Solar Collection Question

Mike T., Swampeast MO

Completely agree that you "can't count on the sun" other than it will rise tomorrow...

Six years of insolation and temperature data should give me a reasonable "average" when it comes to:

1) Determining days that are candidates (by average daytime temperature) for solar heating and;

2) Determining how many of those days (on average) can be supplied via solar.

These are the reasons I'm looking at least daily (if not hourly) averages. Will though say that the numbers are confusing to say the least.

Most amazing to me is that I find NOTHING regarding my original question. With solar energy measured FLAT ON THE GROUND how do I translate that to a tilted collector? Surely it's somehow related to the shadow of the collector...

49%-51% of the sizing is going to wind up based on my personal observations and experience and I won't even touch on the 2.5 sq. meter aperture area of the 2 meter rated panel...

The fortunate thing is that no matter how I play with the numbers I wind up with a system that will nearly eliminate daytime "pulsing" of the boiler with 4 square meters (rated) of panel.

Mike T., Swampeast MO

Only real difference I see here is that you might not have TRVs or similar to provide a real-time load to the boiler.

Solarstar

As to the answer to your origional question I'm pretty sure its calculated at perpendiccular to the sun. since that is Max BTU's at full sun/exposure. Correct me if I'm wrong.!!!

dr al

I agree

I agree, max performance.

Uni R_3

Maybe it's too simple...

If you now believe they level the measuring devices, why not use simple trig? You know the angles.

Ed_26

collector performance

The ability of a solar collector to capture radiation varies a small amount with the angle the sun strikes it.
Called the "incident angle modifier". Measured from 0 to 90* vert. & horiz. across the face. the values are reported as a % of expected performance.
For flat plate, always reps a decrease. For ETC, often can be a small increase.
Best performance is measured @ right angle to the sun.

sg_4

re insolation data

ISTR that the book 'from the ground up' had all the data needed, but I havent looked at it in a while

jp_2

??????????????

what could be more real time than the structure itself?

if panel rads are used, yes, i'll put trvs on them. floors will run on their own off buffer temp.

jp_2

wikipedia?

i assume you've been here?

http://en.wikipedia.org/wiki/Insolation

The insolation into a surface is largest when the surface directly faces the Sun. As the angle increases between the direction normal to the surface and the direction of the rays of Sunlight, the insolation is reduced in proportion to the cosine of the angle. This is known in optics as Lambert's cosine law. This 'projection effect' is the main reason why the polar regions are much colder than equatorial regions on Earth. On an annual average the poles receive less insolation than does the equator, because at the poles, the Earth's surface is angled away from the Sun.

from:

http://en.wikipedia.org/wiki/Sunlight

The World Meteorological Organization defines sunshine as direct irradiance from the Sun measured on the ground of at least 120 W·m−2.

lots to read on this site!

Mike T., Swampeast MO

Been there as well as many other places.

Say you're on the equator with the sun directly overhead. You get X amount of solar energy per square meter of ground. Now move say 40 degees of latitude north. Neglecting the "transmission loss" for the added atmosphere through which the light must pass, isn't there the same amount of energy available to one square meter oriented at a 90 degree angle to the sun? BUT, if you're measuring on flat earth the amount of energy in that square meter on the equator has been "spread out" into a significantly larger area.

Isn't this why we tilt solar panels and sometimes even "track" the sun? Unless a HUGE amount of energy is lost via "transmission loss" through the atmosphere, the insolation data to which I provided the link gives all appearance to me as having been measured flat on the ground. How can one square meter at an angle receive the same amount of energy as one square meter flat on the ground?

This seems to me to be an incredibly basic question and it amazes me that I can't find much in the way of an answer--let alone freely available equations for estimation. Perhaps it's out there in "F-chart" based programs, but from what I've read such cannot be used for real-time solar collection as it's based on the assumption of storage.

Perhaps everything just "averages out" during a given year--provided the collector angle stays fixed and the job (e.g. average fluid temp in the collector) stays about the same. Change that job however (say between a relatively high temp--DHW production--and a relatively low temp proportional space heating at or near its lowest required temps and I'd suspect that things no longer "average out".

Mike T., Swampeast MO

I'm not sure that all control systems give anywhere near the level of feedback from the structure itself as a constantly circulating, satisfied, TRVd system with proportional flow.

The more I study, observe and live with proportional control (e.g. TRVs, pneumatics), the more I'm convinced that the best benefit is its ability to compensate for passive solar gains as they naturally move through the structure each day. Unless passive gain is so great that flow through a given radiator utterly stops, the system gives all appearance of "moving" passive solar gain into other radiators. I'm not saying this actually happens--just that it appears to happen. As recently as a couple years ago, I considered this implausible if not impossible--I learned a LOT from the season-long "Eurocave" experiment...

