The electrochemomechanical energy conversion efficiency has been investigated using a new theoretical and numerical framework for modeling the multiphysiochemical transport in long silica nanochannels. Both the chemical dissociation effects on surface charge boundary conditions and the bulk concentration enrichment caused by double layer interactions are considered in the framework. The results show that the energy conversion efficiency decreases monotonically with the increasing ionic concentration at pH = 8. For a given ionic concentration, there is an optimal channel height for the highest efficiency. The efficiency does not increase with the pH value monotonically, and there is an optimal pH value for the maximum energy conversion efficiency as the other conditions are given. The energy conversion efficiency increases with the environmental temperature. The present results may guide the design and optimization of nanofluidic devices for energy conversion.