Advantages and disadvantages of the solar bike without batteries

There solar bike without batteries it is the only true one pedal assisted velocipede which can boast zero emissions (not even those of batteries) and the same maintenance costs as one bicycle traditional. It is true that the photovoltaic panel wears down reducing performance over time, but the average life of these products is around 20 years and if the panel is of good quality there is a risk that it will last longer than the bicycle. Of course, a vehicle of this type also has disadvantages.

A solar bike without batteries and the Sunbike designed and built by Lorenzo Barbieri of Concordia (MO), who is personally testing it, verifying its merits and indisputable limits. We have already briefly talked about the prototype in a previous article (click here), now let's go into detail with the inventor's words. The story is full of unpublished information and interesting ideas, if you are passionate about the genre it is worth getting to the bottom.

Engineer Barbieri, how did the idea of ​​building a solar bike without batteries come about?

It all started when, 4-5 years ago, in newspapers and various Internet sites I read the first articles about solar-powered cycles, bicycles, tricycles or quadricycles, mostly made by technical institutes, which interested me a lot. To the point that I have not limited myself to reading the articles, but have tried to technically investigate how those vehicles were made and how they worked.

What was the conclusion?

The very idea of ​​being able to exploit the sun's energy to move was a fascinating thing, but the technical insights led me to numerous disappointments: 90% of the cycles announced in large letters in the titles of the articles were equipped with batteries and photovoltaic panels installed did nothing but increase the battery life by 20-50%. The other 10% were even made of velocipedes not even made, only made on paper or computer, and in any case with characteristics that, at best, would have allowed these velocipedes to function exactly like the others.

So solar bikes were actually battery bikes ...

This profound disappointment led me to a basic reasoning: wouldn't you install slightly larger batteries first? The problems of installing photovoltaic panels, charge controllers etc ... would be solved, with all that this entails in terms of costs and practicality in the use of cycles! What sense does it make to call a velocipede a 'solar-powered bicycle' if the motor is powered by a battery which when discharged must be recharged independently of the sun? What is the point of installing a photovoltaic panel, with all the technical problems that this entails, to bring the range from 30 to 40 km? Wouldn't it be easier to install a 480 Wh lithium battery instead of a 360 Wh gel one? For these reasons I started a series of tests to understand where the problems are: first on bicycles, then on various batteries and on photovoltaic panels.

Tell us about the tests ...

First of all, parameters must be established as operating conditions, from which the powers involved can be derived. There are already tables that relate the powers necessary to maintain certain speeds, based on the type of bicycle, the position of the cyclist, etc ..., but many aspects are not mentioned and doubts about the actual application remain. I then checked what the minimum equilibrium speed is and in how many meters must be reached in order to make a start. This speed is 4 km / h and must be reached in about 3 meters. Using a rope, a pulley and gym weights, and marking the references of the next 3 meters and 1.2 meters (4 km / h correspond to 1.11 meters / s, rounded to 1.2 meters), I verified how much weight you need to be able to travel 1.2 meters in 1 second after 3 meters of acceleration. From the kg we get the Newtons, which, multiplied by 1.2 meters give the Joules, which, in these tests are corresponding to the Watts, as they are traveled in exactly 1 second. The resulting necessary power is 40W per wheel. If this power is to be obtained using a permanent magnet motor (of the standard 250W, brushless type) from an electric bicycle with pedal assistance, with an efficiency of about 60%, it is clear that I have to supply the motor with about 70W of electrical power. .

Did you also make other tests?

I have done many other tests on bicycles to understand the powers involved: with smooth wheels or studded wheels, swollen or semi-inflated wheels, bicycles attached together to understand the friction of several wheels, etc .... As for the batteries, there are no data on the performance of the batteries, but only inrush currents, hourly charges, charging cycles, etc…. I then tested some batteries, but above all a 360Wh electric bicycle gel with its charger; I put it on charge by connecting a digital multimeter for user analysis to the charger. After about 6 hours the green 'full battery' light came on and I read how much it absorbed: 550Wh! I do not know the performance of the batteries, but taking into consideration that the charger must be powered, with this check I have detected how much the battery / charger group absorbs. Then I noticed how much energy the battery delivers, and even this figure is disconcerting: 250Wh! Less than half of what is absorbed and less than nominal.

So the battery itself is a limitation of the solar-powered bike ...

