TECHNOLOGY FOCUS - MECHANICAL ENGINEERING - Preferred Components: Each side of the electrolyzer has an inlet and an outlet. Each inlet receives pressurized water from the interior cavity. A first side outlet is connected to a flow path that routes a first side stream out of the pressure vessel, and a second side outlet routes a second side stream into the interior cavity. A valve is provided in the flow path from the first side outlet to selectively allow the first side stream to flow from the first side through the flow path out of the pressure vessel. The valve regulates a pressure of the first side stream. A separator (68) is located in the flow path between the first side outlet and the valve to separate the first side stream into gas and liquid water. The separator has a first outlet through which the gas flows and a second outlet through which the liquid water flows. The second outlet is connected to a flow path that directs the liquid water into the interior cavity of pressure vessel. A pump is located in the interior cavity of pressure vessel. The pump has an inlet communicating with the pressurized water in the interior cavity, and an outlet communicating with inlets on each side of the electrolyzer. The pump selectively supplies the pressurized water in the interior cavity to each side of the electrolyzer. The source of pressurized water is a high-pressure water pump. The pressurized water supplied to at least one side of electrolyzer is regulated. A level indicator indicates a level of the pressurized water in the interior cavity, and the pressurized water source selectively supplies the pressurized water based on the level of pressurized water. A pressure indicator indicates a pressure in the interior cavity, and the pressurized water source selectively supplies the pressurized water based on the pressure in the interior cavity. The valve and pressurized water source control a pressure in the interior cavity. The electrolyzer is submerged in the pressurized water in the interior cavity. The system may include a flow path from an upper portion of the interior cavity out of the pressure vessel through which gas in the upper portion can flow, and a valve in the flow path from the upper portion to selectively allow the gas to flow through the flow path and out of the pressure vessel. Preferred Method: The method includes routing the first side stream to a separator, separating the first side stream into liquid water and gas, and selectively routing the liquid water in the separator to the interior cavity of pressure vessel. Regulating pressure in the first side includes selectively venting the gas in the separator to the pressurized storage device. The method may include selectively supplying pressurized water to the interior cavity of pressure vessel, and pressurizing the interior cavity of with the pressurized water. The interior cavity is pressurized to greater than 5000 psi (preferably greater than 10000 psi). Preferred Parameter: A pressure difference between the anode and cathode sides is less than 2000 psi.