DE102023113884A1 - Devices and methods for operating a transcritical cycle - Google Patents

Abstract

The invention relates to devices (1a, 1b) for operating a transcritical cycle process, in particular a heat-power process and a heat pump process, each with a working fluid circuit (2a, 2b) with a conveying device (3a, 3b), an internal circuit heat exchanger (7), a high-temperature heat exchanger (4), an expansion device (5a, 5b) and a low-temperature heat exchanger (6). The internal circuit heat exchanger (7) is designed with a first flow path (7a) for passing through working fluid at a high-pressure level and a second flow path (7b) for passing through working fluid at a low-pressure level in such a way as to transfer heat between the working fluids flowing through the flow paths (7a, 7b). The working fluid is in each case designed as a mixture of at least two components such that the working fluid, when flowing through the second flow path (7b) of the internal circuit heat exchanger (7), depending on the device (1a, 1b), has a desuperheating and a subsequent partial condensation in each case as the temperature decreases or exclusively a partial condensation in each case as the temperature decreases or exclusively a partial evaporation in each case as the temperature increases or a partial evaporation and a subsequent overheating in each case as the temperature increases.
The invention also relates to methods for operating a transcritical cycle.

Description

The invention relates to devices for operating a transcritical cycle, in particular a heat-power process and a heat pump process, each with a working fluid circuit with a conveying device, an internal heat exchanger, a high-temperature heat exchanger, an expansion device and a low-temperature heat exchanger. The invention also relates to methods for operating a transcritical cycle.

Cyclic processes are known from the state of the art in which heat is transferred isothermally at a low temperature level of the cycle, while heat transfer with a change in temperature occurs at a high temperature level of the cycle. These cycles have a temperature difference between the low temperature level and the high temperature level of the cycle. The cycle of the working fluid can be implemented both as a heat-power process and thus as a so-called clockwise cycle, and as a heat pump process and thus as a so-called counterclockwise cycle.

Such a cycle has a higher efficiency than a known Joule-Brayton process, especially when heat is transferred to the environment at the low-temperature level of the cycle, since the environment can be regarded as a latent heat source or latent heat sink. In comparison to the cycle with isothermal heat transfer at the low-temperature level, the heat in the Joule-Brayton process is transferred at the low-temperature level of the cycle with a change in the temperature of the working fluid.

The cycle process with heat transfer during temperature change at the high temperature level of the cycle process and isothermal heat transfer at the low temperature level of the cycle process has gained importance for thermal electricity storage, in which regenerative surplus electricity is temporarily stored in the form of thermal energy.

In the technology, also known as the Carnot battery, the heat is stored at high storage temperatures in molten salts or in bulk material. However, the molten salts or bulk material are sensitive heat storage devices, which are each charged and discharged using a cyclic process with a corresponding temperature change during heat transfer.

It is known from the state of the art to implement the cycle with isothermal heat transfer at the low temperature level and with temperature change during heat transfer at the high temperature level as a transcritical cycle. The heat is transferred between the secondary medium or the external heat reservoir and the working fluid at the low temperature level of the cycle when the working fluid is in a subcritical state.

An internal heat exchanger, also known as a recuperator or internal heat exchanger, is flowed through by supercritical steam of the working fluid on the high-pressure side or high-temperature side and by superheated steam of the working fluid on the low-pressure side or low-temperature side. The heat is transferred between the working fluid at the high-pressure level and the working fluid at the low-pressure level.

Carbon dioxide is usually used as the working fluid. US 2022 056 817 A Systems and methods for charging a thermal energy storage system. A system can have a compressor or a pump as a conveying device to circulate the working fluid within a fluid circuit. In addition, the system is designed with three heat exchangers, a turbine as an expansion device, a high-temperature storage device thermally connected to a first heat exchanger, a low-temperature storage device thermally connected to a second heat exchanger and a waste heat storage device thermally connected to a third heat exchanger. The turbine is arranged between the first heat exchanger and the second heat exchanger in order to expand the working fluid as it flows through.

In the transcritical process control of the working fluid, the heat transfer is very low compared to a heat transfer with a phase change of the working fluid due to the exposure of the internal circuit heat exchanger with superheated steam on the low pressure side. Consequently, the internal circuit heat exchanger must be designed with a large heat transfer surface and therefore with very large dimensions.

Carbon dioxide as a working fluid also has a very high pressure level. For example, the critical pressure is around 74 bar. This means that the components of the fluid circuit, such as the connecting lines, heat exchangers and conveying devices as well as expansion devices, must be designed for high pressures or be high-pressure resistant, which causes high costs.

In addition, the critical temperature of carbon dioxide is only about 31°C. This means that it is not always certain that a fluid circuit operating as a heat-power process can be directly coupled with ambient heat sinks, since at high ambient temperatures, especially the ambient air, the heat can no longer be dissipated subcritically from the working fluid. In order to ensure subcritical heat transfer from the working fluid to the environment, an ice storage unit can be provided as a heat sink, which serves as a heat sink in the case of a clockwise cycle and as a heat source in the case of a counterclockwise cycle. This always ensures heat transfer with subcritical working fluid at the low temperature level, but this requires an additional complex storage unit, which is particularly disadvantageous when using a fluid circuit as a cycle according to a Carnot battery, which has to compete economically with other storage technologies.

