1. Introduction

Energy efficiency of buildings is important from both an economic and environmental point of view.  The technology exists for sustaintial energy savings in the building sector but the potential for such savings has still to be fully exploited (Rosenfeld and Hafemeister 1988).  To achieve efficiency goals, building designers require effective design tools for analysing and understanding the complex behaviour of building energy use.  In the past decade, computer simulation and modelling has been used for providing an accurate and detailed appraisal of building energy design.  It is believed that building energy simulation is a powerful, analytical method for building energy research and evaluation of architectural design (Hensen, et al. 1993; Seth 1989; Newton, James and Bartholomew 1988; Clarke 1985; Nall 1985). However, the application of simulation in building design is problematic because the simulation tools are complicated and many building designers are not familiar with their properties and limitations.  In real-life, the nature of the building design process and the shortcomings of current simulation tools have made it difficult for the architect to use such tools efficiently (Holm 1993; Mathews and Richard 1993).  There is a need to develop a better understanding of energy simulation and to put into practice the techniques for achieving energy efficient buildings.  It is hoped that the information in this paper can help increase an understanding of energy simulation techniques and encourage building designers to have the confidence to use the simulation based design tools.   Content

 

2. Basic Concept of Building Energy Simulation

The evolution of building energy simulation over the past few decades has encouraged architects to apply this technology to building design (Clarke and Maver 1991).  From a traditional handbook approach to computer simulation method, building designers are trying to extend the limits of architectural design. 2.1 The Basic Theory The theory of building energy simulation is based upon the traditional methods of load and energy calculations in heating, ventilating and air-conditioning (HVAC) design (ASHRAE, 1993)(Amercian Society of Heating, Ventilating and Air-Conditioning Engineers, http://www.ashrae.org/ ).  The purpose of load calculation is to determine the peak design thermal loads of the HVAC systems in order to size and design the equipment and plant; the purpose of energy calculation is to estimate the energy requirements of the building to meet the required loads throughout the year. Building energy simulation is performed to analyse the energy performance of a building dynamically and to understand the relationship between the design parameters and energy use characteristics of the building.  The effects of all kinds of changes can be simulated and observed in a fraction of time and for a fraction of the cost it would take to study these alternatives in real life.  Detailed information about building energy consumption, indoor environmental conditions and equipment and plant performance can be obtained for design evaluation and system selection.  2.2 Major Elements Fig. 1 shows the major elements of building energy simulation (Clarke and Irving 1988).  Within the simulation system, four basic models are used to represent the major components that affect the building's energy flow: · Building model · HVAC system model · HVAC plant model · Control system model Fig. 1.  Major elements of building energy simulation  The inputs to the simulation system are the building descriptions and design parameters; the boundary condition is the climatic context of the location.  The outputs are the data for building energy consumption, peak demand and indoor environmental conditions.  Usually, the modelling target is to provide comfortable indoor conditions while maintaining acceptable levels of fuel consumption; to optimise the system performance; or to compare different design options based on their life cycle costs.  An additional module is required for the economic analysis. To implement the simulation system on computer programs, different modelling approaches and solution techniques can be used (Hensen 1995; Clarke 1985).  Although the method and level of detail may vary in different programs, the general approach to the energy simulation task is similar.  The way energy modelling is expressed and carried out will determine the accuracy and properties of the simulation tool.   Back Content

 

