0. Pei-Chun, Liu*, Brian Ford and David Etheridge
The MEGS Christmas Seminar, 15th December 2010
Modelling on the naturally ventilated tall office buildings of a hot and humid climate: The thermally conflated mass flow network approach
Pei-Chun, Liu*, Brian Ford and David Etheridge

1. INTRODUCTION Problems: Fully air-conditioned tall office
The sick building skin
Problems:
Fully air-conditioned tall office
buildings in a hot and humid
climate.
Challenge of close control due
to the dynamic nature of natural
ventilation .
This project was inspired by a fact that a significant amount of electricity consumption in buildings of a hot and humid climate goes toward fully air-conditioned tall office buildings for providing desired thermal comfort. The central hypothesis of this study is that, by natural ventilation strategies, building design will be able to reduce energy consumption and provide comfortable conditions in the occupied spaces of tall office buildings. However, due to the dynamic nature of natural ventilation as well as the constrain of weather conditions in the hot and humid climate, the challenges for close control of natural ventilation in tall buildings are much greater. Failure in ventilation system design may result in this “the sick building skin”.

2. NATURALLY VENITLATED TALL BUILDINGS
Liberty Tower of Meiji University/Tokyo, Japan/ 119m (23 stories)
Central core for stack effect / Wind Floor opens to 4 directions
This study start by looking at some real buildings which demonstrate the possibility of naturally ventilated tall buildings.
The first one is the liberty tower of Meiji university in Tokyo. There is one central core for stack effect and the wind floor is designed to enhance driving forces from the wind. The air enters from the envelope opening in each floor and is exhaust through openings at the top of central core. The wind floor opens to 4 directions, the driving force is expected to be stable through the year regardless of wind direction.
Source: S. Kato & T. Chikamoto (2002)

3. NATURALLY VENITLATED TALL BUILDINGS
Air intake ventilation grilles
Air exhaust ventilation grilles
(spent air extracted to sky gardens via vents located at slab level
Source: H. Jahn (2003)
NATURALLY VENITLATED TALL BUILDINGS
Deutsche Post Tower/ Bonn, Germany/ 163m (41 stories)
Atriums and skygardens as air exhaust
/ double façade admits cross ventilation
Source: R. Salib (2008)
4th skygarden level
3rd skygarden level
2nd skygarden level
1st skygarden level
Spent-air exhaust through vents at topmost level of the skygarden
Sky gardens as spent-air shaft
Double-skin façade as supply-air shaft
Deutsche Post tower in Bonn adopted both the ventilated facades and sky gardens as spent-air shaft which provide the driving force for natural ventilation. Atriums and sky gardens divide the two offset arcs of floor space on almost the entire height of the tower. The vertical ventilated façade allows the stack effect to draw heat off at high levels, hence decreasing the likelihood of overheating.
However, the DSF mechanism only comprises part of the ventilation concept. The overall ventilation strategy relies on cross-ventilation where air enters the building through the DSF, flows through the offices and into the corridors. The corridors act as exhaust air collectors, allowing the stale air to pass into the central atrium. The exhaust air is finally extracted through windows and vents located on the topmost level of the nine-storey-high sky gardens.
Source: H. Jahn (2003)

4. NATURALLY VENITLATED TALL BUILDINGS
Commerzbank/ Frankfurt, Germany/ 259m (53 stories)
limited stack effect by segmented atrium space
/ individual cross-ventilation via ventilated cavity
Natural ventilation in commerzbank occurs at multiple scales ranging from individual rooms to several building stories, all of which based on climatic conditions. The entire central atrium space acts like a chimney with air being exhausted out the top of the building by way of stack effect. The atrium is segmented by glass decks into four twelve-story space to minimize the undesired updraft of air. Fresh air enters the building through the windward garden at the bottom of every twelve-storey segment, and exhausted out of leeward gardens at the top of them. The natural ventilation system includes both cross-ventilation and a limited amount of stack effect.
Central atrium ventilation
Winter sky garden ventilation
summer sky garden ventilation

