Gradient utilization analysis method and system for low-temperature waste heat of 30-150 DEG C

Abstract

The invention discloses a 30-150 DEG C low-temperature waste heat gradient utilization analysis method and system, and the method comprises the steps: obtaining the inlet and outlet temperature, flow and composition of all heat flow strands needing to be cooled in an industrial process, calculating the heat capacity flow rate and heat load of each heat flow strand, and drawing the temperature and enthalpy line of each heat flow strand on a temperature and enthalpy diagram; combining all the heat flow strands according to the temperature interval to form a thermal recombination curve; an organic Rankine cycle power generation subsystem is constructed according to the waste heat in the temperature range of 70-150 DEG C, and the power generation efficiency eta orc and the net power generation amount of the organic Rankine cycle power generation subsystem are calculated; a steam compression cycle refrigeration subsystem is built according to the waste heat within the temperature range of 30-70 DEG C, and the refrigeration performance coefficient COP is calculated; and a heat energy recovery subsystem connected with the first condenser and the second condenser is arranged to recover condensation heat released by the first condenser and the second condenser.

Description

Technical Field

[0001] This invention relates to the field of industrial waste heat recovery technology, and in particular to an analysis method and system for the cascade utilization of low-temperature waste heat from 30°C to 150°C. Background Technology

[0002] The industrial sector is a core consumer of energy. According to the *China Energy Statistical Yearbook* and data from the International Energy Agency (IEA), approximately 28%–58% of global industrial energy consumption is lost as waste heat. Of this, low- and medium-temperature waste heat (30℃–150℃) accounts for over 65%, encompassing various forms such as waste heat from chemical plant reactions, metallurgical steel rolling cooling, power plant turbine exhaust, and building material kiln flue gas. This type of waste heat, due to its wide temperature range and significant differences in energy quality, presents a major technical challenge in waste heat recovery. Existing waste heat recovery technologies suffer from problems such as temperature gradient mismatch and severe secondary waste of condensation heat. A waste heat recovery technology that balances temperature gradient utilization, precise flow matching, and condensation heat recovery is needed to address these shortcomings of current technologies.

[0003] The information disclosed in the background section is only for enhancing the understanding of the background of this invention, and therefore may contain information that does not constitute prior art known to those skilled in the art. Summary of the Invention

[0004] To address the shortcomings, this invention provides an analysis method and system for the tiered utilization of low-temperature waste heat from 30℃ to 150℃, overcoming the problems of low utilization efficiency, serious energy waste, and poor system compatibility in the existing technology for waste heat from 30℃ to 150℃.

[0005] A method for analyzing the cascade utilization of low-temperature waste heat from 30℃ to 150℃ includes:

[0006] Step S100: Obtain the inlet and outlet temperatures, flow rates, and compositions of all heat streams requiring cooling in the industrial process; calculate the heat capacity flow rate and heat load of each heat stream; and plot the temperature-enthalpy lines of each heat stream on the temperature-enthalpy diagram; merge all heat streams according to temperature ranges to form a thermal composite curve.

[0007] Step S200: Construct an organic Rankine cycle power generation system for the waste heat in the temperature range of 70℃ to 150℃. The organic Rankine cycle power generation system includes a first evaporator, an expander, a first condenser, and a working fluid pump connected in sequence to the waste heat source at 70℃ to 150℃. The circulating working fluid is R245fa, and the pinch point temperature difference is 10℃. Based on the properties of R245fa, determine the temperature and enthalpy change of the working fluid in the first evaporator to obtain the temperature and enthalpy change curve, and calculate the power generation efficiency η of the organic Rankine cycle power generation system. orc and net electricity generation ;

[0008] Step S300: Construct a vapor compression cycle refrigeration subsystem for the waste heat in the temperature range of 30℃ to 70℃. The vapor compression cycle refrigeration subsystem includes a compressor, a second condenser connected to the waste heat source of 30℃ to 70℃, an expansion valve and a second evaporator connected in sequence. The refrigerant is R134a and the pinch temperature difference is 10℃. Based on the evaporation temperature and pressure of the refrigerant, determine the temperature and enthalpy change curve of the refrigerant in the second evaporator and calculate the coefficient of performance (COP).

[0009] Step S400: Set up a heat recovery subsystem connected to the first condenser and the second condenser to recover the condensation heat released by them.

[0010] In the aforementioned method for analyzing the cascade utilization of low-temperature waste heat from 30℃ to 150℃, in step S100, the relationship between the heat load of the heat flow stream and the inlet and outlet temperatures, flow rates, and composition is expressed as follows:

[0011]

[0012] and

[0013]

[0014] in, The heat load of the heat flow stream is the heat released by the heat flow stream. The heat capacity flow rate of the heat flow stream; The inlet temperature of the hot flow stream; The outlet temperature of the hot flow stream; Component A in the heat flow stream i Traffic; Indicates component A i Specific heat capacity; n represents the number of components in the heat flow stream.

[0015] In the aforementioned method for analyzing the cascade utilization of low-temperature waste heat from 30℃ to 150℃, step S100, which involves merging all heat streams according to temperature ranges to form a thermal composite curve, includes:

[0016] Temperature zones are defined based on the inlet and outlet temperatures of each heat flow stream. The heat flow streams within each temperature zone are then combined to obtain the corresponding composite heat flow. The heat capacity flux and heat load of the composite heat flow and composite cold flow within each temperature zone are the sum of the heat capacity flux and heat load of all heat and cold flow streams present in that respective temperature zone.

[0017]

[0018] in, This represents the heat load of the k-th temperature zone; m is the number of streams in the k-th temperature zone. This represents the heat capacity flow rate of the m-th stream within the k-th temperature range; This is the upper limit temperature of the k-th temperature zone; This is the lower limit temperature of the k-th temperature zone.

