Ahrae temp confusion

Hello,

I have a question about cold temp. and PV module max voltage.

Many people refer to this website for finding EXTREME COLD TEMP, which for Kansas City, where I live is -19 C.

http://www.solarabcs.org/about/publications/reports/expedited-permit/map/

I have heard that it is ok to NOT use this EXTREME COLD TEMP, but to use ASHRAE mean dry bulb temp for system design, because:

1. Irradiance will be low when its THAT cold.
2. When sun hits, panels warm up
3. Inverter regulates with MPPT high voltage.

When I search for ASHRAE design temps, I find this website which lists the extreme annual dry bulb mean as -19.

http://www.scribd.com/doc/137301347/ASHRAE-Temperature-Tables-for-All-US-Cities#scribd

I had hoped that it would be less, therefore allowing me a less conservative system design. Its the same temp.

Where does everybody get their figures from?

Also, when I derate the ampacity of my wire, I need to derate based on height of conduit above roof. On both of the above websites they list the same temps in C for certain heights above the roof. Do I use this figure then refer to 310.15(B)(2)(b) in the NEC for derating?

THANKS, this forum is a vital resource.

Comments

  • VicVic Solar Expert Posts: 2,970 ✭✭✭✭
    Re: Ahrae temp confusion
    Hello,

    I have a question about cold temp. and PV module max voltage ...

    I have heard that it is ok to NOT use this EXTREME COLD TEMP, but to use ASHRAE mean dry bulb temp for system design, because:
    1. Irradiance will be low when its THAT cold.
    2. When sun hits, panels warm up
    3. Inverter regulates with MPPT high voltage.

    When I search for ASHRAE design temps, I find this website which lists the extreme annual dry bulb mean as -19.

    http://www.scribd.com/doc/137301347/ASHRAE-Temperature-Tables-for-All-US-Cities#scribd ...

    THANKS, this forum is a vital resource.

    Hi slurry bowl,

    Regarding the things you have heard on the rationale of why it MIGHT be OK to not use the coldest recorded temps for your area;

    1. Irradiance has little to do with the highest Voc experienced by most solar systems, because, usually, these high Voc values occur before the sun rises, or at least before the sun strikes the PVs ...

    2. So, there is no warming of the PVs from the sun.

    3. Many/most/all inverters will cease operation above a certain voltage, and usually will wait for the Vin to diminish as the temperature rises when the sun actually does start to warm the PVs form direct irradiance. IMO, this is not an MPPT regulation issue, but rather the fact that the inverter will be protecting itself from very high Vocs, and most likely cease operation.

    Dry bulb temperature is the temperature that should matter to the PVs, at least those that I have spoken with.

    Will let others deal with the derating of the roof-top conductors.
    My opinions, Good Luck, Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • HandyBobHandyBob Banned Posts: 31
    Re: Ahrae temp confusion

    Rather than trying to find a guess, how about some personal experience?
    My Helios panels have a Voc of 37.4V. I have 4 in series, which would be 149.6 I saw 162V on the input when it was -20F outside and you know that edge of cloud would be higher. Be cautious. Controllers are expensive.
    I expect that you will be sizing wires based on voltage drop, not ampacity. Realize that the charts are estimates and even if calculating, you will not get exact results because you aren't starting from actual measurements. When I finally hooked mine up and measured I found that I had about .5% more drop than predicted. Not earth shattering, but evidence that all of this is guessing. Most times close enough, but still guessing.
  • BB.BB. Super Moderators, Administrators Posts: 29,511 admin
    Re: Ahrae temp confusion

    Also, the resistance of copper varies with temperature (higher temperatures, higher resistance).

    On a roof/in an attic/sun exposed conduit, the wiring is going run hot and have a higher voltage drop if you do not take the temperature variations into account.

    For a 0C to 20C change in copper temperature, there is about a 9.5% increase in resistance (just a rough/quick back of the envelope calculation).

    National Bureau of Standards Circular No. 31 (1914 Edition)

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Ahrae temp confusion
    I have heard that it is ok to NOT use this EXTREME COLD TEMP, but to use ASHRAE mean dry bulb temp for system design, because:

    1. Irradiance will be low when its THAT cold.

    Voltage does not depend on irradiance. At dawn when first light strikes the array and the inverter is off, the voltage jumps to whatever Voc is for the low temperature that night because the modules are still at that temperature and there is no current flowing. I design conservatively when it comes to maximum cold Voc.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Ahrae temp confusion
    ggunn wrote: »
    Voltage does not depend on irradiance. At dawn when first light strikes the array and the inverter is off, the voltage jumps to whatever Voc is for the low temperature that night because the modules are still at that temperature and there is no current flowing. I design conservatively when it comes to maximum cold Voc.

