System for energy recovery

Abstract

A system for energy recovery has a main installation (1), during operation of which hot exhaust gas (2) is generated. The main plant (1) has a discharge device (3) through which the exhaust gas (2) flows and via which the exhaust gas (2) is discharged from the main plant (1). The system has a compressor unit (6) comprising a plurality of compressors (8), by means of which air (7), which is fed to the compressor unit (6) and has an initial temperature (T1), is compressed from an initial pressure (p1) to a final pressure (p2) and an intermediate temperature (Tc). The system has a heat exchanger (10) arranged downstream of the compressor unit (6), through which air (7) compressed to a final pressure (p2) and to an intermediate temperature (Tc) flows and by means of which heat of the exhaust gas (2) flowing through the discharge device (3) is fed to the air (7) flowing through the heat exchanger (10) in order to heat the air (7) to a final temperature (T2). The system has a compressed air motor (11) arranged downstream of the heat exchanger (10), to which air (7) heated to a final temperature (T2) and having a final pressure (p2) is fed, which is operated with the air (7) and from which the air (7) is discharged at a discharge temperature (T3) and a discharge pressure (p3).

Description

TECHNICAL FIELD

[0001] The invention relates to a system for energy recovery, - wherein the system has a main facility, in the operation of which hot exhaust gases are produced, - wherein the main facility has a lead-out device, through which the exhaust gases flow and via which the exhaust gases are led out of the main facility. BACKGROUND

[0002] Such systems are generally known. Purely by way of example, reference is made to EP 10 770 810 B1.

[0003] In many industrial processes, large amounts of "low-grade" thermal energy are produced. This energy is often not utilized or only to a small extent. The waste heat is instead directly discharged into the environment or, in cold seasons, is used, albeit often only to a small extent, for example for heating office buildings or residential buildings. For example, the thermal power used in slab heating furnaces in a steel rolling mill is often of the order of magnitude of 100 MW. Of this, approximately 70% is estimated to actually flow into the slab. The remainder, in the case of a typical exhaust gas temperature of approximately 300°C, is not utilized. In other industrial facilities, such as electric arc steel mills, sometimes even larger amounts of waste heat are produced.

[0004] In addition, in many industrial facilities, large amounts of compressed air are often also required. For example, in the cooling section of a hot strip mill, valves are often used to apply cooling water to the rolled flat material, which are operated using compressed air. Compressed air is also used, for example, to blow away excess cooling water from the upper surface of the rolled flat material or to generate an air flow that is remote from measuring devices, so that the measuring devices are not contaminated. In both steel mills and other industrial facilities, compressed air is produced by means of a compressor driven by electrical energy. Numerous solutions are known from the prior art for recovering electrical energy from industrial process waste heat. However, these prior art solutions are mostly not used in practice. In particular, the efficiency is often very low and the costs associated with implementing these solutions are not in a reasonable proportion to the energy that can be recovered.

[0005] It is known to install a heat exchanger in the lead-out device, which is used to heat up a heat transfer oil. By means of the heated heat transfer oil, a refrigerant is heated up in another heat exchanger, which in turn drives a gas turbine. The refrigerant can be selected as required depending on the temperature level of the exhaust gases. This prior art solution appears to be the most promising. However, it is also often not used because it is also not economical. The advantage of this solution is that no steam cycle circuit has to be installed. The disadvantage is that a heat transfer oil has to be used. However, heat transfer oils chemically decompose at temperatures above approximately 300°C. Therefore, the application of this technology is in principle limited to exhaust gases of up to approximately 300°C.

[0006] A system for the extraction of electrical energy is known from DE 10 2009 030 146 A1. The system has a compressor block which comprises a plurality of compressors. By means of the compressor block, air is compressed to a final pressure. The final pressure can be at a high value, for example 300 bar. The compressed air is expanded to a significantly lower intermediate pressure, typically 40 bar to 80 bar, by means of a first pressure-reducing valve. The expanded air is heated to ambient temperature by means of a heat exchanger 10 arranged downstream of the first pressure-reducing valve 9 and is delivered to an intermediate store. The air taken from the intermediate store is expanded to a pressure which is again significantly lower by means of a second pressure-reducing valve. This pressure is the working pressure. It can be between 5 bar and 20 bar. The air which is again expanded is again heated to ambient temperature by means of a heat exchanger arranged downstream of the second pressure-reducing valve. Only thereafter is the air heated in the heat exchanger to a higher temperature of approximately 300°C. The thermal energy is delivered to the heat exchanger by a heat source which can be a thermal store which is supplied by a solar thermal plant. The heated air is delivered to a compressed-air motor which can drive an electrical generator. SUMMARY

[0007] The task of the present invention is to create the possibility of improving the recovery of energy from waste heat of industrial technical processes, i.e. from the heat of hot exhaust gases of a main plant.

[0008] The task is solved by a system for energy recovery having the features of claim 1. Advantageous design variants of the system are the subject matter of the dependent claims 2 to 15. According to the invention, a system for energy recovery of the type mentioned at the outset is designed as follows: - the system has a compressor block comprising a plurality of compressors, by means of which a compressed air which is delivered to the compressor block is compressed from an initial pressure to a final pressure and an intermediate temperature, the compressed air having an initial temperature, - the system has a heat exchanger arranged downstream of the compressor block, through which the air compressed to the final pressure and the intermediate temperature flows, and by means of which the heat of the exhaust gases flowing through the extraction device is delivered to the air flowing through the heat exchanger, so that the air is heated to a final temperature, and - the system has a compressed-air motor arranged downstream of the heat exchanger, to which the air heated to the final temperature and having the final pressure is delivered, the compressed-air motor being operated with the air, and the air being discharged from the compressed-air motor at a discharge temperature and a discharge pressure.

