Exterior reflective surface?

[bob-n]

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? I'm thinking about what could be put on the south wall of the house, as shown below. If I'm really lucky, it might give my array a 5% boost, which isn't worth much, but would add up over time.
reflector.jpg
Thank you for your thoughts.

[SunEagle]

I guess you have to decide if the cost of the reflector will pay for itself compared to the % increase in output. If you are only looking at a 5% increase then maybe finding a way to reduce your electric usage by that amount may cost you much less then that reflector.

Historically it seems to be cheaper if you find ways to conserve then to install solar power generating equipment.

[J.P.M.]

Since you ask, yes. Many such thoughts/ideas over the last 45+ years or so, but none that are good, safe, practical, durable or even workable for several reasons. In fact, most are counterproductive.

Reply if you're really interested in reading why.

[bob-n]

Right now, it's a dull off-white. Gloss white paint is cheap. How much better would that be? What could work even better?

By the way, 5% was a really wild guess. The array faces East while the wall faces South. PVWatts doesn't have inputs for this situation. I don't even know how to measure reflectivity. Flashlight and cellphone light meter?

How would you predict this?

[bob-n]

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.

[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

[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.

[bob-n]

Yes, I have microinverters.

[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.

[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]

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?

[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

[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.

[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?

[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.