[RE-wrenches] calculating low string voltage

[Joel Davidson]

...or that most of the time, PV systems do not operate at the array and inverter maximum power points...or that test instruments at the best labs are only accurate to 3% to 5% (K. Emrey, NREL) which brings to question some of the tight module specifications we now see...or that a one-time snapshot is not as accurate as a moving picture. Nevertheless, an experienced diagnostician can tell if everything (but not necessarily any specific thing) is ok or something is amiss.

Joel Davidson
  ----- Original Message ----- 
  From: boB at midnitesolar.com 
  To: RE-wrenches 
  Sent: Tuesday, November 29, 2011 10:02 PM
  Subject: Re: [RE-wrenches] calculating low string voltage




  If you really want to throw a monkey wrench into the mix, bring up partial shading...

  Or, maybe that's just not allowed in that debate ??    Just a thought.

  boB







  On 11/29/2011 8:39 PM, Kent Osterberg wrote: 
    Kirk,

    You've got some good feedback on this from Ray and Bill. I'll try to add a little more. Most module datasheets show a normal operating cell temperature, NOCT, value that's typically 47.5°C. That's 20°C ambient temperature, 800 W/sq m, and calm wind and nothing blocking the airflow on the back of the module. That 27.5°C temperature rise should be pretty close to the temperature rise that occurs for a pole-top mount. It's common to see people use 25°C for modules on a pole and 30°C or 35°C on a roof and there are data that support these "typical" values. With 1000 W/sq irradiance, the temperature rise can obviously be more too. Between the intensity of the sun, the direction of the sun, the color of the roof, the spacing off the roof, and the wind speed there is a lot that is different from one system to the next or even one day to the next.

    Most PV module spec sheets don't give you a temperature coefficient for Vmp. I've seen people use the the Voc coefficient, usually expressed as a percentage, for both Voc and Vmp. Big mistake. Data from NREL indicates as Bill said, the temperature coefficient for Vmp is higher than that the temperature coefficient for Voc. That's particularly true when the coefficient is expressed as a percentage per °C. Since there are very few manufacturer's that give both temperature coefficients, I'll use a value from an old Evergreen module for an example. The Evergreen ES-195 datasheet shows Voc = 30.5 volts with a coefficient of -0.34%/°C and Vmp = 27.1 volts with a coefficient of -0.47%/°C. Since one shouldn't add volts and percents, I'll put the temperature coefficients in volts/°C: Voc = 30.5 V - 0.10 V/°C and Vmp = 27.1 V - 0.13V/°C. So Vmp is moving faster than Voc, but not a lot faster. That's generically true for c-Si or poly-Si.

Kent Osterberg
Blue Mountain Solar, Inc.
www.bluemountainsolar.com
t: 541-568-4882
    On 11/29/2011 11:48 AM, Kirk Herander wrote: 
      I am in debate with a PE over calculation of low voltage of a series string on a hot day. He insists that an arbitrary high cell temp is factored in, not just ambient temperature. Could someone please give an accepted formula for this calculation? Thanks. I cannot find a clear reference to low voltage calculation on a hot day (but every reference material is clear on how to calculate high voltage on a cold day).



      Kirk Herander

      VT Solar, LLC

      dba Vermont Solar Engineering

      NABCEPTM Certified installer Charter Member

      NYSERDA-eligible Installer

      VT RE Incentive Program Partner




       

     

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