Optical Properties

[RomainNouvel]

Optical properties (reflectance, transmittance, emittance, absorptance etc...) are presently spread off in different objects of the Material Module (Construction and Glazing), with different level of details (distinction hemispherical/normalIncidence in Glazing). For more readibility and flexibility, it would be interesting to pick out all optical properties and gather them in a single object "OpticalProperties", possibly assigned to Construction or LayerComponent. In this case, the specification of a "GlazingObject", hybrid object between a layer and material, would not be required anymore. Instead, we could only distinguish Materials between Gas and SolidMaterial. The transparence characteristic would come only from the OpticalProperties...

[JoachimBenner]

The solution sounds reasonable. Only exception is the Glazing attribute "numberOfPanes", which is a structural and not an optical property of a window. I suggest to add "numberOfPanes" to the ADE extension of the CityGML class _Opening.

[RomainNouvel]

Agree! I would also propose:

• an Enumeration "IncidenceType" with the values "Hemispherical" and "Normal", and add the incidence parameter in the dataTypes Reflectance, Absorptance, Emittance and Transmittance. The OpticalProperties Class would be then much simpler.
• combined the WavelengthRangeType "Solar" and "Visible", since their wavelength range is quasi the same (only "Visible"?)
• possibility to assign OpticalProperties also to LayerComponent, in order to model the optical properties of window construction with X% frame (opaque) and Y% glazing.

[JoachimBenner]

I agree with the first two bullet points.

I also agree that different materials combined in a layer might have different optical properties. Optical properties are physical properties of a material like rValue or density. Thus, I would assign the OpticalProperties DataType to the FeatureTypes SolidMaterial and Gas, and not to the DataType LayerComponent. This will allow to instanciate one object e.g. for a specific glas type, which can be used by different LayerComponents.

The FeatureType Gas has an mandatory attribute "thickness". I am sure, we already have talked about this, but why do we need this attribute in addition to the thickness attribute of LayerComponent?

[RomainNouvel]

• concerning the "thickness" issue: the rValue [m2.K/W] of a Material is defined depending on its thickness (contrary to the conductivity). We thought to store it thickness as contextual information, but maybe you're right, this is not required. To be tested in simulation...
• Concerning the OpticalProperties assignment issue: Transmittance properties depends on the thickness of a layer, whereas reflectance and absorptance depends on the layer surface treatment... We can assign OpticalProperties to a low-emissivity 6mm clear glass, or to a triple-glazing, but not to glass in general. A related question is here: do we want to model every single glaze of a window or the glazing as (thermal and optical) system, for urban energy modelling purpose?

[RomainNouvel]

Here a proposition to simplify and generalize the use of OpticalProperties:

• instead of having the attribute transmittance -> 2 attributes: glazingTransmittance (transmittance of the glazing/transparent part of the construction) and glazingRatio (ratio of the glazing surface over the total construction surface, which is somehow also an optical property of the window). Thus, a window would be quickly modelled with a glazingRatio, a visible/solar transmittance (g-Value) and a U-Value. A more detailed modelling would still be possible by using the construction layers.

[RomainNouvel]

Correction to a former statement: even if the "Solar" WavelengthRange mainly include the visible spectrum, a distinction of "Solar" and "Visible" is sometimes required, for instance in the case of glazing with coatings, where visible transmittance and solar transmittance (also called g-Value or Solar Heat Gain Coefficient) is distinguished in every manufacturer data, sometimes different of approx. 10%.

Other questions related to this optical properties issue

• the terms "short-wave" and "long-wave" (radiations) are sometimes used in urban energy modelling. Do they correspond to our "solar" and "infra-red" WavelengthRange type? Do we keep our term?
• presently, we distinguish the optical properties with "normal" and "hemispherical" incidences. However, except in some detailed window manufacturing data where normal transmittance are given, the mostly used optical coefficients relates to the hemispherical incidence. Therefore, should we keep our present type "Incidence"? is it relevant in urban scale energy analyse / modelling?
• Reflectance, Transmittance, Aborptance and Emittance are properties/efficiencies, whereas reflectivity, transmittivity, absorptivity and emittivity are ratio/fraction [0,1] defined and wide-used in litterature. Should we be clearer in the class OpticalProperties and used directly the ratio designation?
• According to the Kirchhoff law, for an arbitrary material at thermodynamic equilibrium, the emissivity is equal to the absorptivity. Therefore, for instance, the "thermal emissivity" would correspond to our {Aborptance / WavelengthRange = infra-red}. Do we skip Emittance / Emissivity from our OpticalProperties then?

[JoachimBenner]

I am not sure I totally understand the usage of "glazingRatio ". Take a LoD2 building, where a complete wall (including windows) shall be reprsented by one construction. In this case, all physical parameters in the Construction (u-value, absorptance, reflectance, emittance and transmittance) need to be weighted mean values between the "transparent" and the "opaque" part of the wall. What is the benefit of splitting the "mean transmittance" into two parts: The transmittance of glazing and the percentage of glacing? Shall the "glazingRatio" also also affect the other optical parameters?

