Composting is an exothermic process from which useful energy can be captured in the form either of electricity or of heat for air or water. What the literature lacks are rigorously validated models of such systems and a systematic approach to their design. Previously published models represented changes in process variables (temperature, humidity, oxygen concentration, etc.) in the solid, liquid and gas phases of the compost. In this study, an updated model was created in the commercial, finite-element, multiphysics software COMSOL®, to simulate mass and energy balances in a cylindrical composter. The model will be validated against empirical data from barrel composters and will be used to develop guidelines for the optimal design of compost heat extraction systems.