    And, FWIW, the cell temperature (inside the glass) can actually be colder than the lowest air temperature on a clear cloudless night, since they are radiating into the cold dark sky. :-)
    SMA SB 3000, old BP panels.
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Ahrae temp confusion
    inetdog wrote: »
    And, FWIW, the cell temperature (inside the glass) can actually be colder than the lowest air temperature on a clear cloudless night, since they are radiating into the cold dark sky. :-)

    Quite correct. You must design for temperatures that are colder than the record low temperatures. This is because of radiative cooling.

    On clear, still nights the air absorbs very little infrared radiation. Surfaces cool by radiation into space, and the cold surfaces cool the air by conduction. If the air temp is cold near your panels (on a still, clear night), it is because your panels are colder than the air and it is your panels that are cooling the air.

    Air temperatures are usually measured several feet off the ground. On cold clear nights the record low temperatures are usually set at dawn. This is because sunrise causes a breeze. The breeze stirs up the coldest air (at ground level) and the thermometer (several feet above the ground) briefly dips a few degrees.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • Ethan BrushEthan Brush Solar Expert Posts: 231 ✭✭
    Re: Ahrae temp confusion
    Voltage does not depend on irradiance. At dawn when first light strikes the array and the inverter is off, the voltage jumps to whatever Voc is for the low temperature that night because the modules are still at that temperature and there is no current flowing. I design conservatively when it comes to maximum cold Voc.
    Quite correct. You must design for temperatures that are colder than the record low temperatures.

    First of all let me say that everyone is free to design in a way that makes them comfortable with the long term function and reliability. Life is full of risks and generally one is free to chose what they feel is the best compromise between risk and cost/expediency/etc.

    Now that that is out of the way, there was an article in solar pro or some other publication a little while ago where the author's thesis (which I agree with) was that designers are often overly conservative with their VOC calculations. One big reason is that irradiance is lower when these sever low temp happen and irradiance absolutely effects VOC - quite a bit especially when you have 12-13 panels in series. Put a voltmeter on a panel facing the sun, then point it the other way and tell me that the difference isnt worth taking into account. Another reason presented was that a CC or inverter is not going to fry at 601 volts. If you get a super low temp concurrent with some high irradiance phenomenon and you voltage gets up to 615 volts, its probably going to be fine.

    Not that it matters what I do and I dont try to push my methodology on anybody, but for me I think it is most logical to take into account other factors than temperature such as whether the system is tracked, what time the sun hits the panels in winter, possibility of array getting reflectance from the ground, etc. If all those factors were on the higher risk side I would be more inclined to go closer to the record low for the VOC calculations.
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: Ahrae temp confusion
    I don't try to push my methodology on anybody.

    Well to be honest, neither do we. But when someone needing assistance and asking questions arrives, we try, based on knowledge and experience, to give them the best advice several of us can, in the hopes they will go on to solve their problems and or end up with an awesome, viable, reliable and safe system. Several experienced and knowledgeable heads are always better than one.
  • Ethan BrushEthan Brush Solar Expert Posts: 231 ✭✭
    Re: Ahrae temp confusion

    ...And just for the complete picture on that article: He wasnt just saying to use a higher than record low temp just to up the risk for the heck of it or just to keep things interesting ;) , the reason is that apparently it is somewhat of a chronic problem in the industry to have systems that dip out of the controllers voltage range in very hot temperatures, perhaps because of excessive focus and worry about the low temp VOC.

    One final note is that, for whatever its worth, the NEC doesnt use the phrase or require VOC adjustment for "record low temp", its says "lowest expected ambient temperature" :)
  • solarvicsolarvic Solar Expert Posts: 1,053 ✭✭✭✭
    Re: Ahrae temp confusion

    I think some inverters record the highest voltage they received. If it is over the limit it could void your warranty. People that know for sure might verify if I am right or wrong. I know I over voltage mine about 50 volts a couple years ago and it still works flawlessly. Solarvic
  • VicVic Solar Expert Posts: 2,970 ✭✭✭✭
    Re: Ahrae temp confusion
    ...
    If you get a super low temp concurrent with some high irradiance phenomenon and you voltage gets up to 615 volts, its probably going to be fine ...

    ... it is most logical to take into account other factors than temperature such as whether the system is tracked, what time the sun hits the panels in winter, possibility of array getting reflectance from the ground, etc. If all those factors were on the higher risk side I would be more inclined to go closer to the record low for the VOC calculations.

