Embodiments provide a heat engine system containing working fluid (e.g., sc-CO2) within high and low pressure sides of a working fluid circuit and a heat exchanger configured to transfer thermal energy from a heat source to the working fluid. The heat engine system further contains an expander for converting a pressure drop in the working fluid to mechanical energy, a shaft coupled to the expander and configured to drive a device (e.g., generator or pump) with the mechanical energy, a recuperator for transferring thermal energy between the high and low pressure sides, and a cooler for removing thermal energy from the working fluid in the low pressure side. The heat engine system also contains a pump for circulating the working fluid, a mass management system (MMS) fluidly connected to the working fluid circuit, and a supply tank fluidly connected to the MMS by a supply line.