Working fluid molecular design method for organic Rankine cycle based on PC-SAFT

Abstract

The present invention provides a method for designing working fluid molecules for an organic Rankine cycle based on the PC-SAFT process, comprising four steps: (1) computer-aided molecular design (CAMD) of the working fluid molecular structure; (2) calculation of the thermophysical properties of the working fluid molecules based on the PC-SAFT equation of state; (3) construction of an organic Rankine cycle thermodynamic model; and (4) calculation of the ORC system performance of different working fluids and ranking of the optimal working fluids. This patent allows for a wide range of working fluid molecular designs tailored to the heat source conditions of specific organic Rankine cycle system applications, resulting in new, highly efficient organic working fluids not found in existing databases. The method also provides high accuracy in calculating the thermophysical properties of working fluids in different states, such as superheated gas, saturated gas, saturated liquid, and subcooled liquid. This method can significantly reduce the number of molecules in the list of molecules to be calculated, significantly reducing the amount of computation required for designing working fluid molecules.

Description

Technical Field

[0001] The present invention belongs to the technical field of organic Rankine cycle power systems, and in particular relates to a method for designing working fluid molecules of an organic Rankine cycle based on PC-SAFT. Background Art

[0002] Patent application publication number: CN111816264A, this invention adopts a method for designing working fluids for an organic Rankine cycle based on machine learning. Machine learning is applied to the prediction of cycle performance and working fluid properties, and a working fluid design that optimizes cycle conditions and molecular groups based on working fluid properties is proposed. Based on the working fluid property-cycle parameter learning library and the basic physical property database, machine learning models for cycle performance and working fluid properties are established respectively to complete the large-scale molecular design of working fluids under different cold and heat source conditions and cycle configurations. This application uses a neural network to predict the physical properties of the working fluid and the working performance of the ORC. When the input training data is limited, there may be large deviations in its prediction accuracy. The applicability of the ORC performance prediction model also needs to be re-verified for different application scenarios.

[0003] Patent application publication number: CN113255211B.

[0004] This invention proposes a method for screening organic Rankine cycle working fluids based on multi-objective optimization and decision-making. It only requires the given parameters such as the heat source temperature and flow rate of the organic Rankine cycle, the specific heat capacity and density of the working fluid, as well as the evaporation pressure, condensation pressure, expander inlet temperature, and the temperature difference between the evaporator and condenser pinch points. By establishing a mathematical model of the ORC system's thermal-economic-environmental performance, the constraint boundaries, the range of the circulating working fluid and the optimization variables are given, and the objective function value is obtained using a multi-objective genetic optimization algorithm. This invention uses a multi-objective optimization algorithm to avoid problems such as conflicts between different objective functions encountered during single-objective optimization and optimization results that do not match engineering practice. However, this invention requires screening based on an existing working fluid database, and the thermal properties of its working fluids are directly calculated using the results of the working fluid database, making it impossible to design new working fluids.

[0005] Paper 1: Lampe M, Stavrou M, Schilling J, et al. Computer-aided molecular design in the continuous-molecular targeting framework using group-contribution PC-SAFT. Computers and Chemical Engineering, 2015, 81: 278–287.

[0006] This paper proposes a two-step approach to organic refrigerant molecular design based on the PC-SAFT equation, as shown in the figure below. First, assuming the three PC-SAFT state parameters are continuously changing variables, the paper combines the thermodynamic model of the ORC system with a continuous molecular targeted design (CoMT-CAMD) to achieve the optimal PC-SAFT state equation parameters for the organic refrigerant in an ORC system using a low-temperature geothermal source. The paper then uses a second-order Taylor approximation, combined with a molecular database derived from the CAMD method, to calculate the optimal molecular structure and its PC-SAFT parameters. This approach improves accuracy somewhat by integrating the objective function along this distance, rather than simply measuring the distance between the actual molecule's PC-SAFT parameters and the optimal parameters. However, the algorithm still essentially determines the optimal molecular design by determining the distance between the actual molecule's parameters and the ideal target values. The Taylor approximation in this method requires calculating the derivatives of the objective function with respect to the PC-SAFT parameters, which is computationally complex and labor-intensive, resulting in low accuracy.

[0007] Paper 2: Schilling J, Lampe M, Gross J, Bardow A.1-stage CoMT-CAMD: An approach for integrated design of ORC process and working fluid using PC-SAFT. Chemical Engineering Science, 2017, 159: 217–230.

