Windows and solar gains

espenlok · Posts: 21

In the output after the simulation, the program lists the two properties "Glazing" and "Solar Gains Exterior Windows".

I wonder if the solar gains are included in the value of "glazing", or if these values should be summed together to find the actual heat gain (or heat loss) through the windows.

I have simultated a buidling in Norway, and I am a little confused because I have a large heat gain through the windows, while I would expect a large heat loss because of the cold climate in Norway. However, the heat gain can (partly) be explained if the solar gains through the windows are included in the value of glazing.

BTW, I had a look at the forum post with the title "solar gains glazing" (your spam-filter prevents URLs to your own site, BTW), but I didn't really understand what was meant there, and if this actually was an answer to my question.

Thank you for all help.

Laurent_Support · Posts: 292 Location: Vincennes

Hello,

You will find exact definition of Glazing/Solar heat gain/loss here : http://www.designbuilder.co.uk/programhelp/simulation_detailed_results.htm

To say it simply, glazing heat gains (losses) are the sum of all conduction through glazing panes and frame. Solar gains are only solar beams entering the zone.
In Norway you should see Glazing heat losses if not it means there are zones colder than outside air!
Please let me know if you can't interpret the problem.

Regards,
Laurent

espenlok · Posts: 21

Thank you for the link to the explanation. I actually tried to search for that page, but I couldn't find it.

From the page, I understand that transmitted solar radiation is excluded from the glazing value.

That means that I have a large heat gain from my windows (in total over the year), which is very strange in the cold climate in Norway. Normally windows should give a large heat loss (as you also commented). I have used very isolated windows to avoid as much heat loss as possible, but still, I would definetly not expect heat gain from the windows.

I will try to experiment a little with other windows (and shading) and see if I can get results that make more sense.

Laurent_Support · Posts: 292 Location: Vincennes

Hi,

Take care of annual total. To ensure your model is correct try a simulation hourly step for a winter week and summer week. At this detail, it could show you if there is or not a problem to solve before starting annual calculation.

Good calculation!
Laurent

Andy Tindale · Posts: 2432 Location: Stroud

Please bear in mind that the Window Heat gain data also includes longwave solar gain (i.e. radiation from the warm inner window pane). You will notice that, for example, triple glazed units can have high 'window heat gains' and quite small 'Solar gains exterior windows'. My understanding is that this is because much of the total solar incident on the outer glazing surface is transformed into longwave radiation during its passage through the glazing unit.

Andy

espenlok · Posts: 21

Thank you you for the replies. I have one more question regarding this issue.

Is it possible to find out how much of the "glazing" losses/gains that are caused by transmission losses, how much is caused by longwave radiation effects and how mugh is caused by other effects?

Best regards,
Espen L�ken

Laurent_Support · Posts: 292 Location: Vincennes

Hello Espen,

DesignBuilder can't display this information but I found some data available in EnergyPlus you could ask. Tod o this you have to add a external idf file to your simulations in advanced options of the Simulation dialog and view output data with xESOView software (free download from E+ site).

In E+ documentation you will find in chapter 'Group - Thermal Zone Description/Geometry' -> 'Window Output Variables' interesting data (I think) :
Window Solar Absorbed: All Glass Layers [W]

I hope this could lead you to a best understanding.
Regards,
Laurent

espenlok · Posts: 21

Thank you for your help. However, I was not able to do as you proposed. When trying to add an external idf-file, DesignBuilder asks for the name of the idf-file I want to add, and that I don't know.

I had a look into the E+ documentaion, and I agree that the data could be interesting to have a look at.

However, I still find it extremely strange that I - over the year - have heat gains through the windows in the Norwegian climate. As said by Laurent earlier, that means that "there are zones colder than outside air!". And that is actually just the case a few hours per year. Maybe someone will have a quick look at the attached file to see if they can find an explanation. In the attached file, I just made a small rectangular box with the same windows as in my original building.

Laurent_Support · Posts: 292 Location: Vincennes

Hi,

This is a very interesting subject. I try a simulation with your model. As Andy wrote before it seems that your glazing are heated by solar beams and bring heat in the zone even when outside air is very low.

I attach a picture of a winter week when you can see the exact correlation of glazing heat gains and solar gains.

The glazing you use is very well treated (LoE x2) and provokes this glaze pane heating. Is it possible your pane's properties (thermal, visible, infrared) are oversized ?
Do you try Window5 program ? It is freely downloadable and allows to compute glazing systems properties in order to ensure your DesignBuilder simulation.
It's only an idea to cross points of view and discover what could be (or not) wrong.

Good luck !
Laurent

espenlok · Posts: 21

Hi again!

Thank you for all your responses.

