E-mail: zhuyx@tsinghua.edu.cn

Building Simulation as Assistance in the Conceptual Design

Chunhai Xia, Yingxin Zhu ( ), Borong Lin

Department of Building Science, Tsinghua University, Beijing, 100084, China

Received: 6 December 2007 / Revised: 27 January 2008 / Accepted: 29 January 2008 © Tsinghua Press and Springer-Verlag 2008

Abstract In order to realize the “design by simulation” concept in the building design, the methodology of applying the building simulation in the building’s conceptual design stage is the main theme discussed in this paper. The conceptual design stage is divided into four sub-stages, and the framework of the design is built by way of the simulation in the conceptual design stage. Moreover, the energy saving potential assessment by the simulation in the preliminary conceptual design stage is also discussed in detail, including the input/output information, the calculation method and procedure, and the requirements and information from architects, etc. The natural ventilation design is used as the first trial in this study, and the difference between the detailed conceptual design and the preliminary conceptual design is also discussed, and the new simulation methodology is further described. The main objective of this paper is to help avoid an incorrect decision in the conceptual design stage, as well as to provide a better base for the energy efficient design in the next stage by means of the building simulation tool.

Keywords building simulation, building performance, conceptual design, natural ventilation

List of symbols

A relevant matrix of natural ventilation network Aout relevant matrix of natural ventilation network used

to describe air flowing from node to branch Bf basic loop matrix of the natural ventilation network Cp specific heat capacity of air (kJ/kg· )DH branches’ pressure head vector (Pa) F branches’ opening area (m2)Fmin the allowable minimum value of the unknown air

paths’ average opening area (m2)F1 known main air paths’ opening area vector (m2)F2 unknown main air paths’ opening area vector (m2)G paths’ air flow volume vector (m3/s)G absolute value of paths’ air flow volume vector (m3/s) GL independent paths’ air flow volume vector (m3/s) i branch iI unit matrix j room jm total number of air paths m1 number of unknown air paths n1 number of the occupied roomsn2 number of the auxiliary rooms

s branches’ resistance coefficient (kg/m3)T air temperature vector of the nodes in the network

( )T1 the occupied rooms’ air temperature vector ( )T2 the auxiliary rooms’ air temperature vector ( )Tbz basic air temperature vector in the rooms ( )Tout outdoor air temperature written as vector ( )Z branches’ vertical height vector (m) Φ0 coefficient matrix that reflects the influence of the

adjacent rooms’ air temperature on the current temperature

Φhavc coefficient matrix that reflects the influence of heating, ventilation, and air conditioning system on the current temperature

ρ air density vector in the paths (kg/m3)ρ0 air density under reference temperature (26 ) (kg/m3)

1 Introduction

Since the energy crisis in 1970s, the building simulation has been widely and quickly developed during previous decades. Nowadays, more and more architects, engineers,

Build Simul (2008) 1: 46–52 DOI 10.1007/s12273-008-8107-y

RESEARCH ARTICLE

Build Simul (2008) 1: 46–52 47

and researchers freely use different kinds of building simulation tools, such as EnergyPlus, DOE-2, DeST (Tsinghua University DeST development group 2006), ESP-r, TRANSYS, ECOTECT (Square One research Ltd 2005), and others. Although great development has been achieved, the current research mainly focuses more on the simulation method rather than on the design process. Most software tools are developed for the preliminary design stage and the detailed design stage, or they are used as building performance assessment tools after the design or construction is completed. In fact, the design process generally includes the conceptual design stage, the preliminary design stage and the detailed design stage. The important parameters affecting the building performance are mainly considered in the conceptual design stage, including the shape, the orientation, the window-to-wall ratio (WWR), the interior space layout, and so on. As a result, the building simulation in the conceptual design stage is crucial in order to improve the building performance, the importance of which has been previously discussed by several authors (de Wilde 2004; Hong et al. 2000; Augenbroe 2001). As a result, in order to improve the building’s performance in the design process, more research should be paid to the conceptual design stage, as well as to the framework of the “design by simulation” in this stage.

