Power generation system and method using cold energy of liquefied gas

Abstract

The present invention relates to a power generation system and method using the cold energy of liquefied gas, which generate power by using the waste cold energy of liquefied gas. The power generation system using the cold energy of liquefied gas, according to the present invention, comprises: a turbine-generator for producing power by driving a turbine with working fluid; a branch part for branching, into a first working fluid stream and a second working fluid stream, the working fluid stream expanded while the turbine-generator drives the turbine; a first economizer for recovering the cold energy of the liquefied gas by means of heat exchange between the first working fluid stream and the liquefied gas; a first compressor for compressing the first working fluid stream cooled while the cold energy of the liquefied gas is recovered in the first economizer; a second economizer for recovering residual cold energy of the first working fluid stream by means of heat exchange between the first working fluid stream compressed in the first compressor and the second working fluid stream transferred from the branch part; and a second compressor for compressing the second working fluid stream cooled by allowing the residual cold energy of the first working fluid stream to be recovered in the second economizer.

Description

Power generation system and method using cold energy from liquefied gas

[0001] The present invention relates to a power generation system and method using the cold heat of liquefied gas, which generates power by using the cold heat of the liquefied gas that is wasted.

[0002] Natural gas is liquefied at extremely low temperatures at its production site, forming liquefied natural gas (LNG). This is then transported long distances to its intended destination. Natural gas is liquefied by cooling it from atmospheric pressure to the cryogenic temperature of approximately -163°C, creating LNG. Liquid natural gas, or LNG, has a volume roughly 1 / 600th that of gaseous natural gas, making it ideal for storage and transportation.

[0003] Natural gas stored in a liquid state is regasified using a regasification facility to produce natural gas in a gaseous state, and is supplied to each demander in a gaseous state.

[0004] Basically, the regasification facility is composed of a high-pressure pump that compresses natural gas, i.e. LNG, in a liquid state stored in a storage tank to the pressure required by the demander, and a vaporizer that vaporizes the high-pressure LNG compressed by the high-pressure pump into natural gas.

[0005] Seawater, which is readily available, is primarily used as the primary heat source for vaporizing LNG in vaporizers. Low-temperature seawater, which recovers the cold energy of LNG through direct or indirect heat exchange with the LNG, is then discharged back into the sea.

[0006] The energy required to vaporize natural gas and undergo a phase change reaches 200 kcal per kilogram. A significant amount of energy is wasted in the process of vaporizing liquid natural gas, leaving it largely unused and being discharged into the ocean by seawater.

[0007] Cold power generation is one of the technologies that effectively utilizes the cold energy of LNG. A typical commercial cold power generation system operates in a closed loop, where high-temperature, high-pressure gaseous refrigerant is reduced to low pressure through an expander, driving a turbine to generate electricity. The refrigerant then undergoes heat exchange with LNG, absorbing its cold energy and liquefying it. It is then pressurized through a pump, vaporized through heat exchange with seawater, and re-circulated back to the expander, a process that repeats.

[0008] A basic LNG cold-heat power generation system is composed of a vaporizer that vaporizes LNG by exchanging heat with a working fluid, a pump that pressurizes the working fluid discharged from the vaporizer after heat exchange with LNG, a heat exchanger that heat-exchanges the working fluid pressurized by the pump with a heat source, a turbine that expands the working fluid discharged from the heat exchanger after heat exchange, and a generator that generates electricity from the expansion work generated by driving the turbine.

[0009] Here, the working fluid circulates through a closed cycle consisting of a vaporizer, pump, heat exchanger, and turbine, undergoing a phase change in which it is condensed while exchanging heat with LNG in the vaporizer and vaporized while exchanging heat with seawater in the heat exchanger.

[0010] Conventional LNG cold power generation systems have the disadvantage of low power generation efficiency due to the relatively large loss caused by latent heat as the working fluid undergoes phase change, and the system becomes large due to the low density of the working fluid in the low-pressure section.

[0011] In addition, conventional LNG cold-heat power generation systems require control of the operating pressure and temperature of the cycle so that the working fluid can be completely condensed in the vaporizer and completely vaporized in the heat exchanger, and the temperature of the working fluid must be kept as low as possible to maximize the expansion ratio while maintaining a temperature above the freezing point, making control difficult and limiting improvements in efficiency.

[0012] Accordingly, the present invention aims to achieve the above-described object, and provides a power generation system utilizing cold heat from liquefied gas, which improves the power generation efficiency of a conventional liquefied gas cold heat power generation system.

[0013] The challenges addressed by the present invention are not limited to those mentioned above. Other technical challenges not mentioned will be readily apparent to those skilled in the art, as described below.