[0008] This paper proposes a one-step method for organic working fluid molecular design based on the PC-SAFT equation. It eliminates the first step in the two-step method above, which requires calculating the ideal optimal PC-SAFT parameters of the working fluid. Instead, it directly uses the molecular structure parameters in the CAMD process as optimization variables and uses the mixed integer nonlinear programming (MINLP) optimization algorithm to obtain the optimal molecular structure parameters and the corresponding PC-SAFT parameters. The optimization variables of this method's one-step optimization algorithm include the optimization variables of the molecular structure and the optimization variables of the ORC system model. The MINLP algorithm optimization process itself is very complex, resulting in a still very large final computational workload. Especially for high-temperature heat sources, the computational workload of molecular thermophysical properties is even greater. At the same time, due to the existence of errors in the calculation of molecular thermophysical properties, the MINLP algorithm may obtain incorrect optimization results.

[0009] Existing refrigerant selection algorithms are often based on existing refrigerant databases, making them incapable of designing refrigerant molecules or predicting the thermophysical properties of new refrigerant molecules. Existing molecular design methods based on equations of state (ESOs) such as SRK suffer from low prediction accuracy, leading to discrepancies between the optimal refrigerant obtained during subsequent ORC system performance prediction and the actual refrigerant.

[0010] The existing PC-SAFT-based working fluid molecular design algorithm requires a large amount of calculation for the thermophysical properties of working fluid molecules in ORC systems using high-temperature heat sources. As a result, the subsequent ORC system optimization design process requires a huge amount of calculation, which is difficult to implement on a PC. Summary of the Invention

[0011] In response to the problems existing in the prior art, the method for designing working fluid molecules for the organic Rankine cycle of the present invention mainly includes four steps: (1) computer aided molecular design (CAMD) of the working fluid molecular structure; (2) calculation of the thermophysical properties of the working fluid molecules based on the PC-SAFT (perturbed chain-statistically associating fluid theory) equation of state; (3) construction of the organic Rankine cycle thermodynamic model; and (4) calculation of the ORC system performance of different working fluids and ranking of the optimal working fluids.

[0012] The present invention provides a method for designing working fluid molecules of an organic Rankine cycle based on PC-SAFT, which includes four steps: (1) computer-aided molecular design (CAMD) of the working fluid molecular structure; (2) calculation of the thermophysical properties of the working fluid molecules based on the PC-SAFT equation of state; (3) construction of a thermodynamic model of the organic Rankine cycle; (4) calculation of the performance of the organic Rankine cycle system of different working fluids and sorting of the optimal working fluids;

[0013] In the step (1), the range of candidate groups for molecular design is set, and a designed molecular candidate library is obtained according to the octagonal rule of molecular structure feasibility;

[0014] In the step (2), the molecular properties of the PC-SAFT state equation are calculated using the group composition information of the molecule, the critical temperature and critical pressure of the molecule are calculated using the PC-SAFT state equation, the candidate molecules in step (1) are selected according to the designed molecular critical temperature screening rules, and the thermophysical properties of the working fluid molecules at different temperatures and pressures are calculated;

[0015] In step (3), the temperature and flow rate of the heat source, the temperature and temperature rise of the cold source for a specific application are set, the system configuration of the organic Rankine cycle is determined, and a thermodynamic model of the organic Rankine cycle is established;

[0016] In the step (4), for the alternative working fluid established in step (2), the thermodynamic properties of the alternative working fluid under different working parameters are calculated using the working fluid thermophysical property calculation method in step (2) and the organic Rankine cycle system thermodynamic model established in step (3), so as to obtain the optimal working parameters of the working fluid; and then the calculated net output power is sorted to obtain a list of the top 10 working fluids.

[0017] Specifically, in step (1), a group set for working fluid molecule design is first set, and then a CAMD design of working fluid molecules is performed to obtain a preliminary working fluid molecule list;

[0018] In the step (2), the result of step (1) is used as input, and the corresponding PC-SAFT state equation parameters of each molecular structure are calculated according to the molecular list obtained in step (1), and then the critical temperature and critical pressure of the molecule are calculated. According to the set molecular screening rules, the working fluid molecules that exceed the set range are excluded to obtain alternative working fluids, and the specific enthalpy and specific entropy of the molecule at the set pressure and temperature are calculated at the same time;

[0019] In step (3), according to the specific application requirements of the organic Rankine cycle system, the temperature, flow rate and specific heat capacity of the heat source, as well as the temperature, specific heat capacity and cooling temperature rise of the cold source are set, and then the specific organic Rankine cycle system configuration is set. Finally, a thermodynamic model of the organic Rankine cycle system of the configuration is established to obtain a calculation formula;

[0020] In step (4), the results of steps (2) and (3) are used as input, and the thermodynamic properties of each working fluid in the list of alternative working fluids screened in step (2) are first calculated to obtain the net output power at different evaporation pressures, and then the optimal evaporation pressure and corresponding thermodynamic properties of the working fluid are determined to obtain a list of the top 10 working fluids.

