NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

A Flat ATIR Optics Approach to 

CPV

December 3, 2009 — December 3, 2010

David Schultz 

Banyan Energy, Inc.

Berkeley, California

Subcontract Report

NREL/SR-5200-51859

Revised October 2011
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, Colorado 80401

303-275-3000 • www.nrel.gov Contract No. DE-AC36-08GO28308

A Flat ATIR Optics Approach 

to CPV

December 3, 2009 — December 3, 

2010

David Schultz 

Banyan Energy, Inc.

Berkeley, California

NREL Technical Monitor: Harin S. Ullal

Prepared under Subcontract No. NEU-0-99010-11

Subcontract Report

NREL/SR-5200-51859

Revised October 2011
This publication was reproduced from the best available copy

submitted by the subcontractor and received no editorial review at NREL.

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1

Table of Contents

1 Q1 SUMMARY ....................................................................................................................................... 2

1.1 Introduction..........................................................................................................................2

1.2 Methods................................................................................................................................3

1.3 Results..................................................................................................................................4

1.4 Discussion............................................................................................................................5

2 Q2 SUMMARY ....................................................................................................................................... 6

2.1 Introduction..........................................................................................................................6

2.2 Methods................................................................................................................................6

2.3 Results..................................................................................................................................8

2.4 Discussion............................................................................................................................9

3 Q3 SUMMARY ..................................................................................................................................... 10

3.1 Introduction........................................................................................................................10

3.2 Optical Design Overview...................................................................................................13

3.3 Project Status .....................................................................................................................13

4 Q4 SUMMARY ..................................................................................................................................... 14

4.1 Introduction........................................................................................................................14

4.2 Methods..............................................................................................................................15

4.3 Results................................................................................................................................16

4.4 Discussion..........................................................................................................................18
2

What follows is a chronological summary of technical progress made from December 3, 2009 

through December 3, 2010 pertaining to subcontract NEU-0-99010-11.

1 Q1 SUMMARY

1.1 Introduction

An agglomeration of factors has stifled the economic promise of CPV. Foremost among these 

factors are: insufficient optical efficiency, misfit with existing solar infrastructure and production 

capabilities, and inadequate reliability of the optic-receiver pairing. These difficulties are largely 

driven by the choice of optic. The CPV industry is constrained in a paradigm of bulky reflective 

or refractive optics that operate best at either low concentration (2-5x) or high concentration 

(100x and above). Low concentration approaches are plagued by marginal economics, while 

high concentration approaches face heightened technical risks. High concentration systems 

inevitably face thermal management hurdles and often do not fit well with the existing solar 

infrastructure. Using Aggregated Total Internal Reflection (ATIR) as the optical mechanism for 

gathering light, a cost effective, line-focus optic can be produced at scale to provide superior 

optical efficiency in a flat profile and operate at a mid level of concentration to mitigate the 

tradeoff between economic benefit and adoptability. Substantiating this motivational premise 

behind the ATIR optics approach to CPV requires performance data.

Figure 1. An early ATIR lens prototype (A) that has an aspect ratio of 7:1 and produces a linefocus silhouette (B). A single-unit module prototype demonstrates module integration (C).

Photos from Banyan Energy

Foremost among the goals for establishing the viability of ATIR optics in solar is demonstrating 

optical efficiency. The first deliverable and milestone defined in the Statement of Work required 

that Banyan Energy deliver prototype optics and demonstrate an optical efficiency in excess of 

78.3%.

Banyan Energy performed an outdoor test of optical efficiency (OE) based on short circuit 

current using the line-focus Lens Step prototype. Optical efficiency of 84.7% and acceptance 

angles of ±2.8 ̊ and ±17.2 ̊ (acceptance angles about the tracked and non-tracked axes, 

respectively) were measured. This validates optical modeling efforts which predicted a nominal 

optical efficiency and acceptance angle about the tracked axis of 84.2% and 3.0 ̊ respectively. 

The achieved optical efficiency mark exceeds the threshold value of 78.3% specified for the 

completion of Milestone #1. Along with the data supporting these measurements, two assembled 

optics were sent to NREL in fulfillment of the first deliverable for the subcontract. 

A B C
3 

In addition to achieving the first set of deliverables, Banyan Energy has begun work towards 

module integration; specifying production processes, demonstrating bonds at all critical 

interfaces and conducting preliminary reliability tests. Demonstrating a multiunit module 

construction with reasonable efficiency is our next milestone. 

