Energy, exergy and exergoeconomic analyses of a combined supercritical CO2 recompression Brayton/absorption refrigeration cycle
Introduction
Many efforts have been devoted to the high efficiency and the cost reduction of electricity generated by the nuclear power plants toward the successful future utilization of the nuclear power [1]. In recent years, the Gas Turbine-Modular Helium Reactor (GT-MHR) [2], [3], [4], [5], [6] and the supercritical CO2 recompression Brayton cycle (SCRBC) [7], [8], [9], [10], [11] have become the advanced technologies in utilizing the nuclear energy. Compared with the GT-MHR, the SCRBC has a reasonable efficiency of 45.3% at a lower reactor outlet temperature of 550 °C, while the GT-MHR obtains a comparable efficiency at a significantly higher reactor outlet temperature of 850 °C [8]. The SCRBC proves to be a more promising approach of the energy utilization for the future power plants because of its compactness, simplicity, better economic aspect and higher efficiency. For the SCRBC, the compressor work can be significantly reduced by using the drastic changes of the CO2 properties near the critical point, leading to a significant increase in the efficiency. It is known that the working fluid should be cooled to some temperature before the compression process. For the SCRBC, CO2 is usually cooled to about 32 °C near its critical temperature (31.1 °C), which leads to a reasonable low-grade thermal energy (about 50% of the input energy) rejected to the pre-cooler [11], [12]. Therefore the performance of the SCRBC can be improved by reutilizing the low-grade thermal energy in the pre-cooler through various waste heat recovery systems.
Much attention has been focused on the utilization of the waste heat from the SCRBC by using the organic Rankine cycles (ORCs). Besarati and Goswami [13] implemented a thermodynamic analysis and comparison of three different SCB/ORC (sunpercritical CO2 Brayton/organic Rankine cycle) cycles. The results presented that the largest efficiency increment was obtained by adopting a simple SCBC (supercritical CO2 Brayton cycle) configuration as the topping cycle. However, the maximum efficiency of the overall system was achieved by the SCRB/ORC (supercritical CO2 recompression Brayton/organic Rankine cycle) cycle. Akbari and Mahmoudi [14] investigated a combined SCRB/ORC by using the exergy and exergoeconomic analyses. They concluded that the exergy efficiency of SCRB/ORC was higher than that of the SCRBC by up to 11.7% and that the total product unit cost of SCRB/ORC was lower than that of the SCRBC by up to 5.7%. Wang et al. [15] conducted a comparative study between the SCRB/ORC and SCRB/CDTPC (supercritical CO2 recompression Brayton/CO2 transcritical power cycle) considering the exergy and exergoeconomics. The results showed that the second law efficiency of the SCRB/CDTPC cycle was comparable with that of the SCRB/ORC cycle. Meanwhile, the total product unit cost of the SCRB/CDTPC was slightly higher than that of the SCRB/ORC. Sánchez et al. [16] studied the performance of SCB/ORC with different pure organic fluids and hydrocarbon mixtures as working fluids in the bottoming ORC cycles. The results indicated that the overall efficiency of the SCB/ORC was 7% higher than that of the simple SCBC when the hydrocarbon mixtures were utilized in the bottoming ORC. Zhang et al. [17] simulated and analyzed a SCRBC combined with an ORC with liquefied natural gas as heat sink. They found that the overall thermal efficiency of the SCRB/ORC could be up to 52.12% under the operating conditions of 20 MPa, 800 K and part-flow ratio 6.8.
A number of studies have also been published on the utilization of the waste heat from the SCRBC by employing a CO2 transcritical power cycle (CDTPC). Yari and Sirousazar [18] investigated the performance of the combined SCRB/CDTPC cycle. They found that the second law efficiency of the SCRB/CDTPC was 5.5–26% higher than that of the single SCRBC. Wang et al. [19] conducted a thermo-economic analysis on the performance of the SCRBC combined with a CDTPC. The results showed that the capital cost per net power output was 6% higher than that of the single SCRBC. Wang et al. [20] also carried out the thermodynamic comparison and optimization of two different configurations of supercritical CO2 Brayton cycles with a bottoming CDTPC. They concluded that the thermal efficiencies of the recompression and simple configurations of the supercritical CO2 Brayton cycles could be increased by 10.12% and 19.34%, respectively by adding a CDTPC. Wu et al. [21] performed a detailed analysis on a cooling and power system combining SCRBC with a CDTPC using liquefied natural gas (LNG) as the heat sink. The results revealed that the thermal efficiency of the SCRB/CDTPC could be achieved as high as 54.47% using LNG as heat sink.
