EP0041244B1 - Apparatuses for the energy-saving recovery of useful ambient heat or of waste heat - Google Patents

Abstract

A process for the energy-saving recovery of useful heat from the environment or from waste heat with the use of a reversible chemical reaction comprising, charging and discharging alternatingly and successively by pressure variation with hydrogen two vessels which are interconnected by lines and filled with a metal hydride and the hydride-forming metal and removing as useful heat the heat of compression and of hydride formation thereby liberated by heat exchange and replacing consumed heat of expansion and hydrogen evolution of the hydride by heat exchange with the environment or by waste heat.

Description

The present invention relates to devices for carrying out a method for the energy-saving production of useful heat from the environment or from waste heat using a reversible chemical reaction.

A number of heat pumps are already known which operate according to the compression or absorption principle. Easily vaporizable liquids with low vapor pressure, such as halogenated hydrocarbons or ammonia, are mechanically or thermally compressed to the point of liquefaction, with the heat of condensation of the respective working substances being obtained as heating energy or useful heat. The useful heat consists of the enthalpy of vaporization, which is denied by environmental energy, and the heat of compression, which comes from the mechanical or thermal drive. Thus, only changes in the state of matter take place, chemical changes are deliberately avoided.

The performance figures, i.e. the ratio of useful heat given off to the auxiliary energy used is between 2 and 4 in the case of electrically operated compression heat pumps. In the case of absorption heat pumps, which are basically operated with fossil energy, this number is approx. 1.3. In comparison, an oil or gas boiler has a coefficient of performance of approximately 0.8.

• 1. Volle Reversibilität der chemischen Reaktion, was gleichbedeutend mit hoher Zykluslebensdauer der Arbeitsstoffe ist.
• 2. Möglichst hohe Reaktionsenthalpie, verbunden mit der Zusatzforderung, daß der energieaufnehmende Prozeß bei möglichst tiefer Temperatur abläuft (Nutzung von Umweltenergie nicdriger Energiestufe) und der energieliefernde Prozeß Wärmeenergie auf einem Temperaturniveau liefert, welches ausreicht, um zumindest Gebäudcheizungen betreiben zu können.
• 3. Der reaktionskinetische Ablauf muß den gestellten Anforderungen voll genügen; d.h. das System darf nicht zu langsam arbeiten.
• 4. Gute Wärmeleitfähigkeit der Arbeitsstoffe, um den Wärmeaustauschprozeß möglichst wenig zu behindern.
• 5. Ungiftigkeit der Arbeitsstoffe, um bei etwaigen Leckagen des an sich voll verkapselten Wärmepumpensystems keine gesundheitlichen Gefahren heraufzubeschwören.
• 6. Vertretbarer Preis der Arbeitsstoffe.

Due to the general shortage of energy, thermochemical heat pumps have recently become interesting, in which attempts are made to exploit the absorption or dissipation of energy in a reversible chemical reaction. The advantage of thermochemical heat pumps over the heat pumps used hitherto is that in order to maintain the enthalpy of a chemical reaction, generally much smaller amounts of auxiliary energy are required than for pure compression and / or condensation processes. Theoretically, this means that thermochemical heat pumps should be capable of higher performance figures than the known heat pumps that operate on a purely physical basis. So far, in particular the alkaline earth metal chloride hydrates or ammonia have been investigated as reversible chemical reactions. These systems appeared particularly interesting in connection with the storage of heat, for example solar energy; see. DE-A 27 58 727 and DE-A 28 10 360. These systems have practically no significance as yet, since a number of requirements have to be met which are not or only incompletely met by these chemical systems:

• 1. Full reversibility of the chemical reaction, which is synonymous with a long cycle life of the working materials.
• 2. The highest possible enthalpy of reaction, combined with the additional requirement that the energy-absorbing process takes place at the lowest possible temperature (use of environmental energy at different energy levels) and the energy-supplying process delivers thermal energy at a temperature level which is sufficient to at least be able to operate building heating systems.
• 3. The reaction kinetic sequence must fully meet the requirements; ie the system must not work too slowly.
• 4. Good thermal conductivity of the working materials in order to hinder the heat exchange process as little as possible.
• 5. Non-toxicity of the working materials in order not to pose any health risks in the event of any leaks in the heat pump system, which is fully encapsulated per se.
• 6. Reasonable price of the working materials.

