# Could waste heat be harnessed in some way? **URL:** \ **Category:** Factual Questions\ **Created:** [June 18, 2016, 5:43pm UTC](https://boards.straightdope.com/t/could-waste-heat-be-harnessed-in-some-way/757711 "2016-06-18T17:43:44Z")\ **Posts on this page:** 8\ **Page:** 3
**Author:** ![sbunny8](https://avatars.discourse-cdn.com/v4/letter/s/c77e96/32.png) [@sbunny8](https://boards.straightdope.com/u/sbunny8)\ **Post date:** [June 20, 2016, 11:16pm UTC](https://boards.straightdope.com/t/could-waste-heat-be-harnessed-in-some-way/757711/41 "2016-06-20T23:16:55Z")
> [@Napier](#): > > We’re missing a simple rule here. > > Think in terms of absolute temperature. If you can let heat flow from a high temperature to someplace cooler where the temperature is x% of the high temperature, then, at a bare minimum, x% of the heat must remain in energy form of heat, and only (100-x)% of the original heat can be converted to another kind of energy – and that is a hard and fast maximum, which real heat engines can only try to approach. > > If something is 6 degrees C above room temperature, its absolute temperature is 299 K and the heat gets to flow to 293 K, which means 98% of the original heat energy must stay in the form of heat and only 2% is accessible for conversion to some other form. At the very best. I don’t see the the point of making that observation. I mean, when a rock rolls down a mountain and its kinetic energy demolishes a car, you could say “That rock used to be 4000 miles from the center of the Earth and now it’s only 3999 miles from the center of the Earth so really it has retained more than 99% of its gravitational potential energy.” Okay, yeah that’s true. But so what? It’s more convenient for me to measure the difference in height between the rock’s starting point and its ending point in order to calculate how much kinetic energy it can gain on its way down. Doesn’t the same apply to temperature differentials? A difference of 6 degrees C represents a certain amount of potential energy, regardless of how far above absolute zero the two points are. ---
**Author:** ![Corry\_El](https://avatars.discourse-cdn.com/v4/letter/c/ecae2f/32.png) [@Corry\_El](https://boards.straightdope.com/u/Corry_El)\ **Post date:** [June 21, 2016, 2:47am UTC](https://boards.straightdope.com/t/could-waste-heat-be-harnessed-in-some-way/757711/42 "2016-06-21T02:47:00Z")
> [@Lemur866](#): > > As has been said, most times the only reasonable thing to do with waste heat is to use it to heat something you want heated. So if you’ve got a refrigerator that makes cold water and a water heater that makes hot water you can use the waste heat from the fridge to preheat the water. A lot of answers have said that, but it’s not always so. The _easiest_ way to use relatively low temperature waste heat is just heat something up which needs to be heated. The classic case of that is district heating systems or factory process heating systems (drying or cooking systems, greenhouses etc) run off the exhaust and/or cylinder cooling water waste heat of diesel generators. That’s a common form of cogeneration. But it’s also common for high temperature waste heat, exhaust gas from gas turbines or diesels, to run rankine cycle engines (ie boil a fluid, most simply water but lower temp systems use other more suitable fluids) like steam turbines. Also very common. Most newer baseload natural gas fired powerplants are of this type, gas/steam turbine combined cycle. And also you can use heat for cooling. That’s how old fashioned gas refrigerators worked, and in many cases now medium to high temp heat from engines is used to run what are called absorption chillers. Another common cogeneration arrangement: a diesel or gas turbine generator provides electricity and the waste heat is used for absorption chillers for some of kind of building complex (office, airport) which has high a/c demand. > **[Absorption refrigerator](https://en.wikipedia.org/wiki/Absorption_refrigerator)** > > An absorption refrigerator is a refrigerator that uses a heat source to provide the energy needed to drive the cooling process. Solar energy, burning a fossil fuel, waste heat from factories, and district heating systems are examples of convenient heat sources that can be used. An absorption refrigerator uses two coolants: the first coolant performs evaporative cooling and then is absorbed into the second coolant; heat is needed to reset the two coolants to their initial states. Absorption refrig... ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [June 21, 2016, 2:49am UTC](https://boards.straightdope.com/t/could-waste-heat-be-harnessed-in-some-way/757711/43 "2016-06-21T02:49:41Z")
