However data is set up to define the situation you wish to model, you will be providing any thermal simulation model with a broadly similar set of data. It is sensible to consider all aspects which define the situation you wish to model to enable its correct transfer into numbers which the model will understand. This will also help you to reduce and detect errors in your set up. Many models will use default values for some of the data and not prompt you for its input. These defaults may or may not be changeable but it is important to be aware of the types of data required for thermal simulation in order to ascertain the usefulness of a given model for your purpose as well as in ensuring the appropriateness of the data used.

We may classify data as defining:

• The external environment
• The physical building being investigated
• The internal environment of the building

1.1 External Environment

The external environment is described by

• Climate
• Location
• Physical features (surrounding buildings/landscape)

1.1.1 Climate

At its most basic level, a model will require the following types of data for thermal simulation:

Air thermal status: Air Temperature (ambient-in the shade) plus a measure of humidity, probably the relative or specific humidity.

Solar Radiation: At least one value of solar radiation and most likely in the form of an irradiance (solar power). If only one value is required this will be the global or beam value, where global is the total radiation received by a surface (horizontal) and beam is the total radiation arriving in the direction of the beam. Most simulation programs will however need to identify separately the direct (sun) and indirect (sky/diffuse) components and will thus require any of the following combinations.

• Global + Direct
• Global + Diffuse
• Direct + Diffuse
• Beam + Diffuse

Only two values are necessary as the third may be easily calculated from the other two.

In most models the climate data will be contained within a file and the user needs only select a close location. Air Temperature and Solar Radiation vary continuously and thus the data will need to be provided in a semi-continuous form. Usually this will be by providing a value representative of a period. For example the period 0930 to 1030 may be represented by a mean value for the period and be assigned as representing the value at 10am.

This data will need to be supplied for the whole period of the simulation. The user may wish to select many different periods for examination, for example different seasons and thus the data is usually provided for a complete year and the user simply selects the required period.

Wind Speed and Direction may be additional parameters entered via a climate file. These are used to determine pressure loadings on the building and thence ventilation rates. Accurate modelling of ventilation is not in general achieved with great accuracy by these models. Much of this being due to the accuracy of the input data. Measurements of wind at a local weather station may not reflect the local flows, particularly in a built up site. Much ventilation modelling is still done using physical models in a wind chamber.

1.1.2 Location

Data about the buildings location will almost certainly be necessary to derive the orientation of building surfaces with the sun. This being necessary to resolve the radiation into horizontal and vertical components and also for deriving by how much each surface is shaded from the direct radiation. The model may contain data about particular locations, specifically the global position (latitude, longitude) and a time reference (time zone or longitude on which the local time is based (time is discussed further, below).

1.1.3 Physical features

Details of the geometry of the external environment are needed in particular for deriving further shading, the effect of which will be to reduce the radiation components on the various surfaces. The shading effect to diffuse radiation will be fixed for each surface (the sky does not move). The shading of direct radiation will vary dynamically.

Where external building geometry is not part of the data input, there may be other ways in which the shading effect may be input. This may involve the pre-processing (ie modifying the data prior to input to the model) of climate data.

If the reflectance of building and ground surfaces is input then the additional solar radiation striking the building by reflection off these surfaces may be determined. Ground Surface reflectance is most significant where winter snow cover is common.

A note about time:
Discrepencies in the time basis between the different inputs of a model is a common problem which may be very difficult to resolve. In general, this will not cause a very significant error in calculations. The problem arises in that a number of time definitions are used, specifically in solar geometry calculations. Solar Time is defined by reference to the position of the sun relative to the longitude of the site. In Solar Time, the sun is at its maximum height (altitude) at solar noon. Any shift in longitude shifts the basis for solar time. Solar Noon in Liverpool (Longitude 3^oWest) occurs 8 minutes after Solar Noon in Portsmouth (1^oWest) though they use the same clock time. The same Clock Time may be used over a wide range of longitudes and could be up to a couple of hours out of phase with Solar Time. Daylight Saving (local time defined differently in Summer and Winter) adds a further complication, which may require a complete respecification of the times of use within the building when winter and summer are modelled.
For more detail on this subject, take a look at its treatment in the Solar Geometry tutorial.

1.2 Physical Building

The physical building is described by

• Material construction
• Geometry

1.2.1 Material Construction

A dynamic model will not expect U value as an input (though it may calculate and use it). U value is a parameter which is used for steady-state modelling/calculations. Calculations with U value are based on fixed internal and external temperatures. In reality of course this does not occur. In addition, conditions on internal and external surfaces vary throughout the day, in particular solar radiation will be absorbed by surfaces, causing the surface to heat up and transfer that heat through the building fabric. Dynamic modelling requires that this dynamic heating activity is represented and requires appropriate data. Specifically for each material used in the fabric, its thermal conductivity, density and specific heat capacity are needed. Description of the construction will require material thickness and position relative to the other materials. Thin, non thermal layers will not need inclusion (eg Breather Membranes) except where they adjoin air (air gap or surface layer), where there surface properties will be important. Each external/internal surface will require information representing its ability to receive and reject heat. This may be by defining heat transfer coefficients, or surface resistances. Convective heat transfer is usually represented in this way, whereas radiative heat transfer will probably require the addition of emissivity (relating to the emission of heat) and absorptance (relating to the absorption of heat). All these terms are defined and explained in the Physics presentation on Heat Flows and more detail on material data definition is in the separate page on Material Data.

1.2.2 Geometry

Many modern simulation tools incorporate a graphical means of entering the 3 dimensional geometrical data or allow input from other visual CAD tools. Not all aspects of this geometry will necessarily be understood by the software. The only information which may be taken into the calculation engine could be orientation and area of each surface and whether or not it is exposed to the outside. If shading is determined by the software it will have a more complete understanding of the external geometry but may still not use the internal details in a particularly meaningful way.

1.3 Internal Environment

Here we define the thermal activities occurring within the building according to:

• Adding/Removing
• Where
• Time
• Type
• Rate

For example, each person working within a building may be described as:

• adding heat to
• the office in which they work
• during their working hours
• giving off a specific mix of radiant, convective and latent heat
• at a level corresponding to their activity (siting at a desk)

Similar information will be required to describe heating and cooling equipment, office equipment and lighting.

1.4 Additional Data

Additional data will be entered to describe specific aspects of the modelling scenario. These may include the particular days you wish to model and any specific investigations. These will be dealt with later.

Introduction

Input Data

Material Data

Use Of Data

Calculations

Run Scheduling

Output Analysis

Other Modelling