WO2023237162A1

WO2023237162A1 - Arrangement and method for converting waste heat into mechanical energy and use of an absorption cooling system as temperature controller

Info

Publication number: WO2023237162A1
Application number: PCT/DE2023/100433
Authority: WO
Inventors: Thiago GOTELIP CORREA VELOSO; Uwe Gampe
Original assignee: Technische Universität Dresden
Priority date: 2022-06-08
Filing date: 2023-06-07
Publication date: 2023-12-14
Also published as: DE102022114343A1

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

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

The invention relates to an arrangement and a method for converting waste heat into mechanical energy, in particular kinetic energy (also referred to as kinetic energy), using carbon dioxide (CO ₂ ) in a supercritical state. CO ₂ in a supercritical state or supercritical CO ₂ 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 CO ₂ , hereinafter also briefly: sCO ₂ . 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 from a renewable source, for example.

The arrangement comprises a) a pump arrangement for increasing the pressure and further transporting the CO ₂ under transcritical conditions from a line, the CO ₂ at the inlet of the pump arrangement having a pressure above the critical CO ₂ pressure and a temperature below the critical CO ₂ 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 CO ₂ as a carrier medium and working medium, so that the CO ₂ can be present in the supercritical state as sCO ₂ , c) an engine for Generation of mechanical energy by means of the sCO ₂ , and d) an absorption refrigeration system for the controlled cooling of the CO ₂ emerging from the engine, from which heat has then already been removed in the recuperator, comprising an expeller and an evaporator in which the CO ₂ is heat in two stages is removed by comprising a line that leads from the recuperator to the expeller and further comprising a line that leads from the expeller to the evaporator.

The method includes the steps a) pressure increase and transport of CO ₂ by means of a pump arrangement), whereby the CO ₂ at the inlet of the pump arrangement has a pressure above the critical CO ₂ pressure and a temperature below the critical CO ₂ temperature, b) transfer of waste heat from a waste heat stream to CO ₂ as a carrier medium and working medium sequentially in at least two stages, so that the CO ₂ is present in the supercritical state as sCO ₂ after compression, and c) Generation of mechanical energy from the sCO ₂ using a power machine.

The invention also relates to the use of an absorption refrigeration system as a bottoming system of an sCO ₂ circuit, the absorption refrigeration system serving as a cooler for the topping system and comprising an expeller and an evaporator. The regulation relates to the heat removal from the CO ₂ system of an arrangement for converting waste heat into mechanical energy or specifically into kinetic energy, the arrangement according to the invention. The arrangement for converting waste heat into mechanical energy is also referred to below as the energy system.

Waste heat is heat that is primarily generated by technical systems and is released into the environment or can also be used. From a thermodynamic point of view, most real processes are irreversible. As a result of the dissipation of energy, heat is inevitably generated during these processes. Technical devices and systems cannot be operated without generating waste heat. This usually has to be diverted to avoid malfunctions due to overheating or to restore the initial state of the working medium in cycle processes.

Due to the special properties of CO ₂ above the critical point and the resulting advantages when used in thermodynamic cycles, experts understand CO ₂ as the working fluid of a new generation of thermal energy machines. Higher efficiencies as well as a significant reduction in the size and complexity of the individual components enable better utilization of existing heat sources and the development of new heat sources that were not previously economically usable.

Carbon dioxide (CO ₂ , ASHRAE number: R744) has a long tradition as a refrigerant, but was largely displaced as a refrigerant with the advent of synthetic refrigerants in the 1930s. It was only in the 1980s and 1990s, when it became clear that the most commonly used synthetic refrigerants were essentially responsible for ozone depletion and the greenhouse effect, that carbon dioxide came back into the spotlight.

CO ₂ is non-flammable, does not contribute to ozone depletion and has a much lower global warming potential than conventional refrigerants. That is in small quantities Gas is also non-toxic, but high concentrations in the air pose a risk of suffocation. However, systems that operate with CO ₂ as a refrigerant must withstand higher system pressures and lower, critical temperatures. A system must be built and maintained accordingly safely.

However, since carbon dioxide has a more favorable pressure ratio than conventional refrigerants and a volumetric output that is five to eight times greater, the system components such as compressors or pipe diameters can be made significantly smaller. Due to the high heat transfer coefficient of CO _2, the heat exchanger can be smaller. Due to the low viscosity, less pumping work is required, which is particularly advantageous for systems with a longer network length. All of this in turn reduces material usage, system weight and defrosting time.

Since the critical point of CO ₂ is close to the ambient temperature at around 31 °C (degrees Celsius), seasonal and daily fluctuations in the ambient temperature have significant effects on the heat removal from the process. For a specific design of a cooler, an increase in the ambient temperature results in reductions in efficiency and performance, as the pressure increase in the process shifts towards higher compressibility and consequently also compression work. How strong these effects are depends on the process architecture and the location of the entry state for the pressure increase at the design point (DP).

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 already exceeded at an ambient temperature of 26 °C. This is then referred to as transcritical operation of the refrigeration system, especially when both subcritical and supercritical conditions occur.

The point, defined by pressure and temperature, at which all three states of aggregation, solid, liquid and gas, occur at the same time, 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 become so similar that there is no longer any difference in pressure and temperature between the two states of aggregation. When this critical point is exceeded, one speaks of a transcritical or supercritical system.

With conventional refrigerants, the thermodynamic points usually do not play a role, but they are important when using CO ₂ . Above the critical point at 31 °C and 73.8 bar, very high pressures occur, which is why in subcritical systems, i.e. those that operate below the critical point, excessive heating of the system (for example due to the ambient temperature) must be avoided , for example through protective measures such as auxiliary cooling, an expansion tank or the release of CO ₂ .

In transcritical systems there is usually no condenser, but rather a gas cooler. In addition, transcritical systems must withstand very high pressures of well over 75 bar. The use of transcritical CO ₂ refrigeration systems is particularly efficient when the ambient temperature or the temperature on the warm side is on average below 15 °C, because the higher the temperature, the lower the efficiency.

It is known from the prior art to link absorption refrigeration systems with sCO ₂ energy systems, energy systems that use supercritical CO ₂ , in order to make waste heat, for example from industrial exhaust gases, usable. The absorption refrigeration system is often referred to as a bottoming system and the sCO ₂ system as a topping system. These terms are also used below in the description of the invention.

