GB2614564A

GB2614564A - Multistage compression system

Info

Publication number: GB2614564A
Application number: GB2200179.6A
Authority: GB
Inventors: Alexander Heinz Justin
Original assignee: Carno Heat Ltd
Current assignee: Carno Heat Ltd
Priority date: 2022-01-07
Filing date: 2022-01-07
Publication date: 2023-07-12

Abstract

A multistage compression heat pump 100 for reheating at least one fluid. The system comprises a first fluid (e.g. CO2 refrigerant) flowing inside a first closed loop 102 and a second fluid (e.g. water) flowing inside a second closed loop 104 partly surrounding the first loop. The system comprises an evaporator 106 for vaporizing the first fluid passing therethrough and a first compressor 108 for compressing said vaporized first fluid passing therethrough. A second compressor 108, for which speed is controlled by a control unit to maintain the temperature of the first fluid between lower and upper limits, a heat exchanger 110 for performing heat exchange between the first and second fluids. An expander 112 (e.g. turbine, reciprocating/centrifugal expander) expands the first fluid passing therethrough to recover energy, potentially electricity to power the compressors. The compressors may be implemented to have n stages as n compressors and there may be n fluid streams routed by a three-way/non-return valve 124. During a first mode, the three-way/non-return valve may be adjusted so that first fluid is passed through each compressor in a first mode.

Description

MULTISTAGE COMPRESSION SYSTEM

TECHNICAL FIELD

The present disclosure relates generally to heat pumps; and more specifically, to multistage compression systems for reheating at least one fluid. The present disclosure also relates to methods for reheating at least one fluid, using the aforementioned multistage compression systems.

BACKGROUND

Heat pumps have found application in various processes such as heating, ventilation, air conditioning (HVAC) processes, domestic processes, industrial processes, and so forth. In this regard, heat pumps typically employ exchange of heat between two fluids flowing in closed loops.

Traditional heat pumps employ a subcritical cycle for the purpose of reheating the fluid in the closed loops for the purpose of space heating. Trans-critical heat pumps, on the other hand, are not suitable or efficient for small temperature differences between inlets and outlets of trans-critical heat pumps.

This problem of a trans-critical cycle has been addressed by imposing maximum limits on space heating demand. In this regard, available heat pumps, such as the SanCO2® water heater and Ecodan R744® heat pump, used for space heating applications employ a stratified water tank that possesses a temperature gradient in the water stored therein to achieve space-heating. However, said heat pumps suffer from restrictions on maximum space heating demand limiting their adaptability.

Recently, heat pumps have started using the concept of direct dedicated mechanical subcooling that employs a second heat pump cycle (namely, a subcooling cycle) using a second subcooling heat pump, which uses the rejected heat of a primary trans-critical heat pump for evaporation. However, said subcooling heat pumps require a complete second sealed cycle, thereby increasing complexity, and require a different refrigerant fluid that might have a higher global warming potential. Moreover, such subcooling heat pumps may lead to a lower efficiency.

The above solutions of using a stratified water tank or a subcooling cycle introduce a limitation of the allowed space heating demand or an increase in complexity, respectively. Even though transcritical cycles offer advantages for high temperature lifts and often can be run with more environmentally friendly refrigerant fluids compared to conventional subcritical heat pumps which rely on fluids such as hydrofluorocarbons, the above restrictions make them unfeasible for reheating of fluids in closed loops.

Therefore, in light of the foregoing discussion, there exists a need to overcome the aforementioned drawbacks associated with existing transcritical heat pumps.

SUMMARY

The present disclosure seeks to provide a multistage compression system for reheating at least one fluid. The present disclosure also seeks to provide a method for reheating at least one fluid using a multistage compression system. The present disclosure seeks to provide a solution to the existing problem of using a heat pump with a trans-critical cycle to reheat a fluid in a closed loop for various applications. An aim of the present disclosure is to provide a solution that at least partially overcomes the problems encountered in prior art, and provides an efficient, environmentally friendly, simple, and cost-efficient system.

A first aspect of the invention provides a multistage compression system for reheating at least one fluid, the multistage compression system comprising: - a first fluid flowing inside a first closed loop and a second fluid flowing inside a second closed loop that at least partly surrounds the first closed loop; - an evaporator for vaporizing the first fluid passing therethrough; - a first compressor for compressing the vaporized first fluid passing therethrough; - a second compressor for maintaining the temperature of the first fluid between a lower limit and an upper limit; - at least one heat exchanger for performing a heat exchange between the first fluid and the second fluid passing therethrough; - an expansion valve or expander (to recover an amount of energy) for expanding the first fluid passing therethrough; and - a control unit configured to control a speed of the second compressor, to maintain the temperature of the first fluid between the lower limit and the upper limit.

A second aspect of the invention provides a method for reheating at least one fluid, using a multistage compression system, the method comprising: - providing a first fluid flowing inside a first closed loop and a second fluid flowing inside a second closed loop that at least partly surrounds the first closed loop; - vaporizing, using an evaporator, the first fluid; - compressing, using a first compressor, the vaporized first fluid passing therethrough; - compressing, using a second compressor to maintain the temperature of the first fluid between a lower limit and an upper limit; - performing, using at least one heat exchanger, a heat exchange between the first fluid and the second fluid; expanding, using an expander, the first fluid and recovering an amount of energy; and operating a control unit to control a speed of the second compressor, to maintain the temperature of the first fluid between the lower limit and the upper limit.

