Dear all,

I want to simulate the PV yield and I am using Type94a (see attachement).
I have some knowledge on the 4-parameter PV model, I have two major questions or remarks:

I am using TRNSYS 17.01.xx

############# FIRST #########################:
I am not entirely clear but I think, that Type94a when operated with
ParameterValue(16) (tau-alpha)>0, that is without the IAM-correction,
e.g. tau-alpha=0.9, does not consider tau-alpha for the calculation of the light-current I_L.

The annual yield is not sensitive wrt tau-alpha ():

- Run the attached project (Project_Type94a_noWeatherFile.tpf) (tau-alpha=0.9) and record the value for Wpv_kWh7m2 in the online plotter at the end of the year (my result 675.8 kWh/m2)

- change tau-alpha to 0.5 and repeat the last line (my result 686.4 kWh/m2), the result is higher which is not really physical with nearly half the radiation from before (reason: the cell-temperature is smaller - see below, and not tau_alpha correction for the light current)

You may repeat these steps for a project (Project_Type94a_.tpf) with real weather data
my results for tau-alpha=0.9 : 232.2 kWh/m2
my results for tau-alpha=0.5 : 226.7 kWh/m2 (the difference is too small)

I cite the relevant source code lines (see attachment) in the following.
Lets assume tau-alpha=0.9;

Line 240: tamax = DABS(getParameterValue(16)) ! --> tamax=0.9 represents the optical efficiency factor
Line 245: If (getParameterValue(16) < 0) Then
Line 248: incangmod = 0 ! incangmod = 0 since getParameterValue(16) > 0
Line 249: tau_al = tamax ! tau_al is set constant
Line 393: sun = getInputValue(1)/3.6d0 ! read the total radiation on the PV-array
Line 498: tc = ta + (sun*(tcnoct-tanoct)/sunnoct*(1.-effref/tau_al)) ! tau_al is used for the calculation of the cell temperature
Line 501: il = (sun/sunr)*(ilr+misc*nprl*(tc-tcr)) ! for the calc. of the light current tau_al is not used; that is, it seems there is no optical efficiency considered
Am I missing sth here, or do I use an old source code file (please see attachement)?
As I understand in this case without IAM-correction there is an error in the source-code.

############# SECOND #########################:
Lets assume tau-alpha=-0.9; --> incangmod = 1

I don't understand where type94a gets a value assigned for sungnd?
Where does the sungnd come from, I mean Type94a has no parameter relating to the ground reflectance.
Line 497: sungnd = sungnd*tagnd
sundir = sundir*tadir
tau_al =(sundiff+sungnd+sundir)/sun
sun = sundiff+sungnd+sundir

Thank you for any responses.
best regards

MArtin
_______________________________________________________________

Mag. Ing. Martin Felix Pichler
Institut fuer Waermetechnik / Institute of Thermal Engineering
TU Graz / Graz University of Technology
Inffeldgasse 25B, A-8010 Graz

Tel.: +43 316 873 7312
Fax : +43 316 873 7305
http://www.iwt.tugraz.at/