[0009] The main plant can be constructed as desired, as long as it generates hot exhaust gases. Typically, the main plant is a plant of the steel industry, in particular a slab heating furnace, a blast furnace, a converter or an electric arc furnace steel plant. However, the invention is not limited to such plants.

[0010] Usually, the electric energy is generated by means of an electric generator driven by the compressed air motor. In some cases, however, the mechanical energy generated by the compressed air motor can also be used for other purposes.

[0011] Preferably, the number of compressors is greater than one and the compressors are arranged in series one behind the other, so that the air is compressed in several stages from an initial pressure to a final pressure. In this case, the system preferably has cooling devices arranged between each two successive compressors, by means of which the air delivered from the compressor arranged upstream of the respective cooling device to the compressor arranged downstream of the respective cooling device is cooled. Thereby, the energy consumption required for operating the compressors can be reduced, at the same time the amount of compressed air is maximized, and furthermore the intermediate temperature can be kept relatively low. Thus, the heat of the hot exhaust air can be utilized to heat the compressed air to a further better extent. At the same time, the exhaust air can be cooled to a greater extent. The cooling of the air in the cooling devices is usually carried out as fully as possible, possibly to the initial temperature. However, no cooling device is arranged after the last compressor which compresses the air to the final pressure and the intermediate temperature. This applies to the only compressor in the case where only one compressor is provided.

[0012] It is possible that the system has a further heat exchanger which is arranged in the exhaust and by means of which the heat of the exhaust air is transferred to an intermediate medium, and the intermediate medium acts directly or indirectly on the heat exchanger which heats the air. The intermediate medium can be, for example, a heat transfer oil. The further heat exchanger is particularly suitable for use when the exhaust air temperature is equal to or below 300°C.

[0013] In the case of the use of a further heat exchanger and an intermediate medium, an advantageous design proposal is that the system has a thermal store, the heat exchanger is arranged at least in the upper region of the thermal store, and preferably the air flowing through the heat exchanger flows from below to above through the thermal store, and at least in the lower region of the thermal store there is arranged a heat discharge device, and preferably the intermediate medium flows from above to below through the thermal store, so that the intermediate medium brings heat into the thermal store via the heat discharge device.

[0014] Due to the thermal store, in particular a certain buffer capacity can be provided, so that a uniform operation of the compressed air motor can be achieved despite a varying amount of hot exhaust air.

[0015] It is possible that the heat discharge device is simply the feed pipe via which the intermediate medium is fed into the thermal store. In this case, there is a portion of the intermediate medium in the thermal store. In this case, the intermediate medium and the storage medium of the thermal store are the same medium. Alternatively, the heat discharge device can be a (further) heat exchanger, so that the medium of the thermal store is separate from the intermediate medium.

[0016] Alternatively, it is possible that the heat exchanger is arranged in the discharge device, so that the heat of the exhaust gas is directly transferred to the heat exchanger. This solution is particularly advantageous when the exhaust gas temperature is higher than 300°C. Because at this temperature, common intermediate media, such as thermal oil, are usually no longer stable.

[0017] Preferably, the system has a compressed air reservoir, which is arranged between the heat exchanger and the compressed air motor. Thereby, compressed air can be generated and stored in advance within the capacity of the compressed air reservoir. Thus, the operation of the compressed air motor can be smoothed.

[0018] In a particularly simple design, the air delivered to the compressor unit is taken from the environment. Thereby, no compressed air reservoir is required at the inlet side of the compressor unit. Also, no pre-compression of the air is required. However, it can be necessary to clean the air before it is delivered to the compressor unit. In a similar manner, in a particularly simple design, the air discharged from the compressed air motor is also discharged into the environment.

[0019] Preferably, the system has a regulating device, by means of which the compressor unit and / or the compressed air motor is regulated, so that the discharge temperature and the discharge pressure at least approximately correspond to the initial pressure and the initial temperature. Thereby, the efficiency of the entire device can be optimized.

[0020] The compressors of the compressor unit can be configured as desired. In some cases, the compressor unit can contain a gas turbine as a compressor. However, in general, other designs are more advantageous.

[0021] For example, the compressors can be configured as follows, respectively: - the compressor has a first shaft and a second shaft, whose axes are arranged parallel to each other with a lateral offset, and in operation are rotated synchronously and with the same rotational speed, so that the two shafts roll against each other and are sealed against each other, - the compressor has a housing, which annularly surrounds the first shaft, - the first shaft has a nose on its outer side, which annularly seals on the housing, - the second shaft has a groove on its outer side, into which the nose of the first shaft temporarily dips when the shafts are rotated, after which the nose of the first shaft again comes out of the groove, - an input opening and an outlet opening are arranged in the housing, through which the nose passes shortly after leaving the groove and shortly before immersing into the groove, seen in the direction of rotation of the first shaft, so that the nose divides the respective cavity formed by the first shaft and the surrounding housing into a first section and a second section, wherein the first section extends from the angular position of the nose of the first shaft leaving the groove of the second shaft to the instantaneous angular position of the nose and the second section extends from the instantaneous position of the nose to the angular position of the nose of the first shaft immersing into the groove of the second shaft, - the input opening is permanently open so that air is continuously fed to the first section, - the compressed air is discharged via the outlet opening, - the outlet opening is equipped with a closing device which opens the outlet opening for a short time shortly before the nose of the first shaft immerses into the groove of the second shaft and then closes again.

