CN116798542B - Refrigerant selection and process optimization integrated design method for cascade refrigeration system - Google Patents
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Abstract
The invention discloses a refrigerant selection and process optimization integrated design method of an cascade refrigeration system. The invention also provides an optimization solving method based on model decomposition, which can stably obtain the optimal refrigerant combination in a shorter time. Based on the obtained optimal refrigerant combination and process operation conditions, the cascade refrigeration system can be operated under the optimal conditions, and the performance of the refrigeration system in the practical application scene, such as energy efficiency, can be improved.
Description
Technical Field
The invention relates to a computer-aided product-process integrated design of an cascade refrigeration system, in particular to a refrigerant selection and process optimization integrated design method for the cascade refrigeration system.
Background
With the development of industry and the increasing awareness of environmental protection, there is an increasing need for more efficient and environmentally friendly chemicals. Meanwhile, with the rapid development of computer technology, methods of computer aided design are used in more and more fields.
The computer aided design is to use computer modeling and operation optimizing algorithm to optimize the compound product or process operation condition based on some artificially selected objective function, so that the compound product or industrial process is in optimal state and the industrial requirement is met to the greatest extent. The objective function includes energy consumption, economic cost or environmental indexes which are gradually paid attention to. Computer aided design can select the best solution from a number of product or process alternatives, one of which is to aid in the experiment. Especially for some experiments with high cost or very time consumption, the design of the scheme is finished in the computer in advance, so that the trial-and-error times of the real experiment can be reduced, the experiment time is greatly shortened, and the experiment cost is greatly reduced.
Computer-aided design includes computer-aided product design and computer-aided process design. The optimal compound obtained by product design is finally used as working medium in the process, and the optimal working medium corresponding to different process demands is often different, so that a certain compound product cannot be considered optimal from the process demands in a general way. Meanwhile, the design of the process cannot be made without considering the influence of the working substance on the process performance, and therefore, it is necessary to integrate the product design with the process design.
Integration of product design and process design, some researchers have conducted related research. However, the previous integrated designs are still dominated by sequential integrated designs. That is, in the first step, the product is designed, and the designed optimal product is used in a system; in a second step, the system operating conditions are optimized in the case of using such an optimal product. The integration of product design and process design in this manner, sequentially following the idea of designing the product first and then designing the process, is called sequential integrated design. Although the optimal product and process can be designed integrally, the core of the design method is to respectively optimize the product design and the process design, and the method belongs to a greedy algorithm, and can easily miss the globally optimal solution.
With the development of technology and the improvement of the living standard of people, the refrigerant plays an increasingly important role in our lives. Many scenarios from home air conditioning to industrial refrigeration systems leave out excellent performance refrigerants.
Researchers have made many experiments to investigate various properties of some alternative potential refrigerants. However, the experimental investigation of various potential refrigerants is time consuming and labor intensive and very costly. Therefore, it is necessary to design the refrigerant using a computer-aided design method.
The former have made some single computer-aided refrigerant designs or single computer-aided refrigeration process designs and have achieved good results. However, considering that the optimization of the refrigeration process and the design of the refrigerant are mutually influencing, for example, the optimal refrigerant may be different for different refrigeration requirements, and even for the same refrigerant, the refrigeration performance may be different in different refrigeration processes. Therefore, it is necessary to integrate the design of the refrigerant and the design of the refrigeration process. Furthermore, the technology of product-process integrated design of cascade refrigeration systems is currently very rare, i.e. multiple compound products are designed simultaneously, which are used in combination in one industrial system to optimize the performance index of the system.
Disclosure of Invention
The invention aims to provide a refrigerant product selection and refrigeration process integrated design method of an overlapping refrigeration system based on an operation optimization technology.
