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Renewable Energy: Keep those PV arrays clean or this will happen!!

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    • CommentAuthorTriassic
    • CommentTimeJan 26th 2012 edited
     
    Thought some of you would be intrested in what happens when you put PV arrays on Mars - They get dusty and you can't go and clean them.

    http://roadtoendeavour.files.wordpress.com/2012/01/polar_project_array.jpg

    and if you want to learn more

    http://roadtoendeavour.wordpress.com/2012/01/24/different-views/
    • CommentAuthorCWatters
    • CommentTimeJan 26th 2012 edited
     
    Despite there being no rain on mars those panels magically cleaned themselves allowing the mission to be greatly extended. They only expected these rovers to last 90 days when they landed in 2004. The cleaning effect of the wind allowed one to operate for more than 5 years and the other is still going..

    http://marsrover.nasa.gov/home/index.html

    Latest images scroll down..

    http://marsrover.nasa.gov/gallery/press/opportunity/20120124a.html

    http://marsrover.nasa.gov/gallery/press/opportunity/20120124a/PIA15282_Greeley_north_L257F_br2.jpg

    http://marsrover.nasa.gov/gallery/press/opportunity/20120124a/PIA15281_Greeley_north_L257atc_br2.jpg
    •  
      CommentAuthorSteamyTea
    • CommentTimeJan 27th 2012
     
  4.  
    :bigsmile::bigsmile::bigsmile::bigsmile::bigsmile:
    • CommentAuthorpmusgrove
    • CommentTimeMay 28th 2012
     
    Nothing about Mars but noticed today that my panels had got rather dusty. They are set up as two identical arrays so cleaned one at a time to find out the difference a clean would make. The answer is about 7% of output so it is worth giving them a spray down occasionally.
  6.  
    Yes , clearly dusty on one of my arrays and only 7 ish since last rain .
    pms , no hoespipe ban in your area then ?
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 28th 2012
     
    Parts of Cornwall were the wettest place in the UK yesterday, so I wonder if it is the extra sunshine or the rain that makes them perform better here.
    • CommentAuthorJeff B
    • CommentTimeMay 28th 2012
     
    Side-tracking here a bit, but we got significantly less output on two days last week (Thursday and Friday) when the temperature reached 28C compared with Saturday which was just as sunny but noticeably less warm with strong winds. 21 kWh compared to 25 kWh - presumably down to the fact that the panels are less efficient as the temperature increases?
  9.  
    The effect on PV of ambient heat is gobsmacking I can get 30 plus kWh in spring and just break 20 in high summer despite many more hours sun in a day (mid-Italy mind). Of course I need to look at the air temp around my inverter too.
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 29th 2012
     
    The sun is higher in the summer than the equinox, are your panels tilted for maximum annual generation rather than maximum at one time of year. You may also find that the sun rises and falls 'behind' the panels in the summer, why a NE and NW can work in the summer, but not in the winter.
    • CommentAuthorpmusgrove
    • CommentTimeMay 29th 2012
     
    I have still got 4,000 litres odd of rain water to play with and it goes into the tanks off the roof with the PV on so it is being re-circulated (less a bit from evaporation). I did wonder whether the cooling was also having an effect but it was first thing in the morning when I cleaned them. Will try another experiment and cool one side in the heat of the day to determine the difference. If there is one could fix up a permanent irrigation trickle over the panels.
    • CommentAuthorJoiner
    • CommentTimeMay 29th 2012
     
    What would be the problem with using whatever it is that makes self-cleaning glass self-cleaning? Or is it the fact that it's DUST rather than (proper) dirt that makes such a move problematic?:confused:
    • CommentAuthorJoiner
    • CommentTimeMay 29th 2012
     
    Paul's measurement of the loss of efficiency of his panels makes you wonder how roadside and rural installations fare. The former subject to all the dust kicked up by passing vehicles, with the added ingredient of the rubber particles that make road surfaces so dodgy immediately after a summer shower; the latter subject to the incredible amounts of dust from any activity on a neighbouring field during dry weather, even one some distance away.
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 29th 2012
     
    Self cleaning glass relies on rain/water to wash the dirt away. It does this because the glass has tiny, tiny grooves cut into it that reduce the surface adhesion of the water droplets, the droplets then carry the dirt away.
    Chemical treatments take a different approach, they are a molecule/polymer that has an attraction to glass at one end and at the other end, a molecule that repels water. Trouble is that the end that is meant to stick to the glass is not very good. A silicone wax will work just as well.
    Would be interesting experiment to have two panels, both with micro-inverters and data logging, clean them both up, start logging for a week, then clean one every day and see the difference after calibrating them. Actually be better to have an array to do it with. Anyone got a system like this and willing to wash them?
    Word of caution, a string of 8 panels can produce over 300V DC, at around 8A any wiring problems and it will be a nasty jolt.
    •  
      CommentAuthorJSHarris
    • CommentTimeMay 29th 2012 edited
     
    <blockquote><cite>Posted By: Gotanewlife</cite>The effect on PV of ambient heat is gobsmacking I can get 30 plus kWh in spring and just break 20 in high summer despite many more hours sun in a day (mid-Italy mind). Of course I need to look at the air temp around my inverter too.</blockquote>

    For crystalline type panels the power loss with temperature is around 0.5% per deg C, for amorphous panels it's about half that. If the sun increases your panels temperature (and assuming they are crystalline panels) to around 50 deg C, say, then the power output will drop by about 15% from that when the panels are at 20 deg C, with the same amount of insolation.

