Proposed combined cold and power system integrated SOFC, sSCO2 power cycle and compression-absorption refrigeration with [Na(Tx-7)]SCN/NH3 as working fluid

doi:10.1016/j.enconman.2021.115029

Energy Conversion and Management

Volume 251, 1 January 2022, 115029

https://doi.org/10.1016/j.enconman.2021.115029 Get rights and content

Highlights

•
A novel combined cold and power system with integrated SOFC is designed and optimized.
•
The sCO₂ power cycle is introduced to replace the gas turbine of the SOFC/GT system.
•
A novel ionic liquid working fluid of [Na(Tx-7)]SCN/NH₃ is used in the CSSC system.
•
Thermal performance of CSSC system is better than SOFC/AR and SOFC/GT-ORC systems.

Abstract

A combined cold and power system integrated SOFC, sSCO₂ power cycle, and compression–absorption refrigeration (CAR) is designed and optimized to utilize the high-temperature waste heat of solid oxide fuel cell (SOFC). On the premise of retaining the preheating capacity, the waste heat of the gas from after burner is first used to drive the sCO₂ power cycle. The inlet temperature of compressor in sCO₂ power cycle is appropriately raised to avoid the pinch point. The residual heat of sCO₂ is used to drive CAR with [Na(Tx-7)]SCN/NH₃ as working fluid. On the basis of the electrochemical reaction model of SOFC and the first and second laws of thermodynamics, the steady state characteristics are simulated by using the engineering equation solver. The reliability and energy balance of the model is verified. The optimizations, comparison, and exergy analysis are conducted and discussed. The optimal n_CH4, T_FC,in, and γ_SC are determined to be 0.004136 mol/s, 1029.1 K, and 2.5, respectively. The circulation ratio should be controlled within the range of 3.9 to 4.0. Comparison results reveal the exergy efficiency of the proposed system is higher than that of the SOFC/GT-ORC system by at least 0.035. The main reason for exergy destruction is due to the chemical reactions. The exergy destruction of the SOFC subsystem is the largest, which accounts for 73.7% of the total exergy destruction.

Introduction

Fuel cells (FCs) can convert the chemical energy in the fuel into electrical energy through chemical reactions and output large amounts of heat energy at different temperatures [1]. The operating efficiencies of FCs can overcome the limitations of Carnot cycle efficiency [2]. FCs will be a key technology for future energy development [3].

In accordance with different electrolytes, FCs can be divided into proton exchange membrane fuel cell (PEMFC) [4], phosphoric acid fuel cell (PAFC) [5], alkaline fuel cell (AFC) [6], molten carbonate fuel cell (MCFC) [7], and solid oxide fuel cell (SOFC) [8]. PEMFC belongs to low-temperature FCs, with the operating temperature ranging from 60 °C to 90 °C [9]. PAFC and AFC belong to mid-temperature FCs, with the maximum operating temperature higher than 200 °C [10], [11]. MCFC and SOFC belong to high-temperature FCs, with the operating temperature of 650 °C [12] and 1050 °C [13], respectively. The power generation efficiencies of different types of fuel cells are similar. When the working temperature of the fuel cell is higher, the available value of the generated heat is higher. The operating temperature of SOFC is higher than the other FCs. Therefore, hybrid systems integrated with SOFC possess higher overall efficiencies.

Recently, the studies of SOFC hybrid systems have made encouraging progress. Rosner et al. [14] investigated the influences of the cell design variables on the thermal cell management and economics of SOFC/gas turbine (GT) hybrid system. Lai et al. [15] compared two SOFC/GT hybrid systems with a standalone SOFC system. The SOFC/GT hybrid system with the steam bottom cycle is the best design, with a total efficiency of 44.6%. Koo et al. [16] performed energy and exergy analyses on a SOFC-based hybrid power generation system using homogeneous charge compression ignition (HCCI). Kim et al. [17] conducted a proof-of-concept test for a 5 kW SOFC-engine hybrid system. The exergy efficiency of SOFC is 53.9%, and the exergy efficiency of the whole system can be increased by 5.3% due to the engine. Zhu et al. [18] proposed a high-efficiency conversion and poly-generation system of cooling, heating, and power based on a biomass-fueled SOFC hybrid system. The energy efficiency and the net electrical efficiency can reach 77% and 52%, respectively. The fuel and air entering the fuel cell must be preheated by the high-temperature exhaust gas to maintain the high-temperature steady operating condition of the SOFC [19], [20]. In the existing studies, the exhaust gas is usually first used for preheating and then drives the absorption refrigeration or organic Rankine cycles [14], [15], [16], [17], [18], [19], [20]. From the perspective of the second law of thermodynamics, large heat transfer temperature difference between fuel, air, and exhaust gas results in large exergy destruction. Under the premise of not affecting sufficient preheating, the exhaust gas can be first used to drive the high-temperature cycle. The heat transfer temperature difference and exergy destruction can be reduced, and the system efficiency can be improved.