The object of the present invention is to provide fluid circuits operated in a cyclic process of a working fluid, in which the cyclic process has a temperature difference between the low temperature level and the high temperature level and an isothermal heat transfer of the working fluid at the low temperature level and a heat transfer with a change in temperature of the working fluid at the high temperature level. A clockwise heat-power process and a counterclockwise heat pump process are to be designed. The cyclic process should be able to be operated with maximum energy efficiency for converting sensible heat into mechanical energy or from mechanical energy into sensible heat. The environment, in particular the ambient air, surface water or geothermal energy, or other latent heat reservoirs should be able to be used as a heat sink or heat source. In addition, the components of the heat exchanger within the circuit should have a minimal installation space and the pressure levels and temperature levels of the working fluid should be adaptable for the respective heat transfer.

The object of the invention is achieved with devices and methods for operating a transcritical cycle with the features according to the independent patent claims. Further developments are specified in the dependent patent claims.

The object is achieved by a first device according to the invention with a working fluid circuit for a transcritical cycle, in particular a heat-power process or a heat-power cycle in which a working fluid circulates.

The working fluid circuit of the first device has, in the direction of flow of the working fluid, a conveying device for conveying liquid working fluid, an internal circuit heat exchanger, at least one high-temperature heat exchanger for transferring heat to the working fluid, at least one expansion device for expanding the working fluid and at least one low-temperature heat exchanger for dissipating heat from the working fluid. The internal circuit heat exchanger is designed with a first flow path for passing through working fluid at a high-pressure level and a second flow path for passing through working fluid at a low-pressure level and is arranged in the working fluid circuit in such a way that heat is transferred from the working fluid flowing through the second flow path to the working fluid flowing through the first flow path.

According to the concept of the invention, the working fluid is designed as a mixture of at least two components such that the working fluid, when flowing through the second flow path of the internal circuit heat exchanger, undergoes desuperheating and subsequent partial condensation, also referred to as partial condensation, in each case at a changing temperature, in particular at a decreasing temperature, or exclusively a partial condensation at a decreasing temperature.

The mixture can be adjusted in the number and proportions of the components as required.

The expansion device is advantageously designed as an expansion machine, in particular as an at least one-stage turbo machine, for converting technical work into mechanical work. The conveying device can be designed as a pump.

According to a development of the invention, for a multi-stage expansion of the working fluid with heat transfer to the working fluid between the stages of expansion, at least two high-temperature heat exchangers and at least two expansion devices are arranged alternately in series with one another.

The object is also achieved by a second device according to the invention with a working fluid circuit for a transcritical cycle process, in particular a heat pump process. The problem is solved by using a heat pump cycle in which a working fluid circulates.

The working fluid circuit of the second device has, in the flow direction of the working fluid, a conveying device for compressing vaporous working fluid, at least one high-temperature heat exchanger for dissipating heat from the working fluid, an internal circuit heat exchanger, at least one expansion device for expanding the working fluid and at least one low-temperature heat exchanger for transferring heat to the working fluid. The internal circuit heat exchanger is designed with a first flow path for passing through working fluid at a high-pressure level and a second flow path for passing through working fluid at a low-pressure level and is arranged in the working fluid circuit in such a way that heat is transferred from the working fluid flowing through the first flow path to the working fluid flowing through the second flow path.

According to the concept of the invention, the working fluid is designed as a mixture of at least two components such that the working fluid, when flowing through the second flow path of the internal circuit heat exchanger, only exhibits partial evaporation with increasing temperature, also referred to as partial evaporation, or partial evaporation and subsequent overheating, each with increasing temperature. The temperature change is not due to a pressure loss of the working fluid when flowing through the heat exchanger.

The mixture can be adjusted in the number and proportions of the components as required.

The expansion device is advantageously designed as an expansion element or an expansion machine, in particular as an at least single-stage turbomachine, for recovering expansion work in the form of mechanical work. The conveying device can be designed as a compressor, in particular as an at least single-stage turbomachine.

According to a further development of the invention, for a multi-stage compression of the working fluid with heat transfer from the working fluid interposed between the compression stages, at least two high-temperature heat exchangers and at least two conveying devices designed as compressors are arranged alternately in series with one another.

According to a preferred embodiment of the invention, the mixture is formed from at least one main component, in particular exactly one main component or a main fluid, and at least one secondary component. The at least one main component has a lower normal boiling point and a significantly higher molar fraction than the at least one secondary component. The molar fraction of the at least one main component is preferably at least 90% of the working fluid. The working fluid is preferably formed as a zeotropic mixture, at the low pressure level of the cycle, in particular with a temperature glide of at least 50 K. The normal boiling point is the boiling point of the respective component at ambient pressure.

The working fluid can be a mixture comprising butane as the main component and dodecane as the secondary component. The working fluid is preferably formed as a mixture of 95 mol% butane and 5 mol% dodecane or as a mixture of 96 mol% butane and 4 mol% dodecane.

The at least one high-temperature heat exchanger is preferably designed as a countercurrent heat exchanger, while the at least one low-temperature heat exchanger is preferably designed as a cross-current heat exchanger or as a countercurrent heat exchanger.

