 °F slight heat gain in riser to outlet, but still 16.5°F below OAT.
Outlet temperature profile at is nearly flat and is shifted ~12 hours relative to OAT.
Ground-coupled thermal labyrinth
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
• Labyrinth zones can be added added to inlet of any HVAC system network.
• Bulk-airflow model is needed only if flow is driven by wind or buoyancy, as fan-forced flow will typically overwhelm natural ventilation.
Ground-coupled thermal labyrinth
Labyrinth bypass damper is modulated according to RA and zone temperature sensor-based “votes” for heating or cooling.
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
• Inlet temperature 78 °F at design condition • Outlet temperature 60–68 °F, depending on conditions for preceding 12 hours
Ground-coupled thermal labyrinth
Inlet
Labyrinth segments or thermal zones plotted
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
• Thermal labyrinth for regional hospital • Bi-directional flow (day vs. night back-flush)
• Alternate seasonal configurations for inlet, bypass, flow direction, etc.
Ground-coupled thermal labyrinth
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
UFAD plenum model — Heat transfer in UFAD systems
Total system heat gain
100%
Through ceiling
Through floor
Through slab
65°F
Ceiling-floor radiation
To
From To
Room air extraction
60-70% (RCLR)
Heat gain into supply plenum 35-45%
through floor deck and raised floor
Heat loss from return plenum (gain into supply plenum above ) 10-15%
Ceiling-slab radiation
— UC Berkeley Center for the Build Environment (CBE) 2007
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
UFAD plenum and stratified zones
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
UFAD airside network
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
UFAD temperature and RH control
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Afternoon cooling peak
• Peak OA temp: 95 °F
• SAT leaving AHU: 60 °F
• UFAD SA temp: 66 °F
• Occ. Space temp: 76 °F
• Stratified zone: 79.2 °F
• RA plenum: 77.5 °F
• Weekend run with raised temp setpoint as “hot soak” before weekday cooling operation.
UFAD results for peak cooling days
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Primary ventilation air intake duct to underfloor (UFAD) plenum
User controlled operable vent
Primary air outlet to stack vent in building core
Secondary air outlet Cascading airflow
• Ventilation driven by pressure differentials
• Wind-driven
• Stack-effect
• +/- HVAC pressure
• BAS control of facade ventilation openings
• Mixed-mode operation
• MacroFlo runs at every simulation time step, as does ApacheHVAC
• Thermal stratification
• UFAD and thermal displacement ventilation
Façade-integrated passive conditioned nat-vent UFAD system
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
MacroFlo results can inform design and control of facade elements
Façade-integrated passive conditioned nat-vent UFAD system
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Passive downdraft cool towers
• ApacheHVAC for cooling coils or evaporative cooling and heating coils
• Fans are optional for assist, but not required
• Example model is fully buoyancy driven (no fans)
• Buoyancy-driven air movement through building modeled via MacroFlo
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Passive downdraft cool towers
• Two downdraft towers: both have controllable inlets; one has wind baffles. • UFAD supply air plenums
• 1st floor provides example of controlled vent/cooling diffusers; heat by room units. • 2nd floor uses controlled inlets from towers to plenum; heat by coils in plenum.
• Occupied & stratified zones with typical internal gains • RA plenums discharge to atrium • Outlets placed high on atrium façades controlled to open only on downwind side
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Complete network with all zones and nodes included.
• Facilitated adding damper sets and heating coils where needed.
Passive downdraft cool towers
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Minimal network — reduced to just the necessary zones and nodes.
Passive downdraft cool towers
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Confirming downdraft flow between cells in cool towers
• Passive downdraft outflow from cooling coil cell of the tower (North and South towers shown) to the next cell or tower segment below.
Passive downdraft cool towers
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Confirming flow into the towers and the majority of the flow down from there
• Passive downdraft outflow from cooling coil cell of the tower (North and South towers shown) to the next cell or tower segment below.
