Attaining Thermal Comfort in. Buildings with Predominantly. Glazed Facades. presented to: ANSYS Boston Regional Conference

Transcription

1 Attaining Thermal Comfort in Buildings with Predominantly Glazed Facades presented to: ANSYS Boston Regional Conference September 14, 2011

2 Case Study Background High floor to ceiling windows Large skylight area Automatic Shading Devices External Shades Chilled Beam HVAC design The space evaluated is the most dangerous area for thermal discomfort

3 Designing for Comfort vs. Energy Reduction Why is this good practice? Allows the space to keep an active chilled beam HVAC design rather than switching to a Variable Air Volume system Reacts to peak cooling issues with low static pressure fan coil units rather than a medium pressure VAV system So why increase energy for comfort? Ignoring comfort only increases energy in operation Operators will adjust the system until occupants are satisfied These adjustments are a big reason for buildings not living up to their modeled performance By acknowledging thermal comfort and providing an approach that is operationally realistic, the design building can achieve the modeled energy target

4 Chilled Beam Design Unit use induction from primary air supply to create flow Allows for less primary supply air while maintaining lower static pressure than a standard VAV system

5 Chilled Beam Approximation and Boundary Conditions Actual flow leaving the chilled beam is not directly measured 2D simulation used to approximate chilled beam slot leaving velocity Chilled beam slot width was varied to match value from Reference point VH1 from vendor s calculator

6 Physical Conditions Rng k-e turbulence model Ideal incompressible gas Radiation DO August 10 th, 10 AM (Fair Weather Conditions) Peak Solar Time August 10 th 10 AM (determined from zone peak in energy model) Design outdoor temperature 90 F Direct Normal Solar Irradiation 853 W/m2 Solar Flux East Wall 71 Btu/hr-sf South Wall 10 Btu/hr-sf Perimeter Skylight 100 Btu/hr-sf Core Skylight 95 Btu/hr-sf Only ~10 Btu/hr-sf on the floor This results in a relatively cool floor but hot walls and ceiling 54.7 kw of Solar load into the space

7 Boundary Conditions Core Skylight Triple Glazed ( U-value of 0.21 Btu/hr-sf ) 5.22% trans, 53.25% reflect, 41.53% abs Perimeter Glass Wall Triple Glazed ( U-value of 0.19 Btu/hr-sf ) 23 % trans, 32% reflect, 45% abs Automatic Shades ( activated by glare and light levels ) 3% trans, 72% reflect, 25 % abs Adjusted Perimeter Glass Wall to include shades Transmissivity = 23 % * 3% = 0.69% Reflectivity = 23% % + 32 % = 54.31% Absorptivity = % % = 45% 8 Occupants (450 Btu/person) 8 Computers (400 W each, recirculation inlets/outlets)

8 Computational Summary Cut Cell Mesh due to the orthogonal nature of the space 2.8 million elements Minimum Orthogonal quality 0.23 (0.975 Avg,.0527 std) Simulation progression Initialized and iterated in Steady State Continued until oscillation due to unsteady nature of thermal plumes in the space Continued in Transient for at least 1 air change and 1% energy and mass convergence

9 Chilled Beam Only Breathing Plane Temperature

10 Chilled Beam Only Breathing Plane Velocity

11 Chilled Beam Only Airflow

12 Chilled Beam Only Airflow

13 Chilled Beam Only Comfort Analysis Temperature is at a reasonable level, but is it really comfortable? Refer to ASHRAE (Thermal Comfort) to check Use Predicted Mean Vote Calculation to analyze comfort Inputs are Air Temp, Mean Radiant Temp, Relative Humidity, Air Velocity, Clothing Level, Activity Level, and External Work Output is a number between 2 and 2 to describe how people experience the environment

14 The Problem We can extract the Peak values from the CFD simulation and energy model to analyze the comfort in the space Design Level F Air Temperature 76 F Radiant Temp 95.0 % Rel Hum 50 ft/min Air Velocity 40 Clo Clothing Level 0.97 Met Activity 1 Met External Work 0 Atm Pressure 0 PMV 1.38 PPD We find that the space is actually uncomfortable due to the high radiant temperature in the space

15 Fan Assisted CB Breathing Plane Temperature

16 Fan Assisted CB Breathing Plane Velocity

17 Fan Assisted CB Airflow

18 Fan Assisted CB Airflow

19 The Design Results Extracting the peak values again, we can see comfort is vastly improved Design Level F Air Temperature 69 F Radiant Temp 95.0 % Rel Hum 50 ft/min Air Velocity 60 Clo Clothing Level 0.97 Met Activity 1 Met External Work 0 Atm Pressure 0 PMV 0.40 PPD 8.34 The space is now comfortable at peak times

20 Operation What about part load? Theoretical models are great at sizing systems, but how can they be controlled in the real world? Using EnergyPlus to analyze the cooling season we can see how the space will react With some custom code that controls the building s BMS system, GenOpt (LBNL) can optimize the operative temperature (weighted average between mean air and mean radiant temperature) to find the ratio that results in the most comfortable cooling season Recalling the PMV calculation, some inputs are easy to monitor (Air temp, Relative Humidity), some can be adjusted by the operator (Clothing level, Activity level, work level), and some will require further analysis to control (Mean radiant temperature, velocity) Using this data, we can plot the glazed façade s temperature against the mean radiant temperature In design, thermostats can measure the temperature of the glazing and report back the data to the BMS system

21 GenOpt Output

22 Operation Approximating MRT and airflow The CFD simulation results can allow us to set a simple function that will return the air velocity at the breathing plane based on the part load of the fan coil motors (can be adjusted during balancing) 40 ft/min 60 ft/min Range The mean radiant temperature will require more analysis due to the fact that it is more difficult to measure Client will not install MRT sensor, now what? Using this data, we can plot the glazed façade s temperature against the mean radiant temperature In operation, thermostats can measure the temperature of the glazed wall and report back the data to the BMS system

23 South Zone Controls South Zone MRT Graph Mean Radiant Temp y = x R 2 = South Wall Temperature

24 East Zone Controls East Zone MRT Graph Mean Radiant Temp y = x R 2 = Skylight Temperature

25 The Predicted Operational Results What do we Gain? The seasonal PMV with the modified control system is far superior to the standard setpoint control Predicted Mean Vote Over Cooling Season (South) Base Avg PMV Base Min PMV Base Max PMV Design Avg PMV Design Min PMV Design Max PMV PMV Hour

26 The Predicted Operational Results What do we Gain? The seasonal PMV with the modified control system is far superior to the standard setpoint control 1.5 Predicted Mean Vote Over Cooling Season (East) Base Avg PMV Base Min PMV Base Max PMV Design Avg PMV Design Min PMV Design Max PMV 1 PMV Hour

27 Summary Active chilled beams save energy when compared to a traditional VAV system Special controls are required for high load spaces By analyzing comfort criteria in design phase, we can establish an energy efficient response to the environment without having the operator make costly adjustments The space now not only benefits from an energy efficient design but also has a smart control system that reacts to comfort rather than simply dry bulb temperature This use of ANSYS FLUENT and energy modeling software is one method to ensure a zone will be well designed and able to maintain an appropriate comfort level for the occupants