performance of 3sun mirror modules on a sun tracking carousel

Performance of 3-Sun Mirror Modules on Sun Tracking Carousels on

Flat Roof Buildings Lewis Fraasa, James Averya, Leonid Minkina, Curt Maxeyb, Tony Gehlb, Rick Hurtc, Robert Boehmc

aJX Crystals Inc, 1105 12th Ave NW, Issaquah, WA, USA 98027; bOak Ridge National Lab, P.O. Box 2008, Oak Ridge, TN 37831;

cUniversity of Nevada Las Vegas, Department of Mechanical Engineering, Las Vegas , NV 89154

ABSTRACT

Commercial buildings represent a near term market for cost competitive solar electric power provided installation costs and solar photovoltaic module costs can be reduced. JX Crystals has developed a carousel sun tracker that is prefabricated and can easily be deployed on building flat roof tops without roof penetration. JX Crystals is also developing 3-sun PV mirror modules where less expensive mirrors are substituted for two-thirds of the expensive single crystal silicon solar cell surface area. Carousels each with four 3-sun modules have been set up at two sites, specifically at Oak Ridge National Lab and at the University of Nevada in Las Vegas. The test results for these systems are presented.

Keywords: Solar Concentrator, Concentrator Photovoltaics, CPV, Solar Tracker, Carousel Tracker, 1-axis solar tracker

1. INTRODUCTION

Given straight forward evolutionary innovations, solar photovoltaic (PV) systems can be economical for commercial customers in the sunny Pacific Southwestern US with buildings with flat roofs. This follows because these customers pay retail prices for electric power, need electric power during the days, and require larger PV systems where installations can be standardized. Herein, a lower cost PV system that will serve the commercial-building flat rooftop market is described. This system implements two simple evolutionary innovations. These innovations are, first, lower cost 3-sun PV mirror modules that are, second, mounted on a simple low-profile carousel sun-tracker. Lower cost PV panels can be made now by using less of the expensive single-crystal silicon cell material in simple 3-sun mirror modules operating at low solar concentration ratios. Using less of the expensive single crystal silicon in these 3-sun modules also addresses the present silicon feedstock supply shortage problem for today’s standard planar PV panels. Using low profile carousel solar trackers on commercial building flat rooftops will increase the number of kWh per kW install reducing the cost-of-electricity (LCOE) immediately. In the first section of this paper, these two innovations are described along with a brief history of their development to date. The focus of this paper is on beta site test results. Carousels each with four 3-sun modules have been set up at two sites, specifically at Oak Ridge National Lab and at the University of Nevada in Las Vegas. The test results for these systems are presented in the second section of this paper. The third and fourth sections of this paper describe lessons learned and future improvements.

High and Low Concentration for Solar Electric Applications III, edited by Martha Symko-Davies,Proc. of SPIE Vol. 7043, 704305, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.793179

Proc. of SPIE Vol. 7043 704305-1

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2. BACKGROUND PV mirror module concept

The www.solarbuzz.com web site tracks the history of the standard planar silicon solar module price since 2002. The lowest module price of $4.32 per W was attained in 2004. Since 2004, the standard planar silicon module price has actually gone up, not down, reaching $4.83 per W today. Given a mark up of $1.30 per W suggest that the standard module cost is approximately $3.53 per W. The silicon cell cost of approximately $3 per W accounts for most of this with the cost of the glass, frame, junction box and labor accounting for the remaining approximately $0.53 per W. Figure 1 shows the evolutionary 3-sun PV mirror module concept in comparison with the standard planar PV module.

Standard planar PV module with

expensive single crystal cells

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material

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Figure 1: Standard PV Module vs JXC 3-Sun Mirror Module. The 3-sun mirror module has the same size and power using 1/3 as much expensive single crystal silicon.


