AbstractThermochemical energy conversion is analysed on a thermodynamic basis with particular interest in obtaining guidance as to solar thermochemical absorber design at an early stage of development when technical and cost information is often unreliable. An earlier‐used thermodynamic equivalence technique which is equally applicable to both the endothermic and exothermic reactions has been developed to the point where it gives a clear insight into all sources of heat and work. The method is applied in particular to separating endothermic reactions of which ammonia dissociation is a prime example. A reversibility ratio is defined as the ratio of irreversible to reversible work and it is shown that in a practical solar thermochemical absorber design the reversibility ratio should be minimized, corresponding to reaction temperature minimization and therefore to tower energy losses and to potential for use of lower cost reactor materials and more active catalysts. Values of reversibility ratio are calculated for the ammonia system and are discussed in relation to solar thermochemical absorber design. In a final analysis employing the thermodynamic equivalence technique, it is shown that the apparent paradox between liquid and alternative gas pump work requirements in a liquid/gas thermochemical system is thermodynamically consistent with the internal generation of effective work from the heat source used to drive the endothermic reaction syste