[0019] Plot the composite heat flux enthalpy lines in each temperature range on the enthalpy-temperature diagram, and connect them end to end to obtain the thermal composite curve of multiple heat flux streams.

[0020] In the aforementioned method for the cascade utilization of low-temperature waste heat from 30°C to 150°C, in step S200, both the first evaporator and the first condenser are plate heat exchangers; the expander is a magnetic levitation turbine, and the generator efficiency in the organic Rankine cycle is... The efficiency of the working fluid pump is 0.90–0.95. The values ​​are 0.6-0.8, the evaporation temperature is about 65% of the heat source temperature, the condensation temperature is the ambient temperature plus 15℃, the superheat is 5℃, and the subcooling is 3℃.

[0021] In the aforementioned method for the graded utilization of low-temperature waste heat from 30°C to 150°C, in step S200, the temperature-enthalpy change of the working fluid in the first evaporator is determined based on the physical properties of the circulating working fluid. A suitable temperature-enthalpy change curve is determined in the temperature-enthalpy diagram, and the waste heat from 70°C to 150°C is introduced into the first evaporator of the organic Rankine cycle power generation system. The thermal power of the waste heat is... The organic working fluid absorbs heat in the first evaporator and changes from a subcooled liquid to a superheated vapor. The amount of heat absorbed is expressed as follows:

[0022]

[0023] in, The heat absorbed by the organic Rankine cycle in the first evaporator; As residual heat, The heat exchange efficiency of the first evaporator;

[0024] Superheated steam enters the expander and expands to do work. The actual output work is expressed as:

[0025]

[0026] in, This represents the actual output power of the expander; m orc The mass flow rate of the organic working fluid is kg / s; The inlet enthalpy of the expander is expressed in kJ / kg. The actual enthalpy value at the expander outlet, in kJ / kg.

[0027] The expander drives the generator to generate electricity, and the net power generation is:

[0028]

[0029] in, For generator efficiency; The power consumption of the working fluid pump is expressed as:

[0030]

[0031] in, The enthalpy of the working fluid at the pump outlet is kJ / kg. The enthalpy of the working fluid at the inlet of the working fluid pump is expressed in kJ / kg. For the efficiency of the working fluid pump;

[0032] The expanded organic working fluid enters the first condenser for condensation. The condensation pressure corresponds to the saturation pressure of the working fluid, and the released heat is expressed as:

[0033]

[0034] in, The heat is released by the first condenser;

[0035] After being pressurized by the working fluid pump, the working fluid pressure is increased from the condensing pressure to the evaporating pressure, with a pressure ratio of 3-8, and then returns to the first evaporator to complete the organic Rankine cycle;

[0036] The organic Rankine cycle power generation efficiency is expressed as:

[0037] .

[0038] In the aforementioned method for analyzing the cascade utilization of low-temperature waste heat from 30℃ to 150℃, in step S300, the temperature and enthalpy change of the refrigerant in the second evaporator is determined based on the refrigerant's physical properties. A suitable temperature and enthalpy change curve is then determined from the temperature and enthalpy diagram, and the temperature difference between the curve and the pinch point of the heat flow stream is extracted. The waste heat at 30°C to 70°C is introduced into the second evaporator of the vapor compression cycle refrigeration subsystem, and the thermal power of the waste heat is... The refrigerant absorbs heat in the second evaporator and evaporates from a low-pressure gas-liquid two-phase system into low-pressure superheated vapor. The evaporation pressure is... The heat absorbed by the second evaporator is expressed as the saturation pressure at the corresponding temperature.

[0039]

[0040] in, The heat absorbed by the vapor compression cycle in the second evaporator. As residual heat, The heat exchange efficiency of the second evaporator;

[0041] Refrigerant flow rate is expressed as:

[0042]

[0043] in, The mass flow rate of the refrigerant is kg / s; The enthalpy of the refrigerant at the compressor inlet is expressed in kJ / kg. The enthalpy of the refrigerant at the expansion valve outlet is expressed in kJ / kg.

[0044] The evaporated refrigerant enters the compressor, and the compression work is expressed as:

[0045]

[0046] in, For compression work; The enthalpy of the refrigerant at the compressor outlet is expressed in kJ / kg.

[0047] The compressed, high-temperature, high-pressure gas enters the second condenser, and the heat of condensation is expressed as:

[0048]

[0049] in, This is the heat released by the second condenser;

[0050] The condensed refrigerant is throttled by the expansion valve. During the throttling process, the enthalpy remains approximately constant, and the pressure is measured from the outlet pressure of the second condenser. Down to The cycle returns to the second evaporator to complete.

[0051] The coefficient of performance (COP) of a vapor compression cycle is expressed as:

[0052] .

[0053] In the aforementioned method for the graded utilization of low-temperature waste heat from 30°C to 150°C, the refrigerant evaporation temperature in the evaporator is about 5°C lower than the waste heat outlet temperature, the suction superheat is 10°C, and the liquid subcooling is 5°C.

[0054] In the aforementioned method for analyzing the cascade utilization of low-temperature waste heat from 30°C to 150°C, in step S300, both the second evaporator and the second condenser are plate heat exchangers; the compressor is a scroll compressor.

[0055] In the aforementioned method for the tiered utilization of low-temperature waste heat from 30℃ to 150℃, in step S400, the heat recovered by the heat recovery subsystem is expressed as follows:

[0056] ,

[0057] in, For heat recovery efficiency (0.90-0.98);

[0058] When the recovered heat is used for domestic hot water supply, the flow rate will be... cold water from Heat to ,satisfy

[0059]

[0060] in, The specific heat capacity of water is 4.186 kJ / (kg·℃).