5. RESEARCH QUESTIONS
How many possibilities can the naturally ventilated tall
office buildings to be applied in a hot and humid climate?
What building configurations should be adopted for the
advanced natural ventilation strategies?
How the ventilation related parameters responds to
overall thermally comfortable conditions in the occupied
spaces? T

6. Tools for ventilation assessment
The envelope flow model: MS Excel
--Size openings at the chosen design condition
--Off-design condition
Integrated building simulation tool:
ESP-r_V9
--Thermally conflated air flow network model
--Hourly base data output for the whole year
Tools for ventilation assessment in this study include the envelope flow model, integrated building simulation and the computational fluid dynamics. Envelope flow models are the simplest tool developed from theoretical calculation and are recommended for the initial sizing of openings at the chosen design conditions. The other two tools provide more detailed information and are usually more suitable for later stages in the design.
The envelope flow model is implemented in MS Excel with an iterative procedure which is initially developed by Dr. Etheridge.
The integrated building simulation tool, ESP-r, was a research orientated freeware developed by Energy Systems Research Unit in University of Strathclyde. It can be used to obtained the hourly base data output for the whole year. Airflow network and CFD module are involved and can be conflated with the thermal model. The coarse grid CFD alone approach is also allowed but with insufficient resolution.
As for the commercial CFD program, FLUENT, it can be used to get the refined resolution of the thermal model. The simulations can be done with advanced physical models for the whole building.

7. METHOD—Envelope flow model
Envelope flow models solve the equations that govern the airflow through openings in the envelope of a building. An implicit method solves the equations by an iterative procedure.
One equation for the building envelope
One equation for each opening
Envelope flow models solve the equations that govern the airflow through openings in the envelope of a building. The equations used include one mass balance equation for the building envelope, one equation for each opening which is dependant on the pressure difference across that opening. An implicit method is used in MS Excel to solve the equations by an iterative procedure for obtaining the off-design condition of that particular opening.
One equation for each opening

8. METHOD—Air flow network modelling8
To discretize the building into zones by nodes.
Components are defined to represent leakage
paths and pressure drops associated with openings.
3) The nodes are linked together through components
to form connections which establish a flow network.
4) A mass balance is expressed for each node in
the building.
Component_window
Boundary node
Component_door
zone node
As for the integrated building simulation tool ESP-r, an air flow network module is involved in its thermal domain. The concept of air flow network within ESP-r is that a building is represented by a series of zones interconnected by flow paths. Components are defined to represent leakage paths and pressure drops associated with openings. The nodes are linked together through components to form connections which establish a flow network. A mass balance is expressed for each node in the building.
The idea for envelope flow model and air flow network module inside of ESP-r are essentially the same, but they are used for different purpose during the design stage.

9. Criteria for ventilation performance9
Desired airflow pattern :
---Q> 0 m^3/s when follows the conceptual design
Desired volume flow rates for ventilated cooling :
Heat gains are balanced by the heat removed with ventilation air
Q=H/ ρ∙Cp ∙∆T
Where H=(30W/m^2) ∙400m^2 ;ρ=1.2kg/m^3 ; Cp=1006 J/kgK ; ∆T=3.3K
---Q=3 m^3/s may suffice for cooling purpose
The criteria for evaluating the ventilation performance in this study include the desired flow pattern for preventing the unwanted warm air to be driven into the occupied space. The minimum flow rates for cooling purpose are also examined. It is assumed that the resultant flow rates are positive when follow the conceptual air flow pattern.
For cooling purpose, it is assumed that the internal heat gains are balanced by the heat removed with ventilation air. An equation above is used for calculating the minimum flow rates for cooling. In this study, when all the boundary conditions are applied, the ventilation rates of 3 m3/s, may suffice for cooling purpose.

