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A heat accumulator includes a thermally insulated heat storage vessel having an opening formed in a bottom wall thereof, a quantity of heat accumulating medium arranged within the vessel, and a fluid circulation tube of double tube structure inserted from the opening into the vessel and extending ve

A heat accumulator includes a thermally insulated heat storage vessel having an opening formed in a bottom wall thereof, a quantity of heat accumulating medium arranged within the vessel, and a fluid circulation tube of double tube structure inserted from the opening into the vessel and extending vertically upward toward a top wall of the vessel for introducing a fluid into the vessel or discharging the fluid out of the vessel. The heat accumulator further includes a heat-insulating layer provided on an outer peripheral surface of the fluid circulation tube and extending over at least a longitudinal portion of the fluid circulation tube extending from the bottom wall of the vessel in an upward direction to a predetermined height of the vessel.

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1. A heat accumulator, comprising: a thermally insulated heat storage vessel having a top wall, a bottom wall and an opening formed in the bottom wall;a quantity of heat accumulating medium arranged within the vessel;a fluid circulation tube of double tube structure inserted from the opening of the

1. A heat accumulator, comprising: a thermally insulated heat storage vessel having a top wall, a bottom wall and an opening formed in the bottom wall;a quantity of heat accumulating medium arranged within the vessel;a fluid circulation tube of double tube structure inserted from the opening of the bottom wall into the vessel and extending vertically upward toward the top wall of the vessel for introducing a fluid into the vessel and discharging the fluid out of the vessel; anda heat-insulating layer provided on an outer peripheral surface of the fluid circulation tube and extending over at least a longitudinal portion of the fluid circulation tube extending from the bottom wall of the vessel in an upward direction to a predetermined height of the vessel;wherein the heat-insulating layer extends over an entire length of the fluid circulation tube. 2. The heat accumulator according to claim 1, wherein the heat-insulating layer comprises a vacuum layer provided between the outer peripheral surface of the fluid circulation tube and a hollow cylindrical inner wall of the vessel extending from the bottom wall toward the top wall of the vessel. 3. The heat accumulator according to claim 1, wherein the heat-insulating layer comprises a layer of heat-insulating material. 4. The heat accumulator according to claim 1, further comprising a support member mounted to connect the fluid circulation tube to the vessel. 5. The heat accumulator according to claim 4, wherein the support member is mounted to connect an upper end of the fluid circulation tube to the vessel. 6. The heat accumulator according to claim 1, wherein the heat accumulating medium is received within capsules. 7. The heat accumulator according to claim 6, wherein the capsules are spherical. 8. The heat accumulator according to claim 1, wherein the heat accumulating medium comprises a latent heat accumulating material selected from the group consisting of paraffin, sodium acetate trihydrate and erythritol, and the fluid is introduced via the fluid circulation tube to the heat accumulating medium, flows through the heat accumulating medium, and is discharged from the vessel via the fluid circulation tube. 9. The heat accumulator according to claim 8, wherein a heat transfer occurs between the fluid and the heat accumulating medium as the fluid flows through the heat accumulating medium. 10. A heat accumulator, comprising: a thermally insulated heat storage vessel having a top wall, a bottom wall and an opening formed in the bottom wall;a quantity of heat accumulating medium arranged within the vessel;a fluid circulation tube of double tube structure inserted from the opening of the bottom wall into the vessel and extending vertically upward toward the top wall of the vessel for introducing a fluid into the vessel and discharging the fluid out of the vessel; anda heat-insulating layer provided on an outer peripheral surface of the fluid circulation tube and extending over at least a longitudinal portion of the fluid circulation tube extending from the bottom wall of the vessel in an upward direction to a predetermined height of the vessel;wherein the heat accumulator is operable to perform a heat-retaining operation in which the heat accumulating medium releases heat to the fluid to thereby keep an internal temperature of the vessel at a constant value, wherein during the heat-retaining operation of the heat accumulator, the fluid circulation tube provided with the heat-insulating layer is capable of forming temperature stratification in which a body of the fluid held within the fluid circulation tube is arranged into three horizontal layers of different temperatures, the three horizontal layers consisting of a top layer of a first temperature which is substantially equal to the internal temperature of the vessel, a bottom layer which is lower than the first temperature and substantially equal to the outside temperature, and an intermediate mixed layer disposed between the top and bottom layers and having an average temperature which is intermediate between the first and second temperatures, and wherein said longitudinal portion of the fluid circulation tube provided with the heat-insulating layer contains only the bottom layer and the intermediate mixed layer. 11. A heat accumulator, comprising: a thermally insulated heat storage vessel having a top wall, a bottom wall and an opening formed in the bottom wall;a quantity of heat accumulating medium arranged within the vessel;a fluid circulation tube of double tube structure inserted from the opening of the bottom wall into the vessel and extending vertically upward toward the top wall of the vessel for introducing a fluid into the vessel and discharging the fluid out of the vessel;a heat-insulating layer provided on an outer peripheral surface of the fluid circulation tube and extending over at least a longitudinal portion of the fluid circulation tube extending from the bottom wall of the vessel in an upward direction to a predetermined height of the vessel; and,a support member mounted to connect the fluid circulation tube to the vessel;wherein the support member has an upper surface facing the top wall of the vessel, a lower surface facing the heat accumulating medium, and a number of through-holes extending between the upper and lower surfaces and held in fluid communication with the fluid circulation tube, wherein the fluid circulation tube of double tube structure includes an inner tube, and further comprising an extension tube extending from an upper end of the inner tube toward the bottom wall of the vessel across a thickness of the support member. 12. The heat accumulator according to claim 11, wherein the heat accumulating medium is contained in spherical capsules having an outside diameter larger than an inside diameter of the through-holes of the support member and smaller than inside diameters of the inner tube of the fluid circulation tube and the extension tube.