Exterior reflective surface?

[bob-n]

I just checked and the warranty says not to add reflectors. Solarix is right. Oh well.

[solarix]

I believe you will find that your PV panel manufacturer discourages adding reflectors and even prohibits it in the warranty. PV panels are not designed to handle more than one sun of radiance and the back sheet can get damaged from excess temps, the internal wiring and solder joints can come undone from excess amps, and the cells themselves can become scorched and discolored.
I realize that many people in the solar game are independent minded cusses, but really, do try to color within the lines....

[J.P.M.]

Bruce had a great thought. I never considered putting panels vertical on that wall.

I just ran PVWatts two ways. With the same ideal assumptions and my location:
East at 30 degrees gives 4236 kWh per year - My current array configuration.
South at 90 degrees gives 3824 kWh per year - Vertical panels facing south.
Not much difference, but the current array placement is slightly better than a vertical array on that south wall, so I'm not going to move my panels.
It's not clear whether I face E or ESE, but that's a minor difference, and any additional south would benefit the current array over a vertical placement.

White paint seems like the next experiment. How can I devise a controlled experiment to tell me how much better gloss white paint would be than my current dull off-white stained wood?

Given our latitude, I'm not surprised at the similarity in outputs.
However, one question: Were the arrays you ran the same STC size ?

On the idea of glossy paint being a better reflector than matte finish:

If you paint a non specular surface such as wood siding or any surface that doesn't produce a reflection - such as slate/brick/fibreboard, etc., the reflectivity difference between glossy paint and flat paint of the same color will not be noticeable. Silver surfacing a rough surface won't produce a specular reflection but the reflectivity will be about the same - maybe a tad - a % or 2 less - than the highest available specular reflective surface - a piece of glass that's front silvered, i.e., a front silvered (but to be clear, the silver exposed to the air will begin to tarnish almost immediately). In the case of the rough surfaces, the reflectivity will be diffuse but still reflective of the irradiance hitting it. A surface does not need to be a specular reflector to be highly reflective of light.

However, since your asking: One possible experiment:
1.) Do this on consecutive, sunny days.
2.) Measure/record panel/array output for the day.
3.) Paint the side of the house next to the array flat white.
4.) Measure the panel/array output as above.
5.) Paint the side of the house next to the array gloss white.
6.) Measure record the panel/array output as above.
7.) Compare the results for the 3 days.

You'll have the best chances of less daily output variation due to diurnal changes solar zenith angle if the above is done near(er) the solstices, but doing so at the solstices and equinoxes will give you a SWAG of annual output changes.

The above method is probably not practical, realistic or even possible for any number of reasons, but my opinion is it will give you a decent chance of getting some 1st approx. numbers for differences in the reflective value of different surfacesgt.

But before I did all that , or anything else, and for as easy it is to get a pretty hard number, I'd make sure of, or at least get closer to your array's and other surfaces actual azimuth.

If you're interested in your array's orientation (and also the walls' orientations) with respect to azimuth, shoot an azimuth (look at shadows cast) at solar noon and guesstimate how far off south the array is. Get a reasonable first approx. of solar noon as halfway between sunrise and sunset. Solar noon and civil noon only agree (at most) 4 times/year.
Or, see if the vendor has the info used to estimate array output.
Or use Google maps.
Or a phone app.

Why array and surface azimuth are important for this application: POA irradiance and insolation on devices and surfaces and so outputs from devices that are more vertically oriented are more sensitive to azimuth than devices that have a closer to horizontal tilt. For this application however, my guess is that shading from the wall will reduce output and reflection from the wall will help, but the reflection off the wall won't help as much as the shading by the wall hurts.

[Ampster]

.......
White paint seems like the next experiment. How can I devise a controlled experiment to tell me how much better gloss white paint would be than my current dull off-white stained wood?

As I mentioned earlier, if you can see individual panel output on Enphase you can try a gloss white. If you have a light weight panel you can put up there on a sunny day to see if there is any instantaneous increase. Otherwise it might be hard to account for day to day differences for an expected benefit of 5%.

[bob-n]

To answer a different question, I see virtually the same output per panel, regardless of location. I'm using APsystems microinverters and their apps, but it gives roughly the same amount of info as the Enphase tools.

Here's a snapshot from late morning on a sunny day, with all panels producing roughly 300 watts. You could argue that the 318-watts fromn one panel is significantly more than the rest, but I'm not sure of the measurement accuracy inside the microinverters. Without doing some controlled measurements, I don't trust them to better than 5%.
array.jpg

[NewBostonConst]

White is the next best thing to a mirror for reflection. Mirrors have to be aimed, so I think you are at the best you can be.....That is why globes of most lights are white, they provide good diffused reflection.

[bob-n]

Bruce had a great thought. I never considered putting panels vertical on that wall.

I just ran PVWatts two ways. With the same ideal assumptions and my location:
East at 30 degrees gives 4236 kWh per year - My current array configuration.
South at 90 degrees gives 3824 kWh per year - Vertical panels facing south.
Not much difference, but the current array placement is slightly better than a vertical array on that south wall, so I'm not going to move my panels.
It's not clear whether I face E or ESE, but that's a minor difference, and any additional south would benefit the current array over a vertical placement.

White paint seems like the next experiment. How can I devise a controlled experiment to tell me how much better gloss white paint would be than my current dull off-white stained wood?

[J.P.M.]