The technical scheme adopted by the invention is as follows:
the refrigerant selection and process optimization integrated design method of the cascade refrigeration system is characterized by comprising the following steps of:
constructing a refrigerant selection and process optimization integrated model according to the number m of the cascade refrigeration system to be designed; the refrigerant selection and process optimization integrated model is formed by combining an objective function, a single-stage refrigeration cycle model of each stage cycle constructed based on a refrigerant molecule selection model and constraint conditions between stages added on the basis; the refrigerant molecule selection model represents physical parameters of one refrigerant selected from the candidate refrigerant library in each stage of circulation; the single-stage refrigeration cycle model obtains an evaporator outlet enthalpy value of the jth stage cycle, a compressor outlet enthalpy value and a condenser outlet enthalpy value according to the input evaporation temperature in the evaporator of the jth stage cycle, the condensation temperature in the condenser and physical property parameters of the refrigerant; the constraint conditions include: the condensation temperature of the latter cycle must be greater than the evaporation temperature of the former cycle; the condensation temperature of the stage 1 loop must be greater than the cooling water temperature; the condensing temperature of each stage cycle must be less than the critical temperature of the refrigerant used in that stage cycle; the evaporation temperature of the mth stage cycle must be less than the ambient temperature to be refrigerated; the refrigerant boiling point of the j-th stage cycle must be less than the condensation temperature of the j+1-th stage cycle; the refrigerant boiling point of the mth stage cycle must be less than ambient temperature and the heat released by the j+1th stage cycle is equal to the heat absorbed by the j stage cycle;
searching an alternative refrigerant library, and optimizing the refrigerant selection and process optimization integrated model through an optimization objective function to obtain the optimal refrigerant combination and process operation conditions of the cascade refrigeration system to be designed.
Further, the physical property parameters of the refrigerant include each parameter in the relationship of the boiling point, critical temperature and specific heat capacity temperature of the refrigerant.
Further, the single-stage refrigeration cycle model is represented as follows:
where j is the series index, j=1, 2 …, m, m is the total number of stages of the cascade refrigeration system to be designed;respectively representing the outlet enthalpy value of an evaporator, the outlet enthalpy value of a compressor and the outlet enthalpy value of a condenser of a j-th-stage cycle of the cascade refrigeration system to be designed; />Respectively representing the evaporating temperature in the evaporator and the condensing temperature in the condenser of the j-th stage cycle of the cascade refrigeration system to be designed; />Indicating the selection of the refrigerant corresponding to the j-th stage cycle of the cascade refrigeration system to be designed.
Further, the objective function is a refrigeration coefficient, a refrigeration capacity or an operation cost of the cascade refrigeration system to be designed.
Further, the process operating conditions include an evaporation temperature in each stage of evaporator and a condensation temperature in the condenser.
Further, the refrigerant molecular selection model adopts Boolean variable to represent the selection condition of the refrigerant.
Further, the searching of the candidate refrigerant library optimizes the refrigerant selection and process optimization integrated model through an optimization objective function to obtain an optimal refrigerant combination and process operation conditions of the cascade refrigeration system to be designed, specifically:
step 1: searching the candidate refrigerant library to obtain a group of refrigerant combinations conforming to feasible solutions of the first sub-model; the boiling point of the first sub-model from the former-stage refrigerant must be smaller than the critical temperature of the latter-stage refrigerant; the cooling water temperature must be less than the critical temperature of the first stage cycle and the boiling point of the mth stage refrigerant must be less than the constraint condition of the refrigeration environment temperature to be reached;
step 2: optimizing the second sub-model through the optimized objective function according to the refrigerant combination obtained in the step 1, judging whether a better objective function is found, if so, updating the refrigerant combination and the process operation condition, otherwise, iteratively executing the steps 1-2 until the search of all the refrigerant combinations of the alternative refrigerant library is completed, and obtaining the optimal refrigerant combination and the process operation condition of the cascade refrigeration system to be designed;
the second sub-model consists of an objective function, m single-stage refrigeration cycle models and a condensation temperature of a next-stage cycle, wherein the condensation temperature of the second sub-model is required to be larger than the evaporation temperature of a previous-stage cycle; the condensation temperature of the stage 1 loop must be greater than the cooling water temperature; the condensing temperature of each stage cycle must be less than the critical temperature of the refrigerant used in that stage cycle; the evaporation temperature of the mth stage cycle must be less than the ambient temperature to be refrigerated and the boiling point of the refrigerant of the j stage cycle must be less than the condensation temperature of the j+1th stage cycle.
