Arrangement and method for converting waste heat into mechanical energy and use of an absorption cooling system as temperature controller

Abstract

The invention relates to an arrangement and a method for converting waste heat into mechanical energy using supercritical sCO2, comprising a pump arrangement (1) for increasing the pressure of the CO2 through a line (2), two heat exchangers, a pre-heating device (7) and a main heating device (12), for transferring waste heat from a waste heat stream in a line (14, 15, 16) to the CO2 as working medium, such that the CO2 can be present in the supercritical state, an engine (17) for generating mechanical energy by means of the sCO2, and an absorption cooling system for controlled cooling of the CO2 from the engine (17), comprising an expeller (23) and an evaporator (31), wherein a line for the CO2 (19) exiting the engine (17) goes from the engine (17) to the expeller (23) and to the evaporator (31) of the absorption cooling system. According to the invention, the absorption cooling system keeps the temperature at the inlet of the pump arrangement (1) constantly below the critical temperature, a recuperator (6) is arranged between the engine (17) and expeller (23) for the transfer of heat from the CO2 exiting the engine (17) to the sCO2 going to the engine (17), and a flow divider (3) is arranged between the pump arrangement (1), the recuperator (6) and the pre-heating device (7) to divide the stream of CO2 into a first partial stream to the recuperator (6) and a second partial stream for the transfer of waste heat by means of the two heat exchangers (7, 12).

Description

[0001] Arrangement and method for converting waste heat into mechanical energy and using an absorption refrigeration system as a temperature controller

[0002] The invention relates to an arrangement and a method for converting waste heat into mechanical energy, in particular kinetic energy, using carbon dioxide (CO2) in a supercritical state. CO2 in a supercritical state or supercritical CO2 refers to carbon dioxide in a supercritical state, above the critical point at a temperature of 31 °C and a pressure of 73.8 bar, also called supercritical CO2, hereinafter also referred to as sCO2. Any heat source is considered waste heat within the meaning of the invention, even if it is not residual heat from an industrial process, but rather, for example, from a renewable source.

[0003] The arrangement comprises a) a pump arrangement for increasing the pressure and transporting the CO2 under transcritical conditions from a line, wherein the CO2 at the inlet of the pump arrangement has a pressure above the critical CO2 pressure and a temperature below the critical CO2 temperature, b) at least two heat exchangers, a preheater and a main heater for transferring waste heat from a waste heat stream in a line to the CO2 as a carrier medium and working medium, so that the CO2 can be present in the supercritical state as sCO2, c) a power engine for generating mechanical energy using the sCO2, and d) an absorption refrigeration system for the controlled cooling of the CO2 emerging from the power engine, from which heat has already been extracted in the recuperator, comprising an expeller and an evaporator in which heat is extracted from the CO2 in two stages by means of a line,which leads from the recuperator to the expeller and further includes a line which leads from the expeller to the evaporator.

[0004] The method comprises the steps of a) increasing the pressure and transporting CO2 by means of a pump arrangement, wherein the CO2 at the inlet of the pump arrangement has a pressure above the critical CO2 pressure and a temperature below the critical CO2 temperature, b) transferring waste heat from a waste heat stream to CO2 as carrier medium and working medium sequentially in at least two stages, so that the CO2 is present as sCO2 in the supercritical state after compression, and c) generating mechanical energy from the sCO2 by means of a power machine.

[0005] The invention also relates to the use of an absorption chiller as a bottoming system of an sCO2 cycle, wherein the absorption chiller serves as a cooler for the topping system and comprises an expeller and an evaporator. The control relates to the heat removal from the CO2 system of a system for converting waste heat into mechanical energy, or more specifically, into kinetic energy, of the system according to the invention. The system for converting waste heat into mechanical energy is also referred to below as the energy system.

[0006] Waste heat is heat that is primarily generated by technical systems and released into the environment or can be used. From a thermodynamic perspective, most real-world processes are irreversible. As a result of the dissipation of energy, heat is inevitably generated during these processes. Technical devices and systems cannot operate without generating waste heat. This heat must usually be dissipated to prevent malfunctions caused by overheating or to restore the initial state of the working fluid in cyclic processes.

[0007] Due to the special properties of CO2 above the critical point and the resulting advantages in its application in thermodynamic cycles, experts consider CO2 to be the working fluid of a new generation of thermal energy machines. Higher efficiencies and a significant reduction in the size and complexity of individual components enable better utilization of existing heat sources and the development of new, previously uneconomical heat sources.

[0008] Carbon dioxide (CO2, ASHRAE number: R744) has a long history of use as a refrigerant, but was largely displaced with the advent of synthetic refrigerants in the 1930s. It wasn't until the 1980s and 1990s, when it became clear that the most commonly used synthetic refrigerants were primarily responsible for ozone depletion and the greenhouse effect, that carbon dioxide returned to the spotlight.

[0009] CO2 is non-flammable, does not contribute to ozone depletion, and has a much lower global warming potential than conventional refrigerants. In small quantities, the gas is also non-toxic, but high concentrations in the air pose a risk of suffocation. Systems that use CO2 as a refrigerant, however, must withstand higher system pressures and lower, critical temperatures. A system must be constructed and maintained accordingly safely.

[0010] However, since carbon dioxide has a more favorable pressure ratio and five to eight times greater volumetric capacity than conventional refrigerants, system components such as compressors and pipe diameters can be significantly smaller. Due to the high heat transfer coefficient of CO2, the heat exchanger can be smaller. Due to its low viscosity, less pumping work is required, which is particularly advantageous for systems with longer network lengths. All of this, in turn, reduces material usage, system weight, and defrosting time.

[0011] Since the critical point of CO2 is close to the ambient temperature at approximately 31 °C (degrees Celsius), seasonal and diurnal fluctuations in the ambient temperature have a significant impact on heat removal from the process. For a given chiller design, an increase in the ambient temperature results in efficiency and performance reductions, as the pressure increase in the process shifts toward higher compressibility and, consequently, compression work. The extent of these effects depends on the process architecture and the location of the inlet condition for the pressure increase at the design point (DP).

[0012] The critical point is therefore very close to the operating range. In fact, the critical point, which is 31 °C and 73.8 bar, is exceeded at an ambient temperature of just 26 °C. This is referred to as transcritical operation of the refrigeration system, especially when both subcritical and supercritical conditions occur.

[0013] The point, defined by pressure and temperature, at which all three states of matter—solid, liquid, and gas—exist simultaneously, i.e., are in equilibrium with each other, is called the triple point. The critical point is reached when the densities of the liquid and gaseous phases equalize to such an extent that there is no longer any difference in pressure and temperature between the two states. When this critical point is exceeded, the system is referred to as a transcritical or supercritical system.

[0014] Thermodynamic points usually play no role with conventional refrigerants, but they are important when using CO2. Very high pressures occur above the critical point at 31 °C and 73.8 bar. Therefore, in subcritical systems, i.e., those operating below the critical point, excessive heating of the system (e.g., due to the ambient temperature) must be avoided, for example, through protective measures such as auxiliary cooling, an expansion tank, or the venting of CO2.

[0015] Transcritical systems typically do not have a condenser, but rather a gas cooler. Furthermore, transcritical systems must withstand very high pressures well over 75 bar. The use of transcritical CO2 refrigeration systems is particularly efficient when the ambient temperature or the temperature on the warm side is below 15 °C on average, because the higher the temperature, the lower the efficiency.

