Modified Brayton refrigeration cycles for forced-flow cooling of HTS fusion system

Abstract

Thermodynamic studies are conducted to identify suitable refrigeration cycles for application in High Temperature Superconductor (HTS) fusion systems. According to recent reports on small and efficient fusion systems, HTS magnets are estimated to operate at approximately 20K via forced-flow cooling of helium gas. In addition to the primary cryogenic load of the magnets, there are other cooling requirements, including the cooling of thermal shields and current leads. To construct a closed-loop refrigeration system without liquid nitrogen supply or boiling losses, a modified Brayton cycle was designed to encompass the cooling loads through coolant circulation loops for the magnets, thermal shields, and current leads. Since helium gas is used as both the coolant and refrigerant, an innovative design integrating the cooling loop with the refrigeration cycle has also been proposed. Various modified cycles utilizing forced-flow cooling are optimized through iterative analysis using a process simulator (Aspen HYSYS) and the actual gas properties of helium. It has been verified that the integrated refrigeration cycle with a cooling loop offers significant advantages in terms of thermodynamic efficiency and is simpler to operate without cryogenic pumping devices. In addition, it has been proven that a single refrigeration cycle can be designed to fully meet all cooling requirements of the HTS fusion system.

Description

Technology Field

[0001] The present invention relates to a modified Brayton refrigeration cycle for forced flow cooling of an HTS fusion system, and more specifically, to an integrated cooling cycle that efficiently cools an HTS magnet, a thermal shield, and a current lead simultaneously by using gaseous helium as both a refrigerant and a coolant. Background Technology

[0002] Fusion power generation using High-Temperature Superconductor (HTS) magnets is a worthwhile endeavor to realize clean energy solutions. Because HTS can deliver more current at higher temperatures than Low-Temperature Superconductors (LTS), HTS fusion systems are expected to be smaller and more efficient for cryogenic refrigeration compared to ITER (International Thermonuclear Experimental Reactor) or other LTS tokamak systems. Remarkable advancements in second-generation Rare-Earth Barium Copper Oxide (REBCO) wires, combined with appropriate cooling methods, have provided a realistic outlook for HTS fusion magnets.

[0003] According to the leading group, the operating temperature of HTS fusion magnets must be in the 10-30K range for miniaturization and efficiency. Cryogenic cooling at this temperature differs significantly from conventional 4-5K cooling because liquid helium cannot be used and forced flow of gaseous helium is required. To meet large-scale cooling requirements (over 10kW) at approximately 20K, a modified Brayton cycle, which is effectively used for the supercooling of liquid hydrogen in the target moderator of the European Fracture Neutron Resource Laboratory, could be a reasonable choice.

[0004] In addition to the primary cooling load of the cooling body using HTS magnets, there are other cooling requirements, including thermal shielding and current leads. Liquid nitrogen has been widely used for cooling thermal shielding and current leads in large-scale cryogenic systems. The cooling load of superconducting magnets is significantly reduced by thermal shielding at liquid nitrogen temperatures. For high-magnetic-field superconducting magnets, it is common practice to use binary HTS leads (a series combination of an HTS conductor as the cold part and a metal conductor as the warm part). For most high-current leads (over 10 kA), the junction between the HTS and the metal is cooled by liquid nitrogen, and the evaporated gas flows up for convective cooling of the metal conductor. However, significant design efforts have recently been made for forced-flow cooling of thermal shielding devices and current leads, which can operate without liquid nitrogen supply or boiling losses. For continuous gas cooling with a circulation loop, a modified Brayton refrigeration cycle can also be a viable option. Prior art literature

[0005] Registered Patent Publication No. 10-1761378 (Registered July 19, 2017) The problem to be solved

[0006] The present invention aims to identify a refrigeration cycle suitable for forced-flow cooling of HTS fusion systems and to seek optimal operating conditions for efficient refrigeration. In particular, standard and modified Brayton cycles are investigated in conjunction with a forced-flow circulation loop. Since gaseous helium is used simultaneously as both the refrigerant (working fluid) and the coolant (cooling gas), the present invention proposes an integrated design of the refrigeration cycle and circulation loop to enable easy and simple operation without the need for cryogenic pumps or blowers for forced flow. In addition to obvious operational advantages, the integrated cycle also offers potential benefits in thermodynamic efficiency because it can eliminate the generation of entropy caused by the temperature difference between the refrigerant and the coolant. The present invention proposes an integrated cooling cycle for each of the three requirements (HTS magnet, thermal shield, and current lead), and finally, a fully integrated cooling cycle that satisfies all requirements. means of solving the problem

[0007] An integrated refrigeration cycle for forced flow cooling of an HTS magnet in a nuclear fusion system of the present invention for achieving the above objective comprises: a compressor for compressing a refrigerant; a post-cooler for cooling the compressed refrigerant; a first heat exchanger for heat exchange between the high-pressure refrigerant that has passed through the post-cooler and the low-pressure refrigerant before it has passed through the compressor; a first expander for expanding the refrigerant that has passed through the first heat exchanger; and an HTS magnet that is cooled to an ultra-low temperature by passing the expanded refrigerant through an internal flow path.

[0008] Helium gas can be used simultaneously as the above refrigerant and coolant.

[0009] It may further include a second heat exchanger that exchanges heat between the refrigerant that has passed through the first expansion unit and the low-pressure refrigerant; and a second expansion unit that expands the refrigerant that has passed through the second heat exchanger.

