Azimuth, Solar Elevation, And Solar Irradiance

What you need is the angle between the perpendicular to the panel and the sun. Given that angle — which, of course, changes with the time of day and the season — then the effective area of the panel is equal the the gross area time the cosine of that angle (not the cosine squared). Using that effective area you can calculate the gross solar radiation.

A useful reference for the math for the angle to the sun is here;

zenith-angle-calculation-Guoyuan-Li.pdf

Now.. That is not.. Repeat NOT related the the efficiency of a solar collector. The efficiency of the collector is, simply, the usable power output from the collecter divided by the net effective area of the panel and the instantaneous solar power — which, of course, varies with atmospheric condition. This efficiency varies with the design of the collector, and may — or may not — also be influenced by the angle between the perpendicular and the direction to the sun. This may be closely related to the cosine of that angle, in the case of collectors with flat glass surfaces or some (but not all) types of photovoltaic arrays. Or it may not…

you need a solar simulation software to make it quick and easy to do those calculations and comparisons.

RET Screen out if Canada had a powerful, free program with solar calcs

Possibly some freebie at the NREL site also

Hi @desert_sasquatch , I likely am that guy making coils 😉 Here's an article in the help center: https://www.heatinghelp.com/systems-help-center/another-solar-myth-bites-the-dust/ More recently, I used with this concept to help a local high school win a sustainability competition. https://www.pbs.org/newshour/brief/518687/rancho-cielo We built a coil using 3/4" NSF rated black poly pipe about ten feet in diameter, and used it as a shade structure over a door. Works!

Yours, Larry

@hot_rod "It seems like if you have the ability to store the energy in some thermal tank, or thermal mass, you would want to build the most, not least efficient collector?"

On site thermal storage for more than about a day rarely pencils out.

The graphs you post show an important limitation of solar collectors — the colder it is outside, the lower the efficiency. So in the summer:

• The days are longer, so you get more sunlight
• You need less heating
• The air is warmer, so collectors work more efficiently

I ran the numbers years ago for my home, Washington, DC. Even if I angle the collector to maximize collection on the shortest days of the year, I still would collect three times as much heat per day in June as in December. In June my only heating load is domestic hot water. January is my coldest month, and in January my heating load is over eight times my hot water load.

If I size a collector to provide 100% of my load on a summer day, on a January day it provides 3% of the load. If i size it to provide 100% of the load on a January day, on a July day it's 35 times oversized.

Do the math for your climate, in my climate there's no way around it.

I tend to agree, @Larry Weingarten — and I also agree with the Martin Holliday quote you mentioned. Not, however for any inherent problem, but because utillising solar for domestic space heating or cooling (and yes, properly designed in some climates but not all, essentially passive solar systems can be used for space cooling) requires adapting to the specific site and climate, and that is just too much trouble for the architects and building owners. Further, there is a rather strong fixation on having some sort of collector with some sort of complicated plumbing and tanks or it isn't solar — and that isn't true either.

(Fun fact: the Persians and Nabateans and Israelites were cooling their civic buildings and palaces and some houses and were making ice to cool their beverages… 2500 years ago… with passive structures…) (Have you ever visited some of the cliff dwellings in the southwest, or Chaco Canyon?)

@Jamie Hall : "Have you ever visited some of the cliff dwellings in the southwest?"

Last year I visited Taos Pueblo. It was February, a pretty raw day, not super cold but mid-30's, sunless and a stiff breeze. Just about every building in the place was downright miserable to be in. The ones without fires were bone-chilling, even the ones with fires were cold and smoky. The only building in the place that was bearable was the church, which had a propane heater.

Yes — the cooling is dependent on the dew point. Having said that, I remember one summer — I think it was July — camping in southwest New Mexico… and ice on the water buckets in the morning!

I haven't been at Taos in years, but I do vaguely remember it as being rather dank. Acoma doesn't seem to be — but Acoma is a lot lower in elevation, and has NO shade!

Hi, About longevity, I used coils after this type of system had been under test by the maker for sixteen years, and undergone multiple freezes along with good sun in San Jose, CA. suffering no apparent damage. This is NSF rated black poly tube which is thicker wall than some of the irrigation tubing. Also, I've read that if you can find tubing with 2% carbon black in it, the sun doesn't degrade it an any appreciable rate. The system described in the article has now been in service about 11 years and is doing fine, except some of the glazing got blown off once. One more tidbit is that I've never seen black poly tubing that was brittle and fractured, while I have had to replace UV damaged PEX. I haven't tested coils under a hard freeze, or subjected them to temps over 170F. So, aside from extreme conditions, it seems pretty durable stuff.

Yours, Larry