[0016] It is known from the prior art to combine absorption chillers with sCO2 energy systems, energy systems that utilize supercritical CO2, to utilize waste heat, for example, from industrial exhaust gases. The absorption chiller is often referred to as a bottoming system, and the sCO2 system as a topping system. These terms will also be used below in the description of the invention.

[0017] In Yang et al. (2019), only a small amount of waste heat is utilized, as the heat source has a temperature of only 140 °C. An external temperature source is proposed to operate the expeller (sometimes also referred to as a generator) of the LiBr / H2O (lithium bromide / water) absorption chiller. A similar approach is disclosed by Wu et al. (2017), using an NH3 / H2O (ammonia-water) absorption chiller. Furthermore, the system in Yang et al. (2019) operates in the subcritical CO2 range with temperatures below 10 °C and pressures of approximately 4 bar. In CN 112412555 A, sCO2 intercooling is used, and the operating temperature of the CO2 is higher than required for the supercritical state. Combined sCO2 / LiBr / H2O systems for heat sources such as nuclear reactors, solar power plants and combustion engines operate at high temperatures (over 600 °C), as described, for example, in publication CN 112412555 A.There, the cooling water is used to lower the CO2 temperature in the intercooler before secondary compression. Temperature control under subcritical conditions before compression is not planned. Supercritical operation, i.e., temperatures and pressures higher than the critical CO2 pressure, is considered.

[0018] Ma / Liu et al. (2018) and Ma / Zhang et al. (2018) disclose a combination of sCO2 cycle with LiBr absorption chiller, whereby only a part of the CO2 mass flow is cooled by the LiBr system.

[0019] The sCO2 cycle is designed as a recompression architecture in all three previously known solutions. The recompression architecture is one of the most common configurations of the sCO2 cycle in application studies of this cycle, as it offers high thermal efficiency, primarily made possible by high heat recovery. This property is due to the use of at least two heat recovery units: high-temperature (HTR) and low-temperature (LTR) recuperators. Due to the use of a recompression system and the high power consumption of the compressor, the specific work of this type of system is very high. This means that the recompression system is not economical for applications involving medium-temperature heat sources, such as waste heat recovery.

[0020] The solutions by Ma / Liu et al. (2018) and Ma / Zhang et al. (2018) are designed for high-temperature heat sources for applications in concentrated solar power systems. A thermal oil transfers the energy from the heat source to the sCO2 cycle in the main heater. Suitability for waste heat recovery, where energy is primarily recovered from exhaust gas, is not apparent. Ma / Zhang et al. (2018) consider supercritical operation, i.e., temperatures and pressures higher than the critical CO2 pressure, while Ma / Liu et al. (2018) present a process with lower temperatures, specifically for a mixture of CO2 and krypton.

[0021] Publication CN 112412555 A and Ma / Zhang et al. (2018) describe internal heat exchangers with SHX equipment (solution heat exchanger), each shown in Fig. 1. This device is a heat exchanger between the expeller and the absorber of the absorption chiller system. The use of this equipment is standard for the absorption system but requires additional effort.

[0022] The solution described in publication CN 109519243 B uses an sCO2 system combined with an NH3 / H2O absorption chiller. Superheated ammonia drives a dedicated turbine. The sCO2 cycle is a simple cycle. The NH3 / H2O vapor is used as the heat source for a LiBr absorption chiller.

[0023] The publication CN 109519243 B describes an energy plant based on the interconnection or combination of two Rankine cycles, consisting of a simple sCO2 cycle as the upstream process (topping cycle) and an ammonia / water (NH3 / H2O) steam cycle as the downstream process (bottoming cycle). The non-working heat from the two thermodynamic cycles is removed using a LiBr absorption chiller and an additional, separate CO2 cooler. In the sCO2 cycle, the heat removal is as follows:

[0024] After exiting the turbine, the heat from the sCO2 is first utilized in the expeller (generator) of a LiBr absorption chiller. The carbon dioxide is then further cooled in a separate CO2 cooler with an undisclosed external cooling medium and a downstream cooler, the evaporation condenser, until it is completely liquefied and then fed to the pump. The latter cooler, or CO2 condenser, represents the evaporator of the LiBr absorption chiller. In the sCO2 process, heat removal thus occurs via three devices.

[0025] The publication CN 1 09 519 243 B does not provide any information on state and process variables, so that the process control and the step-by-step heat removal cannot be reproduced thermodynamically.

[0026] All known systems with absorption chillers utilize only a portion of the primary heat source, usually the waste heat intended for use, as the driving energy source for the absorption chiller, which ultimately reduces the potential of the topping system, i.e., the sCO2 energy system. Gotelip et al. (2022) describe the heat exchange between CO2 exiting the turbine and CO2 entering the turbine as a preheating process. A disadvantage is that the system is dependent on external cooling water and its constant temperature. Seasonal fluctuations cannot be compensated for.

[0027] In most known recompression architectures, the CO2 partial streams are combined shortly before the high-temperature recuperator. Therefore, in such a system (HTR), the mass flows of both streams (hot side and cold side) are equal.

[0028] In all known systems, it is difficult to operate the cooling capacity so consistently and effectively that the subcritical range (after expansion of the working medium in the turbine) can be operated all year round and thus a high process efficiency can be ensured.

[0029] The object of the present invention is therefore to provide an arrangement and a method for converting waste heat into mechanical energy using carbon dioxide (CO2) in the supercritical state (sCO2), as well as a method for its operation and the use of an absorption refrigeration system, which overcome the disadvantages of the prior art and achieve a constant cooling capacity of a CO2 energy system combined with the highest possible efficiency and useful performance in the conversion of waste heat into kinetic energy. The system should be as simple as possible and require as few components as possible. These components should be as robust and less prone to failure as possible, while also being able to operate independently of daily and seasonal fluctuations in the ambient temperature.It is also intended to provide a robust system for off-design conditions, i.e. conditions that differ from those for which the system was designed.

[0030] According to the invention, the object is achieved by the subject matter of the independent patent claims. Advantageous embodiments of the invention are specified in the dependent claims. The subject matter of the invention is an arrangement for converting waste heat into mechanical energy, in particular kinetic energy, an energy plant in which CO2 in its state as sCO2 is used to convert waste heat into mechanical energy, comprising a) a pump arrangement for increasing the pressure and transporting the CO2 under transcritical conditions from a line, wherein the CO2 at the inlet of the pump arrangement has a pressure above the critical CO2 pressure and a temperature below the critical CO2 temperature, b) at least two heat exchangers, a preheater and a main heater, for transferring waste heat from a waste heat stream in a line to the CO2 as a carrier medium and working medium, so that the CO2 can be present in the supercritical state as sCO2,c) a power engine for generating mechanical energy using the CO2, and d) an absorption refrigeration system for the controlled cooling of the CO2 emerging from the power engine, from which heat has already been extracted in the recuperator, comprising an expeller and an evaporator in which heat is extracted from the CO2 in two stages by including a line leading from the recuperator to the expeller and a line leading from the expeller to the evaporator.