In Yang et al. (2019), only a small amount of waste heat is used because the heat source only has a temperature of 140 °C. An external temperature source is proposed to operate the expeller (sometimes also referred to as a generator) of the LiBr/H ₂ O (lithium bromide/water) absorption refrigeration system. A similar approach is revealed by Wu et al. (2017), using an NH ₃ /H ₂ O (ammonia-water) absorption refrigeration system. Furthermore, the system in Yang et al. (2019) in the subcritical range of CO ₂ with temperatures <10 °C and pressures of approx. 4 bar. In CN 112412555 A an sCO ₂ intercooling is used and the operating temperature of the CO ₂ is higher than necessary for the supercritical state. Combined sCO ₂ /LiBr/H ₂ O systems for heat sources such as nuclear reactors, solar systems and internal combustion engines work at high temperatures (above 600 ° C), as also described, for example, in the publication CN 112412555 A. There the cooling water is used to reduce the CO ₂ temperature in the intercooling before secondary compression. There is no provision for controlling the temperature in subcritical conditions before compression. Supercritical operation, ie temperatures and pressures higher than the critical CO ₂ pressure, is considered.

Ma/Liu et al. (2018) and Ma/Zhang et al. (2018) disclose a combination of sCO ₂ cycle with LiBr absorption refrigeration system, whereby only part of the CO ₂ mass flow is cooled using the LiBr system.

In all three previously known solutions, the sCO ₂ circuit is designed as a recompression architecture. The recompression architecture is one of the most common configurations of the sCO ₂ cycle in application studies of this cycle, as it offers high thermal efficiency, mainly enabled by high heat recovery. This characteristic is due to the use of at least two heat recoverers: 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 suitable for use with medium-temperature heat sources, such as: B. the use of waste heat is not economical.

The solutions from Ma/Liu et al. (2018) and Ma/Zhang et al. (2018) are designed for high-temperature heat sources for applications in concentrated solar systems. A thermal oil transfers the energy from the heat source to the sCO2 circuit in the main heater. A suitability for waste heat recovery, where energy is mainly recovered from an exhaust gas, is not apparent. Ma/Zhang et al. (2018) consider supercritical operation, i.e. temperatures and pressures that are higher than the critical CO ₂ pressure, while Ma/Liu et al. (2018) present a lower temperature process for a mixture of CO ₂ and krypton.

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

The solution according to publication CN 109519243 B uses an sCO ₂ system combined with an NH ₃ /H ₂ O absorption refrigeration system. Superheated ammonia drives its own turbine. The sCO ₂ cycle is a simple cycle. The NH ₃ /H ₂ O vapor is used as a heat source for a LiBr absorption cooler.

The publication CN 109519243 B therefore describes an energy system based on an interconnection or combination of two Rankine cycle processes, consisting of a simple sCO ₂ process as a preliminary process (topping cycle or topping process) and an ammonia/water (NH ₃ /H ₂ O) steam cycle process as a downstream process (bottoming cycle or bottoming cycle). The non-workable heat from the two thermodynamic cycle processes is dissipated using a LiBr absorption refrigeration system and an additional, separate CO ₂ cooler. In the sCO ₂ process, the heat dissipation is as follows:

After leaving the turbine, the heat from the sCO ₂ is first used in the expeller (generator) of a LiBr absorption refrigeration system. The carbon dioxide is then further cooled in a separate CO ₂ cooler with an unnamed 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 CO ₂ condenser represents the evaporator of the LiBr absorption refrigeration system. In the sCO ₂ process, heat is dissipated using three devices.

The publication CN 1 09 519 243 B does not provide any information about state and process variables, so that the process control and the gradual heat dissipation cannot be thermodynamically reproduced.

All known systems with absorption refrigeration systems only use part of the primary heating source, usually the waste heat intended for use, as the driving energy source for the absorption refrigeration system, which as a result reduces the potential of the topping system, ie the sCO ₂ energy system. Gotelip et al. (2022) describe the heat exchange between CO ₂ leaving the turbine and CO ₂ leading to the turbine in the form of preheating. It is a disadvantage that the system is dependent on external cooling water and its constant temperature. Seasonal fluctuations cannot be compensated for.

In most known recompression architectures, the CO ₂ 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 the same.

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

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 (CO ₂ ) in the supercritical state (sCO ₂ ), as well as to offer a method for its operation and the use of an absorption refrigeration system in which the disadvantages of the prior art are overcome and a constant cooling performance of a CO ₂ energy system is combined with the highest possible efficiency or useful power when converting waste heat into kinetic energy. The system should be as simple as possible and use as few components as possible. These should be as robust and less susceptible to failure as possible and at the same time be able to work independently of daily and seasonal fluctuations in the ambient temperature. In addition, a robust system should also be provided for off-design conditions, i.e. under conditions that differ from those for which the system was designed.

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 of the invention is an arrangement for converting waste heat into mechanical energy, in particular kinetic energy, comprising an energy system in which CO ₂ is used in its state as sCO ₂ to convert waste heat into mechanical energy a) a pump arrangement for increasing the pressure and further transporting the CO ₂ under transcritical conditions from a line, the CO ₂ at the inlet of the pump arrangement having a pressure above the critical CO ₂ pressure and a temperature below the critical CO ₂ 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 CO ₂ as a carrier medium and working medium, so that the CO ₂ can be present in the supercritical state as sCO ₂ , c) an engine for generating mechanical Energy by means of the sCO ₂ , and d) an absorption refrigeration system for the controlled cooling of the CO ₂ emerging from the engine, from which heat has then already been removed in the recuperator, comprising an expeller and an evaporator, in which heat is removed from the CO ₂ in two stages, by comprising a line that leads from the recuperator to the expeller and further comprising a line that leads from the expeller to the evaporator.

The use of the cooling system with the main components: expeller, condenser, evaporator and absorber serves primarily to remove heat from the two streams that pass through the absorption refrigeration system. The first stream, in a preferred embodiment, is the heat source in the expeller for evaporating water from the mixture of water (refrigerant) and lithium bromide (absorbent) represented by the stream. The current after the recuperator is therefore the main drive for the expeller. The flow from the condenser to the evaporator with a temperature of 15 °C is the cooling medium for the second stage of heat removal from the CO ₂ stream in the evaporator. In this case, the evaporator is the main cooler of the sCO ₂ circuit.

