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Abstract

The competitiveness of concentrated solar power technology in the near-future electricity generation scenario, requires a substantial reduction of the Levelized Cost of Energy which can be achieved with an increase of the energy conversion efficiencies while maintaining or reducing the investment costs. This paper discusses the use of pure Dinitrogen tetroxide N2O4, and N2O4/CO2 mixture, as working fluids in supercritical Brayton cycles applied to solar tower power plants. When N2O4 is combined with CO2, the resulting mixture has a compara- tively higher critical temperature than pure CO2, allowing a condensing cycle even at the fairly high ambient temperatures of desert areas, where solar power plants are typically installed. This allows the adoption of simpler cycle configurations than the one used in sCO2 cycles (cost reduction) while achieving very high ther- modynamic efficiency (47% at 700 °C). The N2O4/CO2 mixture with optimized composition, integrated in a solar tower unit, increases the solar-electric efficiency by 1% with respect to commercial plants based on steam cycle with 550 °C maximum temperature (22.3% vs. 21.3%). At 700 °C, the overall solar-electric efficiency can reach 24.5% which is slightly higher than supercritical CO2 cycles, yet with a foreseeable reduction of the investment costs as consequence of the simpler plant lay-out.

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... sCO 2 cycles are characterized by extremely compact machinery and simple layouts: no bleedings, no steam drums, and a minimum operating pressure above the atmospheric pressure (higher than the CO 2 critical pressure (73.8 bar)) together with the possibility of increasing the maximum operating temperature of the power block are the main advantages of sCO 2 cycles when compared with the more traditional steam cycles [2][3][4]. At the current state of the art, the sCO 2 recompressed cycle can reach efficiencies up to 41.8%, at T max = 550 • C using molten salts as heat transfer fluid (HTF) [5,6], and up to 48.9% at T max = 700 • C with liquid sodium as innovative HTF [7,8]. Different national and European projects are working on this topic both from simulation, lab scale or pilot plant demonstration point of view. ...
... cold water) is available. As already suggested in previous works [7,[21][22][23], a blend of CO 2 with a certain dopant, characterised by a higher critical temperature than pure CO 2 , can result in liquid phase conditions at the inlet of the compression step for typical CSP application with a minimum cycle temperature around 50 • C while keeping the same advantages of pure CO 2 over steam Rankine cycle (e.g. the cycle compactness, a maximum operating temperature up to 700 • C). ...
... Starting from the thermal stability test results, the maximum temperature is set at 550 • C, thus coupling the power cycle with a solar tower based on commercially available molten salts [5,8]. On the other hand, the minimum cycle temperature of 51 • C is chosen in order to ensure that the mixture is in saturated liquid condition at the outlet of an air-cooled condenser in hot and arid regions (ΔT mixOUT,airIN = 10 • C with a maximum ambient temperature equal to 41 • C in Seville, Spain, a typical site used as European reference for CSP plants [7]). The other design parameters of the simple recuperative transcritical cycle, such as fluid machines efficiency, heat exchangers pressure drops and the minimum internal temperature approach in the recuperator, are taken in agreement with previous studies [7,23,26,27]. ...
Article
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Nowadays supercritical CO2 cycles are considered as a promising alternative to the traditional steam cycle for the power block in CSP plants with the aim of enhancing the system efficiency and reducing costs. This work deals with the experimental characterisation of a CO2 blend as working fluid in transcritical cycle: the addition of C6F6 as a dopant increases the fluid critical temperature allowing for a condensing cycle in hot environment with ambient temperature higher than 40°C. The potential benefits on adopting this mixture passes through thermal stability test for identifying its maximum operating temperature and Vapour Liquid Equilibrium measurements for tuning the Equations of State, thus having a good prediction of the thermodynamic properties. The static method with thermal stress test at different operating temperatures shows that the mixture can withstand to about 600 °C in an Inconel 625 vessel. Furthermore, the standard Peng-Robinson with the optimised binary interaction parameter is selected for a preliminary thermodynamic assessment of the power cycle. An efficiency of 41.9% is found for an optimum mixture composition with a CO2 molar content of 84% considering a turbine inlet pressure of 250 bar and a maximum and minimum cycle temperature of 550°C and 51°C respectively.
