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.
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.
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.
Isn’t a lot of the steam generated by utilities like Con Edison sent out in underground pipes and used to heat buildings?
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.
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!
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.