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Modelling The Environmental Performance of Buildings and the Built Environment

~~#0099CC:Professor Phil Jones
Centre for Research in the Built Environment
Welsh School of Architecture
Cardiff University
tel 44 1222 874078 fax 44 1222 874623 emailprotectEmail('jonesp', 'cardiff.ac.uk', '@');

~~#0099CC; background:">Abstract

In order to develop the concept of sustainability in the construction industry there must be increased use of detailed predictive tools, both at an individual building level and at urban scale. In the UK, the Egan Report, â€˜Rethinking the Construction Industryâ€™, recognises that in order to promote a sustainable construction industry, the industry will need to change its procedures and adopt â€˜key performance indicatorsâ€™ that will benchmark building projects in relation to cost, customer needs and sustainable development. There is therefore a need to develop the use of a framework of predictive tools that can predict the performance of buildings and their internal and external environments in order to promote sustainable design through the use of innovative design features. This paper reviews some of the main prediction tools related to environmental design and energy use, using case studies to illustrate their use.

1. Introduction

The increase in awareness of the impact that buildings have on the environment has resulted in a growing interest in the use of more advanced prediction models for design. Models can be used to predict the energy use of buildings, which can have a major global impact in relation to carbon dioxide and other emissions from burning fossil fuels. Models can also be used to predict other aspects associated with building environmental design, such as ventilation and internal heat movement, daylight and sunlight, and external wind and its impact on pedestrian comfort. These are aspects, which, in addition to having an association with energy use, also affect the health and comfort of the building occupants. In the UK the government has made a commitment to sustainable construction through the â€˜rethinking constructionâ€™ initiative, which was launched with the Sir John Egan report â€˜Rethinking the Construction Industryâ€™ 1. This calls for a change in the approach to building design, construction and operation, with the introduction of â€˜key performance indicatorsâ€™ (kpiâ€™s) to benchmark buildings against.

Models can be used on a larger scale, say up to city scale, or to assess sustainability across arrange of built environment sectors. This paper reviews modelling tools and frameworks developed to consider buildings and the built environment, and how these feed into the decision-making processes.

2. Energy and Environmental Models

A range of models that are used for energy and environmental prediction are summarised below, together a brief description of how they can be used. The models presented are examples of â€˜advancedâ€™ modelling methods that are based either on computational numerical prediction or physical scale modelling. They can be applied at increasing scale, from individual buildings, building estates to whole cities.

2.1 Energy Modelling

There are a range of models that can predict the dynamic thermal performance and energy use of buildings. They can be used to predict hourly values of internal temperatures and energy use, based on external climate conditions, the building construction, the mechanical services and occupancy patterns, including the internal heat gains from occupancy. An example of such a model is HTB2, developed at the Welsh School of Architecture, and used on a number of research and design projects, in the UK and elsewhere 2,3,4. Models such as HTB2 have in the past been somewhat difficult to use and therefore have mainly been used by researchers. However, recent developments in user interfaces for ease of pre- and post-processing have made these more complex models easier to use and therefore more appropriate for general use by the design community. A new version of HTB2 has been developed for use in a PC Windows environment. Figure 1 illustrates the use of the HTB2 Windows pre-processing program to construct a building using a thermal network approach. Figure 2 illustrates the results from HTB2 in relation to predicting the performance of innovative facades. The model is able to predict surface and air temperatures through the faÃ§ade and relate that the energy use and thermal comfort.

Figure 1: Thermal network approach used by HTB2 to describe a building.

Figure 2: Dynamic thermal modelling used to predict internal temperatures, solar gains and surface temperatures associated with innovative facades.

2.2 Airflow Modelling

Airflow modelling using computational fluid dynamics (CFD) prediction methods are increasingly being used in the design of internal air movement and ventilation in buildings. They can predict the detailed patterns of internal air movement and heat distribution. Figure 3 presents results from the prediction of internal air temperature and velocity for a naturally ventilated office in Penang (T. R. Hamzar and K. Yeang). They can also be used to predict the natural ventilation rate, driven by wind and the stack effect, and the movement of a pollutant throughout the space or smoke movement in the event of a fire. It can predict 3-dimensional effects that are either constant or variable over time. It is therefore an extremely versatile and useful technique in the field of environmental prediction. They are particularly useful when designing complex buildings, especially where there is a 'hybrid' mix of natural and mechanical ventilation. They are quite complex in their operation. However, they are becoming more 'user friendly' and with some training can be used by building services engineers and architects.

