Why do Electric Aircraft Seem to Eschew Photovoltaics?

Question

I work in climate policy so I'm always excited for things that could help me feel less guilty about being into aviation such as biojet and the progress being made in electric propulsion.

To date, however, all of the examples of electric aircraft that I've seen lack photovoltaic equipment (i.e. solar panels). Given the concerns with electric propulsion seem to stem from a lack of energy/power density in current generation battery technology, this feels like a glaring omission. Any amount of PV (sunk into the top of the wings, for example) would produce positive power and mitigate consumption of stored energy in batteries - even on cloudy days.

It's obvious that the systems are nowhere near sufficient to power flight, but given how on-the-margin electric propulsion operates it feels like low hanging fruit from an electrical power perspective. You'd gain the operational benefits of refueling anywhere you could park it in the sun for long enough (to be fair, that'd likely take a LONG time).

I assume there's technical reasons to eschew the equipment, but I'm curious what the constraining factor(s) are, specifically.

The candidates I can think of up front are weight (silicon wafer isn't that heavy, plenty of PV can be made extremely lightweight and you need the wing to be rigid already so balance of structure isn't adding much, if anything), volume (silicon wafer is, pardon me, 'wafer thin'), technical complexity (this is an experimental aircraft we're talking about, remember) or cost (see previous) - all of which seem readily dismissed.

EDIT: I'm not asking about PV as primary power, I'm asking why engineers are leaving 'money on the table' as it were, by not including PV in battery-primary designs.

What am I missing?

Answer 1

It depends on the specific use case, but in general, it's not worth it given the alternatives. The power delivered by solar panels is too low to have a meaningful impact.

Solar irrandiance is about 1361 W/m2 (best case, at the "top of atmosphere" irradiance. The surface irradiance is roughly 1000 W/m2 due to particles etc, thanks for the comment, Mark). If we assume about 10% efficiency for thin-film cells, that leaves about 136 W/m2 or 0.136 kW/m2.

So for the Pipistrel Velis Electro, with a wing surface of 9.51 m2 -even though some of that is flaps/ailerons that cannot as easily be used for solar cells-, that would give you 0.136 * 9.51 = 1.29 kW, at best (best case irradiance, full wing surface and ignoring cable/charging losses).

Compare this with the 35 kW delivered by the engines in cruise.

This is a bit over 3% at the price of extra costs/weight/complexity. It's probably much less trouble to add 3% (~4.2kg) batteries.

Answer 2

Standalone recharge anywhere... As said, the wing surface is too small, the aircraft will likely get grounded several days... Why not just put an array of solar panels finely calibrated for efficiency at many airfields ? Charge would complete fast, and work for a broad variety of aircraft size, ground vehicles and equipments ? The ground array would be easier to maintain and upgrade.

Wings aren't rigid ! Common misconception is "it has internal structure, therefore, it will remain straight..." Actually, over a couple meters, a wing bends. At microscopic level, it slowly deteriorates over time (wrincles/cracks). A layer of solar panels will rub the wing surface at contact level, especially along the edges and at the corners (is it glued or what ?). That's a technical issue, how often it has to get fixed to keep that upperwing clean (ie no aerodynamical disturbance) ?

Weather ! I know solar panels can be reinforced to withstand hail. In aviation, this translates to weight and extra costs. That's usually a big NO.

Efficiency. Solar panels lose efficiency with dust and scratches (we ignore the other causes to focus on those). Snow, dust, rain, hail... Even if it's just for supplement purposes, it has to be maintained well to have a meaning : how much hassle to care for ? What to do before each flight, what to do between flights ? What could go wrong if not properly handled ? Practically, the amount of care is much concerning than adding another battery, as mentionned.

Maintainance : One way to gain or lose money in the aviation industry is how much of a cost is maintaining engines and systems. Who's (company/subcontractors) in charge of ensuring maintainance of those solar panels ? If it's a small company only present in a few states in the US... You either need a company that can handle problems whatever the country, or a company that can qualify personnel, or a company that uses OEM components any developped country can produce (and not charge too much for patents...) - likely a combination of those.

Please rest assured I'm not here to discourage all efforts in building environmental friendly systems, components and behavior, but for practically everything, the purpose must have a meaning. An aircraft is meant to move people or goods over terrain or faster than any other way. To be efficient, weight has to get "donated" to payload as much as possible instead of structure, equipment and systems (unless safety is a concern), and the vehicle has to be as less of a hassle as possible to operate (unless safety is a concern). Your question ask for the "why not" rather than the "how to", therefore, we focus on the purpose, not the technical issues that can be solved in a way or another.

Experimental aircraft are nice, but sometimes, they fail to remember what was the purpose... You can build a self sustained fully electric aircraft, but if you can't put people and cargo inside, and/or it requires special care (qualified people, frequent checks, logistics/equipements, spare parts manufacturing...) on a daily basis which translate to utterly expensive costs the end user (you and me) has to pay...