As is evident in this figure, the 3-sun PV mirror module uses 1/3 of the expensive single crystal silicon. The 3-sun concentrator module design uses existing planar cells. In the actual implementation employed here, standard 125mm x 125mm SunPower A300 cells are cut into thirds as shown in figure 2,. In addition, the 3-sun module design uses standard circuit lamination procedures and equipment. However, as also shown in figure 2, a thin aluminum sheet is added at the back of the laminated circuit for heat spreading. While a standard planar module contains rows of 125mm x 125mm cells, the low concentration modules consist of rows of third-cells with each row now 41.7 mm wide. Linear mirrors with triangular cross sections between the cell rows (figure 1) deflect the sun’s rays down to the cell rows. The benefit of this concept is potentially lower module cost. The cell cost is now reduced by 3 to $1 per W. Adding the $0.53 per W for the cost of the glass, frame, junction box and labor along with a projected mirror cost of $0.3 per W suggest a potential 3-sun module cost of $1.83 per W in high volume production.

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Figure 2: TOP RIGHT: View from the back side of a A300 SunPower cell before and after being cut into 3rd cells. TOP LEFT: Cross-section view of the standard planar silicon module lamination. BOTTOM LEFT: Cross section view of the 3-sun

lamination with the addition of a metal sheet heat spreader to spread the heat uniformly over the whole back plane so that the air contact area for heat removal is preserved. BOTTOM RIGHT: Perspective view of a 3-sun mirror module.

2.2 PV Mirror Module History and Technology Status Today.

The 3-sun mirror module was invented by L. Fraas et al [1] at JX Crystals Inc (JXC). The development of these 3-sun PV mirror modules was enabled by a purchase order from the Shanghai city government through the Shanghai Import & Export Trading Company. This project has taken place in 4 phases over a 2 year period. In the 1st phase, the 3-sun modules were designed and the first 30 were fabricated and tested under calibrated conditions. In the 2nd phase, the two post-mounted 2-axis tracking systems shown in figure 3 were deployed in Shanghai, each with 12 of our 3-sun modules. Then, in a 3rd phase, a roof mounted 100 kW system was deployed at the Shanghai Flower Park. Finally, in the 4th phase, the two rooftop beta-site systems described here were deployed in the USA. Figures 3 and 4 show the systems operating in Shanghai. The 4 kW system has now been operating for 2 years and the 100 kW system has been operating for 18 months. The performance of these systems has been described previously [2, 3, 4].


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Figure 3: JX Crystals Inc 4 kW installation in Shanghai with 2 Array Technologies trackers with 12 mirror modules per tracker.

Figure 4: Photo of modules on 100 kW installation (left) and module power yields with average at 190 W and high of 205 W.

2.3 JXC Sun-Track Carousel

The China 100 kW project really has laid the foundation for the 3-sun mirror module development to date. However, there is a significant difference between the market requirements for China vs. the US. China’s interior does not have the transmission line infrastructure already installed in the US. So China’s solar need is for village electric power. For China, then, modules can be used on horizontal beam trackers in fields. China has recognized this and next JXC project in China is going to be a 300 kW system on horizontal beam trackers in a field. Meanwhile, JXC believes that the economical US market will be for PV integrated into commercial buildings with the target initial market being in the sunny South West. Horizontal beam trackers or post mounted trackers are not suitable for this building integrated market. So, JXC has designed, fabricated, and deployed a carousel tracker for use on commercial building flat rooftops [5, 6]. Figure 5 shows a design drawing for a 1-axis carousel tracker that JXC has designed for easy deployment on commercial building flat rooftops. The prototype carousel shown is a low profile tracker designed to distribute its weight evenly over a large area and for low wind resistance on the roof.


_!_: _•-f—

.-i. -

Referring to the drawing in figure 5, the JXC carousel is designed as a compact pre fabricated unit where the module-support arms fold down for easy shipping. When these arms are folded down, it is only 8” tall. Without mounted modules, it is slightly less than 8 ft wide and slightly less than 10 ft long so that 24 units can fit in a standard 20 ft shipping container. We believe that it can be easily lifted up to a roof and placed in position with no roof penetrations required. Once lifted onto the roof, the module support arms are lifted up as shown in the lower drawing and the modules are then installed.

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Figure 5: JXC SunTrack Carousel. The upper left drawing shows the design for this carousel without modules in its pre-fabricated form for compact shipping. The module support arms fold down for shipping. After lifting the carousel onto the roof, the support arms are then lifted up and modules are mounted as shown in lower left and in photo. The photo at the upper right shows a carousel installed at Oak Ridge National Labs in Sept of 2007 and the I vs V curve shows its performance outdoors on Feb 14 of 2008. The outdoor peak power output for the array of four 3-sun modules of 616 W is consistent with a PTC individual module rating of 154 W. Given a 3-sun module area of 1.26 square meters, this corresponds to a module efficiency of 12.2%.