1.2 Methods 

The primary technical challenge for Q1 was to formulate a rigorous measurement of optical 

efficiency. The short circuit current of a solar cell, Jsc, is proportional to the amount of incident 

irradiance. Therefore, optical efficiency can be measured as the resource-adjusted proportion of 

short circuit current readings from the cell with and without the optic. Measurements were taken 

outdoors on a custom 2-axis tracker (Small Power Systems Inc., Covelo, CA). A custom silicon 

solar cell designed to accommodate 8 sun incident power was provided by NaREC and used as 

the detector. For each measurement a full IV curve was recorded, as well as the Global and 

Direct Normal Irradiance (GNI and DNI, respectively). The input measurement was taken with 

the detector cell mask-delimited. The lens step optic is then coupled to the cell with the noncuring silicone elastomer base (Sylgard 184 Base, DOW, Midland, MI) facilitating the output 

measurement. Optical efficiency is computed as the ratio of input to output measurements after 

the input measurement has been normalized for the differences in area and solar resource 

between the two tests: 

Optical efficiency measurements were complimented by acceptance angle measurements 

generated by measuring peak power output at varying degrees of tracker misalignment about the 

axis of the optic. 

The implemented test method departed from the test method described in the SOW in three 

areas: 1) A bare reference cell is used instead of a packaged cell due to unexpected optical 

effects in the extra packaging layers; 2) The test was preformed outdoors under the sun instead of 

indoors with a solar simulator due to the solar simulator not meeting specifications; 3) The 

Figure 2. (A) Masked cell baseline testing on custom two-axis tracker (Note: DNI and GNI 

sensors mounted in background) (B) Lens Step Prototype optically coupled to cell (C) 

Minimodule mounted on a custom acceptance angle testing platform. 

Photo from Banyan Energy 

௟௔௡௜௙,௖௦ܬ ൌ ܧܱ

ܬ௦௖,௜௡௜௧௜௔௟

ൌ 

ܬ௦௖,ଶ

൬ܬ௦௖,ଵ כ

ܣ௣௢௧௜௖ ௜௡௣௨௧

ଶܫܰܦ כ ௞௦௠௔ܣ

ଵܫܰܩ

൰

ൌ ܬ௦௖,ଶ

ܬ௦௖,ଵ

ଵܣ כ

௨௧௜௡௣ܣ

כ

ଵܫܰܩ

ଶܫܰܦ
4

82.5%

83.0%

83.5%

84.0%

84.5%

85.0%

85.5%

86.0%

86.5%

87.0%

Optical Efficiency

Measured and Predicted 

Optical Efficiency

Measured 

Value

Predicted 

Value

reference cell used for testing was not sent to NREL for verification of Jsc linearity with respect 

to incident flux because the metallization was designed to be sufficient to insure linearity past 

our incident flux levels. 

In addition to the testing required in the Statement of Work, Banyan Energy conducted 

experiments to measure the acceptance angle envelope for the Lens Step prototype optic. 

Acceptance angles were measured at Banyan Energy’s indoor testing facility. An acceptance 

angle profile is generated by first measuring peak power while aligned with the light source 

followed by power measurements over an array of misaligned orientations. The acceptance 

angle envelope is defined as the range of primary and secondary axes misalignments over which 

a solar module achieves greater than 90% of its peak power production. 

Figure 3. Minimodule prototype primary and secondary axes illustrated.

1.3 Results

As indicated in the introduction, the average optical efficiency measurement was 84.7% with a 

standard deviation of 1.3%. The relatively small amount of variability was primarily driven by 

the limitations of the pyranometer used for measuring GNI. Given this variability, the measured 

value of 84.7% was not significantly different from the predicted value of 84.2%. 

Figure 4: Input measurement with delimited cell mounted on 2 axis tracker. (N=9)

Primary Axis

Secondary Axis
5

Figure 5. Acceptance angle profiles for the Lens Step primary and secondary axes (Left) and a 2-

D minimum volume ellipse that defines the misalignment envelope for achieving greater than 90% 

Pmax (Right).

Primary axis acceptance angles and secondary axis acceptance angles had ranges of ±2.8 ̊ and 

±17.2 ̊, respectively. The resolution of these measurements was within 0.17 ̊ limited primarily 

by the resolution of the angle encoders used to gauge the orientation of the test platform.

1.4 Discussion

The results of these tests indicate industry leading optical efficiency and wide acceptance angles. 

This proves the performance of the Banyan Energy ATIR concept and validates the modeling 

approach. An optical efficiency of 84.7% is competitive with the optical efficiencies offered by 

competing optical systems, such as Fresnel lenses or reflective troughs. Next generation optics 

promise improved optical performance that may enable optical efficiency closer to 90%. 

