1 Building Energy Simulation 1Introduction 1.1A Brief History of Building Ener gy Simulation The changing importance of building physics in design, along with improved technological capabilities, has led to an evolution in the attempts to model the complex dynamics of the energy flows in buildings. Ultimately, the need for accurate modelling and simulation techniques is to aid design decisions. Early modelling attempts would generally be “steady-state” models, whereby a building could be broken down into an array of points or “nodes”, with energy flows between different nodes, as shown inFigure 1.Such a system of nodes can be thought of as an electrical network: each node is at a different temperature (analogous to voltage), and there are heat flows between nodes (analogous to current), with the rate of transfer dependent on the thermal resistance (analogous to electrical resistance). Figure 1 –Energy flows in buildings 1 1 Clarke J.A. (2001), “Energy Simulation in Building Design, Second Edition”
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
The changing importance of building physics in design, along with improved technologicalcapabilities, has led to an evolution in the attempts to model the complex dynamics of the energy
flows in buildings. Ultimately, the need for accurate modelling and simulation techniques is to aid
design decisions.
Early modelling attempts would generally be “steady-state” models, whereby a building could be
broken down into an array of points or “nodes”, with energy flows between different nodes, as
shown in Figure 1. Such a system of nodes can be thought of as an electrical network: each node is
at a different temperature (analogous to voltage), and there are heat flows between nodes
(analogous to current), with the rate of transfer dependent on the thermal resistance (analogous to
electrical resistance).
Figure 1 – Energy flows in buildings1
1 Clarke J.A. (2001), “Energy Simulation in Building Design, Second Edition”
The main problem with such a steady-state model is that the environment clearly varies with time;
weather variations, both daily and annually, result in significantly varying external temperatures,
wind speeds, and incident solar radiation. Not only are there weather variations, but the activity
within the building also varies, and thus casual gains are constantly changing. Meanwhile, the effect
of thermal mass in the building allows energy to be stored and released, adding yet another
temporal variation.
As computing power increased, dynamic models of energy flows in buildings began to appear. Thesedynamic models are based on the equations governing energy and mass transfer, and avoid many of
the assumptions and limitations of previous simplified models. For example, dynamic models can
capture the time-dependency of energy flows, such as climactic conditions, thermal mass, and
internal gains. The added complexity of dynamic models comes as a cost though: they are more
time-intensive than steady-state models, both in terms of the time needed to construct the model
and also the time needed to run the simulations, whilst they also require a greater level of details in
terms of inputs to the model.
Ultimately it is up to the individual to decide which type of model is more appropriate on a case-by-
case basis. For some studies, a quick estimate of the monthly energy use will be adequate. In thesecases, steady-state models (often spreadsheet based) would be most appropriate. In other studies,
an accurate profile of the energy use and internal conditions will be required. In these cases a
“Virtual Environment” by Integrated Environmental Solutions (IES-VE) is a modern example of
dynamic building energy simulation software. IES-VE consists of a suite of integrated analysis tools,
as shown in Figure 2, which can be used to investigate the performance of a building either
retrospectively or during the design stages of a construction project.
Figure 2 – Modules and analysis tools available in IES Virtual Environment
The VE software does not require the user to have any knowledge of computer programming or ofthe mathematics and equations that govern building physics, as all the interaction between the user
IES-VE includes a navigator that is designed to guide you through the process of constructing a
model and conducting energy or lighting analysis. The navigator, shown in Figure 4, also enables you
to quickly find the appropriate menu, button, or tab for the task you are working on by simply
clicking on the description of the task that you want to carry out. This can be very helpful, since itcan be difficult for beginners to find their way around the software otherwise. The navigator also
includes check boxes and text boxes to allow you to add notes and tick off completed stages in a
project. This can be useful when collaboratively working on a project.
Figure 4 – IES-VE navigator
IES have also created a simplified version of IES-VE called “VE Gaia” that is aimed at architects and
those less used to energy modelling. It contains most of the same functionality as IES-VE, but with
fewer numerical inputs and more emphasis on presentation of models, results, and analysis.
