Patent Description:
A trans-critical cycle is a thermodynamic cycle where a working fluid goes through the critical point into the supercritical state in part of the cycle. This is often the case when carbon dioxide (CO<NUM>) is the working fluid. Supercritical carbon dioxide (sCO<NUM>) is a fluid state of carbon dioxide where it is held at or above its critical temperature and critical pressure. sCO2 as a working fluid is typically a very good solvent. Supercritical CO2 has been used for extracting flavours in food processing, such as coffee bean processing, due to its solvent properties. Furthermore, sCO<NUM> dissolves oils rapidly and comprehensively.

As sCO2 dissolves oils rapidly and tends to effuse into components due to a very low fluid viscosity, sCO2 may cause problems for rotating or sliding machinery, such as bearings, compressors, or pistons, by dissolving lubricants rapidly. The dissolved oils can also modify the properties of the working fluid. It is therefore necessary to remove solutes from a working fluid in a trans-critical cycle. Conventional methods of removing solutes generally involve complete purging of the system and filtration systems that interfere with normal operation. <CIT> describes an oil separator placed in a non-critical position before the compressor in a trans-critical circuit.

The present invention, in its various aspects, is defined in the appended claims. In one aspect, there is provided a trans-critical thermodynamic system including an expansion device and a separator. The expansion device receives a supercritical fluid containing solutes. The expansion device is operable to expand the supercritical fluid to produce a sub-critical gas by reducing a temperature and/or a pressure of the supercritical fluid. The separator removes the solutes from the sub-critical gas.

The trans-critical thermodynamic system may allow contaminants in the working fluid to be removed during normal operation as opposed to complete purging of the system. Oil-lubricated components may be used without any risk of damaging downstream components. The working fluid may be purified periodically to achieve optimal performance. Further, the controlled periodical removal of the contaminants may have a lower impact on system efficiency compared with conventional methods. Moreover, the trans-critical thermodynamic system may not require additional separating components, such as filters or strainers that have an associated pressure drop and are susceptible to flow damage.

The trans-critical thermodynamic systems of the present disclosure may be used for thermal management and/or waste heat recovery in various applications, for example, but not limited to, gas turbine engines, internal combustion engines, computing facilities, and heating, cooling and ventilation (HVAC) applications.

According to the invention, the trans-critical thermodynamic system includes a high-pressure circuit and a fluid extraction point. The supercritical fluid flows through the high pressure circuit. The fluid extraction point is operable to extract a portion of the supercritical fluid from the high pressure circuit. The expansion device is operable to expand the portion of the supercritical fluid.

By extracting only the portion of the supercritical fluid, an amount of working fluid being passed through the separator is reduced. For example, the portion of the supercritical fluid may be a minimum amount required to maintain an amount of solute (e.g., dissolved oils) in the working fluid below a threshold. This may advantageously reduce energy losses in the trans-critical thermodynamic system.

In some embodiments, the trans-critical thermodynamic system further includes a low pressure circuit and a compressor. The compressor is operable to compress a working fluid from the low-pressure circuit into the high-pressure circuit such that the working fluid becomes the supercritical fluid.

In some embodiments, the trans-critical thermodynamic system further includes a first heat exchanger in the high pressure circuit receiving the supercritical fluid from the compressor. The fluid extraction point is located after the first heat exchanger. The first heat exchanger is configured to cool the supercritical fluid to a thermodynamic state such that the reduction in the temperature and/or the pressure when the portion of the supercritical fluid is passed through the expansion device produces the sub-critical gas having a thermodynamic state matching a position in the low pressure circuit. The thermodynamic state of the sub-critical gas has a temperature less than a temperature at an inlet of the compressor.

The first heat exchanger may advantageously allow control of the thermodynamic properties of the portion of the supercritical fluid that is extracted for passage through the separator. For example, the thermodynamic state of the portion of the supercritical fluid may be chosen so that there is minimal energy loss through the expansion device. Further, the thermodynamic state of the portion of the supercritical fluid may be controlled to avoid returning hot fluid to the inlet of the compressor which can otherwise pose a risk of an unstable supercritical temperature of the working fluid. In some cases, an energy transfer in the first heat exchanger can be adjusted based on a desired thermodynamic state of the portion of the supercritical fluid. Various control strategies may be used to control the energy transfer in the first heat exchanger.

In some embodiments, the trans-critical thermodynamic system further includes a controller operable to control a rate of energy transfer in the first heat exchanger based on a measure of one or more thermodynamic properties of the supercritical fluid at the fluid extraction point.

In some embodiments, the trans-critical thermodynamic system further includes a bypass circuit, a mixing valve and a controller. The bypass circuit diverts a fraction of the supercritical fluid around the first heat exchanger. The mixing valve mixes the supercritical fluid that has passed through the first heat exchanger with the supercritical fluid that has bypassed the first heat exchanger. The controller controls the mixing valve based on a measure of one or more thermodynamic properties of the supercritical fluid at the fluid extraction point.

In some embodiments, the trans-critical thermodynamic system further includes one or more control members operable to control a rate of flow of a heat transfer fluid across the first heat exchanger. The trans-critical thermodynamic system further includes a controller to control the one or more control members based on a measure of one or more thermodynamic properties of the supercritical fluid at the fluid extraction point.

