ECONOMIC EVALUATION OF ENERGY PRODUCED BY A BIFACIAL PHOTOVOLTAIC ARRAY IN THE ERA OF TIME-OF-USE PRICING J. Johnson, K. Hurayb, Y. Baghzouz Electrical & Computer Engineering Department University of Nevada, Las Vegas, NV (USA) International Conference on Clean Electric Power Alghero, Sardinia – Italy ● June 11 th -13 th, 2013

Overview  Background/Introduction  Bifacial-Photovoltaic Power System Electrical Characteristics.  Bifacial PV Power Generation Curve under Different Orientations  Local Utility Load Characteristics  Local Utility Tariffs  Bifacial PV Performance under Different Orientations  Economic Analysis  Conclusions

Introduction  Bifacial photovoltaic modules are designed to produce power from light striking their front as well as light incident on their rear side.

Introduction  Bifacial modules are often installed as sound barriers in fence- integrated systems in motorways and railways. Other applications include architectural awnings, partial patio and parking lot covers.  Bifacial PV panels are typically two to 3 times more expensive than their mono-facial counterparts because of their “added ornamental value” that allows some of the light to pass though, thus resulting in a pleasing environment.

Introduction  With the Time-Of-Use (TOU) pricing of electricity becoming an option to electricity customers, a bi-facial array is expected to provide an advantage over conventional mono- facial arrays as it receives more sunlight.  This article analyzes the performance of a 1 kW, grid- connected, bifacial PV array that is installed on the roof of a local building over a one-year period. Two orientations are considered:  West orientation with 90 o tilt angle (vertical)  South orientation with 30 o tilt angle (equal to latitude angle)

Description of Bifacial PV Array  Array Composition: 5 series bifacial PV modules.  Module Model: Sanyo HIP-195DA3  Array Electrical Characteristics:  Peak Power: P max = 975 W,  Voltage at P max : V m = 275 V,  Current at P max : I m = 3.5 A,  Open Circuit Voltage: V oc = 343 V,  Short circuit Current: I sc = 3.73 A,  Power Temp. Coef: T P =-0.35%/ o C,  Module Efficiency: η =16.1%.

What is the efficiency of the back side?  The figure below shows the measured power-voltage curve of each panel side (with the back covered).  The back side is nearly 15% less efficient than the front side of the panel. Test Conditions: Solar irradiance: 950 W/m 2 Cell operating temp: 57 o C

Circuit Model  Circuit parameter values (best fit):  I L = 4.45 A,  I o = 1 x A,  R s = 12 mΩ,  R sh = 6 Ω.

Bifacial PV Power Generation Orientation: Vertical East/West Mismatch due to shade of frame

Bifacial PV Power Generation Orientation: South with 30 o tilt

Utility Load Characteristics  The load is dominated by summer (June - September) and winter (November - April) patterns, with May and October being the transition months.

Utility Load Characteristics  The peak demand period is defined from 1:00 pm to 7:00 pm during the four summer months.

Utility Tariffs  Historically, residential customers are charged a fixed rate of $0.113/kWh for the electric energy consumed throughout the year.  More recently, the company installed an Advanced Metering Infrastructure (AMI) which allowed it to provide variable rates that depend on time.  The Time-Of-Use (TOU) rate rewards the customer for reducing the consumption during the peak summer period.  Current TOU rates for single-family homes are as follows:  During peak demand period (1:00 pm – 7:00 pm), 6/ - 9/30: $0.329/kWh.  During summer off-peak period (7:00 pm – 1:00 pm), 6/1 - 9/30: $0.071/kWh.  During rest of the time of the year, 10/1 - 5/31: $0.049/kWh.

Utility Tariffs  Note that relative to the flat standard rate of $0.113/kWh, the TOU rate is  Increased by nearly 300% during peak demand period: 720 hrs or 8% of the time,  Reduced by nearly 37% during off-peak summer hours: 2,160 hrs or 26% of the time,  Reduced by nearly 57% during the rest of the year: 5,760 hrs or 66% of the time.

Bifacial PV Power Generation Comparison of two orientations during peak demand  The vertical East-West orientation generates significantly more energy during the latter half, but less energy during the first half of the peak demand period when compared to the south-facing orientation.

Bifacial PV Power Generation Comparison of two orientations during the summer months  The vertical east/west orientation generates more energy in June & July, but less energy in August and September.

Bifacial PV Power Generation Comparison of two orientations during entire year  The South-facing orientation outperforms the vertical East/West orientation except during the months of May, June and July.

ANNUAL ENERGY COST PRODUCED BY PV ARRAY ARRAY ORIENTATIONCOST (FIXED RATE) COST (TOU RATE) South with 30 o Tilt all Year$259.22$ West with 90 o Tilt all Year$226.90$ South with 30 o Tilt (Oct – May), and West with 90 o Tilt (June – Sept.) $257.30$234.03

Conclusions  The vertical East/West orientation produces significantly more energy during the late afternoon hours of the summer months, but less energy during the early afternoon hours.  The total amount of energy produced by the vertical East/West orientation during the 4 summer months in nearly the same as that of south-facing orientation.  For obvious reasons, the south-facing orientation produces significantly more energy during the winter and shoulder months.  The fixed South-facing orientation performs best under both fixed and TOU energy rates.  A slight improvement can be made by re-orienting the array to a vertical East/West orientation during the months of May, June and July, but this is not worth the effort.  The recessed frame design caused significant shading in the late morning hours, thus not suitable for vertical east-west orientation.

Acknowledgement The work presented in this paper is sponsored by the National Renewable Energy Laboratory (NREL), Golden, CO, (USA).

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