Comment by rsynnott | Hacker News Reader

Comment by rsynnott | Hacker News Reader

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> Then there's the diesel cost. A large bus must get ~ 10 mpg highway, but a city bus is all stop and go - disastrous for a vehicle as massive as a bus. Let's say it gets 4 mpg.

Increasingly, most buses would be either electric, or heavy hybrid (ie electric with a diesel generator).

But also, consider the money they take in. My local bus system's buses take up to 100 people. The charge is 2 euro for a journey where the last leg starts within 90 minutes of the first leg, on any mode of transport. However in practice the average journey would be in the 30 min range. So if the buses are constantly full, that's 400 euro per hour takings!

Of course, it's not really that high; the buses are not always full, some people are using monthly or annual tickets, which are cheaper, some people are kids or over 65 or otherwise get cheap or free travel, the trains and trams also have to be paid for, and so on. But it's sufficient that they were actually able to cut the price (it used to be up to 3.50 for a single bus journey); the increased use from the cheaper simpler system offset the reduction in revenue per journey.

The problem with buses is that utilization is very low. Most buses, most of the time, run near empty.

This also significantly i creases their overall passenger mile GHG emissions.

Hybrid buses and CNG probably make sense. But I doubt, fully electric buses make sense: as long as dispatch-able power (ie your marginal producer) is a thermal plant you get 60ish% thermal efficiency at best minus transformer and transmission losses.

A bus engine for use in a hybrid can be made more than 40% efficient (miller cycle, high compression, high octane CNG, full throttle PWM operation, etc).

> The problem with buses is that utilization is very low. Most buses, most of the time, run near empty.

Huh. That doesn't resemble my experience, except for rural/small-town services, which are very much provided as a form of subsidy, not a viable business.

> But I doubt, fully electric buses make sense: as long as dispatch-able power (ie your marginal producer) is a thermal plant you get 60ish% thermal efficiency at best minus transformer and transmission losses.

That's still quite a bit better than diesel engines, where for realistic engines you're talking about 40% _in ideal conditions_, but much worse in stop-start conditions (you will do better with hybrids, granted). Gas also have considerably lower CO2 intensity per thermal watt than diesel does (your fossil fuel electricity production is probably mostly gas in most countries).

But, also, in countries with a lot of unreliable renewables, buses charging overnight can be used as a power sink. For instance, in Ireland, our wind generation can be anything from almost nothing to greater than total system demand; reasonably often, wind turbines actually have to be stopped to reduce output, especially at night. At that point, charging the bus is for practical purposes free, or may even have a modest negative cost (the network will sometimes pay for power consumption).

"That's still quite a bit better than diesel engines, where for realistic engines you're talking about 40% _in ideal conditions_" and so on

Which is where hybrid drive trains and PWM comes in. Run the engine at its ideal and only at its ideal with no care for the actual load requested.

Furthermore I mentioned CNG - ie methane. Methane has more energy per CO2 gram than any hydrocarbon and has an insanely high octane number -> it can run an Otto cycle at diesel compression ratios.

Run it as a miller cycle and the thermal efficiency starts to go up even highed.

Really my 40% is a low ball of what is possible in ICEs to not offend anyone's preconceptions. F1 claims to have achieved >50% thermal efficiency in a race engine but Im usually loath to mention that because of their secrecy and therefore lack of indee verification.

And no one talks that lines and transmission is lossy. No one talks that dispach-able power is often coal or oil. No one talks about the environmental and social cost of batteries.

Hybrids ameliorate (the most, btw) just about every aspect of this while minimizing external costs

> No one talks that dispach-able power is often coal or oil.

... Because they're generally not, nearly anywhere. Oil power plants are, in general, rare today nearly everywhere in the world; natural gas really did a number on them. And dispatchable power plants are almost _never_ coal; the startup lead-time is too high (hours at least). Ignoring weird stuff like grid batteries and pumped storage plants, they're nearly always gas turbines.

For short term stabilization, say within time constant of fifteen minutes, of course you use hydro or nat gas.

But the electrical industry makes very careful predictions of the next day's power consumption; that is within a time frame that coal can easily respond to.

Since coal is expensive and dirty, they end up supplying the marginal production. Ie the buses run on coal.

But why keep coal at all though? One advantage coal over has is that you can store massive amounts of energy in a pile just outside the plant. Only nuclear plants can store such massive amount of energy locally.

This storage ability is heavily used in the North East to toughen the grid in the winter where gas pipeline pressure drops due to heating demand.

As to oil, it is actually still used, albeit intermittently and then rarely [1] It is usually used in plants that are primarily non-oil burning. Again, the advantage is that oil is far easier to store than nat gas.

Note that, since oil has a lot more carbon than methane and since oil extraction, at least in the USA, is very energy intensive, oil contribution to GHG is far greater than it's 1% of the energy mix would suggest.

And its all in the margins where the EV bus gets charged.

[1] https://www.eia.gov/todayinenergy/detail.php?id=31232

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