A waste heat recovery system for solid oxide fuel cells

Abstract

This invention relates to a waste heat recovery system for solid oxide fuel cells (SOFCs), comprising a SOFC stack, a tail burner, an alkali metal thermoelectric converter (AMTEC), a thermoelectric generator (TEG), an air preheater, and a fuel preheater. The cathode inlet gas first flows through the cold side of the AMTEC, where it undergoes initial heat exchange with the high-temperature exhaust gas at the hot end of the AMTEC, and then enters the air preheater for further heating to the SOFC's high-efficiency reaction temperature. The anode fuel gas exchanges heat with the exhaust gas at the air preheater outlet through the fuel preheater, raising its temperature from ambient to the reaction temperature. The SOFC exhaust gas is combusted in the tail burner to generate high-temperature gas, which first enters the hot end of the AMTEC to generate electricity through temperature difference. The cold end of the AMTEC releases heat for initial preheating of the cathode inlet gas. After being cooled by the air preheater and fuel preheater, the high-temperature exhaust gas from the AMTEC enters the hot end of the TEG for secondary power generation.

Description

Technical Field

[0001] This invention belongs to the field of solid oxide fuel cells, and specifically relates to a waste heat cascade recovery system that integrates an alkali metal thermoelectric converter and a thermoelectric generator. Background Technology

[0002] Solid oxide fuel cells (SOFCs), as highly efficient and clean power generation devices, possess significant advantages such as high energy conversion efficiency (up to 60%-80%), strong fuel adaptability, and high waste heat quality, showing broad application prospects in distributed energy systems and large-scale power plants. Their operating temperature is typically between 600-1000℃, and the high-temperature waste heat generated during power generation accounts for approximately 30%-40% of the total input energy. If this waste heat is not effectively utilized, it will not only result in energy waste but may also affect the lifespan and stability of the fuel cell due to excessively high system operating temperatures. Therefore, efficiently recovering and utilizing the waste heat resources of SOFCs is of great significance for improving the overall energy efficiency of the system and reducing operating costs.

[0003] Current waste heat recovery technologies for SOFCs suffer from problems such as unreasonable energy grade matching, low system integration, and insufficient utilization of medium- and low-temperature waste heat. With the development of energy cascade utilization theory, multi-stage waste heat recovery technologies based on different temperature ranges have become a research hotspot: Alkali Metal Thermal-to-Electric Converters (AMTECs) can achieve direct thermoelectric conversion in the high-temperature range of 600-1000℃, offering advantages such as high energy conversion efficiency, compact structure, and no moving parts; Thermoelectric Generators (TEGs) are suitable for the medium- and low-temperature range of 50-500℃, directly converting heat energy into electrical energy through the Seebeck effect, characterized by high reliability and low maintenance costs. However, existing technologies lack solutions for integrating these two technologies to systematically recover SOFC exhaust gas in a cascade manner, resulting in insufficient temperature range adaptability and inefficient integration. Summary of the Invention

[0004] The purpose of this invention is to provide a waste heat recovery system for solid oxide fuel cells, which couples the AMTEC power generation module with a preheating function to reduce heat loss in the pipeline. The technical solution is as follows:

[0005] A waste heat recovery system for solid oxide fuel cells includes a solid oxide fuel cell stack (SOFC), a tail-end burner, an alkali metal thermoelectric converter (AMTEC), a thermoelectric generator (TEG), an air preheater, and a fuel preheater. These components are connected via pipelines to form a gradient energy processing network.

[0006] The cathode inlet gas first flows through the cold side of AMTEC, where it undergoes initial heat exchange with the high-temperature exhaust gas at the hot end of AMTEC to raise its temperature. Then, it enters the air preheater for further heating to the SOFC high-efficiency reaction temperature.

[0007] The anode fuel gas exchanges heat with the exhaust gas from the air preheater through the fuel preheater, raising it from room temperature to the reaction temperature, and then enters the SOFC with air to carry out an electrochemical reaction.