... Over the last years, many studies focused on pure sCO 2 cycles starting from simple recuperative cycle configuration and exploring new and alternative configurations [7,10e12]. At the current state of the art, the sCO 2 recompressed cycle can reach efficiencies up to 41.8%, at T max ¼ 550 C, and up to 48.9% at T max ¼ 700 C [4]. Most of the efforts in the scientific literature is related to the reduction of the compressor power consumption that strongly affects the cycle efficiency. ...
... On the other hand, inorganic compound such as TiCl 4 or N 2 O 4 are suitable for blending the CO 2 when high temperature heat sources (above 550 C) are available leading to thermodynamic efficiencies of the resulting transcritical cycles in the range of 49% with a maximum temperature equal to 700 C. This efficiency is higher than the corresponding one calculated for pure CO 2 cycles [4,25,26]. Moreover, the reduction in the costs of the power block was shown to be significant, especially if compared to traditional steam Rankine cycles [25]. ...
... The two maximum temperatures are selected with respect to the assumptions on HTFs characteristics: a base case at 550 C with the adoption of molten salts; an advanced configuration with liquid sodium reaching a maximum temperature up to 700e750 C [3,7]. The other parameters of the simple recuperative transcritical cycle, such as pump efficiency and the minimum internal temperature approach, are taken from literature [4]. The maximum pressure of the cycle, at pump outlet, is assumed equal to 250 bar without pressure drop, while it is increased up to 257.5 bar, accounting for the total cycle pressure drop of 7.5 bar distributed in the various cycle components, as reported in Table 6. ...
Article
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sCO2 power cycle is the most investigated and most promising technology for replacing conventional steam cycle in CSP plants. Nevertheless, the efficiency of sCO2 power cycle is strongly penalized by high ambient temperatures which are typical of favourable CSP locations. This paper focuses on a new working fluid for power cycles which consists of CO2 blended with C6F6. The addition of C6F6 increases the fluid critical temperature allowing for a condensing cycle for ambient temperatures up to 45°C. The calculated gross mechanical efficiency of the innovative cycle is around 42% when adopting a typical Peng Robinson equation of state with van der Waals mixing rules for a maximum operating temperature of 550°C and a minimum cycle temperature of 51°C. This performance varies just of ±0.1% if the prediction of the binary interaction parameter of the Peng Robinson is over- or under-estimated by 50%, but more significantly if other equations of states are adopted (up to 1% points). Moreover, a detailed analysis on the operating conditions of the cycle components highlighted that components design is affected by the adopted EoS. A sensitivity analysis is then performed to identify where the largest differences in predicting the efficiency of the cycle occur.
... Supercritical carbon dioxide cycles are today seriously reconsidered for high temperature applications in nuclear and solar plants [22][23][24]. The frontier of the research activity consists in the adoption of mixture of working fluids [25][26][27]. Within this context, the use of carbon dioxide mixtures as working fluids deserves to be investigated [28]. The identification of promising mixtures in concentrated solar power applications is the goal of the SCARABEUS project funded by the EU within the H2020 research programme. ...
... On the other hand, perfluorocarbons are very expensive fluids and with a high global warming potential, but, they are not flammable, not toxic and show a high thermal stability [29] The adoption of mixtures provides an additional opportunity in the selection of the working fluid: mixing two fluids is an effective method to change the critical point and thermodynamic properties according to the requested ambient conditions, [28,30]; the mixture will have the critical points in between the two pure fluids, depending on the concentrations, hence this characteristic can be properly tuned by varying its composition. Recently, mixtures using carbon dioxide as the main component have been considered as working fluids in thermodynamic cycles [26,27]. The fluid to be added to the CO 2 must be miscible and must improve the thermodynamic characteristic of CO 2 with:(i) the increase of critical temperature, (ii) the exploitation of the temperature glide on the condenser side and (iii) the design of the turbomachines modifying the compression ratio. ...
... Therefore, another set of EoS must be adopted. In this work, the classic cubic Peng-Robinson equation [38] is considered for the purpose of a preliminary comparison between the fluids (pure and mixtures), given its good accuracy in the prediction of a fluid behaviour [26,27]. Furthermore, when applied to mixtures, it is able to predict also the retrograde condensation. ...
