Benefits of Energy and Performance Modelling of Buildings

Author: Quentin Jackson
e Cubed Building Workshop Ltd
Wellington

Reviewed by Greenbuild.

TABLE OF CONTENTS

1        Why do we Simulate?
1.1          The Output
1.1.1       Hourly Temperature
1.1.2       Temperature Distribution
1.1.3       Discomfort Period
1.1.4       Heating and Cooling Loads
1.2          The Tools

 

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1 Why do we Simulate?

In order to support the design of revolutionary buildings we must be able to assess the effect on the environment (reductions in energy loads for example) of these buildings.  This is where simulation tools can be used to develop:

“… a simplified model of a complex system and using the model to analyse and predict the behaviour of the original system.”

In New Zealand, there are many different tools being used to help assess and evaluate energy efficiency, renewable energy, and sustainability in buildings.  These tools help to support the decisions of designers at various points throughout the life cycle of building design and operation.   Thus allowing us to visualise complex calculations to show us what is actually going on in the building – before it is built.  In turn providing a clear validation mechanism by which one can objectively assess and judge your design decisions, taking you closer to optimal building performance.

It must be remembered that computer simulation is an abstraction of reality.  Simulation tools allow you to assess and calculate the behaviour of building control systems and the resultant impact on energy use, peak demand, equipment sizing and occupant comfort.  However, one has to understand that none of the tools used to predict the benefits of energy and performance modelling of buildings fully represent all aspects of the real world situation.

“Why simulate?  The key reasons are that real-life systems are often difficult or impossible to analyse in all their complexity, and it is usually unnecessary to do so anyway.  By carefully extracting from the real system the elements relevant to the stated requirements and ignoring the relatively insignificant ones (which is not as easy as it sounds), it is generally possible to develop a model that can be used to predict the behaviour of the real system accurately.”

So how do I relate this simulation and its outputs to my project and what might I get out of it?

• Building thermal simulation allows one to model a building before it is built or before renovations are started.
• Simulation allows various energy alternatives to be investigated and options compared to one another.
• Simulation can lead to an energy-optimised building or inform the design process.
• Simulation is much less expensive and less time consuming than experimentation (every building is different).

1.1 The Output

As one can imagine scientists and computer software are prone to throwing out large amounts of data that have little meaning to the untrained eye, so what kind of information should we expect to see and how might I use it?

The key to all this is that simulation uses weather data.  This file summarises sometimes up to 30 years of historical data into one average year for you to test your design against in your location.  Hourly data is used for most energy simulation and at a minimum contains:

• Dry-Bulb Temperature
• Relative Humidity
• Solar Radiation
• Wind Speed
• Wind Direction
• Cloud Cover
• Rainfall

Once you have got all this data and run your simulation, what can I expect out of it?

1.1.1 Hourly Temperature

The tool will produce data for every hour of every day – what you really want from this is to understand how well the building fabric works as a filter from the external environment.  This data can then tell us what proportion of the year the inside of the building can be said to be ‘comfortable’.  You might get a graph like the one following that shows the percentage of hours of the year each room is comfortable (between 18 and 25°C), too cold (under 18°C) and too hot (above 25°C):

Image Source: e Cubed Building Workshop

1.1.2 Temperature Distribution

In order to investigate the likelihood of extreme internal temperatures, an average daily internal temperature profile can be created.  This can help investigate why a temperature change occurred.  The output may look something like the following graph (note that the blue line shows the outdoor temperature so we can see how the indoor environment relates to the external weather conditions:

Image Source: e Cubed Building Workshop

1.1.3 Discomfort Period

Comfort can be defined as ‘the condition of mind that expresses satisfaction within the environment’.  Thermal comfort depends on a combination of factors including: indoor air temperature, mean radiant temperature, relative air speed, relative humidity, clothing and the physical activity of the occupants.

Therefore, from the simulations we can find out how often an occupant is likely to be uncomfortable in a given space.  All the available tools can do this in different ways but one type of graph you could expect is one like the following which shows that significant heat discomfort occurs for seven months of the year:

Image Source: http://squ1.org/wiki/Thermal_Design

1.1.4 Heating and Cooling Loads

The annual heating energy of a building can be predicted using energy models - however these results are indicative only.  Actual energy use depends largely on the occupants’ behaviour and standards – which are extremely hard to replicate with a computer.  Predicted energy usage is best used when examining the effect of different building options in a comparative study.