    A 30% loss implies a temperature rise of around 60 deg C, higher than is likely I would have thought, as it would mean panel temperatures of up around 80 to 100 deg C. Panels tend to dissipate heat reasonably well if spaced off a surface, plus they are converting some of the energy to electricity. The highest temperature I've seen on the panels on my boat (which aren't ventilated on the underside, so get far hotter than house roof mounted panels) is about 60 deg C.

    My guess is that the high temperature isn't the only cause of the reduced output you're seeing, and that there must be some other effect at work, too.
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 29th 2012
     
    Posted By: JSHarrisand that there must be some other effect at work, too.

    Cloud is a good one, as is haze. Also the spacial-temporal distribution. Or in English, if it gets cloudy/hazy at the wrong time of day your output will be lower.

    Are the generation readings taken from the generation meter or the inverter? They should be similar.
    • CommentAuthorJoiner
    • CommentTimeMay 29th 2012
     
    So what hope for the arrays "planned" to tap into the resources of the world's desert areas, where temperatures are reportedly higher than anything seen even in Cornwall, as incredible as that might seem. :bigsmile:
    •  
      CommentAuthorDamonHD
    • CommentTimeMay 29th 2012
     
    Many of them will be concentrating molten-salt system, some/most with air cooling, so a trifling 60C would be nothing to them...

    Rgds

    Damon
    • CommentAuthorJoiner
    • CommentTimeMay 29th 2012
     
    Sorry, should have said "pv arrays".

    Know about the molten-salts, as per... http://www.guardian.co.uk/environment/2010/jul/22/first-molten-salt-solar-power

    ...but was thinking along the lines of the pv-farms that were discussed on here last year, when cooling was the biggest problem then - along the lines of how do you get water to the Sahara (?). The concern this year would be how to keep them clean as well as cool.

    (Incidentally, as a result of this thread I checked the pv array that's supposed to charge up the batteries in the parking ticket machine on the country park and found it filthy with dust and pollen. Have now resolved to give it a wipe at least once a month to hopefully extend the time between charges, although the situation isn't helped by the fitters orienting the slanted pv array to face north!)
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 29th 2012
     
    Hate to say it but you have to look at the stats when it comes to heating and cleaning.
    Yes more direct sun will make something hotter, but it often makes it windier too. Then there are the times when the sun is at a less than optimum angle to the panels, this causes less heating. Then there is the radiant surface and what it is irradiating to, generally a shadowed area with lower than ambient temperature. Then there is the maximum irradiation that the panel is exposed to and if the panel has been designed to cope with this.
    I would think that all this has been thought about on a multi-million Dollar project.

    There are people that get hung up on the temperature performance of panels in Cornwall, often point out that we do not have the extreme temperatures that happen in London but we do have clearer skies when it is clear, are further South, and lots of diffused light (overcast days) that make up for it.
    • CommentAuthorpmusgrove
    • CommentTimeMay 29th 2012
     
    More than happy to add loggers to the arrays if you tell me what I need to do. Put them up with an electrician friend so sure that we can manage to add some bits. The measurements I took were purely instantaneuos output readings off the inverter.
    • CommentAuthorJeff B
    • CommentTimeMay 29th 2012
     
    <blockquote><cite>Posted By: SteamyTea</cite>Self cleaning glass relies on rain/water to wash the dirt away. It does this because the glass has tiny, tiny grooves cut into it that reduce the surface adhesion of the water droplets, the droplets then carry the dirt away.
    Chemical treatments take a different approach, they are a molecule/polymer that has an attraction to glass at one end and at the other end, a molecule that repels water. Trouble is that the end that is meant to stick to the glass is not very good. A silicone wax will work just as well.
    Would be interesting experiment to have two panels, both with micro-inverters and data logging, clean them both up, start logging for a week, then clean one every day and see the difference after calibrating them. Actually be better to have an array to do it with. Anyone got a system like this and willing to wash them?
    Word of caution, a string of 8 panels can produce over 300V DC, at around 8A any wiring problems and it will be a nasty jolt.</blockquote>

    We have an 3 x 7 PV array with microinverters. I just tried washing the lowest row of 7 with cold water from the garden hose. Unfortunately it is very overcast here today and not hot (18C) so unsurprisingly didn't notice any significant change in output from these 7 panels (around 35 W per panel). Will try again when (if!) we get the sort of sunny days we had up til Sunday!
    • CommentAuthorJoiner
    • CommentTimeMay 29th 2012
     
    I think Sod's Law is at work! :bigsmile:
    • CommentAuthorskyewright
    • CommentTimeMay 29th 2012 edited
     
    Posted By: SteamyTeaHate to say it but you have to look at the stats when it comes to heating and cleaning.
    Yes more direct sun will make something hotter, but it often makes it windier too.

