tree shading and solar viability

scott markle_2

I'm interested in how leafless tree shading effects the economic viability of solar DHW applications.

latitude is 41 deg. roof 45deg. pitch azimuth 30 deg. west of south.

I haven't been on the roof yet but I'm estimating that the trees will probably begin shading the roof at a solar elevation of around 30 deg. looking at sun charts for my location I see that by mid october the location will start to see some diminishment of the desired 9am to 3pm exposure "window". November December and February will probably be shaded by tree branches for most of day. It's my understanding that leafless trees can diminish radiation by 25%-60%. This coupled with low ambient temperatures and a shorter day will likely result in a very little usable energy available for this period of the year.

Since so much of the total annual radiation is in the sunny half of the year (when there will be no shading problems) I wonder to what extent this loss of winter capacity will effect the overall economic viability of a dhw system in this situation. Lot's of factors I know, just looking for general opinions and advice, like an estimate of the percentage of annual solar collection capacity that takes place in the 3 darkest coldest months of the year.

Robert O'Brien

Solar Pathfinder

Use a pathfinder.Some judicious trimming might resolve the problem.You can use a modeling program like Polysun to give a full professional projection.
http://www.velasolaris.com/vs2/index.php

scott markle_2

Thanks Robert, not quite ready to spend the $300 on the path finder and then the software. Seems like it might be a bit of hassle leveling the device on a 12/12 roof any experience with this? Eventually I would like to invest in these tools.

I'm thinking about using a less expensive clinometer to get a sense of when the trees will start to interfere. Perhaps even simpler although a bit skewed by a wider summer azimuth would be to visit the site (now) at the time of day when the roof begins to become shaded and then using sun charts and the current date and time of day calculate the suns actual elevation at this time. If I know that shading begins at a certian elevation I can extrapolate (in a general sense) what my full exposure window is for the rest of the year. What I'm interested in knowing is how critical performance in the worst 90 days of the year is to the overall capacity of the system to harness useful energy.

90 days is roughly a quarter of the year but these are the worst preforming months even if exposure was ideal. Intuitively it seems to me that a site that had negligible gains for 90 days of the year but utilized the other 275 very effectively might only be loosing 15-20% of what it would be capable of if fully exposed. Could any one run a simplified simulation of this for me?

Robert O'Brien

How about this?

http://www.builditsolar.com/References/SunChartRS.htm

CC.Rob_15

not much

You have nearly described my SDHW installation (lat, inclin, azimuth). We even have one conifer that does a bit of late day blocking in the winter. There used to be three , so I second the call for some pruning.

In looking at my records of theoretical and observed incoming radiation (with a very fancy pyranometer a couple miles from the house), the largest impact in the winter months is clouds. Much more so than losing 30-45 minutes/day from some shade. 3-4 days of cloudy weather and the tank will drop well below 70F. First sunny day it's back up to 90-100. Winter incoming water temperature is ~45F.

The useful winter hours are around 10a-2p. Goes a bit more if sunny all day.

So maybe figure you can average a daily ~20-30F rise of the tank during the dead of winter. Call it 20-30k BTU/day? Not huge, but slow and steady. (All math done on the fly in my head assuming about 400 BTU/sf/day for the collectors in winter, so please check/correct.)

hot_rod

online tools

of course nothing can see your roof shading, without a "Pathfinder" type of device. Maybe a real tight "google earth" view

There is a rule of thumb arms straight out, thumb up, etc, etc. Be careful on a 45 degree roof, however.

You can build a shading calc device, google around for a cardboard homemade version.

But you can model some things with RET Screen, a free downloadable software. Here is an example for Milwaukee, showing how much it changes across 12 months. Dec. about 3% 12,000 BTU/ day. June, July around 52,000 BTU/ day.

Plenty of sites like this U of Oregon that show the suns movement through the sky for various locations. NASA, NORA, etc have useful data.

Really 10- 2 is the period for 90% of your energy harvest. If you have a good look at the sun in that 4 hours you do well.

But the bottom line will always be much less solar 'available' in winter months. Plus the ambient air temperature chips away at performance, regardless of the sunshine.

hr