[0021] Specifically, the molecular properties in step (2) include the number of molecular segments m, the diameter of the molecular segment σ, and the depth of the ε / k potential well.

[0022] Specifically, in step (2), it is determined simultaneously whether the working medium is in a superheated state, a saturated gas state, a saturated liquid state, a gas-liquid mixed state, or a supercooled state.

[0023] Specifically, the thermophysical parameters in step (2) include enthalpy, entropy, specific heat capacity, and fugacity.

[0024] Specifically, the calculation method of the thermophysical property parameters in step (2) includes using the PC-SAFT state equation and the Joback-Reid method.

[0025] Specifically, in step (3), the net output power and thermal efficiency, working fluid flow rate and expansion ratio parameters of the system are calculated.

[0026] Specifically, in step (4), the calculated net output power is sorted to obtain a list of the top 10 working fluids, and then the cycle thermal efficiency, expander volume expansion ratio and working fluid flow rate of these working fluids are comprehensively considered to obtain a list of the top 10 working fluids.

[0027] Specifically, the calculation formula obtained in step (3) includes: calculation formulas for net output power, thermal efficiency, expander volume expansion ratio and working fluid flow rate.

[0028] Specifically, in step (4), after obtaining the list of the top 10 working fluids, the cycle thermal efficiency, expander volume expansion ratio and working fluid flow rate of these working fluids are comprehensively considered to obtain the list of the top 10 working fluids.

[0029] The present invention provides a method for designing working fluid molecules for an organic Rankine cycle. The method adopts a molecular design method to design new working fluid molecules. By setting a set of molecular groups, new working fluids that are not included in the existing working fluid database can be designed.

[0030] The PC-SAFT equation of state is used to calculate the thermophysical properties of the working fluid. This equation of state can calculate the thermophysical parameters of the working fluid molecules in the gas and liquid states based on the molecular structure of the working fluid and the principles of statistical mechanics. Compared with traditional SRK equations of state, it has higher calculation accuracy.

[0031] By using the newly proposed molecular screening rules and screening the molecular list obtained by CAMD based on the critical pressure value of the molecule, the number of alternative working fluids that need to be considered can be greatly reduced, greatly reducing the computational workload;

[0032] The ORC thermodynamic performance calculation is performed on each working fluid in the screened working fluid list, and the top 10 working fluids are given after comprehensive evaluation. This can avoid the disadvantage of traditional optimization algorithms that only calculate one optimal working fluid, and can effectively reduce the inaccuracy of calculation results caused by errors in molecular thermophysical property prediction.

[0033] Beneficial effects:

[0034] (1) This patent can be used to design a wide range of working fluid molecules based on the heat source conditions of specific ORC system applications, and obtain new and efficient organic working fluids that do not exist in existing databases, providing a reference for the next step of industrial application.

[0035] (2) Compared with the existing machine learning molecular design methods, the present invention uses the PC-SAFT state equation based on statistical mechanics to calculate the thermophysical properties of the working fluid. It has high computational accuracy for the calculation of the thermophysical properties of the working fluid in different states such as superheated gas, saturated gas, saturated liquid, and supercooled liquid, thereby greatly improving the accuracy of new molecular predictions.

[0036] (3) Compared with the existing two-step method or one-step method, the present invention can greatly reduce the number of molecules in the molecular list that needs to be calculated, and can significantly reduce the amount of calculation when designing the working fluid molecules. At the same time, the working fluid molecules within the optimization range are examined one by one, avoiding the inaccuracy of the calculation results caused by the calculation errors of molecular properties in the traditional MINLP optimization algorithm. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The above and other objects, features and advantages of the present disclosure will become more apparent through a more detailed description of exemplary embodiments of the present disclosure with reference to the accompanying drawings, wherein like reference numerals generally represent like components throughout the exemplary embodiments of the present disclosure.

[0038] Figure 1 The working fluid molecule design method of the organic Rankine cycle based on PC-SAFT in an embodiment of the present invention is shown.

[0039] Figure 2 A simple organic Rankine cycle system configuration in an embodiment of the present invention is shown.

[0040] Figure 3 The figure shows a temperature-entropy diagram of the working process of a simple organic Rankine cycle system in an embodiment of the present invention.

[0041] Figure 4 The screening criteria for working fluid molecules in the embodiment of the present invention are shown.

[0042] Figure 5 The graph shows the variation curve of the net output power of the top 10 working fluids with the evaporation temperature in the embodiment of the present invention.

[0043] Figure 6 The graph shows the variation curve of the thermal efficiency of the top 10 working fluids with the evaporation temperature in the embodiment of the present invention.