Acceptance angles beyond ±2 ̊ are sufficient to accommodate tracking imprecision, structural 

imprecision and capture circumsolar radiation. The Lens Step optic provides an envelope that 

goes beyond this minimum and secondary axis acceptance angles on the order of ±20 ̊ enable 

more tracking options. Next generation designs also promise improvements with respect to 

acceptance angles. These results not only fulfill initial tasks outlined in the Statement of Work, 

but enable a viable product and further motivate the module integration work underway.
6 

2 Q2 SUMMARY 

2.1 Introduction 

Technical progress in Q2 focused on module design and integration of optics into a working 

prototype. Task 2 outlined in the Statement of Work called for Banyan Energy to integrate Gen 

1 optics produced and tested during Task 1 into several multi-unit modules suitable for on-sun 

testing. 

In Banyan Energy’s Q1 report, optical efficiency (OE) was calculated based on short circuit 

current (Jsc) because short circuit current tends to be insensitive to temperature changes and 

proportional to radiant energy flux. It was necessary to normalize current readings by the 

proportion of cell area (A1) and optic area (A2) and normalize for differences in radiant energy 

resource. The bare cell garners both direct and diffuse light resource which can be cumulatively 

measured via a Global Normal Irradiance (GNI) reading from a pyranometer. The concentrator 

unit (cell + optic) is expected to capture primarily direct light which can be measured with a 

Direct Normal Incidence (DNI) reading from a pyrheliometer. Optical efficiency was calculated 

as follows: 

௟௔௡௜௙,௖௦ܬ ൌ ܧܱ

ܬ௦௖,௜௡௜௧௜௔௟

ൌ ܬ௦௖,ଶ

ܬ௦௖,ଵ

כ

ଵܣ

ଶܣ

כ

ଵܫܰܩ

ଶܫܰܦ

 ሺ1ሻ

The threshold module performance for the fullfillment of Task 2 was based on this equation and 

short circuit current readings) with some modifications. First, optical efficiency was fixed at 

Banyan Energy’s target value of 90%. Second, a module integration loss factor of 0.7 was 

applied in order to give sufficient error budget for potential integration losses. The module 

output current was required to be greater than the product of the normalized baseline cell current, 

optical efficiency and the integration loss factor. Expressed more concisely: 

ܬ௦௖,ଶ ൐ 0.7ሺܱܧሻ ሺܬ௦௖,ଵ כ

ଶܣ

ଵܣ

כ

ଶܫܰܦ

ଵܫܰܩ

 ሻ ሺ2ሻ

2.2 Methods 

In order to establish baseline peformance without the optic, short circuit current (Jsc,1) was 

measured for a series of four cells attached to a corrugated backplane; the relevant resource basis 

for these readings was GNI. The backplane was fitted with dielectric in order to insulate 

electrically active connections. Cells were wired in serries, fixed in location and laminated to the 

backplane. Black electrical tape was temperarily affixed over the laminant material to the noncell areas in order to minimize the effect of light being reflected from the corrugated backplane 

to the cell. The backplane assembly was then mounted normal to the sun on a two axis tracker 

(Figure 1). Throughout testing, tracking precision was verified to be within ±0.25 ̊ via a visual 

alignment indicator consisting of a mirror coupled to an offset aperture. A series of ten I-V 

readings were taken reccording: short circuit current (Jsc), open circuit voltage (Voc), maximum 

power (Pmax), fill factor (FF), conversion efficiency, direct normal irradiance (DNI) and global 

normal irradiance (GNI). The electrical tape was then removed, optics were coupled to the cells 

using a silicone elastomer, a frame was affixed, the module was mounted on the tracker and a 

new serries of ten electrical peformance readings was taken; the relevant resource basis for these 

readings was DNI. Due to assembly time, the second set of readings was often taken at a later 
7 

date. By pairing simultaneous resource data with electrical peformance data it was possible to 

normalize based on resource, mitigating the effect of differing test times and changing resource 

conditions.

Figure 1. Milestone 2 outdoor test photos from baseline test and module test. Baseline test with 

cells mounted on a corrugated backplane (left). Note: the aluminum was masked with electrical 

tape to minimize reflection from the angled facets of the backplane. The same backplane 

(electrical tape removed) with optics mounted on the cells and frame attached (right). Resource 

sensors, including a pyranometer, pyrheliometer and a reference cell are mounted in the 

background. Visual alignment indicator and tracker controller are mounted in the foreground. 

Photos from Banyan Energy

The ratio of active cell area to effective optic apperture area can be calculated with reference to 

the module geometry denoted in Figure 3 as follows: 

ଵݕ כ ଶݔכ4ൌ ଶܣ

ଵݕ כ ଵݔכ4ൌ ଵܣ

ଶܣ

ଵܣ

ൌ ݔଶ

ଵݔ

Figure 2. Schematic of a four-unit module showing the dimensions used to calculate the ratio of 

active cell area and effective optic aperture area. x1 is equivalent to the active width of the cell, y1 

is equivalent to cell length and x2 is equivalent to the width of the optic. 

