Tuesday, April 07, 2009

Recovering Waste Heat

One of the largest opportunities for improving energy efficiency is the recovery of waste heat for productive use and transduction to other useful forms of energy. The key is to match the recovery method with the application.
Key to making heat recuperation viable is understanding the nature of the energy involved. The temperature distribution of waste heat depends largely on the type of industry. For example, 95% of the waste heat in the electric power industry has a temperature below 150 Celsius...In contrast, 45% of the waste heat in the chemical industry can be up to 50 Celsius above this.

Plant operators usually look at thermal energy in terms of simple enthalpy - the heat content - and conclude that capturing heat of low temperature is not viable for powering other processes....Zhang and Akiyama, however, suggest that exergy - the ability of the waste heat to do useful work - should also be taken into consideration when planning an energy-saving strategy from the viewpoint of quality of energy.

They point out that high-temperature waste heat, with an adequately large exergy value exists in many manufacturing industries. For example, slag and exhaust gases from steelmaking reside at well over 1000 Celsius, representing a powerful energy source.

They explain that latent heat storage, chemical storage, and thermoelectric conversion could be used as effective ways of recovering waste heat, either individually or in combination. _energydaily
The ability to utilise marginal levels of heat -- whether industrial, solar, or geothermal -- will open up vast new worlds of energy generation. More clever transducers of heat to other forms of energy will multiply the available quantities of energy worldwide by a substantial factor.

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Blogger Ronduck said...

Private power plants use reheaters to further heat this hot water in order to use it to run a turbine and generate electricity. If you add the heat in this water with coal heat you can extract the trapped energy that is left over from the previous industrial process.

5:46 PM  
Blogger al fin said...

Good point.

There is another cycle that uses variable mixes of water and ammonia that can apparently extract even lower level heat in a heat engine. Starts with a "K"

6:10 PM  
Blogger Ronduck said...

That could be a variation of the same process that propane powered refrigerators use to deep freeze meat.

Extracting the last bit of heat from a working fluid like water, helium or CO2 may require materials that have become so rare as to be exotic, like freon. Since freon boils at -12F it can extract the last bit of energy, if it weren't considered a threat to the ozone.

11:10 AM  
Blogger al fin said...

Yes. The environmental movement continues to devastate large parts of the economy -- without showing significant benefit. The war against refrigerants is one of the most absurd inventions-from-whole-cloth pseudo-science ever created.

Perhaps one day we'll have the opportunity to return the favour.

8:44 PM  

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