FUTURE POTENTIAL MANUFACTURING AND DEPLOYMENT

While the goal of lower-cost solar electric power can be reached when carousels and 3-sun CPV modules reach high-volume production, unfortunately, this hard­ware is not yet in high-volume production. The first step has been to perform life testing and performance testing at various beta sites as has been discussed above. The performance and durability results are very promising. The next step is a com­mitment to manufacturing.

The 3- sun LCPV approach has three advantages that promise to facilitate manufacturing. The first advantage is ready cell supply with a variety of planar silicon cell options where high-volume cell manufacturing is already in place. The second advantage is that the automated planar module manufacturing equipment is readily adaptable to 3-sun module manufacturing. And the third advantage is that a variety of one-axis trackers already in use for planar silicon module systems can be used for 3-sun systems.

With regard to cell supply, a variety of planar silicon cell types can be used. In our previous work, SunPower A300 cells cut into thirds were used as shown in

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Noon IVs

image178

Volts

 

4/15/08 Pmax

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GMT

2/5/09 Pmax

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12:0013:0014:0015:0016:0017:0018:0019:0020:0021:0022:0023:00

Time

 

JXC Carousel at UNLV 4/9/2008

image180

Time

 

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Figure 12.16. 3-Sun module test data. Top/left: illuminated current versus voltage curve for ORNL array taken on March 9, 2008. Bottom/left: Pmax at UNLV on April 9, 2008. Top/right: Pmax at ORNL on April 15, 2008. Bottom/right: Pmax at ORNL on February 5, 2009. IVs = Illuminated current vs Voltage curve; I = Solar Illumination Intensity; GMT = Greenwich Mean Time.

Figures 12.6 and 12.17a. Based on cell efficiency, these cells are preferred. However, based on cell cost, more traditional silicon cells with front-side grids are less expensive, and there are many more potential cell suppliers. A second cell supplier has fabricated cells as per the front-side metallization design shown in Figure 12.17b. Note that in this design, the grid metallization is screen printed on the front side of the cell, and there are two edge bus bars that will fall underneath the mirror edges.

In addition to cell type, there are efficiency options within each cell type. For example, in the work with SunPower A300 cells to date, cells that had higher than normal leakage currents at 1-sun were used. As a starting point, this was acceptable because 3-sun cells operate at higher light-generated currents, and therefore, higher dark level leakage currents are more acceptable. These cells had efficiencies in the 18-19% range at 3-suns. Nevertheless, SunPower has 22% efficient A300 cells that they reserve for their internal use.

For the dual-bus, front-side, screen printed cell case, there are also different efficiency options. For example, SANYO has reported 22% efficient cells made with 102 x 102 mm cells. The contacts on these cells could potentially be modified to make 22% efficient dual-bus cells. Table 12.2 summarizes various future 3-sun cell options.

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Figure 12.17. (a) A 125-mm A300 cell cut into thirds. (b) A 156-mm multicrystal wafer with eight screen printed patterned 3-sun dual-bus cells.

TABLE 12.2. Derivative 3-Sun Cell Types and Dimensions

Cell Origin

1-Sun Cell Efficiency (%)

1-Sun Cell Dimensions (mm)

3 – Sun Cell Dimensions (mm)

3 – Sun Active Dimensions (mm)

SANYO

22

102 x 102

51 x 102

46 x 102

SunPower

23

126 x 126

42 x 126

42 x 120

China

18

156 x 156

52 x 156

47 x 156

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Step 1:

Step 2:

Step 3:

Pull Ribbons;

Place Cell, Advance Cell

Repeat step 1 until the last cell is placed;

Solder ribbons front & back;

Pull ribbons last time for the trailing end

Figure 12.18. The cells in standard planar silicon modules are electrically interconnected using automated cell stringing equipment where two metal ribbons are soldered from the back of one cell to the front of the next cell. Dual-bus 3-sun cells can be interconnected with two metal ribbons with the same automated stringing equipment.

TABLE 12.3. 3-Sun Module Cell Configuration and Dimensions

Cell Origin

Cell Layout

3 – Sun Module Dimensions

STC Power (W)

SANYO

10 rows with eight cells per row

51.2 x 35 in. 1300 x 890 mm

200

SunPower

10 rows with seven cells per row

50 x 37 in. 1270 x 940 mm

210

China source

10 rows with six cells per row

51.2 x 38in. 1300 x 965 mm

180

The dual-bus cell design shown in Figure 12.17b is very attractive because it can be readily adapted to standard planar module cell stringing equipment as shown in Figure 12.18. This can lead immediately to high-volume, low-cost 3-sun module manufacturing.