3. CAROUSEL BETA SITES IN THE USA 3.1 Oak Ridge National Labs & University of Nevada at Las Vegas

The photo in figures 5 shows a JXC carousel with 180 W 3-sun modules installed on a building flat rooftop at Oak Ridge National Laboratory. This unit has been successfully operating under test since September of 2007. Figure 5 also shows illuminated current vs. voltage curves for the four series connected 3-sun modules on this carousel. Figure 6 shows


another carousel of this type installed on a rooftop at the University of Nevada Las Vegas. This second carousel has been operating outdoors under test since March of 2008. The four 3-sun modules on this second carousel are being individually monitored. The four 3-sun modules on each of these carousels are rated at 180 W under standard test conditions (STC) equivalent to 720 Wstc per carousel. Under practical test conditions (PTC) outdoors, each module is rated at 154 Wptc.

Figure 6: Four 3-sun CPV modules on a carousel at UNLV.

Figure 7 a shows a graph of the daily energy output in kWh for the ORNL carousel since its installation in September of 2007 and figure 7 b shows the average energy density produced per day per 3-sun module on the UNLV carousel since its initial operation in March of 2008.

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Figure 7a: daily energy output in kWh for the ORNL carousel from 9/07 through 6/08.

Figure 7b: energy density produced per day per 3-sun module for the


Referring to figure 7, several facts are evident. First, the weather in Tennessee is much more variable with considerable cloud cover. However, there is a lot of sunshine in Nevada. Referring to figure 7b, the daily energy output is nearly constant. The dip in early May is artificial and not actually caused by bad weather or a carousel or module failure but simply a data transmission failure.

The benefit of solar tracking can be seen in the solar irradiance data shown in figure 8. Four irradiance curves are shown in this figure. The lowest curve is the diffuse radiation on a horizontal surface. The next sinusoidal curve is the global irradiance on a fixed South facing 30 degree tilted surface. The flatter almost square wave curve is the direct irradiance on a 2-axis tracked sensor. The final curve with the slight dip in the middle is the total irradiance on a sensor on the sun tracking carousel. Since the modules on the carousel are mounted at a fixed 30 degree tilt and the sun at noon is almost vertical, the dip results because the sun rays hit the panels slightly North of normal incidence at noon. A comparison of the area under the curve with the dip to the fixed tilt global curve demonstrates the advantage of solar tracking. Over a year in Las Vegas, the carousel azimuth tracker will produce approximately 1.4 times more kW hours per installed. kW relative to a fixed tilt PV system.

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Figure 8: Typical global, direct, diffuse and total irradiance for a day in the summer in Las Vegas.

3.2 Representative Data

There are sunny days in Tennessee. Figure 9 shows the carousel array power output for the ORNL carousel on April 15, 2008. The array produced a peak power of 616 W consistent with the module PTC rating of 154 W. The power output over the day is nearly flat or constant consistent with expectations for a solar tracking system.

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Fig.9. ORNL carousel peak power output over the day on April 15 of 2008.


Figure 10 shows a representative complete set of data for a 3-sun mirror module on the carousel in Las Vegas. This data was taken on June 20 of 2008. This figure shows module short circuit current, Isc, maximum power, Pmax, voltage at maximum power, Vmax, fill factor, FF, temperature on the rooftop, and cell temperature as calculated from the Vmax.

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Figure 10: 3-sun mirror module short circuit current, Isc, maximum power, Pmax, voltage at maximum power, Vmax, fill factor, FF, temperature on the rooftop, and calculated cell temperature over the day measured on June 20 of 2008 at UNLV.

These data reveal two interesting points. First, it can get hot on the roof in Las Vegas. In this case, the roof temperature reaches 50 degrees C. The good news is that the module cell temperature is only 80 degrees C, 30 degrees over the ambient temperature. This is nearly equivalent to the temperature rise above ambient for a standard planar module. The backside aluminum sheet heat spreader described in figure 2 is functioning as expected. The 3-sun module continues to operate without problems at this temperature.