3. Properties of Simulation Design Tools

The variety and diversity of simulation tools give rise to a practical need to distinguish the best applications and limitations of the existing programs. 3.1 Existing Tools in the Market Information about the energy design tools in the market is diverse and changing rapidly.  Some related professional bodies have prepared directories of energy programs, like Evans (1992), Williams (1992), ASHRAE (1991), Association of Energy Engineers (1991) and Weiss and Brown (1989).  Reviews of some energy software can be found in Scientific Computing (1996 & 1995), Anzoategui (1994), Amistadi (1993), State Projects (1993), IBPSA (1992) (International Building Performance Simulation Association, http://next1.mae.okstate.edu:80/ibpsa/ ) , Gale C. Corson Engineering (1990) and Merriam (1989). Many simulation programs for building energy analysis are now available; they range from the simple and approximate to the detailed and sophisticated.  It is difficult to categorise simulation programs since many of them have multiple features and the programs themselves are continuously evolving.  Contemporary programs such as BESA, BLAST, BUNYIP, DOE-2, ESP-II, ESP-r, HVACSIM+ and TRNSYS are able to model a building in detail, but they also require great effort and heavy input from the user.  Programs from commercial bodies such as Carrier HAP and TRACE 600 are more popular in design offices because they are easier to use and have better user interface and default features; but their independence sometimes may be questioned. The selection of a simulation program for a given task depends on the project requirements, time and cost of the analysis, experience of the user and availability of suitable simulation tools and data (ASHRAE, 1995).  Some selection guidelines are provided in ASHRAE (1995 & 1993), State Projects (1993) and Howard, Wager and Winterkorn (1994).  The most important consideration is the capability of the program to deal with the application required. 3.2 Range of Applications In practice, simulation tools can be utilised for the following functions: · To evaluate design options and investigate design optimisation. · To facilitate the investigation of new ideas (cognitive). · To check compliance with building energy codes. · To perform economic analysis for determining the impact of energy conservation measures.  Three types of applications most commonly found in architectural design are: (a) Building energy simulation (whole building) This is the largest and most important application.  Full hourly analysis (like BLAST, DOE-2 and ESP-r) or reduced hourly analysis (like Carrier HAP and TRACE 600) can be performed. (b) Lighting and daylighting simulation It focuses on the analysis of the daylighting aspects and their effects on energy and visual performance (Baker, Fanchiotti and Steemers 1993).  Examples of these programs are ADELINE, RADIANCE and SUPERLITE. (c) Solar system simulation Passive and active solar systems can be modelled using the programs that are designed for solar components and equipment, such as TRNSYS. Apart from energy analysis, many simulation programs also allow for standard design load calculations to determine the design capacities of equipment and plant.  This is a feature often considered useful by casual users.  Table 1 shows a list of common simulation programs for building energy design. Application Program Program Developer and URL Address Building energy simulation BLAST University of Illinois at Urbana-Champaign http://www.bso.uiuc.edu DOE-2 Lawrence Berkeley National Laboratory http://eande.lbl.gov/BTP/DOE2/DOE2.html ESP-r Energy Simulation Research Unit, University of Strathclyde http://www.strath.ac.uk/Departments/ESRU/Ad_Env_Sim.html Carrier HAP Carrier Corporation http://www.carrier.com/WP/nao_rc1/index.html TRACE 600 Trane Company http://www.trane.com/cds/trace600.html Lighting  and daylighting simulation ADELINE Lawrence Berkeley National Laboratory http://radsite.lbl.gov/adeline/home.html RADIANCE Lawrence Berkeley National Laboratory http://radsite.lbl.gov/radiance/home.html SUPERLITE Lawrence Berkeley National Laboratory http://eande.lbl.gov/BTP/WDG/SUPERLITE/Superlite2.html Solar  system  simulation TRNSYS Solar Energy Laboratory, University of Wisconsin-Madison http://www.engr.wisc.edu/centers/sel/trnsys/ Table 1.  Common simulation programs for building energy design  3.3 Limitations Using simulation models for building design has its limitations as existing models fail to tackle issue regarding data preparation in the face of uncertainty in the design environment. Mathews and Richards (1993) and Seth (1989) have commented that the programs available today are far from ideal.  Major shortcomings of current simulation tools include: · The program input is voluminous and scientifically detailed.  Data, which is usually unavailable during early design stages, has to be assumed when doing the analysis. · Program output consists of bulky computer printouts that confuse the user.  Understanding and interpretation of the simulation results is difficult. · Many detailed design tools are research orientated.  Learning to use them is difficult and a long time is required to become competent. · The user interface of the tools is often neglected.  Architects who are trained to express themselves graphically become frustrated by the strict data structure and requirements. · The software does not allow users the flexibility to do any programming easily to meet particular needs. · Program validation and accreditation are lacking.  People are confused and uncertain about which programs will give better simulation results.  These shortcomings must be overcome (in the long run) by program improvement driven by the market and the users.  To work effectively with the simulation tools, practitioners should learn to work within the limitations and understand the role of simulation in the building design process.     Back Content

 