To summarize, I think we have agreed on the following changes of version 0.5.0:

• Elimination of featureType "Glazing", and attaching optical properties to "Construction" and "LayerComponent";
• Concentration of all optical properties of a material in a new data type "OpticalProperties";
• Integration of an enumeration "IncidenceType" (with items "hemispherical" and "normal") into the "Reflectance", "Emittance" and "Transmittance" types;
• Elimination of the "thickness" attribute in featueType "Gas"
• Renaming of "SolidMaterial" into "OpaqueMaterial"

Romain, the issues you mention in your last comment shouls be discassed during the workshop. Will you prepare a corresponding presentation?

What to do with attribute "numberOfPanes", shall it be defined as ADE-attribute of "_Opening"

[RomainNouvel]

Hi Joachim, All windows and openings have a frame and a glazing. In most cases, a window will be a Construction object. While the given Uvalue applies generally to the whole window (frame + glazing), the Transmittance properties apply to only the glazing part. Therefore, the attribute "glazingRatio" aims at defining, the part of a construction which is transparent (specificied with the attribute Transmittance) from generally the frame part (which is opaque, so with Transmittance = 0).

Instead of "glazingRatio", we could refer to a "frameRatio" which = 1 - glazingRatio if it's clearer for you.

• Concerning the "numberOfPanes", I'm actually not sure this brings information useful for the modelling ans simulation. If this is only a descriptive information, can we rather include it in a name / description field of the Construction object?
• Concerning the "incidence" issue, I would like to have the opinion of Jerome Kaempf, which is a specialist in urban-scale radiation exchange.

[JoachimBenner]

No, that was not my point.

The "Construction" object normally will not be related with a CityGML Window object, but with a CityGML WallSurface object. In this case, the "frame" of the window is the complete non-transparent part of the wall.

But, my principal problem is independent from assigning a Construction to a wall or a window. If we have a wall/window with 30% glazing and 70% wall and frame, and the glazing material has a transmittance of x,why not write 0.3*x into the transmittance attribute of OpticalProperties? This would be consistant with the meaning of the other optical (Absorptance, Reflectance, Transmittance) and thermal (U-value) parameters.

[RomainNouvel]

In the case of a wall, either there is no optical properties (optional), or no transmittance, or transmittance = 0, or GlazingRatio = 0. However, GlazingRatio should remain an optional attribute.

Your solution 0.3*x would be ok for the modeller. However, in order to judge/understand the performance of the window (in particular to get the typical g-value), the 3d city model user need to have this frameRatio (or glazingRatio) information. This information data should be "close" from the Transmittance data because it directly relates with its calculation/interpretation.

Would it be preferable to have in OpticalProperties the optional attributes: Absorptance, Reflectance, Transmittance, GlazingRatio, With the 3 three first attributes defined for the whole construction?

[JoachimBenner]

I am still not convinced we really need the glazingRatio. A very detailled model of a window's performance better should be realized by ADE extensions of _Opening. But, I propose to leave the attribute at the moment and discuss the issue at the workshop.

[jkaempf]

Hello guys,

Just a quick note concerning the solar/visible thing.. The visible transmittance (tau_v) takes only into account the visible part of the spectrum AND takes the eye sensitivity curve Vlambda: This metric is used to know the amount of DAYLIGHT that passes through the glazing.

The solar transmittance (tau_e) takes the whole solar spectrum into account: and finally the g-value (or SHGC for Americans) takes in addition to tau_e the heat absorbed by the glazing and transmitted inside the building (Q_i):

For manufacturers (in our cold dominated countries) it is important to have a low U-value (with more panes, rare gas, infrared coating etc.), together with a high tau_e and a high tau_v. The three metrics are important to assess the performance of the glazing, as a yellow tinted glazing may lead to correct tau_e values but impact the tau_v drastically so that the lights have always to be ON inside the rooms.

In a simplified fashion for simulation tools, we generally rely for energy calculations on normal g-values (g_orthogonal to the glazing), and use functions of the type of glass together with the number of panes to compute the g-value as a function of the light impinging angle on the glazing. Sometimes (more rarely), the g-value is mentioned for an altitude of 30° of the impinging light, it is therefore important to specifiy exactly what we mean by the glazing g-value in this Energy ADE. I believe the first one (g_orthogonal) is the most adequate one as widely used.

[RomainNouvel]

After a discussion with Jerome, here is a list of modification that we ask, if possible for the Energy ADE v0.6:

• Removal of Absorptance (which is redundant with Reflectance and Transmittance)
• Change the name Emittance to Emissivity, and delete its attribute WaveLengthRange (Emissivity is defined only for long-wave irradiance (Infrared)