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Comment by jdsully | Hacker News Reader

The majority of your savings are from switching to gas from electric. Gas is significantly cheaper per BTU. Gas tank heaters are still pretty efficient. Especially if insulated.

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It's easy to understand.

First, a gas-fired power plant converts about 40% of thermal energy to electricity, discarding 60% to further heat the planet. Then a few more percent are lost in transmission lines and voltage-lowering transformers. Only then that electricity is used to heat water or air in one's house.

If you just directly burn that same gas in the house to produce heat, it's nearly 3x as efficient.

Gas-fired power plants are already known to be particularly low-efficiency. They're not representative of electricity production in general.

I wonder what kind of thermal electricity generation achieves efficiency above, say, 45%. Efficiency of thermal engines, even when you recycle the waste heat of the gas turbine exhaust with another cascade using a low-boiling substance and another turbine, is limited.

I only mentioned the gas-fired power station to make it an apples-to-apples comparison. It would be hard to compare e.g. to solar panels, even though direct hearing by concentrated sunlight might be feasible.

https://www.ge.com/gas-power/resources/articles/2016/power-p... is over 62% efficient.

If you use the electricity to run a heat pump you can get a couple more Joules out of the gas. Ideally your gas-plant is also connected to a district heating network, so the 60% heat are not completely lost.

A thought occurs to me just now: Why not also use that 40-60% waste heat to drive district cooling during the hot season? There's the absorption refrigeration cycle[1], which requires only a a heat input to drive the cycle to pump heat from the cold side to the hot side. Is the difference between the coefficient of performance between the more common vapor-compression cycle and the absorption cycle such that it is more cost effective to use the generated electricity to pump heat out of conditioned spaces during the hot season?

1. https://en.wikipedia.org/wiki/Absorption_refrigerator

Combined cycle power plants do use the waste heat:

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

Both cycles are limited by the same fundamental value (the Carnot coefficient) so I don't think that it would be a huge difference between the two cycles. Rather, I suspect that there are not all that municipalities that have (or need) both district heating AND district cooling. I expect that in the future we'll see a lot more industrial use of waste heat from power plants though, and part of that might take the form of on-site absorption chillers.

The modern gas power plants archive over 60% efficiency. It is with coal that one gets at best 45%.

Wow, TIL! Thanks.

In a post parallel to yours, lmm says that gas-powered plants are low efficiency. Can either of you supply some documentation or references for your respective positions?

I already posted to another sub thread, https://www.ge.com/gas-power/resources/articles/2016/power-p... - gas power plant with 62% efficiency.

The best coal plants can archive 47% efficiency, https://www.brighthubengineering.com/power-plants/72369-comp...

Using gas generates fixed additional costs though, apart from the energy price. You have to pay an additional provider fee, the annual maintenance of the gas heater, regularly change the CO detectors, etc.

It's worth it for a larger volume though. Not so much for a small flat.

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