A Case-Study

Bifacial Carport Performance

Bifacial Carport Installation in Tucson, AZ with >20% Bifacial Gain

A bifacial system was deployed as an East-facing carport installation in Tucson, AZ (Lat.=32.25, Long.=−110.75). The system consisted of 36 Prism Solar Bi60-343BSTC bifacial modules with a bifacial ratio of 90% and a 270W STC rating, a Sunnyboy 8000US inverter, and a PowerSteel canopy system which mitigated shadowing of the rear of the bifacial modules.  Photos of such carport system can be seen in Figure 1.

 
 
 
 

Figure 1. Four-space carport system with 9.72kW STC of Bifacial modules (12.35kW Bifacial STC) and a 8kW SMA inverter.

 

SYSTEM & PERFORMANCE

The system was modeled using the Prism Solar bifacial model, verified in [Ref. 1], available in [Ref. 2-3]. The system was installed facing perfectly East over a light-colored crushed rock surface (albedo ~40%), a tilt of 7 degrees, a minimum height of 10ft, a system width/depth of 20.5ft, and a system length of 33.6ft; the Height/Depth ratio used in the Prism Solar model and in this case was 10’/20.5’=0.5. Because the system deviates from south by 90 degrees (East-facing), the azimuthal correction presented in [Ref. 3] was used to model the system performance, corresponding to a correction factor of 1.62.  One of the primary drivers for the installation of the system was to observe the bifacial performance of a system when not oriented South, previous works [Ref. 1,4] have shown an overall gain in the bifacial performance of system as they deviate from pointing South.

The DC photovoltaic module annual bifacial gain was estimated by the Prism Solar bifacial model as 21.4%, which is the additional energy the modules would produce over the value indicated by their STC rating; this bifacial gain value was used in the Helioscope PV simulation software to estimate the annual production of the system and compare it to the measured energy yield at the site, shown in Figure 2.

Measured Monthly Energy Production 2016-2017 (kWh)

Figure 2. Monthly energy production as reported by the SMA monitoring system and webportal.


Two Helioscope simulations were conducted, each with an annual soiling rate of 1.5% and other parameters that matched the installation; one simulation used the closest TMY3 (typical meteorological year) weather dataset (Davis-Monthan Air Force base) and the other using the NREL prospector 10 km database.  The results of the simulations, as well as their comparison to the SMA measured data is shown in Table 1.

 

 

Measured (SMA)

NREL Prospector

TMY3 D-M AFB

Year 1 (kWh-AC/KW-DC)

1,831.7

1,881.9

1,781.7

Difference (%)

+2.74

-2.73

 

Table 1. Measured and Simulated Annual Energy Yield, and the Difference in Percentage.


CONCLUSIONS

The measured bifacial field data, and its comparison to the simulated performance of the bifacial carport structure, strongly indicates a bifacial gain/performance in the 20%+ range. The measured field data fell in line between the NREL Prospector and TMY3 simulations, with no difference exceeding 2.74%.


REFERENCES

  1. Stein, J.S., Burnham, L., and, Lave, M. (2017). One Year Performance Results for the Prism Solar Installation at the New Mexico Regional Test Center: Field Data from February 15, 2016 – February 14, 2017. Albuquerque, NM, Sandia National Laboratories. SAND2017-5872. Link: http://energy.sandia.gov/download/41505/

  2. J. E. Castillo-Aguilella and P. S. Hauser, "Multi-Variable Bifacial Photovoltaic Module Test Results and Best-Fit Annual Bifacial Energy Yield Model," in IEEE Access, vol. 4, pp. 498-506, 2016.

  3. J. E. Castillo-Aguilella and P. S. Hauser, "Bifacial Photovoltaic Module Best-Fit Annual Energy Yield Model with Azimuthal Correction," presented at the IEEE Photovoltaics Specialists Conference (PVSC), Portland, OR, 2016.

  4. J E Castillo-Aguilella, “Multi-Year Study of Bifacial Energy Gains Under Various Field Conditions”. 4th PV Performance Modelling Workshop.  Cologne, Germany on October 23rd, 2015.

     

 
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