Figure 3: Airflow modelling used to predict internal air temperature and velocity for an office in the UMNO building designed by Hamzah & Yeang 5.

2.3 Wind Tunnel Modelling

Wind tunnel modelling can be used to predict the wind pressure at openings in relation to ventilation design, and wind flow around buildings and groups of buildings, in relation to pedestrian comfort. A boundary layer wind profile can be induced in the wind tunnel by means of blockages. The profile should be comparable to that appropriate for the site, ie. urban, rural, etc, . The wind tunnel will also have a characteristic turbulence, which again is controlled by a gridded blockage. This should be appropriate for the scale of model used. Most building application wind tunnels operate at scales of between 1:100 to 1:500. As long as the Reynolds Number is kept high (through high tunnel air speeds - usually about 5 to 10 m/s) the turbulent regime is ensured and scaled and real flows will match.

Figure 4. Wind driven flows in an atrium as predicted by wind tunnel studies, shows the effect of a wind shield placed before the upper opening, encouraging flow in the desired direction (SAGA Building by Sir Michael Hopkins and Ove Arup).

The main use of wind tunnels for ventilation studies is to predict pressure coefficients, Cp's, over the building envelop, especially at location where openings are planned (Figure 4). A physical model of a building and its surroundings can be constructed and placed in a wind tunnel where it is subjected to a controlled wind flow. Pressure sensor taps can be installed at various points on the building envelope, corresponding to ventilation openings. The pressure at each opening can be related to the free wind pressure, at a point of known height above the surface, in order to obtain the Coefficient of pressure (or Cp) value for the opening. This can be carried out for a range of wind directions, taking full account of the disturbance of surrounding buildings.

Figure 5. Wind tunnel used to predict relative wind speeds at pedestrian level in a high rise residential development in Hong Kong.

They can also be used for flow visualisation, by introducing smoke or other tracers in the wind tunnel, and observing the flow characteristics. A fine grain can be introduced to investigate wind flow profiling at pedestrian level (as the air speed is increased the grain will be cleared from windy areas, indicating exposure to wind) (Figure 5).

Due to limitations imposed by scaling effects, wind tunnel modelling cannot normally be used to investigate internal flows of air within a building or a space, and they cannot be used to investigate air-flows generated by buoyancy. However, often they are used in combination with CFD methods, with the scale physical modelling used to provide boundary pressure conditions to the CFD model, which can be used to predict the internal conditions.

2.4 SkyDome? Modelling

The provision of daylight, the effect of overshadowing, the control of sunlight penetration and glare are all areas for the application of physical models. While simple daylight calculation methods are generally useful for simple designs, they are often limited in their scope for accounting for complex room geometry, in allowing for site obstructions by surrounding buildings or vegetation, or in the use of modern materials or devices.

The SkyDome? at the School (Figure 6) is one of the largest artificial skies available. It consists of an 8m. half-dome, containing 640 individually controllable luminaires to model the sky dome. These light a 2m rotating model stage, capable of carrying models up to 200 Kg. Integral to the dome is a heliodon, capable of carrying a range of artificial suns, from low power parabolic mirrors, used for visualisation of shadows, to high power arc-lamp projectors, used to simulate solar irradiance to solar collectors. The control of the dome lighting and heliodon is capable of modelling any sky condition anywhere in the world. Again this can also be used in conjunction with computer modelling of daylight and sunlight, and the example in Figure 7 shows a radiance calculation of external daylight factors on the facades of the high-rise development.

Figure 6. SkyDome? facility at the Welsh School of Architecture being used to study sun penetration on a high-rise housing estate in Hong Kong.

Figure 7. Surface daylight factors calculated using Radiance.

