A study of new method and comprehensive evaluation on the improved performance of solar power tower plant with the CO2-based mixture cycles
Introduction
The development of solar thermal power technology promotes the process of saving energy and emission reduction. To develop a high efficiency and effective energy conversion system has been a research frontier and arose hot discussion. As the essential subsystem to convert energy, performance of the power cycle block mainly determines the efficiency of the overall system. Considering the potential to further improve the efficiency of conventional power cycle is limited, a novel cycle is required and the supercritical carbon dioxide (S-CO2) cycle is recommended as one of the effective and economic cycle applied to the solar thermal power plant. Compared to the conventional steam Rankine cycle, the S-CO2 cycle is advantageous in thermal and economic performance especially when the turbine inlet temperature is higher than about 550 °C [1]. First, critical point parameters of CO2 (7.38 MPa/30.98 °C) is greatly lower than those of water (22.06 MPa/373.95 °C). Lower critical point parameters make it easier to operate in the supercritical state and have broad operational working conditions. Second, the corrosion of CO2 has less effect on the metal material than water, making it convenient to select the suitable metal and improve the system safety [2], [3]. Finally, S-CO2 Brayton cycle is more compact and simpler [1], [4], [5]. The configuration of S-CO2 Brayton cycle is not complex with a smaller number of components and volume of the heat exchangers [6] resulting that S-CO2 Brayton cycle is more economical than steam Rankine cycle. As all the characteristics mentioned above, it is demonstrated that S-CO2 Brayton cycle presents great potential in the reduction of investment and improvement in thermal performance.
The S-CO2 cycle was firstly proposed by Sulzer [7]. Then various S-CO2 cycle layouts are proposed with the development of turbomachinery technology, and arose great attention especially in the application of nuclear power plant and solar power plant [8], [9], [10]. Feher [11] added a recuperator to the basic S-CO2 cycle to improve the cycle efficiency that the recuperative cycle was developed. Angelino [12] further suggested the recompression cycle with the high-temperature recuperator and low-temperature recuperator introduced, and Dostal et al. [13] applied the recompression cycle into the nuclear reactor industry. Al-Sulaiman et al. [14] performed a comparative study and concluded that recompression S-CO2 cycle yields the best thermal performance when applied to solar power tower system. Besides, Seidel et al. [15] also found similar results and revealed recompression S-CO2 cycle is an advantage in thermal efficiency with wide operational pressure conditions. Atif et al. [16] pointed out that net output work of recompression S-CO2 cycle is higher than that of recuperation cycle. Neises et al. [17] revealed that temperature difference of turbine in the partial-cooling S-CO2 cycle is >20% higher than that of recompression S-CO2 cycle, which is helpful to design a compactness thermal storage system. In order to exhaustively evaluate the performance of S-CO2 cycles, Wang et al. [18] proposed to compare the performance of cycles using efficiency and specific work as criteria simultaneously, and they found the optimal hot salt temperature for S-CO2 cycle applied in SPT system. They also pointed out that the novel salts with high upper limit temperature (higher than 650 °C) are recommended to further improve the system performance. Li et al. [5] gave a detailed review on S-CO2 cycle applied in various energy industries including nuclear power plant and solar power plant. The theoretical analysis and experimental analysis of S-CO2 cycles are summarized, and their study provides a beneficial complement to understanding the recent development trends of S-CO2 power cycle. Li et al. [19] also investigated the economic performance of recompression S-CO2 cycle coupled to lead-cooled fast reactor based on investment models including turbine, compressor and different types of heat-exchangers. They proposed a design of miniaturized LFR integrated with S-CO2 cycle to promote the implementation of the technology. The thermodynamic performance and economic performance of the overall system are evaluated and they summarized that electricity production cost of LFR coupled to the S-CO2 cycle is 0.0536$·(kW·h)−1. Finally, Li et al. [20] recommended the suitable S-CO2 cycle layout with maximum efficiency and minimum electricity production cost for the LFR based on the multi-objective results. The study provides a comprehensive comparison between various S-CO2 cycles for nuclear power plant. Sandia National Laboratories (SNL) [21], [22] established the S-CO2 cycle experimental platform, and the printed circuit heat-exchanger (PCHE) is used to make the system compactness. Some tests about system performance varied with various operational conditions were performed.