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 CO ₂ emerging from the engine to the sCO ₂ 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 (also called an evaporator), from which the line runs to the pump arrangement. For example, a pump or a compressor or compressor is used as a pump arrangement. Furthermore, in the pumping direction, immediately after the output of the pump, thus between the pump arrangement, the recuperator and the preheater, there is a flow divider for dividing the CO ₂ 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, arranged. The arrangement according to the invention is a thermoeconomically optimized SH (Sequential Heating) architecture.

The present invention also includes the fact that the line can lead to further, additional components along its course. The expeller and evaporator are therefore coolers for a topping process, whereby the evaporator is known to evaporate the coolant of the absorption refrigeration system. According to the invention, the evaporator serves as a condenser for the CO ₂ emerging from the turbine, so that it is then present as subcritical CO ₂ , preferably with a temperature of 20 ± 5 °C, in particular 20 ± 2 °C. This temperature range represents a central part of the invention.

It has proven to be advantageous if the CO ₂ is transported through or enters the pump arrangement under transcritical conditions. The CO ₂ at the inlet of the pump arrangement has a pressure above the critical CO ₂ pressure and a temperature below the critical CO ₂ temperature. Under transcritical conditions, the CO ₂ preferably has a pressure above the critical pressure, but at least 75 bar, and a temperature between 20 ° C and 28 ° C.

Advantages also arise from an embodiment in which the flow divider is designed or set in such a way that a mass flow ratio of 55 to 65% for the first partial flow of CO ₂ under high pressure in the first line in relation to the partial flow in the second line, which both 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 CO ₂ flowing there can release the residual heat from the engine to the first partial flow.

The division into partial streams is an important feature of the preheating architecture.

In the preheating architecture, the CO ₂ mass flow is passed through the flow divider after Main compressor divided into two streams. Thus, in contrast to the prior art, the entire CO ₂ mass flow is routed in the heat dissipation circuit and then to the main compressor. In the previously known recompression architecture, however, only part of the total mass flow is directed to heat removal and then to the main compression and only this part of the mass flow is used in the absorption system.

In the architecture proposed according to the invention, a booster is not required, allowing system simplification and better economic performance. In the previously known solutions, the CO ₂ flow is divided according to the low-temperature recuperator as the main feature of the recompression architectures. With such an arrangement, only part of the CO ₂ mass flow is passed into the absorption refrigeration machine, while the rest goes 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 directed to the recuperator and subsequently completely to the driver of the absorption refrigeration machine.

In the prior art, the precooler regularly takes over the heat dissipation, with part of the heat also being dissipated by supplying energy to the absorption refrigeration machine. 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 of the CO ₂ directly to the evaporator and keeps the CO ₂ temperature at the pump or compressor inlet below the critical CO ₂ temperature as indicated.

According to a preferred embodiment, CO ₂ under high pressure in a first partial stream from the recuperator, CO ₂ under high pressure in a further partial stream, which is brought in from the preheater, are combined through a mixer into a line that runs to the main heater. The preheating architecture thus created allows better utilization of the heat source by recovering heat at two temperature levels, with the main heater acting as a heat exchanger at higher temperatures. In this way, the preheater allows additional heat to be recovered from the heat source downstream of the main heater. This creates an additional mass flow, which increases the performance of the Engine (or electricity generation by a connected generator) and a more efficient cycle.

The line from the engine leads via the recuperator to the absorption refrigeration system. It has proven to be advantageous if a line between the expeller and the evaporator of the absorption refrigeration system runs past a sub-cooler, whereby the CO ₂ is further cooled and the cooling capacity at the evaporator is increased.

A particularly advantageous embodiment of the absorption refrigeration system is a LiBr/H ₂ O absorption refrigeration system. The Li Br absorption system generally works with a heat source temperature of over 75 ° C on the engine. Therefore, the absorption system with LiBr 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 mean a loss of performance for the topping system (sCO ₂ ).

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.

The invention also relates to a method for converting waste heat into mechanical energy, in particular kinetic energy, comprising the steps: a) increasing the pressure of CO ₂ and transporting it through a line by means of a pump arrangement, the CO ₂ having a pressure at the inlet of the pump arrangement (1). above the critical CO ₂ pressure and a temperature below the critical CO ₂ temperature, b) transferring waste heat from a waste heat stream to CO ₂ as a carrier medium and working medium sequentially in at least two stages, so that the CO ₂ after compression in the supercritical State as sCO ₂ is present, c) generation of mechanical energy from the sCO ₂ using a power machine.

According to the invention, process step d), e) and f): d) by means of a recuperator arranged between the engine and the extractor, a heat transfer takes place from the CO ₂ emerging from the engine through the line to the sCO ₂ in a line leading to the engine, and that the CO ₂ after transport through the pump arrangement in step a ) is divided by a flow divider, a first partial flow being passed through the line to the recuperator in, and a second partial flow being passed through a line for the transfer of waste heat in step b), e) controlled cooling of the CO ₂ emerging from the engine by means of an absorption refrigeration system, comprising an expeller and an evaporator, the CO ₂ emerging from the engine via a line being 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 one Temperature maintains, a heat transfer from the CO ₂ emerging from the engine or turbine to the sCO ₂ leading to the turbine in the flow direction of the CO ₂ before step d) by means of a recuperator arranged between the turbine and the expeller.

Reference is made to the previously designated process steps a) to f) in the following statements. After transport through the pump arrangement in method step a), the CO ₂ is divided by a flow divider, with a first part (as sCO ₂ leading to the engine) being passed to the recuperator (in step e) and a second part for the transfer of waste heat in method step b) (ie to the heat exchanger in paragraph b) of the energy system). The transfer of waste heat in process step b) expediently takes place by means of at least one heat exchanger.

According to the invention, the flow of CO ₂ is split after cooling at the extractor and evaporator and immediately after the output of the pump by the flow divider, which is arranged between the pump, recuperator and the preheater, with a first part going to the recuperator (in step e) of the method or paragraph e) of the energy system), and another, second part is directed to the heat exchanger according to the invention in step b) of the method or paragraph b) of the energy system. It therefore makes sense to have lines leading 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 system components). Operating the recuperator with two mass flow rates enables better thermodynamic performance of the system. The mass flow rate that comes directly from the flow divider corresponds to 55 to 65% of the mass flow rate that comes from the engine or turbine. This means that this mass flow ratio allows for greater heat transfer between the low pressure and high temperature flow coming from the compressor and the high pressure and low temperature flow coming directly from the flow divider. The lower irreversibility means that the recuperator has better thermodynamic performance.