4. Building Design and Simulation Environment

Energy efficient buildings are the result not of only a responsible attitude towards energy but also of how successful the designer has been in applying the technology and the energy analysis tools during the design process. 4.1 Building Design and Development Process Papamichael (1991) pointed out that design is feeling and thinking while acting.  The building design process is characterised by one 'E' and three 'M': Evolutionary, Multi-criteria, Multi-discipline and Multi-solution.  Inappropriate modelling of the design process may result in ineffective design tools and solutions.  In general, there are two main categories of design: · Architectural design that works on graphical images to determine the architectural form, shape, facade, etc. · Engineering design that works on system schematics to perform thermal and HVAC calculations. Fig. 2 shows the relationship between design and simulation.  The left hand side of the figure is the design context and the right hand side is the simulation context.  There are interactions between the two that will facilitate transfer of data and knowledge for the purpose of design and performance evaluation. Fig. 2  Relationship between design and simulation Architects usually develop their designs in drawing-based, graphical forms; prototypes are used to investigate the design concepts.  What is important here is that building design is a creative process based on iteration: it consists of a continuous back-and-forth process as the designer selects from a universe of available components and controls options to synthesize the solution within given constraints.  Fig. 3 illustrates the evaluation cycle of architectural design. Fig. 3.  Evaluation cycle of architectural design A full range of architectural issues and criteria have to be considered simultaneously.  The goal of design in architecture is to achieve the best balance of performances in a complete set of application criteria (Gero, D'Cruz and Radford 1983).  Understanding the design and performance relationships is essential and this can be facilitated through simulation.  In real-life, however, building design often happens in a disorganised fashion and frequently jumps from concept to concept.  Energy design is only one consideration amongst many and often not as important and prominent as the others.  Since energy performance has usually been invisible, the most that could be hoped for in the past was that the architect would follow some general guidelines for energy efficiency and make sure the design fell within certain constraints (Nall 1985).  Since architectural design decisions have a significant impact on building energy performance, it is desirable to improve this area by an efficient simulation environment. 4.2 Efficient Simulation Practice Fig. 4 shows the possible applications of energy analysis at various stages of the building design process.  At the early design stages, only conceptual sketches and schematics, often rough and incomplete, are available.  As the design proceeds, more information and detail will be developed.  If energy analysis starts early in the generative design phase, then energy considerations can be integrated into the building form and design concept.  It is believed that the best opportunities for improving the energy performance of a building occur early in the design process (Nall and Crawley 1983). Fig. 4.  Energy analysis in the building design process Because of the possible time and effort required for a full thermal analysis, detailed simulation tools are not efficient for all design exercises.  Provisions of other design tools for a quick assessment of design strategies would be very useful.  For instance, the use of solar path and shading facility can allow the architect to repeatedly evaluate a design concept on solar shading with a minimum amount of effort.  Design tools with different levels of sophistication should be used to meet the needs at various design stages. To solve a design problem using simulation, care should be taken to consider, inter alia, the nature of the problem and the approach of the investigation.  Explicit knowledge on how to translate the problem into proper input and how to use the tool for evaluation is currently lacking.  Newton, James and Bartholomew (1988) have suggested seven major steps as a good framework for a successful analysis: · Step 1 – defining the problem. · Step 2 – specifying the model. · Step 3 – data acquisition. · Step 4 – implementation. · Step 5 – planning. · Step 6 – experimentation. · Step 7 – analysis of results and reporting. 4.3 Integrated Building Design System Integration of simulation into the building design process can ensure that important data and information for each major design decision is provided in a timely fashion.  By establishing design links and exchange between architecture and engineering, an integrated building design system (IBDS) can be developed.  Some researchers have taken the initiative to develop future IBDS for efficient and flexible use of simulation tools.  The COMBINE (Computer Models for the Building Industry in Eurpoe, http://erg.ucd.ie/combine.html ) project in Europe (Clarke, et al. 1995; Kennington and Monaghan 1993) and the AEDOT (Advanced Energy Design and Operation Technologies, http://apc.pnl.gov:2080/0projects_and_capabilities/aedot/html/aedot.html )project in USA (Brambley and Bailey 1991) are typical examples. With the development of computer-aided design, building energy simulation and analysis is an important component in an integrated building design methodology (Augenbroe and Winkelmann 1991).  Program development for future simulation tools consists of some of the following features: · Fully integrated and interactive. · Graphical user interface to streamline the data and knowledge transfer. · Link with computer-aided design & drafting (CADD) tools. · Data transfer between various building design software tools (then the design tools can be used in cooperative mode). · Development of database and standard for building products. Back Content

