[Bldg-sim] Real-time pv yield as a surrogate for irradiance

[Chip Barnaby]

In addition to dirt, there is snow cover.  I live in NH and have a 5 kW
PV system with a real-time display of power output.  Even minor snow
cover drops the output to essentially 0. Chip Barnaby

--
Chip Barnaby
chipbarnaby at gmail.com
 
 
 
On Mon, Jul 20, 2015, at 07:49 PM, David Reddy wrote:
> Yes, interesting idea and glad to see it discussed here.  
>  
> 
    This idea also occurred to us when evaluating PV systems on a number
    of buildings in Hawaii.  In that project, we were fortunate enough
    to have a solar
      monitoring site[1] right down the street, and he had 15min and
    1hour PV  energy generation for every panel (micro-invertors) on 15
    different buildings, each oriented in the same direction.  However,
    some other issues, besides the one's Joe described, came to mind in
    trying to develop a model that linked the two:
>  * In addition to temperature, PV panels energy production also
        degrades as the panels get dirty.  That seemed like the variable
        that is toughest to compensate for.  
>  * Data connection between the inverters to the cloud was pretty
        good, but we did find gaps a number of gaps; sometimes for up to
        a month or two.  
> Still yes, an interesting idea, and sounds like a good research
      project...
> 
      David


>  
> On 7/20/2015 4:07 PM, Joe Huang wrote:
>> This is an intriguing idea (using PV output as a surrogate for
      irradiance), but there are a number of complications that need to
      be investigated:
>> 
      1. PV efficiency degrades with temperature; therefore, you have to
      have some way to adjust the output by the temperature of the PV
      (not the air temperature)
>> 
      2. you also need to adjust for the tilt and azimuth of the PV
      panels to derive the equivalent global horizontal radiation.
>> 
      3. you also need to split the global horizontal into direct and
      diffuse radiation, which could be done using a number of
      direct/diffuse split models, which is what's done for 99% of the
      weather files anyway because I've yet to find any measured direct
      normal radiation outside of strictly research applications.
>>  
>> 
      When I look at this list, all three could be done using a building
      energy simulation program,  provided that you know the
      absorptivity of the PV and its conversion efficiency as a function
      of temperature,  but at this point it's getting more hairy than
      calculating the solar radiation from standard weather station data
      or, beginning in Sept 2015 for US locations, downloading the solar
      radiation from NREL. 
>>  
>> 
      I'm not being critical, and actually I am quite intrigued by this
      technique, and would be happy to help evaluate how well it works.
>>  
>> 
      Joe
>>  
>> Joe Huang    
White Box Technologies, Inc.
346 Rheem Blvd., Suite 205A
Moraga CA 94556
>> yjhuang at whiteboxtechnologies.com http://weather.whiteboxtechnologies.com for simulation-ready weather data
>> http://www.whiteboxtechnologies.com
(o) (925)388-0265
(c) (510)928-2683
"building energy simulations at your fingertips"
>> On 7/21/2015 2:21 AM, Justin Spencer
        wrote:
>>> You can use anything you want as a surrogate, as
          long as you have some way of proving it works. In this case,
          if you could somehow get real time PV data for a set of sites
          that also had actual insolation and then looked at your
          results, I think you could derive a typical relationship that
          wouldn't lead you too far astray. The problem will come when
          there are passing clouds and the PV system is "hunting" all
          the time trying to catch up with the changing insolation. I
          bet you could find those and apply a bias correction that
          could then be used in multiple sites with real time PV data. 
>>>  
>>> On Mon, Jul 20, 2015 at 10:55 AM,
            Chris Yates <chris.malcolm.yates at gmail.com>
            wrote:
>>>> Ok, so there's no substitute for doing
                weather data right.


>>>> However, in the absence of such data, could
                real-time pv yield be used as a surrogate?


>>>> Chris


>>>> Sent from my Android device. Please excuse
                typos, etc.


>>>>  
>>>> 
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Links:

  1. http://www.nrel.gov/midc/nelha/
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