Cell 

Optic 
8 

The relevant area ratio (A2/A1) is equivalent to the ratio of average optic width (x2 = 89.0mm) to 

average cell active area width (x1 =13.4mm). Reducing equation 2 by substituting known 

constants (module integration loss factor, optical efficiency and area ratio) provides a simple 

expression to evaluate performance relative to the threshold defined for Milestone 2: 

ܬ௦௖,ଶ ൐ 4.18 ሺܬ௦௖,ଵ כ

ଶܫܰܦ

ଵܫܰܩ

 ሻ ሺ3ሻ

2.3 Results 

Figure 3. Device 214 I-V curves before and after prototype optic integration. 

Table 1. Device 214 representative electrical performance data before and after prototype optic 

integration (corresponds to I-V curves shown in Figure 4).1

DUT Pmax Imp Vmp Isc Voc FF Eff AVG_DNI AVG_GNI 

Backplane 

214 0.609 0.301 2.025 0.324 2.374 0.791 15.5 806.1 1069.1 

Module 214 2.432 1.171 2.078 1.224 2.471 0.804 12.1 855.3 1067.4 

Representative V-I readings indicate a baseline short circuit current of 0.324 Amperes with a 

global normal irradiance of 1069 W/m2

 (Table 1). With the optics integrated, the short circuit 

current exceeded 1.22 Amperes under a direct normal irradiance of 855 W/m2 (Figure 3, Table 

1). These representative numbers indicate that the output short circuit current exceeded the 

threshold value of 1.08A (Equation 3) by 12.9%. The performance threshold was exceeded for 

all 10 measurements in the test series.2 The full dataset supporting this claim is provided as an 

addendum file. 

 

1

 Data has the following corresponding units: W, A, V, A, V, %, %, W/m2, W/m2 – for each of the respective 

columns in Table 1. 

2

 The full datasets for each of the two modules delivered are provided in separate files. 

0

0.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5 2 2.5 3

Current

Voltage

Device 214 backplane (cell) versus module (optic integrated)

peformance

Backplane 214

Module 214
9

2.4 Discussion

Results indicated that the module electrical performance exceeded the threshold defined in the 

SOW in all cases. The Gen 1 prototype optics continue to perform well and can be readily 

assembled into a module. ATIR optics can be integrated into a flat module form factor and 

mated with cells to produce energy. With low volume assembly methods, module integration 

losses were observed as expected. For example, cells for Module 214 were individually 

measured to be between 16.0-16.3% efficient, however, when laminated in series on the 

aluminum backplane, the module-integrated string efficiency decreased to 15.5%. Imperfections 

in solder joints and series resistance likely decreased receiver performance. Previously, the Gen 

1 prototype optic was measured to be approximately 84%. Given this optical efficiency, the 

maximum achievable efficiency on a 15.5% efficient receiver is 13.0%. Module 214 achieved 

12.1% efficiency. This 0.9% absolute efficiency loss was, at least in part, due to variability in 

optic performance. Currently, prototype optics consist of individual tiles that each have a set of 

imperfections that decrement optical efficiency. Optical losses are also likely to increase due to 

imperfections in the encapsulant layer. Just as cells in series are current limited by the lowest 

performing cell, so too are concentrator units in series limited by the lowest performing opticcell pair.

During module assembly, process control is paramount. A redesign effort is underway focused 

on design for manufacturing and design for assembly in order to minimize cost and improve 

reliability. 

Figure 4. Left: multi-unit module. Right: modules on sun. Photos from Banyan Energy
10 

3 Q3 SUMMARY 

3.1 Introduction 

Technical progress in Q3 focused on module design and integration of production optics into a 

working prototype. Banyan Energy is currently in the process of acquiring prototype parts and 

working through low scale assembly methods. Task 3 outlined in the Statement of Work called 

for Banyan Energy to design and integrate production optics into a multi-unit module for test. 

Recall that in Q2 of the program optical efficiency (OE) was determined based on short circuit 

current (Jsc) because short circuit current tends to be insensitive to temperature changes and 

proportional to radiant energy flux. Accordingly, the threshold module performance for the 

fullfillment of Task 2 defined in the SOW was based on the following equation: 

ܬ௦௖,ଶ ൐ 0.70ሺܱܧሻ ሺܬ௦௖,ଵ כ

ଶܣ

ଵܣ

כ

ଶܫܰܦ

ଵܫܰܩ

 ሻ ሺ1ሻ

Where Jsc is the short circuit current, A is the active aperture, DNI is the direct normal irradiance 

and GNI is the global normal irradiance. The subscripts 1 and 2 indicate pre-optic and post-optic 

configurations, respectively. In this threshold relationship for Task 2, optical efficiency (OE) 

was fixed at Banyan Energy’s target value of 90% and a module integration loss factor of 0.70 

was applied in order to give sufficient error budget for potential integration losses. For the 

fullfillment of Task 3 the peformance metric is much the same as that for Task 2; the difference 

being that the module integration loss factor is less lenient having a value of 0.85. 

