This is very clever: instead of focusing on electric only flight, just make the existing engine fly in the most efficient window while the electric engine buffers the flight profile.
Cars have a wide operating band, which is why hybrids make sense. Fundamentally, hybrid cars want to run the engine at a narrow set of operating points that are most efficient, and use a reservoir to do time arbitrage on the energy generated during driving. It's more than just regen to recover energy already spent -- the motor also lets you navigate the engine map to pick your point of operation. Ideally, the engine and the wheels are fully decoupled, at which point the engine rpm would just be constant. The battery and motor just move energy around.
Planes aren't really like this, they're actually a lot closer to boats. They move from fixed operating point to fixed operating point, and the engine is optimized around this. There's a lot less fat to trim, because the engines already have a narrow operating band.
That's not to say that this won't work; Pratt obviously knows what they're doing. But it's probably not an accident they've gone for smaller turboprops. I actually think the biggest opportunity for hybrid propulsion is in smaller drones where you want high performance piston engines, because tip clearance becomes an issue for gas turbines. A turboprop might suffer similar performance challenges that would make a hybrid configuration make sense.
Edit: just saw a post below on the relevant patents. Looks like they're using hybridization to decouple the two shafts. Without the motors, they're coupled via the gas path. With the motors, you can navigate the maps of the two shafts independently. I'm guessing this lets them pick a combination of shaft states that saves fuel. Most of that is pretty uncontroversial, but I'm surprised the claimed fuel savings are that high. As I mentioned above, it's probably because it's a turboprop on a small aircraft.
I think they have gone for turboprop for a few reasons:
There's an economy of scale with turbofans to some degree. Large high bypass turbofans are extremely efficient, particularly at their cruising mode.
Smaller regional aircraft have more of their fuel consumption dominated by takeoff - the cruising altitude portion of the flight is smaller in proportion.
When doing something experimental, do it on something small to start with. Then scale up when you've got it working.
I kind of wonder how the orders of magnitude work out for solar and wing area.
Solar cells are actually quite thin and could be almost like paint on the wings of aircraft. I wonder if the energy generated vs required is even ballpark. There is plenty of sun at 30,000 feet during the day.
Man this is a middle-schooler level napkin math question.
Take a Boeing 777, it has a wingspan of about 60 meters, and I'll ballpark an average upper wing chord of about 7 meters, for a total upper flat area of about 420 square meters.
High quality modern but standard single sided solar panels can do about 220 watts per square meter is full sunlight (around 22% efficiency tested at a 1000w/m^2 irradiance).
So that is 92,400 watts at full power.
92kW is less than the peak power of a Nissan Leaf. 92kW is 123 horsepower.
The two GE turbofans of a 777 are generating something like 40-50 MW of shaft power during cruise. MW.... megawatts. 50-60,000 horsepower.
Plastering the wing surface of a commercial plane with solar panels would make up less than a quarter of a percent of the total power it uses to produce thrust at cruise, at best case with them fully-lit.
Fully solar sailplanes do exist (NASA's Helios prototypes are an example) but that isn't anything close to a 'normal' aircraft with any appreciable payload/passengers.
Cheers- I guess I'll add, the turbofans that all modern airliners use are almost always referred to by their 'thrust' and you ofter see a lot of published numbers of takeoff thrust and such - it is harder to find numbers at cruising seed and altitude, and then in reality what you need to know is the actual power needed to generate that thrust.... there is some complexities there but in general the turbines generate what is known as 'shaft horsepower' which is a good stand in number we're looking for to compare.
The real takeaway is that power is power and energy is energy and regardless of how it gets to do the 'pushing' of the air, if you want to use solar energy (power, at any given moment, not integrating over time) those are the numbers you are comparing. It's all just unit conversions, at least when you're attempting such napkin math. Anything more - taking into account the actual systems, losses, efficiencies, etc.. just makes it all worse, not better.
A cool exercise is - given the 777 wingspan I esitmate and the power output from solar of said size... what kind of current aircraft use piston-driven engines with similar horsepower? Assuming we had motor inverter electronics and an electric motor that was 100% efficient, you could imagine trying to build a similar weight aircraft of that size that has such a wingspan.
This is why you wind up with the only solar powered aircraft out there being superlight, high altitude craft with super high aspect wing surfaces- maximizing wing area to weight / lift capability such as:
https://en.wikipedia.org/wiki/AeroVironment_Helios_Prototype
(note the largest of those prototypes and the tiny amount of power output they were making. All that wing area and it would have barely been able to carry a single passenger as payload and cruise a day without battery power)
Planes aren't really like this, they're actually a lot closer to boats. They move from fixed operating point to fixed operating point, and the engine is optimized around this. There's a lot less fat to trim, because the engines already have a narrow operating band.
That's not to say that this won't work; Pratt obviously knows what they're doing. But it's probably not an accident they've gone for smaller turboprops. I actually think the biggest opportunity for hybrid propulsion is in smaller drones where you want high performance piston engines, because tip clearance becomes an issue for gas turbines. A turboprop might suffer similar performance challenges that would make a hybrid configuration make sense.
Edit: just saw a post below on the relevant patents. Looks like they're using hybridization to decouple the two shafts. Without the motors, they're coupled via the gas path. With the motors, you can navigate the maps of the two shafts independently. I'm guessing this lets them pick a combination of shaft states that saves fuel. Most of that is pretty uncontroversial, but I'm surprised the claimed fuel savings are that high. As I mentioned above, it's probably because it's a turboprop on a small aircraft.