The object of the invention is also achieved by a first method according to the invention for operating a device with a working fluid circuit in which a working fluid circulates in a transcritical cycle, in particular a heat-power process or a heat-power cycle. The working fluid circuit of the device has a conveying device for conveying liquid working fluid in the flow direction of the working fluid, a heat exchanger inside the circuit with a first flow path and a second flow path, at least one high-temperature heat exchanger for transferring heat to the working fluid, at least one expansion device for relaxing the working fluid and at least one low-temperature heat exchanger for dissipating heat from the working fluid.

• - Ansaugen des flüssigen Arbeitsfluids auf einem niederen Druckniveau durch die Fördervorrichtung und Fördern des Arbeitsfluids auf ein höheres Druckniveau, insbesondere auf einen überkritischen Druck,
• - Erwärmen des Arbeitsfluids beim Durchströmen des ersten Strömungspfades des kreislaufinternen Wärmeübertragers,
• - Erwärmen des Arbeitsfluids beim Durchströmen des mindestens einen Hochtemperatur-Wärmeübertragers unter Aufnahme von Wärme auf eine maximale Hochtemperatur T_H,max und
• - Entspannen des Arbeitsfluids beim Durchströmen der mindestens einen Expansionsvorrichtung,
• - Enthitzen des als überhitzter Dampf vorliegenden Arbeitsfluids und zumindest teilweises Kondensieren des Arbeitsfluids oder zumindest teilweises Kondensieren des als gesättigter Dampf vorliegenden Arbeitsfluids beim Durchströmen des zweiten Strömungspfades des kreislaufinternen Wärmeübertragers jeweils unter Änderung der Temperatur des Arbeitsfluids sowie
• - vollständiges Kondensieren des Arbeitsfluids beim Durchströmen des mindestens einen Niedertemperatur-Wärmeübertragers unter Abgabe von Wärme.

The procedure involves the following steps in the order given:

• - Suction of the liquid working fluid at a low pressure level through the conveying device and conveying the working fluid to a higher pressure level, in particular to a supercritical pressure,
• - Heating of the working fluid as it flows through the first flow path of the internal heat exchanger,
• - Heating the working fluid as it flows through the at least one high-temperature heat exchanger while absorbing heat to a maximum high temperature T _H,max and
• - Relaxing the working fluid as it flows through the at least one expansion device,
• - deheating the working fluid present as superheated steam and at least partially condensing the working fluid or at least partially condensing the working fluid present as saturated steam as it flows through the second flow path of the internal heat exchanger, in each case with a change in the temperature of the working fluid and
• - complete condensation of the working fluid as it flows through at least one low-temperature heat exchanger, releasing heat.

The change in temperature during deheating and condensing in the second flow path of the internal heat exchanger preferably takes place in such a way that the numerical value of the heat capacity flow of the superheated steam has the same order of magnitude as the difference quotient of enthalpy flow to temperature difference during the subsequent condensation.

According to a development of the invention, a temperature glide of the working fluid during condensation in the second flow path and thus on the low-pressure side of the internal circuit heat exchanger corresponds to a temperature change of the working fluid during heating in the first flow path and thus on the high-pressure side of the internal circuit heat exchanger based on an identical enthalpy difference of the working fluid.

According to a preferred embodiment of the invention, the change in state of the working fluid when heat is released in at least one low-temperature heat exchanger occurs essentially isothermally. A temperature glide of the working fluid when condensing within the low-temperature heat exchanger is smaller than the temperature glide of the working fluid when condensing in the second flow path of the heat exchanger within the circuit.

A further advantage of the invention is that a proportion of at least 50% of the heat which is transferred from the working fluid flowing through the second flow path of the internal circuit heat exchanger to the working fluid flowing through the first flow path of the internal circuit heat exchanger is dissipated during the condensation of the working fluid flowing in the second flow path of the internal circuit heat exchanger.

During the process of expanding the working fluid as it flows through the at least one expansion device, mechanical work can be performed. The change in state of the working fluid occurs preferably isentropically, in particular without additional heat input. The working fluid advantageously flows into the expansion device in a supercritical state.

When condensing the working fluid in the at least one low-temperature heat exchanger, the heat is preferably transferred to an external first heat reservoir, while when heating the working fluid in the at least one high-temperature heat exchanger, the heat is absorbed from an external second heat reservoir.

The object of the invention is also achieved by a second method according to the invention for operating a device with a working fluid circuit in which a working fluid circulates in a transcritical cycle, in particular a heat pump process or a heat pump cycle. The working fluid circuit of the device has, in the flow direction of the working fluid, a conveying device for compressing vaporous working fluid, at least one high-temperature heat exchanger for dissipating heat from the working fluid, an internal circuit heat exchanger with a first flow path and a second flow path, at least one expansion device for expanding the working fluid and at least one low-temperature heat exchanger for transferring heat to the working fluid.

