Conclusions: Where, how and when a solar powered mobile phone works best

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.

How well do solar powered phones work in Northern Europe?

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.

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22 Responses to Conclusions: Where, how and when a solar powered mobile phone works best

  1. richard says:

    do you now have plan to make a phone with solar panel? Or are you going to continue to improve the panel to generate larger energy?

    • Joel says:

      We’ve gained some valuable insights into solar charging, Richard, but our research on improving energy efficiency and looking for more sustainable alternative continues.

  2. Jari says:

    So where does this leave Nokia and solar power? Will we ever see a solar panel integrated with a Nokia phone or is gain too small? How about changes in solar tech, if it gets better will that make a big enough difference.

    • Joel says:

      Good questions, Jari. A smartphone, for example, would need a much larger solar panel that can fit on the phone back panel to harvest enough energy to keep it going.

      However, it’s clear that a basic phone, with the solar panel integrated in the back cover, succeeding in harvesting enough energy to keep the phone on standby mode and even provide some talk time if carefully positioned to capture the available sunlight.

      But is that good enough for phone owners? What do you think?

      • Keivan says:

        Hi Joel
        I am interested to the subject. I have some points:

        1. New solar panels have efficiency of about 40%. In your research it was 20%.
        Also, New cell phones use less energy.
        so I think you need to repeat your research.

        2. I think unlimited standby time would be an interesting subject. I use my mobile phone occasionally. It is annoying to see that you did not use your phone but its charge drops.

        3. Do you have any conclusion about the efficiency of solar panels in the office condition under the artificial light?

  3. If consumers used their phones like my daughter does, we’ll need at least 10 suns to power it.

    I’m not sure that phones are the best things to have as solar-powered.

    • Joel says:

      We hear you, Jason :)

      Heavy users are probably not going to benefit from solar charged mobile phones, but there are plenty of people worldwide who would.

  4. SDcupcake says:

    As much as my phone eats up battery, any help would be good to keep it charged. I believe solar charging is a great alternative to and adjunct to the existing power that it currently now offers.

  5. LS says:

    Use of solar energy to charge phones for use best bet as opposed to getting directly charged during the day.

  6. Smart phones would take a lot more charging…I’m wondering though; I’ve seen this thing at Best Buy that allows you to put an iPhone, iPad, Nintendo DS, PSP and a whole bunch of other devices on it to charge. It does plug into the wall, but you put some type of contraption on your device, then set it on the charging pad. Could we work something like this to work with a solar charger?

  7. jjj says:

    Was curious why use a curved panel,you do get a larger area but do you really get more exposure ,if heat was a problem and how was that problem solved.
    Guess on a smartphone a 60-80 cm² panel would fit assuming iphone size and up and not covering the entire back to leave room for the camera.
    Would be interesting to see tests with a solar smartphone using Mirasol and a 28nm dual core clocked at 600-800MHz (not that i would mind tests with tablets and e-readers)

    • Matti says:

      Hi jjj
      We didn’t really get larger panel due to curved shape, but we were able to keep the original thickness in the edges though in the center we had extra 4mm space taken by the data recording electronics. Heat seemed to stay just within the acceptable limits in our testers’ use, though charge would have been terminated if the limits were exceeded.
      Thanks for your contribution.

  8. Pingback: Nokia Lokki: Handy mit Solarzelle im Langzeitversuch »

  9. alberth says:

    I want to congatulate you all on a valiant effort. Even though you weren’t able to show practical solar-only phone charging, you did show what was possible now and how much more would be needed to get there.

    No doubt solar is our future. In fact our common energy sources are all solar right now differing only in how long ago the solar energy was captured. Coal is 300M year old solar, oil is 60M years old, wood biomass about 15years, hydro 1yr, corn ethanol 10months, windpower 1month, ocean tidal 1day, photovoltaic 10seconds. All your energy options are solar, you just choose how fresh you want it.

    Everything is powered by the sun. Even humans are solar powered, it was captured sun energy that grew the wheat in your bread that you ate this morning. Humanity’s future lies with solar as it always has, we just need to transition to fresher, less polluting forms.

    • Matti says:

      Hi Alberth
      I love your solar energy time scale. Never thought that even photovoltaic energy is so old, 10 seconds, the time light travels from the Sun to Earth :-)

  10. Solix says:

    I have a problem with all the devices who combine a solar module and a battery within a slim housing: The battery gets heated up which drastically reduces its lifetime. And after 1-2 years you have a dead battery which in case of most of the solar chargers cannot be exchanged since it is a proprietary type or there is no exchange service.

    Another problem is a housing in direct solar irradiation – who wants to use a 70°C hot mobile phone?

    For me a much smarter solution would be to integrate the solar panel e.g. into a hinged front cover that shadows the device, or into a separate phone case. With such a solution – combined with a 20% solar module – you could easily ramp up your rated charging power to 1Wp or more which enables a stand-alone operation.

    Where to get a 20% solar module in industrial quality and superior design? Just ask me…

    • Matti says:

      Very relevant comments, thank for sharing your thoughts Solix

      • I´m not quite sure about this (because I´m not a scientist or a specialist in energy), but what I´m reading through this past few years is about different resources of energy being studied.

        Besides solar power, we (humans) are moving creatures, so I believe that the same concept of sustainable houses could be applied for sustainable citizens – why couldn´t we make something (like moving a wheel or a handle or squeezing a ball, etc) to use our own energy towards a device that can capture it to our gadgets?
        Maybe a combination of Piezoelectricity + solar + mechanic movement properly leveraged and stored can be our self-generators.

        Like the regenerative breaks that hybrid cars are using, we must remember that we are already energy producers. If we could focus this energy, combine with what nature gives to us, then store it, we could have energy for our needs.

        And if someone needs 10 suns? Let he/she pay for extra energy or even reevaluate what matters most on a daily basis. Certainly some other person would be producing more than it uses, so they could trade (as friends trades many things).

        Power to the people (litteraly).
        (Sincerely) Hope it helps.
        Congrats for all Nokia team.
        Brasil (not Brazil)

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  14. I’m a sunshine person myself, and get so down during the winter months. I think I’ll move to Yuma and enjoy some of that sunshine! Nice Article!

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