[0022] The opening and closing of the outlet opening can be achieved by the shape design of the first and second shafts. As soon as the outlet opening is open, the compressed air can be removed from the respective compressor.

[0023] Such a design of the compressor is known per se. In particular, the compressor part of a space engine is designed in this way. A detailed description of a space engine can be found, for example, in US 2021 / 0 040885 A1, US 10 844 782 B1, US 2022 / 0056 802 A1 and US 2023 / 0 092 617 A1.

[0024] However, it is particularly advantageous if the compressor is designed as a screw compressor. Screw compressors are generally known to the person skilled in the art. Purely by way of example, reference is made to the German Wikipedia entry "Zahnradpumpe (gear pump)" and the section "Schraubenpumpe (screw pump)" therein, accessed on 5 May 2023. Screw compressors are also available on the market. In general, a screw compressor has two counter-rotating shafts which enclose air between one another in a chamber. When the two shafts rotate, this chamber moves in the axial direction, wherein a reduction in the volume of the chamber occurs.

[0025] The compressed air motor can also be designed as required. For example, the compressed air motor can be designed as a gas turbine. Likewise, the compressed air motor can be designed as follows: - the compressed air motor has a first shaft and a second shaft, the axes of which are arranged parallel to one another with a lateral offset and rotate synchronously and at the same rotational speed in operation, so that the two shafts roll on one another and are sealed against one another in an air-tight manner, - the compressed air motor has a housing which annularly surrounds the first shaft, - the first shaft has a nose on its outer side, which nose seals around on the housing, - the second shaft has a groove on its outer side, into which groove the nose of the first shaft temporarily dips when the shafts are rotating, after which the nose of the first shaft again disengages from the groove, - in the housing there are arranged an inlet opening and an outlet opening, through which openings the nose passes shortly after disengaging from the groove, i.e. shortly before dipping into the groove, as seen in the direction of rotation of the first shaft, so that the nose divides the annular cavity formed by the first shaft and the surrounding housing into a first section and a second section, wherein the first section extends from the angular position at which the nose of the first shaft disengages from the groove of the second shaft to the instantaneous angular position of the nose, and the second section extends from the instantaneous position of the nose to the angular position at which the nose of the first shaft dips into the groove of the second shaft, - the inlet opening is equipped with a closing device, which opens the delivery opening for a short time shortly after the nose of the first shaft disengages from the groove of the second shaft, and then closes again, so that compressed air is temporarily delivered to the first section, - the outlet opening is permanently open, so that air is continuously discharged via the outlet opening.

[0026] This construction essentially corresponds to the motor part of a space engine. The construction principle of such a compressed air motor is the opposite of the compressor part of a space engine.

[0027] However, it is particularly advantageous if the compressed air motor is constructed as a screw motor. Screw motors are generally known to the person skilled in the art. Purely by way of example, reference can be made to the doctoral thesis "Grundlagen des Zweiphasen-Schraubenmotors" (Fundamentals of the two-phase screw motor) by Bernhard Paul Kliem, Institute of Mechanical Engineering, University of Dortmund (Germany). In principle, the construction of a screw motor is the opposite of a screw compressor. Typically, a screw motor has two counter-rotating shafts, which enclose air in a chamber between one another. When the two shafts are rotating, this chamber moves in the axial direction, wherein a volume increase of the chamber occurs.

[0028] In a particularly preferred design of the system, the system has an electric motor, which itself has a housing, in which housing the first shaft and the second shaft are supported coaxially. In this design, the first shaft and the second shaft each carry an active part of the electric motor, so that an electromotive force can be generated between the two active parts. In this design, the first shaft is connected to the compressed air motor and the second shaft is connected to the compressor of the compressor unit. The electric motor thus works as a differential motor.

[0029] This design has the special advantage that instead of one electric generator connected to the compressed air motor and (at least) one electric motor connected to the compressor, only one electric machine is required, and in addition this machine can be dimensioned smaller than the two separate machines (electric generator and electric motor) would require.

[0030] The last-mentioned design can be further improved in that the system has a further electric machine whose stator is arranged in a rotationally fixed manner and whose rotor is connected to the compressed air motor or to the compressor of the compressor unit. Thereby, the operation of the compressed air motor and the operation of the compressor unit can be decoupled from one another within the power limits of the further electric machine. The further electric machine can generally be dimensioned much smaller than the first-mentioned electric machine, for example one order of magnitude (= 10 times) smaller.

[0031] In another preferred design, the system has at least one air extraction point at which air can be extracted from the compressor unit, wherein the air extracted from the compressor unit has an extraction pressure which is greater than the initial pressure and at most equal to the final pressure. Thereby, it is not necessary to generate compressed air independently for the main facility. If necessary, the existing compressed air system of the main facility can also be retrofitted and upgraded so that it forms a system according to the invention. BRIEF DESCRIPTION OF DRAWINGS

[0032] The features, characteristics and advantages of the invention described above, as well as the manner and method of realizing them, will become more clear and intelligible in conjunction with the following description of embodiments and in conjunction with the attached drawings. In which: Figure 1 A main facility and a recovery device are shown, Figure 2 The structural configuration of the recovery device is shown, Figure 3 The main facility and a heat exchanger are shown, Figure 4 The heat transfer structure is shown, Figure 5 The regulation structure is shown, Figure 6 The compressor unit, the compressed air motor and the electric machine are shown, Figure 7 A variant of Figure 6 is shown, and Figure 8 Another variant of Figure 6 is shown. DETAILED DESCRIPTION

[0033] According to Figure 1The system for energy recovery has a main installation 1. Hot exhaust gases 2 are generated during operation of the main installation 1. The hot exhaust gases 2 flow through a lead-out device 3 of the main installation 1 and are thereby led out of the main installation 1. The main installation 1 is usually an installation of the steel industry, for example a slab heating furnace or an electric arc steel plant. However, the invention is not limited to installations of the steel industry.

