#earth2tech

Katie Fehrenbacher (@katiefehren), editor of Earth2Tech recently published an article titled "How Apple could revolutionize solar" (see article here: http://gigaom.com/cleantech/how-apple-could-revolutionize-solar/).

In the article, she quotes Bloomberg New Energy Finance solar analyst Nat Bullard (@SolarConstant), who points out "If any company could reliably integrate PV into consumer portable electronics, it is Apple. Given its other devices, it would likely make the simplest, most elegant integration. It may sacrifice some nominal performance (and greater freedom of choice) for the sake of simplicity and robustness – as it has done time and again in the past decade."

Nat is absolutely right about this - the idea of solar integrated into consumer electronics products - by Apple - is fantastic, because Apple has uniquely been able to do things that that people (and Apple's competitors) thought were foolish or impossible, until Apple showed otherwise (a shuffle-only MP3 music player? a smartphone without a physical keyboard? a tablet computer?). And sold tens of millions of iPhones, iPads, and other seemingly magical devices.

Reading Katie's article makes me wonder, however, whether even Apple's magic can overcome the relatively low power output of today's best commercially available solar cells. Let's do some quick napkin math on this, shall we?

Let's consider the case of an iPhone 4, which uses a 3.7V 1420 mAh  Li-Polymer battery. Would say putting a solar cell on the entire back panel of the iPhone 4 have enough juice to charge the battery in any kind of reasonable way?

For argument's sake, let's put the question of cost aside. (I'm also discounting hero cells, expensive multi-junction gallium arsenide cells and the like.) Let's look at the highest efficiency commercially produced solar module, from SunPower, their new E20 / 435 panel, which are 20.1% efficient (module efficiency). This module has 128 Maxeon cells which are rated at 22.4% efficient (cell efficiency). The nominal power from the module is 323Wp, with a rated voltage of 67.2 V and current of 4.81 A.

Since I couldn't find a spec sheet for the Maxeon cell, to make things simple, let's just do a oversimplified calculation to reverse the typical losses from cell to module efficiency to get a nominal power figure for the cell:

323 Wp / 20.1% x 22.4% = ~360 Wp per module (reversing module losses)

360 Wp / 128 cells per module = 2.8 Wp / cell

In other words - each Maxeon cell can produce 2.8 Wp.

Now each cell is around 150 sq cm each. The back of the iPhone 4 is 67.5 sq cm. So assuming the cells just simply be downsized to the size of the iPhone 4:

67.5 sq cm (iPhone 4 back) / 150 sq cm (Maxeon cell) = 45%

So we could generate 45% of the Wp of a Maxeon cell, or 45% x 2.8 Wp = 1.26 Wp.

Now, the iPhone 4's wall charger is rated at 5W (5V at 1A). So an iPhone 4 with a built-in Maxeon solar cell, exposed to full sunlight in ideal testing conditions for temperature and humidity - would take 4x as long as a wall charger (5 W for the wall charger / 1.26 W output for the Maxeon cell the size of the iPhone 4 = 3.97, or 4 rounded).

It takes around 130 minutes using the Apple wall charger to charge the iPhone 4 from a fully discharged state to 100% (see here: http://dailyiphoneblog.com/2010/07/13/image-iphone-4-charging-time-wall-charger-vs-usb-charger/).

The solar cell - in ideal conditions - would take 4 times as long - so 4 x 130 minutes = 520 minutes, or almost 9 hours. (Reality is likely far short of ideal conditions - there won't be direct sunlight all the time, the phone will be in a purse or pocket, the owner will have the phone in his or her hand and will be covering the solar cell, etc.)

So - I'd love to dream that Apple's magic will make a fantastic solar-integrated iPhone case that would charge the phone at a sufficient rate to make a meaningful difference in talk time, web surfing time, etc. However, today's best commercially available monocrystalline c-Si solar cells are simply not efficient enough to make this a practical reality.

Public officials might be well advised to steer clear of controversy, but when it comes to energy storage startup EEStor, its investor Zenn Motor — and the companies’ bold claims about the disruptive potential of EEStor’s ultracapacitor technology — a couple of Congressmen are plowing right ahead.

Representatives John Carter of Texas and Mark Schauer of Michigan, who have lent their support to EEStor and Zenn in recent months, are the latest example of how some politicians have rushed to align themselves with clean energy ventures whether or not the technology and business models are proven. California Governor Arnold Schwarzenegger hailed solar thermal startup Ausra as “one of the best companies in the world,” for example, before the startup laid off staff and was forced to dramatically scale back its ambitions. The trick to currying this favor? Promising to create green jobs.

V2G to Go Postal?: Broad use of electric vehicles in the U.S. Postal Service delivery fleet would be operationally feasible, according to a new study from the USPS Office of the Inspector General, but due to the Postal Service’s current financial distress, government funds and vehicle-to-grid revenue would likely be needed to make this transition economically feasible. — Green Car Congress