In some embodiments, the trans-critical thermodynamic system further includes a solute sensor operable to measure a value representative of an amount of solute in the supercritical fluid. The portion of the supercritical fluid extracted to pass through the expansion device is controlled based on the value to maintain the amount of solute in the supercritical fluid below a threshold.

In some embodiments, the solute sensor is operable to measure a rate of solute collection in the separator.

According to the invention the fluid extraction point is in fluid communication with a cooling circuit. The supercritical fluid in the high-pressure circuit that is not extracted at the fluid extraction point is circulated through the cooling circuit. The cooling circuit further includes at least one heat exchanger and another expansion device.

In some embodiments, the trans-critical thermodynamic system further includes a second heat exchanger receiving the sub-critical gas from the separator.

An entire flow of the working fluid can be purified in situ within the main loop.

Therefore, contaminants may be quickly removed and not re-circulated. Since the separator is positioned downstream of the expansion device, fouling of the first heat exchanger by the contaminants can be prevented.

In another aspect, there is provided a method of removing solutes from a working fluid in a trans-critical circuit. The method includes identifying a position in the trans-critical circuit where the working fluid is a sub-critical gas. The method further includes positioning a separator such that the separator receives at least a portion of the working fluid when the working fluid is the sub-critical gas. The separator is operable to remove solutes from the sub-critical gas.

The method further includes identifying a fluid extraction point in the trans-critical circuit where the working fluid is a supercritical fluid. The method further includes extracting a portion of the supercritical fluid from the fluid extraction point in the trans-critical circuit. The method further includes passing the portion of the supercritical fluid through an expansion device such that the portion of the supercritical fluid becomes the sub-critical gas.

In some embodiments, the method further includes compressing the working fluid upstream of the fluid extraction point such that the working fluid becomes the supercritical fluid. The method further includes passing at least a fraction of the supercritical fluid through a first heat exchanger located upstream of the fluid extraction point. The method further includes controlling a rate of energy transfer in the first heat exchanger based on a measure of one or more thermodynamic properties of the supercritical fluid at the fluid extraction point.

In the invention, the working fluid is preferably carbon dioxide, however any working fluid that dissolves contaminants more significantly when in a supercritical state may benefit from the invention disclosed herein.

With reference to <FIG>, a trans-critical thermodynamic system <NUM> (hereinafter referred to as "the trans-critical system <NUM>") is provided. The trans-critical system <NUM> includes an expansion device <NUM>, a separator <NUM>, a compressor <NUM>, a first heat exchanger <NUM>, a fluid extraction point <NUM>, a second heat exchanger <NUM>, a third heat exchanger <NUM>, and a fourth heat exchanger <NUM>. The trans-critical system <NUM> uses a working fluid. In some embodiments, the working fluid is carbon dioxide (CO<NUM>). <FIG> illustrates a plot <NUM> of temperature (T) versus entropy (s) of the trans-critical system <NUM>. Specifically, the plot <NUM> is a T-s diagram of the trans-critical system <NUM>. <FIG> also schematically illustrates a critical point PC of the working fluid, a saturated vapour line L1 of the working fluid, a saturated liquid line L2 of the working fluid, and a critical boundary line BL between a supercritical state and a sub-critical state of the working fluid. The critical point PC for CO<NUM> is at <NUM> MPa (<NUM> psia) and <NUM> degrees Celsius (<NUM> degrees Fahrenheit) such that the supercritical state for CO<NUM> occurs at or above the critical point PC. A trans-critical cycle is a thermodynamic cycle where the working fluid goes through both sub-critical and supercritical states.

Referring to <FIG> and <FIG>, the trans-critical system <NUM> includes a trans-critical circuit <NUM> and a cooling circuit <NUM>. The trans-critical circuit <NUM> includes the expansion device <NUM>, the separator <NUM>, the compressor <NUM>, the first heat exchanger <NUM>, and the second heat exchanger <NUM>. The trans-critical circuit <NUM> may be a closed-loop circuit. The cooling circuit <NUM> includes the third heat exchanger <NUM>, the fourth heat exchanger <NUM> and another expansion device <NUM> (hereinafter referred to as "the second expansion device <NUM>").

Various points in the flow path of the working fluid are defined in the trans-critical system <NUM>. Point P1 is defined in the flow path of the working fluid where a flow of the working fluid is provided at an inlet 106A of the compressor <NUM>. Point P2 is defined in the flow path of the working fluid where a flow of the working fluid is received from an outlet 106B of the compressor <NUM>. Point P2' is defined in a flow path of the working fluid where a portion of a flow of the working fluid is extracted and provided to the expansion device <NUM>. Point P2' coincides with the fluid extraction point <NUM>. Point P3 is defined in the flow path of the working fluid where a flow of the working fluid is received from an outlet of the third heat exchanger <NUM>. Point P4 is defined in the flow path of the working fluid where a flow of the working fluid is received from an outlet of the second expansion device <NUM>. Point P5 is defined in the flow path of the working fluid where a flow of the working fluid is provided to an inlet of the second heat exchanger <NUM>. The working fluid may be in different thermodynamic states in the trans-critical system <NUM>, for example, supercritical state, sub-critical gas, sub-critical liquid, sub-critical liquid and gas mixture, and so forth.