[0061] A system for performing the method includes:

[0062] Organic Rankine cycle power generation system, which is used to recover waste heat of 70℃–150℃ to generate electricity, includes R245fa working fluid, first evaporator, magnetic levitation expander, first condenser and working fluid pump;

[0063] The vapor compression cycle refrigeration subsystem, used to recover waste heat of 30℃–70℃ for refrigeration, includes R134a refrigerant, a second evaporator, a scroll compressor, a second condenser, and an expansion valve;

[0064] A heat recovery subsystem, which connects the first condenser and the second condenser, is used to recover condensation heat;

[0065] The control module connects the organic Rankine cycle power generation system and the vapor compression cycle refrigeration subsystem to optimize the flow and temperature matching of the two subsystems based on the enthalpy diagram and thermal composite curve.

[0066] Compared with the prior art, the present invention has the following advantages: The present invention utilizes waste heat from 30°C to 150°C in a tiered manner based on temperature gradients using temperature enthalpy diagrams, organic Rankine cycles and vapor compression cycles, and establishes a quantitative model of "waste heat power - power generation - cooling capacity". By accurately calculating power generation and cooling capacity, the utilization efficiency of low-temperature waste heat is improved. Attached Figure Description

[0067] Various other advantages and benefits of the present invention will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiments below. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. It is obvious that the drawings described below are merely some embodiments of the invention, and those skilled in the art can obtain other drawings based on these drawings without any inventive effort. Furthermore, the same reference numerals denote the same parts throughout the drawings.

[0068] In the attached diagram:

[0069] Figure 1 This is a schematic diagram illustrating the steps of a feasibility analysis method for the cascaded recovery and utilization of low-temperature waste heat from 30°C to 150°C according to an embodiment of this disclosure;

[0070] Figure 2 This is a schematic diagram of the enthalpy of two heat flow streams according to an embodiment of this disclosure;

[0071] Figure 3 This is a schematic diagram of a thermal composite curve formed by merging two heat flow streams according to an embodiment of this disclosure;

[0072] Figure 4 This is a schematic diagram of the temperature and enthalpy of an organic Rankine cycle and vapor compression cycle subsystem according to an embodiment of this disclosure;

[0073] Figure 5 This is a schematic flow diagram of an organic Rankine cycle and vapor compression cycle subsystem according to an embodiment of the present disclosure.

[0074] The present invention will be further explained below with reference to the accompanying drawings and embodiments. Detailed Implementation

[0075] Specific embodiments of the invention will now be described in more detail with reference to the accompanying drawings. While specific embodiments of the invention are shown in the drawings, it should be understood that the invention may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this invention will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0076] It should be noted that certain terms are used in the specification and claims to refer to specific components. Those skilled in the art will understand that different terms may be used to refer to the same component. This specification and claims do not distinguish components based on differences in terminology, but rather on differences in function. The terms "comprising" or "including" used throughout the specification and claims are open-ended and should be interpreted as "comprising but not limited to." The following descriptions are preferred embodiments for carrying out the invention; however, these descriptions are for the purpose of understanding the general principles of the specification and are not intended to limit the scope of the invention. The scope of protection of this invention is determined by the appended claims.

[0077] To facilitate understanding of the embodiments of the present invention, further explanations and descriptions will be provided below with reference to the accompanying drawings and specific embodiments. The accompanying drawings do not constitute a limitation on the embodiments of the present invention.

[0078] like Figures 1 to 5 As shown, the analytical method for the cascade utilization of low-temperature waste heat from 30℃ to 150℃ includes the following steps:

[0079] Step S100: Obtain the inlet and outlet temperatures, flow rates, and compositions of all heat streams requiring cooling in the industrial process; calculate the heat capacity flow rate and heat load of each heat stream; and plot the temperature-enthalpy lines of each heat stream on the temperature-enthalpy diagram; merge all heat streams according to temperature ranges to form a thermal composite curve.

[0080] Step S200: Construct an organic Rankine cycle power generation system for the waste heat in the temperature range of 70℃ to 150℃. The organic Rankine cycle power generation system includes a first evaporator, an expander, a first condenser, and a working fluid pump connected in sequence to the waste heat source at 70℃ to 150℃. The circulating working fluid is R245fa, and the pinch point temperature difference is 10℃. Based on the properties of R245fa, determine the temperature and enthalpy change of the working fluid in the first evaporator to obtain the temperature and enthalpy change curve, and calculate the power generation efficiency η of the organic Rankine cycle power generation system. orc and net electricity generation ;

[0081] Step S300: Construct a vapor compression cycle refrigeration subsystem for the waste heat in the temperature range of 30℃ to 70℃. The vapor compression cycle refrigeration subsystem includes a compressor, a second condenser connected to the waste heat source of 30℃ to 70℃, an expansion valve and a second evaporator connected in sequence. The refrigerant is R134a and the pinch temperature difference is 10℃. Based on the evaporation temperature and pressure of the refrigerant, determine the temperature and enthalpy change curve of the refrigerant in the second evaporator and calculate the coefficient of performance (COP).

[0082] Step S400: Set up a heat recovery subsystem connected to the first condenser and the second condenser to recover the condensation heat released by them.

[0083] In a preferred embodiment of the analytical method for the cascade utilization of low-temperature waste heat from 30℃ to 150℃, in step S100, the relationship between the heat load of the heat flow stream and the inlet and outlet temperatures, flow rates, and composition is expressed as follows:

[0084]

[0085] and

[0086]

[0087] in, The heat load of the heat flow stream is the heat released by the heat flow stream. The heat capacity flow rate of the heat flow stream; The inlet temperature of the hot flow stream; The outlet temperature of the hot flow stream; Component A in the heat flow stream i Traffic; Indicates component A i Specific heat capacity; n represents the number of components in the heat flow stream.