On a related yet distant topic, it's very similar to "flying cars" : the purpose is to safely transport people over cities... quite a major safety concern : how would you ensure the everyday lambda anyone is qualified enough to actually fly that thing ? That's the answer why over 2 centuries of dream, yet, the drones-like advertised vehicles are still a long way before taking off from our backyard. The purpose, it being safety and/or "economically viable", should be at the center of the design process, from the start, not a concern one solve later. For an aircraft, you freeze payload and range first (for example), then you solve every major safety issue (means a LOT of dead weight), then you solve all other problems and assert if adding solar panels is a bonus or a malus. Not the other way around.

Answer 3

As you state, "nowhere near sufficient to power flight". And that includes charging.

The Airbus Zephyr 8 has a 25 meter wingspan, and the top surface of the wing is ALL solar panels. It can keep itself aloft sort of indefinitely.

Change that flying to recharging, and scale down to realistic aircraft sizes, and you'd probably have to leave it outside for a week or two, just to recharge once.

Answer 4

Any amount of PV (sunk into the top of the wings, for example) would produce positive power and mitigate consumption of stored energy in batteries - even on cloudy days.

That's fallacious. Sure, any amount of PV would increase gross power, but it's hardly guaranteed to increase net power. Adding PV means increasing the weight, which means higher power requirements. PV runs at about 20W/kg. A Boeing 737 takes about 100W/kg to stay airborne. With more efficient aircraft and PV with better power/weight ratios, you can make this work, but it's far from trivial.

You'd gain the operational benefits of refueling anywhere you could park it in the sun for long enough (to be fair, that'd likely take a LONG time).

Why fly the PV around with the plane? Why not just put PV on the roof of the hangar? It's not like first-stage electric flight is going to be flying into the middle of nowhere. Departure and destination is going to both be to places with well-developed infrastructure.

Answer 5

Solar panels were used to power AeroViroment unmanned aircraft developed under NASA's Environmental Research Aircraft and Sensor Technology (ERAST) program.

Four of them were developed from the middle of the '80s till the end of the last century, namely the Pathfinder, Pathfinder Plus, Centurion and Helios.

Wikipedia has a good entry about them.

Answer 6

As a supplement to ROIMason's answer, what if we used top-of-the-line solar panels?

The Airbus Zephyr 8/S is an unmanned vehicle weighing 65 kg with a payload of 5 kg and it can fly for over a month. We can use this as an example of what top-of-the-line solar panels can do on an aircraft.

...Solar cells are high-efficiency, lightweight, and flexible inverted metamorphic multi-junction epitaxial lift-off GaAs sheets manufactured by MicroLink Devices, with specific power exceeding 1,500 W/kg and areal powers greater than 350 W/m2.

Using the example of the Pipistrel Velis Electro, its wing area of 9.5 m^2 would deliver at best 3.3kW vs the 35kW cruising power, or 10% more power. For about 2kg (plus the extra hardware).

These are top-of-the-line "call us if you want pricing" components. We can assume they're not cheap.

The endurance of the Pipistrel Velis Electro is listed at 50 minutes. Is the cost to install and maintain top-of-the-line solar panels worth an extra 5 minutes of flight time? Probably not.

Answer 7

It's worth noting that there are solar-powered aircraft. The linked Wikipedia article lists 11 different models/ranges of aircraft, going back to 1974.

However, the limited amount of power one can get from solar cells compared to the power required for the aircraft, means that those aircraft are very specific, and:

• Have a very large surface (Zephyr 8/S has a 25 m wingspan, Solar Impulse 71.9m)
• Are extremely light-weight (25 kg for Zephyr 8/S, 2000 kg for solar Impulse)
• Can only carry a very limited payload (5 kg for Zephyr 8/S, one single pilot in an unpressurised cockpit for Solar Impulse 2)
• Usually fly very high to get maximal solar irradiance above the clouds
• Usually fly very high to avoid turbulence, as they tend to be very fragile
• Are usually quite slow (Solar Impulse's average speed was 76 km/h)
• Usually fly pretty long missions so the proportion of flight in cruise is maximised
• Many can't even take off by themselves and need to be launched with balloons or towed by other aircraft
• Are experimental and are able to achieve their objectives only in very specific circumstances (locations, weather...)

Trying to scale that to anything useful to carry passengers on "normal" flights quickly breaks down completely: if you need a 71.9m wing span for an aircraft capable of carrying only a single pilot, imagine what you would need for even a few dozen people! And even more if you want that aircraft to be as sturdy and capable of flying in nearly any weather as a conventional airliner.

The end result is that, as demonstrated in other answers, for a "regular" aircraft, the amount of power you can get from solar cells on the existing wing span is just negligible, and also extremely variable (depends on latitude, time of the year, time of the day, weather...). Compared to the added cost and complexity, it's just not worth it.

In the very best of cases (aircraft in cruise at very high altitudes, far above the clouds, in daylight, with top of the line solar cells which only exist in research labs at this time), you could get probably 500 W/m2.

A 737 requires 7.2 MW just to keep altitude and speed. If we want to achieve 10% of that power (remember, in cruise, at high altitude) using these best-of-the-line-not-available-yet solar cells, we would need 1440 m² of wing surface, about 10 times the actual wing surface.