    BUT, in an attempt to try to be clear of under what conditions highest Voc occurs;

    Generally the highest Voc of the day is in the EARLY morning, before any sun, whatsoever strikes the PV cells. There is enough light for the PV cells to output voltage, but there is almost NO CURRENT available, and the inverter or CC is presenting NO LOAD (therefore Voc) on the PV arrays.

    The HIGH irradiance (implied onto the PV cells) that you mention would mean that the PV cells would be warmed significantly by the sun, AND that there would be a load on the PV arrays. Both of these factors would significantly reduce the voltage produced by the PVs -- they would no longer be Voc (Voltage, Opec Circuit)

    It has seemed that you are equating high Irradiance with highest PV voltage. This is not the case. High irradiance heats the PV cells, and significantly reduces the PV voltage, as does loading of the PVs by an Inverter, or CC.

    There are limits on the input voltage that inverters and CCs can tolerate. The manufacturers know best what are these limits. If one designs a system to exceed these high voltage limits, that is fine, as long as that designer accepts the responsibility of the results, In My Candid Opinion. Many CCs and GT inverters record the highest Voc. Above their specified Limit, many/most/all can or will void the manufacturer's warranty.

    Just to beat this poor old dead horse a bit more. FWIW, Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • Ethan BrushEthan Brush Solar Expert Posts: 231 ✭✭
    Re: Ahrae temp confusion
    Vic wrote: »
    BUT, in an attempt to try to be clear of under what conditions highest Voc occurs;

    Generally the highest Voc of the day is in the EARLY morning, before any sun, whatsoever strikes the PV cells. There is enough light for the PV cells to output voltage, but there is almost NO CURRENT available, and the inverter or CC is presenting NO LOAD (therefore Voc) on the PV arrays.

    The HIGH irradiance (implied onto the PV cells) that you mention would mean that the PV cells would be warmed significantly by the sun, AND that there would be a load on the PV arrays. Both of these factors would significantly reduce the voltage produced by the PVs -- they would no longer be Voc (Voltage, Opec Circuit)

    It has seemed that you are equating high Irradiance with highest PV voltage. This is not the case. High irradiance heats the PV cells, and significantly reduces the PV voltage, as does loading of the PVs by an Inverter, or CC.

    There are limits on the input voltage that inverters and CCs can tolerate. The manufacturers know best what are these limits. If one designs a system to exceed these high voltage limits, that is fine, as long as that designer accepts the responsibility of the results, In My Candid Opinion. Many CCs and GT inverters record the highest Voc. Above their specified Limit, many/most/all can or will void the manufacturer's warranty.

    Just to beat this poor old dead horse a bit more. FWIW, Vic

    There are two different things at play: one is temperature vs VOC, the other is irradiance vs VOC. The VOC listed on the back of the module is at standard test conditions which is 25 degrees C and 1000W/sqm irradiance (I know you all knew that). We adjust for the temperature, but we do not adjust for the irradiance. I agree with your statement about when highest VOC occurs, however the module will be seeing much much less than 1000w/ sqm irradiance then so the voltage will be significantly lower than our temperature adjusted VOC calculation shows. I used the adverb "significantly" so what does that mean? Well it varies based on the installation type and location, but enough to add that extra module in the string that didnt quite fit and often enough to add an entire module. Say we have 90% of STC temperature adjusted VOC which would mean about 200W/ sqm; that would make a calculated 600V string 540 volts Try it: Measure the VOC of an array sometime just before sunrise and compare it with the temperature adjusted value from the back of the module and tell me what you get. Really, if anyone has done this I would be curious what the results are. I dont get a chance to test that much because Im not an early person :)

    Note that I am NOT saying to feel free to design a system whose VOC exceeds the equipment rating a little bit. I argue that it will still never get there if you are a reasonable amount less conservative than using all time record low, thats all.
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: Ahrae temp confusion
    Vic wrote: »
    Just to beat this poor old dead horse a bit more. FWIW, Vic

    And although you can lead that poor horse to the water of enlightenment, you can't make him drink if he doesn't realize he's thirsty. :D
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Ahrae temp confusion
    Really, if anyone has done this I would be curious what the results are. I don't get a chance to test that much because Im not an early person :)
    No need to make the measurement, since many panel makers provide graphs of I versus V for different levels of irradiance at a fixed temperature.
    If I recall correctly the difference in Vmp was only a percent or two from 1000w/m2 to 100w/m2. The difference in Voc was similar, possibly slightly larger.
    So if you are trying to push the string voltage limit to the last volt or two, you need to figure out what the expected irradiance is when the morning sun first hits the panels full force (i.e. after dawn, with sun fully over local horizon) and do the full calculation. Then factor in that panel to panel (batch to batch) variation may easily be as much as 5% and you are back in trouble again.
    SMA SB 3000, old BP panels.
  • Ethan BrushEthan Brush Solar Expert Posts: 231 ✭✭
    Re: Ahrae temp confusion
    inetdog wrote: »
    No need to make the measurement, since many panel makers provide graphs of I versus V for different levels of irradiance at a fixed temperature.
    If I recall correctly the difference in Vmp was only a percent or two from 1000w/m2 to 100w/m2. The difference in Voc was similar, possibly slightly larger.
    So if you are trying to push the string voltage limit to the last volt or two, you need to figure out what the expected irradiance is when the morning sun first hits the panels full force (i.e. after dawn, with sun fully over local horizon) and do the full calculation. Then factor in that panel to panel (batch to batch) variation may easily be as much as 5% and you are back in trouble again.


