Self-Driven Chemical Heat Pipe

Technical Field

The present invention relates to the method of and apparatus for transporting thermal energy ^' and more par¬ ticularly to the transport of heat as chemical energy. More specifically, the present invention relates to a self-driven system for the transport of heat in the form of chemical energy.

Background Art

Various techniques have been employed for trans¬ ferring or transporting thermal energy between a ther¬ mal source and a thermal sink or load. In some in¬ stances, there is relatively little concern for the ^• amount of heat which may be lost, as by radiation, convection or conduction during the transport process, either because the transport distance is small and the loss is slight or because of an excessive amount of thermal energy available for transport. In other in- stances however, it is desired to transfer the heat as efficiently as possible and heatpipes have been espe¬ cially useful in accomplishing that end.. A heat pipe is generally considered a closed-loop two cycle system. In a vaporization/condensation type of heat pipe, there is a rapid heat transfer into the pipe resulting in evaporation of a working fluid therein. ^• he evaporated working fluid builds up sufficient pressure to be trans¬ mitted along the pipe and is then condensed at the other end thereof. The cycle is completed by returning the condensate to the evaporating end by means of capil¬ lary or other action through a wick or other suitable means within the pipe. Typically, the working fluid

may be water, freon, methyl alcohol, acetone or the like. However, because the working fluid is at an elevated-temperature vaporized state while transporting thermal energy from the heat source to the heat sink, it may experience significant radiation, convection and conduction losses to the environment which is normally at a significantly lower temperature. These losses may be considerable if the transport path length is particularly long and therefore such vaporization/ condensation heat pipes are generally used only where the distance over which the heat is to be transported is relatively short or insignificant, as for instance less than ten feet.

In many instances, however, it is desired or re- quired to transport thermal energy over relatively long or significant distances from tens or hundreds of feet to as much as tens or hundreds of miles. To accomplish such transport in a relatively efficient manner, resort has been made to chemical heat pipe systems in which a reactant or reactants within the system undergo a first chemical reaction at the heat source and a second chemical reaction at the heat sink. That first chemi¬ cal reaction is of an endothermic nature in which heat is chemically absorbed by the reaction process and the second reaction is an exothermic reaction during which heat is chemically liberated during the reaction pro¬ cess. Importantly, the reactant and/or reaction prod¬ ucts may exist and be transported at temperatures which do not differ substantially from that of the environ- ment, thereby greatly reducing the potential for ther¬ mal loss from the system. These chemical reactions are generally reversible and are generally effected and/or accelerated by a catalyst at one or both of the reaction sites. Examples of such chemical heat pipes,. -especially

for the delivery of thermal energy over long distances as from a nuclear reactor to remote industrial and/or residential heat sinks, are disclosed in the following U.S. Patents: U.S. 3,198-,710 issued August 3, 1965 to R. B. Long; U.S. 3,558,047 issued January 26, 1971 to Wolfgang et al; U.S. 3,690,550 issued September 12, 1972 to Hilberath et al; U.S. 3,967,676 issued July 6,

1976 to H. S. Spacil; U.S. 4,044,821 issued August 30,

1977 to J. C. Fletcher; U.S. 4,091,864 issued May 30, 1978 to Cocuzza et al; and U.S. 4,169,499 issued Oσto-

^• ber 2, IS IS to LeFrois. The U.S. Patents 4,044,821 and 4,169,499 are examples of systems in which thermal energy from a source at one temperature is transported for use at a substantially higher temperature at the thermal sink. The .remainder of the above cited U.S. patents generally transfer thermal energy from a high temperature heat source to a relatively lower tempera¬ ture heat sink. A variety of reactants have been used in those systems including cyclohexane and/or possibly methylcyclohexane in U.S. Patents 3 ,198,710 and 4,044,821, phosgene in U.S. 3,967,676, urea in U.S. 4,169,499, sulphur trioxide in U.S. 4,091,864 and ethane and/or methane in U.S. Patents 3,558,047 and 3,690,550. The systems disclosed in the aforementioned patents nor- mally provide various supplemental means for transport¬ ing the reactant or reactants and the products of re¬ action between the heat source and heat sink. Typi¬ cally, one or more pumps and/or blowers are provided in the system' s flow path for effecting the requisite reactant transport. These driving or transport mecha¬ nisms represent a cost in the system in terms of the equipment itself and/or the energy consumed to drive the pumps. This cost may be considerable if the sys¬ tem is of large capacity and/or is to transport the reactant for signi icant distances , and in some

instances may be such as to negate the advantages other¬ wise afforded by such a system.

