Preliminary conceptual exploration about performance improvement on supercritical CO₂ power system via integrating with different absorption power generation systems

Highlights

• Two types of combined sCO₂/APG system are proposed.

• Quantitative parameter analysis is conducted based on the decision variables.

• Genetic algorithm is adopted to obtain the optimization results.

• Exergy analysis and comparison are carried out for different system layouts.

Abstract

Supercritical CO₂ (sCO₂) power system has been investigated by many scholars due to its attractive advantages of higher efficiency, compact system structure and eco-friendly working fluid. In this paper, some preliminary conceptual exploration about performance improvement on sCO₂ power system by integrating with two types of absorption power generation (APG) systems are conducted. Parameter analysis, genetic algorithm (GA) optimization and exergy analysis are carried out quantitatively for the proposed combined sCO₂/APG systems based on the self-built simulation platform from the viewpoints of thermodynamics and economics. Parameter analysis results reveal that there exist optimal compressor pressure ratio to maximize the thermal efficiency or minimize the total product unit cost. Higher turbine inlet temperature and lower absorber temperature could contribute to the overall system performance. In addition, compared with the stand-alone sCO₂ system, improvements of 5.98% and 5.07% in thermodynamics as well as promotion of 4.24% and 2.19% in economics can be obtained for sCO₂/LiBr-H₂O system and sCO₂/ammonia water system, respectively. Furthermore, exergy analyses show that the main exergy destructions occur in the reactor and the cooler and the proposed combined sCO₂/APG system could effectively reduce around half of the exergy destruction within the cooler of the stand-alone sCO₂ system.

Introduction

Nowadays, energy situation and environmental pollution problems become increasingly severe, especially in developing countries. Exploring renewable energy and developing high-efficiency energy conversion system are effective methods to relieve current situation. The sCO₂ power system, as a type of promising energy converter, attracts a great deal of attention due to the advantages of high efficiency, compact construction. Besides, the CO₂ working fluid is eco-friendly, safe and non-toxic [1], [2], [3].

Various heat sources including nuclear energy [4], solar energy [5], geothermal energy [6] and other industries [7] can be exploited by the sCO₂ power system. Nuclear energy, as a kind of environmentally friendly, economical and reliable energy, has been considered as the potential alternative to currently widely used fossil fuels. The sCO₂ power system is much more appropriately applied to the conventional pressurized water reactors [8] and nuclear fusion reactors [9]. Dostal [10] found that the recompression layout was the best configuration for the next generation nuclear reactor application.

The main work about the recompression sCO₂ power system mainly focuses on the basic theoretical analysis and key devices investigation, including system thermodynamic analysis [11], economic analysis [12], off-design analysis [13], dynamic behaviors analysis [14], turbomachinery design [15], and heat transfer enhancement [16] up to now. Jahar [11] performed an exergetic analysis and optimization for the recompression sCO₂ power system. He found that the system second law efficiency was much more sensitive to isentropic efficiency of the turbine than that of the compressor. Floyd et al. [13] studied the system off-design behaviors for the seasonal variation in the heat sink diversification on the basis of the preliminary design of the main components. They revealed that a degree-of-freedom of the compressor performance was needed to gain high efficiency and constant thermal power under the elevated heat sink temperature conditions. Minh et al. [17] made an investigation on the advanced control strategies of the recompression sCO₂ power system driven by solar energy and presented its dynamic behaviors. They found that compared with the traditional process, a significant improvement up to 37.1% in total energy output can be provided by means of the inventory control scheme.

Furthermore, it has already been a well-accepted fact from the literature that the overall performance of the recompression sCO₂ power system can be enhanced via integrating with different low-grade waste heat recovery systems to make the utmost of the heat of cooling. Many scholars have done amounts of work on this. Akbari et al. [18] proposed the combined sCO₂/ORC (organic Rankine cycle) system and performed a detailed thermodynamic and exergoeconomic analysis. They observed that the most cost-saving operation condition could be got when RC318 refrigerant was used. Wang et al. [19], [20] suggested a combination of sCO₂ system and tCO₂ system to strengthen the performance of stand-alone recompression sCO₂ power system. Their optimization results pointed out that the combined sCO₂/tCO₂ system had a comparable exergetic efficiency with the sCO₂/ORC system. Li et al. [21] integrated the recompression sCO₂ power system with a low-temperature regenerative Kalina system. Their results showed that the second law efficiency and total product unit cost of the proposed combined sCO₂/Kalina system were able to gain 5.50% and 8.02% improvement, respectively.

Apart from the above investigation, some scholars devoted to investigate the combination of recompression sCO₂ power system with absorption systems. As is well-known, absorption system mainly using the LiBr-H₂O solution or ammonia water as working fluids can be driven by low-grade heat to produce refrigeration or more low-temperature heat. The mixture working fluids can provide a better thermodynamic match in temperature with the heat source and heat sink [22]. Wu et al. [23] connected the absorption refrigeration system with the recompression sCO₂ power system to produce power and cooling together. They found that the combined system could produce 71.76 MW cooling at the expense of 0.36 MW electric power under the basic design conditions. Li et al. [24] made a comparative study utilizing LiBr-H₂O solution and ammonia water as working fluids to recover the heat of cooling from the recompression sCO₂ power system. They revealed that the combined sCO₂/LiBr-H₂O system had a greater potential in terms of generating cooling and power. Recent years, a novel conception, called absorption power generation system, was proposed to recover the low-grade heat based on the characteristics of mixture [25]. Shokati et al. [26] made a comparative analysis between Rankine cycle system and APG cycle system. They argued that LiBr-H₂O system with the lowest exergy destruction cost rate had the highest thermal efficiency.

The above section briefly reviews the research and potential of the recompression sCO₂ power system and the APG system. It is noteworthy that APG is a kind of promising and competitive system in terms of utilizing the low-grade waste heat due to the better mixture characteristics than pure working fluids. In addition, the sCO₂ power system can not only obtain higher thermal efficiency when compared with the Rankine cycle and helium power cycle under the mild temperature conditions, but also avoid the reaction of Na and water in the nuclear reactor. Nevertheless, up to now, nobody adopts the APG system to enhance the recompression sCO₂ power system performance. More power production, higher efficiency and cost-effective product may be obtained and it is necessary to be explored quantitatively to indicate its feasibility and advancement.

In this paper, some preliminary conceptual exploration about performance improvement on supercritical CO₂ power system in the nuclear power plant are conducted from the viewpoints of thermodynamics and economics. Two types of combined sCO₂/APG systems are proposed and compared utilizing LiBr-H₂O or ammonia water as working fluids in the low-grade heat recovery system, respectively. Parameter analysis, single-objective optimization, multi-objective optimization and exergy analysis are performed quantitatively based on the self-built thermodynamic simulation platform. The work of this paper exhibits a set of integrated methodology, containing thermodynamic analysis, economic analysis, optimization, exergy analysis and comparison, for the combined power system. Moreover, the quantitative results in this paper could provide some effective reference for the practical operation of nuclear power plant in the near future.