Design and Evaluation Issues
What is a design tool ?
The following answer is extracted from European Commission report 'Tools and techniques for the Design and Evaluation of Energy Efficient Buildings', UCD-OPET, 1995.
As used here, the term includes a diversity of tools, from those used to inform the process by indicating trends in energy use associated with strategic design decisions, to tools to predict the energetic performance of detailed architectural and engineering proposals. In some cases design tools have been developed to replace laborious calculation procedures used in the design process. In using the design tool, the 'number crunching' exercise is either carried out by the computer or has been simplified by following a number of pre-defined steps in the case of a manual tool. They can save considerable time if used correctly, cutting a week's work on paper to possibly an hour or less in the case of a computer-based tool.
Other tools have been developed to
determine the behaviour of physical phenomena which would have been too complex
to examine by 
hand.
In some cases this extends to assessing interactions between design elements
which were previously treated in isolation. The diagram opposite details the
various energy flowpaths that any advanced tool must be capable of accurately
and dynamically handling. Simple tools will typically handle only one or a few
of these flowpaths.
Thus, the use of design tools makes practical the study of matters not previously considered in many building design processes, either because it is now feasible due to lower time and cost requirements, or because the level of complexity has been reduced. This can help lead to energy related issues being given fuller consideration in the process of design.
Design tools are not always calculation methods. Many other forms of tools have been developed to assist the building designer in arriving at more energy efficient solutions. Handbooks and data tabulations have been compiled. The computerisation of information sources allows designers to locate desired information quickly.
Who should use a design tool ?
The answer to this question dependent on what is to be studied and the stage of the design process reached. Design tools can greatly assist where specialist knowledge is not available or where the required study would be prohibitively complex or time consuming.
Most design tools are based on either mathematical or empirical relationships. However, the user does not necessarily have to understand these formulae in order to use the tool. With an awareness of the limitation of the tool, and the help of guidance of documentation and/or training, users should be able to conduct studies of overall or component performance.
While architects have begun to use energy design tools, at the present time it is the engineer who is most familiar with their use. Until recently, the architect has been poorly served by design tools. Computer aided design (CAD) has been one of the few applications widely taken up in architectural practice. CAD is not a design tool in the sense used here. Other applications now available for the architect include tools which indicate the energy performance of designs where only scant information is available, and tools that permit the visualisation of the flow of light, heat and air.
Tools also have their limitations. They are often mistakenly used with the assumption that they can predict reality. This is a most misleading assumption and is often the basis for tool misuse. While some tools can achieve accurate predictions, they are based on approximations of reality. Similarly, users will bring to a tool their own assumptions and simplifications of the design problem. For the user of a design tool, awareness of the assumptions and simplifications being made by the tool is a prerequisite of effective application.
With simple design tools it is likely that, once the use of the tool is understood, reuse at a later date will require only a brief review of the documentation. With complex tools it is likely that an extensive re-learning exercise will be required each time a new project commences. It is best to dedicate staff to such tools so that, in practice, specific modelling tasks can be carried out efficiently. A smaller practice usually cannot afford to take this course of action and must employ a specialist to provide a modelling service as and when required.
Many design issues can be analysed through the use of design tools, such as
All of these issues are inter-related, in that they can directly or indirectly affect each another and the overall performance of a building and its environmental control systems. Lless sophisticated tools will often focus on only one or a few of these issues. The more issues taken into account, the more complex the model. Usually design tools that are suited to the early stages of the design process will consider only a few issues. It is also usual within such tools to reply on assumptions to reduce the information required as input from the user. Where a detailed design tool is applied at the early design stage, the user will be required to explicitly set default values to many of the input data items.
Where issues are complex or critical it is likely that the use of detailed simulation tools will become necessary. Such tools are also useful where the user has no no previous experience with a particular technology. In such a case detailed simulation can provide the designer with the confidence to proceed with an ideas and thus advance building innovation. (Don't imitate, innovate).
The main aim in the use of tools for energy efficient design is to achieve an optimum solution which balances conflicting factors to minimise energy consumption, maximise performance and mitigate environmental impact. No design tool can do this automatically. Tool use is an iterative process that lays equal stress on the quality of the tool and the user.
In applying a design tool several factors are generally taken into account as follows.
Location
The location of the project is important as energy performance is affected by factors such as topography, surrounding structures, micro-climate etc. Such factors are usually given cursory treatment by the simpler tools. While the micro-climate will have a significant effect on performance, describing data is usually not available and so assumptions need to be made. Because different tools adhere to defferent weather file formats, establishing suitable micro-climate data can be a time consuming task. Some efforts has been made to develop standard weather data formats but these are not yet widely used in practice. The ambient air temperature is required by all design tools although the collection frequency will vary (e.g. monthly, daily, hourly or less). Other typical weather parameters include wind speed and direction, solar radiation and relative humidity.
Geometry and construction
The geometry of the building will play an important part in any analysis. Most simple tools will operate with areas only or be restricted to vertical walls and sloping roofs. As tool sophistication grows, the complexity of the geometric model increases so that arbitrarily complext shapes may be modelled. The input of such models can be time consuming although translations from CAD are sometimes supported. Detailed studies will require zone specific control. Simple tools may prohibit this by restricting the user to a few representative zones.
A most important element of the building model is the constructional description. Detailed tools will consider the dynamic behaviour of such constructions while simple tool are often restricted to steady state or steady cyclic conditions.
Standard calculations
There are many alternative modelling methods used to assess energy consumption, lighting, ventilation, emissions and so on. It is usually necessary to employ a method that is acceptable in terms of some national or international standard. This is particularly important when the aim is to test for compliance with some Building Regulation.
Services systems
The services to be included in the building (space heating, ventilation, air-conditioning, lighting) become important energy issues at the later design stages . Many detailed tools offer systems modelling capabilities and some tools have been specifically developed for this purpose.
Results analysis
The information output from tools varies considerably. Simple tools will typically provide only indications while detailed tools will provide information on the time evolution of the model state variables and performance indeces derived therefrom.
CAD is standard in most practices
and while not a design tool per se can assist in the preparation of building
geometry for use by an external design tool. Some design tools are CAD integrated
so that the user's perception is of one seamless application. Typically, a 
geometrical model
is produced in the CAD package and then exported to the energy program via file
exchange. The user is then required to attribute this geometry to complete the
model. An alternative approach is to arrange that the CAD package controls the
attribution prior to the export phase.
The integration of CAD and performance assessment tools brings the added benefit of supporting better team working by bringing together the different members of the design team. CAD systems also provide facilities for the 3D visualisation of the building model and support for animations.
Regardless of the tool chosen, it is necessary to devise an approach to performance assessment which will ensure quality of outcome. A typical approach might include the following steps.
Establish a base case computer
model to a level of resolution which is sympathetic to the particular design
features being tested. For example, illuminance-based light switching would
require that the characteristics of the photocell be included, while an
atrium used to achieve `borrowed light' would require a rigorous treatment
of geometry and inter-reflections. For a daylight only focus, some aspects
of the model can be abstracted, e.g. the control applied to the ventilation
system might be idealised.
Where possible the model should be 'calibrated' by comparing program outputs with measurements, simplified calculations, expectations etc and the model parameters tuned as necessary.
Corresponding reference models might then be developed by removing or adding design features as required. The results from the base case and reference models can then becompared in order to quantify the benefits (or otherwise) of the proposed design features.
Where possible, the results should be collated in the form of an 'integrated performance view' (IPV) which quantified overall performance using a range of relevant criteria.
As required, new models can be established in order to optimise the performance of specific design features which have merit on the basis of IPV comparison.