[0010] An integrated refrigeration cycle for forced flow cooling of an HTS magnet in a fusion system according to one embodiment of the present invention comprises: a compressor for compressing a refrigerant; a post-cooler for cooling the compressed refrigerant; a first heat exchanger for heat-exchanging the high-pressure refrigerant that has passed through the post-cooler with the low-pressure refrigerant before it has passed through the compressor; a first expander for expanding the refrigerant that has passed through the first heat exchanger and sending it to a low-pressure refrigerant flow path; a second heat exchanger for heat-exchanging the refrigerant that has passed through the first heat exchanger with the low-pressure refrigerant; a third heat exchanger for heat-exchanging the refrigerant that has passed through the second heat exchanger with the low-pressure refrigerant; a second expander for expanding the refrigerant that has passed through the third heat exchanger; and an HTS magnet that is cooled to an ultra-low temperature by passing the expanded refrigerant through an internal flow path.

[0011] Helium gas can be used simultaneously as the above refrigerant and coolant.

[0012] The first expansion device can expand the refrigerant that has passed through the first heat exchanger and send it from the low-pressure refrigerant flow path to the flow path before it flows into the second heat exchanger.

[0013] An integrated refrigeration cycle for forced flow cooling of a heat shield in a nuclear fusion system of the present invention comprises: a compressor for compressing a refrigerant; a post-cooler for cooling the compressed refrigerant; a heat exchanger for heat exchange between the high-pressure refrigerant that has passed through the post-cooler and the low-pressure refrigerant before it has passed through the compressor; an expander for expanding the refrigerant that has passed through the heat exchanger; and a heat shield that surrounds an HTS magnet and cools the refrigerant expanded by the expander to an ultra-low temperature by passing it through an internal flow path.

[0014] Helium gas can be used simultaneously as the above refrigerant and coolant.

[0015] An integrated refrigeration cycle for forced flow cooling of a current lead in an HTS fusion system of the present invention comprises: a compressor for compressing a refrigerant; a post-cooler for cooling the compressed refrigerant; a first heat exchanger for heat exchange between the high-pressure refrigerant that has passed through the post-cooler and the low-pressure refrigerant that has passed through the compressor; a first expander for expanding the high-pressure refrigerant that has passed through the post-cooler and sending it to a flow path in contact with the current lead; a second expander for expanding the refrigerant that has passed through the first heat exchanger; a second heat exchanger for heat exchange between the refrigerant expanded by the first expander and the refrigerant expanded by the second expander; and a current lead that is cooled by passing the refrigerant that has passed through the first expander and the second heat exchanger through an external flow path in contact with the surroundings.

[0016] Helium gas can be used simultaneously as the above refrigerant and coolant.

[0017] The refrigerant path in contact with the current lead is arranged to surround the current lead and can be connected to flow into the compressor. Effects of the invention

[0018] According to the modified Brayton refrigeration cycle for forced flow cooling of the HTS fusion system of the present invention described above, an integrated cooling cycle is provided that efficiently cools the HTS magnet, thermal shield, and current lead simultaneously by using gaseous helium as both a refrigerant and a coolant.

[0019] In addition, it can be operated easily and simply without a cryogenic pump or blower for forced flow. Brief explanation of the drawing

[0020] FIG. 1 is a schematic diagram showing a refrigeration system for forced flow cooling of a magnet (M), a heat shield (TS), and a current lead (CL) in an HTS nuclear fusion system according to one embodiment of the present invention. Figure 2 is a diagram showing six cooling cycles for forced cooling of an HTS magnet. Figure 3 is a graph showing the FOM (performance index) as a function of medium pressure (P4) and inlet temperature (Ti) with selected values ​​of low pressure (P1) in cycle M2. Figure 4 is a graph showing FOM as a function of inlet temperature for various high pressure values ​​in cycle M6. Figure 5 shows the temperature-entropy diagrams and exergy consumption for six optimized M cycles. Figure 6 shows a separate cycle TS1 and an integrated cycle TS2 for forced flow cooling of a thermal shield. Figure 7 shows the temperature-entropy diagram and exergy consumption for the optimized cycles TS1 and TS2. Figure 8 shows separate cycles CL1 and CL2 and an integrated cycle CL3 for forced flow cooling of the current leads. Figure 9 shows the temperature-entropy diagrams and exergy consumption for the optimized cycles CL1, CL2, and CL3. Figure 10 shows a fully integrated cycle F for forced flow cooling of a magnet, thermal shield, and current lead. Figure 11 is a graph showing FOM as a function of low pressure (P1) for various values ​​of high pressure (P2) in cycle F. Figure 12 shows the temperature-entropy diagram, exergy consumption, and a table of temperature, pressure, and flow rate at each point of the optimized cycle F. Specific details for implementing the invention

[0021] The present invention is capable of various modifications and may have various embodiments, and specific embodiments are illustrated and described in detail in the detailed description. However, this is not intended to limit the present invention to specific embodiments, and it should be understood that it includes all modifications, equivalents, and substitutions that fall within the spirit and scope of the invention.

[0022] The terms used in this invention are used merely to describe specific embodiments and are not intended to limit the invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In this invention, terms such as "comprising" or "having" are intended to indicate the existence of the features, numbers, steps, actions, components, parts, or combinations thereof described in the specification, and should be understood as not precluding the existence or addition of one or more other features, numbers, steps, actions, components, parts, or combinations thereof.