The trans-critical system <NUM> further includes a high pressure circuit <NUM> and a low pressure circuit <NUM>. The high pressure circuit <NUM> is defined from point P2 to point P3. The low pressure circuit <NUM> is defined from point P4, through point P5, to point P1. The high pressure circuit <NUM> generally operates at a greater average pressure than the low pressure circuit <NUM>. For a given value of entropy, a point in the high pressure circuit <NUM> has a higher pressure than a corresponding point in the low pressure circuit <NUM>.

The compressor <NUM> receives the working fluid from the low pressure circuit <NUM> at the inlet 106A. The compressor <NUM> receives the flow of working fluid from point P1. At point P1, the working fluid in a sub-critical gas <NUM>. In some embodiments, the sub-critical gas <NUM> is CO<NUM> in the sub-critical gaseous state. The compressor <NUM> is operable to compress the working fluid from the low pressure circuit <NUM> such that the working fluid becomes a supercritical fluid <NUM>. In some embodiments, the supercritical fluid <NUM> is supercritical carbon dioxide (sCO<NUM>). The compressor <NUM> may be directly or indirectly driven by a shaft of a gas turbine engine. The compressor <NUM> compresses the flow of the working fluid and increases the temperature and pressure of the working fluid at point P2. The compression of the working fluid may be substantially isentropic. The compressor <NUM> may be any form of mechanism or device capable of compressing the working fluid such that working fluid received at a lower pressure by the compressor <NUM> is output at a higher pressure. Point P2 may be above the critical point PC and the critical boundary line BL of the working fluid.

The supercritical fluid <NUM>, from the outlet 106B of the compressor <NUM>, flows through the high pressure circuit <NUM>. The first heat exchanger <NUM> is disposed in the high pressure circuit <NUM> after the compressor <NUM>. In other words, the first heat exchanger <NUM> is disposed downstream of the compressor <NUM>. At least a fraction of the supercritical fluid <NUM> provided at the outlet 106B of the compressor <NUM> is provided to the first heat exchanger <NUM>. The first heat exchanger <NUM> may be any device that allows heat exchange between the working fluid and a heat transfer fluid (another liquid or gas) without mixing the working fluid and the heat transfer fluid together. The first heat exchanger <NUM> is configured to cool the supercritical fluid <NUM> from point P2 to point P2'.

The fluid extraction point <NUM> is operable to extract a portion <NUM> of the supercritical fluid <NUM> from the high pressure circuit <NUM>. In some embodiments, the portion <NUM> of the supercritical fluid <NUM> that is extracted at the fluid extraction point <NUM> can be varied based on various parameters. The expansion device <NUM> is operable to expand the supercritical fluid <NUM> to produce a sub-critical gas <NUM> by reducing a temperature and/or a pressure of the supercritical fluid <NUM>. Specifically, the expansion device <NUM> reduces the temperature and the pressure of the portion <NUM> of the supercritical fluid <NUM> from point P2' to point P5.

The fluid extraction point <NUM> is in fluid communication with the cooling circuit <NUM>. The supercritical fluid <NUM> in the high pressure circuit <NUM> that is not extracted at the fluid extraction point <NUM> is circulated through the cooling circuit <NUM>. Specifically, a portion <NUM> of the supercritical fluid <NUM> is not extracted at the fluid extraction point <NUM> and is circulated through the cooling circuit <NUM>. The portion <NUM> of the supercritical fluid <NUM> passes through the third heat exchanger <NUM>. The third heat exchanger <NUM> cools the portion <NUM> of the supercritical fluid <NUM> to a sub-critical liquid <NUM> at point P3. The sub-critical liquid <NUM> at point P3 is passed through the second expansion device <NUM>. The second expansion device <NUM> reduces a temperature and/or a pressure of the sub-critical liquid <NUM> to a sub-critical liquid and gas mixture <NUM> at point P4. The sub-critical liquid and gas mixture <NUM> at point P4 is passed through the fourth heat exchanger <NUM>. The fourth heat exchanger <NUM> heats the sub-critical liquid and gas mixture <NUM> to a sub-critical gas <NUM> at point P5. Point P5 may lie beyond the saturated vapour line L1. The fourth heat exchanger <NUM> may add a degree of superheat to the working fluid.

The separator <NUM> removes one or more solutes <NUM> from the sub-critical gas <NUM> received from the expansion device <NUM>. The solutes <NUM> may condense out of the sub-critical gas <NUM> and flow out of the separator <NUM>. The separator <NUM> may be any device that can separate the solutes <NUM> (e.g., oil) from a gas (e.g., the sub-critical gas <NUM>). The solutes <NUM> from the separator <NUM> may flow to a drain <NUM>. An outlet valve <NUM> may be provided to control a flow of the solutes <NUM> from the separator <NUM> to the drain <NUM>. In some embodiments, the separator <NUM> may be a vapour-liquid separator. In some embodiments, the separator <NUM> can be an oil separator.

The second heat exchanger <NUM> receives at least the sub-critical gas <NUM> from the separator <NUM>. At point P5, the sub-critical gas <NUM> from the separator <NUM> may mixed with the sub-critical gas <NUM> from the fourth heat exchanger <NUM>. The sub-critical gas <NUM> and the sub-critical gas <NUM> may be in substantially a same thermodynamic state. The sub-critical gas <NUM> and the sub-critical gas <NUM> mix to form the sub-critical gas <NUM>. The sub-critical gas <NUM> from point P5 is provided to the second heat exchanger <NUM>. Point P5 may lie beyond the saturated vapour line L1 to ensure that no liquid is provided at the inlet 106A of the compressor <NUM>. The second heat exchanger <NUM> heats the sub-critical gas <NUM> to point P1. Point P1 may lie below the critical boundary line BL. The compressor <NUM> compresses the sub-critical gas <NUM> at point P1 to the supercritical fluid <NUM> at point P2. After compression, at least a fraction of the supercritical fluid <NUM> is provided to the first heat exchanger <NUM>.