[0031] The cooling system, with its main components of expeller, condenser, evaporator, and absorber, primarily serves to remove heat from the two streams passing through the absorption chiller. According to a preferred embodiment, the first stream serves as the heat source in the expeller for evaporating water from the mixture of water (refrigerant) and lithium bromide (absorbent) represented by the stream. The stream downstream of the recuperator is therefore the primary drive for the expeller. The stream from the condenser to the evaporator, at a temperature of 15 °C, is the cooling medium for the second stage of heat removal from the CO2 stream in the evaporator. In this case, the evaporator is the main cooler of the sCO2 cycle.

[0032] According to the invention, a recuperator, a heat exchanger, is arranged between the engine (or in particular the turbine) and the expeller. The recuperator, as a special heat exchanger, serves to transfer heat from the CO2 escaping from the engine to the sCO2 leading to the engine. This means that a line leads from the engine to this recuperator, from there to the expeller, and from the expeller to an evaporator, from which the line runs to the pump arrangement. A pump or a compressor, for example, is used as the pump arrangement. Furthermore, a flow divider for dividing the CO2 flow into a first partial flow to the recuperator and a second partial flow for transferring waste heat by means of the first heat exchanger, the preheater, is arranged in the pumping direction.The arrangement according to the invention is a thermo-economically optimized SH (Sequential Heating) architecture.

[0033] The present invention also encompasses the possibility that the line can lead to further, additional components along its course. The expeller and evaporator are thus coolers for a topping process, with the evaporator known to evaporate the coolant of the absorption refrigeration system. According to the invention, the evaporator serves as a condenser for the CO2 exiting the turbine, so that it is subsequently present as subcritical CO2, preferably at a temperature of 20 ± 5 °C, in particular 20 ± 2 °C. This temperature range represents a central component of the invention.

[0034] It has proven advantageous if the CO2 is transported through the pump assembly or enters it under transcritical conditions. The CO2 at the inlet of the pump assembly has a pressure above the critical CO2 pressure and a temperature below the critical CO2 temperature. Preferably, the CO2 under transcritical conditions has a pressure above the critical pressure, but at least 75 bar, and a temperature between 20 °C and 28 °C.

[0035] Further advantages arise from an embodiment in which the flow divider is designed or adjusted to ensure a mass flow ratio of 55 to 65% for the first partial flow of CO2 under high pressure in the first line relative to the partial flow in the second line, both of which lead to the recuperator. The first line leads directly from the flow divider to the recuperator, while the second line leads from the engine or turbine to the recuperator, so that the CO2 flowing there can transfer the residual heat from the engine to the first partial flow.

[0036] The division into partial flows is an important feature of the preheating architecture.

[0037] In the preheating architecture, the CO2 mass flow is split into two streams by the flow divider downstream of the main compressor. Thus, in contrast to the state of the art, the entire CO2 mass flow is routed through the circuit for heat removal and then to the main compressor. In the previously known recompression architecture, however, only a portion of the total mass flow is routed for heat removal and then to the main compression stage, and only this portion of the mass flow is used in the absorption system.

[0038] The architecture proposed by the invention eliminates the need for a booster compressor, enabling system simplification and improved economic performance. In previously known solutions, the CO2 flow is split after the low-temperature recuperator, the main feature of the recompression architecture. With such an arrangement, only a portion of the CO2 mass flow is fed into the absorption chiller, while the remainder is fed into the recompressor. In contrast to the prior art, in the present application, the entire mass flow expanded in the engine, in particular the turbine, is fed to the recuperator and subsequently completely to the exhauster of the absorption chiller.

[0039] In the current state of the art, the precooler usually takes over heat removal, with some of the heat also being removed by supplying energy to the absorption chiller. Therefore, the previously known application differs in terms of cooling water consumption and temperature levels. In the proposed solution, the absorption chiller instead transfers the heat from the CO2 directly to the evaporator and keeps the CO2 temperature at the pump or compressor inlet below the critical CO2 temperature, as specified.

[0040] According to a preferred embodiment, high-pressure CO2 in a first partial stream from the recuperator and high-pressure CO2 in a further partial stream supplied by the preheater are combined through a mixer into a line leading to the main heater. The preheater architecture created in this way enables better utilization of the heat source by recovering heat at two temperature levels, with the main heater serving as a heat exchanger at higher temperatures. In this way, the preheater enables the recovery of additional heat from the heat source downstream of the main heater. This creates an additional mass flow, enabling higher power output from the engine (or power generation by a connected generator) and a more efficient cycle.

[0041] The line from the engine continues via the recuperator to the absorption chiller. It has proven advantageous to route a line between the expeller and the evaporator of the absorption chiller past a sub-cooler, which further cools the CO2 and increases the cooling capacity at the evaporator.

[0042] A particularly advantageous embodiment of the absorption chiller is a LiBr / H2O absorption chiller. The LiBr absorption system generally operates with a heat source temperature above 75 °C at the engine. Therefore, the LiBr absorption system offers better operating characteristics for the proposed system. On the other hand, absorption systems based on ammonia / water, for example, require a heat source with temperatures above 100 °C, which would result in a performance loss for the topping system (sCO2).

[0043] A preferred embodiment of the arrangement according to the invention comprises a generator for generating electricity, mechanically connected to the engine designed as a turbine.

[0044] The invention also relates to a method for converting waste heat into mechanical energy, in particular kinetic energy, comprising the steps of: a) increasing the pressure of CO2 and transporting it through a line by means of a pump arrangement, wherein the CO2 at the inlet of the pump arrangement (1) has a pressure above the critical CO2 pressure and a temperature below the critical CO2 temperature, b) transferring waste heat from a waste heat stream to CO2 as carrier medium and working medium sequentially in at least two stages, so that the CO2 is present as sCO2 in the supercritical state after compression, c) generating mechanical energy from the sCO2 by means of a power machine.

[0045] According to the invention, as process steps d), e) and f): d) by means of a recuperator arranged between the engine and the expeller, a heat transfer takes place from the CO2 exiting the engine through the line to the sCO2 in a line leading to the engine, and that the CO2, after being transported through the pump arrangement in step a), is divided by a flow divider, whereby a first partial flow is passed through the line to the recuperator, and a second partial flow is passed through a line for transferring waste heat in step b), e) controlled cooling of the CO2 exiting the engine by means of an absorption refrigeration system comprising an expeller and an evaporator, whereby the CO2 exiting the engine via a line is cooled first by means of the expeller and then by means of the evaporator of the absorption refrigeration system,f) the absorption refrigeration system keeps the temperature at the inlet of the pump arrangement constantly below the critical temperature, a heat transfer from the CO2 exiting the engine or turbine to the sCO2 leading to the turbine in the flow direction of the CO2 before step d) by means of a recuperator arranged between the turbine and the expeller.

[0046] The previously described process steps a) to f) are referred to in the following explanations. After being transported through the pump arrangement in process step a), the CO2 is divided by a flow divider, with a first portion (as sCO2 leading to the engine) being directed to the recuperator (in step e) and a second portion being directed to transfer waste heat in process step b) (i.e., to the heat exchanger in paragraph b) of the energy plant). The transfer of waste heat in process step b) is expediently carried out by means of at least one heat exchanger.