Embodiments of the present disclosure may substantially eliminate or at least partially address the aforementioned problems in the prior art, and provide a multistage compression system for reheating of at least one fluid. Suitably, the at least one fluid may comprise the first fluid and/or the second fluid. The multistage compression system may solve the problems associated with a heat pump based on a trans-critical cycle or subcooling cycle to reheat a fluid in a closed loop for various applications, such as space heating, without causing global warming.

In this regard, the multistage compression system may comprise multiple compressors and heat exchangers arranged therebetween that may be used with fluids that are more environmentally friendly with a lower global warming potential. Moreover, the multistage compression system may provide an energy-efficient, economical, practical, and scalable system for reheating of the at least one fluid by keeping the temperature of the at least one fluid between a specific upper limit and a lower limit.

Furthermore, the multistage compression system may efficiently provide heat in two modes -reheating the fluid and hot water demand, by utilizing all compressor stages or skipping all except the first one. In this, the multistage compression system may be configured to route the at least one fluid to flow into a plurality of fluid streams to pass through one or more compressors, based upon the application.

As mentioned above, the multistage compression system may advantageously be compatible with fluids that are environmentally friendly with a lower global warming potential. Additionally, the at least one fluid may be a low flammability and low toxicity refrigerant fluid, that may not be affected by the hydrofluorocarbon phase-out, thereby providing a future proof heat pump. Beneficially, the multistage compression system may employ fluids such as carbon dioxide (CO2) for space heating without imposing restrictions on maximum space heating demand, by maintaining the temperature of the at least one fluid between a lower limit and an upper limit, thereby, increasing the efficiency of the multistage compression system.

In this regard, the multistage compression system may employ a multistage compressor having n stages (or a plurality of compressors) for compressing the first fluid passing therethrough. Moreover, the multistage compression system may comprise a three-way valve and a non-return valve to route the at least one fluid to flow into the plurality of fluid streams to achieve reheating in a first mode and a second mode by adjusting at least one three-way valve in order to utilize the system efficiently.

Besides, the known components (such as compressor, heat exchanger, vaporizer, and so on) of a typical compression-based heat pump, the multistage compression system comprises an expander for expanding the first fluid passing therethrough and recovering an amount of energy. Advantageously, the recovered energy may be used to power the at least one component of the multistage compression system, such as drive the at least one compressor and to generate electricity.

Moreover, the amount of energy recovered during the expansion step may lead to very high efficiency. Advantageously, even without energy recovery, based on initial simulations, the multistage compression system may lead to an efficiency increase of around 10% for space heating and 80% for hot water demand provision depending on outdoor conditions compared to conventional heat pumps, such as a heat pump run with R32 (difluoromethane).

The term "multistage compression" as used herein refers to a compression process that is completed in multiple stages in order for the cooling and heating of fluids, for example. In this regard, compression of fluids occurs in multiple stages, wherein said multiple stages may feature a series of compressors or a single compressor having multiple compression stages. It will be appreciated that the multistage compression system enables the efficient transfer of energy therein. Thus, making the multistage compression system mechanically feasible and thermodynamically efficient.

The term "reheating" as used herein refers to heating of a fluid or a substance in a closed loop or continuous cycle. In this regard, when in operation, the multistage compression system continues to reheat the at least one fluid. The term "at least one fluid" as used herein refers to a gas, a liquid, or a substance that possesses the ability to deform continuously (flow) under applied shear stress, or external force. Notably, every fluid possesses specific thermodynamic properties such as pressure, temperature, enthalpy, entropy, specific volume, internal energy, and so forth, that may be used to design or modify the multistage compression system for various applications.

The at least one fluid may comprise a first fluid flowing inside a first closed loop and a second fluid flowing inside a second closed loop that at least partly surrounds the first closed loop. The term "first fluid" as used herein refers to a refrigerant or a coolant that undergoes a repeated phase transition from a liquid to a gas and vice versa repeatedly in a closed loop in the multistage compression system. The term "second fluid" as used herein refers to a heat transfer fluid such as a gas or liquid that takes part in heat transfer by serving as an intermediary in cooling at one stage of the multistage compression, transporting and storing thermal energy, and heating at another stage of the multistage compression.

The term "first closed loop" and "second closed loop" as used herein refer to cyclic processes that are designed to enable the continuous flow or reheating of the first fluid and the second fluid, respectively, in the multistage compression system. It will be appreciated that various designs of the multistage compression system may have the second closed loop partly or completely surrounding the first closed loop.

Optionally, the at least one fluid may be at least one of: carbon dioxide, and water. Carbon dioxide may be used as the first fluid, i.e. a refrigerant, due to high vapor pressure that leads to a low-pressure ratio with the advantage of high compressor efficiency, high heat transfer coefficients and low relative pressure loss, that are highly favourable for cooling, refrigeration, and heating purposes.

Notably, the use of carbon dioxide as a refrigerant fluid is more environmentally friendly than conventionally used refrigerant fluids such as hydrofluorocarbons. Advantageously, carbon dioxide possesses high volumetric capacity due to high working pressure thereof, and thereby reduces the size of the multistage compression system. Water may be used as the second fluid due to the high heat capacity and thermal conductivity thereof. Moreover, when in use, the multistage compression system allows the water to exchange heat from the refrigerant fluid.

The multistage compression system comprises an evaporator for vaporizing the first fluid passing therethrough. The evaporator transforms the first fluid in its liquid form into a gaseous or vapor form thereof. In this regard, the first fluid is fed into the evaporator and passed through a heating medium arranged inside the evaporator to convert the first fluid into vapor, thereby increasing the entropy and enthalpy of the first fluid in a two-phase region (having both liquid and gaseous form of CO2).

