Waste not, want not

Tapping industrial waste heat could reduce fossil fuel demands in the short term and improve efficiency of countless manufacturing processes, according to scientists in Japan writing in the International Journal of Exergy.

Lihua Zhang and Tomohiro Akiyama of Hokkaido University, Sapporo, explain that heat waste from industrial processes, such as combustion and electricity generation is sometimes of low energy and diffuse. Capturing this low-quality heat for re-use elsewhere on an industrial plant is usually not practical. However, given current environmental and economic pressures the recuperation of such heat energy could become viable.

The team has investigated three promising technologies for heat recovery: latent heat, reaction heat, and the use of a Thermoelectric Device. The aim of their study was to find a way to capture the heat from industrial furnaces and other systems without the constraints of time and space associated with simply using the heat to produce steam to drive other processes at precisely the same site. They say their approach can “recuperate industrial waste heat beyond time and space.”

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.

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