As long as they're not too massive (say tube-in-concrete) floors controlled via FHVs [should] be able to do similar. Warmboard would seem to be an ideal candidate as it combines relatively low mass with a relatively large and highly conductive emitter.

jp_2

Go back to wikipedia

I've scanned mainy topics there, if you read closely you should find all your answers.

as others have said use trig to figure projections, or carefullly draw diagrams.

your first paragraph, as i said with the flashlight and paper.

power= I cosine theta : I=sun intensity approx 1000watts

theta = your latitude + earth tilt, 23 degrees mid winter.

2nd par. look at flashlight experiment again. also answer found from one of my posts, info i found in wikipedia

3rd par. real time & storage basically the same thing.
say you have 'good' sun from 9am-3pm. thats 6 hours.
break that up into 180 deg. you get peak output only during 1 hour or at noon. (example only, it could be at 1.30pm)
so now you have an equation relative to time of day, Power=I*sin theta, 0-180 degrees.

thats a close estimate of hourly data, you could easily derive this from the monthly charts thermomax has.

last par. I don;t understand.

you should be clocking your gas meter, from 9am - 4pm, do that for a week or two. look at the thermomax charts to see how many tubes you'd need. its all about btu need, system doesn't know if you are heating DHW or radiators!

I DO agree, doing all those back calculations can be fun.

3rd par.

jp_2

goto bottom post

sorry, i put my suggestions at the bottom of the thread.

read "go back to wikipedia"

jp_2

23Mj/m^2/day

gotta fix my numbers here.

i would check that number again. most sources i see suggest the equator gets 1,000 watts/m^2.

at your lattiude that comes to about 760 watts/m^2, peak, summer. or 2,600 btu's.

1,000 watts * cos 40 deg.

Mike T., Swampeast MO

I've definitely done the trigonometry to determine the area of the "shadow" to include actual sun elevation depending on day-of-year and time-of-day. The first and last two hours of insolation are usually so low that I just forget about them as there's not even enough energy to overcome the transmission loss.

When the sun is lower in angle than the collector I re-calculate as if the collector were ideally oriented and then apply the vertical incident angle modifier based on the actual difference in angles. The nature of the insolation data [seems] to be self-compensating for the horizontal incident angle as I'm not tracking the panels left and right.

Still haven't heard from the MO dept. of agriculture. Will write them again.

jp_2

original post

what values of sun intensity did you use for the 3 sketches you posted?

the 3 pictures seems correct, just unsure of the math you used?

what does 'self compensating' mean?

Mike T., Swampeast MO

Have tried with the meter, but it's really tough because during the daylight hours of "candidate days" I'm generally using less than 1 therm and have to guess as 10ths of therms. Appears to be somewhere between 1/10th and 1.5 therms but I'm not utterly positive.

I may be going at this wrong by attempting to find these "candidate" days where daytime temperature averages between 38 and 55F. If cloudy with nearly steady temps near the low end of that scale, real-time load and gas consumption are the highest with consumption at about 60% of that predicted by HVAC-Calc. If the day is quite sunny however passive solar gain [seems] to reduce the real-time load and consumption to about 1/3 of that predicted by HVAC-Calc.

The passive solar gain seemed very high to me until I began to really pay attention to the sun and the surroundings. The front south-facing facade (not counting the 10/12 pitch hip roof) is 40' wide by 21' high. The street in front of me is a true boulevard--a divided street. All blacktop. The cross street dead ends (actually passes through to the far side of the divided street) at this boulevard right in front of my house--it's also blacktop. The boulevard and cross street have an upwards slope away from the house--probably about 5 degrees but increasing somewhat the further away you go.

Considering the general angle of the winter sun, I seem to be getting a big reflection of energy from those blacktop streets onto the front facade of the house. Since I've "tuned" my body to be able to detect IR radiation from relatively low temperature sources I'd nearly swear that I can feel the radiation from the street--even on quite cool days.

Mike T., Swampeast MO

Wasn't using any particular value of intensity for those sketches--just the area of flat ground that's in the shadow of the collector itself.

By "self-compensating" I mean that there's no attempt to adjust the collector angle side-to-side (as sun goes across the sky each day)--just to adjust the collector angle front-to-back to compensate for the elevation of the sun. Again, this assumes insolation as measured flat on the ground. Evacuated tube collectors seem to be quite tolerant with regards to horizontal incidence angle. By the time the sun is high enough that a decent amount of insolation (again assuming measured flat on the ground) is measured, the horizontal incidence angle modifier seems to be above 90% and growing quite rapidly.

