Category: Education

This morning, I came across this picture: it makes me dream !

This is the cover photo of a book by Raphaëlle JAVET and Raphaël DOMJAN.
I just ordered it, obviously …

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It was during the course of preparation for Sun trip 2019 and 2020, in Lans-en-Vercors, that Christophe Dugué gave us important information about solar panels. In a very clear presentation, he explained to us the design and operation of a solar panel, the definition of its technical characteristics (maximum power) etc…
Christophe also informed us about practical aspects important for solar cyclists: the influence of temperature on performance, the impact of shading, the consequences of micro-cracks in photovoltaic cells…

The solar panels are its domain: Christophe works at Photowatt, a french manufacturer of solar panels for houses and buildings.
And most importantly, Christophe designed and manufactured prototype solar bike panels. These panels are both light and rigid, therefore resistant to the hazards of a solar bike trip. Christophe tested his first prototype during the Sun trip Tour 2017. Then Stéphane Bertrand was equipped for the Sun trip 2018, Lyon-Canton. For the Sun trip Tour 2019, Christophe will also equip his wife who will participate with him.

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After several days of rain, the Sun came back! It is slightly veed but no matter: I was too eager to test the solar panels received three days earlier! I compared them to the panels bought in 2017 and 2018. Although this test has no scientific value, the observations have seemed interesting enough to be published.

Test panels

	Panel A specified power: 50Wc nominal number of cells: 4 x 4 = 16 cell manufacturer: unspecified cell class: not specified provenance: purchase from a reputable reseller on internet price: 2 x €142 = €284 for 100Wc in 2017
	Panel B specified power: 100Wc nominal number of cells: 5 x 6 = 30 cell manufacturer: unspecified cell class: class C (visible defects) provenance: purchase on a famous auction website price: €135 in 2018
	Panel C specified power: 110Wc nominal number of cells: 5 x 6 = 30 cells: SunPower™ Maxeon cell class: class A provenance: purchase from Linksolar manufacturer price: €350 (~ 395 USD) in 2019

Measuring the instantaneous power of the panels

Not having a solar energy measuring device, it was not possible for me to measure the actual performance of each Panel. So I chose to measure the instantaneous power of the panels with wattmeters, in the sunshine conditions of the moment. And I compared 2 to 2 the panels connected each to an identical charge regulator charging the same battery.

Measured power with panels B and C

Measured power with panels A and C

Reservations about the test protocol

Various elements could impact the outcome of the measures and their analysis:

• • difference in the years of panel manufacturing, in a context of steady progression of cell performance;
  • difference in State of obsolescence and use of panels: Panel C totally new, Panel B having travelled several thousand km (vibrations + shocks), Panel A of 2017 never used and stored;
  • calibration difference of non-certified wattmeters;
  •  measurements made by veted Sun (repeat the test in other conditions of sunshine).

However, these elements can not fundamentally call into question the results obtained.

Calculations and results

The C Panel being the most powerful, I took it as a reference.  I calculated the ratio of the power produced by Panel A or B to the power produced by Panel C. The table above shows the relative efficiency of panels A and B in reference to Panel C, which therefore has an efficiency of 100%. Pushing the reflection further, I imagined an increase in the surface of panels A or B to get the same power as the C Panel. I then calculated the cost in corresponding A or B panels, and the corresponding panel surface.

NB: I took into account the difference in the number of cells (Panel A: 16 cells, panels B and C: 30 cells).
When observing this table, the following remarks can be issued:
Real power:

• panels A and B have almost identical power while panel A is 2 times more expensive than panel B;
• the power of panel C is almost double that of panels A and B.

Price to get the same real power:

• at equal power, panel A is 71% more expensive than panel C;
• at equal power, panel B is 21% cheaper than panel C.

Surface to achieve the same real power:

• at equal power, panels A and B occupy 90% more surface area than panel C.

Personal notes

Objective and subjective criteria had guided my purchases:

• in 2017, I totally discovered the universe of the solar panel, it had reassured me to buy at a large recognized website, the many exchanges by phone and email had given me confidence.
• in 2018, I wanted to experiment with a low-spread panel format (6×5 = 30 cells) and I had found on the market only these panels at low prices.
• in 2019, I had the urge and the opportunity to invest in quality equipment, to increase the performance of my bike.

Conclusions

• A high price is not always a guarantee of the performance of the equipment.
• Lower yield panels (50%) have a strong impact on the solar bike: less autonomy or increased dimensions. In both cases, this will degrade the rider’s comfort and safety.
• A higher expense for quality equipment ultimately represents an economy: the value for money is much better.
  Not to mention the other benefits: autonomy, maneuverability, performance…

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Justin works in an innovative Canadian company specializing in electric bikes. It creates equipments: batteries, motors, controllers, control consoles etc… Many SunTripers are equipped!
He participated in the Sun trip 2018, with his wife Anne-Sophie, on a solar tandem that he fully invented, where one pedal while the other oar!
Here they are at the Bourget-du-Lac campsite in June 2018:

Justin participated in the BC bike show 2019, the largest bike show in Western Canada. In this video, it provides a complete panorama of the various technologies available on the market, their pros and cons… See it, it’s very informative…

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This article tries to shed some light on the reader on an important element for the choice of its solar panels: their performance.
For any proposal for enrichment or correction, write to

What is the efficiency of a photovoltaic cell?

This is the ratio between the amount of electrical energy obtained by converting the light energy and the amount of solar energy received by a cell.

What is the yield of the cells available on the market?

When I first searched for panels, I read that the yield of this monocrystalline silicon cell was 15%, another 20% and some 22% (in 2017). Today in 2019, the best reaches 25%. Unfortunately, this feature is often absent from the panel specifications.
Questioned by phone, a manufacturer had replied to me: “my company, like many others, is sourcing from a fluctuating international market, we buy lots of good quality but without performance specification. By the way, every delivery, the country of manufacture is different! In this context, how to indicate a reliable data? ”

What’s the difference for the solar cyclist?