Considering the high SOFC operating temperature, the exhaust gas is very suitable for driving the sCO₂ power cycle [21], [22], [23], [24]. The cycle efficiency of sCO₂ should be higher than the GT cycle using fuel gas from after burner (AFB) because the pressure of sCO₂ is higher than the fuel gas from AFB. The research of this work also confirms this hypothesis. In the case of using sCO₂ power cycle, the operating pressure of the SOFC can be reduced close to the ambient pressure, which is beneficial for the longevity and operational stability of SOFC.

For the sCO₂ power cycle, there is a pinch point for the heat exchange between the high temperature sCO₂ from the turbine and low temperature sCO₂ from the compressor [25]. Two feasible solutions are used to avoid the pinch point. The traditional solution is that the sCO₂ is partially cooled to ambient temperature and partially compressed directly [26]. The compression of sCO₂ at high temperature consumes more compression work, thereby reducing the efficiency of the power cycle. And there is no residual heat which can be further used. Properly increasing the minimum cooling temperature of sCO₂ can also avoid the pinch point. The relatively high cooling heat can be used to drive the absorption refrigeration at low evaporation temperature. It is indicated that the latter solution is more reasonable.

For the absorption refrigeration at low evaporation temperature, NH₃/H₂O is the widely used working fluid [27]. The rectifier is an essential component of the NH₃/H₂O absorption refrigeration because the absorbent water is volatile [28]. The backflow of condensed liquid ammonia in rectifier largely reduces the coefficient of performance (COP) of NH₃/H₂O system. NaSCN and LiNO₃ are used as absorbent to improve the efficiency of NH₃ absorption refrigeration [29], [30], [31], [32]. The COPs of NaSCN/NH₃ and LiNO₃/NH₃ systems are higher than that of NH₃/H₂O system [30]. However, NaSCN/NH₃ and LiNO₃/NH₃ systems have the drawback of crystallization, thereby limiting their application [33]. Ionic liquids (ILs) are proposed to be used as the absorbent for NH₃ absorption refrigeration to overcome the above limitations [34].

Ionic liquids (ILs) are a type of ionic compounds in liquid form at room temperature [35]. The chemical and thermal stabilities of ILs are good with temperature lower than 400 °C [36]. The vapor pressure and corrosion of ILs are mostly negligible [37]. ILs are miscible with NH₃, and ILs can obviously reduce the saturated vapor pressures of IL/NH₃ solutions [38]. IL/NH₃ absorption systems have no risk of crystallization due to the liquid form of ILs at room temperature. IL/NH₃ absorption systems do not require a rectifier due to the negligible vapor pressure of ILs. IL/NH₃ absorption systems fundamentally overcome the two main limitations of crystallization and rectification in traditional NH₃ absorption systems.

Common ILs, such as imidazole-based ILs and pyridyl-based ILs, were proposed as absorbents in ammonia absorption refrigeration [26], [27], [28], [29], [30]. However, the thermal performance of IL/NH₃ systems is unsatisfactory [31]. The reason is that the interactions between common ILs and ammonia are weak. The weak interactions lead to the low solubility of NH₃, which finally result in high circulation ratio and low COP of the absorption system. The outer empty orbital of the alkali metal element can hold the lone pair of electrons of the NH₃ molecule. The interactions between alkali metal and NH₃ are extremely strong, indicating the reason why the vapor pressure of NaSCN/NH₃ and LiNO₃/NH₃ is low. If the alkali metal cation is introduced in the ILs, the interactions between ILs and ammonia can be significantly enhanced, thereby largely increasing the solubility of NH₃ in ILs. In 2014, a function IL with metal cation Na⁺ of [Na(TX-7)]SCN was synthesized by Ding [39]. The viscosity and density of [Na(TX-7)]SCN can meet the requirements as an absorbent in absorption refrigeration systems [39]. Subsequently, the vapor–liquid equilibrium (VLEand thermal properties of [Na(TX-7)]SCN/NH₃ solution were studied, and the thermal performance of [Na(TX-7)]SCN/NH₃ was predicted [40]. The COP of [Na(TX-7)]SCN/NH₃ is slightly lower than the NaSCN/NH₃ system, but is obviously higher than the NH₃/H₂O system. Considering the additional advantages of non-crystallization, the [Na(TX-7)]SCN/NH₃ is selected as the working fluid of the absorption refrigeration cycle of the proposed system.