Recently, researchers try a lot to further improve the efficiency of S-CO2 cycle. The literature shows the main method to improve system performance is recovery waste heat from S-CO2 cycle [23], [24]. Because the mediate and low temperature of waste heat, the ORC cycle and transcritical CO2 (T-CO2) cycle are usually treated as the bottom cycle to recover the waste heat from the top S-CO2 cycle. Akbari et al. [25] analyzed the performance of combined S-CO2/ORC cycle with the nuclear reactor. Li et al. [26] optimized the thermo-economic performance of S-CO2/Kalina cycle, and found that combined cycle has higher system cost. Wang et al. [27] proposed S-CO2/T-CO2 cogeneration cycle and revealed the combined cycle is advantageous in higher energy and exergy efficiencies over the S-CO2 cycle alone. They et al. [28] also examined the performance of the S-CO2/ORC cogeneration cycle. Wu et al. [29] combined recompression S-CO2 cycle with absorption refrigeration cycle and confirmed the cogeneration system is helpful in elevating the thermal efficiency and exergy efficiency of recompression S-CO2 cycle.
Adoption of other gas into CO2 is also proved to be an effective way to improve the performance of S-CO2 cycle. The S-CO2 cycle is advantageous in less work consumption of the compressor because it operates in supercritical region above the critical point of CO2 with high density of CO2. This indicates that the critical point of the working fluid limits the minimum operating parameters of the S-CO2 cycle. Blending the CO2 provides a way to shift the critical point of the CO2 working fluid and extends the operational range of cycle. It is also beneficial for the CO2 cycle to have a good match with the environment. For example, using lower critical point parameters of the CO2-based mixture as the working fluid for the power cycle is suitable to be located in cold area. Lower allowable operational parameters make the cycle have potential to improve the thermal efficiency of the cycle from the aspect of thermodynamic analysis. If the power cycle is positioned in where is hot and arid, the CO2-based mixture with higher parameters of the critical point as working fluid is recommended to have a good match with the air-cooling system. The CO2-based mixture cycle is firstly studied on the industry of nuclear reactor. Sandia National Laboratories [30], [31], [32] performed an experimental study to investigate the performance of CO2/butane, CO2/Ne and CO2/SF6 Brayton cycles. Jeong et al. [33], [34] conducted a comparative study with nitrogen, oxygen, helium and argon as additives and revealed that the efficiency is improved by 3% with helium added. They also investigated the performance of CO2-based mixture cycle coupled to the sodium-cooled fast reactor, and recommended H2S and cyclohexane as additives for their higher efficiency. Hu et al. [35] confirmed the helium and krypton are helpful to improve the efficiency of S-CO2 cycle when integrated with the nuclear reactor. There are also some studies on the performance analysis of the cycle using CO2/refrigerants as working fluids to utilize the low-temperature heat sources. To improve the thermodynamic performance of the CO2 transcritical Rankine cycle, Shu et al. [36] investigated the performance improvements of tanscritical Rankine cycle using CO2 mixtures for the waste heat recovery of engine, finally they recommended eight refrigerants as the candidate additives into CO2 with the optimal power output, thermodynamic and economic performance treated as the comparison metrics. Sánchez et al. [37] presented the performance of CO2/refrigerants mixtures basic and recuperative transcritical Rankine cycle. They discussed the effects of mixtures’ composition, turbine inlet pressure and temperature of heat source on the performance of cycle, and concluded that the cycle with higher mass fractions of refrigerant as working fluid outperforms nearly pure CO2 cycle. Xia et al. [38] performed a detailed comparative study of thermodynamic and exergoeconomic performance for numerous CO2-based mixtures and pure CO2 using as the working fluids for transcritical power cycles. The parametric analysis was conducted to reveal the effects of turbine inlet temperatures, condensing temperatures and mass fractions of organic fluids on the cycle thermodynamic performance. They pointed out that the CO2-based mixtures could achieve better performance than the pure CO2, and selected the CO2/R32 and CO2/R161 mixtures for low-temperature transcritical power cycle. While the recommendation working fluid for high-temperature cycle is CO2/propane. In order to solve he condensing problem, Pan et al. [39] analysis the transcritical power cycle using CO2-based binary mixture as working fluid for the viewpoint of the theoretical method and the experimental method.