5. Developing Simulation Techniques in Hong Kong

To realise the benefit and potential of simulation technology in Hong Kong, it is necessary to develop more information, experience and skills about building energy simulation and analysis (Lam and Hui 1993).  Research work is now being conducted in the Department of Architecture of The University of Hong Kong to provide support to architects and building designers on the application of building energy simulation tools (BEST) (Further information about BEST can be found at:http://arch.hku.hk/research/BEER/ ).  Current research activities focus on the analysis of building climatic data (ABCD), building up of simulation experience and development of simulation know-how. 5.1 Analysis of Building Climatic Data Buildings must be designed with the local climate in mind.  To carry out building thermal design and energy analysis, climatic data is required for every energy design tool.  Research is being carried out to establish the necessary data and information.  Specific research topics include: · Building up local climatic data and information. · Analysis of the local climate and its influence on building design. · Development of weather files for building energy simulation. · Development of simplified design tools for architects (such as physical solar design tools and bioclimatic charts). 5.2 The Hong Kong Experience Little help and guidance is available on simulation methodology and analysis techniques.  Since a large part of the cost and effort of a building energy study is in the analysis, advances in computer software is not enough by itself.  It is time-consuming, ineffective and error-prone for an inexperienced person to carry out detailed simulations.  Consequently, the importance of training on building energy simulation in Hong Kong should not be overlooked. Architects and building designers in Hong Kong often use overseas technology and software for energy design.  The aspects in local climatic data and design conditions are based on approximations since well-collated research in this area is lacking.  Initial research by Chow, et al. (1994) indicates that the difficulties include not only the shortcomings of the simulation tools but also some unique problems in Hong Kong, such as the time constraint in commercial projects and lack of real building performance data.  Analysis of office building energy performance by Lam and Hui (1996) shows that sensitivity technique when integrated into building energy simulation can be a powerful tool for building thermal design and energy analysis.  It is believed that technology transfer combined with development of understanding of local conditions and characteristics is the key to simulation technology in Hong Kong. 5.3 Developing simulation know-how To ensure sensible and reasonable results from energy simulation, the program user should have a sound background in building physics, knowledge of computing techniques, insight of simulation logic and intuition for detecting irrational data.  As Hand (1993) pointed out, the efficiency of simulation tools in practice depends not only on the facilities offered and the rigour of the underlying calculations but also on the skills of the user vis-à-vis abstracting the essence of the problem into the model, choosing appropriate boundary conditions, setting up simulations and interpreting the results.  To develop simulation skills in Hong Kong, a number of tasks have been identified: · Setting up support for building energy simulation tools. · Establishing links with advanced energy research institutes and relevant professional bodies. · Development of simulation models for building energy analysis. · Provision of information and training for local building professionals.  Some of the above tasks have already been initiated.  For example, by collaboration with the Lawrence Berkeley National Laboratory in USA, a regional DOE-2 Resource Centre has been set up in Hong Kong to provide support to the DOE-2 users and energy analysis(Please contact the author or Mr K.P. Cheung of the Department of Architecture, HKU for details of the Hong Kong DOE-2 Resource Centre, http://arch.hku.hk/research/BEER/doe2/doe2.htm )  There has been communication between local professional societies, such as the ASHRAE Hong Kong Chapter, to discuss and consolidate the efforts and resources for promoting building energy technology.  It is hoped that a focal point will be formed for providing information on energy conservation measures and techniques to the building industry.   Back Content

 

6. Conclusions

As computer simulation tools are constantly changing and evolving, it is useful at this time to outline the current and future development of building energy simulation.  Knowledge about the properties, applications and limitations of simulation tools is of practical importance because both current and potential users of the tools are, to some extent, frustrated and puzzled by the existing programs.  To apply simulation tools and techniques successfully, a clear understanding of the building design process and its relationship with the simulation environment is advisable since humans (in other words architects) and not computers dictate the creative and evaluation process.  For maximum efficiency, the integrated building design systems such as COMBINE and AEDOT currently being developed in other parts of the world will be an important step for the next generation of simulation tools.  Mathews and Richards (1993) pointed out that the success of a design tool is only proven when many people in the building industry apply the tool successfully in practice.  To evaluate building performance and achieve energy efficiency goals, architects and building designers should take full advantage of computer simulation tools that are readily available.  With a better understanding of building energy simulation through education and training, it is possible for us to establish confidence and efficiency in the use of simulation based design tools.   Back Content

 

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