If I'm doing this at all correctly, I should be getting fairly close to an estimate of the solar energy available to the collector. If this is a "candidate" day I then assume that I'll be using the solar for space heating and compute efficiency using 100F average collector temp. (Believe though that it can be done with an average temp as low as 90F). If not a "candidate" day I assume the solar is being used for DHW with an average collector temp of 140F. Am using the equation 0.82 - 2.19 (Tm-Ta)/G to calculate efficiency where Tm is average collector temp, Ta is ambient temp and G is daily irradiance. (Guess I should adjust the equation for hourly instead of daily irradiance however.)

Reasonably sunny "candidate days" seem to give about 80% efficiency. Sunny winter "non-candidate" days (too cold) have the lowest efficiency--around 75%. VERY sunny summer days have the highest efficiency--around 84% but more typically around 80% as well.

jp_2

theres your answer!

""""Have tried with the meter, but it's really tough because during the daylight hours of "candidate days" I'm generally using less than 1 therm and have to guess as 10ths of therms. Appears to be somewhere between 1/10th and 1.5 therms but I'm not utterly positive."""

how about clocking day and night and subtract the night, should give bigger numbers to read.

so you are saying daily usage from 10,000 - 150,000 btu's?
thats a huge spread.

from thermomax web site, that requires 10 - 70 tubes, in summer.

Mike T., Swampeast MO

Guess I could try that but will still have to estimate 10ths of a therm instead of to the nearest full therm.

Have the high end of that consumption estimate wrong. Was looking at times of constant boiler firing at minimum input through the day--average outside temp around freezing. This is NOT one of my "candidate days". Max on a "candidate day" is more like 3/4 therm or 75,000 btus but when consumption is that high, it's likely rather cloudy even if it is a "candidate" by average daytime temperature.

Don't forget that my real goal here is to eliminate as much of the "pulsing" operation of the Vitodens as possible without greatly oversizing the collectors for summertime DHW. At least I won't have to feel like such an energy hog when the "car wash" shower (it can flow at least 12 gpm if you turn everything on) and 95-gallon minimum Jacuzzi tub are used.

Eric_25

Is that like my

FLUX capacitor powered Delorean at all.
You engineer guys make my head hurt.
great info though I am considering some solar panels myself to experiment with for my radiant. I'll keep reading when my headache goes away.

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

Mike T., Swampeast MO

Paul B & Dr. Al

80 gallons for the solar tank itself and 120 gallons for the indirect DHW tank.

24-7-365 DHW recirculation (currently gravity but will probably have to switch to forced with the Viessmann indirect as it has a down-turning outlet) with a large Myson towel warmer as the highest, farthest element of the DHW recirculation system.

While the DHW recirculation loop is nicely insulated and the towel warmer doesn't appear to be too much of an energy consumer when fully loaded with towels, there's certainly a constant, low-level load.

Personally, I'd prefer that the dual-coil solar tank were about 40 gallons, but I doubt that Viessmann could fit their very generous HX coils in a tank that size...

The Viessmann evacuated tube collectors are designed to be self-limiting and while 200 gallons of storage isn't huge, it's certainly within generally regarded as sufficient specs for the amount of collection I'm considering. Seems to be very little chance of "cooking" the system.

Mike T., Swampeast MO

An Interesting Comparison

CC.Rob kindly graphed our data together. Same period, same unit of measure but I have no idea if the measuring equipment itself is the same. See attachment. Hope you don't mind that I shared Rob.

Light blue line is here--dark blue line is his (about 200 miles north by latitude difference). He's in NJ, I'm in MO.

Would seem to support my idea that average summertime insolation is relatively reduced here while wintertime peaks are relatively high.

Mike T., Swampeast MO

If you look carefully you'll notice that from October through April there's a general delay of about 2 solar days between Swampeast MO and NJ that would [seem] to correspond very closely with the general movement of weather systems.

Then notice the nearly polar opposites around the beginning of April and the beginning of August. Nor'easters while high pressure is building here?

Between April and August notice how the general 2 solar day time lag is nowhere near as regular. This is when a gigantic high pressure "dome" typically builds centered right above Swampeast MO. This "dome" steers weather systems around Swampeast MO--at it's height a "ring of fire" occurs all around us. The air goes virtually stagnant, humidity builds and the sky is in a nearly perpetual haze of ugly grey.

This is complete contrast from October - April when our clear days generally have beautiful blue sky.

Perhaps I'm reaching here, but it sure looks to me like "general" atmospheric conditions can have a significant impact on solar energy received on the ground--much more than can be explained by the difference in length of day between these two latitudes.

Mike T., Swampeast MO

Had a reply from MO Dept of Agriculture.

This is the sensor they use throughout the state.

Sure looks to me like it measures insolation as received on flat earth.