Accordingly, it is a principal object of the pres¬ ent invention to provide a method and apparatus which efficiently transports thermal energy for significant distances. Included in this object is the provision of an improved chemical heat pipe.

It is the further object of the present invention to provide a chemical heat pipe which effects transport of the working medium without inclusion of a conven¬ tional active driving mechanism. Included within this object is the provision of a self-driven chemical heat pipe which utilizes the properties of the reactant and reaction products and system geometry to effect a requi- site flow of the working medium.

It is a still further object to provide a chemical heat pipe in accordance with the invention which is capable of selectively storing and releasing heat re¬ spectively to and from storage. In accordance with the present invention, there is provided a.method and apparatus for the efficient trans¬ port of thermal energy, particularly for relatively long distances in a self-driven system employing a chemical heat pipe. The heat pipe employs reversible endothermic and exothermic chemical reactions at re¬ spectively spaced heat source and heat sink positions therein for chemically transferring heat from a heat source to -a heat sink. The chemical reactant or re¬ actants provide the reversible endothermic/exothermic reactions, particularly in the presence of a suitable catalyst. A preferred type of reaction involves the dehydrogenation and hydrogenation of the reactant and the reaction products respectively, a preferred example of one such reactant being methylcyclohexane. Importantly, the transport of the reactant and the reaction products which comprise the working medium of

the heat pipe, is accomplished without requiring the insertion of conventional pumps into the heat pipe sys¬ tem. Instead, the chemistry of the reaction process and a selective valve technique within the heat pipe system operate to establish a substantially unidirec¬ tional flow of the ^' reactant and reaction products with¬ in the closed circuit of the heat pipe system, which flow is operative to transfer relatively large quanti¬ ties of thermal energy over significant distances. Moreover, operation of the heat pipe results in rela¬ tively little or no loss in the thermal energy trans¬ ferred.

In a preferred embodiment, at least a portion of the flow conduit is occluded between the heat sink and the heat source positions with a liquid medium through which the reactant may flow but which effectively blocks the reverse flow of reaction products. The _liquid medium is preferably caused to flow toward the heat source position, as by gravity, and fills a U-trap or the like, preferably near the heat source position. The liquid medium may preferably be the condensate of the reactant following its recombination from the re¬ action products at the heat sink. External cooling coils may effect the requisite condensation. Alter- natively, the liquid medium may comprise a liquid sol¬ vent into which the recombined reactant is dissolved and through which that reactant is capable of passing to the heat source. The reactant in or from the li¬ quid medium is delivered to the heat source position at which it undergoes vaporization and subsequent en¬ dothermic reaction in the presence of a catalyst.

In a further embodiment of the invention, the en¬ dothermic reaction product, normally in a gaseous form, is selectively diverted to a long-term storage device prior to arriving at the eat sink if no heat is

required at the latter and is selectively retrievable from that storage for use at the heat sink when heat is needed thereat and is unavailable from the heat source. The reaction product constituent or consti- 5 tuents may be condensed and stored in liquid form and/ or absorbed, as by adsorption, and stored. The process may be reversed to return the reaction products to the system for reaction at the heat sink.

lθ ^' 3rief Description of the Drawings

Fig. 1 is a diagrammatic representation of a prior art chemical heat pipe system;

Fig. 2 is a generalized schematical diagram of the self-driven chemical heat pipe of the invention; 15 Fig. 3 is a diagrammatical illustration of a self- driven chemical heat pipe in accordance with one embodi¬ ment of the invention;

Fig. 4 is -an illustration of certain segments of the heat pipe of Fig. 3, shown alinged for graphical 20 purposes;

Fig. 5a is a plot of the temperature of the work¬ ing medium in the heat pipe segments of Fig. 4;

Fig. 5b is a graphical plot of the pressure of the reactant in the heat pipe segments of Fig. 4? 25 Fig. 5c is a graphical plot of the pressure of the reaction product in the heat pipe segments of Fig. 4;

Fig.. 5d is a graphical plot of the combined or total pressures of the reactant and reaction product in the heat pipe segments of Fig. 4; 30 Fig. 6 is a diagrammatical illustration of a self- driven chemical heat pipe in which the heat sink region is lower than the heat source region; and

Fig. 7 is a diagrammatical illustration of a self- driven chemical heat pipe including a storage system in 35 accordance with an aspect of the invention.

