DE2633974A1 - METHOD AND DEVICE FOR THERMAL ENERGY STORAGE - Google Patents

Description

United States Energy Research And Development Administration, Washington, D.C. 20545, V.St.A.

Method and device for thermal energy storage

The invention relates to a method and a device for storing energy. In particular The invention relates to a method and a device for converting thermal energy into potential, chemically bound energy. In particular, the invention relates to a method and apparatus for storage and use of thermal energy from solar sources.

The rising cost of energy from fossil fuels and nuclear fuel has made the study of alternative energy sources stimulated.

Over a quarter of the world's energy is used to heat and cool buildings, using highly concentrated (high temperature) energy sources for this purpose are not required. The use of solar energy

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for heating and cooling could help save nuclear fuel and 'serve fossil fuels so that the latter can be used where they are more difficult are to be replaced, for example as raw materials for chemicals. The changing availability of However, solar radiation makes energy storage necessary for those times when solar radiation is not available is. Usual methods of heat storage are based on the effects of heat capacity or phase change. Although water has one of the highest specific heats of any readily available material, so it has a rather small volumetric energy storage capacity and makes insulated storage tanks necessary. The use of pebbles has also been suggested as a cheap way to store energy, but with the same disadvantages as with water and, moreover, such layers a significant one have lower heat capacity. Salt hydrogenation-dehydrogenation equilibria have a tendency to after many cycles Oversaturation, and the stratification disturbs the reversibility.

Hydrogen can exothermically react with some intermetallic (intermetallic) compounds to form decomposable hydrides. A method using such hydrides is described in the following reference: G. Libowitz at page 322 of "Proc. Of the 9th Intersociety Energy Conversion Engineering Conference, San Francisco, August 1974. In one embodiment of this method, a metal hydride is placed on the roof of a residential house is placed in a sealed metal container that is pipe connected to a hydrogen storage tank in the basement of the house. During the sunshine periods, the metal hydride is ^{heated to approximately 200 °} F and the hydrogen gas resulting from the dissociation is stored in the tank When the outside temperature drops, the hydrogen is released from the tank and flows into the

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~ "3 -,

container back on the roof, where the hydrogen exothermic reacts with the free metal to form the metal hydride, releasing thermal energy which can be used to heat the house.

The present invention seeks to provide a method and apparatus for energy storage that avoids the need to store free hydrogen as a gas. Instead, according to the invention, two hydrogenatable metals are used, and the hydrogen gas is in a circuit between the two metals. By the inventive method, a first metal hydride is heated to an effective decomposition temperature, the first decomposition hydride a pressure of 100-2280 mm Hg at a temperature of 100 - 300 ⁰ C and has a heat of formation of about 12-17 kcal per mole of hydrogen, whereby the hydride absorbs the thermal energy and decomposes or decomposes into a first, hydrogenatable metal and hydrogen gas, wherein the hydrogen gas is compressed to a pressure of 760 2 280 mm Hg and wherein the compressed hydrogen gas is brought into contact with a second hydrogenatable metal, to form a second metal hydride, while the second metal is held at a temperature below the effective decomposition temperature of the hydride under formation, and wherein the second hydride einaiDissoziationsdruck 760 2 280 mm Hg at a temperature of about 0 - 25 ⁰ C and possesses a heat of formation that is in the range of 25 75% of the Heat of formation of the first metal hydride per mole of hydrogen, whereby thermal energy is released from the second hydride, which is recovered. By bringing the first hydrogenatable metal to a temperature

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cools below the effective dissociation temperature
and while heat is supplied to the second metal hydride to maintain it at an effective decomposition temperature, the second hydride absorbs thermal energy and is decomposed, releasing hydrogen gas which is then brought into contact with the first hydrogenatable metal, to form the first metal hydride, thereby releasing additional thermal energy which can then be recovered.

The method and apparatus according to the invention can be used together with any thermal energy source which generates a temperature which is sufficient to cause the dissociation of the first metal hydride. The method according to the invention and the device according to the invention are suitable for the utilization of little waste heat that would otherwise be passed on to the atmosphere;
Exhaust gases can be used. In particular, however, the invention is
useful together with a solar energy collector. One such device is the radiant energy collector
according to DT OS 2 533 530.

The present invention has set itself the goal of a
Method and device for energy storage
specify, in particular for the storage of thermal energy. The thermal energy is preferably generated by solar radiation.

Further preferred embodiments of the invention result
from the claims and from the description of exemplary embodiments with reference to the drawing; the drawing
shows a block diagram of the device according to the invention.

