The self-criticality conditions for the growth of any new CO2-free energy system are derived in a quite general manner. In this model, capital and energy are coupled in the form of a circular causality resulting in a closed system with a strong feed-back. The analysis shows explicitly how high the new energy production costs, in terms of US$/kWh of useful energy, are allowed, to be sure that really an energy source and not an energy sink is being built. (C) 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.