[0044] Figure 7 The graph shows the variation curve of mass flow rate of the top 10 working fluids with evaporation temperature in the embodiment of the present invention.

[0045] Figure 8 The graph shows the change curve of the expander volume expansion ratio of the top 10 working fluids in the embodiment of the present invention as a function of the evaporation temperature.

[0046] Figure numerals: 1-working fluid pump; 2-evaporator; 3-expander; 4-condenser; 5-generator; 6-first pipeline; 7-second pipeline; 8-third pipeline; 9-fourth pipeline; 10-alkanes; 11-olefins; 12-alkynes; 13-cyclopentanes; 14-cyclohexanes; 15-aromatics; 21-heat source inlet; 22-heat source outlet; 41-coolant inlet; 42-coolant outlet; N1-N10 represent the top 10 working fluids. DETAILED DESCRIPTION

[0047] Embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. Rather, these embodiments are provided to make the present disclosure more thorough and complete, and to fully convey the scope of the present disclosure to those skilled in the art.

[0048] As used herein, the term "include" and its variations denote open inclusion, i.e., "including but not limited to". Unless otherwise stated, the term "or" denotes "and / or". The term "based on" denotes "based at least in part on". The terms "connected" and "connected" denote connection or communication directly or indirectly through other components. The terms "first", "second", etc. may refer to different or the same objects, but do not directly indicate a difference in order or importance. Other explicit and implicit definitions may also be included below.

[0049] First, describe in detail with reference to the accompanying drawings:

[0050] Figure 1 The working fluid molecule design method of the organic Rankine cycle based on PC-SAFT in an embodiment of the present invention is shown.

[0051] Figure 2 A simple organic Rankine cycle system configuration in an embodiment of the present invention is shown.

[0052] Figure 3 The figure shows a temperature-entropy diagram of the working process of a simple organic Rankine cycle system in an embodiment of the present invention.

[0053] Figure 2 This is a system configuration of a simple organic Rankine cycle given in a specific embodiment, including components such as a working fluid pump, an evaporator, an expander, and a condenser. The low-pressure, low-temperature saturated liquid working fluid is pressurized by the working fluid pump and sent to the evaporator, where it exchanges energy with the heat source and becomes a high-pressure gas. The working fluid then enters the expander to expand and perform work, driving the generator to generate electricity. The low-pressure gaseous working fluid at the expander outlet enters the condenser, where it is cooled by the cold source and becomes a low-temperature, low-pressure saturated liquid, entering the next cycle.

[0054] Figure 3The temperature and specific entropy corresponding to the working fluid states at the inlet and outlet of each component during the entire working process are given.

[0055] Figure 4 The screening criteria for working fluid molecules in the embodiment of the present invention are shown.

[0056] Figure 4 The critical temperatures and Cp0 / R values of all working fluids calculated for the embodiment are shown in FIG. 1 , where the large marks correspond to the top 20 working fluids in terms of net output power, all of which are included in the region set by the molecular screening criteria set below. It can be seen that there is a certain correspondence between the critical temperature and the heat source temperature of the working fluid with a larger net output power. By using this screening criterion, nearly one-third of the working fluids can be excluded first, greatly reducing the workload of subsequent optimization calculations.

[0057] Figure 5 The graph shows the variation curve of the net output power of the top 10 working fluids with the evaporation temperature in the embodiment of the present invention.

[0058] In the figure, N1 to N10 represent the top 10 working fluids ranked by net output power, as shown in Tables 5 and 6. The net output power of the simple ORC system using these working fluids varies with evaporation temperature as shown in Figure 5 As shown in Figure 2, the corresponding net output power gradually increases with increasing evaporation temperature. Since the maximum evaporation pressure of the ORC system is set to 0.9 times the critical pressure of the working fluid, the entire curve basically shows a monotonically increasing pattern.

[0059] Figure 6 The graph shows the variation curve of the thermal efficiency of the top 10 working fluids with the evaporation temperature in the embodiment of the present invention.

[0060] Figure 6 The trend of the ORC system cycle thermal efficiency is shown, which is generally similar to the trend of the net output power. The thermal efficiency of the top five working fluids is significantly higher than that of the working fluids ranked 6 to 10, especially when the evaporation temperature exceeds 525K.

[0061] Figure 7 The graph shows the variation curve of mass flow rate of the top 10 working fluids with evaporation temperature in the embodiment of the present invention.

[0062] Figure 8 The graph shows the change curve of the expander volume expansion ratio of the top 10 working fluids in the embodiment of the present invention as a function of the evaporation temperature.

