A metal halogen electrochemical energy cell system, comprising: a vessel that: (a) contains a vertical stack of a plurality of horizontal cells, each of the plurality of horizontal cells comprising: at least one positive electrode; at least one negative electrode; and a reaction zone between the positive electrode and the negative electrode; and (b) is adapted to contain an aqueous electrolyte comprising a metal halide; and contains a circulation pump that is configured to convey a flow of the electrolyte through the reaction zone of each of the cells of the stack; and wherein the vertical stack comprises: (c) a stack of cell frames supporting the stack of cells; (d) a feed manifold element and a return manifold element in each cell frame in the stack of cell frames; (e) a feed manifold formed by aligned feed manifold elements in the stack of cell frames; (f) a return manifold formed by aligned return manifold elements in the stack of cell frames; and (g) a plurality of distribution channels located in each cell frame, wherein the distribution channels are configured to introduce the electrolyte from the feed manifold to the reaction zone of each cell, and from the reaction zone to the return manifold. a vessel that: (a) contains a vertical stack of a plurality of horizontal cells, each of the plurality of horizontal cells comprising: at least one positive electrode; at least one negative electrode; and a reaction zone between the positive electrode and the negative electrode; and (b) is adapted to contain an aqueous electrolyte comprising a metal halide; and (a) contains a vertical stack of a plurality of horizontal cells, each of the plurality of horizontal cells comprising: at least one positive electrode; at least one negative electrode; and a reaction zone between the positive electrode and the negative electrode; and at least one positive electrode; at least one negative electrode; and a reaction zone between the positive electrode and the negative electrode; and (b) is adapted to contain an aqueous electrolyte comprising a metal halide; and contains a circulation pump that is configured to convey a flow of the electrolyte through the reaction zone of each of the cells of the stack; and wherein the vertical stack comprises: (c) a stack of cell frames supporting the stack of cells; (d) a feed manifold element and a return manifold element in each cell frame in the stack of cell frames; (e) a feed manifold formed by aligned feed manifold elements in the stack of cell frames; (f) a return manifold formed by aligned return manifold elements in the stack of cell frames; and (g) a plurality of distribution channels located in each cell frame, wherein the distribution channels are configured to introduce the electrolyte from the feed manifold to the reaction zone of each cell, and from the reaction zone to the return manifold. (c) a stack of cell frames supporting the stack of cells; (d) a feed manifold element and a return manifold element in each cell frame in the stack of cell frames; (e) a feed manifold formed by aligned feed manifold elements in the stack of cell frames; (f) a return manifold formed by aligned return manifold elements in the stack of cell frames; and (g) a plurality of distribution channels located in each cell frame, wherein the distribution channels are configured to introduce the electrolyte from the feed manifold to the reaction zone of each cell, and from the reaction zone to the return manifold.