The multistage compression system comprises a first compressor for compressing the vaporized first fluid passing therethrough, and a second compressor for maintaining the temperature of the first fluid between a lower limit and an upper limit. The term " compressor" as used herein refers to a mechanical device that increases the pressure of a gas by reducing its volume by compression thereof. It will be appreciated that compression of a fluid results in an increase in the temperature of the fluid.

The terms "lower limit" and "upper limit" of temperature as used herein refers to optimum or desired values of the temperature of the first fluid that is proportional to a specific volume of the fluid. Optionally, the temperature of the first fluid may be maintained by the second compressor in a range of between 30°C and 60°C. In this regard, the lower limit of temperature of the first fluid entering the second compressor may be 30°C and the upper limit of temperature of the first fluid leaving the second compressor, after being compressed, may be 60°C. Optionally, the temperature of the fluid between the lower limit and the upper limit may be in a range from 30 to 40°C, or from 30 to 50°C, or from 40 to 50°C, or from 40 to 60°C, such as from 50°C to 60°C.

Optionally, the first and/or second compressor may comprise a reciprocating compressor, and/or a centrifugal compressor. The term "reciprocating compressor" as used herein refers to a positive-displacement compressor that uses pistons driven by a shaft to deliver fluids at high pressure. Notably, the pressures of up to 5,000 PSIG (-34474 kPa) are commonly produced by multistage reciprocating compressors. Moreover, the first fluid enters the compressor where it gets compressed by a piston driven in a reciprocating motion via the shaft, and is then discharged to other components of the multistage compression system. The term "centrifugal compressor" as used herein refers to a compressor that is used to achieve a pressure rise by adding kinetic energy/velocity to a continuous flow of a fluid, such as the first fluid, through a rotor or impeller. Optionally, the centrifugal compressor may convert said kinetic energy to an increase in potential energy or static pressure by slowing the flow of the first fluid through a diffuser.

Optionally, the first and/or second compressor may be implemented as a multistage compressor having n stages. The term "multistage compressor" as used herein refers to a compressor having multiple stages (namely, n stages). In other words, the first fluid passing through the multistage compressor is compressed several times in steps or stages, to increase a discharge pressure thereof. Optionally, the second stage may be physically smaller than a first stage, to accommodate an already compressed first fluid without reducing its pressure. Notably, each stage may further compress the first fluid and increase pressure and temperature (when intercooling between stages is not used) thereof. Suitably, the multistage compressor is of an optimum size to accommodate the larger desired temperature drop. Optionally, the n stages may be 2, 3, 4, 5 stages, and so on which involve 2, 3, 4, 5 compressors, and so on.

Optionally, the first and/or second compressor may be implemented as n compressors having at least one heat exchanger arranged therebetween. Said n compressors are n single-stage individual compressors. In this regard, during compression in one of the n compressors, the pressure and temperature of the first fluid may be increased to a specific set point that can be adjusted by varying the compressor speed. The compressed first fluid may subsequently be allowed to pass through the subsequent compressor(s) for further compression thereof.

It will be appreciated that with each compression stage, the temperature of the compressed fluid increases. Notably, hot fluid is difficult to compress as compared to the cooled fluid adiabatically. Therefore, beneficially, multiple compressors may have at least one heat exchanger arranged therebetween.

In this regard, the at least one heat exchanger may be installed between an exit of one compressor (namely, a first compressor or a low-pressure compressor) and an inlet of a subsequent compressor (a high-pressure compressor) in order to remove the heat from compression from the previous stage or stages, thereby increasing the average fluid density over the compression process and reducing power consumption by the first and/or second compressor.

The term "heat exchanger" as used herein refers to a device that facilitates the process of heat exchange between two fluids that are at different temperatures. The term "heat exchange" as used herein refers to a transfer of heat or thermal energy across a well-defined boundary around a thermodynamic system. Optionally, the multistage compression system may employ the at least one heat exchanger for cooling down the at least one fluid and using the rejected heat to increase the suction temperature of any of the compressor stages.

Moreover, the at least one heat exchanger may cool the compressed first fluid from the first compressor by exchanging its heat with the second fluid, and enables further compression, to a desired pressure level, of the cooled first fluid in the second compressor, thereby decreasing the net amount of work required to be done to compress the gas in multistage compression. Optionally, the at least one heat exchangers may influence the overall efficiency and size of the multistage compression system. Optionally, the at least one heat exchanger may be a shell-and-tube heat exchanger, a plate type heat exchanger, and so forth.

In an exemplary implementation of a two-stage compression system, the first fluid may be admitted to the first compressor, i.e. the low-pressure compressor, and may be compressed to some intermediate pressure between an inlet pressure and a delivery pressure. The compressed first fluid may have a higher temperature and pressure compared to an inlet temperature and pressure thereof. Said compressed first fluid may be brought to the heat exchanger (also known as intercooler), where the compressed first fluid may be cooled down nearly to the temperature of inlet first fluid to the first compressor by regulating the supply of the second fluid (i.e. cooling water) in the heat exchanger. Notably, the temperature of the cooled first fluid leaving the heat exchanger depends upon the cooling efficiency of the heat exchanger.