Second, notice the pronounced dip in the Vmax and FF correlated with the increase in ISC and sunlight intensity during the day. This dip suggests an area for future improvements. The cells used here were designed for 1-sun operation, not 3-sun operation. In the future, it should be possible to design and fabricate 3-sun cells with FF at room temperature of


0.78 or at least 0.7 at operating temperature. This would be an improvement from 0.63 to 0.7 or a 10% performance improvement.

4. LESSONS LEARNED 4.1 Cleaning Schedule

How important is degradation associated with dust accumulation for 3-sun mirror modules? This question is partially answered in figure 11. The four 3-sun modules on the carousel in Las Vegas were cleaned on June 13 after 3 months of operation. The average improvement was 5.6%. ORNL provided another data point on cleaning. The modules at ORNL were also cleaned on June 13 for the first time after 9 months in the field. The sunlight intensity was not stable enough for a precise reading but the effect was less than a 10% effect and the system returned to an output of 600 W.

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Figure 11: Modules cleaned at UNLV after 3 months of operation.

4.2 SunPower Cells Require a Positive Ground

The first and most important lesson learned had nothing to do with 3-sun modules but simply relates to SunPower cells. Systems using SunPower cells require a positive ground in order to avoid current degradation via surface polarization [7]. This point was demonstrated at ORNL and is illustrated in figure 12.

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Figure 12: Kinks in I vs. V curves on January 3 disappear in I vs. V curves on January 30 after positive grounding on January 24.


While SunPower had described a surface polarization effect requiring that their modules only see negative voltages, we were somewhat surprised that the effect occurred with voltages as low as 50 V as became evident for our carousel at ORNL. The effect is associated with ion diffusion and occurs slowly. A positive accumulated charge at the front of cells degrades the module short circuit current because the current is collected at the backside of the cells. As can be seen in figure 12, kinks develop without a positive grounded array because some modules in a series string develop lower short circuit currents. The system recovers and the kinks disappear after several days once the array is positively grounded.

5. FUTURE IMPROVEMENTS 5.1 Up Sizing

Initially, we have begun with 180 W modules on carousels. In the future, we plan to develop 250 W modules so that four can be mounted on a carousel to produce a 1 kW carousel. Putting more power on a future 1 kW carousel will lead to a lower cost per W for both the carousel and its installation. Table 1 shows alternate 3-sun module designs. As shown in this table, modules can be designed with either SunPower cells or alternately, with more available multi-crystal silicon cells. It is also possible to arrive at a 1 kW carousel by mounting six 180 W modules with a revised design using multi-crystal silicon cells..

Table. 1. Alternate 3-sun mirror module designs

Cell Type Cell Layout

& Efficiency Module dimensions

& STC Power Modules per carousel

& STC Power Sun Power

125 mm x 42 mm 6 x 12 18%

1.56 m x 0.81 m 180 W

4 720 W

Sun Power 125 mm x 42 mm

8 x 12 24%

1.56 m x 1.05 m 320 W

4 1.28 kW

Multi Crystal 78 mm x 39 mm

12 x 16 16%

1.87 m x 1.04 m 250 W

4 1 kW

Multi Crystal 156 mm x 52 mm

6 x 8 18%

1.31 m x 1 m 180 W

6 1.08 kW

5.2 Lower Cost

The motivation for 3-sun mirror modules on 1 kW carousel trackers is lower cost. Table 2 shows the module power specification and the projected module cost as a function of the assumed cell efficiency. It also includes the 1-sun module cases for comparison. An internally consistent costing methodology was assumed throughout with a mirror efficiency of 90% assumed and a mirror cost of $30 /m2 assumed along with a cost of $10 per A300 cell or 156 mm x 156 mm wafer with 8 multi-crystal cells. As can be seen in this table, the 3-sun module cases are all significantly cheaper (between $1.8 per W and $2.5 per W) compared with the 1-sun module cases (over $3.3 per W). The 3-sun case with 24% cells is the best case at $1.83/W with the second best case being the 18% multi-crystal 3-sun at $2.12/W. Of course, the higher cell efficiency leads to a smaller and more powerful module. Given cooperation at SunPower, 24% 3-sun cells should be possible since they have recently announced a 23.4% 1-sun cell. Cell efficiencies actually increase at higher concentrations since the current goes up linearly with the light intensity and since the voltage also increases logarithmically. However, the grid design needs to be optimized to keep the series resistance down and the FF up.

