According to the Guinness Book of World Records, the city of Yuma in Arizona is the sunniest place on earth. Of the possible 4,456 hours of daylight each year, the sun shines in Yuma for roughly 4,174 hours, or about 94% of the time. But if you’re not one of the city’s 77,000 inhabitants, what’s the best part of the world to have a solar powered mobile phone? After our four-month field study, we have a better idea of the answer to this and many other questions.
How well do solar powered phones work in Africa?
Near to the equator Amos, our Kenyan tester, was able to harvest charge for nearly 12 hours a day with Lokki, our solar powered mobile phone prototype. His best charging current was 32mA. If we assume the peak theoretical harvesting time is * peak*2/π, Amos’s theoretical maximum harvesting is 230mAh. However, his actual best was 134mAh, or 60% yielded capacity of the theoretical maximum. Nonetheless, in 59 days, Amos gained 20 hours talk time or sufficient standby for 41 days. If you ignore the variations in weather, at the equator these values should stay the same throughout the year.
Both Petteri sailing the Baltic Sea and Aino camping in Sweden had enjoyed long Northern summer days, enabling them to charge their Lokkis for 16 hours every day. This gave them the potential to charge 300mAh maximum or 30% more than Amos in Africa. However, in winter both the peak current and the charge time will be much reduced, to maybe as much as 30% less than the equator. If so, in Northern Europe, the winter harvest will be half the summer harvest.
During our research period, Aino’s best peak solar charging current was about 30mA. In 6 days she harvested enough energy for 1.4 hours talk time or standby for 3 days. Petteri’s peak performance was less, around 23 mA. This was because while he was sailing the Baltic, Lokki was constantly behind a transparent window and permanently pointing directly upwards, rather than towards the sun. Nonetheless, in 20 days, his Lokki gained enough solar energy for 11 hours talk time or 22 days of standby time. This means that Petteri’s Lokki could have continuously stayed ready in standby and given three minutes of talk time every single day. It was the only Lokki, which could have supported communication without breaks or without turning the phone off.
How well do solar powered phones work in the Arctic?
In the Arctic, despite the sun shining for 24 hours, it was only powerful enough to charge the phones for 19 hours with a theoretical maximum of 350mAh. For the other 5 hours, the midnight sun is too close to the horizon to provide charging power. Even during the 19 hours the sun turns more than 280 degrees, which means that a permanent installation would miss some of the solar energy that a mobile one can catch. During the three months of winter darkness, the potential energy would drop to zero.
The best Ilkka harvested was 170mAh out of a theoretical 350mAh a day, that’s about 50%. In 65 days, he gained enough energy for 17 hours talk time or 36 days of standby time. Esa had a wider distribution of solar charge between 7am and 10pm, but because he was so often trekking, the coverage was thin. Consequently, his best daily harvest was just 59 mAh, or 17% of the theoretical 350 mAh possible. In 49 days, he harvested 2.2 hours talk time or 5 days of standby time.
What scenarios would work best for solar charging?
So what does this all mean for the potential of solar powered mobile phones? Well, firstly, there are different scenarios when they might be useful. For Arctic explorers like Ilkka and Esa, and sailors like Petteri, a solar phone is a lifeline. It provides a link to society no matter where they are, and could be vital for their safety. Aino, camping in Sweden, and Amos living in Nairobi, can use solar power to get more from their phone, radio or extra calls, but they’re in no danger if they run out of battery. That said, they have no other means of charging it either.
Is solar charging efficient enough to use with a basic phone?
Our study has shown that a basic phone can operate with solar power if the consumption is low and if it is exposed to sun for extended periods. After a good harvesting day, an hour of talk or radio play was possible. However, this did require a full day’s exposure to the sun. When the days were cloudy, the phone needed to be shut down in order to maintain some charge. Just carrying a phone attached to your body harvested between 10 and 20 minutes talk time, but to provide continuous standby would probably require the phone to be better aimed at the sun. For people who live in parts of the world with constant sunshine, such as Kenya, and who can’t just plug in and load up on electricity, solar charging is definitely an option.
Is solar charging efficient enough to use with a smartphone?
The difference between the two types is substantial. The latest S30 phones such as the Nokia 100 and Nokia X1-01, for example, have more than a month stand by time. A smartphone is typically loaded with so many applications and so much hardware, the battery drains in a couple of days. With Lokki’s 30 cm² area, and a high solar panel efficiency of 20%, the available peak power is still just 600mW, that’s 120mA at 5V. In the study, the best we harvested was about 50% of the theoretical maximum, and often when the panel was not continuously pointing to the sun, it was below 10%.
If this was replicated, it would take days to charge a typical smartphone battery, during which time the smart phone couldn’t be used. Consequently, a smartphone like the Nokia N9, needs a much larger solar panel than can fit on a phone. The only realistic solar charging solution for a smartphone is a larger external solar panel that would work similarly to a wall charger.
So there you have it. After four months we’ve gained some really unique insights into the practicalities, and the future of solar charged mobiles. We hope you’ve found the project as useful and interesting as we have. And remember, if you have any opinions or questions, please let us know.