The cooled first fluid may subsequently be admitted to the second compressor, i.e. the high-pressure compressor, and may be compressed to a higher pressure, such as less than or equal to the delivery pressure. In the process, the first fluid may be cooled to the lower limit of temperature between the compression stages by passing the first fluid through the heat exchanger. Optionally, the energy recovered by heat exchanging may be significant, especially when the first fluid is to be compressed to a very high pressure. Optionally, the heat exchanging may be required between multiple stages to reheat the first fluid and maintain the temperature of the first fluid between the lower limit and the upper limit.

It will be appreciated that for low temperature applications, such as domestic heating, a single compressor might be sufficient to reach the set point, while for high temperature applications, such as industrial heating, multiple compressors might be necessary.

The multistage compression system employs an expander for expanding the first fluid passing therethrough and recovering an amount of energy. The term "expander" as used herein refers to a device that operates in a reverse cycle as compared to a compressor. Typically, the expander expands the at least one fluid, preferably the compressed first fluid, and may produce power which can be used to supply energy to the first and/or second compressor and other components of the multistage compression system.

Optionally, the expander may be composed of a series of blades to allow the first fluid to enter the expander. In this regard, the expander may harness the kinetic energy of the first fluid, and translate it into a rotational motion of the blades. In such case, while the blades spin, the first fluid may flow through the expander. Optionally, the first fluid flowing through the expander may lose kinetic energy and exit the expander with less energy than it started with. Moreover, as the expansion step may suitably result in a drastic temperature and pressure decrease of the first fluid, the use of energy recovery options may be feasible leading to a very high efficiency. Beneficially, the said operation may lead to an efficiency increase of around 10% for the first mode and/or 80% for the second mode depending on outdoor conditions.

Optionally, the at least one expander may be at least one of: a turbine, a reciprocating expander, and a centrifugal expander. The term "turbine" as used herein refers to a rotary mechanical device that is configured to extract energy from a fluid flow and convert the extracted energy into useful work. Suitably, the turbine may be configured to receive the first fluid to extract excess energy therefrom and convert the extracted energy to generate electrical power, for example, by using an operatively coupled generator. The turbine may comprise rotating blades that are curved and/or arranged so as to develop a torque from a gradual decrease of fluid pressure. Moreover, the turbine may be mechanically and/or operatively coupled with a constant pressure turbine and/or the generator.

The term "reciprocating expander" as used herein refers to a machine that converts the energy contained in a pressurized fluid into mechanical energy by increasing the volume of a working chamber, then decreasing the pressure of the fluid. Optionally, the reciprocating expander could be extensively utilized in small scale compressed air energy storage (CAES) system, distributed renewable energy utilization, uninterruptible power supply, and so forth.

The term "centrifugal expander" as used herein refers to a machine that works on the principle of a centrifugal force. In this regard, the centrifugal expander may apply an outward force, using a high velocity fluid, to make a shaft rotate in order to generate power. Moreover, the centrifugal expander may provide energy as the output. Optionally, the centrifugal expander may intake a high pressure first fluid and provides a low pressure first fluid as output. Optionally, the at least one expander may comprise a rotary vane expander. A rotary vane expander is a positive displacement expander that requires no valve to control the fluid flow. A rotary vane expander may improve the energy efficiency of the multistage compression system.

The multistage compression system comprises a control unit. The term "control unit" as used herein refers to a software and/or hardware in the multistage compression system that is operable to implement specific algorithms therein. Optionally, the control unit may employ a processor configured to perform the abovementioned operations. The processor may include one or more of a microprocessor, a microcontroller, a complex instruction set computing (CISC) microprocessor, a reduced instruction set computer (RISC) microprocessor, a very long instruction word (VLIW) microprocessor, or any other type of processing circuit. Furthermore, the term "processor" may refer to one or more individual processors, processing devices and various elements associated with the control unit. Additionally, the one or more individual processors, processing devices and elements may be arranged in various architectures for responding to and processing the instructions that drive the system.

The control unit is associated with performing operations such as controlling the speed of the second compressor, to maintain the temperature of the first fluid between the lower limit and the upper limit. The speed of a compressor may be associated with any one of: a piston, a shaft, and the like, that enable compression of a fluid in the compressor. It will be appreciated that the speed of the compressor may be directly proportional to a compression ratio of the compressor.

Notably, the compression ratio is the ratio of the absolute stage discharge pressure to the absolute stage suction pressure of the compressor. Moreover, a higher compression ratio may lead leads to an increase in temperature of the fluid inside the compressor. Beneficially, the speed of the compressor may be controlled using the control unit in order to account for varying outdoor temperatures and required indoor temperatures. Optionally, the speed may be optimized for achieving a desired pressure and temperature more efficiently.

In some embodiments the first and/or second fluid may be configured to flow into a plurality of fluid streams. For example, the system may be configured to split the first and/or second fluid into a plurality of fluid streams, such as a plurality of first and/or second fluid streams. The term "plurality of fluid streams" as used herein refers to a network of pipes or pipelines forming a closed loop, such as the first closed loop and the second closed loop, configured to allow the flow of the at least one fluid through them.

Suitably, the first fluid may be configured to flow into a plurality of first fluid streams and /or the second fluid may be configured to flow into a plurality of second fluid streams. For example, the first fluid may be configured to flow into a plurality of first fluid streams inside the first closed loop and/or the second fluid may be configured to flow into a plurality of second fluid streams inside the second closed loop.

In this regard, the first fluid and/or the second fluid flow may through separate fluid streams from one component of the multistage compression system to another component, in order to exchange heat therebetween. It will be appreciated factors such as a minimum fluid flow velocity, a fluid viscosity, and so forth determine a design and associated operation of the first closed loop and the second closed loop.