[0063] Figure 7The graph shows the mass flow rate of the corresponding working fluids as a function of evaporation temperature. As the evaporation temperature increases, the corresponding working fluid mass flow rate gradually decreases. Among these, working fluids such as 1,7-octanediyne, ethynylcyclohexane, vinylcyclohexane, propenylcyclopentane, and benzene have relatively high mass flow rates, which contributes to improved working fluid pump efficiency. In practice, it is difficult to design and maintain high isentropic efficiency for working fluid pumps and expanders with high pressure ratios and low flow rates. Therefore, selecting these working fluids facilitates the selection and design of working fluid pumps and expanders.

[0064] Figure 8 The following curves show how the volume expansion ratio of the corresponding working fluids changes with evaporation temperature. The volume expansion ratio of the expander shows a monotonically increasing trend with increasing evaporation temperature. Among these, working fluids such as benzene, toluene, and vinylcyclohexane have relatively small volume expansion ratios. In the high evaporation temperature region, the volume expansion ratio of benzene is significantly smaller than that of other working fluids. For expanders, a small expansion ratio helps reduce design complexity and improves the isentropic efficiency of the expander, which is crucial for improving the efficiency of the entire ORC system.

[0065] Secondly, according to Figure 1 The specific implementation method is described in detail below:

[0066] 1) Groups are the basic units that make up molecules. When designing new working fluid molecules, the 14 groups shown in Table 1 are set as the candidate group combinations.

[0067] 2) Using the CAMD design algorithm, according to the set conditions and based on the above 14 groups, a set of molecules with feasible molecular structures is obtained. The specific molecular design rules are as follows: Let the number of groups in the molecule be n, and the valence of the kth group be v k , the total number of groups in the group set is m, and the maximum valence state in the set is s max The maximum number of groups allowed in molecular design is n max There are three variables u in molecular design. ik , z ijp , w. When the i-th group in the molecule is the k-th group in the group set, u ik =1, otherwise 0. If the jth position of the i-th group is connected to the p-th group, z ijp =1, otherwise 0. When the designed molecule contains n groups, the first n elements of w are 0, and the remaining elements are set to 1. The maximum length of w is n max , for the high temperature flue gas waste heat recovery of this embodiment, n max =8.

[0068] The total number of groups is limited to:

[0069]

[0070] According to the octagonal rule of the molecule:

[0071]

[0072] Assume that the molecular groups are connected in sequence, with the i-th group connected to the i-1-th group, then:

[0073]

[0074] According to the actual number of groups in the molecule:

[0075]

[0076] w1=0. (5)

[0077] w i ≤w i+1 ,i=1...(n max -1). (6)

[0078] The number of covalent bonds between a group and other groups is not greater than the chemical valence of the group, so

[0079]

[0080] There are constraints based on the symmetry of the group:

[0081]

[0082]

[0083]

[0084]

[0085] According to the connection order from left to right, the molecular structures without duplication are:

[0086]

[0087]

[0088] The heat source of this embodiment is high-temperature flue gas with a temperature of 400°C. Therefore, the critical temperature of the design working fluid is set to no more than 500°C, and only one isomer structure is considered. A total of 129 molecules are obtained, including 21 alkanes, 67 alkenes, 21 alkynes, 10 cyclopentanes, 6 cyclohexanes, and 4 aromatic hydrocarbons.

[0089] 3) According to the rules in Table 1, calculate the three PC-SAFT state equation parameters corresponding to each molecule: the number of segments m, the segment diameter σ, and the potential well depth ε / k. The specific calculation rules are as follows:

[0090]

[0091]

[0092]

[0093] Table 1 Calculation rules for PC-SAFT state equation parameters:

[0094]

[0095] 4) Calculate the ideal gas specific heat capacity at constant pressure as a function of temperature using the Joback-Reid group contribution method as follows:

[0096]

[0097] The calculation method of coefficients A, B, C, and D is

[0098]

[0099]

[0100]

[0101]

[0102] The coefficient contribution values corresponding to different groups are shown in Table 2.

[0103] Table 2 c of the Joback-Reid group contribution method p Calculation rules:

[0104]

[0105] The residual enthalpy H of the working fluid molecules can be calculated from the PC-SAFT state equation R and residual entropy S R , and then calculate the specific enthalpy and specific entropy of the gaseous working fluid at different temperatures and pressures according to the following formula.

[0106]

[0107]

[0108] The enthalpy and entropy of vaporization are calculated using the following method.

[0109]

[0110]

[0111] The specific enthalpy and specific entropy of the saturated liquid and supercooled liquid can then be obtained according to conventional calculation methods.

[0112] 5) Based on the PC-SAFT state equation, the critical temperature T of different molecules is calculated using the following method: c and the corresponding relative critical density η c .

