Tallinn University of Technology

When do we no longer have to charge our smartphones with electricity and they can be charged using body heat or sunlight, for example? Jaan Kalda, physicist professor at TalTech, answers:

Jaan Kalda
Professor Jaan Kalda

The article was published on the Estonian news portal Novaator.

Although both of the proposed charging methods are feasible, unfortunately neither of them is powerful enough to charge phones.

Let's first look at charging with solar energy

Today’s solar panels can generate about 200 watts of electricity per square meter – assuming that they receive direct sunlight. This figure corresponds to 20% quantum efficiency of a solar panel (the ratio of energy produced to luminous energy captured) and may increase slightly in the future – research laboratories are currently testing solar cells with up to nearly 50% quantum efficiency. Let’s assume that the back of a mobile phone is covered with a solar cell and has an area of 100 cm^2. In this case, the charging power of the phone left in the sun would be 2 W. If the phone's battery capacity is 1100 mAh at 3.7 volts, the battery will charge in about two hours, and that would not be bad at all.

However, if it does not receive direct sunlight, illumination will decrease ten times and even a hundred times in cloudy weather. Even on a well-lit desk, illumination is usually about a hundred times lower than direct sunlight. Therefore, if the sun does not shine directly on the phone, it would take 20 to 200 hours to charge. The first number corresponds to charging in bright daylight and the second to cloudy weather or bright artificial light. This is no longer very realistic, even if the efficiency of the solar cells increases to 50%, reducing the charging time by 2.5 times.

Let’s move on to body heat 

The most efficient devices for transforming thermal energy are so-called heat engines, the best-known example of which is the internal combustion engine. This is not suitable for us, because we want to produce energy without burning anything, at a moderate temperature. The so-called Stirling engines run at moderate temperatures, but they are too big to use with a mobile phone. The only devices small enough for use with mobile phones are thermoelectric generators, also called Seebeck generators.

As a side note, Thomas Johann Seebeck, who was born in Tallinn in 1770, is probably the most famous physicist from Estonia in the world (read also about TalTech: Thomas Johann Seebeck Department of Electronics). Thermoelectric generators are very compact, but have relatively low efficiency. As with other heat engines, the efficiency of thermoelectric generators depends on the difference between the heating body (in this case the human body) and the cooling body (indoor air). If we consider this temperature difference to be 20 degrees, the efficiency of even the best Seebeck generators will not exceed 1%. A person sitting calmly produces heat at about 100 watts. Assuming the phone “captures” 1%, the generator will have a charging power of 10 mW and a charging time of 400 hours.

*As of October, the researchers of Tallinn University of Technology answer the questions of readers of the ERR science news portal Novaator on topics that require clarification or are of current interest.