Some investigations have also been performed on the recovery of the waste heat from the SCRBC by adopting a Kalina cycle. Li et al. [22] conducted an exergoeconomic analysis and optimization of a SCRBC coupled with a Kalina cycle. They reported that the total product unit cost and the exergy efficiency of the SCRB/KC (supercritical CO2 recompression Brayton/Kalina cycle) were 5.5% lower and 8.02% higher than those of the SCRBC cycle. Mahmoudi et al. [23] also investigated a combined SCRB/KC from the viewpoints of exergy and exergoeconomics. They found that the minimum total product unit cost and the maximum exergy efficiency of the SCRB/KC were 4.9% lower and 10% higher than those of the SCRBC cycle. However, little attention has been devoted to the waste heat recovery from the SCRBC by utilizing an absorption refrigeration cycle (ARC). Currently the ammonia-water and lithium bromide-water are the conventional working fluids in the ARC. The refrigerants of the ammonia-water and lithium bromide-water are ammonia and water, respectively. The advantage for the ammonia is that it can evaporate at a lower temperature and in a wider temperature range than water. Meanwhile, the ARC utilizing the ammonia-water as the working pair can be driven by the heat source generally at 50–200 °C [24], which is wider than that of the heat source driving the ARC utilizing lithium bromide-water. Therefore the ammonia-water was selected as the working pair in the ARC. Furthermore, the SCRB/ARC (supercritical CO2 recompression Brayton/absorption refrigeration cycle) can provide power and cooling, while the SCRBC can provide only power.
This paper focuses on the energy, exergy and exergoeconomic analyses of the SCRB/ARC cycle. Firstly, the combined SCRB/ARC is investigated in terms of the energy and exergy compared with the single SCRBC under a basic design case. Then, the parametric analysis is performed to investigate the effects of the key decision variables on the thermodynamic and exergoeconomic performances of the combined SCRB/ARC cycle. Finally, the both the SCRB/ARC and single SCRBC are optimized using the genetic algorithm (GA) from the viewpoints of the energy, exergy and exergoeconomics and then the results are compared. These findings may help to extend the utilization of the waste heat from the SCRBC for the nuclear power plants and enhance the performance of the SCRBC.
Section snippets
System description and assumptions
Fig. 1 exhibits the configuration of the proposed SCRB/ARC (supercritical CO2 recompression Brayton/absorption refrigeration cycle) and a T-s diagram of SCRBC (supercritical CO2 recompression Brayton cycle). The proposed cycle is a combination of the SCRBC and the ARC (absorption refrigeration cycle) so that the heat rejected in the pre-cooler of the SCRBC can be utilized to drive the ARC to provide cooling for the nuclear power plants or for other purpose. The CO2 leaving from the reactor
Thermodynamic analysis
The combined SCRB/ARC cycle was developed on the basis of the conservations of the mass, energy as well as exergy balance of individual components [11], [12], [26]. In this paper, MATLAB and REFPROP9.1 [29] are combined through an interface to simulate the cycle performance based on the mass, energy and exergy balance equations as well as the thermodynamic property relations. Moreover, all the physical and thermodynamic properties of the fluids involved in the present work are evaluated by
Results and discussion
In this section, parametric study was conducted to investigate the effects of the decision variables (four variables for the proposed SCRB/ARC cycle) on the values of the objectives: the energy utilization factor (EUF), exergy efficiency (ηex) and the total product unit cost (). The decision variables are the compressor pressure ratio (PRc), heat-end temperature difference in the generator (), cold-end temperature difference in the generator (), temperature in the
Conclusions
In the present paper, the authors have proposed a novel combined SCRB/ARC (supercritical CO2 recompression Brayton/absorption refrigeration cycle) cycle in which the waste heat from the SCRBC is utilized by an ARC for generating cooling. Detailed thermodynamic and exergoeconomic analyses and optimization have been performed for the SCRB/ARC cycle. Parametric analysis has also been carried out to investigate the effects of the key thermodynamic parameters on the thermodynamic and exergoeconomic
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