Skylight load 3

[krismatt]

I am designing an open space showroom with a central atrium covered by a skylight. The climate is hot, so cooling is the biggest issue.The skylight has two sloping sides over two straight sides,ie the cross section is a triangle over a rectangle. The total area is 216sq m and it sits over a rough opening of 150 sq m.
I followed ASHRAE Fundamentals Chapt 29,and used certain assumptions,calculated the U factor for the skylight (4.4W/sqm K). My questions are : 1.Is there some simple formula which I can use to calculate the solar heat gain?
2. Given the skylight is over a central atrium common to all floors(roof, 2nd,1st,ground floor), and that there will be cooling in summer,would one apportion the heat gain from the skylight all over the floors, or on the upper floor only?
Any links where I can find designer info (technical)?

I would appreciate some guidelines please.
Thanks

 

[gepman]

1. I would use HAP or eQUEST to try to determine the solar heat gain since it will be large. The angle of the sun makes a big difference on solar heat gain. Hopefully you have good glass properties.

2. If all the floors are open with a relatively large area to the atrium then you should distribute the heat load to all floors. eQUEST has a model for this. If the floors are closed to the atrium then the heat load should remain in the atrium and to the first floor (I assume it is open to that floor).

You do not have a trivial task.

 

[ChrisConley]

If you have old copies of ASHRAE Fundamentals (1980 and earlier) they will give you a method called CLTD for working out cooling load.

Basically you use a table to get a peak solar load for your location (based on exposure of the window). Work that with your glass shading coefficient and you get a load.

ASHRAE dumped the method as being inaccurate (no accounting for thermal mass effect) a long time ago and now advocates Radiant Time methods.

These methods are computation intensive and are best done with a computer modelling software.

 

[krismatt]

Thanks gepman and Chrisconley for yr replies.It s good to know that my worries are justified.gepman:The floor area is 1820 sq m and the atrium is open to all 4 floors but the opening is only 150 sq m,would you still apportion the skylight load to all floors?

 

[gepman]

It will actually depend on your airflow. I don't know if you have separate systems for each floor or what but if you set up your pressures and return and supply air so that any required makeup for that floor was lost to the atrium the heat load for the atrium would not be counted towards that floors heat load since the airflow would be in the other direction (of course you will have the makeup air load from ambient but you would have that anyway).

You have a tricky situation since your skylight geometry is difficult and this could be complicated somewhat by the building orientation (i.e. building sides not parallel to north south east west).

If you don't have good glass information from the skylight manufacturer you could try to model the skylight in Window5 which is available at

http://windows.lbl.gov/software/window/window.html#Optics 5.1 Update

There is also a lot of other design tools available there for daylighting so it might have something else to help you.

 

[krismatt]

The skylight manufacturer has not been appointed yet and most probably it will end up being some structure erected on situ.The architects are not being helpful there.
I will be air conditioning each floor separately and my intention was to extract the air of the upper two floors from the atrium, I am not in favour of extracting all the air of the building envelope from the atrium as fans would be extra large and I might end up short circuiting all the air conditioned air of the top floor.
I have downloaded eQuest but the program is USA oriented.So not of much use to my project.But thanks anyway for the tip.

 

[marcoh]

Its a difficult question to answer quickly and easily.

Solar radiation enters the building through the skylight and the heat gain need to be allowed for where the solar radiation hits the actual building structure, ie in the middle of the day directly below the skylight, potentially on upper floors as the sun moves.

Remember that the sun heats the building which in turn heats the air, the sun does not directly heat the air (else the atmosphere would absorb all heat from the sun and it would always be freezing!)

Also skylight needs the lowest possible SC, U value has a much lower effect on a skylight heat gain.

 

[hvacinsider]

Here is what we use for horizontal glass(skylights) and we never have a problem with our loads.

4.2 cfm per sq. ft. of glass.

 

[krismatt]

Thank you hvacinsider.I'll use it as a rule of thumb to check my load. So if I get you right you work out the load on the principle of infiltration load only, right?

 

[hvacinsider]

The 4.2 cfm per sq. ft. of glass is for the solar load only. All other additional loads will have to be added too.

Is this what you were asking krismatt?

 

[krismatt]

Hvacinsider,let me re word my question:You said 4.2 cfm per sq ft of glass. I take it that cfm is the usual cubic ft per min.It seems that you convert the sky light load to a ventilation load. Is that what you do? In figures,if I have say 100 sq ft of glass, then I would end up with 420cfm, and to get a cooling load I would have to apply the same formula as for the ventilation load (1.2 x flow rate x enthalpy change in si units). Can you confirm please?

 

[hvacinsider]

The 4.2 cfm per sq. ft is for the glass load(solar) only. Any people, equipment, or other heating giving equip. must be factored in also.

 

[SAK9]

Kris,

I would suggest use of displacement ventilation principles to solve this ie supplying air at the ground floor and upper floor corridors and extracting it along the perphery of the skylight at the roof level.This would allow you to apportion atleast 50% of solar heat gain to the return air stream reducing your dehumidified air flows by half.Using a single air handler for the ground floor of the atrium and the upper floor corridors open to the atrium will be the most practical solution.