Why not a solar energy powered Twizy ?

The American automaker Ford presents a prototype of solar-electric car that is to say, able to recharge batteries with embedded photovoltaic panels on its roof.

And why not a solar-electric Twizy? The answer is no, but...


An electro-solar Ford ?

The Ford's project seems quite questionable, first because it is associated with a system of fixed charging stations equipped with Fresnel lenses... Innovative solution but we could ask what practical use it may have since their use requires slow recharging while the sun is high in the sky... a moment where the car is statistically the most needed ! (See the video)

The use of solar panels is an old dream perpetuated by events like the Solar Challenge, but is it technically and economically viable ? Flexible and efficient solar panels are very expensive. To this must be added the cost of the electronics. How many refills on outlet these panels will they save before becoming profitable for the motorist ? More, charging on outlet is usually at night, which is a good thing because it helps to absorb part the usual overproduction occuring at that moment.

The only real advantage, not negligible, would to expend the range...

An electro-solar Renault-Nissan Twizy ?

To prove if it is a viable project or not, some calculations are necessary. So dream again a little and let us establish a goal of making a solar-electric Twizy... What surface of panels would be needed? For which use? At what price? When would it be cost effective? Are there no better use for this application?

Some basic facts

- One square meter of Earth's surface receives in France between 3 and 5 kWh per day if being headed south at an angle equal to the latitude. (See the map of average annual solar energy in France.)
- One square meter of solar panels, necessarily flexible, lightweight and efficient (20% yield) is capable of providing between 600 and 1000 Wh per day if properly directed, with a peak power of about 150-200 W when the sun is at its highest. (See Wikipedia)
- The Twizy 80 has a 13 kW motor.
- It has a 6.1 kWh lithium-ion battery .
- The price of electricity in France is on average € 0.13 per kWh. So a full recharge of the battery costs  6x0.13 = 0,8 €

No direct supply of the solar-electric motor

First you must discard the solution  to completely replace the battery by directly power the motor with solar panels. It would be necessary to provide an electric peak power of 6kW corresponding to a minimum of 30 m² of embedded solar panels... Impossible.

only 1 m² to recharge the battery

So it can only recharge the battery ... To recharge the Twizy in a day, we should be able to embark between 6 and 10 m² of solar panels. This car has the quality of being a very small car, but it turns out to be a failure when you need surface... The roof itself is only 0.6 m². Suppose that we are very clever and we get to put photovoltaic cells everywhere... We won't find more than a square meter of solar panel!

between 6 to 10 days for a complete recharge

With 1 m², the Twizy will be charged after an average of 6-10 days, depending on the region of France (in fact with a much wider range depending on the season). So charging on wall socket remains essential, otherwise have to develop a folding system capable to deploy the necessary 6-10 m². Hard to imagine the practical side of that...

For a few 6-10 kilometers more

Well, our square meter will still give a little electricity. As already indicated, it can provide between 600 and 1000 Wh per day in the best conditions. It's a bit like having a battery of 6.7 to 7.1 instead of 6.1 kWh originally. An increase of 10 to 16% corresponding to a 6 to 10 km additional distance... This is not a revolution but it is not nothing. It also means that this system will save us between 10 and 16% of recharges on outlet.

Nice, but at what price ?

Currently the price of a square meter of flexible, lightweight and efficient solar panels is about  1000 €. It should be added the electronics, wires, etc. , which can be estimated at 200 €, for a total of  1,200€ . To become economically viable, the electro-solar system must have saved us 1200/ 0,8 = 1500 refills on outlet.
Consider a favorable case : someone who is traveling a lot and have to reload the Twizy two times a day. The electro-solar system could save him 2 refills per week. It will take 750 weeks ie 62 years to monetize the system... Of course some parameters will change in time such as price reduction cells, increasing of their performance, high level of electricity prices, all of that can lead to a more interesting situation, but certainly not to achieve profitability in 5 or 6 years.

An all-in-one autonomous solar-electric system to be invented

It is clear, to enhance range we must forget the idea of solar panels.
But an autonomous solar-electric system, less efficient but cheaper, around 300 to 500 €, could bring some comfort in the Twizy incorporating a battery, a small heater (for several minutes), ventilation , DEL interior lighting, an USB output,... It remains to be invented.

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