An operation control method of a cascade refrigeration system performs operation control according to an optimal refrigerant combination and process operation conditions obtained by the design of a refrigerant selection and process optimization integrated design method of the cascade refrigeration system.
Compared with the prior art, the invention has the beneficial effects that:
1. the integrated design method provided by the invention can design an optimal refrigerant for each stage of the cascade refrigeration system at the same time, and simultaneously determine the optimal process operation condition of each stage;
2. aiming at the optimization complexity of the integrated design model, the invention provides an optimization solving method based on model decomposition aiming at the model, which can stably find the optimal solution in a shorter time.
3. Based on the obtained optimal refrigerant combination and the process operation conditions, the cascade refrigeration system can be operated under the optimal conditions, which is beneficial to improving the energy efficiency of the refrigeration system in the actual use scene, reducing the operation cost or improving the environmental protection of the refrigeration system.
Drawings
FIG. 1 is a schematic diagram of an m-stage cascade refrigeration system;
FIG. 2 is a schematic diagram of a single stage refrigeration system;
FIG. 3 is a flow chart of the optimization solving method of the present invention.
Detailed Description
Fig. 1 is a schematic structural diagram of an m-stage cascade refrigeration system, and as shown in fig. 1, the m-stage cascade refrigeration system is composed of m single-stage refrigeration cycles, each stage of cycle uses one refrigerant, and the refrigerant flows independently. The evaporator of the j-th stage refrigeration cycle and the condenser of the j+1-th stage refrigeration cycle are commonly referred to as a condensation evaporator. The mth-stage refrigerant evaporates and absorbs the heat of the refrigeration environment in the evaporator of the mth-stage circulation and flows to the condensation evaporator. The refrigerant of the j-th cycle evaporates on one side of the condensing evaporator, while the refrigerant of the j+1-th cycle condenses on the other side of the condensing evaporator, so that the j-th refrigerant absorbs heat released from the condensation of the j+1-th refrigerant, and transfers the heat to the previous stage in the condenser of the j-th cycle, and finally transfers the heat to a cold source (shown as cooling water in the figure) in the condenser of the first stage, thus completing the cascade refrigeration of the environment.
From the temperature drop point of view, the m-stage cascade refrigeration system uses the heat source (refrigeration environment) temperatureTemperature of cold source->The temperature interval between them is divided into m temperature sub-intervals, i.e. +.>. Wherein->Is an intermediate temperature, in case of neglecting the heat transfer temperature difference between stages +.>The evaporation temperature of the j-th stage cycle and the condensation temperature of the j+1th stage cycle. At the heat source temperature->Temperature of cold source->In the fixed case, the refrigeration efficiency of the cascade refrigeration system will follow the m-1 intermediate temperatures +.>Is a modification of (a)Changes and changes, thus->Is the m-1 decision variables of the whole system, also the operating conditions of the process, i.e. there is an optimum intermediate temperature +.>So that the refrigerating efficiency of the system is the highest.
From the refrigerant perspective, m kinds of refrigerants are used in m-level circulation, and the refrigeration efficiency of different refrigerants is different for different temperature intervals, so that changing the kind of the refrigerants used in each level circulation also changes the refrigeration efficiency of the cascade refrigeration system. Thus, it is known that the m refrigerant types used in each stage of circulation are also decision variables of the whole system, namely that the optimal refrigerant combination exists, so that the refrigeration efficiency of the system can be maximized.
On the one hand, the intermediate temperature is changedThe decision of the corresponding optimal refrigerant combination will change; on the other hand, the decision to change the refrigerant combination, and the corresponding optimal intermediate temperature, will also change. The optimization of the two decision variables, intermediate temperature and refrigerant combination, is therefore coupled to each other, and therefore they must be considered as a system as a whole to be optimized in order to obtain both an optimal refrigerant combination and an optimal process operating condition-intermediate temperature.