[0023] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. Note that in the attached drawings, identical components are indicated by the same reference numerals whenever possible. Furthermore, detailed descriptions of known functions and configurations that may obscure the essence of the present invention will be omitted. For the same reason, some components in the attached drawings may be exaggerated, omitted, or schematically depicted.

[0024] FIG. 1 is a schematic diagram showing a refrigeration system for forced flow cooling of a magnet (M), a heat shield (TS), and a current lead (CL) in an HTS nuclear fusion system according to one embodiment of the present invention.

[0025] As schematically illustrated in FIG. 1, a control volume is considered around a refrigeration cycle for forced flow cooling. The system includes a refrigeration cycle and a circulation loop, but excludes a magnet (M), a heat shield (TS), and a current lead (CL) with a spiral cooling channel. In the steady state, the energy and entropy balance equations for the control volume are written as follows.

[0026]

[0027]

[0028] Here, , , and These are the input power of the compressor (C) and the circulation pump (P), and the output power of the expander (E), respectively. is the heat released from T0 to the surroundings, and and are respectively T M In magnet and T TS At is the thermal load of the heat shield. In reality, even if the temperature is not spatially uniform, T M and T TS It can simply be considered as the design temperature (i.e., the maximum allowable temperature). ε is the mass flow rate of the cooling gas for the current lead, and h and s are the specific enthalpy and specific entropy of helium, respectively. Subscripts i and e indicate the inlet and outlet of the forced flow cooling, respectively.

[0029] By combining mathematical equation 1 and mathematical equation 2, the net input power can be derived as follows.

[0030]

[0031] Here, the first three terms on the right are the reversible power for the refrigeration of M, TS, and CL, respectively, and the last term is the additional power due to entropy generation. Total entropy generation can be expressed as the sum of the entropies generated in all components of the system.

[0032]

[0033] Here, the subscripts AC, HX, and MIX represent the post-cooler, heat exchanger, and mixture, respectively. The last three terms of Equation 4 are the entropy generation of forced flow cooling for M, TS, and CL, respectively.

[0034] The thermodynamic performance of a refrigeration system is evaluated by the figure of merit (FOM) or second law efficiency, which is defined as follows.

[0035]

[0036] The goal of thermodynamic optimization is to maximize FOM or minimize entropy generation. The deficit in FOM (= 1 - FOM) can be categorized by the contribution of all components of Equations 3 and 4. This will later be illustrated in a pie chart referred to as "exergy expenditure" for each cycle.

[0037] The following assumptions were made for simplicity and quantitative discussion.

[0038] ① The ambient (0) temperature and the outlet temperature of all aftercoolers (AC) are 300K.

[0039] ② The adiabatic efficiency (diabatic duct) of the compressor (C), expander (E), and circulation pump (P) is all 80%.

[0040] ③ Compression is performed in a multi-stage manner using a post-cooler, and the pressure ratio of each stage is the same (1.5~2.0).

[0041] ④ The lowest temperature approach is 1.5% of the hot absolute temperature of all heat exchangers.

[0042] ⑤ The pressure drop for each flow in all heat exchangers is 10 kPa.

[0043] ⑥ In all cycles, the lower limit of pressure is 100 kPa and the upper limit is 2 MPa.

[0044] In reality, although adiabatic efficiency and pressure ratios may vary to some extent depending on the type of compressor, assumptions ② and ③ are used to roughly estimate the compressor work. Assumption ④ is derived from well-known optimization theories for counter-flow heat exchangers in cryogenic refrigeration, and a value of 1.5% was set for comparison with previous reports on large-scale Brayton refrigeration cycles at 20K. Assumption ⑤ is a highly simplified assumption intended solely for brief comparisons between similar cycles. Pressure drop must be carefully estimated for detailed process design, taking into account the fluid flow and dimensional factors of the heat exchanger. The upper limit of assumption ⑥ is also established from reports on helium refrigeration systems.

[0045] For demonstration in the following section, a series of cooling requirements for forced-flow cooling were selected, as listed in Table 1. While the specified temperatures are of paramount importance in this study, the amount of the heat load is roughly estimated from recent publications on HTS fusion magnets by referencing ITER and other existing tokamak systems. The cooling load is defined as TM and TTS for M and TS, respectively, but for CL, the flow rate of the cooling gas is relative to the inlet and outlet temperatures (T i , T e It should be noted that it is defined as ). Strictly speaking, the cooling requirements for CL must be defined according to the number of pairs of corresponding current levels (e.g., 18 pairs of 30kA leads for TF coils, and 24 pairs of 20kA leads for PF and CS coils). The design of these gas-cooled leads involves sophisticated optimization of conductor dimensions and cooling gas conditions. The cooling requirements for CL in Table 1 are considered a simple example of the optimized results for this thermodynamic study. A general-purpose process simulator (Aspen HYSYS) equipped with the actual gas characteristics of helium (REFPROP) is used for cycle analysis and optimization.