3. The integration of building modelling into the design process

In order to achieve environmental improvement in practice requires attention at design and client levels:

At a design level, prediction tools, good practice guides, assessment methods and benchmarking are needed. Although these are available, they are generally not used, or not used to their full effect, due to lack of money, skills, awareness. Where they are used they can be effective.
At a client level, to provide investment in design and capital costs in order to reduce operating costs: Life cycle costing has generally not been successful in the UK. The construction industry is still capital cost driven and there is little incentive to reduce operation costs at the design stage. There is not normally â€˜up-frontâ€™ environmental design funding to resource the environmental design input when it is most needed.

There needs to be a clear link between the socio-environmental-cost indicators at client body decision making level, and the use of prediction procedures, and other design aids, by designers and planners.

Environmental Assessment Methods such as Hong Kong BEAM 6,7 are useful to provide benchmarking system to check that a design has the required environmental performance. These methods are growing in number, with more countries developing their own specific methods appropriate to their climate and building types. The Hong Kong BEAM for offices specifies benchmarking data for total building energy use, lighting energy use, internal environmental conditions, etc. and this encourages the proper degree of analysis to be carried out at design stage.

It is important that the design prediction tools, Assessment Methods and Performance Indicators are linked through a framework of environmental analysis. Figure 8 suggests a suitable framework, linking the design modelling to assessment methods via appropriate indicators. It indicates that if â€˜high levelâ€™ decision makers such as clients and developers have information on environmental performance in the appropriate form they can have a major influence over the design through the brief to the designers, which can act to â€˜kick-startâ€™ environmental design predictions at the detailed design level in addition to providing indicators at higher level.

Figure 8. Framework for environmental analysis.

4. Modelling the City!!

In order to manage the use of energy of the built environment in a sustainable way and to minimise harmful emissions, the performance of the city in sectors or â€˜as a wholeâ€™ must be considered. To fully understand the inter-relation ships between buildings, transport and industry, and the potential for using renewable energy sources on a city wide basis, a model is required that can predict the various interactive processes. An energy and environmental prediction (EEP) model has been developed in collaboration with Local Authorities in South Wales as part of a unified effort to plan for sustainability and to predict and account for reductions in carbon dioxide and other emissions 8. The computer model will provide information for implementing urban energy management and environmental planning, enabling decision-makers to plan for improved energy efficiency. Initially developed for Cardiff, EEP is now being considered by other local authorities in the UK. can be transferred to other cities to predict the effects of future planning decisions from a whole city level down to a more local level.

The framework for the Energy and Environmental Prediction tool is shown in Figure 9. The user will access the tool via a primary user interface, this will take the user through a decision making process, presenting information and options and the opportunity to enter data in a straightforward manner. The interface will call a range of external procedures or sub-models, selected according to the userâ€™s needs, for example, to predict the cityâ€™s energy and emissions from buildings, transport and industry. It will present results simultaneously through the associated Geographical Information System (GIS) which contains an Ordinance Survey (OS) map of the city describing the buildings and roads. Sub-models will exchange data through a data highway, making all data available to all sub-models. Data can be then mapped using the GIS facilities, for example identifying buildings of high energy use predicted by the energy sub-model in EEP (Figure 10).

Figure 9: Principal Components of the Computing Framework, indicating sub-models existing and under development.

Figure 10. An example the EEP mapping used to identify buildings of high energy use.

5. Practical evaluation tools for urban sustainability (PETUS)

Tools such as EEP are used within a decision making framework, which allows for both quantitative and qualitative aspects of a specific project to be addressed. As part of the framework there is a need to clearly define the sustainability objectives for a specific project, and to track these objectives throughout the design process and in the selection of tools. The PETUS framework is being developed under the EU 5th Framework programme. It provides a framework within which practical evaluation tools can be used to analyse and improve the sustainability of urban infrastructure projects. The evaluation procedure enables a consistent assessment approach across all aspects of urban infrastructure projects including water/sewage, waste, transport, energy, green/blue (vegetation/water) areas and projects of a holistic nature. PETUS will provide access to a set oprocedures and tools that will help to assess projects against a set of standard sustainable criteria and provide information to look at the potential for transferring projects to other areas of the world. This will assist personnel involved in urban infrastructure to develop projects in the most sustainable way possible.