The system may comprise at least one of: a three-way valve and a non-return valve to route the first and/or second fluid to flow into a plurality of fluid streams. Suitably, the plurality of fluid streams may be equal to n stages of the multistage compressor or n compressors.

The term "three-way valve" as used herein refers to a control valve that is used to control fluid flow by varying, by means of opening or closing, the flow passage as directed by a signal from the control unit. In this regard, the control unit may be used to control the flow rate of the first and/or second fluid in the plurality of fluid streams by using a three-way valve. In some embodiments, the multistage compression system may comprise one or more three-way valves to control the flow of at least one fluid into the plurality of streams to flow through or skip one or more components of the multistage compression system.

Beneficially, a three-way valve may be used to shut off flow of the at least one fluid in one fluid stream while opening flow of the at least one fluid in another fluid stream. Moreover, a three-way valve may mix the first fluid from a plurality of first fluid streams into a single first fluid stream and/or divert the first fluid from a single first fluid stream into a plurality of first fluid streams. Alternatively, or additionally, a three-way valve may mix the second fluid from a plurality of second fluid streams into a single second fluid stream and/or divert the second fluid from a single second fluid stream into a plurality of second fluid streams.

Optionally, the control unit may control the process variables such as flow of the first and/or second fluid by controlling a three-way-valve in the multistage compression system. Optionally, a three-way valve may enable the direct control of process quantities, such as pressure, temperature, and so forth, of the first and/or second fluid.

The term "non-return valve" as used herein refers to a valve that allows the fluid to flow therethrough in only one direction. Suitably, a non-return valve may be fitted to ensure that the first and/or second fluid flows through the plurality of fluid streams in the right direction, where pressure conditions may otherwise cause reversed flow. In some embodiments, the non-return valve may be a two-port valve, having two openings therein, one for fluid to enter and the other for fluid to leave.

Optionally, three-way valves and/or non-return valves may be used to divide the fluid stream in n equal streams where n is the number of compressor stages of the at least one compressor. Optionally, a three-way valve and/or non-return valve may be employed to allow the first fluid to skip the individual compressors of the multistage compression system. Beneficially, said operation may allow the multistage compression process to be operated as a conventional trans-critical process in a second mode that makes it better suited for providing a high temperature lift. Thus, the fluid stream that is to be reheated might use the three-way valves and non-return valves in order to achieve this.

Optionally, the reheating may be achieved in a first mode and a second mode by adjusting at least one three-way valve. In such embodiments, in the first mode, the first fluid may be passed through each compressor, and the second fluid may be passed through the heat exchanger. In such embodiments, in the second mode, the first fluid may be passed through a non-return valve, and the second fluid may be passed through the heat exchanger.

In this regard, the term "first mode" as used herein refers to a process of reheating the first fluid in the multistage compression system by passing the first fluid through each compressor, and the second fluid through the heat exchanger. Moreover, the first fluid may be passed through each compressor by using a three-way valve and adjusting it accordingly. Optionally, in the first mode, one three-way valve may be used to split the incoming fluid stream of the second fluid in half that is into two fluid streams that carry the second fluid. Optionally, another three-way valve may be used so that the second fluid follows a third fluid stream without mixing with the incoming fluid stream of the second fluid. Optionally, in the first mode, the multiple compressors may be used iteratively to maintain the temperature between the lower limit and the upper limit, again accordingly. Optionally, the rejected heat of the multistage compression system may be used to again heat up (namely, reheat) the at least one fluid that flows in a closed loop.

The term "second mode" as used herein refers to a process for providing hot water demand using the multistage compression system by passing the first fluid through a non-return valve, and the second fluid through the heat exchanger. In other words, in the second mode, by passing the first fluid through a non-return valve, the first fluid may be allowed to flow through the heat exchanger but only one compressor by skipping the multiple compression stage.

In some cases, a multistage compression process with more than one compressor may not be optimal for providing the hot water demand (such as for lifting up tap water from 10°C to 60°C). Therefore, the implementation of three-way valves and/or non-return valves is beneficial to skip the compressor in the system. In an example, during the second mode, a heat exchanger may be used to pass the first fluid therethrough before feeding the first fluid into the expander that could be used to further cool down the first fluid in the multistage compression system. Optionally, the rejected heat of said heat exchanger could then be used for providing a higher temperature lift in the second mode.

In some embodiments, the system may comprise at least one motor configured to drive the compressors. Suitably, the motor may be an electric motor that converts electrical energy into mechanical energy. Moreover, the electric motors can be powered by direct current (DC) sources, such as batteries or rectifiers, or by alternating current (AC) sources, such as a power grid, inverters or electrical generators. The at least one motor may produce a linear force or a rotary force i.e., torque intended to rotate the driving part, such as the input shaft (or shaft), coupled to it. Suitably, the motors may be efficient, lightweight, robust, are mechanically simple and cheap to manufacture. Furthermore, the at least one motor can provide instant and consistent torque at any speed and can run on electricity generated by renewable sources and do not or negligibly contribute to greenhouse effect. In some embodiments the at least one motor may be selected from, but not limited to, a hydraulic motor, a gear motor, a pneumatic motor and the like.

Optionally, the at least one compressor may be connected to the motor by a shaft. Optionally, the first and/or second compressor may have the motor driving the shaft that passes through the body of the compressor and, optionally, rely on rotary seals around the shaft to retain the internal pressure. Optionally, the first and/or second compressor may be a hermetic and a semi-hermetic compressor, optionally wherein the motor driving the compressor may operate within the pressurized gas envelope of the system. Optionally, the motor may be designed to operate in, and be cooled by, the first fluid being compressed.