• - Ansaugen des im Zustand von gesättigtem oder überhitztem Dampf vorliegenden Arbeitsfluids und Verdichten des Arbeitsfluids auf ein höheres Druckniveau, insbesondere auf einen überkritischen Druck sowie eine Hochtemperatur T_H,max, durch die Fördervorrichtung,
• - Enthitzen des Arbeitsfluids beim Durchströmen des mindestens einen Hochtemperatur-Wärmeübertragers unter Abgabe von Wärme,
• - Abkühlen des Arbeitsfluids beim Durchströmen des ersten Strömungspfades des kreislaufinternen Wärmeübertragers auf eine Temperatur unterhalb des kritischen Punktes nahe eines Niedertemperaturniveaus des Kreisprozesses,
• - Entspannen des Arbeitsfluids beim Durchströmen der mindestens einen Expansionsvorrichtung,
• - zumindest teilweises Verdampfen des Arbeitsfluids beim Durchströmen des mindestens einen Niedertemperatur-Wärmeübertragers durch Aufnahme von Wärme sowie
• - vollständiges Verdampfen des Arbeitsfluids beim Durchströmen des zweiten Strömungspfades des kreislaufinternen Wärmeübertragers unter Aufnahme von Wärme und gegebenenfalls Überhitzen des Arbeitsfluids.

The procedure involves the following steps in the order given:

• - Suction of the working fluid in the state of saturated or superheated steam and compression of the working fluid to a higher pressure level, in particular to a supercritical pressure and a high temperature T _H,max , by the conveying device,
• - Deheating the working fluid as it flows through at least one high-temperature heat exchanger while releasing heat,
• - Cooling of the working fluid as it flows through the first flow path of the internal heat exchanger to a temperature below the critical point near a low-temperature level of the cycle,
• - Relaxing the working fluid as it flows through the at least one expansion device,
• - at least partial evaporation of the working fluid when flowing through the at least one low-temperature heat exchanger by absorbing heat and
• - complete evaporation of the working fluid as it flows through the second flow path of the internal heat exchanger, absorbing heat and possibly overheating the working fluid.

When flowing through the second flow path of the in-circuit heat exchanger, the working fluid evaporates completely, so that the working fluid flows out of the second flow path of the in-circuit heat exchanger either as saturated vapor or superheated.

According to a development of the invention, a temperature glide of the working fluid during evaporation in the second flow path and thus on the low-pressure side of the internal circuit heat exchanger corresponds to a temperature change of the working fluid during cooling in the first flow path and thus on the high-pressure side of the internal circuit heat exchanger based on an identical enthalpy difference of the working fluid.

According to a preferred embodiment of the invention, the change in state of the working fluid when absorbing heat in the at least one low-temperature heat exchanger occurs essentially isothermally. In this case, a temperature glide of the working fluid during evaporation is significantly smaller than the temperature glide of the working fluid during evaporation in the second flow path of the internal heat exchanger.

A further advantage of the invention is that a proportion of at least 50% of the heat which is transferred from the working fluid flowing through the first flow path to the working fluid flowing through the second flow path is absorbed during the evaporation of the working fluid flowing in the second flow path.

When the working fluid evaporates in the at least one low-temperature heat exchanger, the heat is preferably absorbed from an external first heat reservoir, while when the working fluid is deheated in the at least one high-temperature heat exchanger, the heat is transferred to an external second heat reservoir.

In the low-temperature heat exchanger, the heat is advantageously transferred between the working fluid and an environment, in particular surface water, ambient air or geothermal reservoirs.

The advantageous embodiment of the invention enables high heat transfer coefficients of the working fluid, especially in the second flow path of the internal heat exchanger. In addition, the pressure levels and the temperature levels of the working fluid can be adjusted by appropriately combining the components of the mixture as the working fluid.

By carefully assembling the components of the mixture, a highly non-linear overall course of the state changes in the two-phase region is achieved. The mixture has almost no or only a very small temperature glide at low to high steam mass fractions at constant pressure and a large temperature glide at very high steam mass fractions, which is ideally linear in a T-s representation or a T-h representation and corresponds to the increase in the liquid isobars with the same enthalpy difference. The mixture is composed in such a way that the increase in the liquid isobars as a temperature change depending on the change in the enthalpy on the high pressure side and thus within the first flow path of the internal heat exchanger corresponds to the increase in the isobars in the two-phase region at high steam mass fractions as a temperature change depending on the change in the enthalpy on the low pressure side and thus within the second flow path of the internal heat exchanger based on the same absolute temperatures. Thus, the capacity flow of the liquid or supercritical working fluid in the first flow path of the internal heat exchanger corresponds to the difference quotient of the enthalpy change to the temperature change of the working fluid in the second flow path of the internal heat exchanger.

This makes it possible to integrate the internal heat exchanger into the working fluid circuit, through which supercritical or liquid working fluid flows on the high-pressure side, which experiences a temperature change during heat transfer, in particular during cooling, and which is acted upon by the working fluid on the low-pressure side, which at least partially has a phase change and also experiences a temperature change during heat transfer. A phase change with temperature change therefore occurs on the low-pressure side of the internal heat exchanger. As a result Due to the congruence of the temperature curve with the liquid isobar, the destruction of exergy is minimal.

Such mixture properties occur when, in particular, a main component is combined with one or more secondary components in such a way that the secondary component(s) have only a very small molar proportion of the working fluid and a higher normal boiling point than the main component.

The main component(s) of the mixture, which is/are adapted to the low temperature level of the respective cycle and thus the temperature profiles of the working fluid as it flows through the low temperature heat exchanger and the second flow path of the heat exchanger within the circuit, consequently has/have a significantly lower normal boiling point than the secondary component(s). The secondary component(s) can be long-chain alkanes.

