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--- electrical:solar:output [2023/09/24 17:33]
frater_secessus [where and when]
+++ electrical:solar:output [2025/06/18 22:30] (current)
frater_secessus [panel temperature]
@@ Line 19: / Line 19: @@
-The location and season both have profound effects on solar harvest. [[opinion:frater_secessus:panelsizesforinsolation#representative_areas|For example]], Phoenix AZ has **over 8.5x the amount of solar power** available in summer as Seattle WA in the winter.  For any given location, winter harvest will be about half of summer harvest.
+The location and season both have profound effects on solar harvest. [[opinion:frater_secessus:panelsizesforinsolation#representative_areas|For example]], Phoenix AZ has **over 8.5x the amount of solar power** available in summer as Seattle WA in the winter.  For any given location, winter harvest can tiny compared to summer harvest.
-You can estimate solar harvest using [[electrical:solar:pvwatts|models like PVwatts]].
+For more specific numbers you can estimate solar harvest using [[electrical:solar:pvwatts|models like PVwatts]].
@@ Line 51: / Line 51: @@
 ===== panel temperature =====
-Solar panels are dark in color and get very hot.  Unfortunately, voltage (and therefore power) output //decreases// as panel temperature //increases//.  This is the reason an air gap between the panels and the camper's roof is recommended to allow cooling airflow.
+Cell temperature affects the voltage part of the panel's I/V curve; warmer temps drive voltages down and very cold temperatures drive it up.  This is why Voc+20% or similar rules of thumb are used when assessing how much voltage "headroom" the solar charge controller has -- a cold snap could, for example, put a 48Voc rated array over a 50v controller input limit.
+For this reason the array's real-world Vmp is rarely the Vmp given on the lab rating.  It doesn't matter much to PWM but you will observe your MPPT finding a Vmp that does not match rated Vmp.  And since with MPPT ''Vmp * Imp = Mpp'' lower Vmp means lower harvest.  Hence the panel mounting airgap;  it allows cooling airflow underneath.
+==== high cell temperatures ====
 The data below, derived from [[http://digivation.com.au/solar/tempderate.php|this calculator]], show power from a 100W mono/poly panel dropping off as ambient temps rise:
@@ Line 70: / Line 76: @@
 So a [[camping:snowbirding|snowbird]] who "chases 60" will be losing ~10% of panel output during the warmest part of the day.  Snowbirds chasing 70 will be losing ~13% of panel output.
-Note:  sub-freezing temperatures can push Voc/Vmp above their rated values.  If you will use solar in sub-freezing conditions leave plenty Voc headroom in your solar charge controller spec.
 Note: that [[hvac:insulation#heat_from_solar_panels|radiated heat from the underside of panels can raise temperatures inside the camper]].
+==== low cell temperatures ====
+Sub-freezing temperatures can push Voc/Vmp above their rated values.  If you will use solar in sub-freezing conditions leave plenty of Voc headroom in your solar charge controller spec. +20% margin is traditional.
@@ Line 203: / Line 213: @@
 Pro:
-  * +30% daily harvest is possible, depending on the sun's position((peak harvest may be much higher at some times but daily harvest is more applicable for offgrid vanlife))
+  * optimal tilt will increase harvest when the sun is low and skies are clear.((peak harvest numbers are more impressive but of less import.))
-  * can be used to increase harvest if roof space is maxxed (cannot add more panel)
+  * can be used to further increase harvest if roof space is maxxed (cannot add more panel)
 Con:
@@ Line 233: / Line 243: @@
 [[https://www.sciencedirect.com/science/article/pii/S1876610214011874|According to lab testing]], dusty panels cause a derating of 5%-6%.
+===== sensitivity to light spectrum =====
+Note:  these differences exist but are of little practical effect.
+==== differences in light ====
+  * direct sunlight is broad-spectrum (a mix of light wavelengths)
+  * when the sun is near the horizon the available light is more red (longer wavelengths)
+  * when skies are overcast or otherwise diffused the available light is more blue (shorter wavelengths)
+Since the latter point is counterintuitive:
+>> at visible wavelengths, overcasts are far from spectrally neutral transmitters of the daylight incident on their tops. Colorimetric analyses show that overcasts make daylight bluer and that the amount of bluing increases with cloud optical depth. Simulations using the radiative-transfer model MODTRAN4 help explain the observed bluing: multiple scattering within optically thick clouds greatly enhances spectrally selective absorption by water droplets.((at visible wavelengths, overcasts are far from spectrally neutral transmitters of the daylight incident on their tops. Colorimetric analyses show that overcasts make daylight bluer and that the amount of bluing increases with cloud optical depth. Simulations using the radiative-transfer model MODTRAN4 help explain the observed bluing: multiple scattering within optically thick clouds greatly enhances spectrally selective absorption by water droplets. ))
+==== differences in panel sensitivity ====
+{{ https://img.mousetrap.net/misc/Spectral_Response_PV-annotated2.jpg?125}}
+Note:   visible light is roughly 400-750nm.((https://en.wikipedia.org/wiki/Light)).  Below that is UV and above that is infrared.
+  * [redder end]
+  * CIGS (copper indium gallium selenide) thin-film panels have peak sensitivity around 950nm.
+  * monocrystalline silicon panels (mono, c-Si) tend to have maximal spectral response around 900nm
+  * polycrstalline silicon panels (poly, mc-Si,pc-Si)((mc-Si and pc-Si are both made from multiple crystals.  Grain size differes.  See https://www.pveducation.org/pvcdrom/manufacturing-si-cells/types-of-silicon)) slightly lower than mono.  We will stipulate 850nm.
+  * GaAs (gallium arsenide) thin-film panels have peak response around 825nm.
+  * CdTe (Cadmium Telluride) flex panels, the most popular thin-film type, have peak response around 750nm.
+  * amorphous silicon panels (a-Si, the original noncrystalline panel) - have peak response around 550nm.
+  * [ bluer end ]

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