Energy Storage and Retrieval as Heat

The present invention generally pertains to the storage of energy and the retrieval thereof as heat, and is specifically directed to physical-chemical methods and systems for storing and retrieving heat.

There is a need for an improvement in storing energy. Most forms of energy are readily converted to heat and may be stored by heating a storage medium. The storage medium may be an inert substance, such as water or rocks, wherein the heat is stored by raising the temperature of the substance and retrieved by moving a relatively cooler heat transfer fluid over the surface of the storage medium. Heat storage by raising the temperature of chemically inert material has the disadvantage of requiring thermal insulation to prevent heat loss and requiring a large volume of substance to store useful quantities of heat.

Heat also may be stored by reversible physiochemical reactions. In one such system, a salt hydrate is heated in a container to above its latent heat of fusion to its liquid state to store the heat. The heat is retrieved by passing a relatively cooler heat transfer fluid such as air or water around the salt hydrate container, thereby producing an exothermic crystallization process, whereby heat is surrendered to the heat transfer fluid. One difficulty with this system is that a stratification layer of the crystalized salt tends to form at the heat transfer surface thereby creating a barrier which reduces the rate of heat transfer.

According to the present invention, energy is stored by heating a salt to a temperature above its latent heat of fusion to convert the salt to a liquid state; and heat is retrieved by moving a heat transfer fluid that is immiscible with the salt and has a density less than that of the salt over the top surface of the liquid salt at such a velocity that the upper layer of the salt is emulsified with the heat transfer fluid to crystalize the salt in the upper layer to thereby exothermally surrender heat from the salt to the heat transfer fluid. The crystalized salt gravitates from the top surface thereby maintaining the top surface in a liquid state.

The present invention is fully operative with both normal salts and those having a supercooled liquid phase. Use of supercooled liquid phase salts allows heat to be stored at ambient or room temperatures that are lower than the latent heat of fusion of the salt.

By using a heat transfer fluid that is immiscible with the salt and of a lower density than the salt, an emulsion between the two can be formed to thereby increase the heat transfer area; while at the same time a sufficient demarcation between the heat transfer fluid and the salt is defined to prevent significant entrainment of salt crystals in the heat transfer fluid outside of the emulsion layer. A higher heat transfer rate is obtained because of the larger heat transfer area between the salt and the heat transfer fluid incident to their direct contact with one another in the emulsion.

The heat transfer fluid is moved across the salt in such a pattern as to create a large shear rate between them. Two preferred patterns are a vortex pattern and an outward radial pattern. Although the heat transfer fluid is moved across the top surface of the salt at such a velocity that the upper layer of the liquid salt is emulsified with the heat transfer fluid to increase the heat transfer area this velocity must not be so high that crystallized salt is entrained in the heat transfer fluid outside of the emulsion layer. If the salt crystals become so entrained they may coat and clog whatever means, if any, are used to move the heat transfer fluid out of the container containing the salt to a heat exchanger.

In one embodiment both the heat transfer fluid and the salt are enclosed in the same container and a coolant is moved through tubing that makes contact with the heat transfer fluid to remove heat from the heat transfer fluid through the tubing to the coolant. Because the heat transfer fluid is enclosed in the container, a larger emulsion layer can be tolerated since there is no danger of salt crystals being circulated outside of the container; and as a result a greater heat transfer area and crystalization rate can be achieved.

Preferred salts include Mg (NO.sub.3).sub.2, sodium acetate and sodium hyposulphite, all of which have a supercooled phase, and Na.sub.2 B.sub.4 O.sub.7.10H.sub.2 O. A preferred heat transfer fluid is mineral oil.

FIG. 1 is a schematic drawing of one preferred embodiment of an energy storage and heat retrieval system in accordance with the present invention.

FIG. 2 is a schematic drawing of a second preferred embodiment of the system of the present invention.

FIG. 3 is a schematic drawing showing a portion of another preferred embodiment of the system of the present invention.

FIG. 4 is a schematic drawing of a portion of still another preferred embodiment of the system of the present invention.

Referring to FIG. 1, the salt 10 is stored in a cylindrical container 12. The salt is heated by energy transferred through a heating coil 14 inside the container 12 and/or by energy transferred through the container wall 16 from hot water tubes 18 mounted on the outside wall 16 of the container 12. Accordingly, the salt 10 is heated to above its latent heat of fusion to its liquid state (phase). Conductive heating strips can be substituted for the hot water tubes in a system wherein all of the energy to be stored is electrical in form. Alternatively, where all of the energy is stored from hot water, hot water tubes can be substituted for the heating coil 14.

In all of the embodiments described herein the salt is either Mg (NO.sub.3).sub.2, sodium acetate, sodium hyposulphite ie thiosulfate, or Na.sub.2 B.sub.4 O.sub.7.10H.sub.2 O (commonly known by the tradename "Borax." All of these salts have a density greater than one. The heat transfer fluid used in all of these embodiments is mineral oil, which is immiscible with each of these salts and also has a density of less than one. Other combinations of salts and heat transfer fluids having the requisite density and immiscibility relationship also may be used in accordance with the present invention.

The mineral oil 20 is moved rapidly over the top surface 22 of the salt 10 in a vortex pattern.

As the mineral oil flows across the top surface 22, an emulsion layer 23 is formed. The salt in the emulsion layer 23 crystalizes and exothermally transfers to the mineral oil 20 the energy that was stored in the salt 10 by heating the salt 10 to above its latent heat of fusion. The salt which crystalizes then gravitates to the bottom of the container 12 to maintain the top surface 22 of the salt 10 in a liquid state.

