[bruce511]

>> The “dirty electricity” angle is less obvious to me.

A power plant typically gets about 60% of energy from a fossil source. A car does about 30%. So even if the electricity comes from say coal, it's still more efficient than buying gas in a car engine.

Of course, these days, it's likely that a substantial portion (up to 100% in some cases) is not "fossil electricity" but rather comes from solar, wind, hydro etc. Ie "clean" electricity.

[cptskippy]

Our energy aggregator is a non-profit Community Choice Aggregator with over 250,000 members that ensures 50% of the energy they purchase comes from renewables and 75% of all energy purchased is carbon free. And for an extra $0.00750 p/kWh you can opt into your consumption coming 100% from renewable sources.

[roelschroeven]

60% efficiency? How do these power plants manage to circumvent the limit set by Carnot efficiency?

[quickthrowman]

They don’t. they capture the waste heat from the gas turbine and use it to make steam to spin another turbine. A gas turbine is ~40% efficient, add in a HRSG to make a combined cycle plant and you can get up to ~60%. You don’t even need a gas turbine to use a HRSG, any exhaust stream with enough heat will work, a boiler plant or similar.

https://en.wikipedia.org/wiki/Heat_recovery_steam_generator

https://en.wikipedia.org/wiki/Combined-cycle_power_plant

[slavik81]

Natural gas burns at ~2200 K. Ambient temperature is ~300 K. 1 - 300/2200 = 86% as the Carnot limit?

[wqaatwt]

Coal burns dirtier and and is more Co2 intense than gas though.