However, several difficulties exist in applying the current building simulation tools.

) Some building simulation tools claim that they could be applied in the conceptual design stage, but they only provide a little support for architects until the end of the conceptual design stage, when large changes of the design can not be accepted. According to the investigation on the energy saving components applied in 67 buildings by de Wilde (2004), although 57% of the total 303 energy saving components are selected in the conceptual design stage, the tools are not available to support the design decision until the preliminary design stage.

) In the conceptual design stage, on one hand the architects and owners need to know the building’s rough thermal performance, and some building information would still be unavailable; on the other hand the current tools are only able to carry out simulation with all of the detailed input information. Thus, the current tools are not available to fully support the conceptual design stage.

) Most building simulation tools are originally developed for heating, ventilation, and air conditioning (HVAC) engineers, so the input/output parameters and the description method of the available information are not consistent with an architect’s own method of declaring parameters or description methods.

Therefore, the inapplicable nature of the current simulation tools, used to assist in the conceptual design, results in the necessary development of new tools and the elimination of current obstacles.

The opinion of the “design by simulation” is adopted in development of DeST (Designer’s Simulation Toolkits), which proposes the theory that the simulation tool should be developed based on the design process. DeST’s simulation process is divided into several steps in order to meet the requirements of different HVAC system design stages, and the input/output parameters, as well as the simulation algorithms, are different in different steps (Tsinghua University DeST development group 2006). DeST is a good simulation tool for the HVAC designers, and the core theory of DeST can be introduced into the conceptual design stage.

Since the simulation tool is to be developed based on a design process, a new framework of applying simulation tools into conceptual design stage is first proposed. Meanwhile, several issues are discussed in the following sections, including

) the subdivision of the conceptual design stage and their characteristics,

) the architects’ requirements on the building simulation tools in each sub-stage,

) the available information for the building simulation in the different sub-stages, and

) the simulation procedure to assist in the conceptual design.

Furthermore, the natural ventilation simulation is taken as the first trial in order to explain how to apply the new framework in the practical design process.

2 Framework of design by simulation

2.1 Subdivision of conceptual design stage

In the sub-task “Design Process Analysis” of IEA Annex 30 (Warren 2002), the building’s life cycle is divided into six stages: the conceptual design, the preliminary design, the detailed design, the commissioning phase, the management of facilities, and renovations. The conceptual design is the first step undertaken, and the large-scale elements of the building are usually determined during this stage. When an architect finishes the conceptual design, the main parts of the building have been decided preliminarily. Usually they are only modified slightly in the following stages. Meanwhile, the final concept of the building is achieved step by step. Therefore, the conceptual design stage also needs to be divided into several sub-stages. Zhang and Li (2001) suggested that the conceptual design stage can be divided into four sub-stages: the sketch design, the layout

Build Simul (2008) 1: 46–52 48

design, the preliminary conceptual design, and the detailed conceptual design. In detail, the design tasks and available

information for simulation are described, as shown in Table1.

Table 1 Sub-stages of conceptual design

Sub-stage Tasks Known information for simulation

Sketch design Analyze design specification and form the basic concept of the building

None

Layout design Deal with the relationship with surrounding environment, design layout of building cluster, and design landscape

Building size, building cluster layout, as well as waterscape and greening

Preliminary conceptual design

Rough design of building’s shape, facade, interior space, structure, functional area division, etc.

Building function, rough plan drawing, rough elevation drawing

Detailed conceptual design

Detailed deign of building’s plan, façade and shape; selecting building material for structure and main facades

Detailed plan drawing, detailed elevation drawing, building structure materials and their physical properties, room function

2.2 Procedure of “design by simulation”

Based on the theory of the “design by simulation”, the work in the whole process is sorted into two parts: the architect’s design work and the tool’s simulation work. As described in Fig. 1, when an architect achieves a design scheme in each sub-stage, the building simulation tool provides a predicted result in order to assess the design. If the design scheme meets both the architect’s requirements and the owner’s requirements, the design process goes on to the next sub-stage. Otherwise, the current design is modified.