With an equal Panel surface, a solar cyclist with high-efficiency cells (25%) will have 66% more energy than a cyclist with low-yielding cells (15%). The first one will be able to drive faster or longer, farther… or make less effort, according to his choice!

This animation illustrates the impact for the solar cyclist of these differences in performance, it does not have the ambition to have a scientific rigor…
Of course, the solar cyclist can compensate for low performance by increasing the surface of panels. But he knows that this will increase the dimensions of his vehicle, make it heavier, less easy to maneuver…

Yields according to different technologies

With monocrystalline silicon cells, in 2019, the yield reached 25% with the “back contact” technology, in the best panels. With polycrystalline silicon, researchers have succeeded in aachieving a record yield of 22,3%, but this technology is not marketed today (learn more…).

Performance above 30%?
In 2016, German researchers achieved a record performance of 33.3% with a cell called “multi-junction” (learn more…). But beware, you will not find it on the Internet, this technology is dedicated to space applications!
And in the United States, in Colorado, yields of 40.8% are displayed! It makes you dream…. but when will these technologies be available to the general public?

How to choose?

The solar cyclist is therefore tempted to acquire the cells with “high efficiency”. And why not the best of the moment?
Then the question of the budget arises! As you will have guessed, the higher the performance, the higher the cost. Unless you have a very generous budget or partners, the solar bike Designer will have to make choices, and most often, compromises!

Each one must first identify its needs, define its priorities in order to determine the optimal solution for its project. He will be able to equip himself with full knowledge of the cause.

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Photovoltaic panels are the key element of the solar bike: they convert the energy of the sun into electric energy that can be used directly by the engine and/or for charging the battery. When we start the adventure of building his solar bike, we are a little lost in front of the diversity of offers.

I will therefore share here several elements that can guide you and allow you to make a choice with full knowledge of the cause.

Flexible panels or rigid panels?

Flexible panels: this is the type of panel most frequently used for solar bikes. The reasons are their lightness (from 3 to 5 kg for 200W) and the good energy efficiency of the monocrystalline silicon cells that compose them (ratio between recovered electrical energy/solar energy received). More often than not, a supporting structure of the panels is added that rigidifies and protects them. In fact, every shock on a cell decreases its performance locally. With multiplied shocks, the total power of the panel decreases significantly and permanently. That is the weak point of flexibility.

Rigid panels: they are sturdy, weatherproof like hail; they are planned a priori for a building installation. They are very heavy (15 to 18 kg for 200Wc). Their photovoltaic cells are often made of polycrystalline silicon with less energy efficiency than monocrystalline silicon. For these reasons, they are rarely used on solar bikes.

One exception, however: Christophe Dugué, who works at a French manufacturer, has built himself a rigid panel and light, monocrystalline. He experimented with it during the 2017 Sun Trip Tour. Stéphane Bertrand was also able to benefit this prototype during the 12,000 km of Sun Trip 2018. This summer, no doubt we will see these extraordinary panels again since Christophe Dugué and his wife participate in the Sun Trip Tour 2019. Unfortunately, these panels are not yet marketed.

The electrical characteristics of the panels

The power of a panel is proportional to the number of photovoltaic cells that compose it. The photovoltaic cell is the basic unit of a panel, it produces an electrical power of the order of 3 Watts under a voltage of less than 1 volt.
The electrical characteristics are mentioned by the manufacturers. The main features are the operating voltage (VMP in volts) and the maximum current in the event of maximum sunlight (IMP in AMPS). Depending on the Assembly of the cells, in series and/or parallel, panels of the same power may have different operating voltages and maximum currents.

The designer of a solar bike is therefore faced with an equation with many parameters influencing each other:

• selection of the panel surface for the desired total power,
• choice of dimensions and where to install them according to the type of bike,
• serial or parallel installation depending on the solar controller (voltage and current),
• choice of type and number of regulators (MPPT boost or not).

Solar panel photovoltaic cells

Two main types of cells can compose the panels. For a solar bike, because of the higher efficiency, it is the monocrystalline silicon that is most appropriate.

Monocrystalline silicon cells -very good yield, about 200Wc/m² -lifespan of ~ 30 years, -higher cost than polycrystalline silicon.

Polycrystalline silicon cells – yield of about 150Wc/m², thus less than monocrystalline, -lifespan of ~ 30 years. -cost more economical than monocrystalline silicon.

But it all depends on the user’s needs: Barnabas Chaillot, met during the prologue of Sun trip 2018, made the most economical choice [read more…].

The quality of photovoltaic cells

The performance of a panel depends on the quality of the photovoltaic cells that compose it but few panel manufacturers mention the “class” of the cells used. It strongly impacts the cost of construction of the panel, and therefore its selling price. For our use, three levels of cell quality are available:

“class A” cells: they are of very good quality, without visible defect and their electrical characteristics correspond exactly to the specifications defined by the manufacturer. The solar panels built with this cell class are therefore the most expensive.

“class B” cells: they are of good quality and have little visible defects. Their yield is close to or slightly lower than the “class A” cells.

“class C” cells: they show visible defects that affect their performance (e.g. chipped, cracked cells…). Their performance is obviously less good, these cells are used to realize the economic signs.

“class D” cells: they present breaks or are incomplete. They can be resized to produce in smaller sizes, but most often their material is recycled to make new ones.

More… All about the performance of photovoltaic cells!

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Of all the motorised modes of transport, the electrically assisted bicycle is one of the most economical: it is estimated to be able to travel on average over 1000 km for €1 of electricity *. * At the current rate of €0.09 per kwh.

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