The fact that made me understand the reason why all the velocipedes made with batteries and photovoltaic panels, once the batteries were discharged, had to stop and recharge them, is the absorption of the battery in the first hour of recharging: 200Wh! The batteries put under charge, initially, absorb a lot; and photovoltaic panels, even in the best conditions, would divert most of the energy produced to the batteries, leaving very little to the engine. Then I did a lot of tests on photovoltaic panels, especially in conditions of medium / low irradiation, even partial shading on the individual cells, with various temperatures, but which I am not reporting. I can only convey my decision that I have chosen that, in order to obtain the 70W necessary for the motor in most hours of light of a day, I must necessarily install over 140W of photovoltaic panels.

In light of the tests, what do you think is the ideal engineering for a solar-powered bike?

After these tests I realized that everything hinges on dimensional aspects, and not on electronic engineering puzzles. The velocipede must have 2 wheels, as with 3 it would absorb almost 10W more which become almost 20 considering the performance of the motor. It takes wheels with a sliding tread and inflated to at least 5 bar. Given the test results, the batteries I decided not to put them. Obviously this leads has advantages and disadvantages. Let's start with the latter ...


1) motor assistance is available only during the day, about 2 hours after sunrise and before sunset, if the sky is not overly cloudy, and if there are no persistent shading situations (roads lined with trees or alongside palaces, particular valleys, etc ...).

2) the provision of assistance is not always constant, as the photovoltaic panels deliver energy immediately and even the shadow of a pole causes a slowdown.

3) the power that a battery delivers is much higher than that of photovoltaic panels; even if the speed is limited to 25 km / h by law, and can also be reached with the panels alone, with the battery you can tackle climbs (with not exaggerated slopes) and start with lively accelerations


1) it is not impossible to travel 200 km in one day, even with little training, running with the sun on a clear day; which is impossible with batteries (even with an oversized lithium battery pack it would require at least 3 recharges)

2) photovoltaic panels last for decades, while batteries last from 200 to 800 charging cycles depending on the type and criteria of use

3) the lack of batteries and related regulating and recharging devices allows the velocipede to be lighter by a few kg; important thing in case you have to pedal

4) the cost of purchasing and managing / replacing the battery system is avoided, limiting maintenance to that of a normal bicycle

What does it tell us about photovoltaic panels?

The photovoltaic panels of a solar-powered bicycle must have a minimum rated power of 140W, checking the various situations of irradiation. I have noticed that many photovoltaic bicycles have much less installed power, or are too heavy, or with too much friction due to the number of wheels; for this, then, they must necessarily resort to the installation of batteries. Furthermore, the panels must be positioned horizontally and above the cyclist: the velocipede can move towards all cardinal points (directional field of 360 °); I have seen photovoltaic bicycles with tilted panels as if they were positioned on the roof of a house, or like the ailerons of a Formula 1; this implies a higher efficiency when the panel is inclined towards the sun (therefore for 90 °) and a lower efficiency in all other directions (270 °), unless the sun is exactly in the Zenith. When the panels are not positioned above the rider, the rider himself can cause shading. Then the pnnli must be light, as being positioned at the top they cause the center of gravity to be raised, and being distant from it they also have a long lever on which to act to make balance less secure. Finally, they must be arranged longitudinally to minimize lateral dimensions (dangerous in traffic). Some recumbent bicycles are the ones that are best suited for installing a large area of ​​photovoltaic panels. However, the bicycle I made is only a rough prototype, assembled with materials available on the market.

What are its conclusions?

A solar-powered bicycle is not recommended for city trips, at least in Italy. The urban roads, the poor quality and quantity of cycle paths would make their use not very relaxing, then, the presence of houses close to the streets, and tree-lined avenues (and luckily there are still some), would create numerous areas of 'shadow that would allow a poor exploitation of the engine assistance. Even in the mountains it is not in its home, as everything is played on the few Watts available, so uphill, where you need many, the help is scarce (you feel the assistance, but to overcome 8% you have to help the motor pedaling). This bicycle is instead ideal for trips to the countryside, suitable for people who (like me) have the time to ride a bike only on weekends, therefore without training, who would like to travel many km without effort and without looming overhead. all those pains that traditional bicycles cause in just 2 or 3 hours. It is fantastic to use it on the asphalted streets of the river banks. It can be suitable for those who love to take their holidays by bicycle by grinding km and km, or for those who can use it along roads suitable for the characteristics that I have more or less tried to describe. I have already thought of a specially created frame and of the modifications suggested by the tests, which could make it further light, aerodynamic and usable, as well as much more beautiful and protective against wind, sun and rain.

Will the tests continue?

I will use the bike I built all summer, maybe I could come up with other ideas to improve it. It would be nice to find the support of some bicycle manufacturer to create the model I have in mind; otherwise I will have the next winter and my usual garage, but, obviously, with many difficulties.

Video: Which is better: 48V or 52V battery? (June 2021).