[0014] According to one aspect of the present invention for achieving the above-described object, a power generation system utilizing cold heat of liquefied gas is provided, including: a turbine-generator that generates electric power by driving a turbine with a working fluid; a branching portion that branches a working fluid stream expanded while driving the turbine in the turbine-generator into a first working fluid stream and a second working fluid stream; a first economizer that heat-exchanges the first working fluid stream with liquefied gas to recover cold heat of the liquefied gas; a first compressor that compresses the first working fluid stream cooled while recovering the cold heat of the liquefied gas in the first economizer; a second economizer that heat-exchanges the first working fluid stream compressed in the first compressor with a second working fluid stream delivered from the branching portion to recover the remaining cold heat of the first working fluid stream; and a second compressor that compresses the second working fluid stream cooled by recovering the remaining cold heat of the first working fluid stream in the second economizer.

[0015] Preferably, the working fluid stream can be maintained at a pressure higher than the critical pressure while circulating through the turbine-generator, the first economizer, the first compressor, the second economizer, and the second compressor.

[0016] Preferably, the working fluid may be a cryogenic fluid that does not reach its freezing point through heat exchange with the liquefied gas.

[0017] Preferably, the system may further include a joining portion for joining the second working fluid stream compressed in the second compressor and the first working fluid stream from which residual cooling heat has been recovered in the economizer into a working fluid stream; and a heater for heating the working fluid stream joined in the joining portion and circulating it to the turbine-generator.

[0018] Preferably, at least one heater is provided, and when two or more heaters are provided, the heaters may be provided in series or in parallel.

[0019] Preferably, the second economizer and the second compressor may be provided in one or more stages.

[0020] Preferably, the first economizer and the second economizer may be one or more multi-stream heat exchangers.

[0021] Preferably, the first economizer and the second economizer may be a plate type heat exchanger, a plate-fin type heat exchanger, a printed circuit board type heat exchanger, an aluminum bonded heat exchanger, or a coil wound type heat exchanger.

[0022] According to another aspect of the present invention for achieving the above-described object, a method for generating power using cold heat of liquefied gas is provided, comprising: a power generation step for generating electricity by driving a turbine with a working fluid; a branching step for branching an expanded working fluid stream into a first working fluid stream and a second working fluid stream while driving the turbine in the power generation step; a cold heat recovery step for recovering cold heat of the liquefied gas by heat-exchanging the first working fluid stream with liquefied gas; a low-temperature compression step for compressing the cooled first working fluid stream while recovering the cold heat of the liquefied gas in the cold heat recovery step; a residual cold heat recovery step for recovering the residual cold heat of the first working fluid stream by heat-exchanging the first working fluid stream compressed in the low-temperature compression step with the second working fluid stream branched in the branching step; and a high-temperature compression step for recovering the residual cold heat of the first working fluid stream in the residual cold heat recovery step and compressing the cooled second working fluid stream.

[0023] Preferably, the method may further include a joining step of joining the second working fluid stream compressed in the high-temperature compression step and the first working fluid stream from which residual cooling heat has been recovered in the residual cooling heat recovery step into a working fluid stream; and a heating step of heating the working fluid stream joined in the joining step and circulating it to the power generation step.

[0024] Preferably, the working fluid may be a material that is maintained above a critical pressure and does not undergo a phase change while circulating through the steps.

[0025] The power generation system and method using cold heat of liquefied gas according to the present invention can reduce energy consumed for compression by cooling the working fluid introduced into the compressor, maximize the expansion ratio, optimize the system, and improve the cold heat utilization efficiency, i.e., power generation efficiency, of the cold heat power generation system.

[0026] In addition, by branching the expanded working fluid stream from the turbine-generator, cooling some of it by recovering the cold heat of the liquefied gas in the first economizer and then compressing it, and cooling the remaining part by using the working fluid stream compressed after recovering the cold heat of the liquefied gas as a refrigerant and then compressing it, the working fluid before compression can be cooled using only the cold heat of the liquefied gas without having a separate cooling cycle.

[0027] Additionally, by heating the working fluid compressed in the compressor in a heater before supplying it to the turbine-generator, the expansion energy obtainable from the turbine-generator can be maximized.

[0028] In addition, since the working fluid is maintained in a supercritical state while circulating through the cycle, the system can be miniaturized compared to a conventional cold-heat power generation system in which the working fluid undergoes a phase change based on the same capacity.

[0029] The effects of the present invention are not limited to those described above. Other effects not mentioned will be readily apparent to those skilled in the art from this specification and the accompanying drawings.

[0030] FIG. 1 is a schematic diagram illustrating the configuration of a power generation system using the cold energy of liquefied gas according to one embodiment of the present invention.

[0031] Figure 2 is a schematic diagram illustrating the configuration of a conventional LNG cold power generation system using propane as a working fluid.