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According to the invention a first metal hydride having the formula Mg ₃ NiH ₄ to about 150 ⁰ C is heated, whereby the hydride is decomposed or disassembled, in a He-.stes hydrogenatable metal Mg ₃ Ni and hydrogen gas, wherein the hydrogen gas at about 1 520 mm of mercury is compressed and in this state with a second hydrogenatable metal with the formula LaNi _n . is brought into contact while the second metal is kept at a temperature below the effective decomposition temperature of the forming hydride, whereby the hydrogen reacts with the second hydrogenatable metal and forms a second metal hydride with the formula LaNi ₁ -H _6-7 , while thermal Energy is released and regained. The Mg ₀ Ni can then be cooled and maintained at a temperature below about 50 C while _{heat is applied to the LaNi H 6-7 in} order to keep the LaNi ₅ Hg _-7 at an effective decomposition temperature, whereby LaNi ^ H _6-7 is decomposed to LaNi ₁ - and hydrogen gas, and the hydrogen gas is brought into contact with the Mg “Ni, whereby Mg NiH. forms as thermal energy is released and recovered.

The first metal hydride can be heated either directly or indirectly, as long as the effective decomposition temperature, which depends on the particular hydride used, is achieved. For example, the hydride can be placed in a sealed container so constructed is that it serves as a flat plate collector that is directly irradiated by sunlight; or but the container can be heated by any other thermal energy source, such as by the residual heat from fireplace gases. Alternatively, the hydride can be used indirectly using heat transfer media,

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such as hydrocarbon oil, a water and glycol mixture or a silicone oil are heated / the heat here from a heat source such as a radiant heat collector to the metal hydride to be led.

The metal hydrides that can be used with the invention depend on the temperatures available for the decomposition of the first metal hydride. Also important are the effective dissociation pressures or temperatures of the hydride, which are the driving force of the reaction, and the heat of formation, which determines the amount of heat that the system can store. The "effective" dissociation pressure or temperature is that pressure or temperature at which the metal to be decomposed emits hydrogen gas at a usable rate. With currently available solar energy collectors or concentrating devices, temperatures in the range of 130 - 185 ° C are available. It is thus preferable that the first metal hydride has an effective decomposition pressure of 100-2 280 mmHg at a temperature of 100-300 ° C and a heat of formation of 12-17 kcal per mole of hydrogen. Mg _{3 NiH 4} , which has a decomposition pressure of about 150 mm Hg at 150 ° C and a heat of formation of about 15.4 kcal per mole of hydrogen, is a satisfactory first metal hydride, but other hydrides meeting these general criteria are also used can be.

It is preferable that the second metal hydride have an effective decomposition pressure of 760-2 280 mm Hg at about 0-2 5 ° C and a heat of formation of 25-75% of the heat of formation of the first metal hydride

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a heat of formation of 50% is preferred. Consequently

is LaNi ^ Hg, which has a decomposition pressure of about T 520 mm Hg at about 25 ° C and a heat of formation of 7.26 kcal per mole of hydrogen
satisfactory second metal hydride, as is also the case for SmCo ₁ -H ₀ [. and FeTiH _{1 Q [} - applies. It can also
there are other metal hydrides that can be used.

It should be noted that the metal hydrides are not stoichiometric and have a single crystal structure over a wide range the hydrogen composition maintained. The hydrogen to metal ratio therefore changes characteristically over a wide range. ' Differences in the Metal hydride formulas with regard to the hydrogen composition are not intended to represent different metal hydrides.

It should also be noted that the term "metal" as used herein does not apply only to individual metals but also to any metal alloy systems that meet the above criteria for metal hydrides and correspond to hydrogenatable metals.

Depending on the temperature available for the decomposition of the first metal hydride, it may be necessary to compress the hydrogen gas to a pressure
sufficient for the hydrogen to react with the second hydrogenatable metal at ambient temperatures. The exact pressure increase required depends on the composition of the second hydrogenatable metal, which determines the amount of pressure required for hydrogenation at or near ambient temperature. LaNi- requires a hydrogen pressure of 1,520 - 2,280 mm Hg at around 25 ° C,

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to form LiNi ₅ H _6-7 . Since MgNiH ₄ dissociates at 150 ° C with a pressure of about 150 mm Hg, it is necessary to pressurize the hydrogen to at least about 1,520 mm Hg while. at 300 ° C the MgNiH. dissociated at a pressure of about 1,520 mm Hg and little or no additional pressurization of the hydrogen would be necessary.