Fig. 1 Framework of “design by simulation” in the conceptual design stage

The initial concept of the building and the culture element contained in the building are mainly discussed during the sketch design stage. “Building” in this period is expressed as a very simple shape, during which it doesn’t even look like a real building. For example, the Ronchamp Chapel, designed by Corbusier looks like an ear, not a building, in the sketch design stage. On the other hand, the architect expects his/her thinking work, the initial concept of the building, not to be broken as far as possible. Therefore, a building simulation tool is unnecessary in this stage.

In the layout stage, the architect begins to consider the rough building shape, the building height, as well as the landscape environment surrounding the building. Reducing the heat island effect, improving the natural ventilation application potential, and ensuring enough duration of sunshine are the main issues that are taken into account. The single building design includes two stages: (1) the preliminary conceptual design stage, where the architect mainly considers the large-scale elements of the building, such as the building shape, the facade type (WWR), the division of interior space, the structure type, and the building function, as well as the application of passive energy saving technologies; and (2) the detailed conceptual design stage, where building elements are discussed in detail, including the determination of structure materials, the main facades, the thermal parameters of walls and windows, as well as some slight modifications of the preliminary concept. Once the preliminary conceptual design is finished, major changes of the design are not able to be accepted. If energy-waste problems exist in the preliminary conceptual design, the high-performance building components possibly have to be used to counteract this defect, which in turn increases the total cost of investment. Current building simulation tools always require all details of the building information, details which are only available until the end of the conceptual design stage. Therefore, new building simulation tools need to be developed with

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the following functions in the preliminary conceptual design stage: 1. Predicting the possibility of the architectural scheme in

realizing the energy efficiency and its cost 2. Comparing the energy conservation potential of different

building design schemes, aiding the designer in making this decision

3. Analyzing the influence of different parameters on the building performance

4. Helping to clearly make the key points for later design sub-stages, especially the detailed design of the material For function 1, 2 and 4, the energy efficient possibility

assessment method in the preliminary stage needs to be developed, which is discussed in detail in the natural ventilation design. For function 3, the sensitivity analysis of different parameters is applied, which is discussed by many authors (Lomas and Eppel 1992; Maria and Levermore 2001; Thornton et al. 1997).

Several important aspects need to be discussed in the conceptual design in order to improve the building performance, including the thermal performance, the natural ventilation, and the use of daylight. In this paper, the design procedure of natural ventilation by simulation is taken as the trial procedure in order to illustrate how to apply the “design by simulation” into the conceptual design.

3 Natural ventilation design by simulation

3.1 Design procedure

Because of its energy saving potential, most architects prefer to include natural ventilation into their building concepts. Natural ventilation possesses an important influence on the internal space design. Meanwhile, the atrium, solar chimney, ventilation shaft and other natural ventilation components also possess significant influences on the building design. As a result, the energy saving potential of natural ventilation needs to be confirmed as early as possible. Under the framework of “design by simulation”, the natural ventilation’s conceptual design is divided into three sub-stages (Fig. 2).

In the layout design stage, the building cluster’s layout and shape are designed according to the wind pressure coefficient on the facade. In the preliminary conceptual

Fig. 2 Conceptual design process of natural ventilation

design stage, different natural ventilation methods are compared, and the reference opening areas of most air paths are provided. In the detailed conceptual design stage, the natural ventilation’s energy conservation potential is predicted with all the air paths’ detailed information. The main natural ventilation scheme is decided in the preliminary conceptual design stage, which is more important than other stages. The computational fluid dynamics (CFD) simulation tools can predict the wind pressure coefficient distribution on the facade, and current natural ventilation tools can meet the simulation requirement in the detailed conceptual design stage. However, because most air paths’ opening areas are unknown, current simulation tools are unfeasible in the preliminary conceptual design stage. This paper mainly discusses the evaluation index and the simulation method in the preliminary conceptual design stage.