[0032] FIG. 3 is a PT diagram for explaining the phase while the working fluid circulates through a cycle according to one embodiment of the present invention.

[0033] In order to fully understand the operational advantages of the present invention and the objects achieved by the embodiments of the present invention, reference should be made to the accompanying drawings illustrating preferred embodiments of the present invention and the contents described in the accompanying drawings.

[0034] Hereinafter, the configuration and operation of a preferred embodiment of the present invention will be described in detail with reference to the attached drawings. Here, when adding reference numerals to components in each drawing, it should be noted that, as much as possible, identical components are indicated with the same numerals even if they are shown in different drawings. In addition, the following embodiments may be modified in various other forms, and the scope of the present invention is not limited to the following embodiments.

[0035]

[0036] In the embodiments of the present invention described below, the liquefied gas may be a liquefied gas that can be transported by liquefying gas at a low temperature, and may be, for example, a hydrocarbon-based liquefied gas such as LNG (Liquefied Natural Gas), LEG (Liquefied Ethane Gas), LPG (Liquefied Petroleum Gas), liquefied ethylene gas, or liquefied propylene gas. Alternatively, it may be a non-hydrocarbon-based liquefied gas such as liquefied carbon dioxide, liquefied hydrogen, or liquefied ammonia. However, in the embodiments described below, the liquefied gas will be described as an example in which LNG is used.

[0037] In addition, the power generation system and method using the cold heat of liquefied gas according to an embodiment of the present invention described below may be applied on land where a heat exchanger for recovering the cold heat of liquefied gas is provided, such as a liquefied gas introduction terminal, a liquefied gas thermal power plant, a liquefied gas filling station, and a liquefied gas fuel-propelled mobility.

[0038] In addition, the power generation system and method using cold energy from liquefied gas according to an embodiment of the present invention, which will be described later, can also be applied to ships or ocean-going vessels. Here, the term "ship" may include all types of ships equipped with LNG regasification facilities capable of regasifying LNG and supplying it to gas demanders, including ships with self-propulsion capabilities such as LNG RVs (Regasification Vessels) and liquefied gas fuel-propelled ships, as well as offshore structures floating on the sea, such as LNG FSRUs (Floating Storage Regasification Units).

[0039]

[0040] Hereinafter, a power generation system and method using cold energy from liquefied gas according to one embodiment of the present invention will be described with reference to the attached drawings.

[0041] First, referring to FIG. 1, the power generation system according to the present embodiment comprises a first economizer (120) in which a first working fluid stream (NLa) is cooled by heat exchange with LNG, a first compressor (130) which recovers cold heat of LNG in the first economizer (120) and compresses the cooled first working fluid stream (NLa), a second economizer (140) which cools a second working fluid stream (NLb) by heat exchange with the first working fluid stream (NLa) compressed in the first compressor (130), a second compressor (150) which recovers residual cold heat of the first working fluid stream (NLa) in the second economizer (140) and compresses the cooled second working fluid stream (NLb), and a second compressor (150) which compresses the first working fluid stream (NLa) in which cold heat is recovered in the second economizer (140) and the second compressor (150) which compresses the first working fluid stream (NLa) compressed in the second compressor (150). It includes at least one of a heater (160) that heats a second working fluid stream (NLb) and a turbine-generator (110) that drives a turbine with the working fluid stream (NL) heated in the heater (160) to generate power.

[0042] According to the present embodiment, downstream of the turbine-generator (110), a branch section (not given a drawing symbol) is provided to branch the expanded working fluid stream (NL) while driving the turbine of the turbine-generator (110) into a first working fluid stream (NLa) and a second working fluid stream (NLb).

[0043] A branch point may refer to a point where a line through which an expanded working fluid stream (NL) flows is divided into a line through which a first working fluid stream (NLa) flows and a line through which a second working fluid stream (NLb) flows. The branch point may also be equipped with a distributor or valve.

[0044] Fig. 1 illustrates an embodiment in which a branch portion is provided downstream of a turbine-generator (110). However, the branch portion may also be provided upstream of the turbine-generator (110). In the case in which the branch portion is provided upstream of the turbine-generator (110), the turbine-generator (110) is provided corresponding to the first working fluid stream (NL1) and the second working fluid stream (NL2), respectively, and the working fluid stream (NL) discharged from the heater (160) is branched into the first working fluid stream (NL1) and the second working fluid stream (NL2) at the branch portion, and then supplied to the turbine-generator (110) corresponding to each stream, where it is expanded and can circulate the cycle.

[0045] The first working fluid stream (NLa) branched downstream of the turbine-generator (110) is supplied to the first economizer (120) and exchanges heat with LNG, and the branched second working fluid stream (NLb) is supplied to the second economizer (140) and exchanges heat with the first working fluid stream (NLa).