A thermodynamic state at the fluid extraction point <NUM> may be selected to ensure that the working fluid loses the minimum amount of energy while reducing the temperature and/or the pressure by expansion to provide the sub-critical gas <NUM>. This may be achieved by extracting the working fluid at a point where the reduction in the temperature and/or a pressure by expansion results in the sub-critical gas <NUM> just outside the saturated vapour line L1. The degree of superheat of the sub-critical gas <NUM> may therefore be minimized.

Selection of the thermodynamic state at the fluid extraction point <NUM> may be achieved by passing the supercritical fluid <NUM> through the first heat exchanger <NUM>. The first heat exchanger <NUM> may extract heat from the supercritical fluid <NUM> at a rate of energy transfer Q̇out. The thermodynamic state at the fluid extraction point <NUM> may be controlled by regulating the rate of energy transfer Q̇out. The rate of energy transfer Qout (or the energy transfer) in the first heat exchanger <NUM> may be selected to provide a calculated specific entropy of the working fluid greater than the saturated vapour line L1 but colder than a temperature T1 at the inlet 106A of the compressor <NUM>. This may result in minimal loss of energy through the expansion device <NUM>. Further, it may also avoid providing hot fluid to the inlet 106A which can otherwise pose a risk of an unstable supercritical temperature of the working fluid.

In some embodiments, the first heat exchanger <NUM> is configured to cool the supercritical fluid <NUM> to a thermodynamic state such that the reduction in the temperature and/or the pressure when the portion <NUM> of the supercritical fluid <NUM> is passed through the expansion device <NUM> produces the sub-critical gas <NUM> having a thermodynamic state matching a position in the low pressure circuit <NUM>. The thermodynamic state of the sub-critical gas <NUM> has a temperature T5 less than the temperature T1 at the inlet 106A of the compressor <NUM>.

The energy transfer in the first heat exchanger <NUM> may be selected to match desired thermodynamic properties of the trans-critical system <NUM>. In some embodiments, the rate of energy transfer Qout in the first heat exchanger <NUM> may be controlled to maintain the fluid extraction point <NUM> at the desired thermodynamic state. In some embodiments, the trans-critical system <NUM> includes a controller <NUM> operable to control the rate of energy transfer Qout in the first heat exchanger <NUM> based on a measure of one or more thermodynamic properties of the supercritical fluid <NUM> at the fluid extraction point <NUM>. In some embodiments, the controller <NUM> may control flow to or from a bypass circuit (not shown in <FIG> and <FIG>) that bypasses the first heat exchanger <NUM> in order to control the rate of energy transfer Q̇out. In some embodiments, the controller <NUM> may vary the flow of the heat transfer fluid over the first heat exchanger <NUM> in one or more stages in order to control the rate of energy transfer Q̇out.

Further, the second heat exchanger <NUM> may provide heat to the sub-critical gas <NUM> at a rate of energy transfer Qin. In some embodiments, the controller <NUM> may also control the rate of energy transfer Qin in the second heat exchanger <NUM>.

The portion <NUM> of the supercritical fluid <NUM> extracted to pass through the separator <NUM> may be a minimum amount required to maintain an amount of solute (e.g., dissolved oils) in the working fluid below a threshold. This may advantageously reduce energy losses in the trans-critical circuit <NUM>. For example, a percentage of the total flow of the supercritical fluid <NUM> extracted at the fluid extraction point <NUM> for expansion may be less than <NUM>%, less than <NUM>%, less than <NUM>%, less than <NUM>%, less than <NUM>%, less than <NUM>%, less than <NUM>%, or less than <NUM>%. The passage of the supercritical fluid <NUM> through the fluid extraction point <NUM> may be controlled based on an amount of solute collected in the separator <NUM>.

In some embodiments, the controller <NUM> may control the portion <NUM> of the supercritical fluid <NUM> extracted to pass through the expansion device <NUM> based on an amount of solute in the supercritical fluid <NUM>. In some embodiments, a valve (not shown in <FIG> and <FIG>) may be provided at the fluid extraction point <NUM> to control the portion <NUM> of the supercritical fluid <NUM> that is extracted for passage through the expansion device <NUM>.

The trans-critical system <NUM> and the plot <NUM>, as illustrated in <FIG> and <FIG>, are exemplary in nature. Various components of the trans-critical system <NUM> may be selected based on the application requirements of the trans-critical system <NUM>.

In the illustrated embodiment of <FIG>, each of the expansion device <NUM> and the second expansion device <NUM> is an expansion valve, such as a thermostatic expansion valve. In some embodiments, an opening of the expansion valve be variable. In alternative embodiment, at least one of the expansion device <NUM> and the second expansion device <NUM> can be a turbine.