ܬ௦௖,ଶ ൐ 0.85ሺܱܧሻ ሺܬ௦௖,ଵ כ

ଶܣ

ଵܣ

כ

ଶܫܰܦ

ଵܫܰܩ

 ሻ ሺ2ሻ

Banyan Energy continues to work towards exceeding this threshold for short circuit current. 

However, there exist some notable differences between design intent indicated in the SOW and 

current design hypotheses. The initial intent stated in the SOW for the new optic was to 

concurrently increase the acceptance angle profile beyond ±3 ̊, increase the geometric 

concentration towards 10x while maintaining a high optical efficiency (targeting 90%) and 

reducing part count to improve manufacturing economics. Since formulating the SOW, we have 

refined our design process; particularly with respect to selecting an appropriate acceptance angle 

and level of concentration. What follows is a rationale behind revising optic design parameters 

from what was indicated in the SOW. 

For non-imaging optics there exists a fundamental tradeoff between acceptance angle and the 

level of concentration. As the level of concentration increases, the acceptance angle tends to be 

more constrained and vice-versa. Over-sizing the acceptance angle budget for misalignments 

stemming from module assembly, mounting assembly, tracker precision and ground settling will 

result in undue constraint on the level of concentration and aspect ratio. In other terms, 

considering optic design parameters are dependent, there is no incentive to make the acceptance 

angles larger than necessary. Rather, acceptance angle budget needs to be rigorously vetted and 

then used as a fixed design parameter. Our initial acceptance angle target of ±3 ̊ (calculated at 

90% of peak power) is sufficient to accommodate the aforementioned sources of misalignment 
11

and has marketplace validation3

. Therefore this value was not changed in the current design 

effort. Furthermore, although the optic design has changed substantially from the first 

generation design, the target level of geometric concentration remains in the 7-8x range. The 

reason for this target is that our estimates indicate that a 5-10x effective flux increase is an 

optimal range for an LCPV product considering the technical and economic tradeoffs that follow 

from the level of concentration selected. After an optical efficiency of 80-90% is taken into 

account, a 7-8x geometric optical design outputs roughly 5-7x effective flux. We briefly explain 

the rationale behind the target range for concentration as follows:

There are 5 essential relationships that affect a CPV product based on the level of concentration. 

They are: cell cost per unit area (i.e. the capital efficiency benefit), cell efficiency, cell operating 

temperature (which ultimately impacts cell efficiency and reliability), acceptance angle, and 

manufacturing feasibility (Figure 1). Cell cost reduces non-linearly with increasing 

concentration such that there are diminishing marginal returns. Certain cell architectures show

efficiency increases under concentration on the order of 10%. For the current Banyan Module 

architecture this benefit has a broad peak in a range of 5-10x effective concentration (Figure 2). 

Concurrent with this efficiency benefit, cell efficiency tends to decrease with increasing 

concentration due to the temperature effect on output voltage. And module life will decrease 

with more intense thermal cycling associated with higher levels of concentration. This assumes 

that higher concentration designs cause higher nominal operating cell temperatures. As was 

mentioned previously, increasing the level of concentration tends to narrow the acceptance angle 

profile, which must remain above the tracking error budget. Finally, increasing concentration 

tends to require reduced manufacturing tolerances that in turn increase cost or make the optic 

infeasible altogether. For example, increasing the level of concentration from 7x to 12x within a 

module form factor adds a modest capital efficiency benefit, but begins to constrain performance 

and life expectancy due to more intense thermal load. Perhaps even more critical to note in this 

scenario is the simple observation that cell sizes begin to get quite small (on the order of a few 

millimeters). At some point, handling small cells in a manufacturing process becomes too 

costly, especially as yields diminish. The feedback we have received from contract 

manufactures and experts who know cell handing equipment is that cell width should target the 

10mm range in order to avoid yield loss and radical tooling changes. This example highlights 

the tradeoffs that dictate the appropriate level of concentration for a given receiver type.

 3 Confidential discussions with two vertically integrated silicon PV companies who have an interest in our product 

and also produce trackers have verified assumptions regarding viability of a ±3 ̊ acceptance angle design.
12 

Figure 6. Five notable tradeoffs stemming from the level of concentration selected for a CPV 

design. 

