Hacker News Reader

Comment by phonon | Hacker News Reader

Power plants are about 35% efficient, and then you lose another ~10% of what gets sent out from transmission losses...so your total efficiency for electrical generation is around 30%. [1]

Ideally, that waste heat would be put to some use...that's called co-generation [2]

[1] http://blog.schneider-electric.com/energy-management-energy-...

[2] https://en.wikipedia.org/wiki/Cogeneration

View on HN

Its also worth pointing out the scale of the problem, with a simple one stage plant design, like a very stereotypical nuke or coal plant, a cold side around outdoor temps and a hot side around 1000F means 30-something percent efficient. And at 1000F or so, the steam pressure will be handwavy a bit less than 2000 PSI.

So simply running hotter is not a realistic solution to get above 30 or so percent, not at current technology levels. That's where strange staged and cogeneration schemes are applied to boost efficiency.

Low grade process heat is surprisingly useless in practice, unfortunately. Also economics and politics smack up against engineering ideals, where massive vertical integration would result in higher efficiency but short of nationalizing all industry or allowing massive monopolies its hard to integrate a coal generating plant with a steel mill heat treatment plant and a sardine canning plant and a greenhouse all under the same roof. Maybe with infinite scaling nano-technology all industrial plants could do all things to really boost efficiency.

Carnot efficiency is 1 - Tcold/Thot. You must also remember to convert temps to an absolute scale. So from your example, that is 1 - (535/1500) or 66% efficient.

The Carnot cycle is a poor model for steam turbine electric power plants. Rankine cycle efficiency of ~40% is the more relevant theoretical upper limit.

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

> Low grade process heat is surprisingly useless in practice, unfortunately.

For electricity generation, yeah. What it is that stops us from using it for residential heating? Eg, space heating and for hot water. Is it just the proximity to big power plants that makes it unfeasible?

It's hard to distribute heat...you would need steam or at least hot water pipes everywhere. You would also need to vent the steam when it's not needed...so if you only need the heat for a few months of the year, you don't get that much efficiency increase overall.

It's been done though.

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

Reply on news.ycombinator.com