Hybrid Energy Systems Which Include Waste Heat Recovery and Conversion

Waste heat losses arise both from equipment inefficiencies and from thermodynamic limitations on equipment and processes. For example, consider reverberatory furnaces frequently used in aluminum melting operations. Exhaust gases immediately leaving the furnace can have temperatures as high as 2,200°F-2,400°F [1,200°C-1,300°C]. Consequently, these gases have high heat content, carrying away as much as 60% of furnace energy inputs. Efforts can be made to design more energy-efficient reverberatory furnaces with better heat transfer and lower exhaust temperatures; however, the laws of thermodynamics place a lower limit on the temperature of exhaust

TABLE 7.3

2010 Production, Calculated Onsite Energy Consumption, and Energy Savings Potential for Eleven Chemicals [1]

gases. Since heat exchange involves energy transfer from a high-temperature source to a low-temperature sink, the combustion gas temperature must always exceed the molten aluminum temperature in order to facilitate aluminum melting. The gas temperature in the furnace will never decrease below the temperature of the molten aluminum, since this would violate the second law of thermodynamics. Therefore, the minimum possible temperature of combustion gases immediately exiting an aluminum reverberatory furnace corresponds to the aluminum pouring point temperature 1,200°F-1,380°F [650°C-750°C]. In this scenario, at least 40% of the energy input to the furnace is still lost as waste heat. Many examples of industrial waste heat and their possible uses are illustrated in Table 7.4. The Temperature Classification of Waste Heat Sources and Related Recovery Opportunity is illustrated in Table 7.5.

Recovering waste heat requires implementation of hybrid process like cogeneration or CHP. Industrial waste heat can be recovered via numerous methods. The heat can either be “reused” within the same process or transferred to another process through process hybridization. Ways of reusing heat locally include using combustion exhaust gases to preheat combustion air or feedwater in industrial boilers. By preheating the feedwater before it enters the boiler, the amount of energy required to heat the water to its final temperature is reduced. Alternately, the heat can be transferred to another process; for example, a heat exchanger could be used to transfer heat from combustion exhaust gases to hot air needed for a drying oven. In this manner, the recovered heat can replace fossil energy that would have otherwise been used in the oven. Such methods for recovering waste heat can help facilities significantly reduce their fossil fuel consumption, as well as reduce associated operating costs and pollutant emissions. As shown below, waste heat can also be converted to power by thermoelectric generator, TPV system, or piezoelectric system. Waste heat (particularly at low temperature) can also be used in heat pump.

TABLE 7.4

Examples of Waste Heat Sources and End Uses [34]

TABLE 7.5

Temperature Classification of Waste Heat Sources and Related Recovery

(Continued)

TABLE 7.5 (Continued)

Temperature Classification of Waste Heat Sources and Related Recovery Opportunity [34]

Industrial-scale energy systems integration provides a systems approach to optimize energy use at manufacturing facilities through technologies that can increase energy flexibility and reduce/recover/reuse waste energy, leading to reduced energy intensity, and narrowing the gap between current energy use and practical minimum energy requirements. Any option for waste heat recovery results in the overall system to be hybrid.