Preferably, in method step a), the pressure increase and transport by means of the pump arrangement, the CO ₂ is transported to the inlet of the pump arrangement under transcritical conditions. The CO ₂ at the inlet of the pump arrangement has a pressure above the critical CO ₂ pressure and a temperature below the critical CO ₂ temperature. Particularly preferably, under transcritical conditions, the CO ₂ has a pressure above the critical pressure, but at least 75 bar, and a temperature between 20 ° C and 28 ° C.

It has proven to be advantageous if a first partial stream of CO ₂ under high pressure, coming from the recuperator, and a second partial stream of CO ₂ under high pressure, coming from the preheater, are mixed in a mixer to form a stream of CO ₂ under high pressure, which then is led to the main heater.

Further advantages have been shown if the temperature of the second partial flow at the output of the preheater does not deviate from the temperature of the first partial flow at the output of the recuperator by more than ± 10 K. This enables a more homogeneous distribution of the flow.

The imbalance of heat capacity at the (low temperature) recuperator is a common feature of sCO ₂ operation. The higher heat capacity on the cold side of the recuperator limits heat transfer and limits the effectiveness of the system by reducing the power outlet temperature. The preheat 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 CO ₂ partial flow under high pressure in the line from the flow divider to the recuperator in relation to a second flow in the line from the drive machine to the recuperator Enables increasing the thermal performance of the recuperator and offers the best thermo-economic compromise for the sCO ₂ cycle for waste heat recovery applications.

In an advantageous embodiment of the method, the two CO ₂ partial streams, which were previously divided by the flow divider, are combined in the mixer. The first partial flow, after it has passed through the recuperator and there the heat transfer has taken place from the CO ₂ emerging from the engine to the sCO ₂ in a line leading to the engine according to step e) explained above, is mixed with the second partial flow CO ₂ stream combined. The second partial stream had previously passed through the preheater and absorbed heat from the waste heat stream that had already cooled down in the main heater. Subsequently, the combined CO ₂ partial streams can together absorb the waste heat at a higher temperature level in the main heater in the previously explained step b).

It has also proven to be advantageous if the CO ₂ has a temperature between 20 and 28 ° C before entering the pump arrangement and the CO ₂ is present there in the transcritical state. The transcritical condition means that the minimum operating temperature is lower than the critical CO ₂ temperature (T _c =30.9 °C) and the minimum operating pressure is higher than the critical pressure (P _c =73.8 bar). In this way there is no condensation of CO ₂ .

The CO ₂ properties at subcritical temperature enable better thermodynamic performance of the sCO ₂ cycle. In particular, the high specific gravity of CO ₂ under these operating conditions allows a significant reduction in the work of compression, resulting in a higher net power of the cycle. So this is a very desirable property to enable more potential applications of the technology, especially in applications related to medium temperature heat sources. Further advantages have resulted from the fact that in step b) the waste heat is transferred from a medium that has a maximum temperature of 450 ° C. Higher temperatures can also be used in other ways, while the advantages of the method according to the invention are realized precisely when the waste heat has a temperature of not more than 450 ° C.

It has also been shown that a particularly high level of efficiency can be achieved with certain process parameters. Thus, in step f), the temperature of the CO ₂ emerging from the engine or turbine should preferably be 240 ± 20 °C and after the heat transfer in step e) should be 75 ± 10 °C. Furthermore, in step d), the temperature of the CO ₂ to be cooled at the inlet of the expeller is preferably 75 ± 10 ° C. As already mentioned before, operation with different mass flow levels in the individual streams of the recuperator compensates for the imbalance of the CO ₂ properties in the system and enables better operation.

In state-of-the-art systems that operate under transcritical conditions, however, the nonlinearity of the CO ₂ properties near the critical point leads to a smaller temperature difference within the recuperator. This leads to the need for a significant increase in the heat exchanger area and thus to an increase in system costs. This is avoided by the invention.

It has therefore surprisingly been shown that the recuperator outlet temperature of 75 ± 10 °C enables a better thermo-economic compromise for the system application. In addition, the CO ₂ flow in this temperature range at the generator inlet enables smooth operation of the system as a drive heat source for the absorption cooling system.

The object according to the invention is thus achieved by a combined system configuration in which the SH (Sequential Heating) sCO ₂ system, optimized for waste heat utilization with a maximum process temperature of around 450 ° C, is used as a topping system with an H ₂ O/LiBr system. Absorption refrigeration system is combined as a bottom ing system in such a way that the bottom ing system controls the heat dissipation or forms the controller. The process temperature is preferably between 350 and 400 °C. In particular, the invention ensures a constant subcritical inlet temperature (e.g. 20 ° C) in the pump arrangement, e.g. B. the compressor or the pump. This enables efficient use of heat even in a medium temperature range between 350 to 400 °C, e.g. B. in the form of waste heat from a technical process. According to the invention, this is done by means of a recuperated, transcritical sCO ₂ process, ie a process with a subcritical entry temperature into the pressure increase of e.g. B. 20 °C at supercritical pressure, e.g. B. 75 bar. This is not possible or not implemented in this way with a conventional sCO ₂ recompression cycle and represents a significant difference between the invention and the prior art.

The invention therefore relates to recuperated, transcritical sCO ₂ process architectures, which, for. B. can be used for heat sources in the temperature range of 350 to 400 °C, e.g. B. for the utilization of waste heat from gas turbines, engines or industrial processes. The performance of these transcritical sCO ₂ process architectures depends heavily on recooling. In the solution according to the invention, the non-working heat is removed from the sCO ₂ process in such a way that the inlet temperature into the pump arrangement is kept constant regardless of seasonal and daily temperature fluctuations, e.g. B. at 20 °C, and thus the mentioned performance fluctuations can be compensated for during operation.

The arrangement and the proposed method proposed on this basis are therefore essentially characterized by: the combination of a recuperated, transcritical sCO ₂ preliminary or topping process (with the associated topping system) with an absorption refrigeration process (preferably an H ₂ O/LiBr absorption refrigeration process) as a downstream or bottoming process (with the associated bottoming system); the heat removal from the sCO ₂ process through sequential cooling in the expeller and evaporator of the bottoming process; the process architecture without additional precooler and refrigeration circuit, in which the heat dissipation in the entire operating range is controlled exclusively by the bottoming process, so that the bottoming process serves as a controller for the heat dissipation; It has surprisingly been shown that the solution according to the invention achieves a constant subcritical inlet temperature into the pump arrangement while maintaining the cycle architecture that is thermo-economically optimized for the technical application and with minimal expenditure on equipment.