The alkaline earth metal chloride hydrates no longer dissociate and evaporate sufficiently at temperatures below freezing. They can therefore only be operated with the aid of heat from the ground, from running water or groundwater, which considerably limits the area of application. In any case, the ambient air available to everyone cannot be used as an energy source below freezing.

Furthermore, the thermal conductivity of the previously proposed working materials is low, so that there are considerable problems in the heat exchange processes. At least one needs very large heat exchange surfaces with the previously proposed working materials, which leads to undesirably large-volume aggregates.

Material and energy transport pose further significant difficulties. The rate of the reaction slows down to the extent that water-free or ammonia-free salts coat themselves with layers of salt hydrate or ammonia. For this reason too, a large distribution of the working materials is unavoidable.

In recent years, some metal hydrides have been investigated in more detail, in order to be able to use them for the production and storage of hydrogen, which in principle can be used as an alternative energy for both engines and heating. The hydride formation or hydride cleavage is associated with a considerable change in enthalpy, which leads to considerable difficulties and disadvantages in the planned uses of these metal hydrides. It has therefore already been proposed in the test vehicles to use the waste heat from the engine and the exhaust gases to heat the hydride stores. In the summer months, air conditioning can be carried out directly by exchanging heat with the hydride storage. On the other hand, there are great difficulties in the starting phase, since even at low temperatures there must be sufficient hydrogen pressure to start the engine and to bridge the period until the exhaust gases are warm enough to be used to heat the hydride storage. A combined hydrogen storage system has therefore already been proposed, in which the refueling of the vehicle and the heating of the house are connected to one another and the amounts of energy released in the formation of hydride are usefully utilized; see. H. Buchner, The Hydrogen-Hydride Energy Concept, Chemie Technik 7 (1978), pages 371 to 377. Accordingly, around 30% of the heat content of hydrogen at room temperature can be converted into useful heat at a higher temperature by hydride formation. It is therefore recommended that hydrogen and heat recovery are always coupled with one another in these processes.

In reverse of this concept, it has also been proposed to store solar heat for indoor air conditioning with the help of metal hydrides. A flat solar collector with around 100 ° C is assumed as the primary energy source, the earth as an auxiliary heating bath at a temperature level of around 10 ° C, and two metal hydride stores with CaNi _s and Fe _o , ₅ Ti _Q , ₅ powder serve as heat storage and heat transformation, between them Hydrogen gas can be exchanged by opening a valve. Heat exchangers also couple the two hydride tanks to the primary energy source, to the auxiliary heating bath or to the consumer, a house; see. H. Wenzl, Hydrogen in Metals: Outstanding properties and examples for their use, Kernforschungsanlage Jülich GmbH, January 1980, pages 66, 67 and Figure 13. However, a rough calculation shows that this concept has no prospect of realization, since the hydride storage facilities are much too big would have to be large in order to be able to serve in profitable dimensions for the storage of solar energy.

From DE-A-28 08 876 it is known to utilize the heat generation in the formation and decay of metal hydrides, for example with the aid of conventional heat pumps. Since conventional heat pumps have a number of wearing parts and, in addition, a relatively complex regulating and control device is necessary for a device of the type described there, this method has not been used in practice.

From FR-A-691 648 a system of interconnected pipes is known which are filled with a heat transfer medium. It is neither the use of metal hydrides nor the use of heat pipes, since these were not even invented at the time.