Article
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In the last years, several fluids have been proposed to replace steam as working fluid in power cycle for converting thermal power into electricity. This paper describes the procedure to be adopted for the selection of any innovative fluid which can be even mixtures of fluids. The first step consists of the working fluid characterization in terms of thermodynamic properties through equations of state. The equations of state have to be calibrated on experimental Vapour-Liquid Equilibrium measurements while, in the second step, the maximum operating temperature is identified through thermal stability tests. Finally, the impact of the fluid thermodynamic properties on the performance of the power cycle in which it is implemented must be assessed through modelling tools. In this work, the procedure is discussed for the mixture of CO2 and C6F14 as a potential working fluid for gas thermodynamic cycles with liquid phase compression. Results of the application of this mixture in a closed cycle show the benefit of using a CO2/C6F14 mixture which provides 3% points efficiency increase at 400°C with respect to the pure CO2 together with a preliminary design of the expander.
... Within this new perspective Bonalumi et al [22] considered TiCl 4 as possible dopant, finding a 5% gain in cycle efficiency with a simple recuperative cycle and a 3% gain in cycle efficiency with a recompressed cycle using CO 2 + TiCl 4 as working fluid with respect to sCO 2 at cycle maximum temperature of 550 • C. Manzolini et al investigated the CO 2 + TiCl 4 and CO 2 + N 2 O 4 mixtures as working fluid in simple recuperative cycles for a CSP power plant located in Las Vegas (US) and Seville (Spain). The studies demonstrated that the selected CO 2 blends outperforms the sCO 2 cycle and conventional steam Rankine cycle in terms of cycle efficiency [23] and LCOE [21]. ...
... As anticipated, this temperature represents the target for the next generation CSP solar tower power plants. Moreover, in order to reproduce the hot and arid environments typical of CSP locations, a design ambient temperature of 35 • C is established and a corresponding minimum cycle temperature of 51 • C is set along the entire analysis [23]. The temperature differences at the cold end and hot end of the primary heat exchanger (PHE) are reported in Fig. 10 as function of the working fluid temperature at PHE inlet: the purpose of increasing the cold-end temperature difference in the PHE at lower temperature is to increase the ΔT ML of the overall PHE, aiming at a reduction of power block cost. ...
Article
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This paper focuses on the use of the CO2 + SO2 binary mixture as innovative working fluid for closed transcritical power cycles with a minimum temperature above 50 °C. Starting from a literature review of the available experimental data on the mixture, the PC-SAFT EoS is identified as a suitable model to characterize the mixture behavior. Once the proper thermodynamic model is selected for this mixture, a comparison between the innovative transcritical cycle and the sCO2 cycle is proposed for various plant layouts in order to find out the advantages of the innovative mixture. The analysis is presented fixing the cycle maximum temperature at 700 °C and the maximum pressure at 250 bar: the results depict an increment in cycle electric efficiency and cycle specific work, along with a lower temperature of heat introduction in the cycle for any considered configuration of transcritical CO2 + SO2 cycle, when compared to pure sCO2. An economic analysis of the power block is then performed to support the selection of the innovative working fluid. Two of the most promising plant layouts are evidenced: the recompression layout is selected for highly efficient power blocks, while the dual recuperated layout works effectively in applications characterized by higher hot source exploitation. The recompression layout adopting the CO2 + SO2 mixture presents a power block electric efficiency of 48.67% (2.33% higher than the respective sCO2 cycle) and a reduction of the power block CAPEX from 1160 $/kWel to 1000 $/kWel when compared to the sCO2 configuration for a 100MWel size, while the dual recuperated layout exploiting the CO2 + SO2 mixture shows a power block electric efficiency of 39.58% (0.69% above the same sCO2 cycle), a decrease of power block CAPEX from 795 $/kWel to 718 $/kWel and 70 °C of additional heat recovery from the hot source with respect to the analogous sCO2 cycle.
... The annual energy output of the direct steam generation configuration was 20% higher than the molten salt-based configuration. Binotti et al. [24] proposed that the dinitrogen tetroxide and carbon dioxide mixture for power cycle of ST power plant can improve the solar to electric efficiency (SEE) by 1% as compared to steam cycle commercial ST plants operating at 550 C. The SEE can reach to 24.5% at 700 C operating temperature which is even better than supercritical carbon dioxide cycle [24]. Kiwan and Khammash [25] presented an optimization model using PSO algorithm for optimizing the solar field of central ST system to maximize annual weighted efficiency. ...
