When designing a solar system, the most important
calculation is determining the length of the string of solar panels. Solar inverters and charge controllers have
set voltage windows that have to be met by a string of solar panels whose
voltage can vary as much as 40 – 60% throughout the year. With low string voltages, operation is less
efficient and the system can be in danger of shutting off during hot
conditions. Design a string voltage too
high and cold sunny conditions could put the inverter into an overvoltage fault
mode which shuts the inverter down.
Solar designers have to hit the “sweet spot” where their string voltage
will always fall within their equipment’s voltage window while maximizing the
string length for more efficient operation.
This is done by designing solar strings based on the upper voltage limit
of the inverter or charge controller.
Effect of Temperature
on String Voltage
At its basic level, higher temperatures drop voltage and
lower temperatures raise voltage in electronics. For the solar designer, this means string
voltage is at its highest when the temperature is coldest, and the extreme low
temperature is used to design the solar string.
There are two methods for calculating solar string voltage based on
temperature, both outlined in NEC 690.7(A) Maximum Photovoltaic System Voltage:
1) …Maximum
photovoltaic system voltage for that circuit shall be calculated as the sum of
the rated opencircuit voltage of the seriesconnected photovoltaic modules
corrected for the lowest expected ambient temperature …. The rated opencircuit
voltage shall be multiplied by the correction factor provided in Table 690.7…
2) When
opencircuit voltage temperature coefficients are supplied in the instructions
for listed PV modules, they shall be used to calculate the maximum photovoltaic
system voltage as required by 110.3(B) instead of using Table 690.7.
The first method calls for using NEC Table 690.7. To use the table, take your solar panel’s
open circuit Voltage rating (Voc), found in the data sheet, and multiply it by
the temperature correction factor based on your lowest expected ambient
temperature. The lowest expected
temperature can be the record low temperature which can usually be found
online. For example, in Albuquerque, NM,
our record low temperature is 17^{o} F. Converting to C puts it at 27^{o}
C, with a corresponding adjustment factor of 1.21. This means for Albuquerque I would multiply the solar panel’s Voc by 1.21 to find the maximum design voltage for string
sizing. Assuming a typical 60cell solar
panel with a Voc of 37V, the maximum design voltage is 44.77V.
The second method requires using an equation and referencing
the temperature coefficient of voltage found on the solar panel data sheet, but
it gives a more exact answer than using NEC Table 690.7. The temperature coefficient of Voc is usually
between 0.3 and 0.4 % per degree C/K, but it varies from panel to panel. The equation for temperature effect on string
voltage is:
Design Voltage = Voc *(1 + T_{Voc}
* (Design Temperature  25^{o} C))
Using a temperature coefficient of 0.33 %/C and the Voc and
low temperature used in method 1 (37 Voc, 27 C), the design voltage becomes:
Voc *
(1 + (0.0033 * (27  25)) = Voc * (1 +
0.1716) = 43.35V
Note that the voltage determined using voltage coefficient
is slightly lower than that found using the NEC table. The NEC table is the more conservative and
less exact method to use, but it’s also a little easier than using the
temperature coefficient, which gives an exact answer for the extreme minimum
temperature and solar panel. Per NEC
690.7 (A), the temperature coefficient method should always be used if the
temperature coefficient of voltage for the solar panel is known, which it
usually is from the equipment data sheet.
Record Low vs.
Minimum Dry Bulb Temperature
If you continue reading 690.7(A), there is an informational
note on what data can be used for the low temperature in string sizing
calculations:
Informational
Note: One source for statistically valid, lowestexpected, ambient temperature
design data for various locations is the
Extreme Annual Mean Minimum Design Dry Bulb Temperature found in the ASHRAE
Handbook — Fundamentals.
These temperature data can be used to calculate maximum voltage using
the manufacturer’s temperature coefficients relative to the rating temperature
of 25°C.
While it’s a mouthful, the gist of it is if you have access
to the American Society of Heating, Refrigeration, and Air Conditioning
Engineers (AHSRAE) handbook or to their temperature data, you can use the
extreme minimum dry bulb for the low calculation instead of the record low
temperature. This temperature is always
higher than the record low. For example,
in Albuquerque, NM, the record low temperature is 17^{o} F, while the
extreme minimum dry bulb temperature is 10^{o} F. If I run it through the voltage coefficient
equation again, the design voltage becomes 41.52V. This is the difference between a string of 13
and a string of 14 on a 600V input solar inverter, so the improvement by using
this data can be significant.
Sometimes, your exact location isn’t available in the ASHRAE
data tables. In this case, either select
the closest site with similar latitude and elevation, or take an average of
surrounding sites to approximate the minimum dry bulb temperature at that
location. If you want to know the
minimum dry bulb temperature for your location for solar design but don’t have
access to an ASHRAE Handbook – Fundamentals, someone may be able to look it up
for you….
Effect of Mounting
Method on String Voltage
Sometimes the solar system
needs to be designed with shorter strings that are close to the lower bound of
the equipment voltage window, and you need to confirm that the system will work in
the hottest conditions instead of the coldest. The voltage
coefficient equation and NEC Table 690.7 are both only usable for maximum
voltage calculations. This is because
maximum voltage calculations are able to make the assumption that the solar
equipment’s temperature is equal to the ambient air temperature, as the low
design temperature typically occurs in the hour before sunrise. For the minimum voltage, the solar array
needs to be considered when it’s at its hottest, when it’s producing power and
the sun is shining on it. At this point,
the equipment can be much, much hotter than ambient temperature due to the
direct solar radiation it receives.
How do you figure out the design hot temperature for a
minimum voltage calculation? To start,
go back to weather data for your location and find the average high temperature
for the hottest month of the year. This
will become the design temperature in a new temperature coefficient calculation
of voltage. Next, based on the solar
mounting method, select the temperature rise that will be added directly to
this value. These are common values for
temperature rise that the solar industry uses.
Mounting Method

Temperature Rise

< 10^{o} on a flat roof

36^{o} C

> 10^{o} on a flat roof

34^{o} C

Flush mount, pitched roof

32^{o} C

Ground mount

30^{o} C

Pole mount

29^{o} C

Under optimal conditions (pole mounting), the solar array is
assumed to be 29^{o} C hotter than ambient temperature, or 82^{o}
F hotter. It only gets worse the closer
you install to the roof, as air circulation decreases and the array temperature
steadily climbs.
While the max voltage calculations called for using the open
circuit voltage, with minimum the “max power” voltage, or Vmp, needs to be
used. Coupled with the temperature rise
factor, the minimum voltage equation becomes:
Design Voltage = Vmp *(1 + T_{Voc}
* (Design Temperature + Temperature Rise  25^{o} C))
Using an average high temperature of 95^{o} F (35^{o}
C) with a solar panel Vmp of 30V, here’s an example of the minimum voltage of a
solar panel installed on a flush mount:
Vmp *
(1 + (0.0033 * (35 + 32  25)) = Vmp *
(1  0.1386) = 25.84V
This is much, much lower than the 41.5V calculated earlier
for the maximum solar panel voltage. It
highlights the importance of temperature effects on your minimum string size. Based on these numbers, if a solar inverter
with a minimum voltage of 200V were considered, a string of 7 would fail under
hot operating conditions, while a string of 8 would continue to work.
Solar energy will be tomorrow's world's fuel source. So we need to best to do best design for it. It's really most renewable and trusted energy source for future. I'm with Hawaii PV. So I've interest on it.
ReplyDeleteI read your post and i appreciate your efforts. The information that you share in the above article is very nice and useful .All the things that you share with people, are very nice. Thanks for this article Redlands solar
ReplyDelete