The subject of the invention is in particular the use of the energy system according to the invention in the method according to the invention. Furthermore, the subject of the invention is the use of an absorption refrigeration system as a temperature controller. The absorption refrigeration system includes an expeller and an evaporator, in cooperation with a recuperator, which serves to transfer heat between two CO ₂ streams of this energy system. The absorption refrigeration system serves as a temperature controller for the removal of heat from the CO ₂ system of an energy system. The combination of expeller and evaporator, which are parts of the absorption refrigeration system, with the recuperator enables, depending on the mass flow or flow rate of the streams, the constant setting of the cooling capacity of the energy system - independent of external cooling, e.g. B. using cooling water and its temperature. This has the advantage that the energy system can be operated independently of the season.

In particular, the use of an absorption cooling system is envisaged as the bottoming system of an sCO ₂ circuit, which serves as a cooler for the topping system. The first CO ₂ stream that leaves 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 CO ₂ stream. The steam is passed into the condenser and the stream with temperatures in the range of 15 °C ± 5 °C is the cooling medium in the evaporator, which removes the heat from the second CO ₂ stream leaving the expeller. The evaporator controls the temperature of the CO ₂ stream at the inlet of the pump assembly to subcritical temperature conditions of 20 to 28 °C, independent of the seasonal fluctuations in the ambient temperature. In this way, the evaporator is the main cooler of the topping system.

The absorption cooling system works without a solution heat exchanger, which is unnecessary due to the invention and which increases the heat dissipation in the expeller. Both absorption cooling systems are arranged in parallel to the first CO ₂ stream To extract heat, which enables sufficient cooling performance in the evaporator. This makes the absorption cooling system more efficient and can better control the topping system or the topping process.

Statements on the steps of the method a) to f) 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 separately in each case executed. This also applies vice versa.

The waste heat used is, for example, waste heat from an exhaust gas, for example an industrial exhaust gas, which is produced during an industrial process. The operation of the absorption refrigeration system is explained in more detail in the exemplary embodiments using the drawings.

According to the invention, it is also included that the CO ₂ in the entire system of the energy system or over the entire process sequence maintains the supercritical state as sCO ₂ , that is, for example, that the sCO ₂ in front of the pump arrangement still has a temperature of 35 ° C and is therefore in the supercritical state is under an appropriate pressure. The invention provides for the use of sCO ₂ as a carrier medium for the energy of the waste heat and as a working medium in the energy system, whereby the thermal energy is to be converted into mechanical and in particular kinetic energy with the CO ₂ as the working medium in a thermal power process, preferably with subsequent generation of Electric energy.

According to the invention, the CO ₂ system of the energy system is also used to operate the expeller (also referred to as a generator) of the absorption refrigeration system. For this purpose, the CO ₂ that emerges from the 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 system according to the invention, a pre-cooling of the CO ₂ emerging from the turbine takes place beforehand, in return the CO ₂ that goes to the engine or, for example, the turbine is carried out, is preheated. This recuperator, which is also called a regenerator or internal recuperator, is a heat exchanger, preferably a high-temperature recuperator. The recuperator is necessary to be able to increase the mass flow in the CO ₂ circuit. 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. Because it is a significant advantage of the invention that heat transfer media with which the waste heat is discharged at a low temperature, namely a maximum temperature of 450 ° C, can also be used and converted with high efficiency into mechanical energy or, in particular, kinetic energy. The process temperature is preferably between 350 and 400 °C.

In the invention, the cycle of the absorption refrigeration system benefits from the waste heat that the CO ₂ carries after the recuperator from step d) of the method or paragraph d) of the description of the energy system, because it is the energy source that is necessary to run the absorption refrigeration system operate.

As is known, in absorption systems, such as a LiBr/H ₂ O system, an energy source is required in the expeller in order to evaporate the water from the water-LiBr mixture (mixture of refrigerant and salt solution of the absorption refrigeration system). This refrigerant vapor is then condensed in the condenser, expanded (preferably using an expansion valve) and fed into the evaporator in the liquid phase at a low temperature, whereby the coolant water evaporates. In the context of the present invention, the CO ₂ mass flow, which represents a partial flow of the CO ₂ emerging from the engine or the turbine, is used to cool. The condenser of the absorption refrigeration system is cooled with cooling water, whereby the temperature of this cooling water has little to no influence on the stable operation of the energy system.

It should be noted that conventional absorption systems (such as LiBr/H ₂ O absorption systems) typically include internal heat recovery device (SHX equipment) to increase the efficiency of the absorption system.

However, the present invention discloses a simplified absorption system in which the internal heat recovery occurring within the Absorption refrigeration system using an internal recuperator can be omitted. The advantage is that the costs of the energy system can be reduced and a higher level of heat removal from the sCO ₂ system is possible. Finally, the absorption refrigeration system supplies the coolant to keep the topping process at an optimal temperature. This significantly increases the performance of the energy system according to the invention.

Advantageously, no commonly used cooling water is required for cooling the CO ₂ , which can be subject to external, seasonal fluctuations and would require adjustment of the entire system, including efficiency. The CO ₂ can be cooled so effectively and consistently that the subcritical range of approx. 20 ± 5 °C of the CO ₂ in front of the pump arrangement can be reached and operated consistently and over seasonal fluctuations (as is the case with cooling water, for example). above all, it is stable and consistent. The useful performance of the sCO ₂ process for utilizing waste heat is hardly reduced, despite the seasonal increase in outside temperatures.

Due to this consistency, the configuration according to the invention has high thermoeconomic performance even under off-design conditions. Furthermore, local two-phase flows do not occur at the entry into the pump arrangement. This would have a negative impact on the stability of the process and the service life of the energy system. This results in regulated cooling. Expellers and evaporators of the absorption refrigeration system serve as temperature controllers for heat dissipation. The temperature control preferably takes place sensibly by changing the mass flows.

The invention represents a "circuit architecture" through the general arrangement of the engine or turbine, pumps, compressors or compressors, recuperators and external heat exchangers of the sCO ₂ circuit and achieves a stabilization of the operating behavior of the arrangement according to the invention. The evaporator of the absorption refrigeration system works as a cooler of the energy system (ie the topping system), so that heat is transferred from the CO ₂ stream present there to CO ₂ streams with a lower temperature.