In some embodiments, one or more of the first heat exchanger <NUM>, the second heat exchanger <NUM>, the third heat exchanger <NUM> and the fourth heat exchanger <NUM> can be a liquid-to-gas heat exchanger, a gas-to-gas heat exchanger or a liquid-to-liquid heat exchanger. Each of the first heat exchanger <NUM>, the second heat exchanger <NUM>, the third heat exchanger <NUM> and the fourth heat exchanger <NUM> can include, but not limited to, shell and tube heat exchangers, plate heat exchangers, plate and shell heat exchangers, plate fin heat exchangers and microchannel heat exchangers.

In some embodiments, the first heat exchanger <NUM> and the third heat exchanger <NUM> can be part of a single heat exchanger assembly with the fluid extraction point <NUM> located in an intermediate location within the single heat exchanger assembly. In some other embodiments, the first heat exchanger <NUM> and the third heat exchanger <NUM> can be separate heat exchangers, and the fluid extraction point <NUM> is located between the first heat exchanger <NUM> and the third heat exchanger <NUM>.

In some embodiments, the second heat exchanger <NUM> and the fourth heat exchanger <NUM> can be part of a single heat exchanger assembly with point P5 located in an intermediate location within the single heat exchanger assembly. In some other embodiments, the second heat exchanger <NUM> and the fourth heat exchanger <NUM> can be separate heat exchangers, and point P5 is located between the second heat exchanger <NUM> and the fourth heat exchanger <NUM>.

In some embodiments, the compressor <NUM> can be a positive displacement compressor, a dynamic compressor or any other type of compressor. Examples of positive displacement compressors include, but not limited to, reciprocating compressors (single-acting or double-acting), diaphragm compressors, ionic compressors, screw compressors, lobe compressors, vane compressors, scroll compressors, and rolling piston compressors. Examples of dynamic compressors include, but not limited to, air bubble compressors, centrifugal compressors, axial compressors, and mixed-flow compressors. The compressor <NUM> may be hermetically sealed, open, or semi-hermetic.

The trans-critical system <NUM> may include additional components not shown in <FIG> and <FIG>. For example, the trans-critical system <NUM> may include one or more fluid conduits, fluid connectors, fluid seals and reservoirs. Further the cooling circuit <NUM> may include any number of heat exchangers and expansion devices as per application requirements.

<FIG> illustrates a trans-critical thermodynamic system <NUM> (hereinafter referred to as "the trans-critical system <NUM>") according to another embodiment of the present disclosure. The trans-critical system <NUM> is substantially similar in structure and operation to the trans-critical system <NUM> described above. Referring to <FIG> and <FIG>, the trans-critical system <NUM> includes the expansion device <NUM>, the separator <NUM>, the compressor <NUM>, the first heat exchanger <NUM>, the fluid extraction point <NUM>, the second heat exchanger <NUM>, the third heat exchanger <NUM>, and the fourth heat exchanger <NUM>. The trans-critical system <NUM> further includes a bypass circuit <NUM>, a mixing valve <NUM>, a controller <NUM> and a solute sensor <NUM>.

The bypass circuit <NUM> diverts a fraction of the supercritical fluid <NUM> around the first heat exchanger <NUM>. The supercritical fluid <NUM> received from the compressor <NUM> is divided into two flows of the supercritical fluid 128A, 128B. The mixing valve <NUM> mixes the supercritical fluid 128A that has passed through the first heat exchanger <NUM> with the supercritical fluid 128B that has bypassed the first heat exchanger <NUM>. The controller <NUM> controls the mixing valve <NUM> based on a measure of one or more thermodynamic properties of the supercritical fluid <NUM> at the fluid extraction point <NUM>. The mixing valve <NUM> can be provided upstream or before the fluid extraction point <NUM>. In some embodiments, the mixing valve <NUM> may be a three-way electronically controlled valve. The measured thermodynamic properties of the supercritical fluid <NUM> at the fluid extraction point <NUM> may include one or more of temperature, pressure, specific entropy, specific enthalpy and specific volume. The rate of energy transfer Qout in the first heat exchanger <NUM> can be controlled by regulating the fraction of the supercritical fluid <NUM> that bypasses the first heat exchanger <NUM>.

The solute sensor <NUM> is operable to measure a value representative of an amount of solute in the supercritical fluid <NUM>. In some embodiments, the portion <NUM> of the supercritical fluid <NUM> extracted to pass through the expansion device <NUM> is controlled based on the value to maintain the amount of solute in the supercritical fluid below a threshold. In the illustrated embodiment of <FIG>, the solute sensor <NUM> is operable to measure a rate of solute collection in the separator <NUM>. In other embodiments, the solute sensor <NUM> may directly measure the amount of solute in the supercritical fluid <NUM>. Examples of the solute sensor <NUM> include a flow rate sensor, an optical sensor, or any other kind of sensor. The amount of solute in the supercritical fluid <NUM> may be measured as a weight percentage of the supercritical fluid <NUM>. Further, the threshold may be a threshold weight percentage.

<FIG> illustrates a trans-critical thermodynamic system <NUM> (hereinafter referred to as "the trans-critical system <NUM>") according to another embodiment of the present disclosure. The trans-critical system <NUM> is substantially similar in structure and operation to the trans-critical system <NUM> described above. Referring to <FIG> and <FIG>, the trans-critical system <NUM> includes the expansion device <NUM>, the separator <NUM>, the compressor <NUM>, the first heat exchanger <NUM>, the fluid extraction point <NUM>, the second heat exchanger <NUM>, the third heat exchanger <NUM>, and the fourth heat exchanger <NUM>. The trans-critical system <NUM> further includes one or more control members <NUM> and a controller <NUM>.