3.2 Simulation method

The natural ventilation of a whole building depends significantly on the opening area and the resistance coefficient of the air paths, which are usually acquired step by step. In the preliminary conceptual design stage, only some main air paths are considered. Other paths are designed in the detailed conceptual design stage or the preliminary design stage. As seen in Table 2, the ventilation network model and the building model are solved simultaneously in order to obtain the indoor temperature and the air flow volume of each path in the detailed conceptual design

Table 2 Comparison of available information for natural ventilation simulation

Preliminary conceptual stage Detailed conceptual stage

Input Allowable limit of occupied rooms’ indoor temperature, main air paths’ opening areas, and all air paths’ resistance coefficient (rough value)

Air paths’ opening areas and resistance coefficient

Output All rooms’ indoor temperature, all air paths’ air flow volume, and subsidiary air paths’ opening areas

All rooms’ indoor temperature and all air paths’ air flow volume

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stage, when all the information of air paths is obtained. If the indoor air temperature is higher than the requirement, the designer will be able to change the opening area until the entire indoor temperature is lower than that of the upper limit.

However, in the preliminary conceptual design stage, the detailed designs of the rooms, facade, and the roof are not done yet. Only the information regarding the main air paths (such as the atrium and the solar chimney) is obtainable for the natural ventilation simulation. Although other air paths’ opening areas and resistance coefficients can be assumed, and the same simulation method in the detailed conceptual stage can be adopted, several problems still can not be avoided:

The amount of assumed information proves to be too large. Different assumptions lead to different results. From the standpoint of the design process, the main work in the preliminary conceptual stage is focused on deciding the main ventilation scheme. Too much attention on the subsidiary air paths, which are to be designed in a later stage, causes a deviation from the principle of “design by simulation”.

Considering that the design work of following stage is to be on the opening area and the types of the subsidiary air paths, the allowable minimum value of the unknown air paths’ average opening area (Fmin) is used as the index to evaluate the different ventilation networks. Meanwhile, the indoor temperature of the occupied spaces is not to exceed the upper limit.

Because the number of unknown variables (including the indoor air temperature, the air flow volume in the paths, and most of the air paths’ opening areas) is larger than the number of equations, the optimization method is introduced. The optimization model is expressed as the following:

( ) ( ) ( )

( )

[ ]

T0 hvac out out 0 bz

T

1f 2 2 2

1

T1 2

Tf L

1, 1

2, 2

1

min

; (1)

; (2)

; (3)

; (4)0 29, 1, , ; (5)

0 , 1, , ; (6)

; (7)

p

i m

i m

j

j

C diag

s s sdiag diagF F F

g

T j nT j n

F

ρ − + − = −

− =

=

=

=

0

0

A G A T T I T T

B G G

+ Z DH

F = F F

G = B G

F1

1,1

1min (8).

m

ii

F

m=

The operational symbol diag converts a vector into a diagonal matrix and each diagonal value of the matrix is in correspondence with each value of the vector.

Equation (1) is the heat balance equation based on the state space method. Equation (2) denotes the pressure loss of each air flow loop in the network. Equation (8) is the objective of the optimization model, and m1 is the number of unknown air paths. T1 is the temperature of the occupied room, which can not exceed 29 under the natural ventilation. T2 is the temperature of the auxiliary room, which doesn’t need to be controlled. For the optimization issue in the above equations, with the linear objective equation and the nonlinear constraint equations, the Sequential Quadratic Programming (SQP) algorithm is applied (Zhang 2005).

With the solution of Fmin, two kinds of information are able to be provided to the architects:

the comparison result of the different ventilation schemes; the reference opening area of each unknown air path.

Because the objectives of the different natural ventilation schemes are the same (which is to control the occupied rooms’ air temperature to under 29 ), the smaller Fmin

denotes a lower cost and better ventilation scheme. Meanwhile, the Fmin corresponds to a group of values of the unknown air paths’ opening areas, which is the reference for the future design of the air paths.