The arrangement according to the invention advantageously works with higher, supercritical pressures, so that the CO ₂ immediately in front of the engine, for example the turbine, is in the supercritical range, which increases the efficiency of the generator. Another advantage is that there is no need for internal recuperators in the absorption refrigeration system. The invention therefore also requires fewer components.

The invention is therefore easy to implement, reduces the number of components in the system and ultimately the useful absorption and conversion of waste heat is improved. An external heat source for the absorption system is no longer necessary. There is also no need for a pre-cooler for the hot CO ₂ emerging from the turbine with its own cooling circuit. The sCO ₂ is cooled exclusively by the absorption refrigeration system, using 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 hot CO ₂ is pre-cooled. The invention is characterized by, but is not limited to, the following preferred embodiments.

An embodiment of the invention, in which the energy system comprises at least two heat exchangers for the sequential transfer of waste heat to CO ₂ as a carrier and working medium, has proven to be advantageous. This means that in step b) of the method according to the invention, the waste heat is transferred to the CO ₂ as a carrier medium sequentially in at least two stages. In a preferred embodiment, the first partial stream of the CO ₂ stream after the recuperator in method step d) is combined with the second partial stream of the CO ₂ stream, which is led from the flow divider to the heat exchanger in paragraph b), in such a way that both partial streams together pass through the heat exchanger as the first heat exchanger, the main heater to absorb the waste heat through the CO ₂ .

In a particularly preferred embodiment, the other, second partial stream of the CO ₂ stream has previously passed through a second heat exchanger, the preheater - for absorbing the waste heat through the CO ₂ - before this second partial stream is combined with the first partial stream and both together the first Heat exchangers happen. Naturally, the temperature that the first heat exchanger provides is higher than the temperature provided by the second heat exchanger, since the medium that provides the waste heat and brings it in as a waste heat flow is still significantly hotter at the first heat exchanger than at the second heat exchanger. The The last described embodiment therefore uses sequential heating by means of the first heat exchanger and the second heat exchanger.

In a preferred embodiment of the invention, the line between the expeller and the evaporator of the absorption refrigeration system passes a sub-cooler. This means that the stream of CO ₂ emerging from the turbine is led past a sub-cooler after cooling at the expeller before being directed to the evaporator of the absorption refrigeration system. This version is advantageous because it increases the cooling capacity on the evaporator. In a preferred embodiment of the invention, the absorption refrigeration system is a LiBr/H ₂ O absorption refrigeration system. It is particularly advantageous if the absorption refrigeration system works without a solution heat exchanger recuperator between the expeller and the absorber. This is required according to the prior art, but is not required for the system according to the invention, as has surprisingly been shown.

The energy system advantageously comprises a generator for generating electrical power, mechanically connected to the engine, designed as a turbine, or in step c) of the method, the mechanical or kinetic energy of the engine or turbine is additionally supplied to the engine by means of a Mechanically coupled generator generates electricity. The temperature of the CO ₂ before entering the pump arrangement is preferably 20 ± 5 °C, so that the CO ₂ is in the subcritical state there.

It has proven to be advantageous when carrying out the method according to the invention 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 method according to the invention, the temperature of the CO ₂ emerging from the turbine is at 240 ± 20 °C and after the heat transfer in step d) at 75 ± 10 °C, preferably before at 235-245 °C and afterwards at 70-80°C. In return, the sCO ₂ 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.

In a preferred embodiment of the method according to the invention, the temperature of the CO ₂ 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 been established proven to be advantageous, with the temperature range between 70 °C and 80 °C producing the best results. This corresponds to an embodiment in which the CO ₂ flowing from the turbine to the recuperator has these temperatures after exiting the recuperator. The thermal effectiveness of heat exchange is therefore limited and is around 85-95%.

Another advantage of the invention is a 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 in the art for an absorption system, but requires additional effort. The elimination of this device, which the invention makes possible, means a simplification of the system.

In most known recompression architectures (HTR), the CO ₂ partial streams are brought together shortly before the high-temperature recuperator. Therefore, in such a known system, the mass flows of both streams (hot side and cold side) are the same size. In contrast to this architecture, however, in the sCO ₂ preheat cycle of the present invention, the merging of the two CO ₂ streams occurs after the recuperator and the preheater and immediately before the main heater. There is therefore a difference between the mass flows of the two streams (hot side and cold side) in the recuperator. This property also reduces the irreversibility of the system; the preheating architecture also does not require the use of the HTR and the recompressor.

The use of the system according to the invention for utilizing waste heat from industrial processes is particularly advantageous in the thermal use of heat sources classified as medium temperatures. Because their use represents a technical and economic challenge for energy production through sCO ₂ systems. The proposed changes are therefore suitable for creating greater application potential for the combined system. However, for high-temperature heat sources, the operating characteristics are completely different.

To implement the invention, it is also expedient to combine the above-described inventive configurations, embodiments and features of the claims with one another. The invention will be explained in more detail below using exemplary embodiments which 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. Fig. 3 shows the location of the supercritical region in an entropy-temperature diagram.

A circuit of supercritical CO ₂ , sCO ₂ is shown schematically in FIG. 1, in particular the arrangement according to the invention for converting waste heat into mechanical energy. In the following explanations, the CO ₂ is consistently referred to as the working medium, since it represents the working medium of a thermal power process or a heat engine for the method according to the invention, carried out in the arrangement according to the invention, the preferred embodiments of which are described.

In the circuit, the pressure of the working medium is raised above the critical pressure by a pump 1 as an embodiment of the pump arrangement, 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 between 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.

The 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 ratio of mass flows allows greater heat transfer between the low pressure and high temperature flow in line 19 and the high pressure and low temperature flow in line 4. The lower irreversibility means that the recuperator 6 has better thermodynamic performance. The proposed mass flow rate allows a better balance between the heat capacities on both sides of the heat exchanger. This results in a better temperature gradient in the heat exchanger, which increases the overall heat transfer efficiency and the Irreversibility reduced. This therefore leads to improved thermal efficiency of the heat exchanger.

The working medium in line 8, the continuation of line 4 after the high-temperature recuperator 6, recovers the heat from the working medium 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 first heats the working medium by means of the second heat exchanger, the preheater 7, so that the working medium reaches an increased temperature at which it then flows into line 9. A mixer 10 mixes the working media from lines 8 and 9, the resulting working medium 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 flows through line 14 entering the heat exchanger 12 the already preheated working medium is heated further.