[0034] The system further has further installation components by means of which the thermal energy of the hot exhaust gases 2, i.e. the waste heat of the main installation 1, shall be used to first generate mechanical energy and, usually on this basis, electrical energy. These further installation components are referred to in the following as a recovery device 4. The recovery device 4 is thermally coupled to the hot exhaust gases 2 flowing through the lead-out device 3. This thermal coupling is indicated in Figure 1 by a double arrow 5. The structural configuration of the recovery device 4 will be explained in connection with Figure 2 .

[0035] According to Figure 2 , the recovery device 4 has a compressor train 6. By means of the compressor train 6, air 7 which is delivered to the compressor train 6 at an initial temperature T1 and an initial pressure pi is compressed to a final pressure p2 and an intermediate temperature Tc. For this purpose, the compressor train 6 comprises a plurality of compressors 8.

[0036] The number of compressors 8 can be set as desired. In many cases, the number of compressors 8 is greater than 1. In this case, the compressors 8 are arranged in series one behind the other. Figure 2 The number of three compressors 8 shown in is merely exemplary. Due to the series arrangement of the compressors 8, the air 7 is compressed in several stages by means of the compressors 8 starting from the initial pressure pi to the final pressure p2. According to this embodiment, the foremost compressor 8 compresses the air 7 from the initial pressure pi to an intermediate pressure pa. At the output side of this compressor 8, the air 7 has a temperature Ta. The temperature Ta is higher than the initial temperature T1. Similarly, the intermediate compressor 8 compresses the air 7 from the intermediate pressure pa to an intermediate pressure pb. At the output side of this compressor 8, the air 7 has a temperature Tb. Similarly, the last compressor 8 compresses the air 7 from the intermediate pressure pb to the final pressure p2. At the output side of this compressor 8, the air 7 has the intermediate temperature Tc. Thus, a plurality of pressures pa, pb is generated in the chain of compressors 8.

[0037] In the case of a plurality of compressors 8, according to Figure 2As shown, the system preferably also has cooling devices 9, which are arranged between each two successive compressors 8. By means of the cooling devices 9, the air 7, which is conveyed from the compressors 8 arranged upstream of the respective cooling device 9 to the compressors 8 arranged downstream of the respective cooling device 9, is cooled. Where possible, each time to the initial temperature T1. Almost any cooling medium can be used for cooling the air 7, for example cooling water, which is used for cooling the rolled material after passing through the rolling line. However, the cooling of the air 7 takes place only between the individual compressors 8. After the compressor 8, which compresses the air 7 to the final pressure p2 and the intermediate temperature Tc, no cooling device is arranged. This applies both if there are a plurality of compressors 8 arranged in series, as shown, or if there is only one compressor 8. Figure 2 This applies both if there are a plurality of compressors 8 arranged in series, as shown, or if there is only one compressor 8.

[0038] The system also has a heat exchanger 10. This heat exchanger 10 is arranged downstream of the compressor group 6. The air 7, which has been compressed to the final pressure p2 but still has the intermediate temperature Tc, flows through the heat exchanger 10. By means of the heat exchanger 10, the heat of the exhaust gas 2, which flows through the extraction device 3, is transferred to the air 7, which flows through the heat exchanger 10. As a result, the air 7 is heated to the final temperature T2.

[0039] Finally, the system has a compressed-air motor 11. This compressed-air motor 11 is arranged downstream of the heat exchanger 10. At this point, the air 7, which is heated to the final temperature T2 and still has the final pressure p2, is conveyed to the compressed-air motor 11. The compressed-air motor 11 therefore operates with this air 7. The air 7 is discharged from the compressed-air motor 11 at the discharge temperature T3 and the discharge pressure p3.

[0040] Mechanical energy is first generated by means of the compressed-air motor 11. However, in many cases, the compressed-air motor 11 is coupled to an electric motor 12, so that this electric motor is operated in generator mode. The electrical energy generated by the electric motor 12 can therefore be fed into the electrical network 14, for example by means of a converter 13.

[0041] Figure 2 Not only the minimum design of the recycling device 4 set out above is shown, but also some designs. These designs can be implemented independently of one another.

[0042] One design is that the system has a compressed-air reservoir 15. This compressed-air reservoir 15 is arranged between the heat exchanger 10 and the compressed-air motor 11. The air 7, which is heated to the final temperature T2 and has the final pressure p2, is conveyed to the compressed-air reservoir 15. From the compressed-air reservoir 15, the air 7, which is heated to the final temperature T2 and has the final pressure p2, is conveyed to the compressed-air motor 11.

[0043] Another design option involves using air 7 supplied to compressor unit 6, which is taken from the environment. Therefore, before being compressed in compressor unit 6, the air 7 supplied to compressor unit 6 has an initial pressure p1 of approximately 100,000 Pa, which is normal atmospheric pressure, and an initial temperature T1 of the local average temperature, which may vary between -20°C and +40°C depending on the time of day and season.