The technical scheme of the invention is further described below with reference to the attached drawings and specific embodiments.
The invention provides a refrigerant selection and process optimization integrated design method of an cascade refrigeration system, which comprises the following steps:
firstly, constructing a refrigerant selection and process optimization integrated model according to the number m of stages of a cascade refrigeration system to be designed; the refrigerant selection and process optimization integrated model is formed by combining an objective function, a single-stage refrigeration cycle model of each stage cycle constructed based on a refrigerant molecule selection model and constraint conditions between stages added on the basis; specifically, the refrigerant molecule selection model represents a physical property parameter of a refrigerant selected from the candidate refrigerant library, and various physical property parameters of all the candidate refrigerants, such as boiling point, critical temperature, and the like, are recorded in the candidate refrigerant library. Each candidate refrigerant in the candidate refrigerant library is given a boolean variable u with a value of 0 or 1, wherein the boolean variable is used for indicating whether a certain refrigerant is selected or not, 0 is not selected, 1 is selected, and a constructed refrigerant molecule selection model is shown in an equation (1):
(1)
wherein n represents the total number of candidate refrigerants,parameters in the empirical correlation of specific heat capacity and temperature of the liquid of the refrigerant, respectively, +.>Parameters in the empirical correlation of specific heat capacity and temperature of the refrigerant, respectively, +.>Parameters in the empirical correlation of evaporation enthalpy and temperature of the refrigerant, respectively, +.>Represents boiling point, & lt + & gt>Indicating the critical temperature, the subscript i indicates the refrigerant number, wherein,the value is typically zero.
Since the m-stage cascade refrigeration system is formed by cascading m single-stage refrigeration cycles, a model of the single-stage refrigeration system needs to be built first. A block diagram of a single stage refrigeration system is shown in fig. 2. By mechanism analysis of single-stage refrigeration system and refrigerant callingThe three physical properties of liquid specific heat capacity, gas specific heat capacity and evaporation enthalpy are empirically related to temperature, so that a single-stage refrigeration CYCLE model CYCLE constructed based on a refrigerant molecule selection model can be obtained, and the single-stage refrigeration CYCLE model CYCLE is constructed according to the evaporation temperature T in an input evaporator 1 And condensation temperature T in the condenser 3 And refrigerant selectionOutput to obtain evaporator outlet enthalpy value h 1 Compressor outlet enthalpy h 2 Enthalpy value h of condenser outlet 3 Abbreviated as:
the details are as follows:
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
wherein T represents the temperature and the temperature of the mixture,and->Respectively represents the specific heat capacity of liquid and the specific heat capacity of gas, < ->Indicating the enthalpy of evaporation,and->Indicating the evaporation enthalpy at evaporation temperature and condensation temperature, respectively,/->And->Respectively, the entropy of the refrigerant at the inlet and outlet of the compressor, tr represents the reference temperature, 233.15K is generally taken, +.>And->Respectively representing reference entropy and reference enthalpy, wherein 0 is taken in the invention, and sinh and cosh respectively represent hyperbolic sine function and hyperbolic cosine function. Equations (2), (3) and (4) represent empirical correlations of liquid specific heat capacity, gas specific heat capacity and evaporation enthalpy with temperature, respectively, all three empirical correlations and the correlated parameter data set used in the test cases of the present invention are from computer aided chemical engineering design software ica s (Gani R, hysoft G, jaksland C, jensen AK. An integrated computer aided system for in)tegrated design of chemical processes, vol. 21.1997.).
Further, the single-stage refrigeration cycle model of each stage of the cascade refrigeration system is identical to the model of the single-stage refrigeration system, and therefore, for the j-th stage refrigeration cycle of the cascade refrigeration system, the following model is satisfied:
(11)
unlike single stage refrigeration systems, cascade refrigeration systems have stage-to-stage constraints, which are specifically as follows:
(12)
represents the condensing temperature of the j+1st stage refrigeration cycle, < >>Indicating the evaporation temperature of the j-th stage refrigeration cycle,representing the heat transfer temperature difference in the condensing evaporator. The constraint of formula (12) indicates that the condensation temperature of the subsequent stage must be greater than the evaporation temperature of the previous stage so that the heat released by the condensation of the refrigerant in the subsequent stage condenser is absorbed by the evaporation of the refrigerant in the previous stage in the evaporator.