My suggestion is, that putting a panel with its own micro inverter on that wall will likely
give a much better payback than a reflector. Bruce Roe

Without some site and system analysis, particularly in New England and facing ESE (If the orientation given in Bob-n's original post is close to correct as I might guess from the latest photo), and also what looks like might be a fair amount of shade roughly off to the west, it's hard to speculate.

However, when all's said and done with respect to what's gained long term with respect to greater energy generation/lower energy expenditures for the toil and the value of the material required compared to a completely passive and dumb reflector, I'd kind of doubt that PV panels will be as cost effective as a passive reflector, particularly if some $$ value is assigned to the labor involved.

That's not to say that either is cost effective, just an educated guess that active PV will be less cost effective - to the point of being cost ineffective - than a passive reflector.

But, opinions vary. While still less expense/hassle and probably more cost effective than adding vertical PV panels to a SW facing wall, based on what I think I've learned, been taught and what I've done, reflective surfaces - while looking like "free energy" - don't do all or as much as one might think, particularly for what are the often/usual unknown/unconsidered drawbacks (with one of those being, for example, the possible effect a mirrored exterior wall might have on resale values).

If reflective surfaces are considered at all, to my overall experience and opinion only, the best efforts at utilization for the least intrusive/obnoxious outcomes seem to be those that consider the albedo from mostly white, mostly diffuse reflective surfaces already in place. Getting the most from such surfaces then often comes down to some perhaps very minor array siting considerations (if any) for any such preexisting albedo enhancing surfaces. Often in such shenanigans, the baby gets thrown out with the bath water and less than optimum array orientations result when ignorance with respect to the paucity of possible production enhancement via reflection gets in the way of common sense and good design.

[bcroe]

My suggestion is, that putting a panel with its own micro inverter on that wall will likely
give a much better payback than a reflector. Bruce Roe

[Ampster]

Based on the above the OP may already be at 5% with that white wall behind his panels. @bob-n, any luck interrogating your micro inverters to see if the southern most ones are giving off more power?

[J.P.M.]

J.P.M. Yes, I'd be interested in learning more, but with the hope of improving. If it really is hopeless, there's no need to teach me too much.

Not necessarily hopeless, but usually not as much benefit as those w/B.S. to peddle might even suspect. If still interested, read on.

1.) The easiest/cheapest way is adhesive backed aluminum - like sheets of metalized duct tape on some stiff backing, firmly affixed to the siding.
Benefits:
Cheap, DIY, reasonably durable.
2.)Drawbacks:
Among the things your mother never told you but some of us have learned the hard way is that such less than completely planar (and so non-specular) reflectors will have hot spots. Those hot spots will be a potential fire hazard for the roof and probably damage the PV cells. A somewhat similar example: An early façade at the Playboy club on the Atlantic City boardwalk (after gambling was legalized) had fancy but rather gaudy plexiglass reflective surfaces that were mostly, but not quite flat. They started some sections of the boardwalk on fire when some concave sections of the plex acted as concentrators and randomly got focal spots on the boardwalk.

Truly specular reflectors are heavy and usually made of glass. Hanging them on a wall can get tricky if staying affixed is a priority.

Also, your array will be differentially irradiated most all of the time. Unless the irradiance on the array is uniform, the array will, in effect, be in partial shade compared to the reflector augmented sections. That'll be mostly OK because you have micros, but the reflected irradiance will still differentially heat the augmented panels. The lower resulting voltage in the warmer panels may/may not be a problem.

Then there's the potential problem of reflectors throwing irradiance where it's not wanted - such as at/in neighbors' lines of sight, not to mention being a possible traffic hazard - usually when the solar zenith angle is high (closer to sunrise/sunset times). Be careful of that as it can be a safety issue.

Your guess/SWAG of 5 % augmentation in energy production may be a bit light, but probably not too far off. My experience/experiments w/specular reflectors leads me to think that 10% or so enhancement of annual production due to POA irradiance enhancement is about as much as can be expected for a stationary flat plate application and one specular reflector. Flat plate PV applications will most likely show less than that due to increased panel temps. and how that increased temp. will affect cell efficiency.

After all the hassle, I found the best compromise is to forget the mirrors/specular stuff and the pipe dream traps that go with them, and go with simple painted white surfaces. The amount of irradiance addition is ~ 1/2-2/3 as much as a good specular reflector, it's a lot easier to keep clean, usually doesn't concentrate direct (beam) radiation, and will also get more of the diffuse radiation onto a receiver than will a mirror. There's other stuff as well, for example, having to do with difference in the nature of the frequency of reflected radiation between specular and diffuse reflectors and how PV glazing and cells can (or cannot) utilize it, but that's a longer subject than room here permits.

[Ampster]

Yes, I have microinverters.

Then, if they are Enphase, check your output over time and see if the ones closest to that south facing white wall are producing more. That view is available from my installer login but not from the view provided by one system that was installed by another installer. You may have to ask them to upgrade your monitoring access.

[bob-n]

Yes, I have microinverters.

[Ampster]

Can you think of something relatively inexpensive that could be put against an outside wall to reflect onto PV panels, and not lose reflectivity after a few winters?..........

The effectiveness may depend on whether you are using microinverters or a string inverter. With a string inverter the extra insolation on some panels may not have the result of more power because the lower output panels may may drag down the lower powered panels. It may be a similar result that a shaded panel does to the rest of the panels in a string. The white wall is already somewhat reflective and if you have microinverters, you may see some difference. Please note I used MAY because this is just a wild ass guess.

[Mike90250]

You must reflect evenly on all panels in the string, otherwise the extra amps will be throttled back by the next panel producing nominal power