[0044] Another design option involves releasing the air 7 from the compressed air motor 11 into the environment.

[0045] Another design option involves a system with at least one air intake point 16. At this intake point 16, air 7 can be drawn from the compressor unit 6. The intake point 16 can be configured as, for example, a valve. The drawn air 7 has an intake pressure greater than the initial pressure p1 and at most equal to the final pressure p2. Figure 2 As shown, for example, air 7 with intermediate pressure pa, air 7 with intermediate pressure pb and air 7 with final pressure p2 can be drawn from compressor unit 6.

[0046] To transfer the heat from exhaust gas 2 to air 7, several different design schemes are possible. The simplest design scheme is based on... Figure 3 As shown, the heat exchanger 10 is arranged in the outlet device 3, so that the heat exchanger 10 is directly circulated by the exhaust gas 2, thereby the heat of the exhaust gas 2 is directly transferred to the heat exchanger 10.

[0047] Or, according to Figure 4 As shown, the system may have another heat exchanger 17, which is arranged in the outlet device 3 and circulated by the exhaust gas 2. In this case, the heat of the exhaust gas 2 is first transferred to the intermediate medium 18 flowing through the other heat exchanger 17. In this case, the intermediate medium 18 acts on the heat exchanger 10, that is, the heat exchanger through which the air 7 flows and by means of it heats the air 7. This action can be direct. This is in Figure 4 Not shown. Instead, an advantageous design is shown in which an intermediate medium 18 indirectly acts on the heat exchanger 10 for heating the air 7. The intermediate medium 18 may be, for example, heat transfer oil.

[0048] In this advantageous design, the system has a thermal store 19. In the thermal store 19 there is a suitable storage medium 20. In this case, the heat exchanger 10 is arranged at least in the upper region of the thermal store 19. It is even better if the heat exchanger 10 extends in the height direction over a significant height range of the thermal store 19. In this case, the air 7 flows through the heat exchanger 10 from below upwards and thus through the thermal store 19. Furthermore, there is arranged at least in the lower region of the thermal store 19 a heat discharge device 21. By means of the heat discharge device 21, the intermediate medium 18 brings heat into the thermal store 19 or the storage medium 20. It is even better if the heat discharge device 21 extends in the height direction over a significant height range of the thermal store 19. In this case, the intermediate medium 18 flows through the heat discharge device 21 from above downwards and thus through the thermal store 19. The storage medium 20 can also be a heat transfer oil.

[0049] The heat discharge device 21 is shown in Figure 4 as a (further) heat exchanger, so that the intermediate medium 18 is separated from the storage medium 20. As long as the intermediate medium 18 and the storage medium 20 are of the same type, the heat discharge device 21 can also be configured as a simple tube, which opens into the thermal store 19. In this case, there is (of course) also a further tube from the thermal store 19 back to the heat exchanger 17.

[0050] During the compression process in the compressor 8 of the compressor unit 6, the air 7 is heated. This is unavoidable. However, in order to minimize the mechanical energy (and thus in the usual case also the electrical energy) required for the compression of the air 7, according to Figure 2 is shown, the compression of the air 7 is preferably carried out in stages, wherein the air 7 is repeatedly cooled between the individual compression processes. Furthermore, the air 7 is expanded in the compressed-air motor 11 and cooled in the process. This is also unavoidable. In the compressed-air motor 11, an adiabatic expansion of the air 7 also usually takes place. If V2 denotes the volume of the air 7 delivered to the compressed-air motor 11 at the final pressure p2 in one individual working cycle and V3 denotes the volume of the air 7 discharged from the compressed-air motor 11 after the working cycle, i.e. after the expansion, the following relationship holds with very good approximation: (V2 / V3) 1,4 = (T3 / T2) 3,5 = p3 / p2 (1) The air 7 is usually discharged by the compressed-air motor 11 into a storage container (which can also be the normal environment), in which there is an initial temperature T1 and an initial pressure p1. Therefore, in the ideal case the following relationship should be fulfilled: (V2 / V3)1,4 = (T1 / T2)3,5 = p1 / p2 (2) In other words: In the ideal case, the discharge temperature T3 and the discharge pressure p3 at least approximately coincide with the initial pressure pi and the initial temperature Ti: p3 = p1 and T3 = T1 (3) Therefore, in the ideal case, the operating mode of the compressor unit 6 and the operating mode of the compressed-air motor 11 are coordinated with each other in such a way that the air 7 at the moment of leaving the compressed-air motor 11 has exactly the pressure and the temperature that it had at the beginning of the cycle, i.e. the initial pressure pi and the initial temperature Ti. The aforementioned moment is the moment at which the air 7 in the compressed-air motor 11 expands transitions into the discharge of the air 7 in the compressed-air motor 11.

[0051] Preferably, according to Figure 5 As shown, the system has a regulating device 22 by means of which the compressor unit 6 and / or the compressed-air motor 11 is regulated in such a way that, within the possible range, the condition is fulfilled that the discharge temperature T3 and the discharge pressure p3 at least approximately coincide with the initial pressure pi and the initial temperature Ti. Thereby, the efficiency can be optimized.

[0052] According to Figure 5 , the initial temperature Ti, the final temperature T2 and the initial pressure pi are delivered as input variables to the regulating device 22. Because these variables have to be accepted in their true state.

