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Summary

  • It was recently reported that a GT Advanced Technologies process could be used to grow high-efficiency, thin-film solar cells with possible applications for mobile devices.
  • Solar-charging mobile devices would be desirable for manufacturers and consumers.
  • Here, I present calculations showing that solar charging would also provide significant augmentation to battery power.
  • Based on these calculations, I conclude that it is only a matter of time until solar-charged mobile devices become a reality.
  • I believe the companies that supply the solar-charging technology stand to benefit much more than the companies that make the mobile devices that use it.

A recent article on Seeking Alpha suggested that a GT Advanced Technologies (NASDAQ:GTAT) patent with applications for epitaxial-grown, thin-film, multi-junctional, high-efficiency PV cells will lead to solar-charged mobile devices. I have no doubt that such technology would be desirable to consumers and manufacturers, so long as it was not just a gimmick. By that, I mean it must actually provide a meaningful benefit. In particular, solar-charging capability would have to provide a meaningful increase to battery longevity. I was skeptical of a high-efficiency mobile-device-sized solar cell being able to supply enough power to do this, so I did some calculations to find some clarity. My conclusion is that solar charging would provide significant power advantages. So, if a method to manufacture durable, thin-film, high-efficiency solar cells in mobile-device-sized form-factors is developed, it is likely to be commercialized. Moreover, the companies (including perhaps GTAT) that can cost-effectively provide this technology stand to benefit.

What Are Multi-Junctional PV Cells, And Why Are They So Efficient?

Multi-junctional solar cells have multiple semiconductor layers with three or more P-N junctions. Such cells are more efficient because each junction can be tailored to capture a different band of wavelengths. Such technology is not new. Sharp (OTCPK:SHCAY) recently demonstrated a 44%-efficient triple-junction PV cell. But historically, the cost/performance of such cells was too high to justify their widespread use. One exception is the aerospace industry, where the performance/weight of such devices is much more important than cost. If the GT Advanced Technologies patent mentioned above makes such technology easier and cheaper to manufacture, then it stands to make a lot of money selling it.

Why Would Solar Charging Of Mobile Devices Be Desirable?

Consumers desire mobile devices that have the capacity to operate with high performance for extended periods of time. The longer a mobile device can operate between charges, the better. At the same time, a mobile device's battery constitutes a very significant portion of the device's size and weight. So, there is a trade-off between performance, battery longevity, performance, size, and weight. The incorporation of solar-charging capabilities into mobile devices to augment battery power could allow the performance and/or time-between-charges to be increased, without increasing battery size and/or weight. Moreover, such a feature is likely to be desirable for energy-conscious consumers and manufacturers who are looking for ways to give their devices market differentiation. For all these reasons, solar-charging technology would be a desirable feature for mobile devices, so long as it provides meaningful augmentation to battery power. Using the specifications for Apple's (NASDAQ:AAPL) iPhone 5s and iPad Air, the following calculations show that high-efficiency (40%) solar cells would do this.

Solar-Charging the iPhone 5s

The iPhone 5s has a 5.92 Whr battery and a surface area of 7254.68 mm2 on each side. On a clear summer day, the sun irradiates the Earth's surface with about 1000 watts of solar power per square meter (1000 W/m2). Assuming that one entire side of the iPhone 5s (the back side, probably) were covered with a 40% efficient thin-film solar cell, like the ones that may be possible with GT Advanced Technologies' process, that solar cell could deliver approximately 2.9 W of power.

1000 W/m2 x (1 m/1000 mm)2 x 7254.68 mm2 x 0.40 = 2.9 W

Such a solar cell would take (under ideal conditions) approximately 2 hours to completely charge the 5.92 Whr iPhone 5s battery.

Solar-Charging the iPad Air

The iPad Air has a 32.9 Whr battery and a surface area of 40680 mm2 on each side. Repeating the calculations above shows that under ideal conditions, a high-efficiency solar cell completely covering one side of the iPad Air could deliver approximately 16.3 W of power.

1000 W/m2 x (1 m/1000 mm)2 x 40680 mm2 x 0.40 = 16.3 W

Such a solar cell would also take (again, under ideal conditions) approximately 2 hours to completely charge the 32.9 Whr iPad Air battery.

Conclusions

The above calculations show that, if equipped with a high-efficiency solar cell, two hours on a sunny window sill could completely charge your iPhone or iPad. So long as your phone's solar cell gets two hours of direct sunlight every day, you would never have to plug it in. Even if less sunlight is available each day, the battery power would be meaningfully augmented, extending the time between charges.

Of course, such technology would not be limited to iOS devices. Apple may be first to innovate in this direction, especially if GTAT develops the technology to make it possible, considering its recent collaborations with Apple. However, such solar-charging will provide similar benefits to Android and Windows phones too. And, as the adoption of solar-charging increases, the companies that supply the technology to make high-efficiency solar cells (again, perhaps GTAT) will probably benefit much more than the companies that manufacture the mobile devices. The latter will be too busy fighting it out in the trenches.

PS: An Interesting Physical Insight

The fact that both the iPhone and iPad would take approximately 2 hours to charge is a consequence of the power consumption of each device (and the battery capacity) scaling almost linearly with surface area. That wasn't something I would have expected a priori, but it is an interesting observation. At least to a physicist.

Source: Solar Charging Would Significantly Augment Mobile Device Battery Power