[0047] According to the invention, the CO2 flow, after cooling at the expeller and evaporator and immediately after the pump outlet, is split by the flow divider located between the pump, recuperator, and preheater. A first portion is directed to the recuperator (in step e) of the process or paragraph e) of the energy plant), and a second portion is directed to the heat exchanger according to the invention in step b) of the process or paragraph b) of the energy plant. Therefore, it is advisable to have lines from the flow divider to the recuperator and from the flow divider to the heat exchanger (from paragraph b) of the description of the energy plant components). Operating the recuperator with two mass flow rates enables better thermodynamic performance of the plant. The mass flow rate coming directly from the flow divider corresponds to 55 to 65% of the mass flow rate coming from the engine or turbine.This means that this mass flow ratio allows for greater heat transfer between the low-pressure, high-temperature flow coming from the compressor and the high-pressure, low-temperature flow coming directly from the flow divider. The lower irreversibility results in better thermodynamic performance of the recuperator.

[0048] Preferably, in process step a), the pressure increase and transport by means of the pump arrangement, the CO2 is transported to the inlet of the pump arrangement under transcritical conditions. The CO2 at the inlet of the pump arrangement has a pressure above the critical CO2 pressure and a temperature below the critical CO2 temperature. Particularly preferably, the CO2 under transcritical conditions has a pressure above the critical pressure, but at least 75 bar, and a temperature between 20 °C and 28 °C.

[0049] It has proven advantageous if, in a mixer, a first partial stream of CO2 under high pressure, coming from the recuperator, is mixed with a second partial stream of CO2 under high pressure, coming from the preheater, to form a stream of CO2 under high pressure, which is then fed to the main heater.

[0050] Further advantages have been demonstrated when the temperature of the second partial flow at the preheater outlet deviates from the temperature of the first partial flow at the recuperator outlet by no more than ± 10 K. This enables a more homogeneous flow distribution.

[0051] The heat capacity imbalance at the (low-temperature) recuperator is a common feature of sCO2 operation. The higher heat capacity on the cold side of the recuperator limits heat transfer and restricts the plant's efficiency by reducing the stream exit temperature. The preheater architecture offers an alternative to overcome this limitation by employing different mass flows between the two streams passing through the recuperator.A multi-criteria optimization of the proposed system has surprisingly shown that a mass flow ratio of 55 to 65% for a first CO2 partial flow under high pressure in the line from the flow divider to the recuperator relative to a second flow in the line from the prime mover to the recuperator allows an increase in the thermal performance of the recuperator and offers the best thermo-economic compromise for the sCO2 cycle for waste heat recovery applications.

[0052] In an advantageous embodiment of the process, the two CO2 partial streams, previously divided by the flow divider, are combined in the mixer. After the first partial stream has passed through the recuperator, where the heat transfer from the CO2 exiting the engine to the sCO2 in a line leading to the engine has taken place according to the previously explained step e), it is combined with the second partial stream of the CO2 stream. The second partial stream had previously passed through the preheater and absorbed heat from the waste heat stream already cooled in the main heater. Subsequently, the combined CO2 partial streams can absorb the waste heat at a higher temperature level in the main heater in the previously explained step b).

[0053] It has also proven advantageous if the CO2 has a temperature between 20 and 28 °C before entering the pump assembly and if the CO2 is in the transcritical state. The transcritical state means that the minimum operating temperature is lower than the critical CO2 temperature (T c =30.9 °C) and the minimum operating pressure is higher than the critical pressure (P c =73.8 bar). This way, no condensation of CO2 occurs.

[0054] The properties of CO2 at subcritical temperature enable better thermodynamic performance of the sCO2 cycle. In particular, the high specific density of CO2 under these operating conditions enables a significant reduction in compression work, resulting in a higher net cycle power. This is therefore a highly desirable property to enable more potential applications of the technology, especially in applications related to medium-temperature heat sources. Further advantages have been derived from the fact that, in step b), the waste heat is transferred from a medium having a maximum temperature of 450°C. Higher temperatures can also be harnessed in other ways, while the advantages of the inventive process are realized precisely when the waste heat has a temperature of no more than 450°C.

[0055] It has also been shown that particularly high efficiency can be achieved with certain process parameters. For example, in step f), the temperature of the CO2 exiting the engine or turbine should preferably be 240 ± 20 °C, and after the heat transfer in step e), 75 ± 10 °C. Furthermore, in step d), the temperature of the CO2 to be cooled at the inlet of the expeller is preferably 75 ± 10 °C. As already mentioned, operation with different mass flow levels in the individual recuperator streams compensates for the imbalance in the CO2 properties in the plant and enables better operation.

[0056] In state-of-the-art systems operating under transcritical conditions, however, the nonlinearity of the CO2 properties near the critical point leads to a smaller temperature difference within the recuperator. This necessitates a significant increase in the heat exchanger surface area and thus increases system costs. This is avoided by the invention.

[0057] Therefore, it has surprisingly been shown that the recuperator outlet temperature of 75 ± 10 °C provides a better thermo-economic compromise for the system application. Furthermore, the CO2 flow within this temperature range at the generator inlet enables smooth operation of the plant as a driving heat source for the absorption cooling system.

[0058] The object of the invention is thus achieved by a combined system configuration in which the SH (Sequential Heating) sCO2 system, optimized for waste heat utilization with a maximum process temperature of around 450 °C, is combined as a topping system with an H2O / LiBr absorption chiller as a bottoming system in such a way that the bottoming system controls the heat dissipation or forms the controller. The process temperature is preferably between 350 and 400 °C. In particular, the invention achieves a constant subcritical inlet temperature (e.g., 20 °C) in the pump arrangement, e.g., the compressor or the pump. This enables efficient use of heat, which is present, for example, in the form of waste heat from a technical process, even in a medium temperature range between 350 and 400 °C. This is achieved according to the invention by means of a recuperated, transcritical sCO2 process, i.e.a process with a subcritical inlet temperature to the pressure increase of, for example, 20 °C at a supercritical pressure, e.g., 75 bar. This is not possible or not realized in a conventional sCO2 recompression cycle and represents a significant difference between the invention and the prior art.

[0059] The invention accordingly relates to recuperated, transcritical sCO2 process architectures, which are used, for example, with heat sources in the temperature range of 350 to 400 °C, e.g., for the utilization of waste heat from gas turbines, engines, or industrial processes. The performance of these transcritical sCO2 process architectures depends heavily on recooling. In the solution according to the invention, the removal of non-usable heat from the sCO2 process is carried out in such a way that, regardless of seasonal and daily temperature fluctuations, the inlet temperature into the pump arrangement is kept constant, e.g., at 20 °C, thus compensating for the aforementioned performance fluctuations during operation.

[0060] The arrangement and method proposed on this basis are therefore essentially characterized by: the combination of a recuperated, transcritical sCO2 upstream or topping process (with the associated topping system) with an absorption refrigeration process (preferably an H2O / LiBr absorption refrigeration process) as a downstream or bottoming process (with the associated bottoming system); the heat removal from the sCO2 process by sequential cooling in the expeller and evaporator of the bottoming process; the process architecture without an additional precooler and refrigeration cycle, in which the heat removal throughout the entire operating range is controlled exclusively by the bottoming process, so that the bottoming process serves as the controller for heat removal;It has surprisingly been shown that the solution according to the invention achieves a constant subcritical inlet temperature in the pump arrangement while maintaining the cycle architecture that is thermo-economically optimized for the technical application and with minimal equipment expenditure.