Annual academic licenses of VE Gaia can be downloaded for free from www.iesve.com. There arealso demonstration videos of the software on YouTube.
2.2
Creating a Geometrical Model
When using IES-VE, the first step in creating a full energy model of a building is to define the
geometry of the building by creating a geometrical model. This specifies the dimensions of the
building, including the floor area and height of every room, as well as the positions and sizes of any
glazing or doors.
Geometrical models can be created in the ModelIT module of VE, and are constructed in a similar
manner to other 3D building modelling programs such as Google SketchUp. In fact, for those whoare comfortable using Sketchup, an IES-VE plug-in is available for Sketchup, which allows the user to
specify building parameters inside Sketchup and then export the model to IES-VE for the full dynamic
energy calculations. This isn’t necessarily advisable, however, as importing models from Sketchup
often introduces a number of compatibility errors. In most cases, it’s easier just to build your model
in ModelIT. Existing models can also be imported using gbXML.
Figure 5 – ModelIT in IES-VE
Figure 5 shows some of the toolbars that are available in the ModelIT module. The modules box
contains a list of all the various modules that are available. For now we will stay in the ModelIT
module, but we’ll use other ones later. Also displayed is a room list , which shows all the spaces thathave been created. So far we haven’t created any spaces, so it’s empty, but soon it will start to fill
up.
Various toolbars are also available. Placing the mouse over any icon should display a short
description of the icon’s function. For more details on each icon, or i f you ever encounter any
trouble and are unsure of how to proceed, press F1 to get the (very useful) help menu.
To create a new space, you can use any one of the draw icons, shown in
Figure 6. The draw prism icon can be used to construct simple cuboids,
by clicking and dragging to achieve the required area. You can specify
the height of the room, and also give the room a reference to help you
Figure 7, above, shows some of the features for drawing a room, along with a simple cuboid room
that is 5m x 5m x 3m. If you tick the box marked “create inner volume”, the room is not modelled as
a thin-walled structure. You can also specify the plane at which you wish to have the room created.
For example, if we wanted to add a second room on top of this first one, we can create another
room at plane 3.0m. You can also use the different viewing options in the view toolbar to create
rooms from different perspectives.
You can also check (and admire) your model as you go along using the model viewer , as shown inFigure 8. And don’t forget to save your work regularly as you go along!
Figure 10 – Alternative methods for adding windows/doors
2.3 Applying Properties
The easiest way to apply properties to your building is to create templates using the building
template manager (shown as (A) in Figure 11), which can be accessed from the Tools tab. The
building template manager is the gateway to accessing other databases, such as the constructions
database, the openings database, and the profiles databases. Once a template has been created, it
can be applied to rooms or surfaces, in order to specify properties for that space. Multiple templatescan be created and assigned to different rooms, to represent the different activities of each room.
2.3.1
Creating constructions
We can begin by going into the constructions database, in which we can create new constructions
that define the materials of the building. This allows for insulation and glazing to be specified, which
will have an important affect on the energy efficiency of a building. After opening the constructions
database, the project constructions box should open up (as shown in (B) in Figure 11), listing all the
different constructions that are available. New constructions can be created by clicking on the add
default construction icon, which can then be edited by double-clicking on the construction
description. This should open up a new box where the construction is defined, as shown in (C) in
Figure 11.
The construction definition box shows the various layers that make up a construction (from outside
to inside), including the material, the thickness, the conductivity, and the density. Based on what
layers are defined, IES-VE will automatically calculate the U-value (measure of insulation ability) for
the layer. To add layers, use the buttons marked copy, paste, add, insert, etc. Thicknesses can be
altered by typing in the required thickness of each layer (in metres). To change the material of a
layer, use the systems materials database, shown as (D) Figure 11, which contains a list of hundreds
of different materials, organised into 14 categories. To change the material of a layer, right-click on a
Back in the building template manager , templates can be created to specify heating systems,
internal heat sources, and air exchanges of a room. This can be done in the thermal conditions tab,
as shown in Figure 14. The building regulations tab is only used if you plan on conducting a “Part L”
assessment. The other four tabs are likely to be more useful.