[0113]

[0114]

[0115] The physical properties of toluene and benzene molecules were calculated using the above algorithm and compared with the literature values, as shown in Table 3. Overall, the parameters of the PC-SAFT state equation and the critical temperature of the molecules are calculated with high accuracy.

[0116] Table 3 Comparison of the calculation accuracy of the algorithm used in the present invention with the results in the literature:

[0117]

[0118] a: Reference Gross J, Sadowski G. Perturbed-chain SAFT: An Equation of State based on a perturbation theory for chain molecules. Ind. Eng. Chem. Res., 2001, 40: 1244-1260;

[0119] b: Results from the NIST Refprop database.

[0120] 6) Under the condition of known heat source temperature Texh, the obtained molecular set is preliminarily screened according to the following rules to obtain a list of candidate working fluids.

[0121] 0.78T exh <T c <0.92T exh . (28)

[0122] Figure 4 Screening criteria for working fluid molecules.

[0123] Figure 4: The critical temperatures and Cp0 / R values of all working fluids calculated for the embodiment, where the large marks correspond to the top 20 working fluids in terms of net output power. It can be seen that there is a certain correspondence between the critical temperature and the heat source temperature of the working fluid with a larger net output power.

[0124] 7) In this embodiment, the heat source conditions are as shown in Table 4: the mass flow rate of the high-temperature flue gas is 0.493 kg / s, the temperature is 400°C, and the organic working fluid is assumed to be in a saturated liquid state at the condenser outlet at a temperature of 20°C. In the ORC system, the isentropic efficiency of the expander is 0.8, the isentropic efficiency of the pump is 0.6, the pinch point temperature difference of the evaporator is 30K, the maximum pressure of the organic working fluid in the evaporator is 0.9 times its critical pressure, the superheat of the working fluid at the expander inlet is 0, and the temperature of the high-temperature flue gas after passing through the evaporator is above 100°C.

[0125] Table 4 Boundary conditions of the ORC design case:

[0126]

[0127]

[0128] 8) In this embodiment, the configuration of the ORC system is set to be simple, specifically as follows Figure 2 、 Figure 3 shown.

[0129] Figure 2 Simple organic Rankine cycle system configuration;

[0130] Figure 3 Temperature-entropy diagram of the working process of a simple organic Rankine cycle system.

[0131] Figure 2 This is a system configuration of a simple organic Rankine cycle given in a specific embodiment, including components such as a working fluid pump 1, an evaporator 2, an expander 3, a condenser 4, and a generator 5. The low-pressure, low-temperature saturated liquid working fluid is pressurized by the working fluid pump and fed into the evaporator 2 through the second pipeline 7, where it exchanges energy with the heat source and becomes a high-pressure gas. It then enters the expander 3 through the third pipeline 8 to expand and perform work, driving the generator 5 to generate electricity. The low-pressure gaseous working fluid at the outlet of the expander 3 enters the condenser 4 through the fourth pipeline 9, where it is cooled by the cold source and becomes a low-temperature, low-pressure saturated liquid, and enters the next cycle through the first pipeline 6. Figure 3 The temperature-entropy diagram shows the temperature and specific entropy corresponding to the working fluid status at the inlet and outlet of each component during the entire working process.

[0132] 9) Based on the simple ORC system, the established thermodynamic model is as follows:

[0133] For a simple organic Rankine cycle, Figure 3The diagram shows the compression process from 31 to 32, the evaporation process from 32 to 34, the expansion process from 34 to 35, and the condensation process from 35 to 31. 31 to 32 seconds represent the isentropic compression process from 31 to 32, point 33 represents the saturated liquid phase point of the working fluid in the evaporator, 33 to 34 is the working fluid phase change process, and 34 to 35 seconds represent the isentropic expansion process from 34 to 35. For the above processes, a simple mathematical model of the organic Rankine cycle system can be established based on the first law of thermodynamics.

[0134] For the compression process, the isentropic efficiency of the working fluid pump is set to η P , then the power consumption of the compressed working fluid is:

[0135]

[0136] For the evaporation process, the heat absorption of the evaporator is:

[0137] Q e =m wf (h4-h2)

[0138] Q e =m exh (h exh,in –h exh,out )

[0139] For the expansion process, the isentropic efficiency of the expander is set to η T , then the output work of the expander is:

[0140] W s =m wf (h4-h5)=m wf (h4-h 5s )η T (30)

[0141] For the condensation process, the heat released by the condenser is:

[0142] Q c =m wf (h5-h1) (31)

[0143] The net output power of the organic Rankine cycle system is:

[0144] Wn=Ws-Wp (32)

[0145] The thermal efficiency of the organic Rankine cycle system is:

[0146]

[0147] The volume expansion ratio EPR of the organic Rankine cycle expander is:

[0148] EPR=ρ4 / ρ5 (34)

[0149] 10) Based on a simple ORC system, the net output power of each candidate working fluid at different evaporation temperatures is calculated for the selected set of working fluids, and the maximum net output power within the constraints is obtained. Data on cycle thermal efficiency, working fluid mass flow rate, and volume expansion ratio at different evaporation temperatures are also obtained.