In order to improve the thermal performances of the absorption refrigeration systems, assisting compressors are usually introduced in the absorption refrigeration to form compression-absorption refrigeration (CAR) system [47–50]. It is found that the thermal performances of CAR system are improved, especially when the assisting compressors are installed between the absorber and the evaporator [41]. The SOFC and sCO₂ power cycle generate amounts of electricity that can be used to drive the assisting compressor for the [Na(TX-7)]SCN/NH₃ CAR system.

In accordance with the above research ideas, a combined cold and power system integrated SOFC, sSCO₂ power cycle and CAR with [Na(Tx-7)]SCN/NH₃ as working fluid (CSSC) was proposed. In the proposed system, the exhaust gas from after burner is first used by the bottom cycle, which can largely reduce the heat transfer temperature and the exergy destruction of the SOFC. The sCO₂ power cycle is introduced to the system to replace the gas turbine of the SOFC/GT system, which results in two advantages. On one hand, the sCO₂ power cycle possesses high efficiency than gas turbine. On the other hand, the operating pressure of the SOFC is largely reduced, which is beneficial for longevity and operational stability of SOFC system. In addition, the condensation heat of the sCO₂ power system is also recovered and utilized by the CAR system. Overall, the proposed system provides new ideas for the integration and development of the SOFC cogeneration system. On the basis of the first law of thermodynamics, the thermal performances of the proposed CSSC system were predicted, optimized, and compared with other CCP systems. On the basis of the second law of thermodynamics, the exergy destructions of each component were calculated and analyzed.

Section snippets

Description of the proposed CSSC system

Fig. 1 shows the schematic for the proposed CSSC system. The CSSC system is composed of three subsystems, namely, SOFC subsystem, sCO₂ power subsystem, and CAR subsystem.

In the SOFC subsystem. The water, fuel, and air are first preheated by the exhaust gas in the heat exchanger for water (WX), heat exchanger for fuel (FX), and heat exchanger for air (AX), respectively. The preheated water and fuel enter the anode of the SOFC after passing through the mixer (MX), and the preheated air enters the

Thermodynamic properties of working fluids

For the SOFC subsystem, the working fluids are treated as ideal gas mixtures. The mixtures can be composed of CH₄, H₂O, N₂, O₂, H₂, CO or CO₂. For the sCO₂ power cycle, the working fluid is the supercritical carbon dioxide. The thermodynamic properties of above mentioned elemental component can be called in the Engineering Equation Solver (EES).

For the CAR subsystem, the working fluid is [Na(Tx-7)]SCN/NH₃. The vapor pressure of the [Na(Tx-7)]SCN/NH₃ can be predicted by the NRTL model. In the

Modeling and validation

To build up the mathematical model for the proposed CSSC system, several reasonable assumptions are made as follows:

(1)
The proposed CSSC system operates under the stable condition.
(2)
The pressure and temperature distributions for SOFC are uniform and set to fixed values.
(3)
The gas mixtures are assumed to be ideal gas mixtures and reach chemical equilibrium in the anode.
(4)
The isentropic expand efficiency of TB and the isothermal compression efficiency of CM1 are set to 0.85, and the isentropic compression

Design condition

For the system design and simulation programming, the design condition is proposed and calculated. The design parameters for the proposed CSSC system are listed in Table 6.

Under the above parameter settings, the operating parameters of each state point for the proposed CSSC system can be calculated, as shown in Table 7. From Table 7, the average temperature of the inlet and outlet for the SOFC is approximately 1085 K.

In subsequent calculations, the average temperature of SOFC is jointly

Conclusion

The thermal performances of the proposed CSSC system were simulated in detail and analyzed on the basis of the first law and second law of thermodynamics. The optimization and comparison were conducted and discussed. Serval main conclusions are drawn as follows:

(1)
The reliability of the simulation model is confirmed. The simulation model of the proposed CSSC system obeys the principles of energy conservation and exergy balance.
(2)
The optimal n_CH4, T_FC,in, and γ_SC are determined to be 0.004136 mol/s,

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is supported by the Key Research and Development Project of Shandong Province (Grant No. 2019GGX102033), and Natural Science Foundation of Shandong Province (Grant No. ZR2020ME172).

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