Every so often there are rumors about Apple integrating solar panels into the displays of their various electronics. It doesn’t sound crazy; I mean, the display is just one giant piece of glass with circuitry behind it. Why not just add some photo-voltaics too? Being clean energy oriented gives some credence I suppose. Apple does tout their environmental record.
Most recently a new patent filing by Apple for integrated solar panels has stirred up that speculation again. I am no insider to Apple’s super secret future plans, so who knows what they are going to do, but I wanted to use a little math to find out how much sense solar panels in phones (or other electronics) would make. I will use the current iPhone 5 form factor as our base case. My assumptions will outline the absolute best case scenario.
You will notice that for the surface area of the iPhone I have used the exterior dimensions of the phone. The surface area, keeping with the best case scenario, is a maximum as it is unlikely Apple would be able to use all of the surface for panels. The home button, speaker, proximity sensors, and camera all make this an impossibility.
Incident sunlight is usually thought of in terms of power per unit area. The typical units are mW/cm2. At the earth’s surface, the nominal value of the solar constant is 137 mW/cm^2. This value corresponds to high noon with the sun directly overhead (as would occur at the equator or in the tropics).
Again 137mW/cm^2 (1370 W/m^2) is a maximum estimation. Relatively few places on earth will have the sun directly overhead on any given day, and… well… high noon can only last so long. But for this analysis let’s assume we are in one of those places and it is always noon.
So how long will it take to charge my iPhone?
1370 W/m2 * 0.00725468 m^2 = 9.9389116 W (Energy from the sun that hits the phone)
3.8 V * 1440 mAh * 1 A/1000 mA = 5.45 Wh (iPhone battery size)
5.45 Wh / 9.9389116 W * 60 minutes/1 hour = 32.90 minutes
Under the theoretical best case it will take over half an hour to charge your iPhone. Now consider some things that make the best case impossible. Latitude: The further away from the equator you are the less direct the sunlight is. So you will not approach 1370 W/m^2. Obstructions: it may be difficult to have complete unobstructed views of the sun due to either your hand, landscape, vegetation, or weather. Solar Panel Inefficiency: most solar panels cannot convert all wavelengths of sunlight into electrical energy so you start at a number lower than the limit. Of the energy that panels are only about 20% efficient. Even if you think panels can reach a higher limit of 50% conversion, panels still won’t capture all the energy. Location: your phone has to be in the sunlight… so it has to be outside or at least in a window (forget about the energy lost through the window). Heat: iPhones have an upper ambient temperature limit of 95 degrees. I have to imagine direct sunlight for extended periods of time would overheat the phone.
I think a more realistic time to charge for an average person on an average day would be closer to three hours given the concerns in the above paragraph and what I mentioned earlier. The key to a solar panel is surface area. So, obviously larger phones will have more room for panels but they will also require larger batteries to drive the bigger displays. I imagine the larger surface area in those cases is a net wash.
And o yeah… forget about the possible theft risk of leaving your phone sitting out in the open.
Given how little the benefit of solar panels in a phone could be, I find it hard to believe that such a solar panels will find their way into phones anytime soon. In order for integrated solar panels to make sense, there would have to be a dramatic increase in the energy conversion rate of panels and/or there would need to be continued reduction in the amount of power consumed by our devices. Or maybe gaining a small charge every time you are in the sun is worth it?
Thanks go out to Aaron Paget for contributions to this post.