This paper presents the results of a numerical simulation of heat distribution in the heat exchanger of a prototype thermoelectric generator constructed and examined in the Thermoelectric Research Laboratory in AGH University, Cracow, Poland. The area of interest was to prepare a numerical model and determine the influence of a dispersion cone on the temperature distribution along the heat exchanger. The role of a dispersion element is to mix the air stream to improve the flow between the internal heat exchanger’s fins in order to enhance heat exchange. The estimation of power output parameters and exchanger efficiency has been performed in order to assess the cone impact for three selected air inlet temperatures. The results show that the presence of the cone increases the efficiency of the thermoelectric generator by at least 25%.