[0053] The regulating device 22, if possible, first controls the ratio of the volumes V2 and V3 in such a way that this ratio fulfils a predetermined condition depending on the ratio of the temperatures Ti and T2, for example according to the above equation. Although the volumes V2 and V3 of the already manufactured compressed-air motor 11 cannot be changed anymore. However, for example, a plurality of compressed-air motors 11 can be arranged in parallel with different ratios of the volumes V2 and V3 to each other and via the actuation of the upstream valves it is determined which compressed-air motor 11 is activated. Similarly, it is also possible to arrange a plurality of compressed-air motors 11 in series in front of each other and to provide the possibility to decouple one or more compressed-air motors 11 from the series arrangement (bridging) in order to optimize the (resulting) ratio of the volumes V2 and V3.

[0054] This way of changing the ratio of the volumes V2 and V3 is also of great importance for the optimization of the operation. If, for example, the final temperature T2 is 300°C or 573 K and the initial temperature Ti, i.e. the ambient temperature, can fluctuate between -20°C and +40°C, then the optimal ratio of the volumes V2 and V3 is between 7.72 and 4.535. In this case, for example, four separate compressed-air motors 11 can be provided for four temperature ranges of 15 K each, wherein the volume ratio of the compressed-air motors 11 can be (approximately) 4.83, 5.47, 6.25 and 7.19. Depending on the temperature range, it is then selected which compressed-air motor 11 is activated for operation.

[0055] Furthermore, the regulating device 22, if necessary after a defined operating configuration of the compressed air motor 11 has been determined, controls the compressor aggregate 6 and / or the compressed air motor 11 such that the final pressure p2 fulfils a predetermined condition depending on the ratio of the temperatures T1 and T2, for example a condition according to the above equation. For example, the volume flow delivered to the compressor aggregate 6 and the volume flow discharged from the compressed air motor 11 can be coordinated, relative to the initial pressure p1, by adjusting the rotational speed nK of the compressor aggregate 6 and / or the rotational speed nM of the compressed air motor 11, in order to set the required final pressure p2.

[0056] Based on the same final temperature T2 (300°C) and a possible initial temperature T1, the optimum ratio of the final pressure p2 to the initial pressure p1 is between approximately 17.5 and approximately 8.3. Since the initial pressure p1, i.e. the ambient atmospheric pressure, can also fluctuate somewhat, the final pressure p2 should be able to vary within a greater range, for example between 800000 Pa and 1820000 Pa when the initial pressure p1 fluctuates upwards and downwards by up to 4%. The ratio between the maximum possible final pressure p2 and the minimum possible final pressure p2 is therefore slightly higher than 2.25. Alternatively, it is also possible to always produce a relatively high final pressure p2, for example 1820000 Pa, but the air 7 is not discharged into the environment at the output side of the compressed air motor 11 in some cases, but is instead fed back to a certain location inside the compressor aggregate 6, for example Figure 2 between the first and intermediate compressors 8 of the compressor aggregate 6.

[0057] As already mentioned before and shown in Figure 6 , the compressed air motor 11 is usually coupled with an electric motor 12, so that the compressed air motor 11 drives the electric motor 12 to produce electrical energy. Furthermore, an electric drive is also required for driving the compressors 8. This can be advantageously achieved according to Figure 6 by the electric motor 12 having a housing 23 in which a first shaft 24 and a second shaft 25 are supported coaxially to one another. According to Figure 6 , the second shaft 25 is supported on the first shaft 24 and thus indirectly in the housing 23, which is merely purely exemplary.

[0058] The first shaft 24 and the second shaft 25 carry a driving portion 26, 27, respectively, of the electric machine 12. Between the two driving portions 26, 27 an electromotive force can be generated. Thus, the two driving portions 26, 27 correspond in principle to a rotor and a stator of the electric machine 12. However, since both driving portions 26, 27 are arranged on the shafts 24, 25, not only the "rotor" can rotate about its axis, but also the "stator". This design can advantageously be utilized in that the first shaft 24 is connected to the compressed-air motor 11 and the second shaft 25 is connected to the compressor 8 of the compressor unit 6. Thereby, it is possible to directly drive the compressor 8 of the compressor unit 6 by the compressed-air motor 11. Thus, the process of converting the mechanical energy provided by the compressed-air motor 11 into electrical energy and converting the electrical energy again into mechanical energy for driving the compressor 8 is omitted. Thereby, also the losses associated with each conversion can be avoided. This makes it possible to increase the part of the remaining mechanical energy which is converted into electrical energy by the electric machine 12.

[0059] If the arrangement does not have a further electric machine, as shown in Figure 6 , a decoupled operation of the compressor unit 6 and the compressed-air motor 11 would not be possible. In order to achieve such a decoupled operation, the system thus has a further electric machine 28, as shown in Figure 7 and Figure 8 . For this electric machine 28, as usual, the stator is arranged in a manner resistant to relative rotation. The rotor of the further electric machine 28 can be connected, as required, to the compressed-air motor 11 ( Figure 7 ) or to the compressor 8 of the compressor unit 6 ( Figure 8 ).

[0060] The compressor 8 of the compressor unit 6 can be configured as required. It is possible that the compressor 8 is configured as a gas turbine. It is generally better that the compressor 8 is configured as a compressor portion of a space propulsion engine. The construction of such a compressor portion has been explained in detail in the introduction to the description. It is generally better that the compressor 8 is configured as a screw compressor. In particular, screw compressors can achieve a very high relative efficiency, far exceeding 90%. The relative efficiency is defined as the quotient between the actual efficiency and the theoretically possible efficiency. If, for example, the efficiency calculated according to the Carnot process under given temperature conditions is 32% and the efficiency of the actual machine is 24%, the relative efficiency is 24% / 32% = 75%.