[0008] The SOFC exhaust gas after the reaction is burned in the tail burner to generate high-temperature gas. The high-temperature gas first enters the hot end of the AMTEC and generates electricity through the temperature difference. The cold end of the AMTEC releases heat for the initial preheating of the cathode intake gas.

[0009] The medium- and high-temperature exhaust gas after AMTEC power generation flows into the air preheater and fuel preheater to preheat the cathode intake gas and anode fuel gas.

[0010] After being cooled by the air preheater and fuel preheater, the high-temperature exhaust gas from the AMTEC enters the TEG hot end for secondary power generation.

[0011] Furthermore, the low-temperature exhaust gas after TEG treatment is supplied with a low-temperature heat source via a heat exchanger.

[0012] Furthermore, the TEG cold end maintains the temperature difference through cooling water.

[0013] This invention couples the AMTEC power generation module with a preheating function, reducing heat loss in pipelines. The equipment is compact with no moving parts, has low maintenance costs, and is suitable for modular deployment of distributed energy resources. Attached Figure Description

[0014] Figure 1 As a first comparative example, this is a SOFC-AMTEC-TEG power generation system.

[0015] Figure 2 As a second comparative example, this is a SOFC-AMTEC-TEG power generation system.

[0016] Figure 3 This is a schematic diagram of the waste heat recovery system for solid oxide fuel cells according to the present invention.

[0017] Appendix Figure 1-3 In the middle; AC: Air compressor; FC: Fuel compressor; AMTEC-H: High-temperature side of alkali metal thermoelectric converter; AMTEC-L: Low-temperature side of alkali metal thermoelectric converter; HE1: Air preheater; HE2: Fuel preheater; HE3: Heat exchanger; SOFC: Solid oxide fuel cell stack; AB: Tail burner; TEG-H: Hot side of thermoelectric generator; TEG-L: Cold side of thermoelectric generator; PUMP: Water pump; AC-DC: Alternating current converter.

[0018] Figure 4 Effect curves of fuel flow on power generation under three configurations

[0019] Figure 5 Effect curves of fuel flow rate on the efficiency and thermal efficiency of the three configuration systems

[0020] Figure 6 The effect curves of fuel utilization rate on the power generation of the three configurations

[0021] Figure 7 Effect curves of fuel utilization rate on the efficiency and thermal efficiency of the three configuration systems

[0022] Figure 8 The effect curves of working pressure on the power generation of the three configurations

[0023] Figure 9 The effect curves of working pressure on the efficiency and thermal efficiency of the three configuration systems Detailed Implementation

[0024] The present invention will now be described in detail with reference to the accompanying drawings.

[0025] The solid oxide fuel cell waste heat recovery system of the present invention can also be called SOFC-AMTEC-TEG power generation system.

[0026] This invention, based on the AMTEC cold-end (AMTEC-L) temperature gradient control principle, proposes three differentiated waste heat recovery system configuration schemes. The core difference lies in the selection of the AMTEC-L heat source and the planning of the heat flow path. The exhaust gas flowing through the preheater, the exhaust gas flowing through the TEG high-temperature side, and the SOFC cathode air intake are respectively used as the AMTEC cold-end gas supply. Through different heat flow path planning, efficient utilization of waste heat across the entire temperature range is achieved. Of these three configuration schemes, the third is the optimal scheme and is the technical solution of this patent application. The first two configuration schemes serve as comparative examples in this patent application.

[0027] Comparative Example 1, i.e., Configuration 1: Series-type waste heat cascade utilization system

[0028] After complete combustion in the tail-end burner, the SOFC exhaust gas flows sequentially through the AMTEC hot end, air preheater and fuel preheater, AMTEC cold end, and TEG hot end, forming a series heat flow path of "high temperature → medium temperature → low temperature". This configuration achieves energy recovery through staged matching of waste heat quality, such as... Figure 1 As shown.

