[RE-wrenches] Inverter oversizing

David Brearley david.brearley at solarprofessional.com
Thu Jan 20 12:35:01 PST 2011 

I see what your looking at; I was comparing Figures 3 & 4, not 2 & 3.


On 1/20/11 2:21 PM, "Kent Osterberg" <kent at coveoregon.com> wrote:

> David,
> 
> Take a close look at the upper and lowers portions of the graphs of the
> 10-second data and the 1-minute data for Freiburg.
> 
> To make it easier to see, attached is reproduction of their graphs on one
> page. Just as fleeting peaks in irradiance (edge of cloud effect) should be
> fewer in the 1 minute averages than in the 10 second raw data; fleeting lows
> (caused by passing clouds) should be fewer in the 1 minute averages than in
> the 10-second data. Inspect the graph and label which you think is 10-second
> data and which you think is 1-minute data. Then compare to the original. The
> two graphs appear to be mislabeled.
> 
> Kent Osterberg
> Blue Mountain Solar, Inc.
> 
> 
> 
> David Brearley wrote:
>>  Re: [RE-wrenches] Inverter oversizing Kent,
>>  
>> The graphs aren¹t mislabeled. The one-minute average data has more
>> granularity, which shows up at the upper limits of the data. It reveals more
>> peaks over 1,000 W/m2, even in excess of 1,200 W/m2. These values get
>> compressed into the lower bins in the hourly data.
>>  
>> The results from the Florianopolis site in Brazil, gives a pretty good idea
>> of how significantly different the results can be based on monitoring
>> frequency:
>>  
>> ³Considerable differences emerge when looking at the high end of the
>> radiation level
>> Distribution, which shows for the one-minute averages that some 9% of the
>> daytime 
>> hours present radiation levels „ 1000W/m2, with a corresponding energy
>> content of
>> some 23%; hourly averages for the same range correspond to around 6% of
>> daytime 
>> hours, and below 11% of the total energy content. Radiation levels above
>> 900W/m2 
>> occur some 16% of the time when looking at one-minute averages, and below 13%
>> of daytime hours when using hourly averages, with corresponding energy
>> fractions 
>> respectively above 38% and 25%.²
>>  
>>> >From the conclusion:
>>  
>> ³we have demonstrated that the estimation of the actual losses due to
>> inverter
>> undersizing increases with increased time resolution of the radiation
>> measurements, 
>> revealing that hourly averages hide important irradiation peaks. In fact,
>> results with 
>> hourly averages are an experimental artifact, and lead to an estimation of
>> the solar 
>> energy resource distribution that does not correspond to reality. Hourly
>> averages of
>> irradiation values lead to inverter undersizing and the associated energy
>> losses.²
>>  
>> Figures 8 & 9 are interesting. You could imagine what these results would
>> look like if overlaid onto your charts.
>>  
>> Best, david
>>  
>> On 1/19/11 8:14 PM, "Kent Osterberg" <kent at coveoregon.com> wrote:
>>  
>>   
>>> David,
>>>  
>>> Thanks for sharing that paper.  The labeling the graphs for the 10-second
>>> and 1-minute data in Freiburg appears to be reversed - the one minute
>>> averaging seems to have more data in all of the bins above 1000 W/sq m.
>>> Basically, these graphs show that irradiance observations above 1100 watts
>>> per square meter are fleeting and disappear in hourly averages.   Such
>>> occurrences are also masked to a small extent by 1-minute averages.
>>>  
>>> Kent Osterberg
>>> Blue Mountain Solar, Inc.
>>>  
>>>  
>>>  
>>> David Brearley wrote:
>>>   
>>>>  Re: [RE-wrenches] Inverter oversizing This reminds me of a scholarly
>>>> article I came across about a year ago while doing some research. Here¹s a