[0088] In a preferred embodiment of the analytical method for the graded utilization of low-temperature waste heat from 30°C to 150°C, step S100, which involves merging all heat streams according to temperature ranges to form a thermal composite curve, includes:

[0089] Temperature zones are defined based on the inlet and outlet temperatures of each heat flow stream. The heat flow streams within each temperature zone are then combined to obtain the corresponding composite heat flow. The heat capacity flux and heat load of the composite heat flow and composite cold flow within each temperature zone are the sum of the heat capacity flux and heat load of all heat and cold flow streams present in that respective temperature zone.

[0090]

[0091] in, This represents the heat load of the k-th temperature zone; m is the number of streams in the k-th temperature zone. This represents the heat capacity flow rate of the m-th stream within the k-th temperature range; This is the upper limit temperature of the k-th temperature zone; This is the lower limit temperature of the k-th temperature zone.

[0092] Plot the composite heat flux enthalpy lines in each temperature range on the enthalpy-temperature diagram, and connect them end to end to obtain the thermal composite curve of multiple heat flux streams.

[0093] In a preferred embodiment of the analytical method for the cascade utilization of low-temperature waste heat from 30°C to 150°C, in step S200, both the first evaporator and the first condenser are plate heat exchangers; the expander is a magnetic levitation turbine, and the generator efficiency in the organic Rankine cycle is... The efficiency of the working fluid pump is 0.90–0.95. The values ​​are 0.6-0.8, the evaporation temperature is about 65% of the heat source temperature, the condensation temperature is the ambient temperature plus 15℃, the superheat is 5℃, and the subcooling is 3℃.

[0094] In a preferred embodiment of the analytical method for the cascade utilization of low-temperature waste heat from 30°C to 150°C, in step S200, the temperature-enthalpy change of the working fluid in the first evaporator is determined based on the physical properties of the circulating working fluid. A suitable temperature-enthalpy change curve is determined in the temperature-enthalpy diagram. Waste heat from 70°C to 150°C is introduced into the first evaporator of the organic Rankine cycle power generation system. The thermal power of the waste heat is... The organic working fluid absorbs heat in the first evaporator and changes from a subcooled liquid to a superheated vapor. The amount of heat absorbed is expressed as follows:

[0095]

[0096] in, The heat absorbed by the organic Rankine cycle in the first evaporator; As residual heat, The heat exchange efficiency of the first evaporator;

[0097] Superheated steam enters the expander and expands to do work. The actual output work is expressed as:

[0098]

[0099] in, This represents the actual output power of the expander; m orc The mass flow rate of the organic working fluid is kg / s; The inlet enthalpy of the expander is expressed in kJ / kg. The actual enthalpy value at the expander outlet, in kJ / kg.

[0100] The expander drives the generator to generate electricity, and the net power generation is:

[0101]

[0102] in, For generator efficiency; The power consumption of the working fluid pump is expressed as:

[0103]

[0104] in, The enthalpy of the working fluid at the pump outlet is kJ / kg. The enthalpy of the working fluid at the inlet of the working fluid pump is expressed in kJ / kg. For the efficiency of the working fluid pump;

[0105] The expanded organic working fluid enters the first condenser for condensation. The condensation pressure corresponds to the saturation pressure of the working fluid, and the released heat is expressed as:

[0106]

[0107] in, The heat is released by the first condenser;

[0108] After being pressurized by the working fluid pump, the working fluid pressure is increased from the condensing pressure to the evaporating pressure, with a pressure ratio of 3-8, and then returns to the first evaporator to complete the organic Rankine cycle;

[0109] The organic Rankine cycle power generation efficiency is expressed as:

[0110] .

[0111] In a preferred embodiment of the analytical method for the graded utilization of low-temperature waste heat from 30°C to 150°C, in step S300, the temperature-enthalpy change of the refrigerant in the second evaporator is determined based on the refrigerant's physical properties. A suitable temperature-enthalpy change curve is determined in the temperature-enthalpy diagram, and the temperature difference between the curve and the pinch point of the heat flow stream is extracted. The waste heat at 30°C to 70°C is introduced into the second evaporator of the vapor compression cycle refrigeration subsystem, and the thermal power of the waste heat is... The refrigerant absorbs heat in the second evaporator and evaporates from a low-pressure gas-liquid two-phase system into low-pressure superheated vapor. The evaporation pressure is... The heat absorbed by the second evaporator is expressed as the saturation pressure at the corresponding temperature.

[0112]

[0113] in, The heat absorbed by the vapor compression cycle in the second evaporator. As waste heat, The heat exchange efficiency of the second evaporator;

[0114] Refrigerant flow rate is expressed as:

[0115]

[0116] in, The mass flow rate of the refrigerant is kg / s; The enthalpy of the refrigerant at the compressor inlet is expressed in kJ / kg. The enthalpy of the refrigerant at the expansion valve outlet is expressed in kJ / kg.

[0117] The evaporated refrigerant enters the compressor, and the compression work is expressed as:

[0118]

[0119] in, For compression work; The enthalpy of the refrigerant at the compressor outlet is expressed in kJ / kg.

[0120] The compressed, high-temperature, high-pressure gas enters the second condenser, and the heat of condensation is expressed as:

[0121]

[0122] in, This is the heat released by the second condenser;

[0123] The condensed refrigerant is throttled by the expansion valve. During the throttling process, the enthalpy remains approximately constant, and the pressure is measured from the outlet pressure of the second condenser. Down to The cycle returns to the second evaporator to complete.