[0061] Similar to the compressor 8, the compressed air motor 11 can also be configured as desired. For example, the compressed air motor 11 can be configured as a gas turbine. It is generally preferred that the compressed air motor 11 is configured as a motor portion of a space propulsion engine. The configuration of such a motor portion has been explained in detail in the introduction of the description. It is generally preferred that the compressed air motor 11 is configured as a screw motor. In particular, screw motors can achieve a very high relative efficiency, far above 90%. The relative efficiency is defined in the same way as explained above for the compressor 8.

[0062] The high relative efficiency of the compressed air motor 11 and the compressor 8 is also of great importance for the economic operation of the system. Since the absolute efficiency of a Carnot cycle process is slightly above 30% at typical initial and final temperatures Tl, T2. At typical temperature conditions, each percentage point of relative efficiency lost in the operation of the compressor 8 and in the operation of the compressed air motor 11 reduces the relative efficiency of the entire system by about 4%. If, for example, the thermal power is 3.84 MW under ideal, completely loss-free conditions, the compressed air motor 11 provides 2.83 MW of power, and the compressor 8 requires 1.66 MW of power, the system can provide as useful power: 2.83 MW - 1.66 MW = 1.17 MW (4).

[0063] However, losses always occur in practice. For example, on the side of the compressed air motor 11, there are friction losses in the compressed air motor 11 and the electric motor 12, electrical losses in the electric motor 12, and converter losses in the inverter 13. Similarly, for driving the compressor 8, there are friction losses in the compressor 8 and the drive electric motor, electrical losses in the drive electric motor and the associated power supply inverter. It is assumed below that the relative efficiency of the entire respective chain is 95% in both cases, which is a very good value in practice. Even at such still very high relative efficiencies, the useful power has already been reduced to: 0.95 - 2.83 MW - 1.66 MW / 0.95 = 0.94 MW (5).

[0064] The useful power is thus reduced to about 80% of its theoretically possible maximum. At a relative efficiency of 90%, the useful power is reduced to about 60% of its theoretically possible maximum, at 85% to about 38.5%, and at 80% to only about 16%. At a relative efficiency of about 76.5%, the useful power drops to 0 and then becomes negative. A high relative efficiency is thus of great importance.

[0065] The application has a number of advantages. Firstly, the waste heat of the main installation 1 can be used effectively. Since air 7 is used as the working medium, no special precautions are required to prevent leakage of the air 7. Such leakage, although it does result in a reduction in efficiency, does not cause environmental pollution. The compressor 8 and the compressed air motor 11 can be constructed simply and robustly. With a suitable design, they can be operated with a relatively high efficiency (significantly more than 90%).

[0066] Although the application has been described and illustrated in detail by reference to the preferred embodiments, it is not intended that the application be limited to the examples disclosed. Other variations of the application will be apparent to those skilled in the art. It is intended that all such modifications and variations be included within the scope of the application, which is defined by the following claims.

[0067] List of reference signs 1 main installation 2 exhaust air 3 extraction device 4 recovery device 5 double arrow 6 compressor unit 7 air 8 compressor 9 cooling device 10, 17 heat exchanger 11 compressed air motor 12, 28 electric motor 13 inverter 14 power grid 15 compressed air reservoir 16 air intake point 18 intermediate medium 19 heat accumulator 20 storage medium 21 heat discharge device 22 regulating device 23 housing 24, 25 shaft 26, 27 active part nK, nM rotational speed pl, p2, p3, pa, pb pressure Tl, T2, T3, Ta, Tb, Tc temperature V2, V3 volume

Claims

1. Systems for energy recovery, - wherein, The system has a main facility (1) that generates hot exhaust gas (2) during operation of the main facility. -The main facility (1) has an exhaust device (3), through which the exhaust gas (2) flows and is exhausted from the main facility (1). -The system has a compressor unit (6) comprising multiple compressors (8), by means of which air (7) delivered to the compressor unit (6) having an initial temperature (T1) is compressed from an initial pressure (p1) to a final pressure (p2) and an intermediate temperature (Tc). -The system has a heat exchanger (10) arranged downstream of the compressor unit (6), through which air (7) compressed to the final pressure (p2) and the intermediate temperature (Tc) flows, and by means of the heat exchanger, the heat of the exhaust gas (2) flowing through the outlet device (3) is transferred to the air (7) flowing through the heat exchanger (10), thereby heating the air (7) to the final temperature (T2), and -The system has a compressed air motor (11) arranged downstream of the heat exchanger (10), air (7) heated to the final temperature (T2) and having the final pressure (p2) is supplied to the compressed air motor, the compressed air motor operates using the air (7), and the air (7) is discharged from the compressed air motor at a discharge temperature (T3) and a discharge pressure (p3).

2. The system of claim 1, wherein, The number of compressors (8) is greater than 1 and the compressors (8) are arranged in series, so that the air (7) is compressed in multiple stages from the initial pressure (p1) to the final pressure (p2), and the system has a cooling device (11) arranged between every two consecutive compressors (8) to cool the air (7) delivered from the compressor (8) arranged upstream of the respective cooling device (11) to the compressor (8) arranged downstream of the respective cooling device (11).

3. The system of claim 1 or 2, wherein, The system does not have a cooling device after the compressor (8) compresses the air (7) to the final pressure (p2) and the intermediate temperature (Tc).

4. The system of claim 1, 2, or 3, wherein, The system has another heat exchanger (17) arranged in the outlet device (3) and by means of the other heat exchanger transfers the heat of the exhaust gas (2) to the intermediate medium (18), and the intermediate medium (18) acts directly or indirectly on the heat exchanger (10) that heats the air (7).