Recently, the application of CO2-based mixture cycle in solar thermal power plant is becoming a hot discussion. Manzolini et al. [40] analysed the thermodynamic and economic performance of the CO2 mixtures cycle using N2O4 and TiCl4 as the added fluids. They found that the innovative cycles present high conversion efficiencies as 43% and 50% with the turbine inlet temperature of 550 °C and 700 °C separately. They also pointed out that the levelized cost of electricity (LCOE) of the proposed CO2 mixture cycle is reduced by 10% compared to that of the conventional steam power cycle. Bonalumi et al. [41] also investigated the CO2/TiCl4 mixture recuperative cycle and recompression cycle for medium/high temperature heat sources (400–800 °C) and cold sinks with above 25 °C. Compared to the pure CO2 cycle, the net electric efficiency is improved by about 5% for CO2/TiCl4 recuperative cycle with 25 MPa/551 °C of turbine inlet parameters. Similarly, the efficiency of CO2/TiCl4 recompression cycle is higher than that of the pure CO2 cycle under 25 MPa/800 °C maximum parametric condition. Another additive to improve the critical point parameters of the pure CO2 is N2O4 and Binotti et al. [42] applied the CO2/N2O4 cycle to solar power plant installed at the fairly high ambient temperatures of desert areas. They recommended the CO2/N2O4 (78%/22%) as the optimal mixture with the efficiency treated as the performance metric. The results of the thermodynamic analysis indicated that the CO2/N2O4 cycle is helpful to elevate the overall solar-electric efficiency by 1% for solar power compared to the steam cycle with 550 °C maximum temperature. Guo et al. [43] also investigated the effects of the crucial parameters on the performance of CO2-based mixture cycle integrated with the SPT system and the detailed distribution of the exergy loss is given.
From above literatures, it can be seen that researchers have attempted a lot to further improve the performance of S-CO2 cycle mainly through two strategies. One of them is the proposal of a cogeneration power system and has been widely applied in nuclear power plant as well as solar power plant. The other one is to use CO2-based mixture cycle instead of the pure CO2 cycle and has been mainly integrated with the nuclear power plant and low-temperature heat resources. The study on the feasibility and performance of CO2-based mixture cycle when applied to solar power plant is limited and still at the early stage. Particularly, the researches mainly focus on the cycle performance without emphasis on the receiver performance. The performance comparison between various supercritical CO2-based mixture cycles has not been fully investigated. In this study, the effects of crucial parameters on the optimal performance of solar power plant and receiver are investigated primarily. Then, the evaluation method for the SPT system is proposed. The performance of various SPT systems is further compared based on the multi-objective optimization in terms of exergy efficiency ηexe,SPT, specific work w and temperature difference of the main-heater ΔTMH simultaneously. Finally, the optimal cycle layout is selected based on the proposed performance evaluation method, and the optimal operational parameters are given. The findings of the study demonstrate a preliminary exploration on improving the performance of solar power tower plant integrated with the S-CO2 Brayton cycle and aim to present a comprehensively systematic performance comparison of CO2-based mixture Brayton cycles coupled to the SPT system.
Section snippets
System description
To simplify and avoid repetition, the recompression cycle is taken as an example and the schematic of the SPT system is shown in Fig. 1. It is an indirect solar power plant that the main-heater is used to transfer the heat carried by molten salt to the working fluid of cycle. The re-heater is adopted that inlet temperature (T3) of low-pressure turbine (LPT) is assumed to be equal to inlet temperature (T1) of high-pressure turbine (HPT). The optimal re-heating pressure is searched to keep the
Simulation method and parametric analysis
To make a comprehensive evaluation of various system configurations, the establishment of integrated system simulation method is the first step and the detailed method of each subsystem in the SPT system integrated with recompression cycle is demonstrated in this section. Then, based on the integrative system model, the parametric analysis is conducted to reveal the effects of crucial parameters on the system optimal performance.
Comprehensively comparative study on performance of various layouts
To perform a comprehensive performance comparative study for various system layouts using S-CO2, CO2/xenon and CO2/butane as the working fluid, the multi-objective optimization method is applied in this section to optimize various performance metrics simultaneously. The effects of crucial parameters on ΔTrec are consistent with those of ΔTMH, and both of ΔTrec and ΔTMH reflect the performance of thermal storage system. Because ΔTMH is more directly related to the performance of power cycle, ΔTMH
Conclusions
In this paper, a new method to improve the performance of SPT system integrated with S-CO2 cycle is proposed via adding the other type of gas. A comprehensively systematic comparison is constructed to evaluate the system performance based on 3 performance metrics considered simultaneously. To find the trade-off relationships of various performance metrics, the effects of turbine inlet pressure, total thermal conductance of recuperator and compressor inlet temperature on the performance of CO2
Acknowledgements
The work was supported by the National Key R&D Program of China (No. 2018YFB1501001) and the National Natural Science Foundation of China (No. 51806165).
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