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In the USA, residential heaters often use a measurement called AFUE, which is supposed to be a percentage of input energy to output energy. It is typical for a two-stage natural gas furnace to achieve a rating of 90% or more. A single-stage natural gas furnace may get around 80%.

When I read about two-stage natural gas electricity power plants, however, it seems they are at best 60% efficient, with a single-stage version being as low as 30-40%.

Is the difference due to the fact that a power plant must turn a turbine with mechanical losses rather than just heating air? Or, maybe the AFUE rating is actually misleading? Or, something else?

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  • $\begingroup$ You will need to be somewhat more specific before anyone can answer your question. $\endgroup$ Mar 28, 2018 at 0:50
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    $\begingroup$ As Martin says, a furnace only needs to produce heat -- it's efficiency is the fraction of total heat that stays inside vs going up the flue. A power plant needs to generate electricity, and there are thermodynamic laws that limit how much mechanical/electrical energy can be extracted, vs the energy that ends up as heat. If you want to understand this better, you need to spend a few weeks studying thermodynamics. $\endgroup$
    – Hot Licks
    Mar 28, 2018 at 2:09

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A power plant efficiency is electricity out / energy in. All the heat generated is by definition wasted energy.

In your house's heater all heat generated is work out - so the only losses are heat which escapes through the chimney, which with good design should be less < 10%.

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    $\begingroup$ Not to be pedantic, but the power plant efficiency should be inverted. $\endgroup$ Mar 28, 2018 at 1:33
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    $\begingroup$ @probably_someone - hence the failure of my perpetual stationary machine to catch on ! $\endgroup$ Mar 28, 2018 at 2:37
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You're seeing the difference between raw thermal efficiency and thermal-to-electrical efficiency.

AFUE is a thermal efficiency. It's how much useful heating you get per unit of fuel heat. It's pretty easy to get that value near 100% in some cases, and if we include heat pumps, then it can go over 100%.

By contrast, electric power plants have to convert thermal energy into electrical energy. This tends to be a pretty lossful process:

Practical thermal efficiency of a steam turbine varies with turbine size, load condition, gap losses and friction losses. They reach top values up to about 50% in a 1200 MW turbine; smaller ones have a lower efficiency.

-"Steam turbine", Wikipedia

50%'s a pretty decent high-end (optimistic) estimate; 40% might be a decent value to assume. Anyway, this conversion factor tends to lower the relative efficiencies in thermal power stations:

The energy efficiency of a conventional thermal power station, considered salable energy produced as a percent of the heating value of the fuel consumed, is typically 33% to 48%. As with all heat engines, their efficiency is limited, and governed by the laws of thermodynamics. Other types of power stations are subject to different efficiency limitations, most hydropower stations in the United States are about 90 percent efficient in converting the energy of falling water into electricity while the efficiency of a wind turbine is limited by Betz's law, to about 59.3%.

-"Thermal power station", Wikipedia [formatting and references omitted]

For a theoretical discussion of these low effiencies, I'd suggest my answer to "Why is it so inefficient to generate electricity by absorbing heat?".

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