[0124] The coefficient of performance (COP) of a vapor compression cycle is expressed as:

[0125] .

[0126] In a preferred embodiment of the analytical method for the graded utilization of low-temperature waste heat from 30°C to 150°C, the refrigerant evaporation temperature in the evaporator is about 5°C lower than the waste heat outlet temperature, the suction superheat is 10°C, and the liquid subcooling is 5°C.

[0127] In a preferred embodiment of the analytical method for the graded utilization of low-temperature waste heat from 30°C to 150°C, in step S300, both the second evaporator and the second condenser are plate heat exchangers; the compressor is a scroll compressor.

[0128] In a preferred embodiment of the analytical method for the cascade utilization of low-temperature waste heat from 30℃ to 150℃, in step S400, the heat recovered by the heat recovery subsystem is expressed as follows:

[0129] ,

[0130] in, For heat recovery efficiency (0.90-0.98);

[0131] When the recovered heat is used for domestic hot water supply, the flow rate will be... cold water from Heat to ,satisfy

[0132]

[0133] in, The specific heat capacity of water is 4.186 kJ / (kg·℃).

[0134] A system for performing the method includes:

[0135] Organic Rankine cycle power generation system for recovering waste heat at 70°C–150°C to generate electricity, which includes R245fa working fluid, first evaporator, magnetic levitation expander, first condenser and working fluid pump;

[0136] The vapor compression cycle refrigeration subsystem, used to recover waste heat of 30℃–70℃ for refrigeration, includes R134a refrigerant, a second evaporator, a scroll compressor, a second condenser, and an expansion valve;

[0137] A heat recovery subsystem, which connects the first condenser and the second condenser, is used to recover condensation heat;

[0138] The control module connects the organic Rankine cycle power generation system and the vapor compression cycle refrigeration subsystem to optimize the flow and temperature matching of the two subsystems based on the enthalpy diagram and thermal composite curve.

[0139] In one embodiment, an analytical method for the cascade recovery and utilization of low-temperature waste heat from 30°C to 150°C includes:

[0140] Step S100: Extract the supply / target temperature, flow rate, and composition of each heat stream requiring cooling from the circulating cooling water distribution system; extract the inlet and outlet temperatures, flow rates, and composition of each heat stream requiring chilled water cooling from the circulating cooling water distribution system; determine the heat capacity flow rate of each heat stream based on the flow rate, composition, and heat capacity flow rate definition; determine the heat released by each heat stream based on its inlet and outlet temperatures and heat capacity flow rate, denoted as heat load; based on the inlet and outlet temperatures and heat load, each stream is represented by a straight-line enthalpy curve, with the vertical axis corresponding to its two endpoints representing the inlet and outlet temperatures respectively, and the difference in the horizontal axis representing the enthalpy difference to represent the heat load of the stream; merge all heat streams to obtain a thermal composite curve, where the difference in the horizontal axis between each endpoint represents the sum of the heat loads of all heat streams within that temperature range, and represent it on the enthalpy diagram;

[0141] Step S200: The organic Rankine cycle power generation system is used to recover waste heat from 70°C to 150°C for power generation. The organic Rankine cycle power generation system includes a first evaporator, an expander, a first condenser, and a working fluid pump connected in sequence. The first evaporator is connected to a waste heat source from 70°C to 150°C. The circulating working fluid of the organic Rankine cycle power generation system is R245fa. Taking a pinch point temperature difference of 10°C, the temperature and enthalpy change of the working fluid in the first evaporator is determined based on the properties of R245fa. The temperature and enthalpy change curve is determined, and the system power generation efficiency η is calculated. orc and net electricity generation .

[0142] Step S300: The vapor compression cycle refrigeration subsystem is used to recover waste heat from 30°C to 70°C. The vapor compression cycle refrigeration subsystem includes a compressor, a second condenser, an expansion valve, and a second evaporator connected in sequence. The second evaporator is connected to the waste heat source from 30°C to 70°C. The refrigerant in the vapor compression cycle refrigeration subsystem is R134a, and the pinch point temperature difference is taken as 10°C. Based on the evaporation temperature and pressure of the refrigerant, the temperature and enthalpy change of the refrigerant in the second evaporator is determined, the temperature and enthalpy change curve is determined, and its COP is calculated.

[0143] Step S400: The heat recovery subsystem is used to recover and utilize the heat energy in the organic Rankine cycle and vapor compression cycle refrigeration subsystem. The heat recovery subsystem is connected to the first and second condensers to recover heat. The recovered heat is used for domestic hot water supply.

[0144] In one embodiment, heat recovery efficiency It is 0.90-0.98.

[0145] The following analysis of an ethylbenzene device, using existing methods and the scheme described in this disclosure, further illustrates the technical effectiveness of this scheme.

[0146] In step S100, based on the relationship between the inlet and outlet temperatures, flow rates, and composition of the heat transfer streams, the heat released by the heat transfer streams in the circulating water cooling water system is calculated as shown in Table 1. All heat transfer streams are combined to obtain a thermal composite curve, where the difference in the horizontal axis between each endpoint represents the sum of the heat loads of all heat transfer streams within that temperature range, and this curve is represented on a temperature-enthalpy diagram.

