Abstract: An energy management system includes an energy storage reservoir and a fluid pump. The reservoir includes a fluid reservoir configured to contain varying quantities of a fluid and to isolate the fluid from the ambient. The fluid reservoir has a top surface, bottom surface, flexible member, and at least one opening for filling or emptying the reservoir. A pressing mass of solid material is positioned above the fluid reservoir with a weight sufficient to press the reservoir and empty the reservoir of fluid when the reservoir opening is open. The pressing mass is supported by the fluid reservoir when the reservoir contains fluid and is closed. The flexible member is disposed around the fluid reservoir to seal the fluid and to facilitate vertical motion of the pressing mass as the reservoir is filled and emptied. The pump supplies fluid to the fluid reservoir via the opening.