The one or more control members <NUM> are operable to control a rate of flow FW of a heat transfer fluid <NUM> across the first heat exchanger <NUM>. In the illustrated embodiment of <FIG>, each control member <NUM> is a blower and the heat transfer fluid <NUM> is a gas, such as air. The control members <NUM> can vary the rate of flow FW of the heat transfer fluid <NUM> in multiple stages (two in <FIG>). In some embodiments, the control members <NUM> may additionally or optionally also include valves, vanes and ducts that control a direction of flow of the heat transfer fluid <NUM>. The number of the control members <NUM> can vary as per application requirements. Further, the type of the control members <NUM> may depend on the properties of the heat transfer fluid <NUM>. For example, in case the heat transfer fluid <NUM> is a liquid, the control member <NUM> may include suitable types of valves, conduits, and other flow control members.

The controller <NUM> controls the one or more control members <NUM> based on a measure of one or more thermodynamic properties of the supercritical fluid <NUM> at the fluid extraction point <NUM>. For example, the controller <NUM> can vary a speed of the control members <NUM> to vary the rate of flow FW of the heat transfer fluid <NUM> across the first heat exchanger <NUM> in multiple stages. The measured thermodynamic properties of the supercritical fluid <NUM> at the fluid extraction point <NUM> may include one or more of temperature, pressure, specific entropy, specific enthalpy and specific volume. The rate of energy transfer Qout in the first heat exchanger <NUM> can be controlled by regulating the rate of flow FW of the heat transfer fluid across the first heat exchanger <NUM>.

Each of the controllers <NUM>, <NUM>, <NUM> described above may include a processor (not shown) and a memory (not shown). The memory may include computer executable instructions that are executable by the processor to perform the various operations that are described above. The processor may be communicably coupled to various sensors and actuators by wired connections and/or wireless connections. Suitable circuitry may be provided to process the signals from the various sensors and provide control signals to the various actuators.

The processor may be any device that performs logic operations. The processor may include a general processor, a central processing unit, an application specific integrated circuit (ASIC), a digital signal processor, a field programmable gate array (FPGA), a digital circuit, an analog circuit, a controller, a microcontroller, any other type of processor, or any combination thereof. The processor may include one or more components operable to execute computer executable instructions or computer code embodied in the memory.

The memory may include at least one computer readable storage medium. Examples of the computer readable storage medium may include a hard disk, a floppy disk, a CD-ROM, a flash drive, a cache, volatile memory, non-volatile memory, RAM, flash memory, or any other type of computer readable storage medium or storage media. The computer readable storage medium may include any type of non-transitory computer readable medium, such as a CD-ROM, a volatile memory, a non-volatile memory, ROM, RAM, or any other suitable storage device.

The trans-critical systems <NUM>, <NUM>, <NUM> described above may be used for thermal management in various applications, for example, but not limited to, gas turbine engines, internal combustion engines, computing facilities, and heating, cooling and ventilation (HVAC) applications. Contaminants (e.g., the solutes <NUM>) in the working fluid can be removed during normal operation as opposed to complete purging of the system. Oil-lubricated components can be used without any risk of damaging downstream components. The working fluid can be purified periodically to achieve optimal performance. Further, the removal of the contaminants may have minimal impact on system efficiency. Moreover, the trans-critical systems <NUM>, <NUM>, <NUM> may not require additional separating components, such as filters or strainers that have an associated pressure drop and are susceptible to flow damage, thereby requiring regular replacement to maintain functionality.

With reference to <FIG>, a trans-critical thermodynamic system <NUM> (hereinafter referred to as "the trans-critical system <NUM>") is provided in accordance with an alternative application of the present disclosure. The trans-critical system <NUM> includes an expansion device <NUM>, a separator <NUM>, a pump <NUM>, a first heat exchanger <NUM>, a second heat exchanger <NUM>, and a third heat exchanger <NUM>. The first heat exchanger <NUM> can also be interchangeably referred to as "the heat recovery heat exchanger <NUM>". The trans-critical system <NUM> uses a working fluid. In some embodiments, the working fluid is carbon dioxide (CO<NUM>). <FIG> illustrates a plot <NUM> of temperature (T) versus entropy (s) of the trans-critical system <NUM>. Specifically, the plot <NUM> is a T-s diagram of the trans-critical system <NUM>. <FIG> also schematically illustrates a critical point RC of the working fluid, a saturated vapour line M1 of the working fluid, a saturated liquid line M2 of the working fluid, and a critical boundary line CL between the supercritical state and the sub-critical state of the working fluid.

Referring to <FIG> and <FIG>, the trans-critical system <NUM> includes a trans-critical circuit <NUM>. The trans-critical circuit <NUM> includes the expansion device <NUM>, the separator <NUM>, the pump <NUM>, the first heat exchanger <NUM>, the second heat exchanger <NUM>, and the third heat exchanger <NUM>. The trans-critical circuit <NUM> may be a closed-loop circuit.

