I just found out about the Solar Impulse project. It's a solar-powered plane, essentially the Tesla's brethren for the sky. Pretty cool stuff! The goal of the project is to demonstrate the viability of solar.
The last leg of the flight traveled from Japan to Hawaii, a distance of about 5000 miles. No stops, no fuel. Just sun. The plane has batteries to store energy for night flights. This is quite practical because the Japan-Hawaii leg took 5 days. Non-stop.
I think there are two major sets of reactions to this:
1. Holy shit, the future is here, and yeah it's slow now but tech gets better!!!
2. 5 days? Fuck that. Is Honey Boo Boo back on?
There's not much sense in talking to set 2 because they won't care about any of this until it's a real product they can buy. Group 1, however, is interesting to me because we can actually quantify what the concrete technical challenges are and assess the viability of overcoming them.
For viability purposes, let's say that a solar-powered craft would need to reach the same customer scenario as a current fuel-powered plane. That is, a cruising speed in the 530-580mph range and the ability to carry hundreds of passengers. How much energy does that take? I'll use the Boeing 787 as my example since it's the most modern fuel-efficient passenger plane. Looking at the 787-9 specifications, we see that it can transport 408 passengers a distance of 9550 miles using 33384 gallons of fuel at a speed of 567mph. We also should know how much energy is stored in a gallon of fuel. Using the two listed options for "jet fuel", I've opted to just call it 120kBTU per gallon.
Time of travel = 9550 miles / 567 mph = 16.8 hours = ~6E4 seconds.
Energy used = 33384 gallons * 120kBTU per gallon * 1055 J per BTU = ~4.2E12 J
Power = 4.2E12 J / 6E4 s = 7E7 J/s = 70 MW.
How much surface area is needed to collect 70MW? In a sunny place like Tucson, AZ, peak solar energy (mid-day) reaches 1000-1100 watts per square meter. A more realistic average over the Earth, over 24 hours, is 164 watts. We can adjust this for daytime averages by doubling it since the night portion contributes practically zero to this aggregate. So, let's say a typical daytime produces 325 watts, with shorter term highs around 1000W. This gives us a factor of 3 bounding, which should be adequate for this study. It's also important to point out that, per the same source, only 8% of solar energy is reflected back to space by the atmosphere, meaning that going higher up doesn't appreciably increase the available solar power.
Based on the range of incoming solar rates and the power requirements, it would take about 70,000 - 200,000 square meters of cells, operating at perfect efficiency. To put this in perspective, a football field is just shy of 5000 square meters. In other words, even on a bright bright day, it would take wings the size of 7 football fields. And for the average day we're looking at about 20 fields each. This seems intractable, even with "improving tech". The biggest improvement is probably the efficiency of the solar cells, and we've already allowed for that.
The other way to slice this is to look at the per-pound requirements and see how much we win by reducing airplane weight.
Power per pound = 70MW / 557k lbs ~= 125 W per pound.
In other words, I could, in an ideal world, attach a newborn to a square meter cell and send them flying around Arizona. Scaling this up to a typical adult requires some assumptions about their size and luggage and stuff, but let's call it 200 pounds. I'm being optimistic about human sizes by the time this hypothetical solar future arrives. The incremental solar area to carry a human is therefore 25-70 square meters. That's about the size of a one-bedroom apartment. The real lesson here is that we can potentially gain a lot by reducing airplane weight. First, we no longer need to carry 25000 lbs of fuel. However, we do need to carry batteries. Let's say, for the sake of the optimistic argument, that we can make the batteries arbitrarily small. We may also be able to make the wings lighter since they no longer need to support all that fuel. Not sure what kinds of savings that gets us. I'm also not sure if we can significantly reduce the weight of the fuselage. So, not much savings there on a 500,000 pound plane.
For the sake of the absolutely optimistic argument, suppose we can make the weight of the airplane zero. To transport 400 typical adults and bags, we'd need around 10-30,000 square meters of wing. That's 1-3 football fields each. And remember, they have to be perfectly efficient. And weigh nothing. And the passengers are sitting inside an ideal, weightless bubble.
