Cleaning of arrays

shputnik

Senior Member
Location
RI
Typically how many times a year does an array need to be cleaned? I realize each area is different. How much snow, leaves, dirt and bird droppings would keep a module working at one hundred percent?
 

jaggedben

Senior Member
First of all ... 100%? That shouldn't be your standard. All panels are going to get some dirt on them and lose a wee bit of production. The question is when does it become worth while to clean that dirt off.

Consider that one solar panel (let's say 280 watts) produces about 420 kWh per year. That's most likely between $40 and $80 worth of electricity, depending on rates where you live. Say dirt causes you to lose 5% of that, which is realistic assumption. Well, that's between $2 and $4 you are saving each year by keeping one solar panel squeaky clean. Say you have a 20 panel system, at most you are saving $80 a year. If you do it yourself and it takes you 8 hours (say a few hours each on multiple occasions), maybe you are making the equivalent of $10/hour. Is that worth it to you to clean your own system? You decide. Would you take that pay to clean other peoples' systems? In many places it'd be hard to find someone who would. There's just not enough systems that need enough cleaning to keep someone fully employed.

One point that comes from this: cleaning too often means putting in more effort (or money) for less and less benefit, since the panels will be less dirty and losing less production. For cleaning to be worth it, you really want to get a good jump in production each time you do it, and then ride that for a while.

Also consider that in rainier places, you get less production because of less sunlight, but the rain washes a lot of the dirt away. Meanwhile in sunnier places, you may get less rain washing but you get more production from more sun. So those things will offset each other to some degree with respect to costs.

Bottom line: we don't tell our residential clients that arrays need to be cleaned. Maybe after a few years, if conditions are bad, dirt could be reducing production by 10%, and it's worth cleaning. But the losses usually seem to be much less than that, and rain washes away a lot of dirt, even when it doesn't rain that often. And we don't want to be telling our customers to get on their roof if they aren't used to the dangers and how to safeguard themselves.

If you've got a Megawatt ground-mount, well, it's probably worth an hour or two of someone's time to do some serious math and figure out a real answer.
 

69gp

Senior Member
Location
MA
The photo below is one of the dirtiest arrays I have ever seen. 3 Meg rooftop in MA built next to a landfill roof only has a 1 degree pitch. Seagulls have a party here. This was baked on for over a month without any heavy rain during the summer of 2014. After the first heavy rain they were as clean as the day they were installed. Price to clean was expensive as even though you can walk on the modules it is not recommended as you make micro cracks in the module itself. Most modules manufactured today are self cleaning and do not require any cleaning.

 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Most modules manufactured today are self cleaning and do not require any cleaning.
How is that? Cleaned by rain is not the same as "self cleaning", and how would more recent manufacturing methods make a difference?
 

jaggedben

Senior Member
How is that? Cleaned by rain is not the same as "self cleaning", and how would more recent manufacturing methods make a difference?
Cover glass coatings that repel the adhesion of dirt are something I seem to recall seeing touted here and there. The idea is that rain or wind washes or blows it all away because it doesn't really stick. Whether 'most modules' have such coatings nowadays is not something I can't speak to though.
 

69gp

Senior Member
Location
MA
How is that? Cleaned by rain is not the same as "self cleaning", and how would more recent manufacturing methods make a difference?

It all has to do with coatings that they apply to the modules, the best right now would be [FONT=Helvetica Neue, Helvetica, Arial, sans-serif]a hydrophobic nano-coating. you can google it and read about it. there is a lot out there on it. it does not really clean the module but makes it so that items do not stick to it. [/FONT]
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
It all has to do with coatings that they apply to the modules, the best right now would be a hydrophobic nano-coating. you can google it and read about it. there is a lot out there on it. it does not really clean the module but makes it so that items do not stick to it.
Hydrophobic nano-coatings are specific to water, and I don't know that they would be transparent enough to UV not to detract from the module's performance. At any rate, I know of no module manufacturers who are using it.
 

ghostbuster

Senior Member
I have a friend with 2 large fixed arrays that has just sent me his comments see below:

Just for the record, I have virtually no issues with dirt on panels. They wash off pretty well.
What does shut down production is a fine layer of frost. Virtually 100% shut down until it melts off, even if only a few molecules thick.
Snow of course stops things too so I end up brushing the panels off when they need it and I have time. This is good exercise by the time one tromps through the snow and manhandles the brush with a 20 foot long handle.
If they could make panels with no metal frame, like frameless glass... they would clean better in all cases!
If they could make the panel so it could heat itself briefly too that would help but once the sun hits any part of the panel they clean themselves off pretty quickly.

If I did it again I’d make the vertical angle adjustable so I could go steeper in winter and lower in summer... better production.

The other thing I’d do is add maybe 15% of panel area looking direct east and the same direct west so that in the morning, when the main array was at low angle and low production, these extras would give an early boost and same in evening. I would not overpower the inverter because at mid day, when main array is at peak these morning and evening additions would be virtually shut down due to angle. Sort of like a tracker but with no moving parts.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
If they could make panels with no metal frame, like frameless glass... they would clean better in all cases!
They do. See http://www.solarpowerworldonline.com/2015/01/411-frameless-solar-modules/

If I did it again I’d make the vertical angle adjustable so I could go steeper in winter and lower in summer... better production.
That may or may not be worth the trouble; tilting at latitude may be nearly as good, and adjustable tilt racking is more expensive and more vulnerable to wind loading. Make a few PVWatts runs to show the differences.

The other thing I’d do is add maybe 15% of panel area looking direct east and the same direct west so that in the morning, when the main array was at low angle and low production, these extras would give an early boost and same in evening. I would not overpower the inverter because at mid day, when main array is at peak these morning and evening additions would be virtually shut down due to angle. Sort of like a tracker but with no moving parts.
This is done quite often; more modules is usually cheaper than a tracking system.
 

Zee

Senior Member
Location
CA
Google did a whole study on it. I am trying to recall........Basically if you spent serious time and HAZARD on a roof with a hose and sweat...you may boost it something like 5-10% immediately, if dirty. Within a week this will drop down closer to what it was when dirty. Overall...... unless you washed it weekly, (prepare to pitch a tent) you won't boost annual output more than 1-2%.

One exception: flat arrays. I mean flat= zero pitch or less than 5 degree pitch. Those were never a good idea with framed modules, and have serious "mud shading" at down slope edges of modules.

Google concluded: not worth washing any array unless it is flat.

In California: "winter rains clean the panels", has been a mantra for 15 years.
 
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