Nonlinear overall temperature profiles only occur when components with large differences in normal boiling temperatures are combined. The exact nature of the temperature profiles, particularly depending on the enthalpy, is achieved by means of the corresponding composition of the main component(s) and the secondary component(s). The selection of the secondary components is based on the desired temperature glide and the desired increase in the isobars within the two-phase region, which correspond to the increases in the isobars in the liquid region.

Due to the essentially free selection of the main component(s) of the mixture, almost any relevant low-temperature level can be set as an isothermal temperature curve on the low-pressure side of the cycle, especially in the low-temperature heat exchanger, in which the working fluid undergoes a partial, almost isothermal phase change.

In the case of cycle processes known from the state of the art, the isothermal heat absorption takes place on the low-pressure side with a subcritical working fluid. The cycle processes can only be operated with a limited number of working fluids with very low critical temperatures and are therefore very limited in setting the isothermal temperature curve on the low-pressure side. For example, carbon dioxide has a temperature at the critical point of 31 °C.

• - höherer Wirkungsgrad durch die isotherme Wärmeübertragung auf dem Niedertemperaturniveau bei direkter Kopplung mit der Umgebung als Wärmereservoir, beispielsweise Umgebungsluft, Oberflächenwasser oder Geothermie,
• - höherer Wärmeübergangskoeffizient innerhalb des kreislaufinternen Wärmeübertragers durch Phasenwechsel des Arbeitsfluids auf der Niederdruckseite,
• - beim Betrieb im rechtslaufenden Wärme-Kraft-Prozess kann die Druckerhöhung im Arbeitsfluid im Vergleich zu komplexen Verdichtern im Joule-Brayton-Prozess durch einfachere Pumpen realisiert werden, sowie
• - superkritische Flüssigkeitsisobaren und subkritische Zweiphasenisobaren sind bei den gewählten Gemischen kongruenter als ausschließlich superkritische Isobaren untereinander bei rein gasförmigen Arbeitsfluiden, sodass im kreislaufinternen Wärmeübertrager geringere Exergieverluste zu verzeichnen sind.

The devices and methods according to the invention for operating a transcritical cycle process have, in particular in comparison to operation as a Joule-Brayton process, further advantages:

• - higher efficiency due to isothermal heat transfer at low temperature levels with direct coupling with the environment as a heat reservoir, for example ambient air, surface water or geothermal energy,
• - higher heat transfer coefficient within the internal heat exchanger due to phase change of the working fluid on the low-pressure side,
• - when operating in the clockwise heat-power process, the pressure increase in the working fluid can be achieved by simpler pumps compared to complex compressors in the Joule-Brayton process, and
• - supercritical liquid isobars and subcritical two-phase isobars are more congruent in the selected mixtures than exclusively supercritical isobars with each other in purely gaseous working fluids, so that lower exergy losses are recorded in the internal heat exchanger.

• - geringere Drucklage möglich,
• - anpassbares Niedertemperaturniveau, damit Vermeiden von kalten Wärmespeichern bei der Verwendung von Carnot-Batterien sowie
• - superkritische Flüssigkeitsisobaren und subkritische Zweiphasenisobaren sind bei den gewählten Gemischen kongruenter als superkritische Isobaren und Isobaren im überhitzten Dampfgebiet, sodass im kreislaufinternen Wärmeübertrager geringere Exergieverluste zu verzeichnen sind.

In comparison to transcritical cycles with carbon dioxide as the working fluid, the devices and methods according to the invention have the following advantages:

• - lower pressure level possible,
• - adjustable low temperature level to avoid cold heat storage when using Carnot batteries and
• - supercritical liquid isobars and subcritical two-phase isobars are more congruent in the selected mixtures than supercritical isobars and isobars in the superheated steam region, so that lower exergy losses are recorded in the internal heat exchanger.

• 1: ein T-s-Diagramm eines idealen thermodynamischen Kreisprozesses mit unterschiedlichen Temperaturniveaus als ein Vergleichsprozess,
• 2a und 2b: ein Schaltbild einer ersten Vorrichtung mit einem ersten Arbeitsfluidkreislauf sowie zugehörige Zustandsänderungen eines Wärme-Kraft-Prozesses im T-s-Diagramm und
• 3a und 3b: ein Schaltbild einer zweiten Vorrichtung mit einem zweiten Arbeitsfluidkreislauf sowie zugehörige Zustandsänderungen eines Wärmepumpen-Prozesses im T-s-Diagramm.

Further details, features and advantages of the invention emerge from the following description of embodiments with reference to the accompanying drawings. They show:

• 1 : a Ts diagram of an ideal thermodynamic cycle with different temperature levels as a comparison process,
• 2a and 2b : a circuit diagram of a first device with a first working fluid circuit and associated state changes of a heat-power process in the Ts diagram and
• 3a and 3b : a circuit diagram of a second device with a second working fluid circuit and associated state changes of a heat pump process in the Ts diagram.

In 1 is a temperature-entropy diagram, referred to as a Ts diagram for short, of an ideal thermodynamic cycle with different temperature levels, specifically two levels of high temperature T _H,max and T _H,min and one level of low temperature T _N , shown as a comparison process. The comparison process serves to represent both a heat-power process and a heat pump process, which ideally approximate this process. The heat transfer processes are shown using the individual arrows. The directions of heat transfer within the heat-power process are shown using arrows with solid lines and within the heat pump process using arrows with dashed lines.