[0046] In this embodiment, an example is provided in which a working fluid stream (NL) is branched into two streams, including a first working fluid stream (NL1) that directly recovers cold heat from LNG, and a second working fluid stream (NL2) that indirectly recovers cold heat from LNG through the first working fluid stream (NL1). However, the number of branched streams is not limited thereto.

[0047] The first economizer (120) and the second economizer (140) according to the present embodiment may each be a plate type heat exchanger, a plate-fin type heat exchanger, a printed circuit board type heat exchanger (PCHE), a brazed aluminum heat exchanger (BAHX), or a coil-wound heat exchanger (CWHX).

[0048] The drawing illustrates an example in which a first economizer (120) and a second economizer (140) are each provided, but the first economizer (120) and the second economizer (140) may be provided as a multi-stream heat exchanger as one or more cold boxes.

[0049] According to the present embodiment, the expanded working fluid stream (NL) in the turbine-generator (110) is divided into at least two streams including a first working fluid stream (NLa) and a second working fluid stream (NLb), and a second economizer (140) is provided downstream of the first economizer (120) to further recover the remaining cold heat of the first working fluid stream (NLa) from which the cold heat of LNG is recovered in the first economizer (120), thereby increasing the flow rate of the circulating working fluid and thus increasing the amount of electricity produced by the turbine.

[0050] As the temperature of the fluid decreases, its density increases, which reduces the compression work, so the expansion energy generated increases compared to the compression energy consumed, allowing for maximum power generation.

[0051] In this embodiment, the first working fluid stream (NLa) and the second working fluid stream (NLb) are names used to describe and distinguish the streams into which the working fluid is branched and divided. The first working fluid stream (NLa) and the second working fluid stream (NLb) are the same substance that flows along different paths after branching off at the branching portion, is compressed, and joins into one working fluid stream (NL) upstream of the heater (160) to circulate in a cycle.

[0052] In this embodiment, the working fluid is circulated through a cycle including at least one of the first economizer (120), the first compressor (130), the second economizer (140), the second compressor (150), the heater (160), and the turbine-generator (110), such that the pressure does not fall below or above the critical pressure. That is, no phase change of the working fluid occurs regardless of the temperature during the cycle.

[0053] Referring to Figure 3, the working fluid remains in the critical region (CA) where the pressure is maintained above the critical pressure while circulating the cycle, and the state (phase) of the working fluid becomes a compressible fluid or a supercritical fluid.

[0054] Since the working fluid is maintained above the critical pressure while circulating in the cycle, the pinch point of the heat exchanger where the working fluid exchanges heat, i.e., the first economizer (120) and the second economizer (140), does not exist inside the device, or at least the effect occurs at a low temperature near the end of the heat exchanger, thereby maximizing the heat exchange performance. In addition, since slugging does not occur, there is almost no piping stress applied to the piping through which the working fluid flows.

[0055] Additionally, in this embodiment, the working fluid may be maintained in a supercritical state while circulating through a cycle including one or more of the first economizer (120), the first compressor (130), the second economizer (140), the second compressor (150), the heater (160), and the turbine-generator (110).

[0056] If the working fluid maintains a supercritical state while circulating the cycle according to the present embodiment, the size of the power generation system can be made small because the working fluid flows in a high density state.

[0057] Additionally, in this embodiment, the working fluid may be a substance that does not reach the freezing point even at the point where the working fluid reaches the lowest temperature while circulating the above-described cycle, i.e., at the point where it is discharged after heat exchange with LNG in the first economizer (120).

[0058] The working fluid of the present embodiment may be a single refrigerant selected from the group consisting of hydrogen, helium, nitrogen, oxygen, neon, argon, carbon compounds having 5 or fewer carbon atoms, and freon refrigerants, or a mixed refrigerant comprising two or more of these. In the present embodiment, the boil-off gas (BOG) generated by natural vaporization of LNG, i.e. methane (CH4), is used as the single refrigerant as the working fluid, or a mixed refrigerant of methane and nitrogen is used, as an example.

[0059] By using the vaporization gas of the liquefied gas as the target of the cold heat recovery as the working fluid, not only can the cold heat of the vaporization of the liquefied gas be recovered without reaching the freezing point while the working fluid circulates through the cycle, but the cost of processing the vaporization gas can be reduced and there is the advantage of easy supply.

[0060] Meanwhile, nitrogen has an extremely low freezing point of approximately -210℃ at atmospheric pressure, making it a suitable working fluid for use as a single refrigerant, as it does not reach its freezing point during the cycle. Furthermore, nitrogen is an inert gas readily available from air, making it inexpensive and safe. Furthermore, because it does not contain carbon, it can be vented into the atmosphere, eliminating the need for a flare device.

