NOVELTY - A closed loop energy storage system comprises a fuel cell stack (117), an electrolyzer stack (115), phase separators (301,303), a fuel tank (215), and an oxidizer tank (217). Phase separator (303) separates liquid product to produce gaseous fuel. Phase separator (301) separates liquid product to produce gaseous oxidizer. The phase separator(s) is configured to receive liquid product from the fuel cell stack and store the liquid product for subsequent separation, without the use of a separate storage tank for the liquid product.    USE - Closed loop energy storage system is used for high-altitude, lightweight, long duration solar powered aircraft.    ADVANTAGE - The closed loop energy storage system comprises phase separators which serve both to separate water from hydrogen and oxygen, and to store water. The system enables repeated storing and discharging of energy, and can be operated up to very high, suborbital altitudes for a long period of time. The system is lightweight and pollution-free, and has high energy storage capacity and excellent economical efficiency. The liquid product can also be used to cool the fuel cell during operation.    DETAILED DESCRIPTION - A closed loop energy storage system comprises a fuel cell stack (117), an electrolyzer stack (115), phase separators (301,303), a fuel tank (215), and an oxidizer tank (217). The fuel cell stack is configured to react a gaseous fuel with a gaseous oxidizer to produce a liquid product and electrical power. The electrolyzer stack is configured to separate the product into mixture (I) including the liquid product and the gaseous fuel, and mixture (II) including the liquid product and the gaseous oxidizer. Phase separator (303) is configured to separate the liquid product from mixture (I) to produce gaseous fuel. Phase separator (301) is configured to separate the liquid product from mixture (II) to produce gaseous oxidizer. The fuel tank is configured to store gaseous fuel for subsequent reaction in the fuel cell stack. The oxidizer tank is configured to store gaseous oxidizer for subsequent reaction in the fuel cell stack. The phase separator(s) is configured to receive liquid product from the fuel cell stack and store the liquid product for subsequent separation in the electrolyzer stack, without the use of a separate storage tank for the liquid product.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the closed loop energy storage system.    Electrolyzer stack (115)    Fuel cell stack (117)    Fuel tank (215)    Oxidizer tank (217)    Phase separators (301,303)