At the level of the low temperature T _N, the heat is transferred in an isothermal change of state. The isothermal change of state is particularly advantageous when the heat is transferred from the working fluid to the environment, which corresponds to heat transfer in the heat-power process, or from the environment to the working fluid, which corresponds to heat transfer in the heat pump process. The environment corresponds to an almost inexhaustible and thus latent or isothermal, external first heat reservoir. In heat-power processes, the environment serves as a heat sink, while in heat pump processes the environment serves as a heat source.

A further change of state with heat transfer between the working fluid and a second external heat reservoir takes place at a temperature that fluctuates between the high temperature levels T _H,max and T _H,min . In the heat-power process, the heat is transferred from the environment to the working fluid and in the heat pump process from the working fluid to the environment. Consequently, the second heat reservoir serves as a heat source in heat-power processes and as a heat sink in heat pump processes.

The minimum level of the high temperature T _H,min differs from the level of the low temperature T _N , whereby the minimum level of the high temperature T _H,min is above the level of the low temperature T _N . The temperature difference between the minimum level of the high temperature T _H,min and the level of the low temperature T _N is achieved by an internal circuit heat transfer between the working fluid at different temperature levels, also referred to as recuperation.

The technical implementation of the thermodynamic comparison process according to 1 is aimed for when a latent or isothermal first heat reservoir at the level of the low temperature T _N is to be thermally coupled with a sensitive second heat reservoir at the level of the high temperature T _H by means of a thermodynamic process.

The technical implementation of such a thermodynamic process depends on the available working fluids and can be implemented, for example, using carbon dioxide as the working fluid, referred to as CO ₂ for short. However, the level of the low temperature T _N in particular is limited to values below 31 °C. In comparison, the temperature levels can be freely adjusted by using certain mixtures as the working fluid.

From the 2a and 2b a circuit diagram of a first device 1a with a first working fluid circuit 2a and state changes of a heat-power process in the Ts diagram associated with the circuit diagram are shown.

The first working fluid circuit 2a has, in the flow direction of the working fluid, a conveying device 3a designed as a pump for increasing the pressure and conveying the liquid working fluid, an internal circuit heat exchanger 7, also referred to as a recuperator, a high-temperature heat exchanger 4 for transferring heat to the working fluid, an expansion device 5a designed as an expansion machine for relaxing the working fluid and a low-temperature heat exchanger 6 for dissipating heat from the working fluid.

The internal circuit heat exchanger 7 is designed with a first flow path 7a and a second flow path 7b. The first flow path 7a is arranged in a connection between the conveying device 3a and the high-temperature heat exchanger 4, while the second flow path 7b is arranged in a connection between the expansion device 5a and the low-temperature heat exchanger 6. Within the internal circuit heat exchanger 7, the heat is transferred from the working fluid flowing through the second flow path 7b to the working fluid flowing through the first flow path 7a.

The conveying device 3a, designed as a pump, sucks in the liquid, in particular supercooled, working fluid present in state A and conveys the working fluid to a higher pressure level, specifically to a supercritical pressure in state B.

When flowing through the first flow path 7a of the internal heat exchanger 7, the working fluid is heated to a temperature in state C above the critical point.

The working fluid is then heated to the level of the maximum high temperature T _H,max as the final fresh steam temperature in state D as it flows through the high-temperature heat exchanger 4 by absorbing heat from the external second heat reservoir and is expanded to state E as it flows through the expansion device 5a designed as an expansion machine. During the expansion process, mechanical work is performed, which is dissipated via a shaft of the expansion machine. In state E, the working fluid is present as superheated steam. Alternatively, the working fluid in state E can also be present as saturated steam.

When flowing through the second flow path 7b of the internal heat exchanger 7, the superheated vapor of the working fluid is deheated while changing the temperature and then at least partially condensed to state F.

The working fluid partially condensed in this way is then completely liquefied to state A as it flows through the low-temperature heat exchanger 6 operated as a condenser, releasing heat to the external first heat reservoir, and is sucked in as a liquid, in particular supercooled working fluid by the conveying device 3a. The working fluid circuit 2a is closed.

From the 3a and 3b a circuit diagram of a second device 1b with a second working fluid circuit 2b and state changes of a heat pump process associated with the circuit diagram are shown in the Ts diagram.

The second working fluid circuit 2b has, in the flow direction of the working fluid, a conveying device 3b designed as a compressor for compressing the vaporous working fluid, the high-temperature heat exchanger 4 for dissipating heat from the working fluid, the heat exchanger 7 within the circuit, an expansion device 5b designed as an expansion element, in particular as an expansion valve, for relaxing the working fluid and the low-temperature heat exchanger 6 for transferring heat to the working fluid.

The first flow path 7a of the in-circuit heat exchanger 7 is arranged in a connection between the high-temperature heat exchanger 4 and the expansion device 5b, while the second flow path 7b of the in-circuit heat exchanger 7 is arranged in a connection between the low-temperature heat exchanger 6 and the conveying device 3b. Within the in-circuit heat exchanger 7, the heat is transferred from the working fluid flowing through the first flow path 7a to the working fluid flowing through the second flow path 7b.

The conveying device 3b designed as a compressor sucks in the working fluid present in state A as saturated or superheated vapor and compresses the working fluid to a higher pressure level, specifically to a supercritical pressure and the level of the maximum high temperature T _H,max in state B.

