Higher and Higher Conversion Efficiencies

With the advent of funding from the DOE in the late 1970s and early 1980s came plans and goals to develop PV technologies through improving performance (efficiency), reducing cost, and assuring reliability of operation. CPV systems offered the possibility of lower cost because expensive solar cells are replaced with less costly structural steel holding mirrors or lenses. However, early CPV systems showed the importance of optical efficiencies as optical losses typically reduced the CPV system efficiency by 15% to 20%. To compensate for optical losses, CPV systems needed the highest-quality, highest-performing solar cells to compete with flat-plate PV systems.

Early PV researchers, principally Martin Green in Australia and Richard Swan-son and Vahan Garboushian in the United States, developed innovative designs for crystalline silicon solar cells, leading to the record efficiencies of the 1980s and 1990s. Today's CPV systems using high-efficiency crystalline silicon solar cells have system efficiencies approaching 20%. Installed CPV system costs are comparable today with those of utility-scale flat-plate PV systems at about $6/watt. 5 But the

Multi Junction Photovoltaic Cell

Fig. 4. The highest-efficiency solar cells, both crystalline silicon and multijunction concentrator devices, have been most suitable for solar concentrator systems. Replace with graph having the new 40.7 % result in 2006.

Fig. 4. The highest-efficiency solar cells, both crystalline silicon and multijunction concentrator devices, have been most suitable for solar concentrator systems. Replace with graph having the new 40.7 % result in 2006.

dramatically higher efficiency solar cells—above 40% now—as shown in Fig. 4, are creating considerable excitement about CPV systems.5

Research on multijunction solar cells began in the 1980s as part of a DOE effort to explore new solar cell materials and new solar conversion processes to improve cell efficiency. A single-junction solar cell is tuned to just one wavelength of the solar spectrum so that maximum efficiency occurs only at that color. (A semiconductor junction refers to an interface between a p-type semiconductor material and an n-type material. P and n refer to semiconductor charge carriers and are a reminder that solar cells behave like batteries in that they have positive terminals, negative terminals, and generate direct current.) Early solar cell researchers calculated that an infinite number of junctions would be the most effective means to harvest energy from each and every color in the solar spectrum and that such a stacked set of junctions could theoretically convert more than 80% of the sunlight into electricity. Yet, the first monolithic two-junction solar cell, made almost three decades after the discovery of the modern solar cell, demonstrated efficiencies less than that of a single-junction cell. The materials and chemical science difficulties encountered in making the first monolithic two-junction solar cells were significant. These multijunction PV technologies are based on elements in columns III and V of the Periodic Table, and they are often referred to as III-V solar cells. Soon thereafter, two-junction III-V solar cells were developed with efficiencies higher than those of the best silicon solar cells.

Table 1. Benchmark (10-MW) System parameters and impact of multijunction (III-V) solar cell efficiency on a CPV utility reference system.7 High-efficiency solar cells are installed in essentially identical solar concentrator structures, and the cost per watt drops from about $6/watt to well under $2/watt while electricity costs fall below 10 cents per kWh. Higher production levels can lead to even lower levelized costs of energy (LCOE).7

Table 1. Benchmark (10-MW) System parameters and impact of multijunction (III-V) solar cell efficiency on a CPV utility reference system.7 High-efficiency solar cells are installed in essentially identical solar concentrator structures, and the cost per watt drops from about $6/watt to well under $2/watt while electricity costs fall below 10 cents per kWh. Higher production levels can lead to even lower levelized costs of energy (LCOE).7

System size

MW

10

12.5

16

Module price

$/Wdc

4.13

3

1.56

Cell efficiency

%

26 (Si)

32 (III-V)

40 (III-V)

Module size

kWpdc

40

50

64

Module efficiency

%

20

25

32

Installed system price

$/Wdc

5.95

4.3

2.52

LCOE

$/kWhac

0.15-0.27

0.10-0.15

0.06-0.11

The U.S. Department of Defense recognized the potential of these new solar cells for powering satellites and supported the development of their manufacturing processes. Again, a new PV technology found a commercial niche in space power markets. Today, almost every commercial and defense satellite—as well as the Mars Rover instrumentation packages—use multijunction III-V solar cells for their electrical power sources. Just before the turn of the century, collaborative research and development by the National Renewable Energy Laboratory and Spectrolab, a division of Boeing, demonstrated a three-junction solar cell with a higher efficiency than that of two-junction cells. Efficiencies are now over 40% in the laboratory, with reasonable quantities of 35% cells available from suppliers. This technology may be coming back to earth more quickly than the early silicon cell technology did as today's governments and investors respond to the world's demands for more and cleaner energy sources. As shown in Table 1, performance pays.7 Capturing those economic benefits involves replacing the crystalline silicon solar cells in essentially identical solar concentrator structures with new high-efficiency III-V multijunction cells.

The pioneering companies of Amonix and Solar Systems developed their CPV structures around crystalline silicon solar cells, but both are rapidly incorporating the new high-efficiency multijunction cells into CPV products that they expect to have available in the near future—within 2 to 5 years. Can the companies making multi-junction III-V solar cells (e.g., Spectrolab and Emcore in the United States) meet this new and imminent market demand? Today's annual manufacturing capacity for multijunction solar cells is about 1 MW under 1-sun illumination. Remember that these multijunction cells are used in non-concentrator versions in space. And as the market for satellites undergoes its own demand cycles, there are periods in which substantial portions of the production facilities are available for other markets, such as the terrestrial CPV market. Concentration provides a huge lever to this production capacity. A solar concentration ratio of 1000 suns means that a manufacturing capacity of 1 MW of flat-plate space PV panels could be the solar power sources for 1000 MW of CPV systems. The total production capacity throughout the world for III-V multijunction solar cells is already about 1 MW/year. The potential capacity there fore exists for CPV technology to make a dramatic leap from megawatts to gigawatts in the market in the very near future.

However, companies want to be sure that these new multijunction solar cells will operate reliably in their CPV systems. After all, the new solar cells typically operate at higher voltages, generate higher current, and behave differently under environmental conditions of temperature cycles and humidity than do crystalline silicon solar cells. And there is a long history for crystalline silicon operation on earth, whereas very few multijunction III-V solar cells have been deployed in field installations. However, early demonstrations are promising. One CPV company, Concentrating Technologies, has operated Spectrolab's triple-junction solar cells for more than one year at an Arizona Public Service test site. Nevertheless, companies integrating these new solar cells into their solar concentrator structures expect it will take 2 to 5 years to assure reliable products for the marketplace.5

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