    Your numbers for VOC decline due to lower irradiance are a bit low. The figure of 90% at 200W/sqm used in my previous post is pretty reasonable. If we chose a 5% reduction due to low irradiance that would be 30 volts on a 600 volt system - that is some substantial "real estate" and in some cases can really make for a much easier and more straight forward install. The increased window would be more pronounced on a 1000 system which are becoming more common. The voltage difference and thus benefit on an off grid system that likely operates at 150 V max are going to be much less of course and perhaps not significant enough to consider irradiance in many cases.

    Here is the article in solarpro. It is very informative and a good read. Perhaps after reading it, you will feel more comfortable about going with a higher string voltage than you otherwise might have done.

    http://solarprofessional.com/articles/design-installation/array-voltage-considerations
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Ahrae temp confusion
    Your numbers for VOC decline due to lower irradiance are a bit low. The figure of 90% at 200W/sqm used in my previous post is pretty reasonable. If we chose a 5% reduction due to low irradiance that would be 30 volts on a 600 volt system - that is some substantial "real estate" and in some cases can really make for a much easier and more straight forward install. The increased window would be more pronounced on a 1000 system which are becoming more common. The voltage difference and thus benefit on an off grid system that likely operates at 150 V max are going to be much less of course and perhaps not significant enough to consider irradiance in many cases.

    Here is the article in solarpro. It is very informative and a good read. Perhaps after reading it, you will feel more comfortable about going with a higher string voltage than you otherwise might have done.

    http://solarprofessional.com/articles/design-installation/array-voltage-considerations
    Thank you, that is for the most part a good source of information.
    One thing that it does not consider is the case of partial shading. if one module in the string is shaded and the bypass diodes are driven into conduction, then that panel will not contribute to the string Vmp at all. This can increase the number of panels required in a string to allow the GTI to continue to operate at full power with a shaded string.
    The other salient point from the article is that (if you use his 85% degradation factor, which I consider far too conservative for voltage calculations) even a range from Vmin to Vmexf of 300V to 600V is not wide enough to handle temperature and degradation extremes.
    With luck the GTI will continue to operate at the low voltage end, but will do so by reducing the current below Imp to allow the string voltage to rise.
    SMA SB 3000, old BP panels.
  • Ethan BrushEthan Brush Solar Expert Posts: 231 ✭✭
    Re: Ahrae temp confusion
    inetdog wrote: »
    Thank you, that is for the most part a good source of information.
    One thing that it does not consider is the case of partial shading. if one module in the string is shaded and the bypass diodes are driven into conduction, then that panel will not contribute to the string Vmp at all. This can increase the number of panels required in a string to allow the GTI to continue to operate at full power with a shaded string.
    The other salient point from the article is that (if you use his 85% degradation factor, which I consider far too conservative for voltage calculations) even a range from Vmin to Vmexf of 300V to 600V is not wide enough to handle temperature and degradation extremes.
    With luck the GTI will continue to operate at the low voltage end, but will do so by reducing the current below Imp to allow the string voltage to rise.

    Yes that example certainly worked out beautifully and I imagine was set up that way. For those who didnt read the article, the max number of modules per string calculated from record low was 13.9. Using ASHRAE low got it up to 14.2. But that is good, many of us were taught that that 13.9 is a "line in the sand" and "irradiance effects current, temperature effects voltage". I assume this has often put excessive fear and focus on the high end, while low voltage is important for long term maximum energy harvest.
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Ahrae temp confusion
    solarvic wrote: »
    I think some inverters record the highest voltage they received. If it is over the limit it could void your warranty. People that know for sure might verify if I am right or wrong. I know I over voltage mine about 50 volts a couple years ago and it still works flawlessly. Solarvic
    That is indeed true for some inverters. They have a "black box" register that records and stores the highest voltage seen by the inputs of the inverter and if you exceed their published maximum allowable voltage, they can revoke the warranty.
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