A system for transferring thermal energy using metal hydrides is known from US Pat. No. 4,044,819, in which solar collectors, heat tanks, heat exchangers and energy sources are combined with one another by burning fossil raw materials. The use of two different metal hydrides that decompose at different temperatures is also described here. The use of heat pipes is not mentioned here. The object of the invention is to provide devices for carrying out a method for the energy-saving production of useful heat from the environment or from waste heat using a reversible chemical reaction, namely the formation and decay of metal hydrides, in first and second containers connected to one another by lines develop, these containers are about half filled with a metal hydride and the other half with the hydride-forming metal or the hydride-forming alloy. The containers are alternately loaded and unloaded with hydrogen by changing the pressure change, and the released energy of compression and hydride is dissipated as heat through heat exchange, and the heat used for relaxation and hydrogen release from the hydride is replaced by heat exchange with the environment or with waste heat. The tanks are roughly the same size and are either filled with the same metal hydride or hydride-forming alloys and permanently connected to one another by a piping system with a switchable suction / pressure pump, or they are filled with two different metal hydrides or hydride-forming metals, which are distinguished by different hydrogen absorption or differentiate the desorption energy (and thus absorb or release hydrogen at different temperatures), the metal hydride with the lower hydrogen desorption energy being present in the first container (1) and the metal hydride with the higher hydrogen desorption energy being present in the second container (2) and the containers ( 1) and (2) through a pipe (3). are permanently connected. This object can be achieved according to the invention when using the same metal hydride or the same hydride-forming alloy in that the heat exchangers are four heat pipes (5, 6, 7, 8), of which a first and a second (7, 8 ) are permanently connected to the supply for waste heat and a third and fourth (5, 6) are permanently connected to the removal of the useful heat, and the first and third (5, 7) are permanently connected to the first container (1) and the second and that fourth (6, 8) are permanently connected to the second container (2).

If the containers (1) and (2) are filled with two different metal hydrides or hydride-forming metals, which differ by different hydrogen absorption or desorption energy, that is Problem can be solved in that the heat exchangers are four heat pipes (5, 6, 7, 8), of which a first (7) permanently with the supply for waste heat and with the first container (1), a second (5) permanently with the Removal of the useful heat and connected to the first container (1), a third (6) can be switched off with the removal of the useful heat and the second container (2), and a fourth (8) can be switched off with the supply for heat from the combustion of fossil fuels and the second container (2) is connected.

The metal hydrides are divided into the low-temperature hydrides and high-temperature hydrides based on their property of decomposing at low or higher temperatures. Especially when it comes to heating houses with the warmth of the surroundings, only the low-temperature hydrides can be used. If, on the other hand, waste heat from power plants or industrial plants is to be used, the high-temperature hydrides are ideal. Iron titanium hydride is particularly suitable for heating residential buildings. This hydride can be rapidly formed in the range from -20 to + 70 ° C. and split again, the pressure range from 0.1 to 12 bar being completely sufficient to control formation and cleavage. The high speed of the reaction, the high metallic thermal conductivity of the metal hydrides and the long metal / metal hydride cycle life, the high energy density enable the use of this metal hydride, provided the system can be hermetically sealed and in particular prevents the entry of oxygen. This problem is significantly alleviated if the heat pump process is carried out according to the absorption principle and there is no need for a leak-sensitive suction / pressure pump. The price of this alloy has already dropped to DM 10 / kg when large quantities are purchased, so that the investment costs for household heating based on this metal hydride can be significantly lower compared to conventional heat pumps.

Another advantage of the metal hydrides is that they have proven to be extremely safe and non-toxic, so that no complex safety measures have to be taken. For a house heating system, for example, it should be entirely sufficient to connect the system to a safety valve and a line leading to the outside, so that, for example, in the event of a fire and the associated overheating of the system, the hydrogen can be safely vented to the outside, where it can be low specific density immediately distributed upwards in the atmosphere and no longer represents a further source of danger.

When using the metal hydrides according to the invention, however, a number of other problems have to be considered. For example, traces of oxygen already lead to inactivation of the metal hydrides, so that the reversible hydride formation is considerably impaired or even completely stopped by small amounts of oxygen. It is therefore absolutely necessary to hermetically seal the entire system from the two containers (1), (2), the switchable piping system (3) and the suction / pressure pump (4) from the environment. D; 3 most metal hydrides can be reactivated with pure hydrogen at elevated temperatures, this part of the device according to the invention should be easily removable and transportable in order to be able to replace and regenerate it in the event of a malfunction due to oxygen. If necessary, the metal hydride could also be protected by upstream oxygen-binding media. These include carrier materials such as silica gel, chromium trioxide in cartridges (Oxisorb, Messer Griesheim).

