The objective of this research work is to investigate the feasibility
of performing energy rating of photovoltaic (PV) modules using natural
sunlight and a controlled thermal test bed. The energy production
of PV modules is dictated by the module temperature, level of irradiance,
angle of incidence and solar spectrum. Five major tasks were identified
and carried out. The five tasks are:
-
Development of thermal test bed (TTB) to vary
the module temperature in conjunction with an air-conditioning
unit. This task allowed the laboratory to carry out the current-voltage
(I-V) measurements of PV modules at different temperature ranging
from 5oC to 65oC.
-
Adjustment of irradiance using a neutral density
filter. Various materials including white cloth, drafting sheets
and perforated black screens were investigated to vary the irradiance
on the test module from 100W/m2 to 1000W/m2. The perforated black
screens were eventually chosen for this task as they served as
ideal neutral density filter.
-
Elimination of angle of incidence effect by
a 2-axis tracker. The TTB was installed on the platform of a 2-axis
tracker so that a zero angle of incidence was obtained. In order
to eliminate the diffused light effect, all I-V measurements were
made on very clear days using a matched reference cell installed
coplanar to the test module.
-
Determination of spectral mismatch error using
the on-site measured spectra.
-
Development of a computer model to translate
the I-V data of known temperature and irradiance conditions to
the unknown temperature and irradiance conditions. The mathematical
model developed in this work has been proved to be accurately
translating all the key parameters (Isc, Voc, Imp, Vmp and Pmax)
of a mono-Si module from reference conditions to test conditions.
The validity of this model for other module technologies is being
investigated.
The on-site solar spectra were collected and spectral mismatch
analysis for different PV technologies (mono-Si, poly-Si, CdTe and
GaAs) was carried out to determine if it was necessary to make spectral
mismatch correction for the measured I-V curves. It was found that
the spectral mismatch error is virtually eliminated because of the
use of matched reference cell and the negligible difference between
the reference spectrum and test spectra collected on clear days
between 9 AM and 3 PM at this location.
Based on the results of our work, we concluded that it was feasible
to perform energy rating of PV modules using natural sunlight.