5. The system of claim 4, wherein, The system has a heat storage tank (19), a heat exchanger (10) is arranged at least in the upper region of the heat storage tank (19), and preferably air (7) flowing through the heat exchanger (10) flows from bottom to top through the heat storage tank (19); and a heat exhaust device (21) is arranged at least in the lower region of the heat storage tank (19), and preferably the intermediate medium (18) flows from top to bottom through the heat storage tank (19), so that the intermediate medium (18) carries heat into the heat storage tank (19) via the heat exhaust device (21).

6. The system of claim 1, 2, or 3, wherein, The heat exchanger (10) is arranged in the outlet device (3) so that the heat of the exhaust gas (2) is directly transferred to the heat exchanger (10).

7. The system of any of the preceding claims, wherein, The system has a compressed air reservoir (15) arranged between the heat exchanger (10) and the compressed air motor (11).

8. The system of any of the preceding claims, wherein, The air (7) supplied to the compressor unit (6) is taken from the environment.

9. The system of any of the preceding claims, wherein, The air (7) discharged from the compressed air motor (11) is released into the environment.

10. The system of any of the preceding claims, wherein, The system has an adjustment device (22) by means of which the compressor unit (6) and / or the compressed air motor (11) are adjusted such that the discharge temperature (T3) and the discharge pressure (p3) are at least approximately the same as the initial pressure (p1) and the initial temperature (T1).

11. The system of any of the preceding claims, wherein, The compressor unit (6) includes a gas turbine or a screw compressor as the compressor (8), or the compressor (8) is constructed as follows: The compressor (8) has a first shaft and a second shaft, the axes of which are arranged parallel to each other with a lateral offset and rotate synchronously and at the same speed during operation, thereby causing the two shafts to roll against each other and airtightly seal against each other. -The compressor (8) has a housing that annularly surrounds the first shaft. - The first shaft has a nose on its outer side, the nose sealingly surrounding the housing. - The second shaft has a groove on its outer side, and the nose of the first shaft is temporarily immersed in the groove when the shaft rotates, and then disengages from the groove. An inlet opening and a outlet opening are arranged in the housing. Viewed from the rotational direction of the first shaft, the nose passes through the inlet opening and the outlet opening shortly after disengaging from the groove, i.e., shortly before immersing into the groove. This causes the nose to divide the corresponding cavity formed by the first shaft and the surrounding housing into a first segment and a second segment. The first segment extends from the angular position where the nose of the first shaft disengages from the groove of the second shaft to the instantaneous angular position of the nose. The second segment extends from the instantaneous position of the nose to the angular position where the nose of the first shaft is immersed in the groove of the second shaft. - The input opening is permanently open, thereby continuously supplying air to the first section. - Compressed air is discharged through the discharge opening. - The discharge opening is equipped with a closing device that briefly opens the discharge opening for a period of time shortly before the nose of the first shaft is immersed in the groove of the second shaft, and then closes it again.

12. The system of any of the preceding claims, wherein, The compressed air motor (11) is constructed as a gas turbine or a screw motor, or as follows: The compressed air motor (11) has a first shaft and a second shaft, the axes of the first shaft and the second shaft being arranged parallel to each other with a lateral offset, and rotating synchronously and at the same speed during operation, thereby causing the two shafts to roll against each other and be airtightly sealed to each other. -The compressed air motor (11) has a housing that annularly surrounds the first shaft. - The first shaft has a nose on its outer side, the nose sealingly surrounding the housing. - The second shaft has a groove on its outer side, and the lug of the first shaft is temporarily immersed in the groove when the shaft rotates, and then disengages from the groove. An inlet and an outlet are arranged in the housing. Viewed from the direction of rotation of the first shaft, the nose passes through the inlet and outlet shortly after disengaging from the groove, i.e., shortly before immersing into the groove. This divides the annular cavity formed by the first shaft and the surrounding housing into a first segment and a second segment. The first segment extends from the angular position where the nose of the first shaft disengages from the groove of the second shaft to the instantaneous angular position of the nose. The second segment extends from the instantaneous position of the nose to the angular position where the nose of the first shaft is immersed in the groove of the second shaft. The input opening is equipped with a closing device that briefly opens the input opening for a short period of time after the lug of the first shaft disengages from the groove of the second shaft, and then closes it again, thereby temporarily supplying compressed air to the first section. - The discharge opening is permanently open, thereby continuously discharging air through the discharge opening.

13. The system according to any one of the preceding claims, characterized in that, The system has a motor (12) with a housing (23) in which a first shaft and a second shaft (24, 25) are coaxially supported. The first shaft and the second shaft (24, 25) respectively carry an active part (26, 27) of the motor (12), thereby generating an electromotive force between the two active parts (26, 27). The first shaft (24) is connected to the compressed air motor (11), and the second shaft (25) is connected to the compressor (8) of the compressor unit (6).

14. The system according to claim 13, characterized in that, The system has another motor (28) whose stator is arranged in a way that prevents relative rotation, and whose rotor is connected to the compressed air motor (11) or the compressor (8) of the compressor unit (6).

15. The system according to any one of the preceding claims, characterized in that, The system has at least one air intake point (16) at which air (7) can be drawn from the compressor unit (6), wherein the air (7) drawn from the compressor unit (6) has an extraction pressure greater than the initial pressure (p1) and at most equal to the final pressure (p2).

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