[0147] Table 1

[0148]

[0149] In step S200, based on the thermal recombination curve, the working fluid of the organic Rankine cycle electron generating system is R245fa, and the pinch point temperature difference is... The temperature is 10℃; the waste heat from 73℃ to 148℃ with a thermal power of 4562kW is introduced into the first evaporator, and the heat absorbed by the organic working fluid R245fa is:

[0150] =4562kW × 0.96 = 4379.52kW;

[0151] The organic Rankine cycle evaporation temperature was selected as 117℃ (corresponding to an R245fa saturation pressure of 1.8MPa and a superheat of 8℃), and the inlet enthalpy was [not specified]. = -8686.5 kJ / kg; The condensation temperature is selected as ambient temperature +25℃ (assuming an ambient temperature of 25℃ and a condensation temperature of 50℃, corresponding to the enthalpy of saturated liquid). =-8917.84kJ / kg); the working fluid mass flow rate is:

[0152] m orc = / ( - )=4379.52kW / (-8686.5+8917.84)kJ / kg≈18.931kg / s;

[0153] Expander isentropic efficiency =0.85, referring to the R245fa property table, the outlet enthalpy after isentropic expansion is... =-8715.49kJ / kg; Actual outlet enthalpy is:

[0154] = - ×( - )=-8686.5-0.85×(-8686.5+8715.49)=8711.14 kJ / kg;

[0155] The expander output power is:

[0156] = ×( - )=18.931kg / s×(-8686.5+8711.14)kJ / kg≈466.49kW;

[0157] Enthalpy at the inlet of the working fluid pump =-8919.36kJ / kg, outlet enthalpy =-8918.08kJ / kg; Pump power consumption is:

[0158] = ×( - ) / =18.931kg / s×(-8918.08+8919.36)kJ / kg / 0.8≈30.29kW;

[0159] generator efficiency =0.93, net power generation is:

[0160] =( - )× =(466.49-30.29)×0.93≈ 405.67kW;

[0161] The heat of condensation is:

[0162] =4379.52+30.29-405.67kW=4004.14kW

[0163] Organic Rankine cycle power generation efficiency = / 4562kW≈8.89%.

[0164] In step S300, based on the thermal composite curve, the working fluid of the vapor compression cycle refrigeration subsystem is selected as R134a, and the pinch point temperature difference is... The temperature is 10℃; waste heat with a temperature of 35℃ to 73℃ and a thermal power of 1500kW is introduced into the second evaporator, and the cooling capacity is:

[0165] =1500kW × 0.95 = 1425kW;

[0166] The evaporation temperature of R134a is 25℃, and the expansion valve outlet temperature is 20℃. The corresponding mass enthalpy... = -8978.5 kJ / kg, compressor inlet temperature is 60℃, corresponding to mass enthalpy = -8757.66 kJ / kg; refrigerant mass flow rate is:

[0167] = / ( )=1425kW / (-8757.66+8978.5)kJ / kg≈6.45kg / s;

[0168] Compressor inlet enthalpy = -8757.66kJ / kg, isentropic enthalpy at the outlet = -8726.60 kJ / kg; Actual outlet enthalpy is:

[0169] = +( - ) / = -8757.66+(-8726.604+8757.66) / 0.8 ≈-8718.84kJ / kg;

[0170] The compression work is:

[0171] = ×( - )=6.45kg / s×(-8718.84+8757.66)kJ / kg=250.39kW;

[0172] The heat of condensation is: = + =1425+250.39=1675.39kW;

[0173] COP = 1425 / 250.39 ≈ 5.69.

[0174] In step S400, the condensation heat of the first and second condensers is recovered through the heat recovery subsystem, with a recovery efficiency of [missing information]. =0.96, recovered heat =(1675.39+4004.14)kW×0.96≈5452.35kW;

[0175] If this heat is used for domestic hot water supply, heating 15℃ cold water to 55℃ (temperature difference 40℃), then the hot water flow rate is: = / ( ×ΔT)=5452.35kW / (4.186kJ / (kg·℃)×40℃)≈117.2m³ / h;.

[0176] The analytical method for the tiered recovery and utilization of low-temperature waste heat from 30℃ to 150℃ disclosed herein is based on rigorous material and energy balance calculations. By introducing pinch point technology, it performs integrated analysis of low-temperature thermal energy. With the help of temperature-enthalpy diagrams, it intuitively reflects the coupling relationship between the heat flow stream and the circulation subsystem in the system. At the same time, by taking advantage of the difference in physical properties of the circulation medium in the two subsystems, different temperature cut-off points are selected to achieve reasonable tiered utilization of waste heat from 30℃ to 150℃.

[0177] In one embodiment, the inlet and outlet temperatures, flow rates, and composition of the heat streams are extracted to determine the heat released, which is recorded as the heat load. All heat streams are merged to obtain a thermal composite curve, which is then plotted on a temperature-enthalpy diagram. A suitable working fluid flow rate for the circulating subsystem is selected, and the pinch temperature difference between the heat streams and the circulating subsystem is extracted to construct an organic Rankine cycle power generation system. Waste heat from 70°C to 150°C is used to generate electricity, and the system efficiency and net power generation are calculated. A vapor compression cycle refrigeration subsystem is established, using waste heat from 30°C to 70°C to heat the circulating medium. Similarly, the refrigerant flow rate is determined, and its COP is calculated. The heat flow from the organic Rankine cycle power generation system and the vapor compression cycle subsystem is recovered and utilized through a heat recovery subsystem to improve the overall energy utilization efficiency.

[0178] Furthermore, this invention utilizes a pinch analysis method based on enthalpy diagrams and thermal composite curves to achieve systematic integration and quantitative characterization of multiple heat streams within a wide temperature range of 30℃–150℃. By discretizing each heat stream according to temperature intervals and superimposing them to form a continuous thermal composite curve, it not only intuitively reveals the "quantity" and "quality" of recoverable waste heat in different temperature ranges, but also provides a scientific basis for the temperature segmentation of subsequent subsystems (with 70℃ as the boundary), avoiding the problems of "high-quality underutilization" or "low-quality abandonment" caused by crude temperature matching in traditional waste heat recovery.

