RSM optimization of heat recovery from the chimneys of natural gas boilers using TEGs array: An approach for simultaneous generation of electric power and preheated water

Abstract

Natural gas as the cleanest fossil fuel has widespread application in households heating systems. However, natural gas has poor radiation and consequently a substantial amount of heat is wasted through the exhaust gas in the chimney of natural gas boilers. The focus of this work is application of Thermoelectric Generators (TEGs) arrays for recovering heat from the chimneys of natural gas boilers aimed at the simultaneous generation of electric power and preheated water and finding the optimum conditions for the process using Response Surface Methodology (RSM) optimization. A TEGs array consisting of 36 TEG modules was installed atop the exhaust gas chimney of a natural gas boiler and output power, flame temperature and energy conversion efficiency were recorded at different equivalence ratios, exhaust gas and water flow rates and co-current and counter-current flows. Finally, the optimum values of the process parameters were determined using RSM optimization. The results indicated that as the resistance of the load was equal to the internal resistance of the TEGs array, the output power was maximized. Furthermore, as the equivalence ratio increases, there is an optimum equivalence ratio such that in this equivalence ratio the output power is maximized. Also, compared to the co-current flow, counter-current flow of water raises the TEGs output power as much as 23.7 %. Finally, an increase in equivalence ratio in the range of 0.4 to 0.7 raises the combined energy conversion efficiency from 32.6 % to 45.8 %. The findings from the RSM optimization reveal a maximum output power of 18.49 W, which is attained by utilizing the optimal values of the parameters analyzed for the boilers. Specifically, these values are an equivalence ratio of 0.7, an exhaust gas flow rate of 201.14 kg/h, and a water flow rate of 2.5 L/min.