Proc. of SPIE Vol. 7043 704305-10

Table 2: 3-Sun and 1-Sun Module Size, Power, and Cost* vs Cell Efficiency.

Module Module Size Cell Efficiency

Module Power

Module Efficiency

Module Cost

1-Sun MultiCrystal

1.94 m x 1 m 15% 275 W 14% $3.35 / W

3-Sun MultiCrystal

1.87 m x 1.04 m 16% 260 W 13.5% $2.39 / W

3-Sun MultiCrystal

1.87 m x 1.04 m 18% 292 W 15% $2.12 / W

1-Sun Sun Power

1.56 m x 1.04 m 22% 315 W 19.4% $3.52 / W

3-Sun Sun Power

1.56 m x 1.05 m 18% 262 W 16% $2.45 / W

3-Sun Sun Power

1.56 m x 1.05 m 24% 338 W 20.6% $1.83 / W

* An internally consistent costing methodology was assumed throughout with a mirror efficiency of 90% assumed and a mirror cost of $30 /m2 assumed along with a cost of $10 per Sun Power 1-sun cell or 156 mm x 156 mm wafer with 8 MultiCrystal cells.

CONCLUSIONS

Large scale cost competitive solar electric power is in sight given the two evolutionary innovations described here. The first innovation is a prefabricated 1-axis azimuth drive carousel sun tracker that can be installed on commercial building flat rooftops or over car ports. It is targeting commercial customers in sunny locations. It can be fitted with either conventional 1-sun modules or with innovative 3-sun CPV modules. Equipped with conventional 1-sun modules today, each carousel can produce over 1 kW. Future, 3-sun mirror modules designs will allow 1 kW carousels to be made.

In the future, once both carousels and 3-sun CPV modules enter high volume production, solar electric power costs are projected to fall to as low as 8 cents per kWh. This should be quite affordable for commercial customers paying retail prices for electricity.

Unfortunately, neither carousel trackers nor 3-sun CPV modules are in high volume production today. The challenge today is qualification testing and beta site testing. Herein, the performance of two beta site systems has been reported. These two systems are located at ORNL and at UNLV.

Both systems consist of a carousel equipped with four 180 W JXC 3-sun mirror modules. Both systems have been successfully operating for several months producing power consistent with predictions.

Both systems are especially noteworthy because they are well instrumented with solar flux measurement equipment and monitored daily on site by competent engineers.

The test results for these 3-sun CPV modules are also especially noteworthy because they validate the novel features of the 3-sun CPV module design. Specifically, no mirror degradation or soiling problems have been observed and the aluminum sheet heat spreader at the back of the laminated module circuit is holding the cell temperatures at levels comparable to those in conventional 1-sun modules. However, observations on lower module fill factors suggest that the performance of these modules can be improved with cells specifically designed to operate at 3-suns rather than 1-sun.

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REFERENCES

[1] L. Fraas et al., US Pateent 7,388,146, “Planar solar concentrator power module”, June 17, 2008; additional patent applications.

[2] L. Fraas et al,”Test Sites and Testing of 3-Sun Mirror Modules”, WCPEC-4, Hawaii, May 2006. [3] L. Fraas et al, “Start-Up of First 100 kW System in Shanghai with 3-Sun Mirror Modules”, ICSC-4, Spain, March

2007. [4] L. Fraas et al, “Field Experience with 3-Sun Mirror Module Systems”, PVSC-33, San Diego, May 2008. [5] L. Fraas, Carousel patent application. [6] L. Fraas et al, “Carousel Tracker with 1-Sun or 3-Sun Modules for Commercial Building Rooftops”, Solar 2008,

San Diego, May 2008. [7] R. Swanson et al, “The Surface Polarization Effect in High-Efficiency Silicon Solar Cells”, SunPower Corp..

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performance of 3sun mirror modules on a sun tracking carousel

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