[0179] Secondly, in the high-temperature range (70–150℃), an Organic Rankine Cycle (ORC) power generation system using R245fa as the working fluid fully leverages the advantages of this working fluid in the medium and low-temperature range, namely high latent heat of vaporization, good thermal stability, and environmentally friendly characteristics. Combining a magnetic levitation turbine expander (no mechanical friction, high speed, high efficiency) with a plate heat exchanger (compact, high heat transfer coefficient), and optimizing the evaporation / condensation temperatures under a 10℃ pinch temperature difference constraint (e.g., evaporation temperature is 65% of the heat source temperature, and condensation temperature is ambient temperature +15℃), the conversion efficiency of low-grade heat energy to electrical energy is significantly improved. Simultaneously, by precisely calculating the working fluid flow rate, expansion work, and pump work, net power generation is maximized.

[0180] Third, a vapor compression cycle is introduced in the low-temperature section (30–70℃) to utilize the previously unusable low-temperature waste heat for refrigerant evaporation and heating. R134a is selected as the refrigerant because it has suitable saturation pressure and high volumetric cooling capacity in the low-temperature evaporation zone. By controlling parameters such as the evaporation temperature being 5℃ lower than the waste heat outlet and the suction superheat being 10℃, the risk of liquid slugging is avoided while ensuring the heat exchange driving force.

[0181] Finally, a unified heat recovery subsystem is set up to centrally recover the waste heat discharged from the condensers of the ORC and refrigeration cycles (with a recovery efficiency of over 90%), which is then used for domestic hot water supply, completely eliminating the secondary energy waste caused by the direct discharge of condensation heat in traditional systems.

[0182] Although embodiments of the present invention have been described above in conjunction with the accompanying drawings, the present invention is not limited to the specific embodiments and application fields described above. The specific embodiments described above are merely illustrative and instructive, and not restrictive. Those skilled in the art can make many other forms based on the guidance of this specification and without departing from the scope of protection of the claims of the present invention, and all of these are within the scope of protection of the present invention.

Claims

1. A method for low temperature waste heat cascade utilization analysis at 30-150℃, characterized in that, It comprises the following steps: Step S100: obtaining the inlet and outlet temperatures, flow rates and compositions of all hot streams to be cooled in the industrial process, calculating the heat capacity flow rates and heat loads of the hot streams, and drawing the temperature-enthalpy lines of the hot streams on a temperature-enthalpy diagram; merging all the hot streams according to temperature intervals to form a heat composite curve; Step S200: Constructing an organic Rankine cycle power generation system for waste heat in the temperature range of 70-150℃, which comprises a first evaporator connected with the waste heat source of 70-150℃, an expander, a first condenser and a working fluid pump connected in sequence, the circulating working fluid is R245fa, the pinch temperature difference is 10℃, the temperature-enthalpy change curve is determined according to the temperature-enthalpy change of the working fluid in the first evaporator determined according to the properties of R245fa, and the power generation efficiency η of the organic Rankine cycle power generation system is calculated orc and the net power generation ; Step S300: constructing a steam compression cycle refrigeration subsystem for waste heat in the temperature interval of 30-70℃, the steam compression cycle refrigeration subsystem comprising a compressor, a second condenser connected to the waste heat source of 30-70℃, an expansion valve and a second evaporator connected in sequence, the refrigerant being R134a, the pinch point temperature difference being 10℃, the temperature-enthalpy change curve of the refrigerant in the second evaporator being determined according to the evaporation temperature and pressure of the refrigerant, and the coefficient of performance COP being calculated; Step S400: setting a heat energy recovery subsystem connected to the first and second condensers to recover the condensation heat released by the two.

2. The method according to claim 1, wherein the temperature range is from 30 °C to 150 °C. Preferably, in step S100, the relationship between the heat load of the hot stream and the inlet and outlet temperatures, flow rates and compositions is expressed as: ； and ； where is the heat flux heat duty, i.e. the heat emitted by the heat flux; is the heat capacity flow rate of the heat flux; is the inlet temperature of the heat flux; is the outlet temperature of the heat flux; is the flow rate of component A i in the heat flux; denotes the specific heat capacity of component A i ; n denotes the number of components in the heat flux.

3. The method according to claim 1, wherein the temperature range is from 30 °C to 150 °C. In step S100, the step of merging all the hot streams according to temperature intervals to form a heat composite curve comprises: dividing temperature intervals according to the inlet and outlet temperatures of the hot streams; merging the hot streams in each temperature interval to obtain a corresponding composite hot stream, the heat capacity flow rates and heat loads of the composite hot stream and composite cold stream in each temperature interval being the sum of the heat capacity flow rates and heat loads of all the hot streams and cold streams existing in the corresponding temperature interval, i.e. ； wherein, Hkrepresents the heat load of the kth temperature zone; m is the number of streams in the kth temperature zone; Ckmrepresents the heat capacity flow rate of the mth stream in the kth temperature zone; Tkmaxrepresents the upper limit temperature of the kth temperature zone; Tkminrepresents the lower limit temperature of the kth temperature zone, drawing the temperature-enthalpy lines of the composite hot streams in each temperature interval on a temperature-enthalpy diagram, and connecting the beginning and end of the lines to obtain a heat composite curve of multiple hot streams.