O-.-Pl

3est Mode for Carrying Out the Invention

Referring to the drawings, there is illustrated in Fig. 1 a chemical heat pipe system generally in ac¬ cordance with the prior art. The system 110 is cora- prised of a closed loop, or closed circuit, flow path extending between a source of thermal energy, such as heat source 112, and a load for receiving- thermal energy such as heat sink 114. Although the distance between heat source 112 and heat sink 114 may be as short as several feet, the advantages of a chemical heat pipe are most evident and best utilized when the distance is relatively long or significant, as for instance at least tens of feet and generally hundreds of eet to hundreds of miles. Reactant A within re- action chamber 116 adjacent heat source 112 is heated to a temperature by the heat source sufficient for it to undergo an endothermic chemical reaction in the presence of a catalyst 118. The product or products of that endothermic reaction have generally been designated B and will hereinafter be referred to as ^• "reaction product". Typically, reactant A may be a hydrogen-containing compound and will undergo a de- hydrogenation reaction and result in reaction product B. • The thermal energy now stored chemically in re- action product B is transported via system arm 120, with the aid of one or more pumps or blowers 122, to a reaction chamber 124 adjacent the heat sink 114. The reaction product B undergoes an exothermic chemical reaction in the presence of a catalyst 126 in reaction chamber 124 to release thermal energy to the heat sink 114. This reaction recombines the reactant previously dissociated in reaction chamber 116, by hydrogenating the reaction product B and thereby reforming the reac¬ tant A. The recombined reac-ant A is then delivered via system arm 128 with the assistance of one or more

-8- pumps or blowers 130 , to the heat source position with¬ in reaction chamber 116 to begin the thermal transfer cycle anew.

The prior art system 110 of Fig. 1 has been effec- tive for transporting large quantities of chemically stored thermal energy for significant distances with relatively little loss; however, such systems have re¬ quired the assistance of pumps or blowers 122, 130 to effect the requisite circulation or transport of the working medium within the system. These driving agents, i.e. pumps 122, 130, represent elements through which or by which energy is irretrievably lost and which represent an additional capital cost to the system as . well as requiring periodic maintenance. In accordance with the present invention, there is provided a self-driven chemical heat pipe system 10 as broadly illustrated in schematical form in Fi,g. 2. The reactant A undergoes -endothermic reaction at a heat source position in reaction chamber 16 adjacent and in heat-transfer relationship with heat source 12 to form reaction product B, which product is transported via system arm 20 to a heat sink position in reaction chamber 24 adjacent, to and in heat-transfer relationship with heat sink 14. In reaction chamber 24, the reaction product B undergoes exothermic reaction to release stored thermal energy to the heat sink 14. Both the dissociation of reactant A in chamber 16 and the recom¬ bination ^' of reaction product B into reactant A in re¬ action chamber 24 are preferably conducted in the pres- ence of respective catalysts. The reactant A is returned from reaction chamber 24 to reaction chamber 16 to re¬ sume the cycle anew; however, in accordance with the invention the system 10 does not require the pumps or blowers heretofore required for such long distance transport of the working medium.

^' X.-Z£

-9 -

Instead, the chemistry of the reaction process and the reactant pressure difference between the source and the sink, combined with a selective valve process with¬ in system 10 operate to establish a substantially uni- directional flow of the reactant and reaction products within the closed circuit of the heat pipe system, which flow is adequate for the transfer of large quantities of thermal energy over significant distances. In fact, a liquid-filled U-tube 40 appearing in return-arm 28 of system 10 between reaction chamber 24 and reaction chamber 16 effectively establishes a unidirectional flow of the medium within the system and thereby acts as a selective valve in a manner to be hereinafter more fully described. Referring to Fig. 3, there is illustrated a pre¬ ferred embodiment of the self-driven chemical heat pipe 10 of the invention. The reactant A is selected to undergo endothermic reaction ^' and become chemically dis¬ sociated into reaction product B within reaction σham- ber '16 in the presence of a suitable catalyst 18 at the heat source temperature. More specifically, reactant A may be a material which undergoes dehydrogenation in re¬ action chamber 16 , examples of such reactants being cy- clohexane and methlcyclohexane which when dehydrogenated form the respective reaction products benzene and toluene, each accompanied by hydrogen. For the purpose of illustration herein, further discussion will be con¬ ducted in terms of meth lcyclohexane as the reactant A herein, though it will be understood that many other suitable reactants might be' substituted. Methylcyclo¬ hexane undergoes the following reaction by which it is dissociated into reaction product B, toluene and hy¬ drogen, which are recombined by endothermic reaction at the heat sink to reform the methylcyclohexane:

^{' C} 7 ^H 14 * ^C 7 ^H 3 ^{+ 3H} 2

The methylcyclohexane reactant A, which may enter reaction chamber 16 in a liquid state as will be here¬ inafter described, is vaporized by the heat source 12 at high temperature, generally in excess of 600°F at 5 atmospheric pressure. The vaporized reactant then reacts endothermically, as promoted by a suitable catalyst 18, as for instance one from the platinum family, in the presence of the heat from heat source 12. This reaction dissociates, or dehydrogenatεs the 0 methylcyclohexane into reaction product B comprised of toluene and hydrogen in the gaseous state. Reaction product B is transported via arm 20 to reaction chamber 24 in a manner to be hereinafter more fully described. Reaction product B undergoes an exothermic reaction in 5 reaction chamber 24, which reaction is promoted by catalyst 26 therein which may be nickel or the like. This exothermic reaction occurs at a temperature of about 300°F at atmospheric pressure and results in the recombination or hydrogenation of the reaction 0 product to reform the methylcyclohexane reactant A. The exothermic reaction releases chemically stored thermal energy to the heat sink 14.

The methylcyclohexane so reformed in reaction chamber 24 issues from an exit thereof into arm 28. 5 Within arm 28 between reaction chamber 24 and reaction chamber 16 ^" is provided the means for effecting a sub¬ stantially unidirectional flow of the reactant A in such quantity as to afford the requisite transfer of thermal energy without the assistance of conventional D- driving or pumping means. The methylcyclohexane in its gaseous state is rapidly cooled upon exiting from reaction chamber 24 and entering arm 28, as by means of a condensing coil 50 positioned in effective heat transfer relation with the reactant A passing through 5 arm 28. Condenser 50 may conveniently be positioned externally of heat pipe arm 28 in encircling

relationship therewith. The rapid cooling of the gaseous reactant A converts it to its liquid state and effects a rapid decrease in its partial pressure. This condensed reactant A is readily conducted by gravity toward the reaction chamber 16 and accumulates as condensate 42 which fills and thereby occludes arm 28 over a portion of its length. In the embodiment of Fig. 3, the duct of arm 28 is of smaller diameter than that of arm 20 such that most of the length of arm 28 is filled with condensate 42, thereby minimizing re- vaporization of the condensate. A U-shaped portion 44 of the arm 28 traps the condensate 42 for eventual re¬ lease to reaction chamber 16. The trapped condensate 42 occluding arm 28 effects the selective valving 40 of Fig. 2. The condensate 42 at the discharge end of U-Trap 44 is preferably confined, as by dam 46, to prevent its unrestrained discharge into chamber 16 and flooding of the catalyst surface 18. In fact, the discharge end of U-trap 44 is preferably sufficiently close to the -heat source 12 as to be in some degree of heat transfer relationship therewith such that the reactant A of condensate 42 -is readily vaporized from the surface 47 of the condensate pool, thereby serving to supply reactant for the endothermic reaction in chamber 16.

Consideration will now be given to the flow dyna¬ mics of the working medium. Reference is ^' made to the segments of system 10 illustrated in Fig. 4 and to the respective graphs of temperature and pressure appearing in Figs. 5a-5d aligned under Fig. 4. The condensate 42 of reactant A extends from near con¬ denser 50, which is positioned relatively close to the exit end of reaction chamber 24 to chill the gaseous reactant A thereat, converting it to its liquid state and thereby rapidly reducing its partial pressure, to

a position in relative proximity with the heat source 12 and reaction chamber 16.

