Cleaning PV Arrays [RE-wrenches]

[Bill Brooks]

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Joel and Dean,

I don't know how everyone is calculating their power or energy improvements,
and it definitely dependents on where in California you live (this is a
rainy season/dry season issue and soiling location issue), but at PVUSA it
was proved that in a typical year, 7% of energy could be improved by regular
washing--11% on the driest year. A single washing in July will increase
annual energy production by 4%(how many things can you do to a PV system to
increase production 4%--not much).

This past summer we washed our 12 systems in Chino on July 22 and there was
15% after eight weeks from the past washing in May (take a look at the data
http://pierminigrid.showdata.org/data/). We just washed the 3 systems at
PVUSA on September 9 (see data) after over 4 months without cleaning and saw
a 20% increase. These are instantaneous increases, but match up with the
annual energy numbers above. Chino is a particularly bad location for
soiling because of the dairy industry and local construction.

I just washed an array near Long Beach that had not been cleaned since
installed nearly 2 years ago and there was a 35% increase in output (tested
the next day, not while the modules are cold). That is in an area of severe
industrial soiling and salt air.

All in all, to recommend to a client to not clean their arrays in a dry
season location is maybe the safest thing, but it will have a significant
impact on energy production. How severe an impact depends on the local
conditions. The value of cleaning depends on the cost of manpower. In my
estimation, the value of cleaning the arrays in Chino was worth at least
$500 in energy savings over 3 months at $0.10/kWh. For customers that pay
more, the value would be higher.

I recommend that residential customers wash their arrays every month during
the dry season. Once in July is an absolute minimum. The breakage Dean
mentioned was due to that particular module--it was probably already cracked
(although I would never recommend a fire hose if that's what they actually
used). With standard tempered glass crystal silicon modules, a light spray
will cool them enough to spray them down without fear of breakage. The
problem with washing on a hot sunny day is not so much thermal shock (some
care is needed there), the bigger problem is minerals in the water
depositing on the modules. Generally it is best to was them in the early
morning or late afternoon--but with the price of electricity in California,
it is absolutely worth it.

Bill.



-----Original Message-----
From: Dean T. Newberry [mailto:deant at dcn.org]
Sent: Sunday, September 19, 2004 9:09 PM
To: RE-wrenches at topica.com
Subject: Re: Cleaning PV Arrays [RE-wrenches]



Hi Joel,
The modules on the Vets Center are   BP Millennia    MST50/850
BP Solar BP850I 50W Millennia 2J a-Si Module (medium voltage, integral
frame) 47.1

cul  deant

Joel Davidson wrote:
>
>Dean, Do you know what brand and model solar module shattered?
>
>----- Original Message -----
>From: "Dean T. Newberry" <deant at dcn.org>
>To: <RE-wrenches at topica.com>
>Sent: Sunday, September 19, 2004 8:14 PM
>Subject: Re: Cleaning PV Arrays [RE-wrenches]
>
>Hi Mark,
>After 1-1/2 years late one evening I washed my 1kW array with citrus
>cleaner solution scrubbed with a mop, rinsed with a hose and squeeged it
>off,  the two day difference was 4%.
>I don't think it is worth the trouble.
>
>The Davis city crews used a firehose midday when the ambient was 95F.
>One of the modules shattered. Definitely not worth the trouble.
>
>I recommend that my customers let the rain clean the modules.
>
>cul  deant
>
>Mark Byington wrote:
>
>

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