[0046]

[0047] [Forced Flow Cooling of HTS Magnets]

[0048] Figure 2 illustrates six different refrigeration cycles for forced-flow cooling of an HTS magnet (M). In all cases, helium serves as both the refrigerant and the coolant for the circulation loop. Cycle M1 is a simple example of a standard Brayton cycle with a cold expander (E) and a counter-flow heat exchanger (HX2) at the cold end. A circulation pump (P) or cryogenic blower drives the forced flow against the pressure drop in the circulation loop. The heat load of the magnet is transferred from HX2 to the refrigeration cycle. As shown in cycles M2 or M3, modifying the cycle with two expanders can significantly reduce the input power required for refrigeration. In the modified cycle, the two expanders can be arranged in series (M2) or parallel (M3), referred to as the "two-stage expansion" cycle or "double expander" cycle, respectively. The cold end configuration is identical in all three cycles M1, M2, and M3. In practice, the circulation pump can be placed at ambient temperature using a double-row heat exchanger for easy and reliable operation. The warming cycle is not considered in this thermodynamic study because it is clearly less efficient than the corresponding colding cycle (due to the generation of additional entropy in the heat exchanger) and the thermodynamic optimization of the main refrigeration cycle must be the same regardless of the pump temperature.

[0049] The present invention proposes an innovative design that integrates a circulation loop with a refrigeration cycle. Cycles M4, M5, and M6 are integrated versions of cycles M1, M2, and M3, respectively. A distinct advantage of the integrated cycle is the elimination of the circulation pump (P) and the cold-end heat exchanger (HX2). Instead of the cryogenic pump, an ambient temperature compressor is responsible for forced flow with additional power. Since a reduction in input power is also expected, a quantitative comparison of FOM for the separated and integrated cycles is therefore an important issue in the present invention.

[0050] From mathematical equations 3 and 5, the FOM of M cycles is simply as follows.

[0051]

[0052] To complete the thermodynamic cycle based on the previous section, the number of unknowns and given conditions must be carefully calculated so that design variables can be identified and optimized. Table 2 lists the number of unknowns, given conditions, and design variables for all cycles considered in this invention. Since the circulation loop is directly connected to the refrigeration cycle, the integrated cycles M4, M5, and M6 have two fewer variables than the corresponding separate cycles M1, M2, and M3.

[0053]

[0054] In the case of cycle M2, the low pressure (P1), high pressure (P2), and intermediate pressure (P4) of the refrigeration cycle, and the inlet temperature (T) of the HTS magnet i Four design variables are used, including ). Generally, in a gas cycle where the working fluid behaves like an ideal gas with constant specific heat, the "pressure ratio" (instead of each pressure level) is the dominant factor. Due to the minor non-ideal gas behavior of helium, higher pressure levels may be slightly preferred for a given pressure ratio. Additionally, the thermodynamic efficiency loss due to the pressure drop of HX may decrease as the operating pressure increases. Under assumption ⑥, the high pressure of the cycle is an upper limit ( P 1 = 2MPa) is set, and the other three variables are optimized.

[0055] FIG. 3 shows the selected low pressure (P1) value and the inlet temperature (T i Shows FOM as a function of intermediate pressure (P4) for ). As indicated by the dots, P4, P1, and T iRegarding this, there clearly exists a single optimal that maximizes FOM. In two-stage expansion, the optimal intermediate pressure is a trade-off between the two expanders, depending on the amount of cooling required at each stage. In this cycle, the optimal P4 is closer to high pressure, which means that the first expander (at a higher temperature) plays a small role. The optimal inlet temperature is the temperature rise (T) for a given cooling load. e -T i It is a compromise between ) and the circulating helium flow rate. T i When the value is higher than optimal, the temperature difference of M decreases, but since the flow rate must increase, more compressor power is required. Consequently, the maximum FOM is 25.7% when the pressure levels are 2.00 MPa, 1.64 MPa, and 0.27 MPa and the inlet temperature is 14.4 K.

[0056] In cycle M4 of FIG. 2, the integrated refrigeration cycle for forced flow cooling of the HTS magnet in the fusion system of the present invention comprises a compressor (C) that compresses the refrigerant, a post-cooler (AC) that cools the compressed refrigerant, a first heat exchanger (HX1) that exchanges heat between the high-pressure refrigerant that has passed through the post-cooler and the low-pressure refrigerant before it passes through the compressor, a first expander (E) that expands the refrigerant that has passed through the first heat exchanger, and an HTS magnet (M) that cools the expanded refrigerant to a cryogenic temperature by passing it through an internal flow path.

[0057] In the refrigeration cycle of the present invention, helium gas can be used simultaneously as a refrigerant (working fluid) and a coolant (cooling gas). Since helium (He) has a boiling point of less than 5K, it exists only as a gas throughout the refrigeration cycle and does not undergo a phase change. Therefore, in the refrigeration cycle of the present invention, the component corresponding to the condenser in the refrigeration cycle where a phase change occurs is called a post-cooler.

[0058] In cycle M5 of FIG. 2, the integrated refrigeration cycle for forced flow cooling of the HTS magnet in the fusion system may further include a second heat exchanger (HX2) that heat exchanges the refrigerant passing through the first expander (E1) with the low-pressure refrigerant, and a second expander (E2) that expands the refrigerant passing through the second heat exchanger.

[0059] In the case of cycle M5, the first heat exchanger (HX1), the first expansion unit (E1), the second heat exchanger (HX2), and the second expansion unit (E2) are connected in series in order in the high-pressure refrigerant flow path. Therefore, cycle M5 can be said to be a two-stage expansion cycle.