4. The method according to claim 1, wherein the temperature range is from 30°C to 150°C. In step S200, the first evaporator and the first condenser are both plate heat exchangers; the expander is a magnetic suspension turbine, and the efficiency of the generator in the organic Rankine cycle is 0.90-0.95, the efficiency of the working medium pump is 0.6-0.8, the evaporation temperature is about 65% of the temperature of the heat source, the condensation temperature is the ambient temperature plus 15℃, the superheat is 5℃, and the subcooling is 3℃. 0.90-0.95, the efficiency of the working medium pump is 0.6-0.8, the evaporation temperature is about 65% of the temperature of the heat source, the condensation temperature is the ambient temperature plus 15℃, the superheat is 5℃, and the subcooling is 3℃.

5. The method according to claim 1, wherein the temperature range is from 30°C to 150°C. In step S200, the temperature-enthalpy change of the working medium in the first evaporator is determined according to the properties of the circulating working medium, a suitable temperature-enthalpy change curve is determined in a temperature-enthalpy diagram, and the waste heat with a temperature of 70-150°C and a thermal power of After the organic working medium absorbs heat in the first evaporator, it changes from a supercooled liquid to a superheated vapor, and the absorption amount is represented as: ； wherein, Qabsorbed is the heat absorbed by the organic Rankine cycle in the first evaporator; Qresidual is the residual heat, η is the heat exchange efficiency of the first evaporator; The superheated steam enters the expander to expand and do work, and the actual output work is expressed as: ； wherein, is the actual output power of the expander; m orc is the mass flow of the organic working fluid, kg / s; is the enthalpy at the inlet of the expander, kJ / kg; is the actual enthalpy at the outlet of the expander, kJ / kg, The expander drives the generator to generate electricity, and the net power generation is: ； wherein, is the generator efficiency; is the work consumed by the working fluid pump, expressed as: ； wherein, Hout is the working fluid pump outlet working fluid enthalpy, kJ / kg; Hin is the working fluid pump inlet working fluid enthalpy, kJ / kg; ηp is the working fluid pump efficiency; The expanded organic working medium enters the first condenser to condense, and the condensation pressure corresponds to the saturation pressure of the working medium, and the released heat is expressed as: ； wherein, Qcondis the heat rejected by the first condenser; After being pressurized by the working medium pump, the working medium pressure is raised from the condensation pressure to the evaporation pressure, and the pressure ratio is 3-8, and then it returns to the first evaporator to complete the organic Rankine cycle; The organic Rankine cycle efficiency is expressed as: 。 6. The method according to claim 1, wherein the temperature range is from 30°C to 150°C. In step S300, the enthalpy change of the refrigerant in the second evaporator is determined according to the physical properties of the refrigerant, a suitable enthalpy change curve is determined in the enthalpy diagram, and the temperature difference between the heat flow and the pinch point is extracted introducing waste heat of 30-70°C into the second evaporator of the vapor compression refrigeration subsystem, the thermal power of the waste heat absorbing heat in the second evaporator to evaporate the refrigerant from a low-pressure gas-liquid two-phase to a low-pressure superheated vapor, the evaporation pressure is the saturation pressure at the corresponding temperature, and the heat absorbed by the second evaporator is represented by ； wherein Qabs is the heat absorbed by the vapor compression cycle in the second evaporator, Qres is the residual heat, η2 is the heat exchange efficiency of the second evaporator; The refrigerant flow rate is expressed as: ； wherein, is the mass flow rate of refrigerant, kg / s; is the compressor inlet refrigerant enthalpy, kJ / kg; is the expansion valve outlet refrigerant enthalpy, kJ / kg; The evaporated refrigerant enters the compressor, and the compression work is expressed as: ； wherein, Pcomp is the compressor power; hout is the compressor outlet refrigerant enthalpy, kJ / kg; The compressed high-temperature and high-pressure gas enters the second condenser, and the condensation heat is expressed as: ； wherein, Qcond2is the heat rejected by the second condenser; The condensed refrigerant is throttled by an expansion valve, and the enthalpy value of the throttling process is approximately constant, and the pressure drops from the second condenser outlet pressure to the second evaporator to complete the cycle. The performance coefficient COP of the steam compression cycle is expressed as: 。 7. The method according to claim 6, wherein the temperature range is from 30 °C to 150 °C. The evaporation temperature of the refrigerant in the evaporator is about 5℃ lower than the outlet temperature of the waste heat, the suction superheat degree is 10℃, and the liquid subcooling degree is 5℃.

8. The method according to claim 1, wherein the temperature range is from 30°C to 150°C. In step S300, the second evaporator and the second condenser are both plate heat exchangers; and the compressor is a scroll compressor.

9. The method according to claim 1, wherein the temperature range is from 30°C to 150°C. In step S400, the heat energy recovery subsystem recovers heat expressed as: ； wherein, is the heat recovery efficiency (0.90-0.98); The recovered heat is used for domestic hot water supply, the flow of cold water is heated from to to satisfy ； wherein, Cp is the specific heat capacity of water, 4.186 kJ / (kg °C).

10. A system for performing the method of any one of claims 1-9, characterized in that, It comprises: An organic Rankine cycle power generation subsystem for recovering 70-150℃ waste heat to generate electricity, which comprises an R245fa working medium, a first evaporator, a magnetic suspension expander, a first condenser and a working medium pump; A steam compression cycle refrigeration subsystem for recovering 30-70℃ waste heat to generate refrigeration, which comprises an R134a refrigerant, a second evaporator, a scroll compressor, a second condenser and an expansion valve; a thermal energy recovery subsystem connected between the first condenser and the second condenser for recovering condensation heat; a control module connected to the organic Rankine cycle power generation subsystem and the vapor compression cycle refrigeration subsystem for optimizing flow and temperature matching of the two subsystems based on a temperature-enthalpy diagram and a thermal composite curve.

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