Editing IPCC:AR6/WGIII/Chapter-11 (section)

==== 11.3.4.1 Heat-use Energy Efficiency Improvement ====

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While about 10% of global GHG emissions originate from combustion to produce high-temperature heat for basic material production processes ( [[#Sandalow--2019|Sandalow et al. 2019]] ), limited efforts have been made to decarbonise heat production. There is still a large potential for using various grades of waste heat and the development of high-temperature heat pumps facilitates its use. [[#NEDO--2019|NEDO (2019)]] applies a ‘Reduce, Reuse, and Recycle’ concept for improved energy efficiency, and we use this frame our discussion of heat efficiency.

''Reduce'' refers to reducing heat needs via improved thermal insulation, for example, where porous type insulators have been developed with thermal conductivity half of what is traditionally achieved by heat-resistant bricks under conditions of high compressive strength (Fukushima and Yoshizawa 2016). ''Reuse'' refers to waste heat recovery. A study for the EU identified a waste heat potential of about 300 TWh yr –1 , corresponding to about 10% of total energy use in industry. About 50% of this was below 200°C, about 25% at temperatures 200°C–500°C, and 25% at temperatures of 500°C and above ( [[#Papapetrou--2018|Papapetrou et al. 2018]] ). A survey conducted in Japan showed that 9% of the input energy is lost as waste heat, of which heat below 199°C accounts for 68% and that below 149°C was 29% ( [[#NEDO--2019|NEDO 2019]] ). [[#McBrien--2016|McBrien et al. (2016)]] identified that in the steel sector process heat recovery presently saves 1.8 GJ per tonne of hot rolled steel, while integrated across all production processes heat recovery with conventional heat exchange could save 2.5 GJ t –1 , and it scales up to 3.0 GJ t –1 using an alternative heat exchange that recovers energy from hot steel. High-temperature industrial heat pumps represent a new and important development for upgrading waste heat and at the same time they facilitate electrification. One recent example is a high-temperature heat pump that can raise temperatures up to 165°C at a coefficient of performance (COP) of 3.5 by recovering heat from unused hot water (35°C–65°C) ( [[#Arpagaus--2018|Arpagaus et al. 2018]] ). Commercially available heat pumps can deliver 100°C–150°C but at least up to 280°C is feasible ( [[#Zühlsdorf--2019|Zühlsdorf et al. 2019]] ). Mechanical vapour recompression avoids the loss of latent heat by condensation, then it acts as a highly efficient heat pump with a 5–10 COP ( [[#Philibert--2017a|Philibert 2017a]] ).

Waste heat to power (WHP), or ''Recycle'' in NEDO’s terms, is also an under-utilised option. For example, a study for the cement, glass and iron industries in China showed that current technology enables only 7–13% of waste heat to be used for power generation. With improved technologies, potentially 40–57% of waste heat with temperatures above 150°C could be used for power generation via heat recovery. Thermal power fluctuations can be a challenge and negatively affect the operation and economic feasibility of heat recovery power systems such as steam and/or organic Rankine cycle. In such cases, latent heat storage technology and intermediate storage units may be applied ( [[#Jiménez-Arreola--2018|Jiménez-Arreola et al. 2018]] ). The development of thermoelectric conversion materials that produce power from unused heat and energy harvested from a higher temperature environment is also progressing, with several possible applications in industrial processes (Gayner and Kar 2016; [[#Jood--2018|Jood et al. 2018]] ; [[#Lv--2018|Lv et al. 2018]] ; [[#Ohta--2018|Ohta et al. 2018]] ). A potential early application in industry is to power wireless sensors, a niche that uses microwatts or milliwatts, and avoid power cables ( [[#Champier--2017|Champier 2017]] ).

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