[ENH] cell temperature from backside of module temperature measurements

[mikofski]

Is your feature request related to a problem? Please describe.
recently a question was posed, given backside module temperature, how to include them in the model chain? Looking at the temperature models available, there isn't one that takes module temperature as input and returns cell temperature. SAPM has a formula for adjusting module temperature to obtain cell temperature based on irradiance and constant "delta-T" parameter that is characteristic of the module. However this formula is bundled into pvlib.temperature.sapm_cellas Eq (2) and the function doesn't take module temperature as input

Describe the solution you'd like
add an optional kwarg to pvlib.temperature.sapm_cell that allows it to bypass the module temperature calculation.

pvlib-python/pvlib/temperature.py

Line 117 in ae98bf9

module_temperature = sapm_module(poa_global, temp_air, wind_speed,

option 1:

add module_temperature as kwarg

def sapm_cell(poa_global, temp_air, wind_speed, a, b, deltaT,
              irrad_ref=1000, module_temperature=None):
    if module_temperature is None:
        module_temperature = sapm_module(poa_global, temp_air, wind_speed,
    else:
        warning.warn('air temp and wind speed parameters ignored, overridden by module temp')
        return module_temperature + (poa_global / irrad_ref) * deltaT

option 2:

add new pvlib.temperature.sapm_module_to_cell func

def sapm_module_to_cell(module_temperature, poa_global, deltaT, irrad_ref=1000):
    return module_temperature + (poa_global / irrad_ref) * deltaT

note: if option 2, then pvlib.temperature.sapm_cell could be updated to call pvlib.temperature.sapm_module_to_cell

Describe alternatives you've considered

• We could explain to users that they can back out the conditions to the use pvlib.temperature.sapm_cell
• we could encourage users to formulate the equation themselves from the literature
• we could implement a 3rd SAPM function to perform this task seperately, call it pvlib.temperature.sapm_module_to_cell?

Additional context
related to this post on the Google group

[wholmgren]

-1 for a new kwarg, +1 for a new function

[cwhanse]

+1 for pvlib.temperature.sapm_cell_from_module rather than sapm_module_to_cell, I'd prefer to keep the output earlier in the name string