Energy simulations can also be used to perform ‘sizing runs’ to calculate how large the heating and cooling loads are and what size the plant items need to be to reach and maintain comfortable internal temperatures.  This allows you to minimise the cost of providing the heating and cooling and reduce energy wastage as much as possible – which basically means reducing the loads on the system as much as possible.  A graph of the results might look like the following which shows us the space loads, not the actual energy load.  This is because the model knows no details of the actual HVAC system, only the amount of heating or cooling it has to supply to the space.  Thus, for a given space load, an efficient system will use less energy meeting that demand than a relatively inefficient one.

Image Source: http://squ1.org/wiki/Thermal_Design

1.2 The Tools

The energy analysis tools listed below include energy analysis software used for both residential and commercial energy performance simulation.  The following is by no means an exhaustive list:

Design Builder

DesignBuilder is a new software tool for assessing and creating building designs.  It provides a range of environmental performance data such as: annual energy consumption, maximum summertime temperatures and HVAC component sizes. The software can be used effectively at any stage of the design process from the conceptual stage where you may want to try out some façade options on a single room, through to production.

http://www.designbuilder.co.uk/

Energy Express

Energy Express estimates energy consumption and cost in commercial buildings, also providing peak load estimation. Offering a choice between a comprehensive air-conditioning model, and a simple generic air-conditioning model, Energy Express allows architects in particular, to evaluate building facade options without having to specify details about the air-conditioning.

http://ee.hearne.com.au/index.htm

Energy Plus

EnergyPlus is a building energy simulation program for modelling building heating, cooling, lighting, ventilating, and other energy flows. While it is based on the most popular features and capabilities of BLAST and DOE-2, it includes many innovative simulation capabilities such as time steps of less than an hour, modular systems and plant integrated with heat balance-based zone simulation, multizone air flow, thermal comfort, and photovoltaic systems.

http://www.eere.energy.gov/buildings/energyplus/

Tas

Tas is a suite of software products, which simulate the dynamic thermal performance of buildings and their systems. The main module is Tas Building Designer, which performs dynamic building simulation with integrated natural and forced airflow. It has 3D graphics based geometry input that includes a CAD link. Tas Systems, is a HVAC systems/controls simulator, which may be directly coupled with the building simulator. It performs automatic airflow and plant sizing and total energy demand. The third module, Tas Ambiens, is a robust and simple to use 2D CFD package which produces a cross section of micro climate variation in a space.

http://www.edsl.net/

 

Adeline

Adeline provides architects and engineers with accurate information about behaviour and the performance of indoor lighting systems. Both natural and electrical lighting problems can be solved, in simple rooms or the most complex spaces. Adeline produces innovative and reliable lighting design results by processing a variety of data (including: geometric, photometric, climatic, optic and human response) to perform light simulations and to produce comprehensive numeric and graphic information.

http://www.ibp.fhg.de/wt/adeline/

Radiance

Radiance is a suite of programs for the analysis and visualization of lighting in design. Input files specify the scene geometry, materials, luminaires, time, date and sky conditions (for daylight calculations). Calculated values include spectral radiance (i.e. luminance + colour), irradiance (illuminance + colour) and glare indices. Simulation results may be displayed as colour images, numerical values and contour plots. The primary advantage of Radiance over simpler lighting calculation and rendering tools is that there are no limitations on the geometry or the materials that may be simulated. Radiance is used by architects and engineers to predict illumination, visual quality and appearance of innovative design spaces, and by researchers to evaluate new lighting and daylighting technologies.

http://radsite.lbl.gov/radiance/

Rayfront

Rayfront is a platform independent toolkit that provides a graphical user interface to the lighting simulation software Radiance. Radiance is the industry standard raytracing engine for physically correct lighting simulations (see comparisons).  Rayfront is equally well applied to electrical lighting and daylighting.   It has no limits for geometry size and complexity, but gives you fast and accurate results for small projects as well.

http://www.rayfront.com/

Flovent

Flovent is a powerful Computational Fluid Dynamics (CFD) software package that predicts 3D airflow, heat transfer and contamination distribution in buildings of all types and sizes.

http://www.flovent.com/

Ecotect

Ecotect is a software package with a unique approach to conceptual building design. It couples an intuitive 3D design interface with a comprehensive set of performance analysis functions and interactive information displays.

http://www.squ1.com

 

Virtual Environment

The IES Virtual Environment is a unique, integrated system for building performance assessment that brings productivity to every aspect of the building design. It operates from a single building model, bringing together all the design strands you need in one unified system.

http://www.iesve.com/content/default.asp