[0061] When the power generation system according to the present embodiment is applied on land equipped with liquefied gas regasification facilities such as liquefied gas terminals, or when the system is applied at sea equipped with liquefied gas regasification facilities such as FSRUs, in any case, equipment for generating and supplying nitrogen is provided for the purpose of purging or maintenance of the regasification facilities. That is, when nitrogen is used as the working fluid of the power generation system according to the present embodiment, the working fluid can be easily supplied even without a separate nitrogen generation / supply device, and installation and operating costs can be reduced by utilizing the provided equipment.

[0062] In the case of a conventional cold power generation system using the propane Rankine cycle (C3 Organic Rankine Cycle), there is a risk of jet fire due to the flammability and heavier-than-air nature of the working fluid, propane.

[0063] The power generation system using the Brayton Cycle, which uses supercritical carbon dioxide as a working fluid, is not suitable for recovering cold heat because the freezing point of the working fluid, carbon dioxide, is -78.5℃ at normal pressure, and thus the problem of freezing may occur during the process of recovering cold heat from LNG supplied at -155℃.

[0064] In addition, carbon dioxide is a greenhouse gas that causes environmental pollution, is an acid gas that causes equipment corrosion, and is dangerous to the human body when exposed to a certain concentration or higher for a long period of time. In addition, in order to maintain carbon dioxide at a pressure higher than the critical pressure, the operating pressure of the system must be maintained in a high-pressure environment of 200 barg or more, making it difficult to apply in practice.

[0065] Meanwhile, the lower the temperature of the fluid, the higher its density, so the energy consumed for compression decreases.

[0066] According to the present embodiment, the working fluid stream (NL) discharged from the turbine-generator (110) is branched into two or more flows including a first working fluid stream (NLa) and a second working fluid stream (NLb), the first working fluid stream (NLa) is cooled by heat exchange with LNG in a first economizer (120) and then compressed in a first compressor (130), and the second working fluid stream (NLb) is cooled by heat exchange with the first working fluid stream (NLa) compressed in the first compressor (130) in a second economizer (140) and then compressed in a second compressor (150), thereby reducing the energy consumed for compression.

[0067] Meanwhile, the higher the temperature of the fluid introduced into the turbine, the more energy is generated by expansion.

[0068] According to the present embodiment, by providing a heater (160), the first working fluid stream (NLa) compressed in the first compressor (130) and from which residual cold heat is recovered in the second economizer (140) and the second working fluid stream (NLb) compressed in the second compressor (150) are heated and supplied before being supplied to the turbine-generator (110), thereby increasing the energy generated by expansion, i.e., the amount of power generated.

[0069] The first working fluid stream (NLa) compressed in the first compressor (130) and from which residual cooling heat is recovered in the second economizer (140) and the second working fluid stream (NLb) compressed in the second compressor (150) may be combined into one working fluid stream (NL) through a combining portion (not given a drawing symbol) before being supplied to the heater (160).

[0070] FIG. 1 illustrates an example in which one heater (160) is provided, and working fluid streams that have branched at a branching section and then gone through each process are combined into one working fluid stream (NL) at a confluence section. However, this is not limited thereto, and one or more heaters (160) may be provided in series or in parallel.

[0071] For example, two heaters (160) are installed in series, and the working fluid streams that were branched into multiple flows are joined into one working fluid stream (NL) at the confluence, and can be heated in two stages while sequentially passing through the two heaters (160) connected in series.

[0072] Meanwhile, two heaters (160) are installed in parallel, and the working fluid streams that were branched into multiple flows are combined into two working fluid streams at the junction, and the two combined working fluid streams may be distributed and supplied to two heaters (160) installed in parallel.

[0073] In the heater (160) of this embodiment, the heat source for heating the working fluid stream (NL) is seawater (SW), as an example.

[0074] Meanwhile, FIG. 1 illustrates an example in which an economizer (120, 140) for recovering cold heat of LNG is provided in two stages, including a first economizer (120) that directly exchanges heat between a working fluid and LNG, and a second economizer (140) that is provided at the rear end of the first economizer (120) and exchanges heat between the working fluid and the working fluid (first working fluid stream) from which cold heat of LNG is recovered in the first economizer (120).

[0075] In this embodiment, as illustrated in Fig. 1, an example is provided in which two stages of economizers (120, 140) are provided to directly or indirectly recover the cold heat of LNG, including a first economizer (120), a first compressor (130), a second economizer (140), and a second compressor (150). However, the number of stages of the economizers (120, 140) is not limited thereto. That is, one or more stages of the economizers (120, 140) may be provided, and the cold heat of LNG may be recovered through a process of one or more stages.

