I looked for a figure and found this suggestion for sunny days: 800W/m².
If the plane uses 20kW while flying, this would mean you’d need 25m² of 100% efficiency solar panels to fly without draining the battery.
10.5m wing span, depth uncertain but looks to be under a metre, and we’re not even halfway there. The fuselage isn’t suitable for a solar array. The tail, maybe a bit.
Verdict: ignoring solar panel mass and fragility, you could probably already design something that would get you another five minutes during your hour of powered flight time, but I think you’d have to change things up a lot to increase it much beyond that.
I think if you were trying to do such a thing seriously, you’d try to increase wing and body top surface area.
The wing area for this aircraft is 9.29m^2, and the glide ratio is 17:1 at 74mph. Let's say you fly for an hour and then glide through a descent of 12,000 feet (ceiling is 18,000 at max weight). That gives you about 90 minutes of solar absorption time.
Assume we only fly at noon on clear sunny days to get 1kW/m^2 of solar energy (nice round number). That's 13.9kWh of incident energy on the wings. If we can capture that at 25% efficiency -- today's best commercial cells are around here, although about twice the efficiency has been achieved by researchers -- it's 10.5 minutes of additional powered flight, or about 3 minutes of climb. (I think that 3 minutes of climb yields an added 3660 ft of altitude or 11 minutes of glide.)
4kW from propeller generation during the glide phase yields another 6 minutes of powered flight.
I agree with your conclusion for this particular aircraft's geometry; it's not enough to sustain flight indefinitely. Still, other solar aircraft have demonstrated that sustained solar flight of manned aircraft is possible, and one such aircraft, Solar Impulse 2, has even circumnavigated the globe (albeit interrupted by a few stops along the way).
That all sounds about right, except for the generation part (see below) which is but a minor factor anyway.
Indefinite solar-powered flight is understandably not a goal of Pipistrel for this craft; it will be interesting once it gets to the point where it’s a more commercially feasible design goal.
I love how you rounded up to 1kW/m² to get a nice round number, then multiplied by 13.9!
It’s worth noting on the propeller generation point that it’s going to be more efficient not to use it if you’re trying to maximise range: it will diminish your glide ratio; it’s mostly for when you actively want to go down, and might as well retrieve and store most of the lost kinetic energy. If the Trainer gets 17:1, I’d expect the Electro would get roughly that 17:1 if merely idling, but lower if regenerating; I don’t care to speculate on the numbers—I’m not a pilot or an electric car expert and it’ll take me too long to calculate the actual energy rates involved. But physics more or less decrees that it can’t regenerate more power than it will take to regain the additional lost altitude. (I say only “more or less” because of things like gravity assist manoeuvres, which are fascinating but not applicable to craft like this.)
I was rather sloppy in the way I brought the regeneration up in my earlier comment. It was true, but not relevant because of this last paragraph. I didn’t think it through when I mentioned it at first.
Agreed. It is probably too generous to assume the ideal glide ratio and simultaneously assume generation via the prop.
Depends on a lot of stuff though. It's not a question of energy creation -- the energy of the system includes energy in the air. Generally freewheeling generates more drag than a stationary propeller (google "ESC brake vs freewheel" to see what R/C hobbyists have to say about this). However, you may have an unusual situation where maintaining the ideal glide velocity requires braking, and in such a case it is presumably more efficient to brake via the propeller.
Suffice it to say that even in the ideal case, this particular aircraft would have trouble maintaining flight via solar power, but other aircraft are proving that this is a viable possibility.
