Solmetric PVA-1000S, User Manual

Page: 98 / 152

3 Software Overview
3-30
Irradiance Measurement
SolSensor measurement of irradiance involves these steps:
•
SolSensor is clamped to the module frame, which orients it in the plane of the
array.
•
The position of the sun relative to the orientation of the irradiance sensor is
calculated based on the array azimuth and tilt and the site latitude and longitude.
•
The irradiance value is temperature corrected based on the reading of a
temperature sensor attached to the irradiance sensor and on the angle of
incidence of the irradiance.
•
One of four spectral corrections is applied, according to the array’s PV module
technology (poly-Si, c-Si, HIT-Si, or CdTe). The corrections are derived from
module/sensor spectral overlap calculated by SMARTS atmosphere model as a
function of air mass and elevation.
This process yields the effective irradiance incident on the PV module. Effective
irradiance is that portion of the total irradiance that the module can convert into
electricity.
Prediction of PV Performance
The PVA software compares the measured I-V curve to the predictions of built-in PV
performance models. The Performance Factor is calculated from this comparison. The
prediction process involves these steps:
•
User creates a Project and inputs all of the required parameters, including
selection of their PV module from the built-in equipment database.
•
The model predicts I
sc
, I
mp
, V
mp
, and V
oc
for the current values of irradiance and
temperature.
•
Under low light conditions, the prediction is adjusted based on 200W/m2
performance data in the database.
•
The Performance Factor – the ratio of measured P
max
to predicted P
max
- is
calculated and expressed in percent.
The Performance Factor is the most important metric of the performance of the PV
module or string.
Translation of Measured Values to STC
Measured values of
Isc
, I
mp
, V
mp
, and V
oc
are translated to STC using irradiance and
temperature dependence factors stored in the model database, and the translation
equations from Chapter 8, Translation of I-V Data to Standard Test Conditions.