Fuel cell and geothermal combined power generation system and method for multi-gradient waste heat utilization

Abstract

The application relates to a fuel cell and geothermal combined power generation system and method with multi-gradient waste heat utilization, and belongs to the technical field of energy comprehensive utilization. The application solves the problems of the existing geothermal power generation system, such as the mismatch between power supply and regional power demand fluctuation and low geothermal power generation efficiency. A fuel compressor, a mixer, an anode of a fuel cell, a separator, the mixer are sequentially connected through a fuel pipeline to form a fuel circulation, and the separator and an afterburner are connected through a fuel pipeline; an air compressor, a first heat exchanger, a cathode of the fuel cell, the afterburner, the first heat exchanger and a gas turbine are sequentially connected through a gas pipeline to form a gas circulation; a geothermal well and a second heat exchanger are connected through a water pipeline to form a geothermal water circulation; the second heat exchanger, a third heat exchanger, an expander, a first thermoelectric power generation device and the second heat exchanger are sequentially connected through a working medium pipeline to form an organic Rankine cycle; and exhaust gas discharged from the gas turbine is connected to the third heat exchanger through a first exhaust gas pipeline for heat exchange.

Description

Technical Field

[0001] The present invention relates to a fuel cell and geothermal combined power generation system and method for multi-gradient waste heat utilization, belonging to the technical field of comprehensive energy utilization. Background Art

[0002] Geothermal energy is a non-carbon, renewable, sustainable energy source. It boasts stable heat output, continuous 24 / 7 energy supply, zero emissions, zero pollution, and vast reserves. It holds untapped potential for mitigating the threat of climate change. Geothermal energy has garnered widespread attention in recent years as a renewable energy source. Geothermal power generation technology, owing to these characteristics, is considered a promising regional power supply solution. The general principle of geothermal power generation involves circulating water through an underground well. After the water has fully absorbed geothermal heat, it is pumped out and then, using a thermodynamic cycle (such as the Organic Rankine Cycle), the absorbed geothermal energy is converted into mechanical energy, thereby generating electricity.

[0003] Large geothermal power plants operate relatively stably, generating relatively stable power output. However, regional electricity load fluctuates significantly over time, making a single geothermal power generation system insufficient to adequately match power supply with regional consumption. This can lead to power shortages during peak periods. Furthermore, most geothermal resources are low-temperature, with low heat quality and limited power generation efficiency, hindering the widespread application of geothermal power generation technology. Achieving flexible energy supply and improving power generation efficiency in geothermal energy systems is a pressing challenge in geothermal development and utilization.

[0004] Against this backdrop, demand for natural gas peak-shaving to ensure stable urban power supply has increased significantly. Natural gas can be used in fuel cells, generating highly efficient electricity through energy conversion, playing a vital role in improving urban power supply systems. However, the exhaust gas temperatures of fuel cell systems are high, and the waste heat in the exhaust is either directly discharged or used only to heat the air, rather than being converted into electricity, resulting in energy waste. Due to these limitations, current fuel cell exhaust gas treatment efficiency is low, and the overall system output characteristics urgently need to be improved.

[0005] In the existing technology, medium and low temperature geothermal resources generate electricity by heating working fluids in different cycles. For example, in the organic Rankine cycle, geothermal water from hydrothermal geothermal resources can be extracted from the ground, and then the geothermal water is used to heat the organic working fluid, which is then used to perform external work to generate electricity. This type of solution only utilizes geothermal energy through a single circulation system, resulting in high return water temperature, insufficient utilization of geothermal energy, low power generation efficiency under the heat source conditions of medium and low temperature geothermal water, and difficulty in selecting a suitable circulating working fluid to match the heat source. Due to the above problems, the current proportion of medium and low temperature geothermal power generation is very small, and the power generation method urgently needs to be broken through. Summary of the Invention

[0006] The present invention aims to solve the problems of mismatch between the power supply of existing geothermal power generation systems and the fluctuation of regional power demand and low geothermal power generation efficiency, and thus provides a fuel cell and geothermal combined power generation system and method with multi-gradient waste heat utilization.

