Disclosed is a thermal storage solution which can operate without any internal tubing or mechanical pumping in the heat reservoir, and features a heat transfer technology based on evaporation and condensation of heat transfer fluids that will prevent hot and cold zones in the thermal storage reservoir. The main advantage is that the reservoir will have a much lower cost, have more degrees of freedom regarding the interplay between storage capacity, input and output power, and can operate without any mechanical or pressurized parts.