This process of rapidly moving the heat transfer fluid (mineral oil 20) directly across the top surface of the salt 10 is also significant when the salt is one which both has a supercooled phase and can be converted from its supercooled phase by agitation. The agitation produced by the rapid movement of the heat transfer fluid causes such a salt to crystalize from its supercooled phase when the mere cooling of the salt 10 by contact with a relatively cooler heat transfer fluid by itself would not be sufficient to convert the salt 10 from its supercooled liquid state, and the introduction of a nucleation agent to effect such conversion would be necessary. The present invention is nevertheless operable, however, even with those salts having a supercooled state as to which agitation does not effect conversion whereby the introduction of a nucleation agent or some other means is required for conversion.

The top of the container 12 has an inlet 24 through which the mineral oil 20 is directed tangentially into the container 12 so as to flow in a free vortex pattern. The mineral oil 20 is moved over the top surface 22 of the salt 10 at such a velocity that the upper layer of the salt 10 is emulsified with the mineral oil 20 in a layer 23 to increase the heat transfer area. The top of the container 12 includes an outlet 26 in its top center for drawing out the mineral oil 20. The mineral oil 20 flows more rapidly at the center of the vortex and thereby affords a higher heat transfer rate near the outlet 26. Also intense centrifugal force near the center of the vortex separates any salt crystals from the mineral oil 20 and prevents such salt crystals from being drawn into the outlet 26, and thereby prevents the outlet 26 and the tubing 28 leading therefrom from becoming coated with and clogged by any such salt crystals.

The heated mineral oil 20 is transferred by tubing 28 to a heat exchanger 46. The heat exchanger 46 essentially includes a coil 50 and a fan 48 for removing the heat from the mineral oil by blowing air across the coil 50. A pump 30 is provided in the system for pumping the cooled mineral oil back through the container 12.

This process may be repeated by again heating the crystalized salt to a temperature above its latent heat of fusion.

Referring to FIG. 2, the salt 32 is stored in a cylindrical container 34. A heating coil 36 inside the container 34 and hot water tubes 38 mounted on the outside of the container wall 40 are provided for heating the salt 32 to store energy therein as described in connection with the embodiment shown in FIG. 1 discussed hereinabove.

The mineral oil 42 (which is the primary heat transfer fluid) is stored within the container 34 in the space above the salt 32. A coolant (which is in effect, a secondary heat transfer fluid) is circulated through the mineral oil 42 without making direct contact therewith by means of tubing 44. The heated coolant is moved through the tubing 44 to a heat exchanger 46 where heat is removed from the coolant by blowing air with a fan 48 across a coil 50. A pump 52 is provided for circulating the coolant.

Still referring to the embodiment of FIG. 2, a propeller 54 is mounted in the top center of the container 34 for moving the mineral oil 42 over the top surface 56 of the salt 32 in an outward radial pattern. The outward radial pattern of the flow of the mineral oil maintains high temperatures at the wall 40 of the container 34 and thereby prevents salt crystals from building up on the wall 40. Any salt crystals having a tendency to form on the wall 40 or the tubing 44 will melt when heated. The propeller 54 is driven by a motor 58.

The mineral oil 42 is moved over the top surface of the salt 32 to form an emulsion with the salt in layer 43. The mineral oil 42 is heated by the exothermic crystallization of the salt 32 and the salt crystals so formed gravitate to the bottom of the container 34 to maintain the top surface 56 in a liquid state. This process may be repeated by again heating the crystalized salt to above its latent heat of fusion. The heat transfer area may be increased by increasing the size of the emulsion layer 43. Because the mineral oil 42 is enclosed in the container 34 a larger emulsion layer can be tolerated than in the embodiment shown in FIG. 1.

Using the system of FIG. 2, with the salt being sodium acetate a heat transfer coefficient of 170 btu/hr/ft1/.degree. F. and a crystalization rate of 80 cm/hr have been observed.

The present invention may also be practiced by linear movement of the heat transfer fluid over the top surface of the liquid state salt, such as with the system partially shown in FIGS. 3 and 4.

In the system of FIG. 3, the mineral oil 60 is moved from an inlet 62 on one side of the container 64 across the top surface 66 of the liquid salt 68 to an outlet 70 on the other side of the container 64. In other respects, this embodiment is the same as that shown in FIG. 1. An extremely high rate of linear flow is desired in order to create an emulsion in the top layer 71 of the salt 68 with the mineral 60 to increase the heat transfer area. Also, with a higher rate of flow the salt crystals that are formed are of smaller size, thereby providing greater heat release and also preventing an accumulation of salt crystals at the top surface 66.

In the system of FIG. 4 the mineral oil 72 is moved within convection cells 74 formed in the top portion of the container 76, thereby creating a linear motion across the top surface 78 of the salt 80 to form an emulsion layer 82. A coolant that circulates through a heat exchanger (not shown) is moved through the mineral oil 72 without making contact therewith by means of tubing 84, as in the embodiment shown in FIG. 2. The heat flows in free convection currents through the mineral oil 72 within the container 76 and is conducted to the coolant through the tubing 84.

The process of storing energy by heating the salt and retrieving the energy by exothermic crystallization of the salt caused by moving the relatively cooler mineral oil across the salt is the same in the embodiments of FIGS. 3 and 4 as in the embodiments of FIGS. 1 and 2.