Referring to Fig. 5a, there is illustrated a plot of the relative temperature of the working medium at various locations along the heat pipe system segments illustrated in Fig. 4. Reaction product B, i.e. toluene and hydrogen, may have a temperature near or somewhat above ambient as it approaches heat sink 14. Upon entering reaction chamber 24 and undergoing exo- thermic reaction, the temperature of the working medium increases to that which will be provided to the heat sink 14. The condenser 50 near the exit from reaction chamber 24 rapidly chills the recombined- gaseous reactantA to such extent that it condenses into a liquid. After leaving the cooling region of condenser 50, the working medium, most of which is condensed reactant A, is conducted, as by gravity, to the pool of ^' condensate 42 which is in turn conducted, as by gravity, toward chamber 16. This distance may range from tens or hundreds of feet to many miles and the temperature of the working medium therealong may be near that of the ambient. Because of the proximity of condensate 42 to the- heat source 12 at its one end, it begins a gradual increase in temperature with rela- tively rapid vaporization at its downstream surface. Within the reaction chamber 16, the endothermically- reacting working medium undergoes a rapid rise in temperature to a level approximating that of the heat source 12. In the illustrated system, this tempera- ture peak is somewhat greater than that experienced in the reaction chamber 24. Upon exiting from reac¬ tion chamber 16 , the reaction product B cools or preferably is cooled by a heat recovery system (not shown) to a temperature approximating that of the ambient for the long distance transport to -the heat

sink 14.

Referring to Fig. 5b, there is graphically illus¬ trated the partial pressure of reactant A at various points along the heat pipe system segments illustrated in Fig. 4. The partial pressure of methylcyclohexane is greatest within reaction chamber 24 wherein reac¬ tion product B is converted into gaseous reactant A. Looking leftward in Fig. 5b, a very gradual decline in the partial pressure of reactant A will be experienced because of the depletion of A upstream. However, in the downstream direction from reaction chamber 24, the partial pressure of reactant A is seen to drop dramati¬ cally as a result of the condensation effected by condenser 50, such that flow is induced in the desired direction. This partial pressure of reactant A remains relatively low so long as the reactant is in its condensed state, and is seen to undergo a gradual rise as it approaches the heat source 12. However, when reactant A exits from the pool of condensate 42 at its downstream side 47, it is immediately vaporized, is reacted endothermically and is converted to reaction product B in reaction chamber 16. As reactant A is converted to reaction product B, its partial pressure decreases. This decreased partial pressure of reactant A holds through most of the transport path through arm 20 and begins to gradually increase as the region of reaction chamber 24 is approached.

Referring to Fig. 5c in which the partial pressure of reaction product B is illustrated along the segments of the heat pipe system illustrated in Fig. 4, it will be seen that the partial pressure of the reaction product is usually substantially greater than that of

the partial pressure of reactant A, as for instance by a factor of 10 to 1, and has a somewhat inverse relationship as a function of location within the sys¬ tem. Specifically, referring to that location along the graph of Fig. 5c which corresponds with the heat source reaction chamber 16 , it is seen that the partial pressure of reaction product B is at a maximum during its formation within that reaction chamber, is reduced slightly upstream toward the stream of condensate as a consequence of the "rightward" flow of reactant A, and slowly declines through arm 20 toward reaction chamber 24. In reaction chamber 24, reaction product B is exo- thermically reacted to reform the reactant A and thus undergoes a relatively rapid reduction in partial pres- sure because of its removal thereat from the system. This state of reduced partial pressure for reaction product B extends to the upstream side of the pool of condensate 42. " It will- b .observed that reaction product B is substantially entirely excluded from the region occluded by condensate 42 and thus exhibits no partial pressure thereat.

A graph of the combined or total pressures of reactant A and reaction. product B is illustrated in Fig. 5 along those segments of the heat pipe system illustrated in Fig. 4. Because the partial pressure of reaction product B is usually so much greater than that of reactant A, the plot of total pressure- within the system generally follows that of the partial pres¬ sure of reaction product B illustrated in Fig. 5c. Thus, it is seen that the pressure differential across the pool of condensate 42 resulting from the partial pressure of reaction product B, and thus the total pressure of 3 plus A is significant, with the greater pressure on the downstream side of the pool in the reaction, chamber 16. This pressure .differential is manifested by the column of condensate 42 in arm 28 having a level higher than that of surface 47 within