(13)
Represents the cooling water temperature (water cooling is an example)>Indicating cooling water side heat transferThe temperature difference, constraint of equation (13), indicates that the condensation temperature of the stage 1 refrigeration cycle must be greater than the cooling water temperature so that the heat absorbed by the refrigeration system can be released to the cooling water.
(14)
Representing the critical temperature of the refrigerant of the j-th stage, the constraint of formula (14) represents that the condensing temperature of each stage must be less than the critical temperature of the refrigerant used in the stage, so that it is possible to prevent the refrigerant from being unable to switch between the gas-liquid two phases due to exceeding the critical temperature.
(15)
Represents the evaporation temperature of the mth stage (stage with lowest temperature) refrigeration cycle, +.>Indicating the temperature of the environment to be refrigerated, +.>Indicating the ambient side heat transfer temperature difference. The constraint of equation (15) indicates that the evaporation temperature of the mth stage cycle must be less than the temperature of the environment to be refrigerated so that the refrigeration system can absorb heat from the environment to cool the environment.
(16)
(17)
The constraint of formula (16) represents the boiling point of the refrigerant in the j-th stageMust be less than the condensing temperature of the j+1th stage refrigeration cycle, and equation (17) indicates that the boiling point of the refrigerant of the mth stage must be less than the ambient temperature. The two-way constraint is to avoid excessive vacuum in the evaporators at each stage, thereby reducing maintenance costs for the equipment.
(18)
Indicating the refrigerant mass flow of the j-th stage cycle. The constraint of formula (18) is a thermal equilibrium constraint of the condensing evaporator, meaning that the heat released by the j+1th stage condenser is equal to the heat absorbed by the j-th stage evaporator, i.e., the heat released by the j+1th stage refrigeration cycle is fully absorbed by the j-th stage refrigeration cycle (ignoring the heat lost to the air).
The objective function is exemplified by the coefficient of refrigeration COP. The COP of the cascade refrigeration system is calculated as follows:
（19）
thus, the refrigerant selection and process optimization integrated design model for the entire cascade refrigeration system can be expressed as follows:
maximizing the objective function COP
Constraint conditions:
single stage refrigeration cycle model for each stage cycle:
constraint from stage to stage:
this is a mixed integer nonlinear programming model, the optimization variables are the condensing temperature of each stageAnd vaporization temperatureAnd selection of refrigerant per stage +.>。
After the refrigerant selection and process optimization integrated design model is completed, searching an alternative refrigerant library, and optimizing the refrigerant selection and process optimization integrated model by using an optimization objective function to obtain the optimal refrigerant combination and process operation condition of the cascade refrigeration system to be designed.