[0061] The invention also particularly relates to the use of the energy plant according to the invention in the process according to the invention. Furthermore, the invention relates to the use of an absorption chiller as a temperature controller. The absorption chiller comprises an expeller and an evaporator, in conjunction with a recuperator, which serves to transfer heat between two CO2 streams of this energy plant. The absorption chiller serves as a temperature controller for heat removal from the CO2 system of an energy plant. The combination of expeller and evaporator, which are parts of the absorption chiller, with the recuperator enables the constant adjustment of the cooling capacity of the energy plant, depending on the mass flow or flow velocity of the streams – independent of external cooling, e.g., by means of cooling water, and its temperature. This has the advantage that the energy plant can be operated seasonally independent.

[0062] Specifically, the use of an absorption chiller system is envisaged as the bottoming system of an sCO2 cycle, serving as a cooler for the topping system. The first CO2 stream leaving the recuperator is the exclusive heat source for driving the expeller, which evaporates the water (refrigerant) from the mixture with lithium bromide (absorbent) and removes the heat from the CO2 stream. The vapor is passed to the condenser, and the stream, with temperatures in the range of 15 °C ± 5 °C, serves as the cooling medium in the evaporator, which removes the heat from the second CO2 stream leaving the expeller. The evaporator regulates the temperature of the CO2 stream at the inlet of the pump assembly, independent of seasonal variations in the ambient temperature, to subcritical temperature conditions of 20 to 28 °C. Thus, the evaporator is the main cooler of the topping system.

[0063] The absorption cooling system operates without a solution heat exchanger, which is eliminated by the invention, thus increasing heat removal in the expeller. Both absorption cooling systems are arranged in parallel to extract heat from the first CO2 stream, enabling sufficient cooling capacity in the evaporator. This makes the absorption cooling system more efficient and allows for better control of the topping system or process.

[0064] Statements regarding the steps a) to f) of the method according to the invention in claim 9 and the description of the method apply equally to the components of the energy system according to paragraphs a) to f) of claim 1 and the description of the energy system, unless specifically stated in each case. This also applies vice versa.

[0065] The utilized waste heat is, for example, waste heat from an exhaust gas, such as an industrial exhaust gas generated during an industrial process. The operation of the absorption chiller is explained in more detail in the exemplary embodiments using the drawings.

[0066] The invention also encompasses the CO2 maintaining the supercritical state as sCO2 throughout the entire system of the energy plant or throughout the entire process sequence, i.e., for example, the sCO2 still has a temperature of 35°C before the pump arrangement and is thus present in the supercritical state under a corresponding pressure. The invention envisages the use of sCO2 as a carrier medium for the waste heat energy and as a working medium in the energy plant, wherein the thermal energy is to be converted into mechanical and, in particular, kinetic energy with the CO2 as the working medium in a thermal power process, preferably with subsequent generation of electrical energy.

[0067] According to the invention, the CO2 system of the energy plant is also used to operate the expeller (also referred to as generator) of the absorption refrigeration system. For this purpose, the CO2 exiting the power engine or, for example, the turbine is cooled sequentially by the expeller and the evaporator using the absorption refrigeration system. By means of step e) of the method or the recuperator according to paragraph e) of the description of the energy plant according to the invention, the CO2 exiting the turbine is pre-cooled beforehand, whereby, in turn, the CO2 that is fed to the power engine or, for example, the turbine is preheated. This recuperator, also called a regenerator or internal recuperator, is a heat exchanger, preferably a high-temperature recuperator. The recuperator is necessary to increase the mass flow in the CO2 cycle.Without the recuperator, the low temperature of the waste heat from some industrial exhaust gases would be an obstacle to efficient conversion into mechanical energy, or in particular, kinetic energy. A key advantage of the invention is that it also allows the use of heat transfer media, which dissipate waste heat at low temperatures—namely, a maximum temperature of 450 °C—and can be converted into mechanical energy, or in particular, kinetic energy, with high efficiency. The process temperature is preferably between 350 and 400 °C.

[0068] In the invention, the cycle of the absorption refrigeration plant benefits from the waste heat carried by the CO2 after the recuperator from step d) of the process or paragraph d) of the description of the energy plant, because it is the energy source necessary to operate the absorption refrigeration plant.

[0069] As is known, absorption systems, such as a LiBr / H2O system, require an energy source in the expeller to evaporate the water from the water-LiBr mixture (mixture of refrigerant and salt solution from the absorption chiller). This refrigerant vapor is then condensed in the condenser, expanded (preferably by means of an expansion valve), and passed in the liquid phase at low temperature into the evaporator, where the coolant evaporates water. Within the scope of the present invention, the CO2 mass flow, which represents a partial flow of the CO2 escaping from the engine or turbine, is used to cool it. Cooling with cooling water takes place at the condenser of the absorption chiller, whereby the temperature of this cooling water has little to no influence on the stable operation of the energy system.

[0070] It should be noted that conventional absorption systems (such as LiBr / H2O absorption systems) typically include an internal heat recovery device (SHX equipment) to increase the efficiency of the absorption system.

[0071] However, the present invention discloses a simplified absorption system in which the internal heat recovery, which occurs within the absorption chiller by means of an internal recuperator, can be eliminated. The advantage is that this reduces the costs of the energy system and enables greater heat removal from the sCO2 system. Finally, the absorption chiller provides the coolant to maintain the topping process at an optimal temperature. This significantly increases the performance of the energy system according to the invention.

[0072] Advantageously, this eliminates the need for conventional cooling water for CO2 cooling, which can be subject to external, seasonal fluctuations and would require adjustments to the entire system, including its efficiency. The CO2 cooling is so effective and consistent that the subcritical range of approximately 20 ± 5 °C of the CO2 upstream of the pump arrangement can be reached consistently, even across seasonal fluctuations (as is the case with cooling water, for example), and operation is, above all, stable and consistent. The effective performance of the sCO2 process for waste heat utilization is barely reduced, despite seasonal increases in outside temperatures.

[0073] Due to this consistency, the inventive configuration achieves high thermoeconomic performance even under off-design conditions. Furthermore, local two-phase flows do not occur at the inlet to the pump arrangement, which would have a negative impact on the stability of the process and the service life of the energy plant. This results in controlled cooling. The expeller and evaporator of the absorption chiller serve as temperature controllers for heat removal. Temperature control is preferably achieved by changing the mass flow rates.

[0074] The invention represents a "cycle architecture" through the general arrangement of the engine or turbine, pumps, compressors or condensers, recuperators, and external heat exchangers of the sCO2 cycle and achieves a stabilization of the operating behavior of the inventive arrangement. The evaporator of the absorption chiller acts as a cooler for the energy system (i.e., the topping system), so that heat is transferred from the CO2 stream present there to lower-temperature CO2 streams.

[0075] Advantageously, the arrangement according to the invention operates at higher, supercritical pressures, so that the CO2 is present in the supercritical range immediately upstream of the engine, for example, the turbine, which increases the efficiency of the generator. Another advantage is that internal recuperators in the absorption chiller system can be dispensed with. The invention therefore also requires fewer components.

[0076] The invention is therefore easy to implement, reduces the number of components in the system, and ultimately improves the useful absorption and conversion of waste heat. An external heat source for the absorption system is no longer necessary. A pre-cooler for the hot CO2 exiting the turbine with its own cooling circuit is also dispensed with. The cooling of the CO2 is carried out exclusively by the absorption refrigeration system, whereby the hot CO2 is pre-cooled by means of step d) of the method or the recuperator in paragraph d) according to claim 1, directed to the arrangement for converting waste heat into mechanical or kinetic energy, or the description of the arrangement according to the invention. The invention is characterized by the following preferred embodiments, without being limited to them.