    I don't think the following stats are not ideally suited to this comparison, but they were easy to pull together...

    Best generation day last week for our 3.9kWp, WSW facing, array was Monday 21st:

    Generation 25.6 kWh
    24hr mean temperature 13.3°C
    24hr mean humidity 69%
    24hr mean windspeed 4.8 mph
    24hr mean gustspeed 6.7 mph
    24hr mean direction 199° (SSW)
    Rainfall 0.0 mm
    Maximum temperature 17.4°C on day 21 at time 16:07
    Minimum temperature 9.6°C on day 21 at time 02:46
    Maximum windspeed 11.5 mph on day 21 at time 11:32
    Maximum gust speed 19.6 mph from 180 °( S ) on day 21 at time 11:31
    Sunshine hours (horizontal) 9.5hrs
    Max solar radiation (horizontal) 960 W/m²

    Friday was the hottest:
    Generation 23.14 kWh
    24hr mean temperature 21.1°C
    24hr mean humidity 69%
    24hr mean windspeed 7.1 mph
    24hr mean gustspeed 9.8 mph
    24hr mean direction 48° ( NE)
    Rainfall 0.0 mm
    Maximum temperature 27.5°C on day 25 at time 16:19
    Minimum temperature 14.3°C on day 25 at time 01:48
    Maximum windspeed 11.5 mph on day 25 at time 09:03
    Maximum gust speed 18.4 mph from 068 °(ENE) on day 25 at time 09:02
    Sunshine hours (horizontal) 13.1hrs
    Max solar radiation (horizontal) 805 W/m²

    N.B. While there was a bit more breeze on Friday, it was from "behind", so the panels didn't "feel the benefit" as much - and the air mass that was passing by was substantially warmer.

    Based on the experience of the last 18 months or so an "ideal" PV day seems to be one where the sun is in the clear pretty much all of the time but the rest of the sky has a good number of nice bright white clouds, and with a good stiff cooling breeze straight on to the array (oh, and perhaps a shower of rain before sunrise to rinse off the surface in advance).

    Not that I'm being picky. A less than "ideal" dawn to dusk blue sky is quite welcome too. :wink:
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 29th 2012
     
    Shall have a look tomorrow and see if I have any decent data kicking about.
    • CommentAuthorJoiner
    • CommentTimeMay 30th 2012
     
    skyewright - Was that whilst clean or dirty? :wink:
    • CommentAuthorskyewright
    • CommentTimeMay 30th 2012 edited
     
    Posted By: Joinerskyewright - Was that whilst clean or dirty? :wink

    I see the :wink:, but...

    They had a good wash (SW gale) a fortnight ago. Dust doesn't tend to be much of an issue here& at the moment no obvious bird poo either, so the surface state of the panels was probably similar over that period.

    On Sat, Sun, Mon, still with ~13hr sunshine but slightly cooler each day, the generation figures were 23.7kWh, 24.4kWh & 24.4kWh.
  28.  
    I see someone over on Navitron was experimenting with cooling their panels with tap water in the afternoon and ended up with bad mineral deposits from their hard water necessitating a trip up to the roof with a bucket and detergent. :wink:
  29.  
    Yes I remember when last year I raised here the issue of my PV panels not performing as well as I expected in high summer and suggesting an automated watering/cleaning solution a couple of people pointed out that the risks and unintended consequences would outweigh any benefits. Right now though I do seem to have a little spare capacity so I might just do some experimentation myself. I only have one inverter for my 2 string 5kW system so one-sided cleaning not an option.

    My panels are mounted in a dip in the roof so as to be the same hight as the surrounding tiless but with a good 40mm gap underneath and are mounted tight so I should be getting signification ventilation currents from the hot air rising albeit only up a 15 deg slope. Ambient shaded air temp high summer is often 35 deg so 60 deg on the panels a very real possibility!
    •  
      CommentAuthorJSHarris
    • CommentTimeJun 1st 2012
     
    <blockquote><cite>Posted By: Gotanewlife</cite> Ambient shaded air temp high summer is often 35 deg so 60 deg on the panels a very real possibility!</blockquote>

    60 deg C on the panels will only result in a loss of about 20% as they are rated at 20 deg C normally, so 60 deg C is only a 40 deg C increase. When I mentioned 60 deg above it was an increase of 60 deg above ambient, so around 80 deg C on the panels, which is way hotter than I believe they get even in pretty hot climates..

    There has to be something else affecting output when you see this big drop in performance, as it seems unlikely to be wholly a temperature effect, as I mentioned above.

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