Optionally, the recovered amount of energy may be used to drive the first and/or second compressor. For example, the recovered amount of energy from the expander may be channelled to drive the first and/or second compressor coupled to the motor via the shaft. In this regard, the recovered amount of energy may be used to power the motor and/or drive the shaft to cause the first and/or second compressor to operate.

Optionally, the recovered amount of energy may be used to generate electricity. In this regard, the expander may drive a generator generating electricity used to power the motor. In an example, after the first fluid has passed all n compressor stages, an energy recovery valve of the expander may be used to reduce the pressure of the first fluid. Optionally, where an energy recovery valve is used, the recovered amount of energy may be converted to mechanical energy that may be used to drive the compressors directly and/or generate electricity in a generator to later power the multistage compression system.

In some embodiments, the system may comprise a stratified fluid tank The stratified fluid tank may be operatively coupled to the multiple stage compression system. The stratified fluid tank may be arranged to store heated second fluid. Suitably, the stratified fluid tank may be arranged to store heated second fluid exiting from the multistage compression system and/or exiting the second closed loop of the multistage compression system. The term "stratified fluid tank" as used herein refers to a layered hot water storage tank. In other words, the stratified fluid tank may be used to store the second fluid used in the second closed loop. Beneficially, said operation may allow the storage of the hot fluid for the purpose of providing hot water demand and space heating.

Moreover, said operation may enable the temperature lift of the fluid to be reheated in the multistage compression process to be less dependent on the temperature drop in the closed loop space heating system. For example, while the temperature drop in the closed loop space heating system might be, say, 60°C to 40°C, the multistage compression system might only need to be optimized for 30°C to 60°C (depending on the expected hot water demand) and not for 40°C to 60°C, when the stratified fluid tank is used. Thus, this may result in fewer required compressor stages, lower complexity and higher efficiency of the system.

Optionally, the second fluid (for example, water) may be fed into a stratified fluid tank having different storage levels, depending on the available feed temperature of the exiting hot second fluid and a current temperature of the hot second fluid collected inside the stratified fluid tank. Moreover, in some embodiments the second fluid may be fed into the stratified fluid tank via a vertical line, for example via three-way valves, in which case the second fluid may be fed into a specific storage layer of the stratified fluid tank corresponding to a specific (e.g. water) temperature. Advantageously, there may be no mixing of the hot second fluid stored based on different storage temperature thereof inside the stratified fluid tank. Beneficially, the stratified fluid tank may be used when the multistage compression system is in a resting phase, i.e. a non-operation phase between subsequent operations of the multistage compression system.

The present disclosure also relates to the method according to the second aspect of the invention described above. Various embodiments and variants disclosed above apply mutatis mutandis to the method.

In some embodiments, the method may comprise dividing the first and/or second fluid to flow into a plurality of fluid streams. For example, the method may comprise dividing the first fluid into a plurality of first fluid streams and/or dividing the second fluid into a plurality of second fluid streams. Suitably, the method may comprise dividing the first fluid into a plurality of first fluid streams in the first closed loop and/or dividing the second fluid into a plurality of second fluid streams in the second closed loop.

EXAMPLES Example 1

In an exemplary implementation, commercial and domestic heating systems may be retrofitted with the multistage (herein, a two-stage) compression system and the stratified fluid tank comprising an antifreeze therein to reheat the water and provide hot tap water, respectively, to at least partially address the drawbacks associated with the existing heating systems that utilize radiators and gas boilers. In this regard, the temperature in the stratified fluid tank is controlled so that the water at the top is around 60°C and the bottom is 15°C. The top of the tank is connected to the at least one heat exchanger heating up the fresh tap water if required, and the cool water is returned to the stratified fluid tank at the bottom. The outlet of the multistage compression system is connected to a three-way valve which feeds the stratified fluid tank at the top and an inlet of a local heating system. The cold water coming back from the local heating system is mixed with water from the bottom of the stratified fluid tank and pumped back to the multistage compression system by utilizing a three-way valve.

When the multistage compression system is operating in "reheating" mode, it draws water at 30°C and returns water with 60°C. In this regard, the multistage compression system mixes the 40°C return water from the local heating system with 15°C water from the bottom of the stratified fluid tank. The water flowing out of the multistage compression system is split accordingly and fed back to the stratified fluid tank and the local heating system. This procedure gradually increases the average temperature of the stratified fluid tank and the stratification layer moves downwards.

When domestic hot water is produced, the average temperature of the stratified fluid tank decreases again and the stratification layer moves upwards.

When operating in "hot water supply mode", which is activated when the stratification layer is high up in the stratified fluid tank (the average temperature is too low), water is drawn from the bottom of the stratified fluid tank at 15°C and returned at the top with 60°C via exit of the multistage compression system. Depending on the temperature levels in the tank, the multistage compression system switches between the two modes. In "hot water supply mode" the heat pump reaches a COP of 4.67 for outside temperatures of 0°C, in "reheating the fluid" mode the COP is still 2.77, while the conventional heat pump based on R32 is simulated to achieve a COP of 2.57 overall. Notably. some heating systems utilize higher flow temperatures of up to 85°C, which can also be served efficiently with the proposed CO2based multistage compression system.