Overall, the idea of solar for mass air travel seems far-fetched, simply because the energy density of solar is not high enough.
The last leg of the flight traveled from Japan to Hawaii, a distance of about 5000 miles. No stops, no fuel. Just sun. The plane has batteries to store energy for night flights. This is quite practical because the Japan-Hawaii leg took 5 days. Non-stop.
I think there are two major sets of reactions to this:
1. Holy shit, the future is here, and yeah it's slow now but tech gets better!!!
2. 5 days? Fuck that. Is Honey Boo Boo back on?
There's not much sense in talking to set 2 because they won't care about any of this until it's a real product they can buy. Group 1, however, is interesting to me because we can actually quantify what the concrete technical challenges are and assess the viability of overcoming them.
For viability purposes, let's say that a solar-powered craft would need to reach the same customer scenario as a current fuel-powered plane. That is, a cruising speed in the 530-580mph range and the ability to carry hundreds of passengers. How much energy does that take? I'll use the Boeing 787 as my example since it's the most modern fuel-efficient passenger plane. Looking at the 787-9 specifications, we see that it can transport 408 passengers a distance of 9550 miles using 33384 gallons of fuel at a speed of 567mph. We also should know how much energy is stored in a gallon of fuel. Using the two listed options for "jet fuel", I've opted to just call it 120kBTU per gallon.
Time of travel = 9550 miles / 567 mph = 16.8 hours = ~6E4 seconds.
Energy used = 33384 gallons * 120kBTU per gallon * 1055 J per BTU = ~4.2E12 J
Power = 4.2E12 J / 6E4 s = 7E7 J/s = 70 MW.
How much surface area is needed to collect 70MW? In a sunny place like Tucson, AZ, peak solar energy (mid-day) reaches 1000-1100 watts per square meter. A more realistic average over the Earth, over 24 hours, is 164 watts. We can adjust this for daytime averages by doubling it since the night portion contributes practically zero to this aggregate. So, let's say a typical daytime produces 325 watts, with shorter term highs around 1000W. This gives us a factor of 3 bounding, which should be adequate for this study. It's also important to point out that, per the same source, only 8% of solar energy is reflected back to space by the atmosphere, meaning that going higher up doesn't appreciably increase the available solar power.
Based on the range of incoming solar rates and the power requirements, it would take about 70,000 - 200,000 square meters of cells, operating at perfect efficiency. To put this in perspective, a football field is just shy of 5000 square meters. In other words, even on a bright bright day, it would take wings the size of 7 football fields. And for the average day we're looking at about 20 fields each. This seems intractable, even with "improving tech". The biggest improvement is probably the efficiency of the solar cells, and we've already allowed for that.
The other way to slice this is to look at the per-pound requirements and see how much we win by reducing airplane weight.
Power per pound = 70MW / 557k lbs ~= 125 W per pound.
In other words, I could, in an ideal world, attach a newborn to a square meter cell and send them flying around Arizona. Scaling this up to a typical adult requires some assumptions about their size and luggage and stuff, but let's call it 200 pounds. I'm being optimistic about human sizes by the time this hypothetical solar future arrives. The incremental solar area to carry a human is therefore 25-70 square meters. That's about the size of a one-bedroom apartment. The real lesson here is that we can potentially gain a lot by reducing airplane weight. First, we no longer need to carry 25000 lbs of fuel. However, we do need to carry batteries. Let's say, for the sake of the optimistic argument, that we can make the batteries arbitrarily small. We may also be able to make the wings lighter since they no longer need to support all that fuel. Not sure what kinds of savings that gets us. I'm also not sure if we can significantly reduce the weight of the fuselage. So, not much savings there on a 500,000 pound plane.
For the sake of the absolutely optimistic argument, suppose we can make the weight of the airplane zero. To transport 400 typical adults and bags, we'd need around 10-30,000 square meters of wing. That's 1-3 football fields each. And remember, they have to be perfectly efficient. And weigh nothing. And the passengers are sitting inside an ideal, weightless bubble.
Overall, the idea of solar for mass air travel seems far-fetched, simply because the energy density of solar is not high enough.
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