[0150] 11) The maximum net output power of each working fluid was ranked from highest to lowest, resulting in a list of the top 10 working fluids, as shown in Tables 5 and 6. Table 5 provides the chemical formula, molecular structure, PC-SAFT state parameters, critical temperature and critical pressure, and ideal gas specific heat capacity at constant pressure for these 10 working fluids. Table 6 provides the net output power data for these 10 working fluids.

[0151] Table 5 List of the top 10 preferred working fluid thermal properties

[0152]

[0153] Table 6: Thermodynamic properties of the top 10 working fluids:

[0154]

[0155] 12) For the top 10 working fluids obtained above, the cycle thermal efficiency, evaporation temperature, turbine output power, working fluid pump power consumption, volume expansion ratio and other data corresponding to the maximum net output power were also calculated, as shown in Table 6.

[0156] Figure 5 The net output power of the top 10 working fluids changes with evaporation temperature;

[0157] Figure 6 The thermal efficiency of the top 10 working fluids changes with evaporation temperature.

[0158] In the figure, N1 to N10 represent the top 10 working fluids ranked by net output power, as shown in Table 5 and Table 6. The net output power of the simple ORC system using these working fluids varies with the evaporation temperature. Figure 5 As shown in Figure 2, the corresponding net output power gradually increases with increasing evaporation temperature. Since the maximum evaporation pressure of the ORC system is set to 0.9 times the critical pressure of the working fluid, the entire curve basically shows a monotonically increasing pattern. Figure 6 The trend of the ORC system cycle thermal efficiency is shown, which is generally similar to the trend of the net output power. The thermal efficiency of the top five working fluids is significantly higher than that of the working fluids ranked 6 to 10, especially when the evaporation temperature exceeds 525K.

[0159] Figure 7 Curves showing the mass flow rate of the top 10 working fluids versus evaporation temperature;

[0160] Figure 8 The curve of the volume expansion ratio of the expander of the top 10 working fluids changing with the evaporation temperature.

[0161] Figure 7 The graph shows the mass flow rate of the corresponding working fluids as a function of evaporation temperature. As the evaporation temperature increases, the corresponding working fluid mass flow rate gradually decreases. Among these, working fluids such as 1,7-octanediyne, ethynylcyclohexane, vinylcyclohexane, propenylcyclopentane, and benzene have relatively high mass flow rates, which contributes to improved working fluid pump efficiency. In practice, it is difficult to design and maintain high isentropic efficiency for working fluid pumps and expanders with high pressure ratios and low flow rates. Therefore, selecting these working fluids facilitates the selection and design of working fluid pumps and expanders. Figure 8 The following curves show the volume expansion ratio of the expander for the corresponding working fluids as a function of evaporation temperature. The volume expansion ratio of the expander shows a monotonically increasing trend with increasing evaporation temperature. Among these working fluids, benzene, toluene, and vinylcyclohexane have relatively small volume expansion ratios. In the high evaporation temperature region, the volume expansion ratio of benzene is significantly smaller than that of other working fluids. For expanders, a small expansion ratio reduces design complexity and improves the isentropic efficiency of the expander, which is crucial for improving the efficiency of the entire ORC system.

[0162] 13) Taking into account the net output power, cycle thermal efficiency, working fluid mass flow rate, volume expansion ratio and other data of the top 10 working fluids, the final working fluid ranking is shown in Table 7:

[0163] Table 7: List of the top 10 working fluids after comprehensive consideration:

[0164]

[0165]