>>>> link to it if anyone is interested:
>>>>  
>>>> www.lepten.ufsc.br/publicacoes/solar/eventos/2005/PSC/burger_ruther.pdf
>>>> <http://www.lepten.ufsc.br/publicacoes/solar/eventos/2005/PSC/burger_ruther
>>>> .pdf> 
>>>> <http://www.lepten.ufsc.br/publicacoes/solar/eventos/2005/PSC/burger_ruther
>>>> .pdf> 
>>>>  
>>>> David Brearley, Senior Technical Editor
>>>>  SolarPro magazine
>>>> NABCEP Certified PV Installer 
>>>>  david.brearley at solarprofessional.com
>>>> Direct: 541.261.6545
>>>>  
>>>> On 1/19/11 12:29 PM, "Bill Brooks" <billbrooks7 at yahoo.com> wrote:
>>>>  
>>>>   
>>>>   
>>>>> Kent,
>>>>>  
>>>>> How often were your data records? To capture edge of cloud effects, you
>>>>> need one-second data. Not many people gather that fast or that much data
>>>>> on inverters. I don¹t think there is that much energy in these spikes, but
>>>>> they are real and make some difference. 15-minute average data will
>>>>> completely wash out this data.
>>>>>  
>>>>> This is also a deficiency in modeling software since most models are using
>>>>> hourly data.
>>>>>  
>>>>> Bill.
>>>>>  
>>>>>  
>>>>>   From: re-wrenches-bounces at lists.re-wrenches.org
>>>>> [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Kent
>>>>> Osterberg
>>>>>  Sent: Tuesday, January 18, 2011 1:18 PM
>>>>>  To: Wrenches; Marco Mangelsdorf
>>>>>  Subject: [RE-wrenches] Inverter oversizing
>>>>>   
>>>>>  
>>>>> Attached is a graph that I produced to document the effect of various
>>>>> ratios between the PV array size and the inverter size.  I extracted
>>>>> output power data for a 1020-watt system located in NE Oregon that is on
>>>>> the Sunny Portal
>>>>> <http://www.sunnyportal.com/Templates/PublicPageOverview.aspx?page=85820a7
>>>>> 3-a347-48fb-b8d1-92e5f9b78ab3&plant=608681a7-ef60-4edb-84ff-07110db0ab6a&s
>>>>> plang=en-US> . The data are publicly accessible so feel free to run your
>>>>> own analysis.  Better yet, analyze the data for a system near you.
>>>>>  
>>>>> Using 2009 data, I looked at how much energy would have been lost if the
>>>>> output was clipped at 800W, 810W, .... 1020W.  I used 2009 data because
>>>>> there was a period in 2010 when the Sunny Webbox didn't have internet
>>>>> access.  At 800 watts, power clipping would have happened on about 25% of
>>>>> the days.  Yet the energy that would have been lost was only 0.38% of the
>>>>> annual total.
>>>>>  
>>>>> The results shown on this graph aren't universal, results would be a
>>>>> little different in 2010, it would be different in some other climate, it
>>>>> would have been different at another elevation, it would be different with
>>>>> a different array angle, ..., and the module tolerance and inverter
>>>>> efficiency also effect the results.  Modules in this system are Suntech
>>>>> 170-watt +/-3%.  The inverter is Sunnyboy 1800 that  should be operating
>>>>> at close to 93% efficiency.
>>>>>  
>>>>> Kent Osterberg
>>>>> Blue Mountain Solar. Inc.
>>>>>  
>>>>>  
>>>>>  
>>>>   
>>>>  
>>>>  
>>>> 
>>>> 
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>>  
>>  David Brearley, Senior Technical Editor
>>  SolarPro magazine
>> NABCEP Certified PV Installer 
>>  david.brearley at solarprofessional.com
>> Direct: 541.261.6545
>> Fax:  541.512.0343
>>  
>>  Visit our Web site at solarprofessional.com
>>  
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David Brearley, Senior Technical Editor
SolarPro magazine 
NABCEP Certified PV Installer 
david.brearley at solarprofessional.com
Direct: 541.261.6545
Fax:  541.512.0343

Visit our Web site at solarprofessional.com

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