[0076] When the process for recovering residual cold heat of LNG is configured as N stages, the power generation system according to the present invention may include a branching section for branching a working fluid stream (NL) into N streams, N economizers including a first economizer for directly recovering cold heat of LNG and a second economizer for recovering cold heat of LNG, and N compressors provided at the rear end of each economizer in one-to-one correspondence with the N economizers.

[0077] For example, the operating method when the economizer is equipped with three stages is as follows. At this time, the working fluid stream is branched into three streams including a first working fluid stream, a second working fluid stream, and a third working fluid stream at the branch section.

[0078] In addition, at this time, the economizer may include a first economizer (not shown) in which a first working fluid stream and LNG are heat-exchanged, a first compressor that compresses the first working fluid stream whose temperature is lowered by heat exchange in the first economizer, a second economizer that heat-exchanges the second working fluid stream and the first working fluid stream compressed in the first compressor, a second compressor that compresses the second working fluid stream whose temperature is lowered by heat exchange in the second economizer, a third economizer that heat-exchanges the second working fluid stream compressed in the second compressor and the third working fluid stream, and a third compressor that compresses the third working fluid stream whose temperature is lowered by heat exchange in the third economizer.

[0079] In addition, at this time, the first working fluid stream whose temperature has been increased by heat exchange in the second economizer, the second working fluid stream whose temperature has been increased by heat exchange in the third economizer, and the third working fluid stream compressed in the third compressor are combined at a junction and introduced into the heater (160).

[0080] As a result of the simulation, in the case where the liquefied gas is LNG and the working fluid is methane (BOG) as in this example, it was confirmed that the process for recovering residual cold heat can be equipped with up to 11 stages including up to 11 economizers and 11 compressors, and when equipped with 11 stages, the efficiency is improved by approximately 42.3% compared to the propane ORC cold heat power generation system.

[0081] Meanwhile, the simulation results showed that when the liquefied gas is liquefied hydrogen and the working fluid is hydrogen gas, the efficiency was improved by approximately 497.5% compared to the conventional propane ORC cold-heat power generation system even if the process for recovering residual cold heat was equipped with four stages, including up to five economizers and five compressors.

[0082] In this embodiment, where the liquefied gas is LNG and the working fluid is methane (BOG), the discharge pressure of the turbine-generator (110) is explained as an example of 60 to 80 barg, or 77 barg.

[0083] In addition, when the discharge pressure from the turbine-generator (110) is 60 to 80 barg, or 77 barg, the discharge pressure of the second compressor (150) and the first compressor (130), i.e., the inlet pressure of the turbine-generator (110), is 130 to 150 barg, or 140 barg, is explained as an example.

[0084] When the introduction pressure of the turbine-generator (110) is high, the amount of power that can be generated by the turbine-generator (110) increases, and at the same time, the temperature of the working fluid introduced from the turbine-generator (110) to the first economizer (120) decreases. Considering the approach temperature of the first economizer (120), when the temperature of the working fluid introduced to the first economizer (120) decreases, the amount of cold heat that can be recovered from the liquefied gas decreases, and the amount of power consumed to compress the working fluid in the compressor (130, 150) also increases, so the overall efficiency of the power generation system may actually decrease.

[0085] According to the present embodiment, when the second economizer (140) and the second compressor (150) are provided in only one stage, and the discharge pressures of the second compressor (150) and the first compressor (130) are 130 to 150 barg, or 140 barg, the ratio of the power generation amount of the turbine-generator (110) to the power consumption of the compressor can be the highest. This is an example of a case where the liquefied gas is LNG and the working fluid is methane, and the discharge pressure of the compressor (130, 150) having a high ratio of generated power to consumed power can vary depending on the liquefied gas and the working fluid.

[0086] The power generation system according to the present embodiment is characterized by maximizing the efficiency of the power generation system by compressing the working fluid at the lowest possible temperature and expanding it at the highest possible temperature.

[0087] The efficiency improvement effect of the power generation system utilizing the cold energy of liquefied gas according to the above-described embodiment was verified through process simulation. The simulation results are illustrated using an example where the liquefied gas is LNG and the working fluid is methane.

[0088] The temperature of LNG introduced into the first economizer (120) was set to -161.5℃, and a process simulation including a cold heat recovery process in which 11 economizers directly and indirectly recover the cold heat of LNG across 11 stages was performed. As a result, it was confirmed that the efficiency increased by approximately 42.3% compared to an ORC power generation system in which the working fluid is propane and a phase change occurs during the cycle. More specific efficiency improvement effects will be described later.

[0089] Referring to Fig. 1, the working fluid in a supercritical state of 140 barg and 10°C expands to 77 barg while driving the turbine in the turbine-generator (110), and the temperature is lowered to -28.04°C, and then branches into 11 streams at the branch section. At this time, the amount of power (E1) generated by the working fluid driving the turbine, as confirmed through process simulation, is a total of 48.7 kW.