In order to carry out the heat exchange on the metal hydride containers quickly and with little loss, extensive contact with the two exchanger systems (5), (6), (7) and (8) should be possible. On the other hand, the mass of the casing and the heat exchanger should be kept small, since otherwise the heat capacity of these parts would become unnecessarily large and considerable delays and heat losses would occur when the system was switched over.

• a) Wasserstoff wird vom Behälter (1) zum Behälter (2) gepumpt. Aus dem Hydrid im Behälter (1) bildet sich wieder Metall, während sich im Behälter (2) Hydrid bildet. Die freiwerdende Wärme im Behälter (2) wird durch den Wärmeaustausch direkt als Nutzwärme abgeführt. Sobald sich praktisch alles Hydrid im Behälter (1) ins Metall und das Metall im Behälter (2) zum Hydrid umgewandelt hat, wird keine weitere Wärme im Behälter (2) mehr frei, so daß das System jetzt umgeschaltet werden muß.
• b) Durch das Rückpumpen des Wasserstoffs vom Behälter (2) in den Behälter (1) kehrt sich die Reaktion der Hydridbildung um, so daß jetzt im Behälter (1) Wärme frei wird. Selbstverständlich wird kurz nach dem Umschalten zunächst keine Nutzwärme anfallen, da der Behälter (1) durch Wärmeaustausch mit der Umgebung maximal die Umgebungstemperatur besitzen wird und erst durch Hydridbildung der Behälter (1) entsprechend erwärmt werden muß, bis die Temperatur auf die gewünschte Höhe angestiegen ist. Diese Umschaltphase wird umso länger sein, je größer die Wärmekapazität des Systems ist und je größer die Differenz zwischen der Temperatur der Nutzwärme und der Umgebungswärme ist. Erst wenn der Behälter (1) die Temperatur der Nutzwärme erreicht oder überstiegen hat, sollte die Nutzwärme entnommen werden. Um die im Umschaltzeitpunkt im Behälter (2) vorhandene Speicherwärme sinnvoll zu nutzen, sollte sie entweder dazu verwendet werden, Brauchwarmwasser zu bereiten oder den Behälter (1) durch Wärmeaustausch mit Behälter (2) bis zur Einstellung der Gleichgewichtstemperatur vorzuwärmen.

Such a heater would have the following cycles:

• a) Hydrogen is pumped from the container (1) to the container (2). Metal forms again from the hydride in the container (1), while hydride forms in the container (2). The heat released in the container (2) is dissipated directly as useful heat by the heat exchange. As soon as practically all the hydride in the container (1) has converted to metal and the metal in the container (2) to the hydride, no further heat is released in the container (2), so that the system must now be switched over.
• b) By pumping back the hydrogen from the container (2) into the container (1), the reaction of the hydride formation is reversed, so that heat is now released in the container (1). Of course, shortly after the switchover, there will initially be no useful heat, since the container (1) will have a maximum of the ambient temperature due to heat exchange with the surroundings, and the container (1) will only have to be heated accordingly until the temperature has risen to the desired level . This changeover phase will be longer, the greater the thermal capacity of the system and the greater the difference between the temperature of the useful heat and the ambient heat. The useful heat should only be removed when the container (1) has reached or exceeded the temperature of the useful heat. In order to make sensible use of the storage heat in the tank (2) at the time of the switchover, it should either be used to prepare domestic hot water or preheat the tank (1) by heat exchange with tank (2) until the equilibrium temperature is reached.