Since the heat of formation of the second hydride is approximately half the heat of formation of the first hydride, the hydrogenation of the second hydrogenatable metal releases an amount of thermal energy that is approximately equal to half the thermal energy input into the first metal hydride. Thus, in order to release one mole of hydrogen from the first metal hydride, one needs the input of about 15 _f 4 kcal of thermal energy, during the hydrogenation of LaNi ₁ . makes about 7.3 kcal of thermal energy available for recovery by one mole of hydrogen. In order to reverse the process and obtain the "thermal energy" which is now stored as potential, chemically bound energy in the first hydrogenatable metal, the first hydrogenatable metal must be brought to a temperature below the dissociation temperature of the hydride, which is approximately 50 ° C. for MgNi , cool and heat the second metal hydride to the decomposition temperature, which for LaNi ₅ Hg _{-7 is} above approximately ⁰ ° C. Because of the higher decomposition pressure of the second metal hydride, hydrogen flows to the first hydrogenatable metal and combines with it to form the first hydride, and while releasing thermal energy which can then be recovered, the amount of heat released by hydrogenation of the first hydrogenatable metal can be controlled by controlling the amount of hydrogen available for recombination.

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When the second metal hydride is decomposed, thermal energy must be devoted to the hydride at a temperature above the effective decomposition temperature for that particular metal hydride. Because this effective temperature is low, thermal energy can be drawn from the essentially unlimited reserves of thermal energy that is present in the environment can be extracted.

In the same way it is important that the temperature of the hydrogenatable metal, since the metal is replaced by the hydrogen is touched, at a temperature below the effective decomposition temperature of the hydride being formed is held to complete the hydrogenation reaction and extract all of the available thermal energy. This is achieved as part of the thermal energy recovery step.

The recovery of the thermal energy generated by releasing the hydrogenation of either the first or the second hydrogenatable metal and maintaining it the temperature below the effective decomposition temperature can be achieved by any conventional heat exchange process, which are known to the person skilled in the art, take place, for example, by using a heat exchange medium such as, for example a liquid or gas through which hydrogenatable metals can flow and that recovered from the medium Heat used for space heating purposes.

Reference is now made to the device according to the invention shown in the drawing. There is a thermal one Energy source 10 shown, for example, a solar energy concentration device can be, and which thermal energy transfers to a heat transfer medium,

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which by a pump 14 through a heating loop 12 is pressed. When the heat transfer medium passes through a first metal hydride bed 16 there is thermal Energy from the metal hydride and decomposes it into a first hydrogenatable metal, which in the first Bed remains and hydrogen gas emitted from the bed flows through a gas line 18, the gas being compressed by a compressor 20. The squeezed Hydrogen gas continues through line 18 and flows through a flow controllable control valve 19 to a second metal hydride bed 22, which contains a second hydrogenatable metal, which with the hydrogen reacts and forms a second metal hydride, thereby releasing thermal energy from the bed is recovered that one is a heat recovery fluid by pump 25 through a first heat recovery loop 24.

By cooling the first metal hydride bed 16 to a Temperature below the effective dissociation temperature for the metal hydride, during the second metal hydride bed 22 Heat is added to keep the hydride at a temperature above the effective dissociation temperature to hold, the second hydride bed decomposes into a second hydrogenatable metal and hydrogen gas. The flow The hydrogen gas from bed 22 is valve controlled when it returns through gas line 18 to the first hydrogenatable metal bed 16 runs where the hydrogen with the metal reacts and forms the first metal hydride, while thermal energy is released thereby is recovered by a pump 28 a heat recovery fluid circulated through a second heat recovery loop 26.

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~ 11 -

The following example is intended to explain the invention, but is not to be understood as restrictive.

example

Using the apparatus described above, silicone oil is circulated through the radiant energy collector / where the silicone oil is heated to about 150C and then passed through a bed containing one foot of Mg ₂ NiH. which is heated to about 150 ° C, and wherein 1,300 moles of H "are released at 150 mm Hg decomposition pressure while the decomposition takes place into the hydrogenatable alloy Mg Ni. This requires approximately 80,500 BTU of energy. The hydrogen gas is compressed by a compressor to a pressure of approximately 1,520 mm Hg, which requires approximately 7,000 BTU of energy, and the compressed hydrogen is passed over a bed of LaNi ₅ which is hydrogenated to LaNi ₅ H ₆ with approval of 38,000 BTU, which can be used to warm a house or a room during the day. If solar energy is no longer available during the night, the Mg ~ Ni bed is cooled to a temperature below its effective dissociation temperature or below approximately 50.degree. The bed of LaNi ₅ H _6-7 , which is at about 25 ° C, is above its dissociation temperature; In this way, the LaNi ₅ H _6-7 now decomposes to LaNi ₅ and hydrogen gas, which is at an effective pressure for recombination with the Mg_Ni alloy and releases 80,500 BTU, which are used to heat a house or room during the night can. Since the dissociation temperature and pressure of LaNi ₅ H _{6-7 are} so low, the heat can be used to maintain the

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Hydrids at a temperature above its effective dissociation temperature at temperatures down to at about 0 ° C to release the hydrogen. By using a control valve to interrupt or regulate the amount of hydrogen available for reaction with the Mg Ni can be used for the Space heating available amount of heat can be controlled in this way.