the reaction chamber 16. Because the reaction product B cannot flow back through the liquid, i.e. counter¬ clockwise as viewed in Fig. 3, the relatively large pressure existing in reaction chamber 16 serves to drive the working medium clockwise toward the heat sink. Moreover, the- largest pressure gradients ex¬ hibited by the reactant A are exhibited immediately downstream of reaction chamber 24 in the region of condenser 50 and within reaction chamber 16. These gradients provide the driving force to the reactant A clockwise around the system as viewed in Fig. 3. Thus, it is seen that the reaction product B is driven by a pressure gradient from reaction chamber 16 to reaction chamber 24 via arm 20, and the recombined reactant A is driven or flows from reaction chamber 24 to reaction chamber 16 through arm 28 b me.ans of the pressure gra¬ dient effected by condensing the reactant and by gravi¬ tational flow of the condensed reactant. The liquid valve 40 effectively prevents the reverse flow of re- action product B.

In view of the foregoing description, it will be seen that a unidirectional flow of the working medium (reactant and reaction product) is established in sys¬ tem 10, which flow is of sufficient capacity to trans- port substantial quantities of thermal energy a sig¬ nificant distance with minimal losses. The enthalpy removed from the working medium by the condensing action of condenser 50 is quite small compared with the heat of reaction and in any case is recovered as additional cooling at the heat source.

Moreover, the aforedescribed embodiment is in¬ herently self-balancing. For instance, if the temper¬ ature at the heat source 12 increases, the rate of vaporization of reactant A from condensate pool surface

47 correspondingly increases, as does its rate of re¬ action in chamber 16. This acceleration carries through the entire system, resulting in increased thermal energy being delivered to heat sink 14. The volume of condensate pool 42 will remain relatively constant, though increased through-put may be re¬ flected by an increased partial pressure of reaction product B on surface 47 which would alter the relative levels of the ends of condensate pool 42. Although the foregoing embodiment utilized the condensed reactant A as the sole constituent of liquid pool 42, it would be possible, either alternatively or supplementally, for all or some of that liquid to be a solvent into which the reactant A is dissolved. The reactant A would then be transferred in solution through the arm 28 and U-trap 44 to surface 47 where it would vaporize and react as heretofor described. Such arrangement does not necessarily require the gaseous reactant A leaving chamber 24 to be chilled and condensed, as in the aforedescribed embodiment. Instead, by dissolving the gaseous reactant A in a liquid solvent, the partial pressure of the reactant is greatly reduced and it also enters the liquid form, as desired. Although the condensed or liquified reactant in return arm -28 preferably occludes that arm for all or most of its length in order to minimize revaporization of the condensate, some voids may exist but- the total liquid pressure head must be maintained.

In those situations in which the heat source is located somewhat higher than the heat sink, on the order of tens of feet, it may not be possible to use gravity to aid in the transport of the liquid-form reactant for the entire distance to the heat sink re- action chamber. However, as illustrated in Fig. 6, in

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which elements similar to like elements in Fig. 3 are similarly numbered, it can be seen that the reactant condensate 42 is readily conducted over the relatively long transport arm 28 to a primary reservoir 60 in which it is collected. The condensate 42 occludes arm 28, preferably over all or most of its length, to provide the selective valve 40. However, because the level of condensate 42 in reservoir 60 is below that of the heat source reaction chamber 16 , that condensate- is then vaporized, as by heat exchange re¬ sulting from passing the heat source end of arm 20 through the condensate in the reservoir. This also has the desirable effect of cooling the reaction product for transport. The reactant is thereby vaporized, forcing it upwardly through relatively short arm 28' to another region of condensation provided by condenser 50' encircling arm 28'. The gaseous reactant is again condensed and is collected as condensate 42 in a secondary reservoir 62 in proximity with reaction chamber 16. A U-trap 44 with reservoir 62 provides a desired secondary valve, and the liquid reactant at surface 47 may be vapor¬ ized and endothermically reacted as previously described. Some control of the rate of thermal transfer to the heat sink may be obtained by the inclusion of one or more selectively variable mechanical con¬ trol valves (not shown) in the flow path and/or by tapering the wall of dam 46 to vary the area of condensate surface 47, and thereby its rate of va¬ porization, as a function of the level of the con¬ densate thereat.