Example 2

In another exemplary implementation, an industrial heating system (where temperatures up to 150°C are required) may be retrofitted with the multistage compression system. In such case, the multistage compression system could be used for the purpose of reheating the second fluid to decarbonize the process. For example, for boiling process like brewing, the at least one fluid, such as steam or oil, is used at input temperatures of around 130°C-150°C. The fluid cools down to around 90°C before being reheated. In this regard, the multistage compressor having three or more stages could be used to reach temperatures of 130°C, then after a first heat exchanger heating up the second fluid, at least one compressor (namely, a booster compressor) could increase the temperature of the first fluid, i.e. CO2, to 130°C again. The booster compressor and the heat exchanger might be used afterwards depending on expected heat source temperatures. Furthermore, an additional industrial use case might be drying with hot air. In this regard, depending on the application hot air of 60°C-120°C is required. The air could be reheated using the multistage compression system.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present disclosure will now be described, by way of example only, with reference to the following diagrams wherein: FIG. 1 is a schematic illustration of a multistage compression system for reheating at least one fluid, in accordance with another embodiment of the present disclosure; FIG. 2 is a schematic illustration of a multistage compression system operatively coupled to a stratified fluid tank, for storing the heated second fluid exiting from the multistage compression system, in accordance with an embodiment of the present disclosure; FIGs. 3A and 3B are graphical representations of a temperature-entropy relationship and a pressure-enthalpy relationship of a first fluid reheated in a first mode, respectively, in accordance with an embodiment of the present disclosure; FIGs. 4A and 46 are graphical representations of a temperature-entropy relationship and a pressure-enthalpy relationship of a first fluid reheated in a second mode, respectively, in accordance with an embodiment of the present disclosure; and FIG. 5 is a flowchart of steps of a method for reheating at least one fluid, in accordance with an embodiment of the present disclosure.

DETAILED DESCRIPTION

Referring to FIG. 1, there is shown a schematic illustration of a multistage compression system 100 for reheating at least one fluid, in accordance with an embodiment of the present disclosure. The multistage compression system 100 comprises the at least one fluid comprising a first fluid (depicted as solid lines) flowing inside a first closed loop 102 and a second fluid (depicted as dashed lines) flowing inside a second closed loop 104 that at least partly surrounds the first closed loop 102. Moreover, the multistage compression system 100 comprises an evaporator 106 for vaporizing the first fluid passing therethrough and at least one compressor 108 for compressing the vaporized first fluid passing therethrough, and to maintain the temperature of the first fluid between a lower limit and an upper limit.

Furthermore, the multistage compression system 100 comprises at least one heat exchanger 110 for performing a heat exchange between the first fluid and the second fluid passing therethrough and an expander 112 for expanding the first fluid passing therethrough and recovering an amount of energy. Additionally, the multistage compression system 100 comprises a control unit (not shown) configured to control a speed of the at least one compressor 108, to maintain the temperature of the first fluid between the lower limit and the upper limit, wherein the at least one fluid is configured to flow into a plurality of fluid streams, such as fluid streams 114, 116, 118, 120, 122.

The multistage compression system 100 further comprises at least one of: a three-way valve 124 and a non-return valve 126 to route the at least one fluid to flow into the plurality of fluid streams, wherein the plurality of fluid streams is equal to n stages of a multistage compressor or n compressors. It will be appreciated that the multistage compression system 100 may comprise one or more three-way valve 124 and one or more non-return valve 126 at different sites of the multistage compression system 100 as shown.

Referring to FIG. 2, there is shown a schematic illustration of a multiple stage compression system 200 operatively coupled to a stratified fluid tank 202, for storing the heated second fluid exiting from the multistage compression system 200, in accordance with an embodiment of the present disclosure. In this regard, the reheating is achieved in a first mode and a second mode by adjusting at least one three-way valve 204. Moreover, in the first mode, the first fluid is passed through each of the at least one compressor 206, and the second fluid is passed through one of the at least one heat exchanger 208.

Furthermore, in the second mode the first fluid is passed through the three-way valve 204. It will be appreciated that in the second mode, the three-way valve 204 may be used to route the first fluid to pass through a non-return valve 210, and the second fluid is passed through each of the at least one heat exchanger 208. Moreover, the outlet of the multistage compression system 200 is connected to a three-way valve 212 which feeds the stratified fluid tank 202 at the top and an inlet of a local heating system 214. The cold water coming back from the local heating system 214 is mixed with water from the bottom of the stratified fluid tank 202 and pumped back to the multistage compression system 200 by utilizing a three-way valve 216.

Additionally, the multistage compression system 200 comprises an expander 218 for expanding the first fluid passing therethrough and recovering an amount of energy and an evaporator 220 for vaporizing the first fluid passing therethrough.

Referring to FIGs. 3A and 3B, there are shown graphical representations of a temperature-entropy relationship and a pressure-enthalpy relationship of a first fluid reheated in a first mode, respectively, in accordance with an embodiment of the present disclosure. In this regard, an evaporator is used to increase the entropy and enthalpy of the first fluid in the two-phase region. Then, at least one compressor compresses the first fluid.

Moreover, at first, the pressure and temperature of the at least one fluid is increased to a specific set point that may be adjusted by varying speed of the at least one compressor. Furthermore, the pressure is released using the expander. Optionally, for low temperature applications such as domestic heating, the first compressor stage might be sufficient to reach the set point.

Optionally, for high temperature applications such as industry, multiple compressor stages might be necessary.

Referring to FIGs. 4A and 4B, there are shown graphical representations of a temperature-entropy relationship and a pressure-enthalpy relationship of a second fluid reheated in a second mode, respectively, in accordance with an embodiment of the present disclosure.