[0060] In cycle M6 of FIG. 2, the integrated refrigeration cycle for forced flow cooling of an HTS magnet in a nuclear fusion system of the present invention may include a compressor (C) that compresses a refrigerant, a post-cooler (AC) that cools the compressed refrigerant, a first heat exchanger (HX1) that exchanges heat between the high-pressure refrigerant that has passed through the post-cooler and the low-pressure refrigerant that has passed through the compressor, a first expander (E1) that expands the refrigerant that has passed through the first heat exchanger and sends it to a low-pressure refrigerant flow path, a second heat exchanger (HX2) that exchanges heat between the refrigerant that has passed through the first heat exchanger and the low-pressure refrigerant, a third heat exchanger (HX3) that exchanges heat between the refrigerant that has passed through the second heat exchanger and the low-pressure refrigerant, a second expander (E2) that expands the refrigerant that has passed through the third heat exchanger, and an HTS magnet (M) that cools to an ultra-low temperature by passing the expanded refrigerant through an internal flow path.

[0061] In the refrigeration cycle of the present invention, helium gas can be used simultaneously as a refrigerant (working fluid) and a coolant (cooling gas).

[0062] The first expander (E1) can expand the refrigerant that has passed through the first heat exchanger (HX1) and send it to the path before it flows into the second heat exchanger (HX2) from the low-pressure refrigerant path. That is, the path connected at the outlet of the first expander (E1) can be connected to the path between the third heat exchanger (HX3) and the second heat exchanger (HX2) from the low-pressure refrigerant path.

[0063] For cycle M6, two design variables are used: high pressure (P2) and inlet temperature (T6). Figure 4 shows the FOM as a function of inlet temperature (T6) for some high pressure (P2) values. For a given P2, there exists a unique optimal T6 that maximizes the FOM, as indicated by the dot. The optimal T i Similar to the case of cycle M2, this is a trade-off between the temperature rise (T7-T6) and the circulating helium flow rate for a given cooling load. As P2 increases up to 2 MPa, the optimal T6 decreases and the maximum FOM tends to gradually increase. Again, under assumption ⑥, the high pressure of the cycle is set to 2 MPa. At a high pressure of 2.00 MPa and an inlet temperature of 15.2 K, the maximum FOM is 27.8%. The gain in FOM for cycle M3 is mainly due to the reduction in entropy generation at P and low temperature HX4, as verified below.

[0064] Figure 5 shows the temperature-entropy (Ts) diagram and the exergy consumption for the six optimized M cycles. Among the standard cycles with one expander, the integrated cycle M4 is less efficient than the separated cycle M1. This is because the optimal low pressure in cycle M1 is as low as 150 kPa, whereas the low pressure in cycle M4 must match the specified inlet pressure (600 kPa) of the forced flow. Conversely, the integrated cycle with two expanders (M5, M6) is much more efficient than the separated cycles (M2, M3). Regarding the arrangement of two expanders, cycle M2 (series) is slightly better than cycle M3 (parallel) in the separated configuration, but cycles M6 (parallel) and M5 (series) are nearly equally efficient in the integrated configuration. For integrated designs featuring the forced flow cooling function of HTS magnets, a two-stage expansion or dual expansion cycle can be recommended.

[0065] The pie chart of exergy costs clearly demonstrates that the larger FOM of the integrated cycle is primarily due to the removal of the pump and cold heat exchanger. For example, the separated cycle M3 has irreversibility of (15.8%) and (1.5% + 2.1%) in C (compressor) and P (pump) + HX4, respectively, while the integrated cycle M6 has irreversibility of (16.9%) in C (compressor) without P (pump) + HX4. Through integration, the irreversibility caused by the separate cycles is reduced by 3.6%, but it can be said that 1.1% more compressor power is required. Overall, the integrated cycle M6 has a 3.0% higher FOM than cycle M3.

[0066] [Forced flow cooling of the heat shield]

[0067] Figure 6 shows a separate cycle TS1 and an integrated cycle TS2 for forced flow cooling of a heat shield (TS). Since the heat shield temperature (TTS) is generally higher than 80K, two-stage expansion or double expansion cycles are not considered here. Although cycles TS1 and TS2 have different cooling temperature levels, they are optimized in the same way as cycles M1 and M4, respectively.

[0068] As illustrated in FIG. 6, the integrated refrigeration cycle (TS2) for forced flow cooling of the heat shield in the fusion system of the present invention may include a compressor (C) that compresses the refrigerant, a post-cooler (AC) that cools the compressed refrigerant, a heat exchanger (HX1) that exchanges heat between the high-pressure refrigerant that has passed through the post-cooler and the low-pressure refrigerant that has passed through the compressor, an expander (E) that expands the refrigerant that has passed through the heat exchanger, and a heat shield (TS) that surrounds the HTS magnet (M) and cools the refrigerant expanded by the expander (E) to an ultra-low temperature by passing it through an internal flow path.

[0069] In the refrigeration cycle of this embodiment, helium gas can be used simultaneously as a refrigerant (working fluid) and a coolant (cooling gas).

[0070] The HTS magnet (M) is not shown in FIG. 6, but is a cryogenic superconducting magnet (M) as shown in FIG. 1. The heat shield (TS) can surround the magnet (M), which must be maintained at 20K, to prevent cold air from being lost to the surroundings.