The turbine 17 relaxes the working medium that flows to it in line 13 until it enters line 19, and transmits the resulting or converted mechanical energy to a generator 18. At the exit of the turbine 17, when it enters line 19, remains the working medium in subcritical pressure. The working medium reaches the high-temperature recuperator 6 through line 19, is cooled there by the working medium from line 4, from the cold side, and flows via the line

20 to an expeller 23 of a lithium bromide water absorption refrigeration system. The heat of the working medium, in the low pressure in line 20, supplies the absorption refrigeration process of the absorption refrigeration system 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 transported via line

21 condenses at reduced temperature in the evaporator 31. Finally, the working medium is returned to the pump 1 via line 22 at a low temperature and completes the preheating cycle.

The use of the cooling system with the main components expeller 23, condenser 25, evaporator 31, absorber 33 primarily serves to dissipate heat from the 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) represented by stream 36. The current 20 after the recuperator 6 is therefore the main drive for the expeller 23. The Stream 30 with a temperature of 15 °C is the cooling medium for the heat removal from the CO ₂ stream 21 in the evaporator 31. In this case, the evaporator 31 is the main cooler of the sCO ₂ circuit.

In the lithium bromide absorption refrigeration system, 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 medium flowing in line 20 in order to evaporate the refrigerant (water). The water vapor then flows from the expeller 23 as refrigerant vapor in 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.

The high-pressure liquid refrigerant flowing through the line 28 expands at an expansion valve 29, whereby the pressure of the refrigerant in the line 30 leaving the expansion valve 29 drops 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.

The refrigerant, here water in the state of saturated vapor, enters through line 32 into an absorber 33 to be absorbed by the strong salt solution (LiBr solution). An absorber pump 35 increases the pressure of the now diluted LiBr-H ₂ O solution flowing to it via line 34 and leads it via line 36 to the inlet into 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 by means of cooling water.

2 shows, in addition to the components and processes already explained in FIG is guided past a sub-cooler 42 in the evaporator 31 before the working medium enters the evaporator 31. The heat is removed from the sub-cooler 42 by means of cooling water 43, 44. In addition, it should be noted that in Figures 1 and 2 no SHX (solution heat exchanger) is used between expeller 23 and absorber 33. The use of such equipment is otherwise standard for such an absorption system. Eliminating this equipment would mean a possible simplification of the system.

3 shows in a Ts diagram with the specific entropy s as the abscissa and the absolute temperature T as the ordinate the schematic supercritical region 50, as used, for example, in CO ₂ processes such as the method according to the invention. In addition, the subcritical region 51 has a higher specific entropy s and the mixed phase region 52 is at a lower temperature? shown.

Non-patent literature cited:

Wu, C.; Shun-sen, W.; Feng, X-j.; Li, Jun:

Energy, exergy and exergoeconomic analyzes of a combined supercritical CO ₂ recompression Brayton/absorption refrigeration cycle

Energy Conversion and Management 2017, 148, 360-377.

Ma., Y.; Liu, M.; Yan., J.; Liu, J.

Performance investigation of a novel closed Brayton cycle using supercritical CO ₂ - based mixtures as working fluid integrated with a LiBr absorption chiller

Applied Thermal Engineering 2018, 141, 531-547.

Ma, Y.; Zhang, X; Liu, M.; Yan, J.; Liu, J.

Proposal and assessment of a novel supercritical CO ₂ Brayton cycle integrated with

LiBr absorption chiller for concentrated solar power applications

Energy 2018, 148, 839-854.

Yang, S.; Deng, C.; Liu, Z.

Optimal design and analysis of a cascade LiBr/H ₂ O absorption refrigeration/transcritical

_CO2 process for low-grade waste heat recovery

Energy Conversion and Management 2019, 232-242.

Gotelip, T.; Gampe, U.; Gloss, S.

Optimization strategies of different SCO ₂ architectures for gas turbine bottoming cycle applications

Energy 2022, 250, 123734.

Reference symbols

1 pump arrangement, pump (of the energy system)

2 line for CO ₂ (high pressure)

3 river dividers

4 Line for CO ₂ (high pressure, first partial flow)

5 Line for CO ₂ (high pressure, second partial flow)

6 recuperator (heat exchanger); High temperature recuperator

7 preheater, second heat exchanger (second heater, waste heat on

_CO2 )

8 Line for CO ₂ (high pressure, first partial flow)

9 Line for CO ₂ (high pressure, second partial flow)

10 mixers with throttle

11 Line for CO ₂ (high pressure)

12 first heat exchanger (main heater, waste heat to CO ₂ )

13 Line for CO ₂ (high pressure)

14, 15, 16 line for waste heat flow

17 engine, turbine

18 generator

19, 20, 21, 22 Line for CO ₂ (low pressure), CO ₂ stream

23 expellers (of the absorption refrigeration system)

24 Line for H ₂ O (refrigerant vapor of the absorption refrigeration system)

25 condenser (of the absorption refrigeration system)

26, 27 Connection for cooling water for the condenser

28 line for H ₂ O (water)

29 Expansion valve (of the absorption refrigeration system)

30 line for H ₂ O (water)

31 evaporators

32 line for H ₂ O (water)

33 absorbers (of the absorption refrigeration system)

34 line for LiBr/H ₂ O (diluted solution)

35 Absorber pump (pump of the absorption refrigeration system)

36 line for LiBr/H ₂ O (diluted solution, mixture of refrigerant and

saline solution)

37 line for LiBr (concentrated solution)

38 throttle valve Line for LiBr (concentrated solution), 41 connection for cooling water for the absorber sub-cooler A/or cooler

Line for cooling water for the sub-cooler of the CO ₂ Line for cooling water for the sub-cooler of the CO ₂ supercritical area subcritical area

Mixed phase area

entropy

temperature

Claims

Patent claims

1. Arrangement for converting waste heat into mechanical energy using carbon dioxide (CO ₂ ) in the supercritical state (sCO ₂ ), comprising a) a pump arrangement (1) for increasing the pressure and for further transporting the CO ₂ under transcritical conditions from a line (22 ), wherein the CO ₂ at the inlet of the pump arrangement (1) has a pressure above the critical CO ₂ pressure and a temperature below the critical CO ₂ 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 CO ₂ as a carrier medium and working medium, so that the CO ₂ can be present in the supercritical state as sCO ₂ , c) an engine (17) for Generation of mechanical energy by means of the sCO ₂ , and d) an absorption refrigeration system for the controlled cooling of the CO ₂ emerging from the engine (17), from which heat has then already been removed in the recuperator (6), comprising an expeller (23) and an evaporator ( 31), in which heat is removed from the CO ₂ in two stages by including a line (20) which comes from the recuperator

(6) leads to the expeller (23) and further comprises a line (21) which leads from the expeller (23) to the evaporator (31), characterized in that e) the absorption refrigeration system is designed to control the temperature at the inlet of the pump arrangement ( 1) to be kept constantly below the critical temperature, f) that between the engine (17) and the expeller (23) there is a recuperator (6) for heat transfer from the CO ₂ emerging from the engine (17) to the sCO ₂ leading to the engine (17). is arranged, and that between the pump arrangement (1), the recuperator (6) and the preheater

(7) a flow divider (3) is arranged for dividing the stream of CO ₂ into a first partial stream to the recuperator (6) and a second partial stream for transferring waste heat by means of the at least two heat exchangers (7, 12).