Advancing oneIn the step, because the model is a highly nonlinear and non-convex mixed integer nonlinear programming model, the direct calling of the optimization solver solves very unstable, and a feasible solution can not be obtained in many times, only one relaxation solution can be obtained, and even if the feasible solution can be obtained, the local extremum is very easy to converge. The present invention is therefore based on refrigerant selection and selection of refrigerant in a process optimization integrated modelIs a discrete variable, is finite for all possible refrigerant combinations, and can be exhausted, but the characteristic that the exhaustion calculation time is too long for all the refrigerant combinations is unacceptable, the searching process of the refrigerant combinations is combined with part of the process mechanism constraint of the cascade refrigeration system to carry out conditional restriction, namely, the refrigerant combinations which do not accord with the process mechanism constraint can be eliminated before the process optimization is carried out, thereby reducing a considerable proportion of the process optimization and greatly shortening the optimization time. Moreover, the global search is performed on the refrigerant combination, so that the globally optimal refrigerant combination can be obtained. Specifically, searching the candidate refrigerant library, and optimizing and solving the refrigerant selection and process optimization integrated model by using an optimization objective function, wherein the flow of the method is shown in fig. 3, and the refrigerant selection and process optimization integrated model constructed by the cascade refrigeration system is split into two sub-models, and the method comprises the following steps:
step 1: first, the first sub-model 1 is operated, if a feasible solution (feasible refrigerant combination) conforming to the first sub-model 1 is obtained, the method can enter step 2 to optimize the refrigeration process, and if the refrigerant combination does not conform to the first sub-model 1, the method is directly skipped, and the next group of refrigerant combinations is searched. The boiling point of the first sub-model 1 from the former stage refrigerant must be smaller than the critical temperature of the latter stage refrigerant，/>The method comprises the steps of carrying out a first treatment on the surface of the The cooling water temperature must be less than the critical temperature of the first stage cycleAnd the boiling point of the m-th stage refrigerant must be less than +.>Is composed of constraint conditions of (1);
step 2: and (3) optimizing the second submodel 2 according to the refrigerant combination obtained in the step (1) by using an optimized objective function, judging whether a better objective function is found, if so, updating the refrigerant combination and the process operation condition, otherwise, iteratively executing the steps (1) to (2) until the search of all the refrigerant combinations in the alternative refrigerant library is completed, and obtaining the optimal refrigerant combination and the process operation condition of the cascade refrigeration system to be designed. The second sub-model 2 is composed of the target function COP and the whole integrated design model exceptAll other constraints (formula (12) -formula (16) and formula (18)) are included.
Tables 1-6 set forth the optimum design results of the above method in two-stage and three-stage cascade refrigeration systems, respectively. An alternative refrigerant library for use herein is a set of 294 pure compounds containing all the physical parameters required for each compound.
Table 1 design results of two-stage cascade refrigeration System (direct call general optimizer)
Table 2 design results (exhaustive) of two-stage cascade refrigeration systems
Table 3 design results of two-stage cascade refrigeration systems (optimized solution method of the invention)
Table 4 design results of three-stage cascade refrigeration system (direct call general optimizer)
Table 5 design results (exhaustive) of three-stage cascade refrigeration systems
Table 6 design results of three-stage cascade refrigeration system (optimized solution method of the invention)
/>
It can be seen from tables 1-6 that the method proposed by the present invention does allow to select an optimal refrigerant for each stage of the cascade refrigeration system, so that the COP of the system as a whole is maximized. Compared with the optimization effect of directly calling the optimizer, the optimization solving method provided by the invention has the advantage of being more stable; compared with an exhaustion method, the optimization solving method provided by the invention can greatly shorten the optimization time while ensuring that the optimal solution can be found, and can respectively save 89.39% and 98.58% of time on average for two-stage and three-stage cascade refrigeration systems.
It is apparent that the above examples are given by way of illustration only and are not limiting of the embodiments. Other variations or modifications of the above teachings will be apparent to those of ordinary skill in the art. It is not necessary or exhaustive of all embodiments. And obvious variations or modifications thereof are contemplated as falling within the scope of the present invention.
Claims (7)
1. The refrigerant selection and process optimization integrated design method of the cascade refrigeration system is characterized by comprising the following steps of:
constructing a refrigerant selection and process optimization integrated model according to the number m of the cascade refrigeration system to be designed; the refrigerant selection and process optimization integrated model is formed by combining an objective function, a single-stage refrigeration cycle model of each stage cycle constructed based on a refrigerant molecule selection model and constraint conditions between stages added on the basis; the refrigerant molecule selection model represents physical parameters of one refrigerant selected from the candidate refrigerant library in each stage of circulation; the single-stage refrigeration cycle model obtains an evaporator outlet enthalpy value of the jth stage cycle, a compressor outlet enthalpy value and a condenser outlet enthalpy value according to the input evaporation temperature in the evaporator of the jth stage cycle, the condensation temperature in the condenser and physical property parameters of the refrigerant; the constraint conditions include: the condensation temperature of the latter cycle must be greater than the evaporation temperature of the former cycle; the condensation temperature of the stage 1 loop must be greater than the cooling water temperature; the condensing temperature of each stage cycle must be less than the critical temperature of the refrigerant used in that stage cycle; the evaporation temperature of the mth stage cycle must be less than the ambient temperature to be refrigerated; the refrigerant boiling point of the j-th stage cycle must be less than the condensation temperature of the j+1-th stage cycle; the refrigerant boiling point of the mth stage cycle must be less than ambient temperature and the heat released by the j+1th stage cycle is equal to the heat absorbed by the j stage cycle;
searching an alternative refrigerant library, optimizing the refrigerant selection and process optimization integrated model through an optimization objective function to obtain an optimal refrigerant combination and process operation conditions of the cascade refrigeration system to be designed, wherein the process operation conditions comprise the evaporation temperature in each stage of evaporator and the condensation temperature in the condenser.