In accordance with a further aspect of the in¬ vention, storage means may be provided for storing

• -18-

the potential thermal energy of the reaction product for an extended period and selectively releasing it to the heat sink region for exothermic reaction when needed. Referring to Fig. 7, there is diagrammati- cally illustrated a self-driven chemical heat pipe system in part similar to that described with refer¬ ence to Fig. 3, which similar portions are given the same reference numbers and will not be described in detail. However, in Fig. 7 a storage reservoir 70. is provided for temporarily storing the reaction product for extended periods and for releasing the stored product when it is required for reaction at the heat sink 14. With no demand for heat but with excess high temperature waste energy, the endothermic section of the heat pipe can be operated with the reaction product transferred to storage reservoir 70. In the methylcyclohexane/toluene system, the toluene may be condensed by condenser 72 and stored as a liquid 74 at room temperature and the hydrogen adsorbed on suitable metal surfaces 76. When heat is needed in the low temperature heat sink, an acti¬ vating heat source can be used to boil off toluen , to desorb hydrogen and to promote the hydrogenation reaction. More specifically, the primary system includes a primary flow circuit comprised of arms 20 and 28 and the storage system includes.- a secondary low circuit comprised of arms 20', 20" and reservoir.70 selectively connectable into and disconnectable from the primary system. Valves 80 and 82 are located in arms 28 and 20 respectively, and valves 84 -and 86 are located in arms 20' and 20" respectively. Arm 20' connects to arm 20 intermediate valve 82 and reaction chamber 16 , and arm 20" connects to arm 20 intermediate valve 82 and reaction chamber 24. The valving is determined

- - by the temperatures of the source 12 and sink 14, and may be con.trolled manually or preferably automatically in response to sensed thermal conditions.

Under conventional operating conditions, in which 5 heat source 12 exhibits ^' a high temperature and heat sink 14 exhibits a low temperature, valves 80- and 82 are open and valves 84 and 86 are closed, such that the primary system is operated in the heat transfer manner as hereinbefore described.

10 However, when the source temperature is high and the sink temperature is also relatively high, heat is available to promote the dehydrogenation reac¬ tion but that heat is not required in the sink.- There¬ fore, toluene and hydrogen are stored by having valves

15 80 and 84 open, and valves 82 and 86 closed. As re¬ action product transits arm 20' through open valve 84, its toluene is condensed to its liquid form by ^' con¬ denser 72 in heat transfer relation with arm 20'. The liquid toluene 74 is collected and stored in the bot-

20 torn of reservoir 70 and the gaseous hydrogen may be adsorbed on suitable metal surfaces 76. Alternatively, valve 86 might be left open, the adsorbing surfaces 76 omitted, and the hydrogen -gas pressure buildup can be accommodated using most or all of the volume of the

25 entire primary and storage system.

Conversely, when the temperatures of both the sink and the source are low and heat is required by the sink, the stored toluene and hydrogen can be released to the exothermic hydrogenation section by closing valves 80,

30 82, and 84, opening valve 86 and supplying "starting" heat. The "starting" heat is provided by a heater 90 having coils 92 adjacent the catalyst surface of re¬ action chamber 24, coils 94 adjacent the adsorption- plates 76 and coils 96 adjacent the liquid toluene 74.

"•S Heater 90 supplies sufficient heat ro coils 96 to

-20- evaporate toluene, to coils 94 to desorb hydrogen and to coils 92 to initiate the exothermic reaction. Once that exothermic reaction has begun, it will be self- sustaining, and no supplemental heat is requiredthereat any longer; however, such "starting" heat is still re¬ quired at coils 94 and 96 so long as the stored com¬ ponents are to be discharged from reservoir 70. As previously described, the exothermic reaction in cham¬ ber 24 produces gaseous recombined reactant which -is condensed by condenser 50 and fills much of arm 28 as condensate 42. However, because the temperature at heat source 12 is too low to -suf iciently support the endothermic reaction in chamber 16 , it may be desir¬ able to provide arm 28 with sufficient storage capaci- ty for all of the liquified reactant which could re¬ sult from a complete discharge of reservoir 70. Ac¬ cordingly, a reservoir 98 is provided in arm 28 closely downstream of condenser 50 for providing such storage capacity and which is preferably of sufficient diameter to retain the additional condensate 42 with relatively little increase in the pressure head in arm 28.

Although this invention has been shown and de¬ scribed with respect to detailed embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail thereof may be made without departing from the ^' spirit and scope of the claimed invention. For instance, the arms 20 and 28 might each comprise plural parallel conduits rather than the singular conduits illustrated; however, their respective functions would not be altered.