> [@sbunny8](#): > > I don’t see the the point of making that observation. I mean, when a rock rolls down a mountain and its kinetic energy demolishes a car, you could say “That rock used to be 4000 miles from the center of the Earth and now it’s only 3999 miles from the center of the Earth so really it has retained more than 99% of its gravitational potential energy.” Okay, yeah that’s true. But so what? It’s more convenient for me to measure the difference in height between the rock’s starting point and its ending point in order to calculate how much kinetic energy it can gain on its way down. Doesn’t the same apply to temperature differentials? A difference of 6 degrees C represents a certain amount of potential energy, regardless of how far above absolute zero the two points are. No. That’s the difficulty. Stuff rolling downhill isn’t a valid model for heat flowing down a temperature gradient. The rock can have nearly all its weight multiplied by its distance fallen converted into some other form of energy. With heat, though, it’s only this fraction that is available. If your heat has only one degree to drop out of a total of 1000, not only is that a small drop, but also 99.9% of the heat has to remain in the form of heat. This is what entropy is about. ---
**Author:** ![astorian](https://avatars.discourse-cdn.com/v4/letter/a/ad7895/32.png) [@astorian](https://boards.straightdope.com/u/astorian)\ **Post date:** [June 22, 2016, 10:46pm UTC](https://boards.straightdope.com/t/could-waste-heat-be-harnessed-in-some-way/757711/44 "2016-06-22T22:46:21Z")
Isn’t a lot of the steam generated by utilities like Con Edison sent out in underground pipes and used to heat buildings? ---
**Author:** ![Melbourne](https://avatars.discourse-cdn.com/v4/letter/m/b5e925/32.png) [@Melbourne](https://boards.straightdope.com/u/Melbourne)\ **Post date:** [June 23, 2016, 2:25am UTC](https://boards.straightdope.com/t/could-waste-heat-be-harnessed-in-some-way/757711/45 "2016-06-23T02:25:57Z")
> [@Whack-a-Mole](#): > > Regarding the faucets I realize adding a little generator and battery/capacitor complicates things but I have had fish pumps for my aquarium pumping water non-stop for years… > Perhaps in the end the energy saved is less than the added cost over the lifetime of the product (add in maintenance costs for people replacing batteries). Probably is else they would do it I would guess. My Great-Uncle, who had a related product under sevelopment 40 years ago, told me that it was illegal to use the drinking water supply for power. It used to be common for cities (eg Chicago, Melbourne) to have hydraulic power supplies. Presumably, using the tap water for power was such an obvious idea that they had to regulate it. ---
**Author:** ![boffking](https://avatars.discourse-cdn.com/v4/letter/b/858c86/32.png) [@boffking](https://boards.straightdope.com/u/boffking)\ **Post date:** [June 23, 2016, 2:35am UTC](https://boards.straightdope.com/t/could-waste-heat-be-harnessed-in-some-way/757711/46 "2016-06-23T02:35:11Z")
> [@Melbourne](#): > > My Great-Uncle, who had a related product under sevelopment 40 years ago, told me that it was illegal to use the drinking water supply for power. Imagine if instead of just drinking the tap water, people used it to run turbines. Consumption would skyrocket, and also this would put undue stress on the sewage system. ---
**Author:** ![iceiso](https://avatars.discourse-cdn.com/v4/letter/i/3be4f8/32.png) [@iceiso](https://boards.straightdope.com/u/iceiso)\ **Post date:** [June 23, 2016, 1:23pm UTC](https://boards.straightdope.com/t/could-waste-heat-be-harnessed-in-some-way/757711/47 "2016-06-23T13:23:00Z")
> [@boffking](#): > > Imagine if instead of just drinking the tap water, people used it to run turbines. Consumption would skyrocket, and also this would put undue stress on the sewage system. I had this idea a while ago so I did the math behind it. Assumptions: - $0.156/kw-hr - Water inlet pressure: 50 psig - Water outlet pressure: 5 psig - Average daily water usage: 100 gal (per person) - Turbine efficiency: 75% Your payback is $1.38 per year per person. This does not include the cost of the water, which would now be available to you at a much lower pressure. Assuming an installed cost of $750 and flat energy prices, it would take a short 543 years to pay back your initial investment! ---
**Author:** ![Bill\_Door](https://avatars.discourse-cdn.com/v4/letter/b/50afbb/32.png) [@Bill\_Door](https://boards.straightdope.com/u/Bill_Door)\ **Post date:** [June 24, 2016, 12:44pm UTC](https://boards.straightdope.com/t/could-waste-heat-be-harnessed-in-some-way/757711/48 "2016-06-24T12:44:05Z")
The thing about heat is that it can’t do work just sitting there. The only way you can get heat to do any useful work is to move it from a higher energy state to a lower energy state while extracting a portion of the difference between the two states. It comes down to driving force, at the end of the day it’s all sources and sinks, you’ve got a source that needs to move towards a sink. As your source gets lower in energy, the difference between your source and your sink grows smaller until eventually there’s not enough driving force to extract energy out of the difference. [Previous page](https://boards.straightdope.com/t/could-waste-heat-be-harnessed-in-some-way/757711.md?page=2)