[0007] The technical solution adopted by the present invention to solve the above technical problems is:

[0008] A fuel cell and geothermal combined power generation system with multi-gradient waste heat utilization includes a solid oxide fuel cell subsystem and a geothermal subsystem, wherein:

[0009] The solid oxide fuel cell subsystem includes a fuel compressor, a mixer, a fuel cell, a separator, an afterburner, a first heat exchanger, a gas turbine, and an air compressor. The fuel compressor, the mixer, the anode of the fuel cell, the separator, and the mixer are sequentially connected through a fuel pipeline to form a fuel cycle. The separator and the afterburner are connected through a fuel pipeline. The air compressor, the first heat exchanger, the cathode of the fuel cell, the afterburner, the first heat exchanger, and the gas turbine are sequentially connected through a gas pipeline to form a gas cycle.

[0010] The geothermal subsystem includes a geothermal well, a second heat exchanger, a third heat exchanger, an expander, a working fluid pump and a first thermoelectric power generation device. The geothermal well and the second heat exchanger form a geothermal water circulation through a water pipeline; the second heat exchanger, the third heat exchanger, the expander, the first thermoelectric power generation device and the second heat exchanger are connected in sequence through a working fluid pipeline to form an organic Rankine cycle; the exhaust gas discharged by the gas turbine is connected to the third heat exchanger through the first exhaust gas pipeline for heat exchange, and the exhaust gas after heat exchange in the third heat exchanger is discharged through the second exhaust gas pipeline.

[0011] Furthermore, in the geothermal water circulation, a second temperature difference power generation device is provided on the water pipeline between the second heat exchanger and the geothermal well.

[0012] Furthermore, a third temperature difference power generation device is provided on the second exhaust gas pipeline.

[0013] Furthermore, a pre-reformer is provided on the fuel pipeline between the mixer and the anode inlet of the fuel cell.

[0014] Furthermore, the solid oxide fuel cell subsystem also includes a fourth heat exchanger, which is connected to the gas pipeline between the air compressor and the first heat exchanger, the gas turbine and the fourth heat exchanger are connected through the first exhaust gas pipeline, and the fourth heat exchanger and the third heat exchanger are connected through the third exhaust gas pipeline.

[0015] A power generation method using the combined power generation system, in the fuel cycle, natural gas is pressurized by a fuel compressor and then enters the system, the anode outlet exhaust gas of the fuel cell contains unused fuel, a part of the exhaust gas is recycled to the mixer to provide steam and heat for the reforming of the imported natural gas, and the other part of the exhaust gas is mixed with the gas flowing out of the cathode outlet of the fuel cell into the afterburner, and the mixed gas is heated in the first heat exchanger and then expanded in the gas turbine to generate power.

[0016] In the gas cycle, air enters the system through an air compressor, is preheated by the high-temperature exhaust gas in the first heat exchanger, and then enters the cathode of the fuel cell, and the cathode outlet gas of the fuel cell is mixed with the anode outlet gas in the afterburner and then enters the first heat exchanger to exchange heat.

[0017] The circulating water extracts heat from the formation, exchanges heat in the second heat exchanger, and then transfers the heat to the organic working medium in the organic Rankine cycle, which generates power using the absorbed geothermal heat.

[0018] Compared with the prior art, the present application has the following effects:

[0019] Through the combined power generation system of the present application, flexible power supply and efficiency improvement of the low-temperature geothermal power generation system can be realized, by integrating the geothermal power generation system with the high-temperature fuel cell system, adjusting the working condition and output power of the fuel cell system under the condition of stable geothermal power generation capacity, the overall power supply capacity of the system can be quickly adjusted to follow the regional power consumption load, solving the problem of energy waste caused by mismatch between power supply and power consumption; at the same time, the heat generated by the high-temperature fuel cell system is utilized in multiple gradients, and the circulating temperature of the geothermal power generation organic Rankine cycle system is improved using fuel cell exhaust gas, thereby improving the efficiency of geothermal power generation.

