[RE-wrenches] calculating low string voltage

boB at midnitesolar.com boB at midnitesolar.com
Tue Nov 29 22:02:30 PST 2011



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
>>
>> NABCEP^TM Certified installer Charter Member
>>
>> NYSERDA-eligible Installer
>>
>> VT RE Incentive Program Partner
>>
>>
>
>
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