The superheated steam of the working fluid is then deheated as it flows through the high-temperature heat exchanger 4, releasing heat to the external second heat reservoir and changing the temperature to state C. During supercritical operation or supercritical heat release in the high-temperature heat exchanger 4, the temperature of the working fluid decreases continuously. The high-temperature heat exchanger 4 is also referred to as a gas cooler.

When flowing through the first flow path 7a of the internal heat exchanger 7, the supercritical or liquid working fluid is further cooled to a temperature in state D or deheated by changing the temperature. The heat dissipated by the liquid working fluid conducted through the first flow path 7a of the internal heat exchanger 7 serves to completely evaporate the working fluid conducted in the opposite direction through the second flow path 7b of the internal heat exchanger 7. The working fluid evaporated in this way can also be superheated.

When flowing through the expansion device 5b designed as an expansion element, the working fluid present in state D in front of the expansion device 5b is expanded to state E. The expansion process can be work-producing.

When the working fluid subsequently flows through the low-temperature heat exchanger 6, which is operated as an evaporator, it is at least partially evaporated to state F by absorbing heat from the external first heat reservoir. The absorption of heat by the working fluid is almost isothermal.

The partially evaporated working fluid is completely evaporated and possibly superheated as it flows through the second flow path 7b of the internal heat exchanger 7, absorbing heat from the supercritical or liquid working fluid conducted through the first flow path 7a of the internal heat exchanger 7 and increasing the temperature to state A, and is sucked in by the conveying device 3b. The working fluid circuit 2b is closed.

LIST OF REFERENCE SIGNS

1a, 1b

device

2a, 2b

working fluid circuit

3a, 3b

conveyor device

4

high-temperature heat exchangers

5a, 5b

expansion device

6

low-temperature heat exchangers

7

in-circuit heat exchanger

7a

first flow path of the internal heat exchanger

7b

second flow path of the internal heat exchanger

TH

high temperature

TN

low temperature

A to F

state of the working fluid

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions.

Cited patent literature

• US 2022 056 817 A [0008]

Claims (20)

