Embodiments provide a power generation device that utilizes a working fluid containing carbon dioxide within a working fluid circuit having high and low pressure sides. Components of the device may include a heat exchanger configured to be in thermal communication with a heat source whereby thermal energy is transferred from the heat source to the working fluid, an expander located between the high and low pressure sides of the working fluid circuit and operative to convert a pressure drop in the working fluid to mechanical energy, a recuperator operative to transfer thermal energy between the high and low pressure sides, a cooler operative to control temperature of the working fluid in the low pressure side, a pump operative to circulate the working fluid through the working fluid circuit, and a mass management system configured to control an amount of working fluid mass in the working fluid circuit.