10. Building bioclimatic charts (BBCCs)10
BBCCs:
A way for testing comfortable conditions in the occupied space.
Adaptive thermal comfort theory:
People naturally make adjustments to themselves and their surroundings to reduce discomfort.
Comfort boundaries :
Still air: 18-29˚C / 50%~80%
Airflow of 1.5m/s: 18-32˚C / 50%~90%
Followed by the modelling, the results will then be interpreted using the building bioclimatic charts. BBCCs offer a way of rapidly testing whether or not natural ventilation is likely to produce comfortable conditions in the office. The key component in the chart is that comfort boundaries would vary in the terms of building design and cooling strategies used.
The boundaries defined in this study are according to previous studies on adaptive thermal comfort for the hot and humid climates. The adaptive thermal comfort theory is that people naturally make adjustments to themselves and their surroundings to reduce discomfort.
The comfort boundary for this study is that for the still air condition, the boundary would lie between degree C with the relative humidity between 50% and 80%. As for the light breeze condition, the upper limit for temperature could be extended to 32 degree C with the RH of 90%.

11. The prototype building with advanced natural ventilation strategies11
However, modern non-domestic buildings tend to be large and deep plan with sealed facades for security and noise control. In such cases, where traditional forms of natural ventilation are unlikely to deliver sufficient ventilation performance, advanced natural ventilation strategies should be considered. That is, the revised prototype of naturally ventilated tall office building is proposed. Further tools for ventilation assessment are used for limited representation of the ANV system in the envelope flow model.
The prototype building is developed according to a current office design in Taiwan. Some modifications are made to utilize the natural ventilation strategies. The architectural feature of the base model is a combination of an external ventilated façade and a central atrium. The prototype building is in square shape and is divided into four portions with similar configuration but different orientation.
A current design of Taipei, Taiwan

12. The base cases
For each portion of the prototype building, various segmentation heights are used as study models. For each study model, the external ventilated façade and central lightwell would run through the full height of the corresponding segmentation.
Proposed naturally ventilated tall office models

13. Conceptual air flow pattern13
Central Atrium
DSF cavity
Atrium-vent
DSF-vent
Lower inlet
Top outlet
Individual office space
The initial study models compose of a ventilated double-skin façade with one outlet at the top of the DSF cavity and several vents connected to individual office space. The DSF-vents locate in the higher-end of the office wall while the atrium-vents are in the lower-end of the wall opposite to the DSF-vent side.
The conceptual air flow pattern for this advanced natural ventilation system is centre-in / edge-out ventilation strategy. The air flow throughout the building is activated by the stack effect and wind force. It is assumed that fresh air enters from the open plan, flow into the lower inlet of the atrium and feeds into each office space through individual atrium-vent. The stale air of each office space is expected to discharge into the high-end vents connected to the DSF cavity. Solar radiation will warm up the cavity and then the accumulated warm air will be exhausted through the outlet on top of the DSF cavity.

14. RESULTS & DISCUSSIONS
This slide presents the statistic analysis from the ESP-r simulation results. They demonstrate the possibility for different segmentation height to have the required flow rates for cooling. We can see the probability is higher for the cases with help of wind. It is also easier for the mid-seasons to reach the design condition than the hot summer. For the buoyancy alone cases as shown in the right hand side, the possibility for cooling would decrease with floor height due to the limited stack effect. And it is challenging for the hot summer to obtain the design performance by using the buoyancy alone ventilation strategy.
The probability for ventilated cooling:
The buoyancy-alone(R) and wind & buoyancy combined (L)ventilation strategies

15. RESULTS & DISCUSSIONS 15
By manual plotting the indoor dry bulb temperature (˚C) and relative humidity (%) into the BBCCs would provide insights for ventilated cooling assessment.
According to the simulation results so far, the initial understanding for indoor thermal comfort during the occupied hours between March and August is clarified.
The results suggest that for still air condition, thermal comfort can be achieved for most of the time in March and April and partially in May and June. As for the light breeze condition with indoor air flow speed of 1.5 m/s could enhance the thermal comfort to some extend. Further cooling effect due to wind-driven ventilation will be examined in the following study.
The worst case scenario

16. EXPECTED OUTCOMES Natural
To investigate the year round feasibility of natural
ventilation in a hot and humid climate with
reference to the proposed building configuration.
To identify the dominated parameters and its range
of influence to the resultant air flow rates and flow
pattern.
To suggest the possible control strategies in terms
of the identified driving forces.
To develop routes for predicting the performance of
advanced naturally ventilated tall office buildings.

17. Any questions/comments ?
THANK YOU FOR YOUR ATTENTION
Any questions/comments ?