Referring to FIG. 5, there is illustrated a flowchart 500 of steps of a method for reheating at least one fluid, in accordance with an embodiment of the present disclosure. At step 502, the at least one fluid is supplied in the multistage compression system, wherein the at least one fluid comprises a first fluid flowing inside a first closed loop and a second fluid flowing inside a second closed loop that at least partly surrounds the first closed loop. At step 504, the first fluid is vaporized, using an evaporator. At step 506, the vaporized first fluid is compressed, using at least one compressor, to maintain the temperature of the first fluid between a lower limit and an upper limit. At step 508, a heat exchange between the first fluid and the second fluid is performed, using at least one heat exchanger. At step 510, the first fluid is expanded, using an expander, and an amount of energy is recovered. At step 512, a control unit is operated to control a speed of the at least one compressor, to maintain the temperature of the first fluid between the lower limit and the upper limit.

The steps 502, 504, 506, 508, 510, and 512 are only illustrative and other alternatives can also be provided where one or more steps are added, one or more steps are removed, or one or more steps are provided in a different sequence without departing from the scope of the claims herein.

Modifications to embodiments of the present disclosure described in the foregoing are possible without departing from the scope of the present disclosure as defined by the accompanying claims. Expressions such as "including", "comprising", "incorporating", "have", "is" used to describe and claim the present disclosure are intended to be construed in a non-exclusive manner, namely allowing for items, components or elements not explicitly described also to be present. Reference to the singular is also to be construed to relate to the plural.

Claims

CLAIMS1. A multistage compression system for reheating at least one fluid, the multistage compression system comprising: - a first fluid flowing inside a first closed loop and a second fluid flowing inside a second closed loop that at least partly surrounds the first closed loop; - an evaporator for vaporizing the first fluid passing therethrough; - a first compressor for compressing the vaporized first fluid passing therethrough, - a second compressor for maintaining the temperature of the first fluid between a lower limit and an upper limit; - at least one heat exchanger for performing a heat exchange between the first fluid and the second fluid passing therethrough; - an expander for expanding the first fluid passing therethrough and recovering an amount of energy; and - a control unit configured to control a speed of the second compressor, to maintain the temperature of the first fluid between the lower limit and the upper limit.
2. The multistage compression system according to claim 1, wherein the first and/or second compressor is implemented as a multistage compressor having n stages.
3. The multistage compression system according to claim 1, wherein the first and/or second compressor is implemented as n compressors having one or more of the at least one heat exchanger(s) arranged therebetween.
4. The multistage compression system according to any preceding claim, wherein the first and/or second fluid is configured to flow into a plurality of fluid streams.
5. The multistage compression system according to claim 4, comprising a three-way valve and/or a non-return valve to route the first and/or second fluid to flow into the plurality of fluid streams, optionally wherein the plurality of fluid streams is equal to n stages of the multistage compressor or n compressors.
6. The multistage compression system according to claim 5, wherein the reheating is achieved in a first mode and a second mode by adjusting the three-way valve and/or non-return valve, and wherein - in the first mode: - the first fluid is passed through each compressor, and - the second fluid is passed through one or more of the at least one heat exchanger(s); and in the second mode: - the first fluid is passed through a three-way valve and/or non-return valve, and - the second fluid is passed through one or more of the at least one heat exchanger(s).
7. The multistage compression system according to any preceding claim, wherein the first fluid comprises carbon dioxide and/or wherein the second fluid comprises water.
8. The multistage compression system according to any preceding claim, wherein the temperature of the first fluid is maintained by the second compressor in a range of from 30°C to 60°C.
9. The multistage compression system according to any preceding claim, wherein energy recovered from the expander is used to drive the first and/or second compressor.
10. The multistage compression system according to any preceding claim, wherein energy recovered from the expander is used to generate electricity.
11. The multistage compression system according to any preceding claim, wherein the expander comprises a turbine, a reciprocating expander, and/or a centrifugal expander.
12. The multistage compression system according to any preceding claim, comprising at least one motor configured to drive the first and/or second compressor.
13. The multistage compression system according to any preceding claim, wherein each of the first and/or second compressors comprises a reciprocating compressor and/or a centrifugal compressor.
14. The multistage compression system according to any preceding claim, comprising a stratified fluid tank optionally wherein the stratified fluid tank is arranged to store heated second fluid.
15. A method for reheating at least one fluid, using a multistage compression system, the method comprising: - providing a first fluid flowing inside a first closed loop and a second fluid flowing inside a second closed loop that at least partly surrounds the first closed loop; - vaporizing, using an evaporator, the first fluid; - compressing, using a first compressor, the vaporized first fluid passing therethrough, - compressing, using a second compressor to maintain the temperature of the first fluid between a lower limit and an upper limit; - performing, using at least one heat exchanger, a heat exchange between the first fluid and the second fluid; - expanding, using an expander, the first fluid and recovering an amount of energy; and operating a control unit to control a speed of the second compressor, to maintain the temperature of the first fluid between the lower limit and the upper limit.
16. The method according to claim 15, comprising dividing the first and/or second fluid flow into a plurality of fluid streams.
17. The method according to claim 15 or claim 16, wherein the reheating is achieved in a first mode and a second mode by adjusting at least one three-way valve and/or non-return valve, and wherein - in the first mode: - passing the first fluid through each compressor, and - passing the second fluid through one or more of the of the at least one heat exchanger(s); and in the second mode: - passing the first fluid through a three-way valve and/or non-return valve, and - passing the second fluid through one or more of the at least one heat exchanger(s).