Figure 7. Cell efficiency profile with increasing concentration. Note that this empirical data takes 

into account the effect of increasing temperatures at increased levels of concentration for the first 

generation Banyan module architecture. The shaded region indicates the broad peak that defines 

an optimal operating range. 

Having proven manufacturability for a certain characteristic feature size in the first generation 

optic and verified cost models with potential customers, four main factors have driven us to 

design for a level of concentration in the 7-8x range rather than increasing the concentration to 

the 10x target put forth in the SOW. Namely, maintaining acceptance angles, maintaining cell 

width, minimizing thermal load (without having to add fins) and maximizing cell efficiency. 
13

3.2 Optical Design Overview

The evolution of the production optic design from the current prototype design can be 

summarized as follows: The concentrating and redirecting functionality of the prototype primary 

optic have been integrated into the cover glass and waveguide optic, respectively; thus, 

eliminating the need for a concerted primary optic altogether. Waveguide parts have been 

lengthened by a factor of 3 to reduce part count and improve throughput. Total optic part count 

has been reduced by a factor of 6 for each square meter of module. The optic parts have been 

made simpler, with fewer, large features; this reduces the need for tight tolerances and allows for 

performance stability with thermal cycling. 

3.3 Project Status

Prototype parts for the production module have been ordered and are expected by mid-August 

2010. Assembly and testing are expected to commence in August with refinements to follow. 

Given the progress thus far, it is anticipated that the deliverables for Task 3 can be achieved 

within the allotted time; the deadline being December 3, 2010. 
14 

4.1 Introduction 

Technical progress in Q3 and Q4 focused on module design and integration of production optics 

into a working prototype. Task 3 outlined in the Statement of Work called for Banyan Energy to 

design and integrate production optics into a multi-cell prototype module for testing. Banyan 

Energy has now produced several 4-cell prototypes that demonstrate the performance and the 

module integration of the next generation of optics. The new optic design shows improvements 

in optical efficiency and acceptance angle profile, but perhaps more crucial to the long term 

success of the product is that it reduces manufacturing costs by relaxing tolerances and requiring 

fewer parts. Also notice that the backplane is now flat, as opposed to the previous design that 

was corrugated. This design change allows the receiver package to be produced with 

conventional lamination process.

In Banyan Energy’s Q1 report, optical efficiency (OE) was calculated based on short circuit 

current (Jsc) because short circuit current tends to be insensitive to temperature changes and 

proportional to radiant energy flux. It was necessary to normalize current readings by the 

proportion of active cell area (A1) and optic area (A2) as well as normalize for differences in 

radiant energy resource. The bare cell garners both direct and diffuse light resource which can 

be cumulatively measured in Watts per square meter via a Global Normal Irradiance (GNI) 

reading from a pyranometer (CMP-3, Kipp & Zonen, Netherlands). The concentrator unit (cell + 

optic) is expected to capture primarily direct light which can be measured with a Direct Normal 

Incidence (DNI) reading from a pyrheliometer (CHP-1, Kipp & Zonen, Netherlands). Optical 

efficiency was calculated as follows: 

௟௔௡௜௙,௖௦ܬ ൌ ܧܱ

ܬ௦௖,௜௡௜௧௜௔௟

ൌ ܬ௦௖,ଶ

ܬ௦௖,ଵ

כ

ଵܣ

ଶܣ

כ

ଵܫܰܩ

ଶܫܰܦ

 ሺ1ሻ

The threshold module performance for the fullfillment of Task 3 was based on this equation and 

short circuit current readings with some modifications. First, optical efficiency was fixed at 

Banyan Energy’s target value of 90%. Second, a module integration loss factor of 0.85 was 

applied in order to give sufficient error budget for potential integration losses. Recall that in 

Task 2 the module integration factor was less stringent at 0.70. The module output current was 

required to be greater than the product of the normalized baseline cell current, optical efficiency 

and the integration loss factor. Expressed more concisely: 

ܬ௦௖,ଶ ൐ 0.85ሺܱܧሻ ሺܬ௦௖,ଵ כ

ଶܣ

ଵܣ

כ

ଶܫܰܦ

ଵܫܰܩ

 ሻ ሺ2ሻ

The relevant area ratio (A2/A1) is equivalent to the ratio of the active module aperture area (not 

including support rib area) to the active cell area. Subscripts 1 and 2 imply pre-optic (receiver 

only) and post-optic assemblies, respctively. In addition to validating electrical performance 

according to this threshold, the acceptance angle profile for the new optic was measured and 

reported. 