[0020] For low-temperature geothermal resources, when the extracted geothermal source temperature is lower than 150 DEG C, the cycle uses the geothermal heat and the fuel cell exhaust gas to heat the organic working medium twice, which is equivalent to upgrading the low-grade heat of the geothermal source to a high-quality heat source with a higher temperature, thereby improving the circulating power generation efficiency of the organic Rankine cycle; at the same time, the first temperature difference power generation device is added to utilize the geothermal energy after being used by the organic Rankine cycle, realizing efficient geothermal gradient utilization. BRIEF DESCRIPTION OF DRAWINGS

[0021] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed in the embodiments will be briefly introduced as follows: obviously, the drawings in the following description are only two embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0022] Figure 1 It is a system composition schematic diagram in the specific embodiment one of the present application.

[0023] Figure 2 This is a schematic diagram of the system composition in the second specific implementation method of this application. DETAILED DESCRIPTION

[0024] The following will be combined with the drawings in this application to clearly and completely describe the technical solutions in the specific embodiments of this application. Obviously, the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments of this application, all other embodiments obtained by ordinary technicians in this field are within the scope of protection of this application.

[0025] In the description of this application, unless otherwise expressly specified or limited, the terms "disposed" and "connected" should be understood in a broad sense. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to direct connections, indirect connections through an intermediate medium, or internal connections between two components. Those skilled in the art will understand the specific meanings of the above terms in this application based on the specific circumstances.

[0026] Specific implementation method 1: Combination Figure 1 This embodiment describes a fuel cell and geothermal combined power generation system with multi-gradient waste heat utilization, including a solid oxide fuel cell subsystem 1 and a geothermal subsystem 2, wherein:

[0027] The solid oxide fuel cell subsystem 1 includes a fuel compressor 1-1, a mixer 1-2, a fuel cell 1-3, a separator 1-4, an afterburner 1-5, a first heat exchanger 1-6, a gas turbine 1-7 and an air compressor 1-8. The fuel compressor 1-1, the mixer 1-2, the anode of the fuel cell 1-3, the separator 1-4 and the mixer 1-2 are connected in sequence through a fuel pipeline to form a fuel cycle. The separator 1-4 and the afterburner 1-5 are connected through a fuel pipeline; the air compressor 1-8, the first heat exchanger 1-6, the cathode of the fuel cell 1-3, the afterburner 1-5, the first heat exchanger 1-6 and the gas turbine 1-7 are connected in sequence through a gas pipeline to form a gas cycle.

[0028] The geothermal subsystem 2 includes a geothermal well 2-1, a second heat exchanger 2-2, a third heat exchanger 2-3, an expander 2-4, a working fluid pump 2-5 and a first thermoelectric power generation device 2-6. The geothermal well 2-1 and the second heat exchanger 2-2 form a geothermal water circulation through a water pipeline; the second heat exchanger 2-2, the third heat exchanger 2-3, the expander 2-4, the first thermoelectric power generation device 2-6 and the second heat exchanger 2-2 are connected in sequence through a working fluid pipeline to form an organic Rankine cycle; the exhaust gas discharged by the gas turbine 1-7 is connected to the third heat exchanger 2-3 through the first exhaust gas pipeline 3 for heat exchange, and the exhaust gas after heat exchange in the third heat exchanger 2-3 is discharged through the second exhaust gas pipeline 4.

[0029] The core components of the solid oxide fuel cell subsystem 1 are the fuel cell 1-3 and the gas turbine 1-7.

[0030] The fuel is natural gas.

[0031] The mixer 1-2 is a gas mixture.

[0032] The cold end of the thermoelectric power generation device is connected with a cooling water inlet pipeline and a cooling water outlet pipeline.

[0033] In the fuel cycle, the natural gas is pressurized by the fuel compressor 1-1 and then enters the system, the anode outlet exhaust gas of the fuel cell 1-3 contains unused fuel, a part of the exhaust gas is recirculated to the mixer 1-2 to provide steam and heat for the reforming of the imported natural gas, and the other part of the exhaust gas is mixed with the gas flowing out of the cathode outlet of the fuel cell 1-3 in the afterburner 1-5 to form high-temperature exhaust gas, which is expanded in the gas turbine 1-7 after heat exchange in the first heat exchanger 1-6.

