Simulation for environmental engineering

Simulation for environmental engineering - Methodology

Key concepts:

performance assessment methodologies;
how to use modelling and simulation in system design and performance evaluation; relative vs absolute;
rank ordering of options;
selection of indoor/outdoor boundary representation;
conversion of problem - input data - results - information;
selecting appropriate modelling and simulation level.

Lecture structure:

What is the question ? ("problem led")
relative vs. absolute
rank ordering
selecting appropriate level

You can access more information under a variety of topics:

Course material (initial):

1.0 Simulation methodology

The most critical aspect of modelling is the planning and organisation of work to address the needs and goals of a project within its resource and time constraints (Hand and Strachan, CIBSE GUIDE, 1996). The procedure involves the following steps:

define design questions;
translate to modelling objectives;
plan and prepare model;
simulate; and
interpret results

A brief synopis of these aspects are:

1.1 Defining the Design Questions

Asking the right questions at the appropriate stage of the design process is fundamental in achieving the maximum benefit from undertaking a building performance assessment. For Example:

Early design stage: are orientation of building and glazing ratio reasonable ?
Mid-design stage: is there a requirement for air-conditioning ?
Late design stage: what control strategy will provide energy efficiency ?

The use of an integrated design approach will help maximise the the return from a modelling exercise, where all members of the design team work closely together to realise the aims and design targets for the building. An integrated approach will enable energy efficient features to be incorporated at all stages at little or no extra cost.

Key issues may be identified by carrying out design checks using simple tools to evaluate orientation options and form issues such as site layout. This will add confidence to the overall approach and highlight the critical issues and unanswered questions.

1.2 Translating Design Questions to Modelling Objectives

Can the design question be expressed as a modelling task ? This will be dependent upon the choice and applicability of the assessment tool used in simulation to address the design question, e.g.

Design Question: Does the building require air-conditioning ?

Modelling Task: Define overheating with respect to peak summertime temperatures and frequency of occurance within a naturally ventilated scheme.

1.3 Model planning and preparation

Once objectives are decided upon, develop a clear procedure for modelling. This enables:

quality assurance
design team member checks on modelling
consistency of approach to similar problems

The procedure should:

identify appropriate design tools/programs and
the required level of modelling resolution(abstraction)

The result is:

specification of necessary data to develop appropriate model and
sequence of assessments necessary to evaluate performance

Figure 1 set outs a typical procedure: in a phased study or when different aspects of the design are assessed with multiple design tools.

Figure 1. Typical Modelling Methodology

Model Abstraction

The representation of the level of physical reality within the model is a key concept. (see Level of Abstraction)

In terms of the level of detail, the aim is to keep the model as simple as possible and avoid significant errors that will result from over simplification. One means to achieve the correct level of detail required for the various building sub-systems, i.e. constructions, lighting, etc. is to use previous case studies or personal experience. In the absence of these the best peocedure is to create a preliminary reference model with:

simplified geometry
known or estimated constructions
known or estimated internal gains(and their variations with time)
design compliant ventilation rates
ideal plant and control

Regarding Modelling Accuracy

design tools need sufficient accuracy for practical purposes
the level of detail will affect accuracy

Reference Model

Parametric investigation requires a reference or base case model for comparison.

for new build projects, the reference model is normally the initial design content
for refurbishment projects, the reference model is normally the existing building

Other modelling issues

climate
zoning
shading and internal solar distribution
ventilation and infiltration
plant and control
occupancy and small power
thermal bridges
computational parmeters
risks and uncertainty

1.4 Results interpretation

The results of the simulation need to translate the reporting features of assessment tools to clear, concise, quantitative information enabling decision making.

Presentation of results should convey the appropriate level of information.

Some general points are

Brief the client based on interative process.
Keep presentation relevant without focus on detailed interpretation.
For parametric assessment, clearly define variation to the reference case.
Quantify where possible.
Explain results.

Elements of presentation

Objective statement
Summary of main findings (Executive summary)
Brief details of design tool capabilities
Model description
Model assumptions and sensitivity analyses
Note of design variations from reference case
Graphical and tabular results
Conclusions set against stated objectives