I have now studied the values from the design winter week in more detail. Based on this, there seems to be a difference between the help file (which Laurent linked to earlier) and the program. According to the help file the Glazing heat gain is �the total heat flow to the zone from the glazing, frame and divider of exterior glazing excluding transmitted short-wave solar radiation�.

However, when looking into the output data, it seems clear that there is a clear correlation between the �Glazing� value and the �Solar Gains Exterior Windows� value. As far as I can understand, this means that the �Glazing� value shows �the total heat flow to the zone from the glazing, frame and divider of exterior glazing including transmitted short-wave solar radiation�.

Do you agree on that, or do you have another explanation? If my interpretation, will that make any mistakes in the rest of the calculations in DesignBuilder?

I am aware that �Glazing� also includes �longwave solar gain (i.e. radiation from the warm inner window pane)�. However, the inside of the window will always be colder than the room temperature when it is so cold outside (except if there is electrical heating in the windows), so this cannot be the explanation.

If we now accept that �Glazing� do include transmitted short-wave solar radiation, there is still another problem. The �Glazing� value when it is sunny is a little higher than �Solar Gains Exterior Windows�, while I would expect it to be a little lower. Actually, I would think that �Glazing when it is sunny� should be about equal to �Solar Gains Exterior Windows� minus �Glazing when it is not sunny�.

Why is that? Again, I am aware that �Glazing� also includes �longwave solar gain�. However, I have checked my windows in Window5 (a very nice and useful program BTW), and there is no way that this longwave solar gain can give a positive contribution to the energy calculations (on the contrary, it will give a tiny negative contribution).

To your other question; yes, I am aware that I have chosen a window with good thermal properties. However, in the case we are trying to model, the project owner wants to choose very energy-efficient solutions, so then we do need to use a good window. (And to be honest, the window is just a little better than the new Norwegian building regulations). However, even if the window is over-dimensioned, it should be able to find an explanation for the calculations in DesignBuilder Smile

.

espenlok · Posts: 21

Hi again!

I am still wondering about this issue (and my last message was not answered).

1. Do you agree that the graphs I attached to my last message show that solar gains is included in the Glazing calculations? (According to the help system, it is not included).

2. If you do agree that it is included; is it possible that this can give some kind of double counting of the heat gain (This is just a proposal, it may wery well do not be so. However, the solar gains in DesignBuilder is extremley much higher than in the other modelling software which we are using. Therefore I am wondering if it is possible that it may be caused by double counting. Another possible reason is that the other software is not very good in treating the solar gains).

3. How is it possible that the �Glazing� value when it is sunny is a little higher than �Solar Gains Exterior Windows�? I would expect it to be a little lower. Actually, I would think that �Glazing when it is sunny� should be about equal to �Solar Gains Exterior Windows� minus �Glazing when it is not sunny�. When I am using windows without low-e, this problem does not exist. (See also my comment about longwave solar gains in my previous message).

I hope this message clarified what I tried to ask in my last message.

Espen

espenlok · Posts: 21

One additional question:

I made a experiment to check how the solar gains were influenced by shading, and this experiment gave some strange results. I have a rectangular building with window only on the southern side. This gives as expected a large solar gain.

Then I put a wall in front of the southern wall of my building (some meters away). The solar gain was then reduced, but less than I expected.

I was curious, and therefore I put the wall directly in front of the southern wall, so the entire window was covered with the wall. This reduced the solar gain, but still there was a relatively large solar gain (about 20 % of the solar gain without the wall).

My question is then: Is it relly possible that short-wave solar radiation is transmitted through a thick wall? Or is a mistake in the calculations?

I am aware that to put up such a wall is a very strange thing do do, and is not relevant in the real world, but I did it just to control the calculations of solar gains. As I have mentioned earlier, compared to other energy simulation models, the solar gain in DB is very large, and I try to find the explanation. As it is now, the difference is so large that we don't trust the results from DB, which is a big problem, of course.

I attached my model of the rectangular test building if you want to see. I did not attach the model without the wall, as it easily can be removed, for instance by using simulation options and remove the mark on "include all buildings in shading calculations".

Andy Tindale · Posts: 2432 Location: Stroud

With regard to the Glazing gain problem, this is now fixed in v.1.4.0.034 released yesterday. Previously DB was not subtracting short-wave solar transmitted radiation from the Glazing gain before displaying it(though documentation said it did). Please note this is just a reporting issue, it does affect the rest of the simulation.

Please try again with the new version.

Andy Tindale · Posts: 2432 Location: Stroud

Your model with the shading problem does not seem to be posted - could you try sending it again or else email it to andy@...

Thanks,

Andy

espenlok · Posts: 21

Sorry, I had some problems with your ftp-server (strange since it has been working earlier). I will send the file by e-mail.

Thank you for fixing the problem, by the way. Actually I had noticed that the window gain was changed in the version, but I wanted to be sure that this mistake didn't influence the calculations in any way.

Espen

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