[0071] By the refrigeration cycle of this embodiment, the heat shield (TS) can be cooled to be maintained at 90 to 100 K.

[0072] From mathematical formulas 3 and 5, the FOM of the TS cycle is as shown in mathematical formula 7 below.

[0073]

[0074] As listed in Table 2, the number of variables to optimize is 3 for cycle TS1 and only 1 for cycle TS2. For cycle TS1, P1, P2, T a The optimal values ​​for are 2 MPa, 0.38 MPa, and 73.0 K, respectively, and the corresponding maximum FOM is 21.9%. For cycle TS2, the optimal P2 is 1.58 MPa, and the corresponding maximum FOM is 25.2%. For the optimized cycle, the temperature-entropy diagram and exergy consumption are shown in Fig. 7. The integrated cycle TS2 has a noticeably higher FOM than the separated cycle TS1, and the reason for the higher efficiency can be explained by the exergy consumption chart. In cycle TS1, the irreversibility ratios for the pump (P) and the cold heat exchanger (HX2) are 2.6% and 2.8%, respectively, which were eliminated in the integrated cycle TS2.

[0075] [Forced flow cooling of current leads]

[0076] Figure 8 shows two separate cycles CL1 and CL2 and one integrated cycle CL3 for forced flow cooling of the current leads (CL). Cycle CL1 is a standard cycle with a single expander, but since the warm gas returns to near ambient temperature, the circulation loop passes through both HX1 and HX2. For high current levels (10 kA or more), the inlet temperature (T i Since the temperature must be low at 50K, as shown in cycle CL2, the thermodynamic performance can be significantly improved by arranging two expanders in series. Therefore, similar to cycle M2, cycle CL2 can be described as a separated two-stage expansion cycle. Similar to cycle M3, two expanders could also be arranged in parallel, but this is not included in the invention because the thermodynamic performance is significantly lower.

[0077] The design of the integrated CL cycle is subject to more constraints than the M and TS cycles because the return of the cooling gas must be nearly at ambient temperature. The integrated version of cycles CL1 and CL2 cannot be simply configured due to the pressure mismatch between the return of the cooling gas (state e) and the compressor suction (state 1). To meet the requirements presented in Table 1, a pump (or pre-compressor) is required between the return location and the outlet of the hot heat exchanger (HX1), which offers no clear advantage to the integrated cycle. Another approach is to arrange two expanders in parallel, as shown in cycle CL3, so that one expander manages the cooling gas and the other manages the main refrigerant. Thus, cycle CL3 can be referred to as an integrated dual expander cycle. While other viable configurations of the integrated cycle could be proposed, cycle CL3, which is the best cycle attempted by the inventor to date, is selected and presented here.

[0078] In the embodiment illustrated in FIG. 8, an integrated refrigeration cycle (CL3) for forced flow cooling of a current lead in an HTS fusion system may include a compressor (C) that compresses the refrigerant, a post-cooler (AC) that cools the compressed refrigerant, a first heat exchanger (HX1) that exchanges heat between the high-pressure refrigerant that has passed through the post-cooler and the low-pressure refrigerant that has passed through the compressor, a first expander (E1) that expands the high-pressure refrigerant that has passed through the post-cooler and sends it to a flow path in contact with the current lead (CL), a second expander (E2) that expands the refrigerant that has passed through the first heat exchanger, a second heat exchanger (HX2) that exchanges heat between the refrigerant expanded by the first expander (E1) and the refrigerant expanded by the second expander (E2), and a current lead (CL) that is cooled by passing the refrigerant that has passed through the first expander and the second heat exchanger to an external flow path in contact with the surroundings.

[0079] In the refrigeration cycle of this embodiment, helium gas can be used simultaneously as a refrigerant (working fluid) and a coolant (cooling gas).

[0080] In this embodiment, the second heat exchanger (HX2) can heat exchange the relatively high-pressure refrigerant expanded by the first expander (E1) with the relatively low-pressure refrigerant expanded by the second expander (E2) and send it to a flow path arranged to come into contact around the current lead (CL).

[0081] The refrigerant path in contact with the current lead (CL) is arranged to surround the current lead (CL) and can be connected to a low-pressure refrigerant path to flow into the compressor (C). The path surrounding the current lead (CL) can be arranged to spirally wrap around the current lead (CL).

[0082] Since the forced flow enters at cryogenic temperatures and returns to near ambient temperature, the cooling load of CL is significantly different from the cooling loads of M and TS. From Equations 3 and 5, the FOM of the CL cycle is as follows.

[0083]

[0084] Here, the reversible power of the molecule is the exergy difference between the inlet and outlet of the cooling gas.

[0085] Cycles CL1 and CL2 are optimized similarly to cycles M1 and M2, respectively. For cycle CL1, P2 is set again to 2 MPa, and the optimal P1 becomes 0.36 MPa. The maximum FOM is 14.6%, which is significantly lower than the FOM of cycles M1 and TS1. The main reason is that there is a large discrepancy in the capacity ratio of counter-flow HX1 due to the additional high-temperature flow of the cooling gas. For cycle CL2, the optimal P4 and P1 are 0.73 MPa and 0.22 MPa, respectively, and the corresponding maximum FOM is 23.1%. It should be noted that due to the increased cooling load of HX1, the pressure ratio of expander E1 in cycle CL2 is much higher than that of cycle M2.