[0029] Comparative Example 2, i.e., Configuration 2: Series-connected waste heat cascade system with TEG pre-installed

[0030] After combustion, the exhaust gas first flows through the AMTEC hot end and preheater, then enters the TEG hot end for power generation, and finally recovers waste heat through the AMTEC cold end. This configuration prioritizes the recovery of medium-temperature waste heat, allowing more exhaust gas waste heat to be allocated to the heating process, such as... Figure 2 As shown.

[0031] The technical solution of this invention, namely configuration 3: a waste heat recovery system coupled with air preheating.

[0032] Integrating the AMTEC cold end with cathode air inlet preheating: Fresh air first flows through the AMTEC cold end to absorb heat, then enters the preheater for further heating, raising the overall system temperature and maximizing power generation efficiency, such as... Figure 3 As shown.

[0033] The solid oxide fuel cell waste heat recovery system of the present invention includes a solid oxide fuel cell stack (SOFC), a tail burner (AB), an alkali metal thermoelectric converter (AMTEC), a thermoelectric generator (TEG), an air preheater (HE1), a fuel preheater (HE2), an air compressor, a fuel compressor, a water pump, and an AC / DC converter. These components are connected via pipelines to form a gradient energy processing network: the cathode intake gas first flows through the cold side of the AMTEC, where it undergoes an initial heat exchange with the high-temperature exhaust gas at the hot end of the AMTEC, and then enters the air preheater (HE1) for further heating to the SOFC high-efficiency reaction temperature; the anode fuel gas exchanges heat with the exhaust gas at the outlet of the air preheater (HE1) through the fuel preheater (HE2), raising its temperature from room temperature to the reaction temperature, and then... Air enters the SOFC for an electrochemical reaction. The SOFC exhaust gas, after the reaction, is completely combusted in the tail burner AB to generate high-temperature gas. This high-temperature gas first enters the AMTEC hot end to generate electricity through temperature difference. The AMTEC cold end releases heat for initial preheating of the cathode intake air. The medium-high temperature exhaust gas after AMTEC power generation flows into the air preheater HE1 and fuel preheater HE2 to preheat the cathode intake air and anode fuel gas. After cooling, it enters the TEG hot end for secondary power generation through the Seebeck effect. The TEG cold end maintains the temperature difference through cooling water. The low-temperature exhaust gas after TEG treatment can pass through the heating heat exchanger HE3 to provide a low-temperature heat source for industrial and heating applications, forming a complete chain of "high-temperature power generation - medium-high temperature preheating - medium-low temperature power generation - low-temperature heating". AMTEC is suitable for direct power generation in the 600-1000℃ high-temperature range, while TEG is suitable for secondary power generation in the 50-500℃ medium-low temperature range, covering the entire temperature range of 200-1000℃ exhaust gas and solving the problem of discontinuous temperature ranges in traditional systems. This invention couples the AMTEC power generation module with a preheating function, reducing heat loss in pipelines. The equipment is compact with no moving parts, has low maintenance costs, and is suitable for modular deployment of distributed energy resources.

[0034] The three configurations exhibit significant performance differences: Comparative Example 1, or Configuration 1, employs a series path of "high temperature → medium temperature → low temperature." When the fuel flow rate is 0.14 mol / s, the TEG power reaches 2.95 kW, but the AMTEC power drops sharply by 12.8% due to the cold end temperature exceeding the critical threshold. Comparative Example 2, or Configuration 3, prioritizes the recovery of medium-temperature waste heat through the TEG, distributing more of the exhaust waste heat to the heating stage, resulting in the highest thermal and system efficiency, making it more valuable for distributed power generation systems with high heating power requirements. Configuration 3, or this invention, integrates AMTEC cold end and cathode inlet preheating, raising the overall system temperature and achieving the highest power generation efficiency compared to other configurations, with a total power generation of 22.72 kW when the fuel flow rate is 0.14 mol / s.