The device according to the invention for carrying out the method uses so-called heat pipes for heat exchange (heat pipes; see P. Dunn and D.A. Reay, Heat Pipes, Pergamon Press, 1976). These are hermetically sealed metal pipes, some of which are filled with an easily evaporable liquid. The heat transfer takes place by evaporating the liquid at the lower end and giving off the heat of vaporization by recondensing the liquid at the upper end of the tube. These heat pipes act as diodes because heat can only be transferred in one direction, namely from bottom to top. If the amount of heat at the lower end is no longer sufficient to evaporate the liquid, steam can no longer rise and condense at the top. As soon as the upper end is warmer than the lower end, heat is no longer transported. These heat pipes also have the advantage that the thermal conductivity is 3 orders of magnitude higher than that of copper.

By using these heat pipes in the devices according to the invention, there is no need to switch the heat exchanger systems, since the heat pipes can only ever transport the heat in the one desired direction. In such a case, only the direction of the hydrogen flow through the pump (4) has to be reversed. This can be done by using appropriate valves or by reversing the direction of pump rotation. With the absorption heat pump, the direction of flow of the hydrogen is reversed by simply switching the fossil heating source on and off in accordance with the working cycle time.

So while the heat exchangers have to be switched with air, water, antifreeze water or other liquids with each phase reversal, the corresponding heat exchangers have to be switched over, which requires a lot of equipment and appropriate control devices, this can be dispensed with when using heat pipes. The reversal of the pumping direction of the hydrogen can be done in this preferred embodiment of the invention by thermostats or even by a simple timer. The useful heat gained can only flow in the desired direction due to the diode effect of the heat pipes, so that a phase-reversed switching can never occur. Of course, even when using heat pipes, it cannot be avoided that useful heat cannot be removed for a certain time after the switchover, since the cooled container must first be brought to the temperature of the useful energy to be removed by hydride formation and, if appropriate, heat exchange.

In the second embodiment of the invention, the pressure change is brought about thermally. This eliminates the suction / pressure pump, but it is necessary to use two different metal hydrides. The two metal hydrides must differ by different hydrogen absorption or desorption energy and thus absorb or release the hydrogen at different temperatures. The metal hydride with the lower hydrogen desorption energy is able to utilize heat from the environment or waste heat, while the second metal hydride with higher hydrogen desorption energy has to be fed with heat, as can be obtained, for example, from the combustion of fossil fuels.

A typical combination of two different metal hydrides is a titanium-iron-manganese hydride and a titanium-zirconium-chromium-manganese hydride. The chemical composition of these hydrides is TiFe _o , ₈ Mn _O , ₂ H ₂ and Tio, gZro "CrMnH3.

The absorption and desorption temperatures of these two metal hydrides are + 65 ° C and + 121 C or - 6 ° C and + 50 ° C. From this a theoretical system coefficient of performance of 1.6 can be calculated.

This device also consists of two containers (1), (2), each filled with about half of metal hydride and the hydride-forming metal of the two different metal hydrides, a connecting pipe (3), mutually switchable heat exchangers (5), (6) , for the dissipation of the useful heat and mutually switchable heat exchangers (7), (8) for the supply of heat to the environment or waste heat or the fossil heat as well as line (13), (14) and switchable shut-off valves (11), (12) .

According to the invention, heat pipes are also used for this. While the heat pipe (7) is still fed with heat from the surroundings or waste heat, the heat pipe (8) is fed intermittently with heat which has arisen from the combustion of fossil fuels. The additional line (13), (14) and switchable shut-off valves (11), (12) are necessary to prevent direct transmission of the fossil-generated heat to the useful heat flow. This would be prevented by putting the heat exchanger of the heat pipe (6) out of operation by bypassing the useful heat flow during the period of hydrogen desorption. This is done by operating the shut-off valve (11) accordingly.

During the decommissioning of the heat pipe (6), there is a heat build-up in the proportion of the current carrying useful heat, which is held in the heat exchanger. This has the desired consequence that the heat-transporting medium overheats in the heat pipe and almost completely changes into poorly heat-conducting steam without the possibility of condensation. As a result, the heat transport to the heat exchanger at the head of the heat pipe is greatly reduced. In principle, it would also be possible to install a second shut-off valve in the bypass line, which in turn opens or closes the bypass line. However, such an arrangement requires additional control effort.