[0086] For the integrated cycle CL3, there is only one design variable selected as P2. Similar to cycle M6, as P2 increases up to a maximum of 2 MPa, FOM gradually increases. The high pressure of the cycle is set to 2 MPa, and the maximum FOM is 21.0%. For the optimized CL cycle, the temperature-entropy diagram and exergy consumption are shown in Fig. 9. While the irreversibility ratio of the warm component (C + AC + P) is nearly the same in all CL cycles, the cryogenic component ( HX 's + E 's) is optimally arranged in cycle CL2 to provide the best thermodynamic performance.

[0087] From a thermodynamic perspective, the separated two-stage expansion cycle CL2 is more efficient than the integrated cycle in forced-flow cooling of CL. On the other hand, the cycle CL3 is simpler in configuration compared to the cycle CL2, and the method of combining two expanders in parallel, as presented below, can be usefully applied to the design of a fully integrated cycle.

[0088] [Fully Integrated Refrigeration Cycle]

[0089] Based on the integrated design results and experience from the previous section, a fully integrated cycle was devised to simultaneously meet all requirements for forced flow cooling for the magnet (M), heat shield (TS), and current lead (CL). The key lesson is that an integrated cycle with two expanders (in series or in parallel) is recommended for the primary cooling of the magnet at 20K, and additional expanders can be effectively integrated in parallel for the cooling of the heat shield and current lead.

[0090] Cycle F was proposed as one of the candidates, as shown in Fig. 10. The fully integrated refrigeration cycle (F) for forced flow cooling of the HTS fusion system of the present invention comprises: a compressor (C) for compressing a refrigerant; a post-cooler (AC) for cooling the compressed refrigerant; a first heat exchanger (HX1) for heat-exchanging the high-pressure refrigerant that has passed through the post-cooler (AC) with the low-pressure refrigerant before it passes through the compressor (C); a first expander (E1) for expanding the refrigerant that has passed through the first heat exchanger (HX1) and sending it to the internal flow path of a heat shield; a second heat exchanger (HX2) for heat-exchanging the high-pressure refrigerant that has passed through the first heat exchanger (HX1) with the low-pressure refrigerant; a third heat exchanger (HX3) for heat-exchanging the high-pressure refrigerant that has passed through the second heat exchanger (HX2) with the low-pressure refrigerant; a second expander (E2) for expanding the refrigerant that has passed through the third heat exchanger (HX3); and a current that expands the refrigerant expanded by the second expander (E2). It includes a third expander (E3) that sends the refrigerant expanded by the second expander (E2) to a path in contact with the lead (CL), a fourth heat exchanger (HX4) that exchanges heat with the low-pressure refrigerant, a fourth expander (E4) that expands the refrigerant that has passed through the fourth heat exchanger (HX4), an HTS magnet (M) that cools the refrigerant expanded by the fourth expander (E4) to an ultra-low temperature by passing it through an internal path, a heat shield (TS) that cools the refrigerant expanded by the first expander (E1) to an ultra-low temperature by passing it through an internal path, and a current lead (CL) that cools the refrigerant expanded by the third expander (E3) by passing it through an external path in contact with the surroundings.

[0091] In the fully integrated refrigeration cycle (F) of this embodiment, helium gas can be used simultaneously as both a refrigerant (working fluid) and a coolant (cooling gas). The refrigeration cycle (F) of this embodiment can simultaneously cool the HTS magnet (M), the heat shield (TS), and the current lead (CL).

[0092] The low-pressure refrigerant is compressed by the compressor (C) and cooled by the post-cooler (AC), then sequentially passes through the first heat exchanger (HX1), the second heat exchanger (HX2), the third heat exchanger (HX3), the second expansion unit (E2), the fourth heat exchanger (HX4), and the fourth expansion unit (E4) in the high-pressure refrigerant flow path, and flows into the internal flow path of the HTS magnet (M) to cool the HTS magnet (M).

[0093] The refrigerant passing through the internal path of the HTS magnet (M) can sequentially pass through the fourth heat exchanger (HX4), the third heat exchanger (HX3), the second heat exchanger (HX2), and the first heat exchanger (HX1) in the low-pressure refrigerant path.

[0094] The refrigerant path passing through the internal path of the heat shield (TS) can be connected to the path before passing through the second heat exchanger (HX2) in the low-pressure refrigerant path. Thus, the refrigerant passing through the first heat exchanger (HX1) and the first expansion valve (E1) in the high-pressure refrigerant path flows into the internal path of the heat shield (TS) to cool the heat shield (TS), and then passes through the second heat exchanger (HX2) and the first heat exchanger (HX1) in the low-pressure refrigerant path to flow into the compressor (C).

[0095] The refrigerant flow path in contact with the current lead (CL) is arranged to surround the current lead (CL) and can be connected to flow into the compressor (C). Thus, a portion of the refrigerant expanded by the second expander (E2) in the high-pressure refrigerant flow path can be expanded again by the third expander (E3) and flow into the refrigerant flow path arranged to surround the current lead (CL) in a spiral shape, thereby cooling the current lead (CL). The refrigerant flow path around the current lead (CL) is connected from the low-pressure refrigerant flow path to the flow path before the compressor (C), so that the refrigerant can be compressed again by the compressor (C).