4 Q4 SUMMARY 
15

4.2 Methods 

In order to establish baseline peformance without the optic, short circuit current (Jsc,1) was 

measured for a series of four cells attached to a backplane along with the relevant resource basis 

for these readings, GNI. The backplane was fitted with dielectric in order to insulate electrically 

active connections. Cells were wired in series and laminated to the backplane. The backplane 

assembly was then mounted normal to the sun on a two axis tracker. Throughout testing, 

tracking precision was verified to be within ±0.25 ̊ via a visual alignment indicator consisting of 

a mirror coupled to an offset aperture. A series of current-voltage sweeps were taken recording: 

short circuit current (Jsc), open circuit voltage (Voc), maximum power (Pmax), fill factor (FF) and 

conversion efficiency. Also recorded for each sweep were direct normal irradiance (DNI) and 

global normal irradiance (GNI). After baseline cell performance was measured, optics were 

coupled to the cells using a silicone elastomer, a frame was affixed, the module was mounted on 

the tracker and a new series of electrical peformance readings were taken; the relevant resource 

basis for these readings was DNI. By pairing simultaneous resource data with electrical 

peformance data it was possible to normalize power output based on resource, mitigating the 

effect of differing test times and changing resource conditions.

In addition to the testing required in the Statement of Work, Banyan Energy conducted 

experiments to measure the acceptance angle envelope for the new prototype optic. An 

acceptance angle profile is generated by first measuring peak power while aligned with the light 

source followed by power measurements over an array of misaligned orientations. Acceptance 

angle profiles were generated about both the primary (daily) axis of rotation and the secondary 

(seasonal) axis of rotation. By convention, the acceptance angle envelope is defined as the range 

of primary and secondary axes misalignments over which a solar module achieves greater than 

90% of its peak power production. 

For the acceptance angle tests, the orientation of the test platform was determined via a ball 

bearing optical shaft encoder (H6-2500-I-S, US Digital, Vancouver, WA) mechanically coupled 

to the solar tracker. The encoder had a resolution of 0.14 ̊. Initial alignment of the test platform 

was determined via a custom built visual alignment indicator consisting of a 1 mm pin hole 

aperture spaced 170 mm from a mirror mounted coplanar to the test platform. Visual alignment 

was established when the aperture and the divergent reflected light from the sun formed 

concentric circles. The resolution of the visual alignment indicator was 0.17 ̊. After testing, an 

error check was performed to validate that peak power production occurred at the initial reading, 

where visual alignment was achieved. Once initial visual alignment was established the encoder 

readings were tared and the test platform was allowed to travel through a range of misaligned 

orientations about each axis while concurrent power measurements were taken.
16

4.3 Results

Figure 1. I-V curves before and after prototype optic integration for device 4C-5 are shown in blue 

and red respectively. Note: cells oversized to ensure overlap. Photos from Banyan Energy 

Cells were oversized in the width dimension by approximately 100% (cell width = 19mm, focal 

area width = 9.4mm) in order to allow for substantial misalignment error during manual 

assembly. Also, Banyan Energy wanted to minimize costs and use up an inventory of pre-diced 

cells from previous prototypes. The area-corrected characteristic I-V curve can be calculated by 

simply scaling the receiver data by the area ratio (Figure 1). This area-corrected data represents 

achievable performance once process controls are in place to precisely match and register the cell 

dimensions and location with the optic focal area. Note that although these results show a 2.3x 

power amplification once the receiver is paired with the optic (Table 1), a 4.7x power 

amplification is expected for this particular prototype if cell size and position are properly 

matched with focal area.

Table 1. Parameters from the test (corresponds to I-V curves shown in Figure 2).4 Parameters in 

bold were used to verify performance exceeded the threshold. Aperture areas for the receiver and 

module were 4750 and 16896 mm2 respectively. 

DUT Pmax Imp Vmp Jsc Voc FF Eff AVG_DNI AVG_GNI

Receiver 4C-5 0.8046 0.3922 2.051 0.419 2.375 80.7 16.04 918 1056

Module 4C-5 1.8410 0.8862 2.078 0.923 2.437 81.7 13.50 807 1026

Characteristic I-V readings indicate a receiver short circuit current (Jsc,1) of 0.419 Amperes 

under an average global normal irradiance (GNI1) of 1056 W/m2 (Table 1). With the optics 

integrated, the short circuit current (Jsc,2) exceeded 0.923 Amperes under a direct normal 

 4 Data has the following corresponding units: W, A, V, A, V, %, %, W/m2

, W/m2 – for each of the respective 

columns in Table 1. Note that receiver and module efficiencies are measured on GNI and DNI bases respectively. 

