Thermal optimization of embedded thermoelectric generators in refractory furnaces

Abstract

Industrial energy consumption represents a large part of the energy flow in the United States and it would probably be similar in many other countries. Coal or gas fired furnaces are a major component of the energy consumption to supply high temperatures. Refractory wall is indispensable along the furnace pathway designed for melting or processing materials. Unfortunately, more than a fraction of the input energy diffuses across the refractory wall due to the high temperatures and the waste heat is dumped to the ambient. Thermoelectric power generator (TEG) has an ultra low profile and is widely scalable with arraying the modules. This configuration is matched very well with the requirement of harvesting energy from the waste heat through the refractory wall without any design change of the current furnaces. We propose a simple water-cooled TEG system replacing a fraction of the refractory wall thickness while maintaining the melt temperature and the heat flux would be the same as the current refractory wall with passive air cooling. Design optimization is conducted, trading-off the TEG module thickness and the furnace wall thickness maximizing the power output while maintaining the above heat flux. We will present a quantitative analysis based on an example of the conventional fire ports that produce furnace gases at a temperature of 1500 °C. The furnace is designed for melting glass pellets and maintaining the temperature of melt glass at 1000 °C in a pool. A facility with 500 ton/day capacity is modeled. There are four fire ports and 54 cm thick aluminum-zirconia-silica (AZS) refractory wall around the ports. The thermal optimization is conducted considering the design of TEG matched to desired heat flux of approximately 10 kW/m2. As shown in our earlier work, the TEG design with a smaller TE element fill factor down to 10% or below, will provide the most cost effective power generation. Interestingly, the additional material cost for the optimum TEG with 10% fill factor and a copper cold plate is much less expensive compared to the cost needed for the AZS refractory wall material that it is replacing. When the remaining thickness of the refractory wall is 25 cm, the power generation from TEG is 1.72 kW/m2 while maintaining the same heat flux. The total power output from the all four fire ports can be 66.1 kW and the cost for the TEG module is estimated to be $0.5 per Watt based on demonstrated robust high temperature TE material.