A plurality of independent, closed Rankine cycle power plants, each of which has a vaporizer, is operated by serially applying a medium or low temperature source fluid to the vaporizers of the power plants for producing heat depleted source fluid. A preheater is provided for each vaporizer; and said heat depleted source fluid is applied to all of the preheaters in parallel. The heat depleted source fluid thus serves to heat the operating fluid to the evaporization temperature, while the source fluid applied to the vaporizers supplies the latent heat of vaporization to the operating fluid of the power plant. The present invention is advantageous, as compared to a conventional cascaded power plant of the type described, because the temperature drop of the source fluid can be increased without reducing the efficiency. Alternatively, the temperature drop can be maintained but the efficiency can be increased. In either case, the power produced by the power plant according to the present invention is increased.