[0034] In the gas cycle, air enters the system through the air compressor 1-8 and is preheated by the high-temperature exhaust gas in the first heat exchanger 1-6, and then enters the cathode of the fuel cell 1-3, and the cathode outlet gas of the fuel cell 1-3 is mixed with the anode outlet gas in the afterburner 1-5 to form high-temperature exhaust gas.

[0035] The circulating water extracts heat from the stratum, and after heat exchange in the second heat exchanger 2-2, the heat is transferred to the organic working medium in the organic Rankine cycle, and the organic Rankine cycle generates power by absorbing geothermal heat. Specifically:

[0036] In the geothermal water cycle, the circulating water extracts heat from the stratum through the geothermal well 2-1, and then releases heat after the second heat exchanger 2-2, and then flows back to the geothermal well 2-1 for heat extraction;

[0037] In the organic Rankine cycle, the organic working medium is first pressurized by the working medium pump 2-5 to the evaporation pressure, and then absorbs the heat of the circulating water through the second heat exchanger 2-2 to reach the saturated gas state; the saturated gas further absorbs the heat of the exhaust gas discharged from the solid oxide fuel cell subsystem 1 in the third heat exchanger 2-3 to reach the supersaturated state; then, most of the heat absorbed from the geothermal water and the exhaust gas is converted into mechanical work by the expander 2-4, and the remaining low-grade heat is further recovered by the third thermoelectric power generation device 6 to generate power, thereby constructing a thermoelectric power generation-organic Rankine combined cycle composite heat recovery system.

[0038] The combined power generation system of the present application can achieve flexible power supply and efficiency improvement of the low-temperature geothermal power generation system. By integrating the geothermal power generation system with the high-temperature fuel cell 1-3 system, the geothermal water cycle and the organic Rankine cycle system maintain stable operating conditions, and the output power remains unchanged. The power generation of the fuel cells 1-3 is increased during peak power consumption, and the power generation of the fuel cells 1-3 is reduced during low power consumption, thereby achieving matching of the combined system with regional power consumption fluctuations. Under the condition of stable geothermal power generation, the operating conditions and output power of the fuel cell 1-3 system are adjusted, and the overall power supply of the system can be quickly adjusted according to the regional power load, thereby solving the energy waste problem caused by the mismatch between energy supply and energy consumption.

[0039] The multi-gradient system utilizes the heat generated by the high-temperature fuel cell 1-3 system, using the exhaust gas from the fuel cells 1-3 to raise the temperature of the geothermal power generation organic Rankine cycle system, thereby improving geothermal power generation efficiency. The exhaust gas generated by the high-temperature fuel cell 1-3 reaches extremely high temperatures after combustion. The high-temperature waste heat of the waste gas first preheats the intake air of the fuel cell 1-3, then recovers most of the waste heat energy through the steam turbine, and finally uses it to heat the superheated gas of the organic Rankine cycle.

[0040] Specifically, the cascaded utilization of waste heat from fuel cells 1-3 can improve the overall energy efficiency of the combined system. First, the high-grade waste heat from fuel cells 1-3 is used to preheat the cathode inlet air to the required temperature, ensuring a stable and uniform temperature for fuel cells 1-3. Then, the mid-range waste heat is converted into electricity via gas turbine 1-7. Finally, the low-grade exhaust waste heat is used to raise the temperature of the organic working fluid in the organic Rankine cycle, which is then used in conjunction with the first thermoelectric generator 2-6 to generate electricity. In this system, the solid oxide fuel cell subsystem 1, the geothermal subsystem 2, and the enhanced organic Rankine cycle are integrated to achieve cascaded utilization of natural gas, geothermal energy, and system waste heat, improving energy efficiency.

