Field measurements under different conditions, and relate these data to STC measurements

1. www.ecn.nl
Field measurements under
different conditions
Bas Van Aken
vanaken@ecn.nl
Bifacial workshop
Chambéry, 27th May 2014

2. Contents
• Indoor measurements
– Standard test conditions (STC)
– Adapted conditions, e.g. angle of incidence, background albedo
• Outdoor measurements
• Outdoor monitoring

3. Rear side material
• n-Pasha solar cells
88% bifaciality: Pmax and Isc
• 2x2 solar cell laminates,
front: AR-glass
• rear: AR-glass 92%
rear: float glass 86%
• 0° is incidence on front
• 180° is incidence on rear
• Indoor measurements under
STC conditions (no albedo)
front rear
Short-circuitcurrent[A]

4. … plus albedo
• Indoor measurements under
STC conditions WITH albedo
front
rear
Short-circuitcurrent[A]

5. Rear side material
All Isc relative to Isc of
laminate with AR-glass
on rear at corresponding
angle of incidence
Indoor measurements
STC conditions + albedo
front
rear

6. Outdoor measurements
Tilt = 30°
• Modules at 30° tilt measured
under clear sky
• Azimuth is measured against the
position of the sun at 27°
elevation
• “model” calculates Isc as a
function of angle of incidence
and azimuth, …
• but ignores the contribution of
the rear side Angle =
25° 45° 60° 45° 25°
-90 -60 -30 0 30 60 90

7. Vertical installation
• STC: front Isc = 9.0 A
rear Isc = 8.3 A
• Gi (front) and Gj (rear)
• Gtot is Gi + Gj x factor
• Factor is 8.3/9.0
• Note the constant value
of rear irradiance due to
diffuse light

8. Roof top system
• Reference cells:
pyrano, c-Si and
c-Si + KG3-filter
• Tilt 30°
Azimuth 170°
• Low albedo location
• IV-curve each 10
minutes then fixed
at Vmpp
• >20 000 IV traces
each year

9. Outdoor data – Q1 2013
monofacial
• Example of full
dataset
• Voc depends
strongly on T_module
• Red Voc data are
corrected to T=25°C
• Isc: linear with
irradiation
• FF: resistive losses
become important for
higher current
Thorough analysis needed to compare datasets for bifacial and monofacial modules

10. Isc as a function of irradiation
Isc,(1000W/m2)[A]
Irradiation [W/m2]
Outdoor Isc normalised to 1000
W/m2 irradiation
Lines indicate Isc values under
standard test conditions (STC)
determined with indoor flash test
Low irradiation: bifacial module has higher normalised Isc than monofacial one
High irradiation: monofacial +1.5%, bifacial +3.0% relative to STC Isc

11. Bifacial effect in detail
Total energy output (kWh) normalised to the peak-power per hour and per month
Bifacial effect highest when mostly indirect light contributes: early mornings, late
afternoons, especially in Summer (daylight > 12 hours, sun in Northern part of sky)
Bifacial:monofacial[-]
Bifacial:monofacial[-]
solar noon
Time of day [hour]

12. Conclusions
AR-glass v float glass as rear side material
– STC 1% difference
– +albedo 2-3% difference
– outdoor @30° tilt 4% difference, more away from South
• East-west: Isc scales with sum front and rear irradiation
corrected for STC bifaciality factor (rear:front)
• Bifacial effect largest when the sun is in the Northern half
of the sky: early morning / late afternoon during the
summer months