It is also necessary to install a bypass line (14) and a shut-off valve (12) in the supply line for fossil heat to the heat pipe (8).

If the respective intended use of the useful heat makes it necessary to be able to remove it continuously, it is necessary either to partially transfer the useful heat in a heat store, for example Glauber's salt heat store, or to use two devices according to the invention in parallel and to take the useful heat out of phase from them. The cycle of such a double system would then follow the rhythm, for example (1), (1 '). (2), (2 '), (1) etc. For normal heating of a house, however, it is readily acceptable that no useful heat can be removed for a certain period of time after the switchover, in particular if these phases are relative without the provision of useful heat are short.

The dimensioning of the device according to the invention and the length of the respective phases depend to a considerable extent on the amounts of useful heat required, the amount of environmental heat or waste heat and the investment costs. If the ambient air were used, it would certainly make sense to run only one cycle per day, since the warmer day air would then be used. Here, however, the investment costs of the plant and the amount of metal hydride required would be considerably higher. According to the invention, it is possible and extremely advantageous to make the cycles significantly shorter, for example in 30 minutes to 3 hours, and thereby to significantly reduce the size and total investment of the system. In theory, it is quite possible to shorten the cycles even more, e.g. B. to 10 minutes, however, this would reduce the investment costs proportionally no longer as much. In addition, the kinetics of hydride formation would be disruptive in the case of even shorter cycles.

The dimensioning results from the following rough calculation: With a maximum heat requirement per heating day in a single-family house of 100 kWatt, a reaction container should contain at least 3000 kg of metal or metal hydride. If the individual phases are shortened to one hour, the hydride requirement drops to 125 kg per container. At the already mentioned price of around 10 DM per kg, the investment sum falls below that of conventional heat pumps, whereby the higher efficiency and the more problem-free use of environmental heat enable almost universal use, at least in the latitudes where the outside temperatures rarely fall below -10 ° C decrease. The device according to the invention can be used particularly advantageously where larger amounts of waste heat are available at a relatively low temperature level, for example cooling water or condensates from power plants, steelworks, coking plants, chemical plants etc. These amounts of heat can be transported relatively easily and with little loss over longer distances and can be converted according to the invention into useful heat of higher temperature at the respective consumer points. Only in this way is it conceivable, for example, to operate district heating lines at relatively low temperatures and to only extract heat from the desired higher temperature in households or at the consumer points. The device according to the invention is thus used like a heat transformer. In contrast to electrical energy, which can only be transported over long distances with low losses when the voltage is high, heat can be transported in a line system with low losses if the temperature differences from the surroundings are low.

It is clear from the explanations that there is no need for further differentiation that the heat pump variants according to the invention can also be used for cooling. The absorption heat pump in particular would be suitable for solar cooling, since the upper temperature level for the process control when appropriate metal hydrides are selected is already in the range of the conductivity of non-concentrating solar collectors.

• Figur 1 zeigt die erste Ausführungsform unter Verwendung von Wärmerohren sowohl für die Zuführung der Umweltwärme als auch zur Abführung der Nutzwärme, bei der aufgrund der Diodenwirkung keine Umschaltungen notwendig sind.
• Figur 2 zeigt die zweite Ausführungsform unter Verwendung von Wärmerohren, bei der die Druckveränderung thermisch erfolgt.

The principle and preferred embodiments of the devices according to the invention are explained in more detail in the following figures.

• Figure 1 shows the first embodiment using heat pipes both for supplying the environmental heat and for dissipating the useful heat, in which no switching is necessary due to the diode effect.
• Figure 2 shows the second embodiment using heat pipes, in which the pressure change takes place thermally.