[0041] For low-temperature geothermal resources, when the temperature of the mined geothermal source is below 150°C, the cycle heats the organic working fluid twice through geothermal heat and the exhaust gas from the fuel cells 1-3, equivalently upgrading the low-grade heat of the geothermal source to a higher-temperature, high-quality heat source, thereby improving the cycle power generation efficiency of the organic Rankine cycle. At the same time, the addition of the first thermoelectric power generation device 2-6 allows for secondary utilization of the geothermal energy after being utilized by the organic Rankine cycle, achieving efficient cascade utilization of geothermal energy.

[0042] In the geothermal water circulation, a second thermoelectric power generation device 2-7 is installed in the water pipeline between the second heat exchanger 2-2 and the geothermal well 2-1. This design, by adding the second thermoelectric power generation device 2-7 to the geothermal water circulation, recycles the lowest-grade waste heat in the system, thereby achieving more efficient cascaded geothermal utilization.

[0043] A third thermoelectric generator 6 is arranged on the second exhaust pipe 4. In this way, the third thermoelectric generator 6 is arranged on the second exhaust pipe 4, further recovering and utilizing the waste heat with the lowest heat grade in the system, improving the heat energy conversion efficiency and reducing the waste of heat energy.

[0044] A pre-reformer 1-9 is arranged on the fuel pipe between the mixer 1-2 and the anode inlet of the fuel cell 1-3. In this way, a part of the exhaust gas separated by the separator 1-4 is mixed with the natural gas sent by the fuel compressor 1-1 in the mixer 1-2 to form mixed fuel gas, and the mixed fuel gas is first pre-converted in the external pre-reformer 1-9 and then further converted and consumed in the fuel cell 1-3.

[0045] Specific implementation method two: combination Figure 2 In this embodiment, the solid oxide fuel cell subsystem 1 further comprises a fourth heat exchanger 1-10, which is arranged on the gas pipe between the air compressor 1-8 and the first heat exchanger 1-6, the gas turbine 1-7 is connected to the fourth heat exchanger 1-10 through the first exhaust pipe 3, and the fourth heat exchanger 1-10 is connected to the third heat exchanger 2-3 through the third exhaust pipe 5. In this way, the air is compressed by the air compressor 1-8 into the system, preheated with the high-temperature exhaust gas discharged from the gas turbine 1-7 at the fourth heat exchanger 1-10, and then enters the first heat exchanger 1-6 for the second heating, further improving the gas temperature entering the cathode of the fuel cell 1-3.

[0046] The first heat exchanger 1-6 is a high-temperature heat exchanger, the second heat exchanger 2-2 is a low-temperature evaporator, the third heat exchanger 2-3 is a high-temperature evaporator, and the fourth heat exchanger 1-10 is a low-temperature heat exchanger.

[0047] Specific implementation method three: combination Figures 1-2 In this embodiment, a power generation method using the above combined power generation system is described. In the fuel cycle, the natural gas is pressurized by the fuel compressor 1-1 and then enters the system, the anode outlet exhaust gas of the fuel cell 1-3 contains unused fuel, a part of the exhaust gas is recycled to the mixer 1-2 to provide steam and heat for the reforming of the imported natural gas, and another part of the exhaust gas is mixed with the gas flowing out of the cathode outlet of the fuel cell 1-3 in the afterburner 1-5, and the mixed gas is expanded in the gas turbine 1-7 after being heated in the first heat exchanger 1-6 to generate electricity.

[0048] In the gas cycle, the air enters the system through the air compressor 1-8, is preheated by the high-temperature exhaust gas in the first heat exchanger 1-6, and then enters the cathode of the fuel cell 1-3, and the gas at the cathode outlet of the fuel cell 1-3 is mixed with the gas at the anode outlet in the afterburner 1-5 and then enters the first heat exchanger 1-6 for heat exchange.

[0049] The circulating water extracts heat from the formation, and after heat exchange in the second heat exchanger 2-2, the heat is transferred to the organic working medium in the organic Rankine cycle, and the organic Rankine cycle generates electricity using the absorbed geothermal energy.