... The annual energy output of the direct steam generation configuration was 20% higher than the molten salt-based configuration. Binotti et al. [24] proposed that the dinitrogen tetroxide and carbon dioxide mixture for power cycle of ST power plant can improve the solar to electric efficiency (SEE) by 1% as compared to steam cycle commercial ST plants operating at 550 C. The SEE can reach to 24.5% at 700 C operating temperature which is even better than supercritical carbon dioxide cycle [24]. Kiwan and Khammash [25] presented an optimization model using PSO algorithm for optimizing the solar field of central ST system to maximize annual weighted efficiency. ...
Article
This paper compares two main technologies of solar to electrical energy conversion, namely solar tower (ST) and photovoltaic (PV). For a fair comparison, a 100 MW same sized ST and PV plants are designed for a region with very good direct normal irradiance (DNI) and global horizontal irradiance (GHI). The initial design of the ST plant is optimized for solar multiple and thermal energy storage hours, and the PV plant is optimized for the optimal distance between parallel PV arrays. The ST plant has superior annual energy output of 513040.16 MWh compared to 270754.6 MWh from PV plant and capacity utilization factor of 58.6% in comparison to 30.9% from PV plant. On the contrary, the land use factor and solar to electric efficiency (SEE) of the PV plant is superior to ST plant while levelized cost of energy of ST plant is 2.83 times higher than the PV plant. Although ST plant has superior technical performance but way better economic performance of PV plant makes it the stand out solar to electrical energy technology for a location with promising GHI and DNI. This paper provides very useful guidelines for the policymakers to select a particular technology for the future solar-based power generation projects.
... In order to properly compare the two heat exchangers, the same heat duty, internal pressure drop and temperature difference on the hot and cold side are assumed. Due to the different thermodynamic properties and the fixed heat duty, the mass flow rate is also different, while the pressure of the CO2 cooler is set to 100 bar as this is an optimum value for sCO2 cycles with minimum temperatures around 50 °C [26]. ...
Conference Paper
Full-text available
CO2 blends provide tremendous advantages when used as a working fluid in transcritical power cycles with respect to pure CO2. The benefits become especially apparent if coupled with concentrated solar power since increasing the critical temperature of the blend with respect to pure CO2 allows dry condensing at high ambient temperatures in locations of high solar radiation. One key cycle component is the cooler, which in this work is designed as an air-cooled condenser with a MATLAB in-house code. The internal, condensation heat transfer model used in this paper relies on a correlation developed by Cavallini (2006). The model itself is validated against experimental data from a test rig for heat transfer measurements on a CO2 + R1234ze(E) mixture. The resulting design of the condenser is compared with the commercial software HTRI for a specific case study which is representative of the condenser of a recuperated cycle working with a CO2 + C6F6 blend. The authors also present an upgraded heat exchanger design with microfinned tubes, the DIESTA tubes, and groovy fins on the air side. The design of the heat exchanger adopting the mixture is compared to a case with pure CO2 as the working fluid.
... The design of the solar tower using solar salts as HTF considers a receiver similar to the one of the Gemasolar plant, while for the high temperature sodium receiver a reduced size (50% with respect to solar salts) is assumed, thanks to the possibility of tolerating higher solar fluxes for this HTF [8]. The field layouts are designed assuming about 190 MW at the receiver surface and a solar multiple equal to 2. The other relevant characteristics of the two solar field are available in [9]. A thermal model developed in MATLAB is used to estimate the receiver convective and radiative loss and the receiver thermal efficiency at design and off design conditions [10] [11]. ...
Conference Paper
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The adoption of CO2-based mixtures as power block working fluid for CSP plant can turn supercritical CO2 cycles into efficient transcritical cycles even at high ambient temperature, with significant performance improvement and potential power block cost reduction. In this work, the use of CO2+C6F6 mixture as working fluid for a power cycle coupled with a solar tower is analyzed. Two different cycle maximum temperatures (550°C and 650°C) are considered and for both configurations the overall plant design is performed. The yearly energy yield is computed with hourly data and the LCOE is minimized varying storage and cycle recuperator sizes. Results show comparable results for the innovative working fluid and for the sCO2 cycles.