Device (1a) with a working fluid circuit (2a) for a transcritical cycle, in particular a heat-power process in which a working fluid circulates, comprising in the flow direction of the working fluid a conveying device (3a) for conveying liquid working fluid, a heat exchanger (7) within the circuit, at least one high-temperature heat exchanger (4) for transferring heat to the working fluid, at least one expansion device (5a) for expanding the working fluid and at least one low-temperature heat exchanger (6) for dissipating heat from the working fluid, wherein the heat exchanger (7) within the circuit is designed with a first flow path (7a) for passing through working fluid at a high-pressure level and a second flow path (7b) for passing through working fluid at a low-pressure level in such a way that heat from the working fluid flowing through the second flow path (7b) is transferred to the working fluid flowing through the first flow path (7a). transferred, characterized in that the working fluid is designed as a mixture of at least two components such that the working fluid, when flowing through the second flow path (7b) of the internal circuit heat exchanger (7), has a desuperheating and a subsequent partial condensation, in each case at a decreasing temperature, or exclusively a partial condensation at a decreasing temperature. Device (1a) according to claim 1 , characterized in that for a multi-stage expansion of the working fluid with heat transfer to the working fluid interposed between the stages of expansion, at least two high-temperature heat exchangers (4) and at least two expansion devices (5a) are arranged alternately in series with one another. Device (1b) with a working fluid circuit (2b) for a transcritical cycle, in particular a heat pump process in which a working fluid circulates, comprising in the flow direction of the working fluid a conveying device (3b) for compressing vaporous working fluid, at least one high-temperature heat exchanger (4) for dissipating heat from the working fluid, a heat exchanger (7) inside the circuit, at least one expansion device (5b) for expanding the working fluid and at least one low-temperature heat exchanger (6) for transferring heat to the working fluid, wherein the heat exchanger (7) inside the circuit is designed with a first flow path (7a) for passing through working fluid at a high-pressure level and a second flow path (7b) for passing through working fluid at a low-pressure level in such a way that heat from the working fluid flowing through the first flow path (7a) is transferred to the working fluid flowing through the second flow path (7b). transferred, characterized in that the working fluid is designed as a mixture of at least two components such that the working fluid, when flowing through the second flow path (7b) of the internal circuit heat exchanger (7), exclusively exhibits partial evaporation with increasing temperature or partial evaporation and subsequent overheating, in each case with increasing temperature. Device (1b) according to claim 3 , characterized in that the expansion device (5b) is designed as an expansion element or an expansion machine, in particular as an at least single-stage turbomachine, for recovering expansion work in the form of mechanical work. Device (1b) according to claim 3 or 4 , characterized in that for a multi-stage compression of the working fluid with heat transfer from the working fluid interposed between the compression stages, at least two high-temperature heat exchangers (4) and at least two conveying devices (3b) designed as compressors are arranged alternately in series with one another. Device (1a, 1b) according to one of the Claims 1 until 5 , characterized in that the mixture is formed from at least one main component and at least one secondary component, wherein the at least one main component has a lower normal boiling point and a substantially higher molar fraction than the at least one secondary component. Device (1a, 1b) according to claim 6 , characterized in that the molar proportion of the at least one main component is at least 90% of the working fluid. Device (1a, 1b) according to one of the Claims 1 until 7 , characterized in that the working fluid is designed as a zeotropic mixture. Method for operating a device (1a) with a working fluid circuit (2a) in which a working fluid circulates, comprising in the flow direction of the working fluid a conveying device (3a) for conveying liquid working fluid, a circuit-internal heat exchanger (7) with a first flow path (7a) and a second flow path (7b), at least one high-temperature Heat exchanger (4) for heat transfer to the working fluid, at least one expansion device (5a) for expanding the working fluid and at least one low-temperature heat exchanger (6) for heat dissipation from the working fluid in a transcritical cycle, in particular a heat-power process, comprising the following steps in the order given: - Suction of the liquid working fluid at a low pressure level through the conveying device (3a) and conveying the working fluid to a higher pressure level, in particular to a supercritical pressure, - Heating of the working fluid as it flows through the first flow path (7a) of the heat exchanger (7) inside the circuit, - Heating of the working fluid as it flows through the at least one high-temperature heat exchanger (4) while absorbing heat to a maximum high temperature T _H,max and - Expanding the working fluid as it flows through the at least one expansion device (5a), - Deheating the working fluid present as superheated steam and at least partial condensation of the working fluid or at least partial condensation of the working fluid present as saturated vapor as it flows through the second flow path (7b) of the internal circuit heat exchanger (7), in each case with a change in the temperature of the working fluid, and - complete condensation of the working fluid as it flows through the at least one low-temperature heat exchanger (6) with the release of heat. procedure according to claim 9 , characterized in that a temperature glide of the working fluid during condensation in the second flow path (7b) of the internal circuit heat exchanger (7) corresponds to a temperature change of the working fluid during heating in the first flow path (7a) of the internal circuit heat exchanger (7) based on the identical enthalpy difference of the working fluid. procedure according to claim 9 or 10 , characterized in that the change in state of the working fluid during the release of heat in the at least one low-temperature heat exchanger (6) takes place essentially isothermally, wherein a temperature glide of the working fluid during condensation is smaller than a temperature glide of the working fluid during condensation in the second flow path (7b) of the internal circuit heat exchanger (7). Method according to one of the Claims 9 until 11 , characterized in that a proportion of at least 50% of the heat which is transferred from the working fluid flowing through the second flow path (7b) to the working fluid flowing through the first flow path (7a) is dissipated during the condensation of the working fluid flowing in the second flow path (7b). Method according to one of the Claims 9 until 12 , characterized in that when the working fluid is expanded as it flows through the at least one expansion device (5a), mechanical work is performed and an approximately isentropic change of state occurs. Method according to one of the Claims 9 until 13 , characterized in that the working fluid flows supercritically into the expansion device (5a). Method according to one of the Claims 9 until 14 , characterized in that when the working fluid condenses in the at least one low-temperature heat exchanger (6), the heat is transferred to an external first heat reservoir. Method according to one of the Claims 9 until 15 , characterized in that when heating the working fluid in at least one high-temperature heat exchanger (4), the heat is absorbed from an external second heat reservoir. Method for operating a device (1b) with a working fluid circuit (2b) in which a working fluid circulates, comprising a conveying device (3b) in the flow direction of the working fluid for compressing vaporous working fluid, at least one high-temperature heat exchanger (4) for dissipating heat from the working fluid, an internal circuit heat exchanger (7) with a first flow path (7a) and a second flow path (7b), at least one expansion device (5b) for expanding the working fluid and at least one low-temperature heat exchanger (6) for transferring heat to the working fluid in a transcritical cycle, in particular a heat pump process, comprising the following steps in the order given: - Suction of the working fluid in the state of saturated or superheated steam and compression of the working fluid to a higher pressure level, in particular to a supercritical pressure and a high temperature T _H,max , by the conveying device (3b), - Deheating the Working fluid as it flows through the at least one high-temperature heat exchanger (4) while releasing heat, - cooling the working fluid as it flows through the first flow path (7a) of the internal heat exchanger (7) to a temperature below the critical point, - relaxation of the working fluid as it flows through the at least one expansion device (5b), - at least partial evaporation of the working fluid as it flows through the at least one low-temperature heat exchanger (6) by absorbing heat, and - complete evaporation of the working fluid as it flows through the second flow path (7b) of the internal circuit heat exchanger (7) while absorbing heat and possibly overheating the working fluid. procedure according to claim 17 , characterized in that a temperature glide of the working fluid during evaporation in the second flow path (7b) of the internal circuit heat exchanger (7) corresponds to a temperature change of the working fluid during cooling in the first flow path (7a) of the internal circuit heat exchanger (7) based on the identical enthalpy difference of the working fluid. procedure according to claim 17 or 18 , characterized in that the change in state of the working fluid when absorbing heat in the at least one low-temperature heat exchanger (6) takes place essentially isothermally, wherein a temperature glide of the working fluid during evaporation is smaller than a temperature glide of the working fluid during evaporation in the second flow path (7b) of the internal circuit heat exchanger (7). Method according to one of the Claims 17 until 19 , characterized in that a proportion of at least 50% of the heat which is transferred from the working fluid flowing through the first flow path (7a) to the working fluid flowing through the second flow path (7b) is absorbed during the evaporation of the working fluid flowing in the second flow path (7b).

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