In the room conditions tab, the desired internal conditions of the room can be specified, such as the
duration and magnitude of the heating and cooling. Time profiles, such as those created in §2.3.2
can be used to specify the length of the heating/cooling period, whilst the simulation set-point can
be used to specify the temperatures at which heating/cooling will be turned on/off. For example, ifthe profile shown in Figure 13 (B) was used, along with a heating set-point of 19°C, then the heating
would be turned on at any point between 9am-5pm when the internal temperature drops below
19°C, and heating will be permanently off between 5pm-9am.
Finally in the thermal conditions section of the building template manager there is the air
exchanges tab, which can be used to specify the volumetric flow rate of infiltration, natural
ventilation, and mechanical ventilation. These are added in a similar manner to the internal gains,
with the magnitude of the air flow defined and the duration specified using activity profiles. If you
are confident that you know the magnitude and duration of the air flows into a space, then it is
reasonable to specify them in the air exchanges tab. However, there is a more accurate way of
defining natural ventilation, via the MacroFlo module or tab, as described below.
Once a thermal conditions template has been created – which includes room conditions, system
details, and information about internal gains and air exchanges – it can be applied to a room in the
Apache module, by selecting the space in question, and then using the assign room thermal
template to selection set button.
2.3.4
Modelling natural ventilation
IES-VE provides separate modules for modelling natural ventilation. If a detailed analysis of the flow
through a single space is required, then the MicroFlo module can be used, which is a CFDprogramme (computational fluid dynamics). If only an estimate of the volumetric flow rate (i.e. l/s or
m³/s) is needed to integrate with the dynamic energy model, then MacroFlo can be used.
MacroFlo will calculate the volumetric air flow rate through different openings (such as windows,
doors, etc.), based on weather conditions, size and orientation of the opening, etc., and can also be
used to model interactions between an occupant and an opening. For example, a window can be
specified to open and provide natural ventilation if the internal temperature of a space exceeds a
certain point. Activity profiles, as described in §2.3.2, can also be used to specify the time periods
that a window/door might be opened. All of this can be done in the MacroFlo Opening Types tab in
The monthly tables (see (B) in Figure 20) can be used to display the monthly (and yearly) totals for
different variables. In Figure 20, the table (B) shows that there is an annual heating load of
approximately 3.3MWh per year for the building, or 112kWh/m² per year – not very good
considering PassivHaus standards recommend less than 15kWh/m² per year.
3.4
Assessing Improvements
When designing a new building or analysing an existing building, the effect of changes to the
building are likely to be of interest. In particular, the effect of different materials and improved HVAC
(heating, ventilation, and air-conditioning) systems will be of interest. The case study building used
so far has un-insulated cavity walls, double-glazed windows, and a natural gas boiler with an
efficiency of 0.81. Clearly there is plenty of room for improvement here.
Firstly, the insulation of the building can be improved to reduce conductive losses, by adding
insulation into the cavity and upgrading the windows to triple-glazing. This is done by changing the
layers of the constructions that were made earlier in the constructions database in the building
template manager , as shown in Figure 11 and described in §2.3.1. Once the constructions have
been altered in the database, they do not need to be reapplied to the building, as changes in the
databases are propagated through to any constructions that have already been applied.
Secondly, the HVAC system can be improved to increase the efficiency with which heating,
ventilation, etc. are provided. This can be done by altering the HVAC system template in Apache
Systems in the building template manager . For example, the natural gas boiler can be replaced with
an electric heat-pump, which has a higher COP (coefficient of performance). Again, it is not
necessary to reapply the template, as changes will be propagated through.
Other options for the HVAC system that can be modelled in IES-VE include having a CHP (combined
heat and power) boiler or a ventilation heat-recovery system. There is also an option in Apache
Systems to have solar thermal DHW, and there is a Renewables icon in the Apache module of IES-VEthat enables simplified modelling of solar PV, wind turbines, and CHP generators.