The gaps between the cells are 1 to 3 mm wide, the cells themselves being 5 to 8 mm thick, with smooth external surfaces. The minimum storage capacity of the packet of cells is 400 watt-hours at a storage temperature of at least 70 deg.C. Each grating comprises parallel members (9) bearing against the cells and running in the flow direction and transverse ones (10) of smaller cross-section generating turbulence in the water flowing round them. Gap width and cell thickness are such that heat discharge is at least 100 watt-hours per minute. The lengthwise members accommodate cell volume alterations without affecting turbulence generated.
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