From the above description and the example mentioned it can be seen that the method according to the invention and the device according to the invention effectively store thermal energy, in particular is an effective way of utilizing and storing thermal energy from solar sources.

It should be noted that the invention is not limited to the metal hydrides specifically specified here, but rather that any other metal hydrides can be used which correspond to the inventive concept.

The use of metal hydrides - such as Mg NiH, for example - falls in particular. or other more suitable metal hydrides under the invention, i.e. hydrides used with solar collectors such as power station towers, where temperatures of 500-700 ° C are expected. The two-metal hydride system described here can also be used for thermal energy storage in these collectors which operate at higher temperatures.

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Claims (1)

- 13 claims (j \ A method of storing thermal energy in which a first metal hydride is heated to an effective decomposition temperature, and the hydride absorbs thermal energy and decomposes to form a hydrogenatable metal and hydrogen, and the hydrogen is stored as a gas thereby characterized in that the first metal hydride has a decomposition pressure of about 100-2280 mm Hg at a temperature of 100-300 ° C, and a heat of formation of about 12-17 kcal per mole of hydrogen, and the hydrogen released by the first metal hydride to a pressure of 760 - 2281 mm Hg is compressed before it is brought into contact with a second hydrogenatable metal while the second metal is kept at a temperature below the effective decomposition temperature of the hydride being formed, in which way a second hydride is formed, namely with a decomposition pressure from 760 - 2280 mm Hg at a tempe Temperature of about 0-25 ° C and a heat of formation of 25-75% of the first metal hydride per mole of hydrogen, with thermal energy being released and recovered in the formation of the hydride. 2- .. The method according to claim 1, characterized in that that the following steps are also provided: a) cooling the first hydrogenatable metal to a temperature below the effective decomposition temperature of the metal with hydrogen; b) adding heat to the second metal hydride to make it Maintaining hydride at an effective decomposition temperature, whereby the metal hydride absorbs thermal energy and decomposes to form a second hydrogenatable metal and hydrogen gas; 709809/1 13S c) bringing the hydrogen gas into contact with the first hydrogenatable metal, the first metal being brought to a temperature is maintained below the effective decomposition temperature of the forming hydride, thereby forming a metal hydride and thermal energy is released, d) recovery of thermal energy. 3. Process according to Claims 1 and 2, characterized in that the first metal hydride is Mg ₃ NiH ₄ and that the second metal hydride is LaNi ^ H ,, or SmCo ₁ -B ₀ _ or FeTiH _{1 QC} -. 4. The method according to claim 4, characterized in that the second metal hydride is LaNi ₅ Hg _-7 , and that the hydrogen gas is compressed to at least 1520 mm Hg. 5. Device for storing thermal energy with the following elements: a source of thermal energy (10), a heat transfer loop (12) comprising a circulating Has heat transfer medium for removing thermal energy from the source, a first bed of a first metal hydride (10) for receiving the thermal energy from the transmission medium, the hydride being decomposed by the thermal energy to form hydrogen and a first hydrogenatable metal, and the first hydride having a dissociation pressure of approximately 100-2280 mm Hg at a temperature of 100 - 300 ⁰ C, and a heat of formation of about 12-17 kcal per mole of hydrogen, a line (18) to conduct the hydrogen away from the first bed, marked by a compression (20) in the line to release the hydrogen gas compress to a pressure of 760 to 2280 mm Hg, 709809/1138 ■ valve means in the conduit compressed to a flow Allow hydrogen from compression while the hydrogen flows in the opposite direction is controlled, a second bed of a second hydrogenatable metal (22) for reaction with the compressed hydrogen gas for Formation of a second metal hydride with release of thermal energy, the second hydride having a dissociation pressure of 760-2280 mm'Hg at a temperature of 0-25 ° C as well as a heat of formation which 25-75% of the heat of formation of the first metal hydride per mole of hydrogen is, and a heat recovery loop (24) in the second bed for removing thermal energy generated by the second Bed is released. 6. Apparatus according to claim 5, characterized in that that the first bed contains heat recovery means to "reduce the thermal released in the first bed Dissipate energy. 7. Apparatus according to claim 6, characterized in that the first metal hydride is Mg ₂ NiH. and the second metal hydride is LaNi ₅ Hg _-7 or SmCo ₅ H _{3 5} or FeTiH _{1 gj} . 8. Apparatus according to claim 7, characterized in that that the thermal energy source is a solar energy collector. 709809/1136 Blank page