Efficiency of AMTEC cells has reached 16% in the laboratory. High-voltage multi-tube modules are predicted to have 20–25% efficiency, and power densities up to 0.2 kW/ L appear to be achievable in the near future. Calculations show that replacing sodium with a potassium working fluid increases the peak efficiency from 28% to 31% at 1100 K with a 1 mm thick BASE tube.