[0090] The first working fluid stream (NLa) is supplied to the first economizer (120) and exchanges heat with LNG supplied through the liquefied gas line (LL). At this time, the temperature of the LNG introduced into the first economizer (120) through the liquefied gas line (LL) is, for example, approximately -157.6°C and the pressure is 80 barg.

[0091] In the first economizer (120), the first working fluid stream (NLa) is cooled to about -154.6°C through heat exchange, and LNG is heated to about -40°C and vaporized.

[0092] Here, the term 'vaporization' not only means a phase change from liquid to gas, but can also be a concept that includes a temperature increase through heat exchange even in a state where it is difficult to distinguish a phase change, such as in a supercritical state.

[0093] For example, if the pressure of LNG introduced into the first economizer (120) is approximately 80 barg, which is the pressure required by the onshore regasification gas demand source, then since LNG is in a supercritical state, it cannot be clearly distinguished that LNG changes phase from liquid to gaseous state, and the temperature rises within the first economizer (120) and moves to a state close to gaseous state and is discharged, but this process is expressed as 'vaporization'.

[0094] The first working fluid stream (NLa) cooled in the first economizer (120) is compressed to about 140.8 barg by the first compressor (130), and the temperature rises to about -152.1°C during the compression process. At this time, the compression energy (E2) consumed in the first compressor (130) is about 4.631 kW.

[0095] Meanwhile, the second working fluid stream (NLb) branched off downstream of the turbine-generator (110) is supplied to the second economizer (140). In the second economizer (140), the first working fluid stream (NLa) of low temperature and high pressure compressed in the first compressor (130) and the second working fluid stream (NLb) undergo heat exchange.

[0096] In the second economizer (140), the second working fluid stream (NLb) at about -28.04°C and about 77 barg can be cooled to about -149.1°C through heat exchange, and the first working fluid stream (NLa) can be heated to about -38.1°C.

[0097] The second working fluid stream (NLb) cooled in the second economizer (140) is compressed in the second compressor (150) to about 140.8 barg, which is slightly higher than the inlet pressure of the turbine-generator (110), and the temperature rises to about -146.4°C during the compression process. At this time, the compression energy consumed in the second compressor (150) is about 1.989 kW (E3).

[0098] The subsequent processes, from the third to the eleventh economizers, repeat the aforementioned processes, so a detailed description will be omitted. The eleventh working fluid stream cooled in the eleventh economizer, the final economizer, is compressed to approximately 140.8 barg in the eleventh compressor.

[0099] Through this process, the high temperature and high pressure working fluid streams compressed in the compressor are mixed into one working fluid stream (NL), thereby generating a working fluid stream (NL) of approximately -39.05°C.

[0100] The mixed working fluid stream (NL) is heated to 10°C in a heater (160). As an example, the heat source for heating the working fluid stream (NL) in the heater (160) is seawater (SW). The temperature of the seawater (SW) supplied to the heater (160) is 16.55°C, and can be cooled to 11.55°C through heat exchange and discharged.

[0101] The working fluid stream (NL) heated in the heater (160) is circulated back to the turbine-generator (110).

[0102] Through the process simulation described above, it was confirmed that the total power generation (E1) generated was approximately 48.7 kW, the total work consumed was approximately 17.4 kW, and the total net power generation was approximately 31.4 kW per ton / hr.

[0103] Figure 2 illustrates a conventional propane ORC power generation system.

[0104] Referring to Fig. 2, a conventional propane ORC power generation system is composed of a condenser (C) that condenses propane by exchanging heat between propane, which is a working fluid, and LNG, a pump (P) that pressurizes the propane condensed in the condenser (C), a heater (H) that vaporizes the propane pressurized by the pump (P), and a turbine-generator (T) that generates electricity by driving a turbine with the working fluid vaporized in the heater (H).

[0105] Through process simulation, it can be confirmed that the amount of power that can be generated using the conventional propane ORC power generation system shown in FIG. 2 is 22.1 kW per 1 ton / hr of LNG, and that the power generation system according to the present embodiment has an increase in power generation efficiency of approximately 42.3% compared to the conventional propane ORC power generation system.

[0106] In a conventional propane ORC power generation system that pressurizes the working fluid to a liquid state, the pump duty (E5) required to pressurize the propane is 0.5 kW, whereas in the power generation system according to the present embodiment that compresses the working fluid to a supercritical state, the compressor duty required to compress the working fluid reaches 17.4 kW.

