waste heat
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CT: Every year, billions of dollars of energy are thrown away as waste heat.
Heat is the natural by-product of energy conversion processes; both conversion of primary fuels to work or energy, and conversion of energy to work. Of the 100 quadrillion BTU’s of energy the US consumes every year, 50 percent to 60 percent is lost as waste heat.
Though higher temperature waste heat in large quantities is often re-used to perform work or provide heat to buildings and processes, currently lower temperature heat is lost. Before the GEN4 system, no technologies were efficient enough to convert low grade waste heat to electricity. It was mostly discarded in cooling towers, ponds, the atmosphere, or discharged into the sewer.
In industrial settings this lost heat, both high temperature and low temperature, is significant. In 2004, the U.S. Department of Energy released a study, “Industrial Loss Reduction and Recovery in Industrial settings” that found that almost 2 quadrillion BTUs of energy could be recovered from industrial waste heat.

S: http://www.ener-g-rotors.com/waste-heat/ (last access: 1 February 2015)

N: 1. waste (adj): c.1300, of land, “desolate, uncultivated,” from Anglo-French and Old North French waste (Old French gaste), from Latin vastus (see waste (v.)). From c.1400 as “superfluous, excess;” 1670s as “unfit for use.” Waste-paper attested from 1580s.
heat (n): Old English hætu, hæto “heat, warmth; fervor ardor,” from Proto-Germanic haita- “heat”, from PIE kaid-, from root kai- “heat.” The same root is the source of Old English hat “hot” and hæða “hot weather” (see hot).
2. In the field of energy: heat energy that has not been utilised in an industrial thermal process and is released to the surrounding air, soil or waters.
3. In the field of energy transformation: by-product heat energy that occurs unavoidably in industrial processes.

S: 1. OED – http://www.etymonline.com/index.php?allowed_in_frame=0&search=waste+heat&searchmode=none (last access: 1 February 2015). 2. GDT. 3. TERMIUMPLUS.

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CR: renewable energy