[0166] While various embodiments of the present disclosure have been described above, the foregoing description is intended to be illustrative, non-exhaustive, and not limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is selected to best explain the principles of the embodiments, their practical applications, or improvements over the prior art, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A working fluid molecular design method for an organic Rankine cycle based on PC-SAFT, characterized in that: It includes four steps: (1) Computer-aided molecular design (CAMD) of the working fluid molecular structure; (2) Calculation of the working fluid molecular thermophysical properties based on the PC-SAFT equation of state; (3) Construction of the organic Rankine cycle thermodynamic model; (4) Calculation of the performance of the organic Rankine cycle system with different working fluids and the optimal working fluid ranking; In the step (1), the range of candidate groups for molecular design is set, and a designed molecular candidate library is obtained according to the octagonal rule of molecular structure feasibility; In the step (2), the molecular properties of the PC-SAFT state equation are calculated using the group composition information of the molecule, the critical temperature and critical pressure of the molecule are calculated using the PC-SAFT state equation, the candidate molecules in step (1) are selected according to the designed molecular critical temperature screening rules, and the thermophysical properties of the working fluid molecules at different temperatures and pressures are calculated; In step (3), the temperature and flow rate of the heat source, the temperature and temperature rise of the cold source for a specific application are set, the system configuration of the organic Rankine cycle is determined, and a thermodynamic model of the organic Rankine cycle is established; In the step (4), for the alternative working fluid established in step (2), the thermodynamic properties of the alternative working fluid under different working parameters are calculated using the working fluid thermophysical property calculation method in step (2) and the organic Rankine cycle system thermodynamic model established in step (3), so as to obtain the optimal working parameters of the working fluid; and then the calculated net output power is sorted to obtain a list of the top 10 working fluids.

2. The method for designing working fluid molecules of an organic Rankine cycle based on PC-SAFT according to claim 1, characterized in that: In the step (1), a group set for working fluid molecule design is first set, and then a CAMD design of the working fluid molecules is performed to obtain a preliminary working fluid molecule list; In the step (2), the result of step (1) is used as input, and the corresponding PC-SAFT state equation parameters of each molecular structure are calculated according to the molecular list obtained in step (1), and then the critical temperature and critical pressure of the molecule are calculated. According to the set molecular screening rules, the working fluid molecules that exceed the set range are excluded to obtain alternative working fluids, and the specific enthalpy and specific entropy of the molecule at the set pressure and temperature are calculated at the same time; In step (3), according to the specific application requirements of the organic Rankine cycle system, the temperature, flow rate and specific heat capacity of the heat source, as well as the temperature, specific heat capacity and cooling temperature rise of the cold source are set, and then the specific organic Rankine cycle system configuration is set. Finally, a thermodynamic model of the organic Rankine cycle system of the configuration is established to obtain a calculation formula; In step (4), the results of steps (2) and (3) are used as input, and the thermodynamic properties of each working fluid in the list of alternative working fluids screened in step (2) are first calculated to obtain the net output power at different evaporation pressures, and then the optimal evaporation pressure and corresponding thermodynamic properties of the working fluid are determined to obtain a list of the top 10 working fluids.

3. The method for designing working fluid molecules of an organic Rankine cycle based on PC-SAFT according to claim 1, characterized in that: The molecular properties in step (2) include the number of molecular segments m, the diameter of the molecular segment σ, and the depth of the ε / k potential well.

4. The method for designing working fluid molecules of an organic Rankine cycle based on PC-SAFT according to claim 1, characterized in that: In the step (2), it is simultaneously determined whether the working medium is in a superheated state, a saturated gas state, a saturated liquid state, a gas-liquid mixed state, or a supercooled state.

5. The method for designing working fluid molecules of an organic Rankine cycle based on PC-SAFT according to claim 1, characterized in that: The thermophysical parameters in step (2) include enthalpy, entropy, specific heat capacity, and fugacity.

6. The method for designing working fluid molecules of an organic Rankine cycle based on PC-SAFT according to claim 1, characterized in that: The calculation method of the thermophysical property parameters in the step (2) includes using the PC-SAFT state equation and the Joback-Reid method.

7. The method for designing working fluid molecules of an organic Rankine cycle based on PC-SAFT according to claim 1, characterized in that: In the step (3), the net output power and thermal efficiency, working fluid flow rate and expansion ratio parameters of the system are calculated.

8. The method for designing working fluid molecules of an organic Rankine cycle based on PC-SAFT according to claim 1, characterized in that: In the step (4), the calculated net output power is sorted to obtain a list of the top 10 working fluids, and then the cycle thermal efficiency, expander volume expansion ratio and working fluid flow rate of these working fluids are comprehensively considered to obtain a list of the top 10 working fluids.

9. The method for designing working fluid molecules of an organic Rankine cycle based on PC-SAFT according to claim 2, characterized in that: The calculation formula obtained in step (3) includes: calculation formulas for net output power, thermal efficiency, expander volume expansion ratio and working fluid flow rate.

10. The method for designing working fluid molecules of an organic Rankine cycle based on PC-SAFT according to claim 2, characterized in that: In the step (4), after obtaining the list of the top 10 working fluids, the cycle thermal efficiency, expander volume expansion ratio and working fluid flow rate of these working fluids are comprehensively considered to obtain the list of the top 10 working fluids.

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