Using the cells shown in Table 12.2, a large variety of module sizes are pos­sible just as is the case for standard silicon planar modules. However, in addition to lower-cost modules, it is also desirable to manufacture and install lower – cost systems at the array level. In Chapter 9, a prefabricated sun-tracking carousel was described for simple and rapid field installation of arrays on commercial building flat rooftops. The 1.2-kW carousel shown in Figure 9.10 uses six SANYO 200-W modules. It may be desirable simply to begin manufacturing by designing a 3-sun module with similar dimensions to the SANYO modules used on that carousel. Table 12.3 shows various 3-sun module cell configurations leading to 3-sun modules with similar dimensions to the SANYO 200-W planar module. Each of these module designs would lead to the carousel depicted conceptually in Figure 12.19 with a power output of over 1 kW.

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Figure 12.19. Sun-tracking 1.2-kW carousel with 3-sun modules for deployment on com­mercial building flat rooftops. This is a single-axis AZ tracker with modules mounted at a fixed tilt. It rotates from east to south to west over a day.

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Figure 12.20. Conceptual drawing of a one – axis tripod sun tracker with array of 3 – sun modules. The beam is aligned approximately with the earth’s axis, and the array rotates from east to west over the daylight hours.

The LCPV modules can also be easily deployed in fields using one-axis tripod sun trackers as depicted conceptually in Figure 12.20 . These tripod trackers are similar to those deployed by SunPower Corporation in Multi MW fields as described by M. Campbell in the previous chapter.

12.2 FUTURE POTENTIAL COST

The key to the 3-sun concept lies in the fact that sheet metal mirrors are at least 10 times cheaper than single-crystal cells today. As was projected in Table 3.2 in Chapter 3, this should remain true into the future even as the cost of the silicon cells comes down. Chapter 2 discussed cost goals for solar electricity system for the future. From these two inputs, it is possible to relate the cost projections for a solar electric power system based on 3-sun modules to the future cost goals enu­merated in Chapter 2. This is done in Table 12.4 as an interesting comparison since one set of numbers is derived from the bottom up from detailed component cost projections, and the other set is derived as system goals from the top down.

Table 12.4 suggests that the LCPV system described here provides a straight­forward prescription for reaching a cost for solar electricity of less than 100 per kilowatt hour in sunny regions of the world as in the southwestern United States.

12.3 CONCLUSIONS

Fifty percent of the cost of a PV system today is in the PV modules, and 70% of the cost of a PV module is in the silicon cells. These cells require very high-purity silicon feedstock along with expensive ingot growth and cell fabrication processes.

TABLE 12.4. Predictions versus Goal Comparison for LCPV

LCPV parameter from Table 3.2

Medium Term Goal from Table 2.2

Module efficiency

20%

20%

Module cost

$1.20 per watt

$1.25 per watt

Annual irradiance

2382 kWh/m2/year

2435 kWh/m2/year

Area BOS

$100 per square meter

$150 per square meter

Inverter cost

$300 per kilowatt

$300 per kilowatt

Additional assumptions from Table 2.2

Fixed charge rate

10%/year

10%/year

Indirect cost rate

22.5%

22.5%

Conclusion

Levelized cost of energy

$0.09 per kilowatt hour

$0.09 per kilowatt hour

The JXC 3-sun CPV module cost is simply reduced by substituting low-cost reflecting aluminum mirrors for two-thirds of the expensive silicon cell area.

The 3-sun mirror module is an evolutionary design based on planar silicon cells already in high-volume production. Its novelty is the use of mirrors and sun trackers.

While the goal of lower-cost solar electric power can be reached when car­ousels and 3-sun LCPV modules reach high-volume production, unfortunately, this hardware is not yet in high-volume production. The first step has been to perform life testing and performance testing at various beta sites as has been reported here with promising results.

Large-scale, cost-competitive solar electric power is now in sight given the two evolutionary innovations described here. The first innovation is the 3-sun PV module, and the second is a prefabricated one-axis AZ drive carousel sun tracker that can be installed on commercial building flat rooftops or over carports. Commercial customers in sunny locations are the prime users for this technology as they pay retail prices for electricity well over 100 per kilowatt hour.

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 90 per kilowatt hour [9]. This should be quite affordable for commercial customers paying retail prices for electricity.

Updated: August 21, 2015 — 9:06 am