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Comment by snek_case | Hacker News Reader

Battery powered electric vehicles are much more efficient than gasoline engines in terms of energy use per mile. With synthetic oil, you're losing energy both during the synthesis step and while using it. However, I think for airplanes or long term energy storage it could still make a lot of sense.

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Agreed, in terms of airplanes. For airplanes, one of the most important considerations is the amount of energy per mass of fuel. Batteries are not practical, in this sense, as they weigh too much. It is simply not possible for batteries to fuel a large plane over long distances.

Also, batteries weigh the same when they're depleted, whereas used fuel doesn't weigh anything.

Used fuel weighs more than the raw fuel, it’s just outside your vehicle in the typical use case. As you have shown, a typical assumption made with fuel burning vehicles is that disposing of the effluent is free and you can just throw it overboard.

In terms of measuring performance of the aircraft, you do indeed just throw it overboard. Obviously the energy system as a whole need to take into account the dumping of hydrocarbons into the atmosphere but I believe the parent commenter does that by suggesting synthetic fuels created through carbon capture.

Anyway, my point is just that when comparing two aircraft, and their fuel sources, it’s probably worth just comparing the closed system of the aircraft and measuring the environmental impact of fuel sources separately.

Wouldn't the fair comparison be tossing depleted batteries overboard? Or less ridiculously, storing rather than exhausting the burned fuel.

Obviously we won't do that, but it's worth considering what the apples to apples comparison would really look like.

The fact that aircraft get lighter as they burn fuel increases their effective range since a lighter aircraft requires less fuel to fly.

It's also important as most long range airliners take off with more fuel than they can safely land with. The air frames are very heavily optimized as every pound matters. If you always landed with the same weight you took off with, you'd end up needing to make the landing gear and surrounding structure significantly stronger to prevent excessive wear over time.

Also landing speeds would have to be higher because stall speed increases with weight. Faster landings are more dangerous.

My point is that achieving energy density parity with jet fuel isn't enough to make battery powered intercontinental flights viable. Such an aircraft will not be able to get to its destination. You can't get a fair comparison unless you are comparing a replacement that is similarly viable in the first place.

Your body throws CO2 and H2O overboard freely all the time, you know.

If you don't have any place to put the excess energy then the comparison is between inefficient synthetic oil and 0% efficient unused capacity.

The nice thing about oil is that it's very energy dense. We could store excess energy in the summer to be used in the winter. In contrast, batteries can maybe provide a few days of storage, but with synthetic oil, we could buffer for multiple months.

It’s probably more efficient to use a fuel like methane as opposed to oil as a medium-long term storage, e.g. for turbines used to address peak power grid usage, or winter storms (especially as more homes electrify HVAC).

I like the idea of globally phasing out fossil fuels for synthetic fuels made from carbon capture (possibly with biomass components if required).

Gas cars might be less efficient than electric, as expected of any kind of shim or polyfill, but they would be good enough that we wouldn't need to incentivize people to ditch their current vehicles.

Perhaps more importantly, it would fill a big gap in our planning for the future. Rather than banking on the ability to massively scale up lithium mining, we build renewable power plants in proximity to natural gas plants and hook up carbon capture systems to those plants. Boom, now they're just overly complicated batteries, and therefore part of the solution rather than part of the problem. Consider them technical debt; at least people get to keep their jobs for a while longer and modernizing of the grid progresses more quickly.

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