[0077] An embodiment of the invention in which the energy plant comprises at least two heat exchangers for the sequential transfer of waste heat to CO2 as a carrier and working medium has proven advantageous. This means that in step b) of the process according to the invention, the waste heat is transferred sequentially to the CO2 as a carrier medium in at least two stages. In a preferred embodiment, the first partial stream of the CO2 stream after the recuperator in process step d) is combined with the second partial stream of the CO2 stream, which is conducted from the flow divider to the heat exchanger in paragraph b), such that both partial streams together pass through the heat exchanger as the first heat exchanger, the main heater, for absorbing the waste heat by the CO2.

[0078] In a particularly preferred embodiment, the other, second partial stream of the CO2 stream has previously passed through a second heat exchanger, the preheater, to absorb the waste heat from the CO2, before this second partial stream is combined with the first partial stream and both pass through the first heat exchanger together. Naturally, the temperature provided by the first heat exchanger is higher than that provided by the second heat exchanger, since the medium that provides the waste heat and delivers it as a waste heat stream is significantly hotter at the first heat exchanger than at the second heat exchanger. The latter embodiment therefore utilizes sequential heating by means of the first heat exchanger and the second heat exchanger.

[0079] In a preferred embodiment of the invention, the line between the expeller and evaporator of the absorption chiller passes a sub-cooler. This means that the flow of CO2 exiting the turbine, after cooling at the expeller, is passed through a sub-cooler before being directed to the evaporator of the absorption chiller. This design is advantageous because it increases the cooling capacity at the evaporator. In a preferred embodiment of the invention, the absorption chiller is a LiBr / H2O absorption chiller. It is particularly advantageous if the absorption chiller operates without a solution heat exchanger-recuperator between the expeller and the absorber. This is required according to the prior art, but, as has surprisingly been shown, is not needed for the system according to the invention.

[0080] Advantageously, the energy system comprises a generator for generating electrical power, mechanically connected to the prime mover, designed as a turbine. In step c) of the process, electricity is additionally generated from the mechanical or kinetic energy obtained from the prime mover or turbine by means of a generator mechanically coupled to the prime mover. The temperature of the CO2 before entering the pump assembly is preferably 20 ± 5 °C, so that the CO2 is in a subcritical state there.

[0081] When carrying out the process according to the invention, it has proven advantageous if, in step b), the waste heat is transferred from a medium, a waste heat stream, which has a maximum temperature of 450 °C, preferably between 350 and 400 °C. In a further advantageous embodiment of the process according to the invention, the temperature of the CO2 exiting the turbine is 240 ± 20 °C and, after the heat transfer in step d), 75 ± 10 °C, preferably 235-245 °C beforehand and 70-80 °C afterwards. In contrast, the CO2 leading to the turbine is particularly preferably heated from 40-60 °C to 225 ± 20 °C, in particular from 50 ± 5 °C to 205-235 °C.

[0082] In a preferred embodiment of the process according to the invention, the temperature of the CO2 to be cooled at the inlet of the expeller is at least 60 °C, preferably at least 65 °C. A temperature range of 75 ± 10 °C has proven advantageous, with the temperature range between 70 °C and 80 °C producing the best results. This corresponds to an embodiment in which the CO2 flowing from the turbine to the recuperator has precisely these temperatures after exiting the recuperator. The thermal efficiency of the heat exchange is thus limited and is approximately 85-95%.

[0083] A further advantage of the invention is the simplification of the absorption system by eliminating the SHX, the internal heat exchanger with SHX equipment (solution heat exchanger). This device is a heat exchanger between the expeller and absorber of the absorption cooling system. The use of this equipment is standard for an absorption system according to the state of the art, but requires additional effort. Eliminating this device, which the invention enables, means a simplification of the system.

[0084] In most known high-temperature recompression (HTR) architectures, the CO2 partial streams are combined shortly before the high-temperature recuperator. Therefore, in such a previously known system, the mass flow rates of both streams (hot side and cold side) are equal. In contrast to this architecture, however, in the sCO2 preheating cycle of the present invention, the two CO2 streams are combined after the recuperator and preheater and immediately before the main heater. Thus, there is a difference between the mass flow rates of the two streams (hot side and cold side) in the recuperator. This property also reduces the irreversibility of the system; moreover, the preheating architecture does not require the use of the HTR and the recompressor.

[0085] The use of the system according to the invention for waste heat utilization from industrial processes is particularly advantageous for the thermal utilization of heat sources classified as medium-temperature. This is because their utilization poses a technical and economic challenge for energy generation using sCO2 systems. Therefore, the proposed modifications are suitable for creating greater application potential for the combined system. However, the operating characteristics for high-temperature heat sources are completely different.

[0086] To implement the invention, it is also expedient to combine the above-described inventive configurations, embodiments, and features of the claims. The invention will be explained in more detail below using exemplary embodiments that describe the invention in more detail without, however, limiting it. Figure 1 shows a first embodiment of the invention. Figure 2 shows a second embodiment of the invention with an additional sub-cooler. Figure 3 shows the position of the supercritical region in an entropy-temperature diagram.

[0087] Fig. 1 schematically shows a cycle of supercritical CO2, sCO2, and in particular the inventive arrangement for converting waste heat into mechanical energy. Throughout the following explanations, CO2 is referred to as the working medium, since it represents the working medium of a thermal power process or a heat engine for the inventive method, carried out in the inventive arrangement, the preferred embodiments of which are described.

[0088] In the circuit, the pressure of the working medium is raised by a pump 1 as one embodiment of the pump arrangement above the critical pressure, which then prevails at the outlet of the pump 1 and in line 2. At a flow divider 3, the working medium is divided into two partial flows into lines 4 and 5, with the first partial flow in line 4 being passed through a high-temperature recuperator 6 and the second partial flow in line 5 being passed through a heat exchanger, a preheater 7.

[0089] Recuperator 6 is designed to operate with two different mass flow rates, with the mass flow rate in line 4, the first partial flow, corresponding to 55 to 65% of the mass flow rate in line 19, the second partial flow after exiting the engine 17. This enables better thermodynamic performance of the entire system because this mass flow ratio enables greater heat transfer between the low-pressure, high-temperature flow in line 19 and the high-pressure, low-temperature flow in line 4. The lower irreversibility results in recuperator 6 having better thermodynamic performance. The proposed mass flow rate enables a better balance between the heat capacities on both sides of the heat exchanger. This leads to a better temperature gradient in the heat exchanger, which increases the overall heat transfer efficiency and reduces irreversibility.This therefore leads to improved thermal efficiency of the heat exchanger.

[0090] The working fluid in line 8, the continuation of line 4 after the high-temperature recuperator 6, recovers the heat from the working fluid leaving the engine 17 in line 19 by means of the high-temperature recuperator 6. The exhaust gas, as a carrier of the waste heat in line 15, initially heats the working fluid by means of the second heat exchanger, the preheater 7, so that the working fluid reaches an elevated temperature, at which it then flows into line 9. A mixer 10 mixes the working fluids from lines 8 and 9; the resulting working fluid flows in line 11 to a first heat exchanger, a main heater 12. While the exhaust gas, as a carrier of the waste heat intended for use as an energy source, further heats the already preheated working fluid through line 14 entering the heat exchanger 12.