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.00 1.00 2.00 3.00

Current

Voltage

Characteristic I-V (DEVICE 4C-5)

Module Current 

(optics integrated)

Receiver Current 

(no optics)

Receiver Current 

(no optics / area 

corrected)

Module Receiver
17

irradiance (DNI2) of 807 W/m2 (Figure 2, Table 1). These representative numbers indicate that 

the output short circuit current exceeds the threshold value of 0.878 Amps (Equation 3) by 

5.14%. The performance threshold was exceeded for all measurements in the test series.5

 The 

full datasets supporting this claim is provided as an addendum file.

Acceptance angles for the first series of production prototypes were comparable to the first 

generation of prototypes having primary and secondary acceptance angles of approximately ±3 ̊ 

and ±18 ̊ at 90% of peak power, respectively. 

Figure 2. Primary and secondary axis acceptance angle profiles for the first series of production 

prototypes (specifically 4C-5 here) indicate adequate acceptance angles of approximately ±3 ̊ and 

±18 ̊ degrees of allowable misalignment at 90% of peak power. Secondary axis data has been 

cosine corrected to account for the change in projected area. 

Figure 3. Primary and secondary axis acceptance angle profiles for a refined first generation 

arrayed module indicate what is achievable for ATIR optic acceptance angle profiles once quality 

parts and assembly processes are implemented. Note: Data taken from an arrayed (36 cell) gen 1 

module (Ulysses) in outdoor 1-sun conditions. The test module achieved an absolute peak power 

(Pmax) of 23.4 W during these tests. Secondary axis data has been cosine corrected to account for 

the change in projected area. 

 5 Datasets for each of the two modules delivered are provided in separate files e-mailed to Harin Ullal
18

4.4 Discussion

Results indicated that the module electrical performance exceeded the threshold defined in the 

SOW in all cases. Beyond the pre-incubator goals of validating performance possibilities for 

solar modules with ATIR optics, Banyan Energy is particularly excited about these performance 

results because the projected costs of parts and assembly at scale for this particular prototype 

design fit with a profitable economic model. Banyan has worked closely with glass, cell and 

injection molding suppliers to verify part costs at scale. Further, there exist many opportunities 

to advance the performance of this particular design. For instance, geometric precision of optic 

parts at scale will be improved with roll-patterned glass parts and molded parts made with hard 

tooling rather than aluminum molds. Also, in the final product, antireflective coatings will be 

applied to the primary optic, increasing optical efficiency considerably. Moving to a backcontact cell in the receiver package will bolster overall power output and may prove to be a more 

reliable construction. Banyan Energy is currently working on these improvements and a larger 

(0.25 m2

) version of the production prototype detailed in this report. Current results indicate a 

4.7x power amplification for an optic with 7x geometric concentration. The final construction 

will achieve a power amplification of well over 5x for the same geometric concentration.

Recall that the first generation prototype had acceptance angles at the 90% power point of ±2.8 ̊ 

and ± 17.8 ̊ (about primary and secondary axes, respectively). After improving optic quality in a 

subsequent series of prototypes these acceptance angles were widened considerably to ±3.1 ̊ and 

±30.1 ̊. Also, both primary and secondary acceptance angle profiles changed to have more of a 

flat-top producing near 100% of peak power from a range of ±2 ̊ and ±20 ̊ (Figure 3). For this 

first production prototype we achieved acceptance angles of approximately ±3 ̊ and ± 18 ̊, 

comparable or slightly better than what was achieved for the first series of prototypes (Figure 2). 

Given the early stage of development for the new production optic, there is reason to believe that 

these profiles will also see improvements similar to the initial optic. 

The pre-incubator program has provided essential technical and financial support. Working with 

NREL has also provided intangible benefits such as the legitimacy that comes with diligent 

oversight from experts. The funding management, testing prowess, expertise and oversight of 

the NREL personnel were essential to building the world’s first solar module with ATIR optics. 

Banyan Energy offers our sincerest thanks for NREL’s diligence and help in seeing our earlystage company successfully achieve all three Milestones negotiated in the Pre-Incubator 

program. 

Particular personnel of note on this particular project were Dr. Harin Ullal who helped to 

negotiate appropriate milestones and shepherd the project to completion within budget and on 

time. Dr. Keith Emery was also helpful providing testing expertise and recommendations for 

setting quantifiable goals. Dr. Sarah Kurtz shared her knowledge of the greater CPV industry. 

Daryl Myers helped in acquiring and understanding solar data; this was essential to Banyan 

Energy’s energy production model. Matt Muller was and continues to be crucial in on-going 

efforts to test our initial series of quarter-panel prototypes. Banyan Energy would also like to 

thank all those administrators at NREL and the DOE that help small companies like Banyan 

begin development of renewable energy technologies in the U.S through the Pre-incubator 

Program.