MULTIOBJECTIVE OPTIMIZATION OF THE DYNAMIC MODEL OF A STIRLING ENGINE COUPLED WITH A FLUIDIZED BED COMBUSTOR

In a companion paper, the development of a new concept cogeneration device for the production of heat and electricity from renewable energy sources is presented [1]. Briefly, the main idea is the use of a Fluidized Bed Combustor (FBC) coupled with a Stirling Engine. Fluidized bed combustion allows a clean burning of biomass. The heat generated is partially adopted to be converted, by using a Stirling Engine, into electricity. One of the most critical components of this device is the head of the Stirling engine, more specifically the elements of the hot side heat exchanger, that in the proposed novel configuration, is posed in direct contact with the sand of the FBC instead that exposed to the hot flue gases. This choice is primarily suggested by the heat exchange coefficients between the multiphase fluidized bed medium and the surface of the heat exchanger, much larger than those attained when the heat exchanger is located in the stream of hot flue gases. Moreover, the mechanical action exerted by the fluidized solid particles substantially reduces the fouling usually caused by impurities in exhaust gases of a biomass combustion process. Furthermore, the heat power dissipated by the cooler of the Stirling engine and the heat flux of the exhaust gases are recovered to produce domestic hot water (DHW). A mathematical model, which includes the fluidized bed and the Stirling engine, is developed and used to optimize design and operating conditions, with specific attention to the Stirling hot side heat exchanger. It is shown that the choice to place the heat exchange in direct contact with the fluidized bed can lead to an improvement of performance in terms of fuel utilization efficiency (FUE) and shaft power output.