[0091] Turbine 17 expands the working fluid flowing into it via line 13 until it enters line 19, and transfers the resulting or converted mechanical energy to a generator 18. At the outlet of turbine 17, upon entering line 19, the working fluid remains at subcritical pressure. The working fluid reaches high-temperature recuperator 6 via line 19, where it is cooled by the working fluid from line 4, from the cold side, and flows via line

[0092] 20 to an expeller 23 of a lithium bromide-water absorption chiller. The heat of the working medium, present at low pressure in line 20, supplies the absorption chiller process of the absorption chiller by separating the absorption mixture of LiBr salt solution and water in the expeller 23 using the energy from the working medium. The working medium is then discharged via line

[0093] 21 condenses at a reduced temperature in the evaporator 31. Finally, the working fluid is returned to pump 1 via line 22 at a low temperature, completing the preheating cycle.

[0094] The use of the cooling system with the main components expeller 23, condenser 25, evaporator 31, and absorber 33 primarily serves to remove heat from streams 20 and 21. Stream 20 is the heat source in expeller 23 for evaporating water from the mixture of water (refrigerant) and lithium bromide (absorbent), which is represented by stream 36. Stream 20 after recuperator 6 is thus the main drive for expeller 23. Stream 30, with a temperature of 15 °C, is the cooling medium for heat removal from CO2 stream 21 in evaporator 31. In this case, evaporator 31 is the main cooler of the sCO2 cycle.

[0095] In the lithium bromide absorption chiller, water (H2O) is the refrigerant, while lithium bromide (LiBr) is the absorbent. In the expeller 23, the weak lithium bromide solution absorbs heat from the working fluid flowing in line 20 to evaporate the refrigerant (water). The water vapor then flows from the expeller 23 as refrigerant vapor through line 24 into the condenser 25 and condenses to the state in which it enters the outlet 28. Coolant, for example, cooling water, is supplied via the coolant connections 26, 27.

[0096] The high-pressure liquid refrigerant flowing through line 28 expands at an expansion valve 29, causing the pressure of the refrigerant in line 30 leading from the expansion valve 29 to drop to the low pressure in the evaporator 31. The refrigerant evaporates to the saturated vapor refrigerant by absorbing the heat from the cooled working fluid in line 21 in the evaporator 31 and enters line 32.

[0097] The refrigerant, here water in the saturated vapor state, enters an absorber 33 through line 32 to be absorbed by the strong salt solution (LiBr solution). An absorber pump 35 increases the pressure of the now diluted LiBr-H2O solution flowing into it via line 34 and conveys it via line 36 to the inlet of the expeller 23, where the water is expelled. The now concentrated LiBr-salt solution leaves the expeller 23 via line 37, expands at a throttle valve 38, and enters the absorber 33 via line 39. Heat is dissipated in the absorber 33 via cooling connections 40, 41, for example, using cooling water.

[0098] Fig. 2 shows, in addition to the components and processes already explained in Fig. 1, which are otherwise identical, a schematic of a line 21 for a flow of the working medium exiting the turbine 17 under low pressure, which, after cooling at the expeller 23, is passed past a sub-cooler 42 in addition to increasing the cooling capacity in the evaporator 31 before the working medium enters the evaporator 31. The heat is dissipated from the sub-cooler 42 by means of cooling water 43, 44. It should also be noted that in Figures 1 and 2, no SHX (solution heat exchanger) is used between the expeller 23 and the absorber 33. The use of such equipment is otherwise standard for such an absorption system. Omitting this equipment would mean a possible simplification of the system.

[0099] Fig. 3 shows a Ts diagram with the specific entropy s as the abscissa and the absolute temperature T as the ordinate, schematically illustrating the supercritical region 50, as used, for example, in CO2 processes such as the method according to the invention. Also shown are the subcritical region 51 with a higher specific entropy s and the mixed-phase region 52 at a lower temperature θ.

[0100] Cited non-patent literature:

[0101] Wu, C.; Shun-sen, W.; Feng, Xj.; Li, Jun:

[0102] Energy, exergy and exergoeconomic analyzes of a combined supercritical CO2recompression Brayton / absorption refrigeration cycle

[0103] Energy Conversion and Management 2017, 148, 360-377.

[0104] Ma., Y.; Liu, M.; Yan., J.; Liu, J.

[0105] Performance investigation of a novel closed Brayton cycle using supercritical CO2- based mixtures as working fluid integrated with a LiBr absorption chiller

[0106] Applied Thermal Engineering 2018, 141 , 531-547.

[0107] Ma, Y.; Zhang, X; Liu, M.; Yan, J.; Liu, J.

[0108] Proposal and assessment of a novel supercritical CO2Brayton cycle integrated with

[0109] LiBr absorption chiller for concentrated solar power applications

[0110] Energy 2018, 148, 839-854.

[0111] Yang, S.; Deng, C.; Liu, Z.

[0112] Optimal design and analysis of a cascade LiBr / H2O absorption refrigeration / transcritical

[0113] CO2process for low-grade waste heat recovery

[0114] Energy Conversion and Management 2019, 232-242.

[0115] Gotelip, T.; Gampe, U.; Glos, S.

[0116] Optimization strategies of different SCO2architectures for gas turbine bottoming cycle applications

[0117] Energy 2022, 250, 123734.

[0118] Reference symbol

[0119] 1 pump arrangement, pump (of the energy system)

[0120] 2 lines for CO2 (high pressure)

[0121] 3 river dividers

[0122] 4 Line for CO2 (high pressure, first partial flow)

[0123] 5 Line for CO2 (high pressure, second partial flow)

[0124] 6 Recuperator (heat exchanger); high-temperature recuperator

[0125] 7 Preheater, second heat exchanger (second heater, waste heat to

[0126] CO2)

[0127] 8 Line for CO2 (high pressure, first partial flow)

[0128] 9 Line for CO2 (high pressure, second partial flow)

[0129] 10 mixers with throttle

[0130] 11 Line for CO2 (high pressure)

[0131] 12 first heat exchanger (main heater, waste heat to CO2)

[0132] 13 Line for CO2 (high pressure)

[0133] 14, 15, 16 Line for waste heat flow

[0134] 17 Power machine, turbine

[0135] 18 Generator

[0136] 19, 20, 21 , 22 Line for CO2 (low pressure), CO2 flow

[0137] 23 expeller (of the absorption refrigeration system)

[0138] 24 Line for H2O (refrigerant vapor of the absorption chiller)

[0139] 25 Condenser (of the absorption chiller)

[0140] 26, 27 Connection for cooling water for the condenser

[0141] 28 Line for H2O (water)

[0142] 29 Expansion valve (of the absorption refrigeration system)

[0143] 30 Line for H2O (water)

[0144] 31 evaporators

[0145] 32 Line for H2O (water)

[0146] 33 absorbers (of the absorption chiller)

[0147] 34 Line for LiBr / H2O (diluted solution)

[0148] 35 Absorber pump (pump of the absorption refrigeration system)

[0149] 36 Line for LiBr / H2O (diluted solution, mixture of refrigerant and

[0150] saline solution)

[0151] 37 Line for LiBr (concentrated solution)

[0152] 38 Throttle valve line for LiBr (concentrated solution), 41 Connection for cooling water for the absorber sub-cooler / or cooler

[0153] Cooling water line for the CO2 sub-cooler. Cooling water line for the CO2 sub-cooler. Supercritical area. Subcritical area.