2. The method of claim 1, wherein the physical properties of the refrigerant include parameters of refrigerant boiling point, critical temperature, and specific heat capacity temperature correlation.
3. The method of claim 1, wherein the single stage refrigeration cycle model is represented as follows:
；
where j is the series index, j=1, 2 …, m, m is the total number of stages of the cascade refrigeration system to be designed;respectively representing the outlet enthalpy value of an evaporator, the outlet enthalpy value of a compressor and the outlet enthalpy value of a condenser of a j-th-stage cycle of the cascade refrigeration system to be designed; />Respectively representing the evaporating temperature in the evaporator and the condensing temperature in the condenser of the j-th stage cycle of the cascade refrigeration system to be designed; />Indicating the selection of the refrigerant corresponding to the j-th stage cycle of the cascade refrigeration system to be designed.
4. The method of claim 1, wherein the objective function is a refrigeration coefficient, a refrigeration capacity, or an operating cost of a cascade refrigeration system to be designed.
5. The method of claim 1, wherein the refrigerant molecular selection model uses boolean variables to represent the selection of refrigerant.
6. The method according to claim 1, wherein the searching of the candidate refrigerant library optimizes the refrigerant selection and process optimization integrated model by optimizing an objective function to obtain an optimal refrigerant combination and process operating conditions for the cascade refrigeration system to be designed, in particular:
step 1: searching the candidate refrigerant library to obtain a group of refrigerant combinations conforming to feasible solutions of the first sub-model; the boiling point of the first sub-model from the former-stage refrigerant must be smaller than the critical temperature of the latter-stage refrigerant; the cooling water temperature must be less than the critical temperature of the first stage cycle and the boiling point of the mth stage refrigerant must be less than the constraint condition of the refrigeration environment temperature to be reached;
step 2: optimizing the second sub-model through the optimized objective function according to the refrigerant combination obtained in the step 1, judging whether a better objective function is found, if so, updating the refrigerant combination and the process operation condition, otherwise, iteratively executing the steps 1-2 until the search of all the refrigerant combinations of the alternative refrigerant library is completed, and obtaining the optimal refrigerant combination and the process operation condition of the cascade refrigeration system to be designed;
the second sub-model consists of an objective function, m single-stage refrigeration cycle models and a condensation temperature of a next-stage cycle, wherein the condensation temperature of the second sub-model is required to be larger than the evaporation temperature of a previous-stage cycle; the condensation temperature of the stage 1 loop must be greater than the cooling water temperature; the condensing temperature of each stage cycle must be less than the critical temperature of the refrigerant used in that stage cycle; the evaporation temperature of the mth stage cycle must be less than the ambient temperature to be refrigerated; the j+1th stage cycle releases heat equal to the heat absorbed by the j stage cycle and the constraint that the boiling point of the refrigerant of the j stage cycle must be less than the condensation temperature of the j+1th stage cycle.
7. An operation control method of a cascade refrigeration system, characterized in that the operation control is performed according to the optimal refrigerant combination and the process operation condition obtained by the refrigerant selection and process optimization integrated design method design of the cascade refrigeration system according to any one of claims 1 to 6.
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