In this investigation, the heat rejected by one heat conversion device has been utilized as the source of power input for another device. A mathematical model has been developed to cascade the two devices under investigation consisting of a thermionic energy converter (TIEC) and an alkali metal thermal to electrical converter (AMTEC) cell. The latter uses the waste heat of the former as the energy input. This cascade system employs a different type of AMTEC, which is a vapor anode system rather the liquid anode used in an earlier work. Also, it uses a more involved algorithm with increased number of nodes than simulated previously. The efficiency of the new cascade has been optimized by solving 16 non-linear equations by MATHCAD. Because of the limitation of MATHCAD the solution is performed in two loops. Nevertheless, the efficiency of this new cascade is thus improved by 28% over the previous one. (c) 2005 Elsevier B.V. All rights reserved.