Referring to <FIG> and <FIG>, various points in the flow path of the working fluid are defined in the trans-critical system <NUM>. Point R1 is defined in the flow path of the working fluid where a flow of the working fluid is provided at an inlet 506A of the pump <NUM>. Point R2 is defined in the flow path of the working fluid where a flow of the working fluid is received from an outlet 506B of the pump <NUM>. Point R3 is defined in the flow path of the working fluid where a flow of the working fluid is received from an outlet of the first heat exchanger <NUM>. Point R4 is defined in the flow path of the working fluid where a flow of the working fluid is received from an outlet of the expansion device <NUM>. Point R5 is defined in the flow path of the working fluid where a flow of the working fluid is provided to an inlet of the second heat exchanger <NUM>. The working fluid may be in different thermodynamic states in the trans-critical system <NUM>, for example, supercritical state, sub-critical gas, sub-critical liquid, sub-critical liquid and gas mixture, and so forth.

The trans-critical system <NUM> further includes a high pressure circuit <NUM> and a low pressure circuit <NUM>. The high pressure circuit <NUM> is defined from point R2 to point R3. The low pressure circuit <NUM> is defined from point R4, through point R5, to point R1. The high pressure circuit <NUM> generally operates at a greater average pressure than the low pressure circuit <NUM>. For a given value of entropy, a point in the high pressure circuit <NUM> has a higher pressure than a corresponding point in the low pressure circuit <NUM>. The high pressure circuit <NUM> and the low pressure circuit <NUM> together form the trans-critical circuit <NUM>.

The pump <NUM> receives the working fluid from the low pressure circuit <NUM> at the inlet 506A. The pump <NUM> receives the flow of working fluid from point R1. At point R1, the working fluid in a saturated liquid <NUM>. In some embodiments, the saturated liquid <NUM> is CO<NUM> in the saturated liquid state. Point R1 may be located on the saturated liquid line M2. In some other embodiments, point R1 may be offset from the saturated liquid line M2 and located in the sub-critical liquid region. The pump <NUM> is operable to pressurize the working fluid from the low pressure circuit <NUM> into the high pressure circuit <NUM> such that the working fluid becomes a pressurized liquid <NUM>. The pressurized liquid <NUM> may be a sub-critical liquid. In some embodiments, the pressurized liquid <NUM> is liquid CO<NUM>. The pump <NUM> may be directly or indirectly driven by a shaft of a gas turbine engine. The pump <NUM> pressurizes the flow of the working fluid and increases the temperature and pressure of the working fluid at point R2. Point R2 may be located in the sub-critical liquid region. The pump <NUM> may be any form of mechanism or device capable of pressurizing the working fluid such that working fluid received at a lower pressure by the pump <NUM> is output at a higher pressure. Point R2 may be below the critical boundary line CL of the working fluid.

The pressurized liquid <NUM>, from the outlet 506B of the pump <NUM>, flows through the high pressure circuit <NUM>. The first heat exchanger <NUM> is disposed in the high pressure circuit <NUM> after the pump <NUM>. In other words, the first heat exchanger <NUM> is disposed downstream of the pump <NUM>. The first heat exchanger <NUM> may be any device that allows heat exchange between the working fluid and a heat transfer fluid (another liquid or gas) without mixing the two working fluid and the heat transfer fluid together. The first heat exchanger <NUM> is configured to heat the pressurized liquid <NUM> to a supercritical fluid <NUM>. Specifically, the first heat exchanger <NUM> heats the working fluid from point R2 to point R3. Point R3 is located above the critical boundary line CL in the supercritical region.

The expansion device <NUM> receives the supercritical fluid <NUM> from the first heat exchanger <NUM>. The expansion device <NUM> is operable to expand the supercritical fluid <NUM> to produce a sub-critical gas <NUM> by reducing a temperature and/or a pressure of the supercritical fluid <NUM>. In the illustrated embodiment of <FIG>, the expansion device <NUM> is a turbine. The expansion device <NUM> expands the supercritical fluid <NUM> at point R3 to the sub-critical gas <NUM> at point R4. Point R4 may lie beyond the saturated vapour line M1.

The separator <NUM> removes one or more solutes <NUM> from the sub-critical gas <NUM> received from the expansion device <NUM>. The solutes <NUM> may condense out of the sub-critical gas <NUM> and flow out of the separator <NUM>. The separator <NUM> may be any device that can separate the solutes <NUM> (e.g., oil) from a gas (e.g., the sub-critical gas <NUM>). The solutes <NUM> from the separator <NUM> may flow to a drain <NUM>. An outlet valve <NUM> may be provided to control a flow of the solutes <NUM> from the separator <NUM> to the drain <NUM>. In some embodiments, the separator <NUM> may be a vapour-liquid separator. In some embodiments, the separator <NUM> may be an oil separator.

The second heat exchanger <NUM> receives the sub-critical gas <NUM> from the separator <NUM>. The second heat exchanger <NUM> cools the sub-critical gas <NUM> at point R4 to a saturated gas <NUM> at point R5. Point R5 may lie on the saturated vapour line M1.

The third heat exchanger <NUM> receives the saturated gas <NUM> from the second heat exchanger <NUM>. The third heat exchanger <NUM> cools the saturated gas <NUM> at point R5 to the saturated liquid <NUM> at point R1.