Previous findings from the COST Action research programme (C8 Urban Infrastructure and Sustainability, 1997) found that many tools and procedures have been developed in the past to assess the sustainability of urban infrastructure but few of these are used in practice. There is therefore a need to fill the gap between theory and practice by allowing stakeholders involved in urban infrastructure at all decision making levels to undertake an appropriate assessment of the likely impact of an infrastructure project on sustainability. There is a need to provide a set of practical methods to â€˜operationaliseâ€™ the concepts of sustainability. Therefore a decision making system is needed that can:

a) provide information to actors engaged in building and infra-structure projects
b) provide a decision making aid for specific projects.

There are various tools and checklists that can be used. It is important that these tools are respond to the needs of practice as well as their intended functions, for example, to predict energy in buildings. They must also be used within some sort of decision making process. Checklists such as BREEAM are only part of solution and the danger is that they constrain thinking to that what is in the tool and to the time the tool was applied. Therefore the use of tools must be contained within an overall decision making process.

PETUS was designed and developed to provide a decision support system, with the two main components, that is, access to information and tools, and a procedural guide for the implementation of a building or infrastructure project. It was based on experience gained from practice. There have been systems in the past, but they have been largely developed from theory, and are often not easily applied in practice. PETUS was developed from a practice based approach using experience gained through case studies and end user involvement. Some 50 end users from the partner countries were consulted during the development of PETUS and many had the opportunity to present their experiences directly to the project. The PETUS system was developed around the following main principles:

- to provide information on sustainability for the different end-use sectors.
- to clarify what the sustainable objectives are for a specific project.
- to ensure that all aspects of sustainability are considered and informed choices made.
- to ensure design intent feeds through the program, and any changes are made as a conscious decision.
- To involve the project team and end users in a multi-disciplinary holistic approach.

The concept behind PETUS is illustrated in figure 11. PETUS is available as a web based framework (Figure 12).

Figure 11 â€“ The Decision Support System for the PETUS project illustrating the interaction between the database and the process

6. Conclusions

ÃŸ Advanced models are available for detailed environmental prediction, to inform design and to provide performance indicators for high level decision making for both buildings and the built environment.
ÃŸ However, due to barriers in the construction industry and local government, relating to the use of models in design, they are still only used on a relatively small number of projects.
ÃŸ A framework for environmental analysis is needed to link the design prediction tools to environmental assessment methods through performance indicators in order to promote environmental issues at high level decision making.
ÃŸ A computer based model framework has been developed for cities which links a range of models through a GIS environment.
ÃŸ The PETUS decision making support system can be used on major urban infra-structure projects.

Figure 12 The PETUS web site

References

1. Rethinking the Construction Industry, UK government report chaired by Sir John Egan, DETR, HMSO (1998)
2. Alexander, D K, HTB2 Users Manual, Welsh School of Architecture, (1996).
3. Alexander, D K, Hassan K A Ku and Jones P J - Simulation of solar gains through external shading devices, Proceedings of Building Simulation 97 - Fifth International IBPSA Conference, Volume I, International Building Performance Simulation Association (IBPSA), Czech Republic (1997) 355-362 IBSN 80-01-01646-3
4. Burnett J, Jones P J and Yik F W H - HTB2/Becon: A Building Energy Prediction Model for Air-Conditioned Commercial Buildings, Proceedings of Air Conditioning in High Rise Buildings 97, Volume I, Tongi University Press, Shanghai (1997) 340-345 ISBN 7 5608 1805 6/Z.60
5. Jones, P J and Yeang K, Use of a wind wing wall as a device for low-energy passive comfort cooling in a high rise tower in the warm-humid tropics, PLEA 99 Conference, Brisbane (1999) Volume 1,501-506 ISBN 1 86499 3480
6. HK-BEAM Version 1/96, An environmental assessment method for new air conditioned office premises, CET Ltd Hong Kong ISBN 962-85076-2-1
7. HK-BEAM Version 2/96, An environmental assessment method for existing air conditioned office premises, CET Ltd Hong Kong ISBN 962-85076-4-8
8. Jones, P J, Lannon, S, Williams, J and Prasad D, An energy and environmental prediction tool for planning sustainable cities â€“ a global model, PLEA 99 Conference, Brisbane (1999) Volume 2,789-794 ISBN 1 86499 3480

Phillip Jones
November 2005