Tablets and On-Device Solar Power

How close are we to powering our tablets with on-device solar cells?

In a recent test I ran a tablet connected to the internet at an average 200 mW. That’s screen off, wifi connected and polling in the background to update emails, location, twitter and other processes. It’s nothing really surprising because most of the tablets are built on smartphone technology. With the screen on though, usage will jump about 5-7x and if you push the CPU and add 3G you can reach 20X that power drain. Still, running everything in 4W is still impressive. It’s just a shame that 20-30% of the in-use power and about 70% of the idle power when the screen is on is used by the screen itself. LED-backlit technology is getting better but only in small amounts. What if you could use ambient light for the screen, just as we do when we read a paper book?

Pixel Qi is one company that are developing these ‘transflective’ screen technologies and in an interesting interview I saw today, Mary Lou Jepsen, the founder, talks about using solar energy to power a low power tablet with a Pixel Qi screen. By combining transflective screen technology with a low power tablet, you’re talking about 1W of power in idle, screen-on situations.

Unfortunately, the 1W panel that Mary Lou shows, isn’t going to be enough.

More after the video.

There are a number of issues here.

1 – No one leaves their devices in the sun unless it is in a secure place. It is usually recommended that devices are not left in the sun at all.
2 – There are, on average during a summer period, about 4 ‘solar’ hours per day in central Europe, rising to about 7, year-long towards the equator. That’s because the sun is only at its strongest for the midday period and clouds and shadows will also impact the overall efficiency.
3 – Battery charging isn’t a simple matter of trickling some energy into a batttery cell. Most Li-Ion battery circuits only trigger charge at a certain level of voltage which again, reduces the number of hours per day that a solar panel can be used
4 – Charging circuitry is innefficient. You can lose 20% of the engery through the charging electronics.

Don’t forget that we tend to be awake and using our devices during sunlight hours and we tend to do that away from the sun so the possibilities for on-device charging are seriously limited.

While a 1W panel sounds like it might be enough, to charge the average tablet (with a 15Wh battery) you’d need to sit the device in the midday sun every day for about a week for a complete charge.
Yes, Pixel Qi screen technology reduces the battery requirements but even with a battery half the size, the time taken and the akwardness would make the process unusable for all but the most niche of cases and even in those cases (off-the-grid, emerging countries) a solar charging station for replaceable batteries is, in nearly all cases, a better idea.

Of course there are some scenarios where this would work today and at some point in the future the cross-over point will be reached where it makes sense to build solar cells into consumer devices. Pixel Qi are definately doing their bit to accelerate this but there’s still a way to go before we see solar panels built into tablets.

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