[0035] Regarding the control of key parameters, increasing fuel flow rate has a positive impact on power generation and heating power, both of which increase with the increase in fuel flow rate; appropriately improving fuel utilization rate can increase power generation to a certain extent, but the power of thermoelectric generators that rely on waste heat from exhaust gas will decrease, and the system thermal efficiency will also decrease accordingly; increasing pressure can improve SOFC power generation efficiency by reducing polarization loss, but the temperature of waste heat from exhaust gas will decrease with the increase in pressure, affecting the thermoelectric conversion efficiency of thermoelectric generators.

[0036] The applicable scenarios for the three configurations are as follows: Comparative Example 1, i.e., Configuration 1 (Series-type waste heat cascade utilization system): It is suitable for scenarios where the temperature fluctuation of industrial waste heat is small and the heating demand is low (such as metallurgical kiln tail gas recovery). Waste heat can be utilized step by step through the series path of "high temperature → medium temperature → low temperature", but attention should be paid to the risk of AMTEC cold end temperature exceeding the critical threshold.

[0037] Comparative Example 2, i.e. Configuration 3 (TEG pre-connected series system): It is primarily suitable for district heating and building cogeneration scenarios. Its TEG pre-connected design can distribute more waste heat from exhaust gas to the heating process. When the fuel flow rate is 0.14 mol / s, the heating power reaches 6.48 kW and the thermal efficiency is 18.43%, which is suitable for scenarios with high heating demand, such as centralized heating in northern regions.

[0038] The solid oxide fuel cell waste heat cascade recovery system of the present invention, namely configuration 3 (air preheating coupling system), is suitable for high power generation demand scenarios (such as grid frequency regulation and industrial park microgrids). Through AMTEC cold end and cathode air inlet preheating integration, the power generation efficiency is the highest (total power generation is 22.72kW when fuel flow rate is 0.14mol / s).

[0039] Through the above design, this invention realizes the graded utilization of waste heat from high-grade power generation to low-grade heating, optimizes the reactor stack reaction conditions, improves system energy efficiency and reliability, and is applicable to distributed power generation, industrial waste heat cogeneration and building thermoelectric coupling scenarios.

Claims

1. A solid oxide fuel cell waste heat recovery system, comprising a solid oxide fuel cell stack (SOFC), a tail-end burner, an alkali metal thermoelectric converter (AMTEC), a thermoelectric generator (TEG), an air preheater, and a fuel preheater, wherein the components are connected by pipes to form a gradient energy processing network, wherein, The cathode inlet gas first flows through the cold side of AMTEC, where it undergoes initial heat exchange with the high-temperature exhaust gas at the hot end of AMTEC to raise its temperature. Then, it enters the air preheater for further heating to the SOFC high-efficiency reaction temperature. The anode fuel gas exchanges heat with the exhaust gas from the air preheater through the fuel preheater, raising it from room temperature to the reaction temperature, and then enters the SOFC with air to carry out an electrochemical reaction. The SOFC exhaust gas after the reaction is burned in the tail burner to generate high-temperature gas. The high-temperature gas first enters the hot end of the AMTEC and generates electricity through the temperature difference. The cold end of the AMTEC releases heat for the initial preheating of the cathode intake gas. The medium- and high-temperature exhaust gas after AMTEC power generation flows into the air preheater and fuel preheater to preheat the cathode intake gas and anode fuel gas. After being cooled by the air preheater and fuel preheater, the high-temperature exhaust gas from the AMTEC enters the TEG hot end for secondary power generation.

2. The solid oxide fuel cell waste heat recovery system according to claim 1, characterized in that, The low-temperature exhaust gas after TEG treatment is supplied with a low-temperature heat source through a heat exchanger.

3. The solid oxide fuel cell waste heat recovery system according to claim 1, characterized in that, The TEG cold end maintains a temperature difference through cooling water.

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