calculating solar gain, windows?

jp_2

to do this accurately, it seems you need to know house orientation, sun angles, sun power and all that stuff???

I can see using tables for estimating yearly/monthly, but what about on a daily basis?

anyone do this?

hydronicsmike

I dont know....

...how to figure how much Energy is claimed by Solar Gain, but I know how to deal with it. Indoor Feedback --> tN4. visit www,tekmarcontrols.com for more info. Call me if I can help you in any way. 250-545-7749, Ext. 214

Best Regards,

Mike

GMcD

Solar gain

The ASHRAE Fundamentals Handbook has a complete chapter on it. Basically to find the peak solar gain through a window, you have to determine your local latitude, then find the sun azimuth at the peak solar incidence angle relative to the windows and then obtain the "Btuh/SF" solar gain number from a vast table in the ASHRAE Handbook and do this calculation:

Window area x shading coefficient x Btuh/SF x 1.17 = Solar Gain into space in Btuh.

The Shading Coefficient is what the window specifications will (should) have. You also have to factor in any external shading and calculate the heat gain through the shaded and the un-shaded portion to get the true total. The "1.17" number is a factor to include the window frame heat gain since there will be some thermal bridging around the window frame. That "1.17" number may be greater or smaller depending on how well or how badly the thermal break in the window frame is done.

Many of the "canned" HVAC load calculation programs will do this based on a standard climate data file that would apply to your specific location. Some of the higher end programs will be able to calculate solar gain through any exposure on an hourly basis. There is no "simple way" to do this, other than via some good software.

Some links:

http://www.srv.net/opt/sunchrt.html

http://www.eere.energy.gov/consumerinfo/factsheets/passive_solar.html

http://www.kahl.net/solarch/sareadme.htm

http://www.ibpsa.org/proceedings/bs95/papers/BS95_653_660.pdf

http://windows.lbl.gov/software/resfen/

http://windows.lbl.gov/software/window/window.html

http://www.energycodes.gov/support/shgc_faq.stm

http://www.eere.energy.gov/buildings/tools_directory/subjects.cfm/pagename=subjects/pagename_menu=other_applications/pagename_submenu=solar_climate_analysis

That should give you a bit of a start!

Constantin

I don't know that it can be done...

... insolation is variable even on a daily basis unless you live in areas near the equator that have no cloud cover 100% of the year. Granted, you can account for the shifting seasons, but I'm not aware of many programs that can achieve the level of sophistication your're describing - yet.

Having said that, HVAC-Calc (and presumably other heat gain programs as well) does take home orientation, etc. into account to calculate the peak load. That is a relatively simple process since ASHRAE and Manual-J give you most of that info. Contractors in turn go and size systems to meet peak demand, whether for heating or cooling applications.

jp_2

good sites

lots of interesting sites geoff!

hydronic mike, if you can not figure out the extent of solar gain I may not want you to sell me a system that controls it! just joking, this is just a general question I'm asking.

I think if you know the angle of the sun with relation to the window and you know approximate watts/sq meter for that month in your area you can get a real good idea of solar gain.

Dale Pickard

Geoff knows of what he writes

There is no good simple methods though there are some rules of thumb for passive solar heating design.

The ASHRAE method that he describes is a good example of an educated engineering guess to determine numbers good enough to provide design information for cooling systems.

The hourly method is the way to go and though it's complicated, it's the type of engineering problem for which an a more of less exact solution can be calculated, at least in terms of gains available to a given plane. All of this data is available for many climates.

However, determining how those solar gains may affect a building's lighting, heating or cooling or electrical loads on an annual basis is another thing. Among the more important concepts is that of "utilizability". Typically, only a small fraction of the energy available is useful.
How the energy available at the collection area is made available to; or interacts with the load over time, temperature or voltage, has a lot to do with the utilizability of the energy that one has endeavored to collect.
A passsive system may collect a great deal of energy at a 80% efficiency but if overheats the building beyond the instantaneous heating load, (balance point), and cannot be stored; it will be ventilated away or represent a cooling load. So the utilizability may be very low. It may even cost you to get rid of excess energy that could not be made useful, ie. ventilation fans.

blah, blah, blah

Dale

Boaz

Energy-10

We use Energy-10 to assess passive solar attributes of a proposed or existing building. This program can be massaged, if one knows how, to provide a range of useful data. For example, have you ever wondered what the "effective R value" of Rastra has to do with actual thermal performance? Instead of using manufacturer's data, we actually "build" Rastra blocks in the software to see how it will really perform under the specific conditions proposed by the architectural plan. This program uses real weather data to model thermal performance hour-by-hour over the course of a year. Sometimes we have to create the weather data from a similar file based on our first-hand knowledge. For example, we may have Albuquerque weather data and need White Rock, NM, which we know is typically X degrees colder and get Y more snow each year. As always, the old adage of "garbage in, garbage out" holds true, so knowing how to get the most accurate data out of the program is essential.

jeff_51

manual J

Deal with it all the time on forced air and ac. You have to use the proper chart for your location and know the orientation of the windows, but this has been used and perfected over many years

jerry scharf_2

Not sure what answer you want

Are you looking to answer "how much heat gain I will get in my house today" or "how much I will get in a theoretical building on the 7th of January 2006?" As with everying else, the later is almost impossible.

Computing the gain for today is much more tractable, and here is how I plan to do it. First I compute the solar track through the sky (lots of free software that will do that), and compute the gain as a geometric function of the projected surface area of the window (the size of the window projected onto a theoretical sphere centered on the sun.) There is a bit of fudge for atmospheric loss, but that's smaller. The next thing you need to do is to calculate shading. One part of this is the local opject shading, such as walls, overhangs, and trees. This is done with simple projected object coverage. (That dip into computer graphics can pay off.) The other part of this the shading is cloud cover. One way to take on cloud cover is to build a tracker tube photosensor and an ambient sensor and look at hte differnetial. Another way is to collect NWS avaiation weather information on cloud cover for local airports. I plan to start with the later and then later see how it compares to local measurements.

enjoy,
jerry

jp_2

jerry ??

""" and compute the gain as a geometric function of the projected surface area of the window (the size of the window projected onto a theoretical sphere centered on the sun.)."""

jerry since the sun is so far away, I think you only have to project the apparent window size that is perpendicular to the sun angle. then use given watts/sq meter.
reason I figure you do not have to think about using spheres is the small size of windows compared to earth radius or sun's radius.

i think of a flashlight shining on a piece of paper, if its perpendicular you get a circle, if the papers at an angle you get a elipse. the flashlight puts out the same power but, now the power per unit area is different. the elipse of course will have more area than the circle, thus the elipse would have lower watts/sqmeter say.

I was curious if anyone calculates close estimates, better than 'rule of thumb' stuff. if you are dealing with new construction and big south/east windows you might want to estimate loading on theoretic cold sunny day, then plan piping from that.

i don't see where outdoor reset can solve this by itself.

jerry scharf_2

we're saying the same thing

The way you described the projection and the way I did look different but turn out to be the same. I was trained as a physicist, so my brain works the way I described it.

You are completely correct that ourdoor reset doesn't solve this. This is the soapbox I continue to stand on, that outsoor reset can't adress things like this or room occupancy issues, so you still need some kind of temperature sensor in the room and htat has to continue to be the "primary control" for heat into the room.

You are starting to see more and more talk about "indoor reset", which makes more sense. It still is playing "behind the curve", but it's better.

My experiments are about seeing how well I can be "in front of the curve" with the radiant controls. Maximum stability and minimum room overshoot are often a hard mix.

jerry


jerry