The working fluid may absorb waste heat in the first heat exchanger <NUM>. The expansion device <NUM> may be used to recover energy from the waste heat absorbed by the working fluid in the first heat exchanger <NUM>. A thermodynamic state at point R3 may be selected to ensure that the temperature and/or the pressure of the working fluid is reduced by expansion in the expansion device <NUM> to provide the sub-critical gas <NUM> at point R4. Further, an amount of energy extracted in the expansion device <NUM> may be maximised while ensuring that the separator <NUM> receives the working fluid as the sub-critical gas <NUM>. Selection of the thermodynamic state at point R3 may be achieved by passing the pressurized liquid <NUM> through the first heat exchanger <NUM>. The first heat exchanger <NUM> may heat the pressurized liquid <NUM> at a rate of energy transfer Q̇1in. The thermodynamic state at point R3 may be controlled by regulating the rate of energy transfer Q̇<NUM> in.

Further, the second heat exchanger <NUM> may extract heat from the sub-critical gas <NUM> at a rate of energy transfer Q̇1out. The rate of energy transfer Q̇1out may be controlled to provide a suitable thermodynamic state at point R5.

Moreover, the third heat exchanger <NUM> may extract heat from the saturated gas <NUM> at a rate of energy transfer Q2out. The rate of energy transfer Q2out may be controlled to provide a suitable thermodynamic state at point R1.

In some embodiments, the pump <NUM> can be a positive displacement pump, an impulse pump, and a velocity pump. Examples of positive displacement pumps include, but not limited to, rotary positive displacement pumps, reciprocating positive displacement pumps, and linear-type positive displacement pumps. Rotary positive displacement pumps can include gear pumps, screw pumps, lobe pumps and rotary vane pumps. Reciprocating positive displacement pumps can include plunger pumps, diaphragm pumps and piston pumps. Velocity pumps can include radial-flow pumps, axial-flow pumps, and mixed-flow pumps.

The trans-critical system <NUM> described above may be used for waste heat recovery in various applications, for example, but not limited to, gas turbine engines, internal combustion engines, computing facilities, and heating, cooling and ventilation (HVAC) applications. Contaminants (e.g., the solutes <NUM>) in the working fluid can be removed during normal operation as opposed to complete purging of the system. Oil-lubricated components can be used without any risk of damaging downstream components. An entire flow of the working fluid can be purified in situ within the main loop. Therefore, the contaminants may be quickly removed and not re-circulated. Further, the trans-critical system <NUM> may not require additional separating components, such as filters or strainers that have an associated pressure drop and are susceptible to flow damage, thereby requiring regular replacement to maintain functionality. Since the separator <NUM> is positioned downstream of the expansion device <NUM>, fouling of the first heat exchanger <NUM> by the contaminants can be prevented.

<FIG> illustrates a method <NUM> of removing solutes from a working fluid in a trans-critical circuit. The method <NUM> will be described with reference to the trans-critical system <NUM> described above with reference to <FIG> and <FIG>. However, the method <NUM> may be implemented by any one of the trans-critical systems <NUM>, <NUM>, <NUM> described above.

At step <NUM>, the method <NUM> includes identifying a position (e.g., point P2') in the trans-critical circuit <NUM> where the working fluid is the sub-critical gas <NUM>.

The method <NUM> may further include identifying the fluid extraction point <NUM> in the trans-critical circuit <NUM> where the working fluid is the supercritical fluid <NUM>. The method <NUM> may further include extracting the portion <NUM> of the supercritical fluid <NUM> from the fluid extraction point <NUM> in the trans-critical circuit <NUM>. The method <NUM> may further include passing the portion <NUM> of the supercritical fluid <NUM> through the expansion device <NUM> such that the portion <NUM> of the supercritical fluid <NUM> becomes the sub-critical gas <NUM>.

The method <NUM> may further include compressing the working fluid upstream of the fluid extraction point <NUM> such the working fluid becomes the supercritical fluid <NUM>. The method <NUM> may further include passing at least a fraction of the supercritical fluid <NUM> through the first heat exchanger <NUM> located upstream of the fluid extraction point <NUM>. The method <NUM> may further include controlling the rate of energy transfer Qout in the first heat exchanger <NUM> based on a measure of one or more thermodynamic properties of the supercritical fluid <NUM> at the fluid extraction point <NUM>.

Claim 1:
A trans-critical thermodynamic system (<NUM>), comprising:
a high pressure circuit (<NUM>) through which supercritical fluid (<NUM>) containing solutes (<NUM>) flows;
a fluid extraction point (<NUM>) operable to extract a portion (<NUM>) of the supercritical fluid (<NUM>) from the high pressure circuit (<NUM>);
an expansion device (<NUM>) receiving the portion (<NUM>) of supercritical fluid (<NUM>), the expansion device (<NUM>) operable to expand the portion (<NUM>) of supercritical fluid (<NUM>) to produce a sub-critical gas (<NUM>) by reducing a temperature and a pressure of the supercritical fluid (<NUM>); and a separator (<NUM>) for removing the solutes (<NUM>) from the sub-critical gas (<NUM>) characterized in that the fluid extraction point (<NUM>) is in fluid communication with a cooling circuit (<NUM>), wherein the supercritical fluid (<NUM>) in the high pressure circuit (<NUM>) that is not extracted at the fluid extraction point (<NUM>) is circulated through the cooling circuit (<NUM>), the cooling circuit (<NUM>) comprising:
at least one heat exchanger (<NUM>, <NUM>); and
another expansion device (<NUM>).