Claims

1. A fuel cell and geothermal combined power generation system with multi-gradient waste heat utilization, characterized by: It includes a solid oxide fuel cell subsystem (1) and a geothermal subsystem (2), wherein: The solid oxide fuel cell subsystem (1) comprises a fuel compressor (1-1), a mixer (1-2), a fuel cell (1-3), a separator (1-4), an afterburner (1-5), a first heat exchanger (1-6), a gas turbine (1-7) and an air compressor (1-8); the fuel compressor (1-1), the mixer (1-2), an anode of the fuel cell (1-3), the separator (1-4) and the mixer (1-2) are sequentially connected via a fuel pipeline to form a fuel cycle; the separator (1-4) and the afterburner (1-5) are connected via a fuel pipeline; the air compressor (1-8), the first heat exchanger (1-6), the cathode of the fuel cell (1-3), the afterburner (1-5), the first heat exchanger (1-6) and the gas turbine (1-7) are sequentially connected via a gas pipeline to form a gas cycle; The geothermal subsystem (2) includes a geothermal well (2-1), a second heat exchanger (2-2), a third heat exchanger (2-3), an expander (2-4), a working fluid pump (2-5) and a first thermoelectric power generation device (2-6); the geothermal well (2-1) and the second heat exchanger (2-2) form a geothermal water cycle through a water pipeline; the second heat exchanger (2-2), the third heat exchanger (2-3), the expander (2-4), the first thermoelectric power generation device (2-6) and the second heat exchanger (2-2) are sequentially connected through the working fluid pipeline to form an organic Rankine cycle; the exhaust gas discharged from the gas turbine (1-7) is connected to the third heat exchanger (2-3) through the first exhaust gas pipeline (3) for heat exchange, and the exhaust gas after heat exchange in the third heat exchanger (2-3) is discharged through the second exhaust gas pipeline (4); In the geothermal water circulation, a second temperature difference power generation device (2-7) is provided on the water pipeline between the second heat exchanger (2-2) and the geothermal well (2-1); A third temperature difference power generation device (6) is provided on the second exhaust gas pipeline (4); A pre-reformer (1-9) is provided on a fuel pipeline between the mixer (1-2) and the anode inlet of the fuel cell (1-3).

2. The multi-gradient waste heat utilization fuel cell and geothermal combined power generation system according to claim 1 is characterized in that: The solid oxide fuel cell subsystem (1) further comprises a fourth heat exchanger (1-10), wherein the fourth heat exchanger (1-10) is connected to the gas pipeline between the air compressor (1-8) and the first heat exchanger (1-6), the gas turbine (1-7) and the fourth heat exchanger (1-10) are connected via a first exhaust gas pipeline (3), and the fourth heat exchanger (1-10) and the third heat exchanger (2-3) are connected via a third exhaust gas pipeline (5).

3. A method for generating electricity using the combined power generation system according to any one of claims 1 to 2, characterized in that: In the fuel cycle, natural gas is pressurized by a fuel compressor (1-1) and then enters the system. The exhaust gas at the anode outlet of the fuel cell (1-3) contains underutilized fuel. A portion of the exhaust gas is recycled to the mixer (1-2) to provide steam and heat for the reforming of the imported natural gas. The other portion of the exhaust gas enters the afterburner (1-5) and is mixed with the gas flowing out of the cathode outlet of the fuel cell (1-3). After the mixed gas passes through the first heat exchanger (1-6) for heat exchange, it is expanded in the gas turbine (1-7) to generate electricity. In the gas cycle, air enters the system through an air compressor (1-8), is preheated by high-temperature exhaust gas in a first heat exchanger (1-6), and then enters the cathode of a fuel cell (1-3). The cathode outlet gas and the anode outlet gas of the fuel cell (1-3) are mixed in an afterburner (1-5) and then enter the first heat exchanger (1-6) for heat exchange. The circulating water extracts heat from the stratum and, after heat exchange in the second heat exchanger (2-2), transfers the heat to the organic working fluid in the organic Rankine cycle, which uses the absorbed geothermal energy to generate electricity.

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