Passive downdraft cool towers
964 l/s 2042 cfm
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Passive downdraft cool towers
Confirming flow at controlled floor diffusers
• Flow at two controlled discharge dampers in the UFAD supply plenum (controlled diffusers in MacroFlo are “doors” with formula profiles).
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Passive downdraft cool towers
Confirming flow through inlet dampers on 2nd-floor UFAD plenum
• Flow into the 2nd-floor UFAD plenum is controlled by dampers at the connection to the cool towers so that plenum can be heated in winter.
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Maintaining cooling setpoint (23°C +/- 1°C) in summer
• Temperature in occupied zones (green, blue, and red lines) vs. outdoor temperature (light teal – Brisbane, AU, Mar 4-6).
Passive downdraft cool towers
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Shoulder season performance of 2nd-floor occupied zone
• Occupied zone temperature (green) is maintained by reducing flow from cool tower (light blue) to vent only and engaging floor plenum heating coil
Passive downdraft cool towers
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Passive downdraft cool towers
Thermal stratification and temperatures on flow path: 1st-floor occupied zone
• Control of both coil LAT in towers and passive flow via dampers results in very tightly controlled space temperature and avoid excessive cooling.
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Passive downdraft cool towers
Thermal stratification and temperatures on flow path: 2nd-floor occupied zone
• Temperature in 2nd-floor occupied zone is slightly more variable as a result of heat transfer from RA below through floor deck into UFAD plenum.
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Thermal stratification and temperatures on flow path: atrium
• Atrium occupied zone temperature is very consistent in spite of substantial solar gain as well as associated gain (1–2°C) in atrium floor plenum.
Passive downdraft cool towers
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Bulk airflow vs. CFD for air movement, buoyancy, and comfort
CFD can be very useful where bulk airflow modeling leaves off
• Bulk-airflow model driven by temperature difference for adjacent fully mixed zones.
• No wind-driven or stack-effect flows within a zone (only at openings between).
• No thermal plumes adding to overhead “pool” of hot air.
• Use CFD to predict air movement and associated local thermal comfort conditions.
• Where temp gradients are significant, use zonal method for energy model.
• Use CFD to calibrate or tune the zonal bulk-airflow model
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Atrium analysis example: CFD model used to “tune” zonal model
Occupied zone
Airflow from DV diffuser on back wall
• Temperature at center of occupied zone was basis for comparison of airflow requirements at a given supply air temperature.
• Single volume (no zonal subdivisions)
• Initial boundary conditions for each test case taken from selected time step in zonal thermal and HVAC model
• Initial DV airflow and temperature at large side-wall diffuser set to the same value used in the zonal thermal and HVAC model
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Zonal model of atrium for bulk-airflow and HVAC network
• Subdivision of atrium into 10 zones:
• 6-in deep “pool” of cooling air (SA introduced here)
• 7.5-ft Occupied zone above the cooling air pool
• 28-ft Lower stratified zone
• 4-ft Upper stratified zone against the ceiling
• 6-in deep Façade zones for each orientation
• Matching height of occupied and lower stratified zones
• Concentrated zone of convective heating at glass surfaces
Upper stratified
zone
Lower stratified
zone
Façade zones
Floor-level “pool” of DV cooling air
Occupied zone
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Atrium HVAC system model
Thermal displacement ventilation system airside network (ApacheHVAC)
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Zone temperatures for the zonal modeling method: 10-zone model with bulk-airflow network.
Atrium zone temperatures: Zonal method (dynamic HVAC model)
Upper stratified zone Lower stratified zone Occupied zone Floor-level SA “pool” Supply air at diffuser
Copyright © 2013 Integrated Environmental Solutions Limited. All rights reserved.
Timothy Moore Senior Product Manager ApacheHVAC Integrated Environmental Solutions [email protected]
Questions?
Modeling methods for building-integrated and mixed-mode HVAC systems