3.3 Case Study

For the buildings in Fig. 3, two atriums are designed in (a), and one atrium is designed in (b). The two schemes have a large difference in their room layout, and the simulation is carried out in order to further aid architects in making the decision as to whether two atriums are necessary.

In Fig. 3 (a), the temperature of rooms N5 to N10 are not to exceed 29 and the opening areas and the resistance coefficients of B4, B5, B7, and B9 are known. In Fig. 3 (b), only one atrium exists.

Applying the optimization method to solve this problem, the optimal opening area of each air path, the indoor temperature, and the indoor air change rate (ACR) are found, as shown in Tables 3, 4, and 5. The air flow direction of each path is displayed, as shown in Fig. 3. The temperature of all the occupied rooms meets the design standard. In the two-atrium scheme (a), the air flows out of building across the paths on the atrium ceilings (B10 and B15). While in the one-atrium scheme (b), the air flows out of the building across the paths on the atrium’s ceiling as well as the paths along the external windows of the second floor (B6 and B13). Furthermore, the volume of air flowing across B10 in scheme (b) is 1167 m3/h, while the volume of air flowing across B6 and B13 in scheme (b) is 5035 m3/h. In

Build Simul (2008) 1: 46–52 51

other words, the contribution of the solar chimney to the natural ventilation of the third floor in scheme (b) is less than that of the two atriums in scheme (a).

The Fmin of scheme (a) and scheme (b) are 2.3m2 and 3.1m2, respectively. Therefore, in order to maintain the

same indoor thermal environment, a smaller opening area is needed in scheme (a). This shows how architects can decide in the future to select the schemes based on the smaller opening area.

Fig. 3 Schemes of natural ventilation

Table 3 Opening areas of air paths (unit: m2)

Scheme B1 B2 B3 B4 B5 B6 B7 B8

(a) 0.1 1.6 1.6 40.0 40.0 3.3 40.0 4.4

(b) 3.7 3.7 2.8 40.0 5.8 1.4

Scheme B9 B10 B11 B12 B13 B14 B15 Fmin

(a) 40.0 3.2 1.1 0.1 4.4 3.2 2.6 2.3

(b) 40.0 0.5 3.3 4.1 1.7 4.4 3.1

Table 4 Indoor temperature (unit: )

Scheme N1 N2 N3 N4 N5 N6 N7 N8 N9 N10

(a) 30.5 32.1 24.7 27.6 25.7 29.0 29.0 24.4 29.0 22.5

(b) 25.8 31.7 27.8 29.0 25.0 21.3 29.0 29.0

Table 5 ACR of room (unit: h–1)

Scheme N1 N2 N3 N4 N5 N6 N7 N8 N9 N10

(a) 31.8 31.8 39.1 21.1 20.5 4.2 8.1 0.9 11.3 19.4

(b) 43.1 8.1 17.3 10.8 43.1 16.1 16.6 5.2

4 Conclusion

A successful application of the building simulation in the conceptual design stage is crucial in order to improve the building performance, and this contributes to the reduction of energy consumption from the beginning. The conceptual design stage is divided into four sub-stages for the procedure of the “design by simulation”. Meanwhile, the simulation stage is defined as consistent with the design stage, including

the microclimate simulation, the rough thermal simulation and the detailed thermal simulation. Furthermore, the framework of “design by simulation” is built, and several important issues are presented, which need further research.

In order to simulate the natural ventilation in the preliminary conceptual design stage, the difference in the available information between the preliminary conceptual design stage and the detailed conceptual design stage is discussed, and an optimization model is developed. The

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minimum value of the unknown air paths’ average opening area is applied to evaluate the different ventilation networks. The results also can aid the detailed design of the air paths in future stages.

It is just a start for the research on applying building simulation tools in the conceptual design stage, and some research obstacles are discussed briefly. Undoubtedly, a great deal of work remains to be done in further research.

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