• (3) das umschaltbare Rohrleitungssystem für Wasserstoff,
• (4) die ggf. umschaltbare Pumpe für den Wasserstoff,
• (5) und (6) die Wärmeaustauscher für die Nutzwärme
• (7) und (8) die Wärmeaustauscher für die Umgebungswärme bzw. Abfallwärme,
• (11) und (12) Absperrventile,durch die die Abnahme von Nutzwärme bzw. die Zufuhr von fossiler Wärme intermittierend unterbrochen werden kann,
• (13) und (14) bypass-Leitungen für die Abnahme von Nutzwärme bzw. die Zufuhr von fossiler Wärme, die gegebenenfalls durch weitere nicht eingezeichnete Absperrventile im Wechselrhythmus mit den Absperrventilen (11) und (12) geschaltet werden können.

In all figures, (1) and (2) mean the containers filled with metal or metal hydride,

• (3) the switchable piping system for hydrogen,
• (4) the possibly switchable pump for the hydrogen,
• (5) and (6) the heat exchangers for the Useful heat
• (7) and (8) the heat exchangers for the ambient heat or waste heat,
• (11) and (12) shut-off valves, through which the removal of useful heat or the supply of fossil heat can be interrupted intermittently,
• (13) and (14) bypass lines for the take-up of useful heat or the supply of fossil heat, which can optionally be switched alternately with the shut-off valves (11) and (12) by further shut-off valves (not shown).

Claims (3)

1. An apparatus for carrying out a process for the energy-saving recovery of useful heat from the environment or from waste heat with utilization of a reversible chemical reaction of forming and decomposing metal hydrides in first and second reservoirs which are connected with each other by lines and about one half of which is filled with a metal hydride while the other half is filled with the hydride-forming metal or the hydride-forming alloy, said reservoirs being alternatingly and successively charged and discharged with hydrogen by pressure variation, and the liberated heat of the compression and the hydride formation being removed as useful heat by heat exchange and consumed heat of expansion and of the hydrogen evolution of the hydride being replaced by heat exchange with the environment or with waste heat; the reservoirs (1 and 2) having about the same size, being filled with the same metal hydride or hydride-forming alloy and being permanently connected with each other by a piping system (3) with a reversible suction/pressure pump (4), characterized in that the heat exchangers are four heat pipes (5, 6, 7, 8), a first and a second (7, 8) of which are permanently connected with the supply for waste heat and a third and fourth (5, 6) of which is permanently connected with the removal of the useful heat, and the first and the third (5, 7) of which are permanently connected with the first reservoir (1) and the second and the fourth (6, 8) of which are permanently connected with the second reservoir (2).

2. Apparatus for carrying out a process for the energy-saving recovery of useful heat from the environment or from waste heat with utilization of a reversible chemical reaction of forming and decomposing metal hydrides in first and second reservoirs connected by pipes and about one half of which is filled with a metal hydride while the other half is filled with the hydride-forming metal or the hydride-forming alloy, said reservoirs being alternatingly and successively charged and discharged with hydrogen by pressure variation, and the liberated heat of the compression and the hydride formation being removed as useful heat by heat exchange and consumed heat of expansion and of the hydrogen evolution of the hydride being replaced by heat exchange with the environment or with waste heat; the reservoirs (1 and 2) having about the same size and being filled with two different metal hydrides or hydride-forming metals which differ by different hydrogen absorption or desorption energy (and, therefore, take up or release again hydrogen at different temperatures), the metal hydride having the lower hydrogen desorption energy being present in the first reservoir (1) and the metal hydride having the higher hydrogen desorption energy being present in the second reservoir (2), the reservoirs (1 and 2) being permanently connected by a line (3), characterized in that the heat exchangers are four heat pipes (5, 6, 7, 8), a first (7) of which is permanently connected with the supply of waste heat and with the first reservoir (1), a second (5) of which is permanently connected with the removal of useful heat and with the first reservoir (1), a third (6) of which is disconnectably connected with the removal of the useful heat and the second reservoir (2), and a fourth (8) of which is disconnectably connected with the supply of heat from the combustion of fossil fuels and the second reservoir (2).

3. An apparatus according to claim 1 or 2, characterized in that two systems having almost the same size are connected in parallel with phase displacement for removing the useful heat.

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