[0154] Mixed phase area

[0155] entropy

[0156] temperature

Claims

Patent claims 1. Arrangement for converting waste heat into mechanical energy using carbon dioxide (CO2) in the supercritical state (sCO2), comprising: a) a pump arrangement (1) for increasing the pressure and transporting the CO2 under transcritical conditions from a line (22), wherein the CO2 at the inlet of the pump arrangement (1) has a pressure above the critical CO2 pressure and a temperature below the critical CO2 temperature; b) at least two heat exchangers, a preheater (7) and a main heater (12), for transferring waste heat from a waste heat stream in a line (14, 15, 16) to the CO2 as the carrier medium and working medium, so that the CO2 can exist in the supercritical state as sCO2; c) a power engine (17) for generating mechanical energy using the sCO2; and d) an absorption chiller for controlled cooling of the CO2 exiting the power engine (17), which then already has heat transferred to it in the recuperator (6). was withdrawncomprising a desorber (23) and an evaporator (31) in which heat is extracted from the CO2 in two stages by including a line (20) that is connected to the recuperator, (6) leads to the desorber (23) and further comprises a line (21) leading from the desorber (23) to the evaporator (31), characterized in that e) the absorption refrigeration system is designed to keep the temperature at the inlet of the pump arrangement (1) constantly below the critical temperature, f) that a recuperator (6) for heat transfer from the CO2 exiting the power engine (17) to the sCO2 leading to the power engine (17) is arranged between the power engine (17) and the desorber (23), and that a line (21) leads from the pump arrangement (1), the recuperator (6) and the preheater (7) a flow divider (3) for dividing the flow of CO2 into a first partial flow to the recuperator (6) and a second partial flow for the transfer of waste heat by means of which at least two heat exchangers (7, 12) are arranged.

2. Arrangement according to claim 1, wherein the CO2 has a pressure above the critical pressure, but at least 75 bar, and a temperature between 20 °C and 28 °C under transcritical conditions.

3. Arrangement according to claim 1 or 2, wherein the flow divider (3) is designed or adjusted to provide a mass flow ratio of 55 to 65% for a first partial flow of CO2 to ensure a high-pressure flow in line (4) relative to a flow in line (19) to the recuperator (6). Arrangement according to one of the preceding claims, wherein CO2 under high pressure in a first partial flow (8) and CO2 under high pressure in a second partial flow (9) are combined in a mixer (10) into a line (11) as CO2 under high pressure. Arrangement according to one of the preceding claims, wherein a line (21) between the desorber (23) and the evaporator (31) of the absorption chiller is routed through a sub-cooler (42) for further cooling. Arrangement according to one of the preceding claims, wherein the absorption chiller is a LiBr / H2O absorption chiller. Arrangement according to one of the preceding claims, wherein the absorption chiller operates without a solution heat exchanger recuperator between the desorber (23) and the absorber (33).An arrangement according to one of the preceding claims, comprising a generator (18) for generating electricity, mechanically connected to the turbine-configured power machine (17). A method for converting waste heat into mechanical energy, comprising the steps: a) pressure increase and transport of CO2 by means of a pump arrangement (1), wherein the CO2 at the inlet of the pump arrangement (1) has a pressure above the critical CO2 pressure and a temperature below the critical CO2 temperature, b) transfer of waste heat from a waste heat stream to CO2 as the carrier medium and working medium sequentially in at least two stages, such that the CO2 is present as sCO2 after compression in the supercritical state, and c) generation of mechanical energy from the sCO2 by means of a power machine (17), characterized in that d) heat transfer from the power machine (17) through the generator (18) is effected by means of a recuperator (6) arranged between the power machine (17) and the ejector (23). The CO2 exiting line (19) is transferred to the CO2 in a line (4) leading to the engine (17), and the CO2, after being transported through the pump arrangement (1) in step a), is divided by a flow divider (3), wherein a first partial flow is directed through line (4) to the recuperator (6), and a second partial flow is directed through a line (5) for the transfer of waste heat in step b), e) controlled cooling of the CO2 exiting the engine (17) by means of an absorption chiller, comprising a desorber (23) and an evaporator (31), wherein the CO2 exiting the engine (17) via a line (19) is first cooled in the recuperator (6), then by means of the desorber (23) and finally by means of the evaporator (31) of the absorption chiller, f) the absorption chiller keeps the temperature at the inlet of the pump arrangement (1) constantly below the critical temperature holds, method according to claim 9,wherein in step a) the CO2 at the inlet of the pump arrangement (1) has a pressure above the critical CO2 pressure and a temperature below the critical CO2 temperature and thus enters the pump arrangement (1) under transcritical conditions. Method according to claim 10, wherein the CO2 under transcritical conditions has a pressure above the critical pressure, but at least 75 bar, and a temperature between 20 °C and 28 °C. Method according to any one of claims 9 to 11, wherein in a mixer (10) a first partial stream in line (8) of CO2 under high pressure, a second partial stream in line (9) of CO2 under high pressure is mixed to form a stream in line (11) of CO2 under high pressure. Method according to any one of claims 9 to 12, wherein the temperature of the second partial stream (9) at the outlet of the preheating device (7) deviates from the temperature of the first partial stream (8) at the outlet of the recuperator (6) by no more than ± 10 K. Arrangement according to one of claims 9 to 13,wherein the flow divider (3) provides a mass flow ratio of 55 to 65% for a first partial flow of CO2, High pressure is established in line (4) relative to a second flow in line (19) to the recuperator (6), thus guaranteeing an optimal thermal capacity ratio between the two flows and thereby reducing irreversibility in the system. A method according to any one of claims 9 to 14, wherein the first of the CO2 partial flows divided in the flow divider (3) in line (4, 8) after the recuperator (6), in which the heat transfer according to step f) has taken place, is combined with the second of the CO2 partial flows divided in the flow divider (3) in line (5, 9), so that both partial flows together can absorb the waste heat. A method according to any one of claims 9 to 15, wherein the CO2 has a temperature between 20 and 28 °C before entering the pump arrangement (1) and is in a transcritical state there. Method according to any one of claims 9 to 16, wherein in step b) the waste heat is transferred from a medium having a maximum temperature of 450 °C.A method according to any one of claims 9 to 17, wherein in step f) the temperature of the CO2 exiting the engine (17) is 240 ± 20 °C and, after heat transfer in the recuperator (6) according to step f), is 75 ± 10 °C. A method according to any one of claims 9 to 18, wherein in step d) the temperature of the CO2 to be cooled at the inlet of the desorber is 75 ± 10 °C. A method according to any one of claims 9 to 19, wherein a higher heat absorption in the desorber (23) is enabled by operating the absorption chiller without a solution heat exchanger recuperator between the desorber (23) and the absorber (33).Use of an absorption chiller as a bottoming system of an sCO2 cycle according to one of claims 1 to 8, wherein the absorption chiller serves as a cooler for the topping system and comprises a desorber (23) and an evaporator (31), characterized in that a CO2 stream (20) is the exclusive heat source for driving the desorber (23), and that the absorption chiller serves as a temperature controller for heat removal from the CO2 system. the energy system and regulates the temperature of the current (22) at the inlet of the pump arrangement (1) independently of the ambient temperature to subcritical temperature conditions of 20 to 28°C, wherein the evaporator (31) is the main cooler of the topping system.

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