Ground/Water reflectance

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Hello everyone,

I've got a building that is located at the edge of a harbor with the water directly to the south of it. How do I model a reflective surface on the ground that represents the harbor? I feel this is key to include, as this reflectivity will send additional heat through the windows. I looked through the archives but didn't see anything related to ground reflectivity.

If anyone knows how to create reflective objects and/or an appropriate value of reflectivity for water in eQUEST, that would be most helpful.

Thanks in advance!

Regards,
Alex Krickx

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Alex:
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I wonder if the turbidity (word ??) of the water will actually not make a
difference. Any harbor will have boats, wind effects, and an occasional sea
monster disturbing the "glass" water.
?
Only in paintings ???
?
John R. Aulbach, PE, CEM

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Alex,

Exterior surfaces have a "Ground reflectance" property -
GND-REFLECTANCE, accessible through the Daylighting - Shading - Other
tab of the exterior surface. Reading the DOE-2 Help description, the
reflectance value for "ocean" is 0.05, implying that most solar
radiation is absorbed, not reflected. I'm guessing that the visual glare
we perceive does not increase the solar load as much as we would think.
Sea monsters do not seem to have been considered by the DOE-2 team.

Regards,
Bill

William Bishop, PE, BEMP, LEED(r) AP

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Thanks Bill and John for your responses.

I guess I would expect that the ocean would be a bit more "turbid" (seems appropriate) - but what about for buildings facing a lake or harbor where the water is calmer? Perhaps the additional reflected solar gain is negligible, but my gut tells me that is not the case.

This project is a retrofit and they talked of having intense heat along the facades facing the water. Perhaps it's more due to the tinted single-pane glass that gets hot (?) but my initial thought was the water is the culprit.

Is it possible to create a 2d shape in the ground plane south of the building, and apply the reflectance to that, or do I need to apply it to the whole ground?

I looked through a previous project and didn't see the term "GND-REFLECTANCE" anywhere in the inp or pd2 files. Is the default to have a ground-reflectance of 0? If so, I guess I'll run one case with 0, and one with 0.05... although I doubt there will be much difference between the two.

Thanks again - any other thoughts on this are much appreciated.

Kind regards,
Alex Krickx

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Found under EXTERIOR-WALK Command.

Play with this.

John

?
GND-REFLECTANCE
Defines the solar reflectance of the ground in the vicinity of the
EXTERIOR-WALL.

Table 6? Typical Ground Reflectance Values
Surface GND-REFLECTANCE
Asphalt (Paved) 0.18
Concrete (Bituminous) 0.10
Concrete (Light-Colored) 0.32
Concrete (Old) 0.22
Field (Green) 0.12 - 0.25
Field (Wheat) 0.07
Grass (Dry) 0.24
Ocean 0.05
Rock (Crushed) Surface 0.20
Soil (Dark) 0.08

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The albedo of an ocean should probably equal that of a lake on an average basis throughout the day (not a peak hourly basis). The radiation reflected by water is very low when the incident wave is perpendicular to the surface of the water, and it approaches 100% as the angle of incidence approaches 90 degrees (morning and evening). As the waves move through the water the incident angle of each zone of water will vary from 0-90 degrees on a constant basis and the reflectance should roughly average out to equal that of a still body of water.
For a lake, in the late afternoon, the reflectance will increase from 0.05 to 0.1 or 0.2. Later in the evening it will reach closer to 0.8 or 0.9, but at this time the radiation intensity from the sun drops off significantly.
To model the lake on one side is a challenge, your peak loads may not change but your annual consumption may. If eQuest uses the gnd reflectance values as a multiplier of solar radiation, it may be worth running a few iterations with different values of gnd reflectance to see the results.

Joe Fleming

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Alex,
The default ground reflectance is 0.20. You have to enter a different
value for it to appear in the inp file.
Bill

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So changing it to 0.05 to try to account for the reflectance of the water, would actually decrease the reflected solar heat...

Hmmm. I'll play with some different values in eQUEST and see how much ground reflectance impacts consumption, and keep in mind the 0.05 value.

Thanks for everyone's quick and thoughtful responses.

Cheers,
Alex

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Hey Alex,

I hate to muddy the waters further, but I'm pretty sure the visible light spectrum (a small band of the sun's energy) does not behave identically to the rest of the electromagnetic spectrum regarding how much energy is reflected for a given incidence angle on water...

To voice my concern another way: sunlight bounced from a lake in the morning/evening may well effectively double the amount of visible light energy incident on a building fa?ade for a certain hour, but this would likely not result in a corresponding doubled thermal solar load - energy at frequencies above/below the visible light spectrum may be more prone to simply dive into the lake...

I understand the term 'albedo' in the context of collected weather data / energy modeling generally refers to the full spectrum of the sun's energy (light inclusive), and you might trip yourself up by assuming terrestrial surface albedo is the same thing as surface reflectivity, for light.

(good catch-up reading with examples: http://en.wikipedia.org/wiki/Albedo)

Water bodies do have a really low albedo - not because they do not reflect light, but mostly because they suck up the rest of the electromagnetic spectrum really well. If on the other hand your lake freezes and is covered with snow during the winter, I'd expect the sun's full spectrum of energy to bounce much more effectively to your cozy lakehouse =).

PS: I am no astro-physicist, nor am I a meteorologist... I'm only throwing out some cautions/thoughts from what I've picked up here and there on the topic...

~Nick

NICK CATON, E.I.T.

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