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

3. Arrangement according to claim 1 or 2, wherein the flow divider (3) is designed or set to have a mass flow ratio of 55 to 65% for a first partial flow of CO ₂ under high pressure in line (4) in relation to a flow in line (19) to the recuperator (6). Arrangement according to one of the preceding claims, wherein CO ₂ under high pressure in a first partial stream (8), CO ₂ under high pressure in a second partial stream (9) are combined in a mixer (10) into a line (11) as CO ₂ under high pressure . Arrangement according to one of the preceding claims, wherein a line (21) between the expeller (23) and the evaporator (31) of the absorption refrigeration system is led through a sub-cooler (42) for further cooling. Arrangement according to one of the preceding claims, wherein the absorption refrigeration system is a LiBr/H ₂ O absorption refrigeration system. Arrangement according to one of the preceding claims, wherein the absorption refrigeration system works without a solution heat exchanger recuperator between the expeller (23) and the absorber (33). Arrangement according to one of the preceding claims, comprising a generator (18) for generating electricity, mechanically connected to the engine (17) designed as a turbine. Method for converting waste heat into mechanical energy, comprising the steps: a) increasing pressure and transporting CO ₂ by means of a pump arrangement (1), wherein the CO ₂ at the inlet of the pump arrangement (1) has a pressure above the critical CO ₂ pressure and a temperature below the critical CO ₂ temperature, b) transferring waste heat from a waste heat stream to CO ₂ as a carrier medium and working medium sequentially in at least two stages, so that the CO ₂ is present in the supercritical state as sCO ₂ after compression, and c) Generation of mechanical energy from the sCO ₂ by means of an engine (17), characterized in that d) by means of a recuperator (6) arranged between the engine (17) and the extractor (23), a heat transfer from the engine (17) through the Line (19) exiting C0 ₂ to the sCO ₂ in a line (4) leading to the engine (17), and that the C0 ₂ after transport through the pump arrangement (1) in step a) through a flow divider (3) is divided, a first partial flow being passed through the line (4) to the recuperator (6), and a second partial flow being passed through a line (5) for the transfer of waste heat in step b), e) controlled cooling of the out the CO ₂ emerging from the engine (17) by means of an absorption refrigeration system, comprising an expeller (23) and an evaporator (31), the CO ₂ emerging from the engine (17) via a line (19) first in the recuperator (6), then is cooled by means of the expeller (23) and finally by means of the evaporator (31) of the absorption refrigeration system, f) the absorption refrigeration system keeps the temperature at the inlet of the pump arrangement (1) constantly below the critical temperature, method according to claim 9, wherein the CO ₂ in step a) at the inlet of the pump arrangement (1) has a pressure above the critical CO ₂ pressure and a temperature below the critical CO ₂ temperature and thus enters the pump arrangement (1) under transcritical conditions. The method according to claim 10, wherein the CO ₂ has a pressure above the critical pressure, but at least 75 bar, and a temperature between 20°C and 28°C under transcritical conditions. Method according to one of claims 9 to 11, wherein in a mixer (10) a first partial flow in line (8) of CO ₂ under high pressure, a second partial flow in line (9) of CO ₂ under high pressure to form a flow in line (11 ) of CO ₂ is mixed under high pressure. Method according to one of claims 9 to 12, wherein the temperature of the second partial stream (9) at the output of the preheater (7) does not deviate from the temperature of the first partial stream (8) at the output of the recuperator (6) by more than ± 10 K. Arrangement according to one of claims 9 to 13, wherein the flow divider (3) has a mass flow ratio of 55 to 65% for a first partial flow of CO ₂ High pressure in line (4) in relation to a second flow in line (19) to the recuperator (6), thereby guaranteeing an optimal thermal capacity ratio between the two flows and thus reducing irreversibility in the system. Method according to one of claims 9 to 14, wherein the first of the CO ₂ partial streams divided in the flow divider (3) in line (4, 8) after the recuperator (6), in which the heat transfer according to step f) took place, with the The second of the CO ₂ partial streams divided in the flow divider (3) is combined in line (5, 9), so that both partial streams can absorb the waste heat together. Method according to one of claims 9 to 15, wherein the CO ₂ has a temperature between 20 and 28 °C before entering the pump arrangement (1) and the CO ₂ is present there in the transcritical state. Method according to one of claims 9 to 16, wherein in step b) the waste heat is transferred from a medium which has a maximum temperature of 450 °C. Method according to one of claims 9 to 17, wherein in step f) the temperature of the CO ₂ emerging from the engine (17) is 240 ± 20 °C and after the heat transfer in the recuperator (6) according to step f) is 75 ± 10 °C . Method according to one of claims 9 to 18, wherein in step d) the temperature of the CO ₂ to be cooled at the inlet of the expeller is 75 ± 10 °C. Method according to one of claims 9 to 19, wherein a higher heat absorption in the expeller (23) is made possible by the absorption refrigeration system working without a solution heat exchanger recuperator between the expeller (23) and the absorber (33). Use of an absorption refrigeration system as a bottoming system of an sCO ₂ circuit according to one of claims 1 to 8, wherein the absorption refrigeration system serves as a cooler for the topping system and comprises an expeller (23) and an evaporator (31), characterized in that a CO ₂ stream (20) is the exclusive heat source for driving the expeller (23), and that the absorption refrigeration system acts as a temperature controller for the heat removal from the CO ₂ system the energy system and for this purpose regulates the temperature of the stream (22) at the inlet of the pump arrangement (1) to subcritical temperature conditions of 20 to 28 ° C, regardless of the ambient temperature, the evaporator (31) being the main cooler of the topping system.