[0096] The main flow of the Brayton refrigeration cycle consists of two-stage expansions (E2 and E4) for M at the low-temperature end, similar to cycle M5. Two additional expanders (E1 and E3) are added to TS and CL, respectively. Since the pressure ratios of E1 and E2 are nearly equal, and the pressure ratios of E3 and E4 are also nearly equal, this arrangement is considered parallel. The flow rates of E1 and E3 must be determined to meet the cooling requirements of the heat shield and current lead, respectively.

[0097] For the optimization of cycle F, the number of design variables is 2, as listed in Table 2. Figure 11 shows FOM as a function of low pressure (P1) for various high pressure (P2) values. For a given P2, there exists a unique optimal P1 that maximizes FOM. As P2 increases, the corresponding maximum FOM gradually increases, and according to assumption ⑥, the high pressure of the cycle is set to 2 MPa. The optimal P1 is 0.44 MPa, and the overall maximum value of FOM is 24.8%.

[0098] Detailed information on the optimized cycle F is shown in Fig. 12. The first diversion flow to E1 is supplied for forced flow cooling of TS at 100K, and the second diversion flow to E3 is supplied for forced flow cooling of CL at 50K. Through two-stage expansion, the main flow is lowered to 16.2K to force flow cooling of M to 20K. The temperature, pressure, and flow rate at each point for the optimized cycle are listed, confirming that all assumptions and refrigeration requirements in Table 1 are correctly met.

[0099] Finally, it is necessary to mention the value and limitations of this study. The FOM (24–28%) of the newly proposed cycle was found to be in a range generally identical to that of modern Brayton refrigerators. The M cycle is similar to recently installed 20K refrigerators used as target moderators for ESS, while the TS cycle is similar to commercial products for the supercooling of liquid nitrogen at 65–75K. Although it is difficult to find existing refrigerators comparable to the CL cycle, hydrogen liquefiers (from 300K to 20K) equipped with helium Brayton cycles currently under construction may be roughly similar in cycle configuration. Large-scale refrigeration at around 20K has not been widely required until now, but it is clearly emerging in high-magnetic-field HTS magnet applications such as nuclear fusion. Modifying and optimizing the Brayton refrigeration cycle is an important thermodynamic task for realizing gas cooling by forced flow, and this study represents a pioneering effort toward the development of practical systems. Although some important technical issues (e.g., operating principles or thermal stability) were not considered in this thermodynamic study, more realistic cooling requirements must be incorporated in the next stage with the advancement of HTS fusion technology.

[0100] Thermodynamic optimization is performed on modified Brayton refrigeration cycles applicable to forced-flow cooling of future HTS fusion systems. Various modified Brayton cycles are selected and optimized to meet cooling requirements for 20K HTS magnets, as well as 100K thermal shields and current leads ranging from 50 to 295K. The thermodynamic performance index, FOM (Factor of Performance), is defined as the ratio of reversible work to actual work for refrigeration. The integration of the forced-flow cooling refrigeration cycle and the circulation loop has been newly proposed, and it has been quantitatively verified that the integrated design can significantly increase the FOM by eliminating the circulation pump and the cold heat exchanger. Full details of the cycle data can be provided for the next steps toward the actual development of large-scale refrigeration systems at 20K.

[0101] Although an embodiment of the present invention has been described above, those skilled in the art may modify and change the present invention in various ways by adding, changing, deleting, or adding components, etc., without departing from the spirit of the present invention as described in the claims, and such modifications and changes are also to be included within the scope of the rights of the present invention. Explanation of the symbols

[0102] FOM : Figure of merit h : Specific enthalpy [kJ / kg] : Mass flow rate [kg / s] P : Pressure [MPa] : Heat transfer rate [kW] s : Specific entropy [kJ / kg-K] : Entropy generation rate [kW / K] T : Temperature [K] : Power or work rate [kW] O : Ambient AC After-cooler C : Compressor CL : Current lead E : Expander e : Exit HX Heat exchanger i : Inlet (Entrance) M Magnet MIX : Mixing P Circulating pump rev : Reversible TM : Thermal shield

Claims

Claim 1 delete Claim 2 delete Claim 3 delete Claim 4 delete Claim 5 delete Claim 6 delete Claim 7 delete Claim 8 delete Claim 9 An integrated refrigeration cycle for forced flow cooling of a current lead in an HTS fusion system comprising: a compressor for compressing a refrigerant; a post-cooler for cooling the compressed refrigerant; a first heat exchanger for heat-exchanging the high-pressure refrigerant that has passed through the post-cooler with the low-pressure refrigerant that has passed through the compressor; a first expander for expanding the high-pressure refrigerant that has passed through the post-cooler and sending it to a flow path in contact with a current lead; a second expander for expanding the refrigerant that has passed through the first heat exchanger; a second heat exchanger for heat-exchanging the refrigerant expanded by the first expander with the refrigerant expanded by the second expander; and a current lead that is cooled by passing the refrigerant that has passed through the first expander and the second heat exchanger through an external flow path in contact with the surroundings. Claim 10 An integrated refrigeration cycle for forced flow cooling of current leads in an HTS fusion system, characterized in that, in claim 9, helium gas is used simultaneously as the refrigerant and coolant. Claim 11 An integrated refrigeration cycle for forced flow cooling of a current lead in an HTS fusion system, characterized in that, in claim 9 or 10, the refrigerant flow path in contact with the current lead is arranged to surround the current lead and is connected to flow into the compressor.

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