[0107] However, while the conventional propane ORC power generation system that expands the working fluid in a gaseous state in a turbine-generator has an expansion work (E4) generated in the turbine of 22.1 kW, the power generation system according to the present embodiment that expands the working fluid in a supercritical state generates an expansion work of 48.7 kW (E1), resulting in a significant increase in power generation efficiency.

[0108]

[0109] As described above, embodiments of the present invention have been described. It will be apparent to those skilled in the art that the present invention can be embodied in other specific forms, in addition to the embodiments described above, without departing from the spirit or scope thereof. Therefore, the above-described embodiments should be considered illustrative rather than restrictive, and accordingly, the present invention is not limited to the above description, but may be modified within the scope of the appended claims and their equivalents.

[0110]

[0111] <Explanation of symbols>

[0112] 110: Turbine-generator

[0113] 120: First Economizer

[0114] 130: 1st compressor

[0115] 140: Second Economizer

[0116] 150: Second compressor

[0117] 160: Heater

[0118] NL: Working fluid stream

[0119] NLa: First working fluid stream

[0120] NLb: Second working fluid stream

[0121] LL: Liquefied gas line

Claims

1. A turbine-generator that generates electricity by driving a turbine with a working fluid; A branch section for branching an expanded working fluid stream into a first working fluid stream and a second working fluid stream while driving a turbine in the turbine-generator; A first economizer that recovers cold heat from the liquefied gas by heat-exchanging the first working fluid stream and the liquefied gas; A first compressor that compresses a cooled first working fluid stream while recovering cold heat of the liquefied gas from the first economizer; A second economizer that recovers residual cold heat of the first working fluid stream by heat-exchanging the first working fluid stream compressed in the first compressor and the second working fluid stream transferred from the branch; and A power generation system utilizing the cold heat of liquefied gas, comprising a second compressor that recovers residual cold heat of the first working fluid stream in the second economizer and compresses the cooled second working fluid stream.

2. In claim 1, The above working fluid stream is, A power generation system utilizing the cold heat of liquefied gas, which is maintained at a pressure higher than the critical pressure while circulating through the turbine-generator, the first economizer, the first compressor, the second economizer, and the second compressor.

3. In claim 1, The above working fluid is, A power generation system utilizing the cold energy of liquefied gas, which is an extremely low-temperature fluid that does not reach its freezing point through heat exchange with the liquefied gas.

4. In claim 1, A joining portion that joins the second working fluid stream compressed in the second compressor and the first working fluid stream from which residual cold heat is recovered in the second economizer into a working fluid stream; and A power generation system utilizing the cold energy of liquefied gas, further comprising a heater for heating the working fluid stream joined at the above joining section and circulating it to the turbine-generator.

5. In claim 4, The above heater is provided with one or more, A power generation system utilizing the cold energy of liquefied gas, wherein when two or more of the above heaters are provided, the heaters are provided in series or parallel.

6. In claim 1, A power generation system utilizing the cold energy of liquefied gas, wherein the second economizer and the second compressor are provided in one or more stages.

7. In claim 1, A power generation system utilizing the cold energy of liquefied gas, wherein the first economizer and the second economizer are one or more multi-stream heat exchangers.

8. In claim 1, The above first economizer and second economizer are, A power generation system utilizing the cold energy of liquefied gas, which is a plate heat exchanger, a plate-fin heat exchanger, a printed circuit board heat exchanger, an aluminum bonded heat exchanger, or a coil wound heat exchanger.

9. A power generation stage that generates electricity by driving a turbine with a working fluid; A branching step for branching the expanded working fluid stream into a first working fluid stream and a second working fluid stream while driving the turbine in the above-mentioned development step; A cold heat recovery step for recovering cold heat of the liquefied gas by heat-exchanging the first working fluid stream and the liquefied gas; A low-temperature compression step for compressing the cooled first working fluid stream while recovering the cold heat of the liquefied gas in the above cold heat recovery step; A residual heat recovery step of recovering residual heat of the first working fluid stream by heat-exchanging the first working fluid stream compressed in the low-temperature compression step and the second working fluid stream branched in the branch step; A power generation method using cold heat of liquefied gas, comprising a high temperature compression step for recovering the residual cold heat of the first working fluid stream in the above residual cold heat recovery step and compressing the cooled second working fluid stream.

10. In claim 9, A joining step of joining the second working fluid stream compressed in the high-temperature compression step and the first working fluid stream from which residual cooling heat has been recovered in the residual cooling heat recovery step into a working fluid stream; and A power generation method using cold heat from liquefied gas, further comprising a heating step of heating the working fluid stream joined in the joining step and circulating it to the power generation step.

11. In claim 9, A method for power generation using cold energy of liquefied gas, wherein the working fluid is a substance that is maintained above the critical pressure and does not undergo a phase change while circulating through the above steps.

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