Heating device with heat storage

A heat storage device comprises a hermetically sealed metallic container having one or more molten alkali metal fluorides and/or alkaline earth metal fluorides included therein. A material having a carbon basis is also included in the metallic container to react with any oxygen present in the metallic container and thereby prevent corrosion of the metallic container by the molten fluorides.

This invention relates to a heat storage device comprising a bath of one or 
more molten alkali metal fluorides and/or alkaline earth metal fluorides 
sealed in a gas-tight manner in a metallic container, heating means in 
thermal contact with the bath, and means serving to control heat transfer 
between said device and the exterior. 
Such heat storage devices are well known in the art and their application 
by the public has increased considerably since the energy crisis and the 
rapid rise of a cost of the kilowatt hour. In fact such heat storage 
devices present the advantage of consuming electrical energy during the 
slack hours, generally at night, for which there is a reduced consumer 
charge and of releasing the resulting stored heat at will, normally during 
the peak hours, generally the day-time. 
In said heat storage devices, the heat storing materials consist of 
substances capable of storing a maximum of calorific energy in a minimum 
volume, which are placed in an insulated container. These materials may be 
of many types, such as solids, for example refractory bricks or otherwise, 
or liquids, for example pressurised water, or even two-phase systems with 
a transition solid/liquid phase, for example salts, which may store a 
comparatively large quantity of heat in the form of latent melting heat. 
The heat storage device according to the present invention relates to this 
latter type comprising substances or mixtures of substances having a 
melting-point in the operating temperature range, more specifically 
fluorides of alkali metals and/or alkaline earth metals. These substances 
or mixtures of substances are known as such in particular from U.S. Pat. 
No. 3,823,305 which describes the use of metal fluorides, for example LiF, 
or eutectic mixtures of metal fluorides, for example lithium fluoride, 
sodium fluoride, potassium fluoride, calcium fluoride and magnesium 
fluoride, the eutectic melting-point of which lies between 600.degree. and 
850.degree. C. 
These prior heat storage devices possess a latent disadvantage due to the 
corrosion of the metallic container in contact with the fluorides, in the 
presence of an atmosphere which is not free from oxygen. A solution to 
this problem consists in using salts from which all traces of oxygen have 
been removed and introducing said salts into a metallic container in an 
atmosphere of inert gas, before sealing it hermetically. This solution is 
complicated practice and comparatively expensive. Another solution, 
described in U.S. Pat. No. 3,962,407 consists in adding metallic aluminium 
to the fluoride bath. 
However, even if the corrosion phenomena are suppressed by these means, 
other equally serious disadvantages still remain. 
In fact, in storing heat energy in the form of latent heat at the high 
temperature melting-points of said fluoride salts, volume variations are 
produced both by voluminal expansion and by phase transition such that the 
walls of the metallic container are subjected to a heavy mechanical load. 
Since the rupture of the metallic container at a high temperature 
constitutes an unthinkable risk in domestic applications, it is desirable 
to provide an arrangement which is also suitable to serve as an expansion 
inhibitor. 
According to the present invention, the heat storage device is 
characterized in that in the metallic container includes a material having 
a carbon basis. 
According to a first embodiment of the invention said carbon material is a 
piece of graphite. 
In this embodiment said piece of graphite ensures the fixation of oxygen 
present in the metallic container and thus prevents the walls of the 
container from being corroded during contact with the fluorides. 
According to a second embodiment, said carbon material is a graphite foil 
applied against the inner walls of the metallic container. 
In this manner the graphite foil also acts as a cushion absorbing 
expansion.

A bath 3 of fluorides, for example, a eutectic comprising a ternary mixture 
of the fluorides of sodium, calcium and magnesium in the molar 
percentages: 64% NaF+23% CaF.sub.2 +12% MgF.sub.2, the melting-point of 
which is approximately 745.degree. C., is provided, as shown in FIG. 1, in 
the metallic container 1, preferably consisting of stainless steel, having 
a cover 2 which can be closed hermetically. 
Generally, a heat storage arrangement is constituted by several storage 
devices, such as that shown in FIG. 1, disposed in a modular block filled 
with an insulating material, for example a porous ceramic material. 
Alternatively, such arrangement has a double wall in which at least one 
radiation screen is provided, while a hydrogen atmosphere the pressure of 
which may be controlled by means of an independent reservoir provided with 
a reversible hydrogen getter and heating means which are independent and 
controllable, may be introduced between the two walls so as to control the 
heat exchange between the storage arrangement and the exterior, which 
principle is described in U.S. Pat. No. 3,823,305. 
If the salts or the atmosphere in the container is not free of oxygen, 
corrosion of metallic parts in contact with the fluoride bath occurs, 
which effect lead to a rupture of the container, more especially since it 
is subjected to a heavy mechanical load due to the increase in volume 
which the storage material experiences by heating and/or phase change. 
According to the present invention, a material having a carbon basis is 
introduced into the container before closing it hermetically. The carbon 
material ensures the fixation of the oxygen present in the container in 
known manner as described for example in the publication "Advances in 
Molten Salt Chemistry", Vol. 3, p. 183, PLENUM PRESS, New York and London 
(1975). In this manner intergranular corrosion of the walls of the 
container no longer occurs. 
According to a first embodiment, the carbon material comprises a piece of 
graphite, as shown at 4 in FIG. 1. Such a graphite piece has the advantage 
of floating on the surface of the fluoride bath 3 and not being wetted by 
it. In this case, however, it serves the same purpose as aluminium, namely 
the fixation of oxygen normally in the form of a gaseous oxide of carbon; 
however, while aluminium deposits at least partly on the surface of the 
metallic parts in contact with the bath in a protective layer, the carbon 
fixes the oxygen in the form of a gas. 
In the case of an excess of aluminium, the metallic surfaces in contact 
with the bath may be attacked due to the dissolution of certain 
components. Graphite does not present this disadvantage since even in 
excess it remains as such, at least in the temperature range employed. 
According to a modified embodiment of the invention, as shown in FIG. 2, 
said carbon material is utilized in the form of a foil of graphite 14 
applied against the inner walls of the metallic container 11. Such foil 14 
may be constituted either in the form of a ribbon of "papyex" 
(commercially available from the firm Le Carbone Lorraine) or preferably, 
because of its greater thickness (up to 1,3 mm) as a graphite felt or a 
graphite foam. 
In this case the carbon material plays the part of a cushion absorbing 
expansion between the metallic container and the fluoride in addition to 
its function of a reducing agent. Due to the significant temperature 
variations between roughly 0.degree. and 1000.degree. C. and the 
differences in the coefficients of expansion, the metallic container is 
subject to heavy mechanical loads. Whereas in the liquid state the molten 
salt may expand within the container 11, which is intentionally not filled 
entirely, in the solid state the salt presses against the walls of the 
container 11 considerably, which effect in certain cases may lead to 
rupture. The graphite felt which can be compressed materially thus permits 
obtaining the few millimeters necessary for the expansion of the fluoride 
salt 13 up to its change into the liquid state. A cover 12 also ensures a 
hermetic seal.