the ice bank shown in the drawings includes a container 10 having a cylindrical wall of composite construction . it may include an outer skin of aluminum foil covering thermal insulation which enclosures a tank member of rigid plastic such as polyethylene . an insulated base 11 defines the bottom of the container 10 . the top comprises a molded cover 12 with a foam insulation core . the periphery of the cover 12 may be secured to a flange 13 around the upper edge of the container 10 as by a circle of twenty - three bolts 14 . typically the container 12 may be somewhat more than eight feet high and over seven feet in diameter . coiled within it are extended lengths of plastic heat exchange tubing , perhaps of 5 / 8 inch outside diameter laid in a series of flat spirals with the turns in a given spiral and the spirals themselves held apart by spacer strips 15 as described in u . s . pat . no . 4 , 671 , 340 . circulated through the heat exchange tubing held by the spacer strips 15 is a brine such as 25 % ethylene glycol . the brine either melts or freezes a pcm which when entirely liquid fills the container to the level indicated in fig1 as level a , just covering the tops of the spacer strips 15 and all of the heatexchange tubes held thereby . inlet and outlet headers 20 and 21 extend off - center down the middle of the bundle of tubes and inlet and outlet headers 22 and 23 extend down the outside of the bundle of tubes , as shown in fig2 . by providing two pairs of inlet and outlet headers it is possible to circulate brine through the heat exchange tubes in a counter - flow fashion from one level to the next . an expansion space 25 is defined below the underside of the cover 12 and above initial level a of the liquid pcm . as noted previously its purpose is to provide space for the liquid pcm to rise as the pcm freezes to ice around the heat exchange tubes during the charging cycle . since there are no heat exchange tubes in the expansion space 25 the liquid pcm occupying that space could freeze during sub - cooling of the ice below , unless preventive measures are taken as in accordance with this invention . the ice - cap preventing means of the invention consists of a thermally insulated barrier 26 extending substantially completely laterally across the interior of the container 10 at the level a . the barrier 26 is formed of waterproof closed - cell polystyrene extruded rigid foam which may be bonded with impermeable foil on both sides . as shown in fig2 the barrier 26 is formed of two half discs 26a and 26b joined along their respective straight edges . the co - joined edges are interlocked in a butt lap as shown in fig3 . they define a central circular aperture 28 through which the inlet and outlet headers 20 and 21 extend . they also define an aperture 29 near their common periphery through which the headers 22 and 23 extend . in addition , to permit easy passage of the liquid pcm through the barrier 26 during alternate freezing and thawing , a suitable array of small drain holes 30 may be formed in each of the half discs 26a and 26b . the barrier 26 may be approximately one - half inch to one inch thick . by itself , however , it lacks sufficient strength to withstand the rising force of the buoyant ice beneath it when freezing is complete . in one form of the ice bank of the invention 1 , 620 gallons of water are held within the container 10 and when fully frozen exert an upward buoyancy force of almost 1 , 600 lbs . to resist this force and to stregthen the barrier 26 a series of ribs 32a , 32b and 32c are provided which depend integrally from and are molded as part of the underside of the cover 12 to abut the top side of the barrier 26 as more clearly shown in fig1 . they are shown to be annular and concentric but they could also be radial . the array of holes 30 in the barrier 26 should be positioned to insure that as the liquid pcm rises it can readily fill all parts of the expansion space 25 which , as should be apparent , is somewhat compartmentalized by the ribs 32a , 32b and 32c . in the operation of the improved ice bank of the invention liquid pcm is added to the container 10 until it reaches level a . each of the half discs 26a and 26b are placed on top of the spacers 15 at level a with their straight edges adjoining and interlocked and thereby fitted around the pairs of inlet and outlet headers 20 - 21 and 22 - 23 . the cover 12 is then bolted in place on the flange 13 of the container 10 and as this is done the ribs 32a , 32b and 32c come into abutting contact with the top side of the barrier 26 . when it is desired to store coolness in the ice bank , brine is sub - cooled below its freezing temperature , for example to 21 ° f . for a water pcm which of course freezes at 32 ° f . this causes ice to form gradually around all of the heat exchange tubes held by the spacer strips 15 . as the ice forms it expands and forces the liquid pcm up through the various apertures 28 and 29 and holes 30 in the barrier 26 until virtually all of the expansion space 25 is filled with liquid pcm . when charging is complete the ice beneath the barrier 26 is solid and may be sub - cooled to as low as 15 ° f . nonetheless the liquid pcm above the barrier 26 does not freeze because the barrier provides sufficient thermal insulation so that the cold entering through the barrier 26 into the expansion space 25 is more than cancelled by the heat gain through the cover 12 and around the edges thereof into the space 25 . consequently an ice - cap does not form in the space 25 . the scope of the invention is to be taken from the following claims rather than from the specifics of the preferred embodiment described above . certain variations in that preferred embodiment within the scope of the invention should be apparent . for example , the harrier 26 may be strengthened and held against the buoyant upward force of the ice beneath by spacers other than the ribs 32a , 32b and 32c , for example by separate spacer elements . the barrier 26 may be formed in more that two half discs or even as a single disc through which the inlet and outlet headers are inserted . the particular cross - sectional configuration of the container 10 may be appropriately modified to be of rectangular shape or any shape other than cylindrical . the manner in which the heat exchange tubes are held within the container forms no part of the invention and indeed they may be vertically disposed rather than horizontally in successive spirals .