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Comment by dragontamer | Hacker News Reader

> Pipeline compressor stations often generate 35MWth+ in waste heat, which we can use at about 20% efficiency to generate electricity.

Can the waste heat be used for distillation instead? To heat up the fuel/water mix to separate the two cleanly?

It would seem to me that the future of "waste heat" is to actually utilize the heat, instead of powering a relatively inefficient heat-engine. Perhaps instead of 20% efficiency, you get closer to 100% efficiency since all you'd be doing is running the fuel-water mix to the heat-source to evaporate the fuel out (I presume anyway, I'm no chemist)

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Your instincts are excellent: using heat directly is always and everywhere better than turning it into a fungible commodity like electricity, only to convert that electrical energy to some other form later. (Creating liquid fuel is no different in this regard.)

The reality is that there's almost never a practical direct use of the heat at the facility where it is created. It is too low quality for the onsite processes. Economizers and Regenerators are two common pieces of equipment that suck out what heat can be used - what's left is too low temperature for whatever they're making onsite.

Sometimes there's an opportunity to broker a deal between two adjacent facilities, where one can use the heat that the other considers waste. But those are rare.

Happy to talk about this until you wished you'd never asked. Feel free to email.

re: waste heat, I've been thinking forever that someone must capture this. I've generally assumed the economics don't work out or no one has cracked it outside of scenarios like regenerative braking. We waste so much energy that we could capture and put back to use.

Regen braking doesn’t use waste heat. It converts mechanical energy back to electrical.

And in so doing, reduces waste heat due to friction, which is likely what the parent commenter meant.

Some say it doubles the lifespan of brake pads.

> We waste so much energy that we could capture and put back to use.

The biggest current waste of energy is residential heating. We take high quality energy in the form of natural gas and fuel oil and use it to produce low quality heat.

Co-gen systems burn fuel to generate electricity and use the waste heat for heating buildings.

Something like 55-60% of the thermal energy created by burning fossil fuels is currently wasted.

It’s kind of ironic that cars and power plants work hard on getting rid of that energy.

Modern power plants work hard at getting every last bit of that energy before it escapes. Current Combined Cycle plants, that generate steam from the exhaust of a gas turbine, are currently getting over 60% efficiency.

One must be a bit careful there. 60% efficiency based on the LOWER heating value of natural gas. That ignores the heat available from condensing the water of combustion. Coal plants typically use the higher heating value when computing efficiency, which does reflect the latent heat of vaporization of that water.

I know ICE engines use turbochargers to "recapture" the energy. Exhaust can be used to power the intake fan, which can increase air-pressure available to the engine's intake.

Turbocharged engined still need a ton of cooling but yes I think they are more efficient overall .

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