... From the above examples, it can be further inferred that supercritical mixtures are more widely used than pure fluids. In energy engineering, the application of CO 2based blends as the working fluids can achieve higher efficiency of Brayton cycle [76,66,65,60,54], through a good matching with ambient temperature [26,86] and reducing compression work. In supercritical pressure fluid heat storage, fluids with different critical points are obtained by mixing to meet the demand for thermal energy storage at different temperature levels. ...
Chapter
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General backgrounds and basic concepts are introduced in this chapter, including critical phenomenon, critical anomalies, and the applications of supercritical pressure fluids. The coupled heat and mass transfer is explained briefly. A literature review is also provided, followed by the motivation and outline of this book.
... Theoretical investigations have focused on laminar thermo-chemical equilibrium assumptions without three-dimensional turbulence interactions [18,19]. Valuable research works in recent years, which focused on evaluating the efficiency of closed Brayton cycle invoking dissociating gases often ignore heat-transfer non-equilibrium processes [20,21]. Although with reasonable arguments, there are strong evidences of insufficient research in applications invoking reactive heat transfer. ...
Article
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Waste heat recovery is an indispensable solution towards high energy efficiency in various industrial processes. While many methods are available to recuperate waste heat of medium-to-high temperature range, limited solutions are applicable at low-temperature (<373K). The present work presents a potentially reasonable cost while less understood method, namely the reactive heat transfer using reversible exo-/endothermic reactions for harvesting low-grade heat. Invoking high-fidelity direct numerical simulation, the interplay amongst turbulence, heat transfer, and chemical reactions is investigated in a heated channel flow. We consider a temperature difference between hot source and reacting working fluid of 100K and show a remarkable improvement of heat transfer coefficient by 600% compared to non-reacting working fluid. This is associated with ~17% higher total energy absorption across the geometry of interest. These improvements are proven to be related to the existence of mild exothermic reactions near the channel core, the molar expansion, and the mild endothermic reaction close to the hot source which contributes to a thin thermal boundary layer, etc.
... To mitigate this drawback, this study proposed novel carbon dioxide mixtures as working fluid in waste heat recovery power cycles. The use of CO2 mixtures to alter the properties of CO2 and to achieve higher cycle efficiency is recently explored by some research works [4], [5]. Here in this paper, we explored possibility of CO2 based mixtures as working fluid in waste heat recovery power cycles to address following main challenges: (i) to improve the heat recovery effectiveness of CO2 power cycle, (ii) to reduce cycle maximum operating pressures in order to avoid mechanical stresses in cycle components and, (iii) to keep power cycle layout simpler. ...
... The dopants were chosen due to their thermal stability and their higher critical temperatures compared to CO 2 . The mass fraction ratios were set to CO 2 -TiCl 4 85%-15% and CO 2 -N 2 O 4 78% to 22% based on a previous optimisation [15]. Cycle optimisation carried out at high turbine entry temperatures of 550 and 700 • C resulted in cycle efficiencies up to 50%, a reduction of 50% and 20% in specific costs of the power block with respect to conventional steam cycle and sCO 2 power blocks, and a reduction of 11 to 13% of levelized cost of electricity (LCoE) with respect to a conventional steam cycle. ...
Article
Full-text available
Supercritical CO2 (sCO2) power cycles have gained prominence for their expected excellent performance and compactness. Among their benefits, they may potentially reduce the cost of Concentrated Solar Power (CSP) plants. Because the critical temperature of CO2 is close to ambient temperatures in areas with good solar irradiation, dry cooling may penalise the efficiency of sCO2 power cycles in CSP plants. Recent research has investigated doping CO2 with different materials to increase its critical temperature, enhance its thermodynamic cycle performance, and adapt it to dry cooling in arid climates. This paper investigates the use of CO2/TiCl4, CO2/NOD (an unnamed Non-Organic Dopant), and CO2/C6F6 mixtures as working fluids in a transcritical Rankine cycle implemented in a 100 MWe power plant. Specific focus is given to the effect of dopant type and fraction on optimal cycle operating conditions and on key parameters that influence the expansion process. Thermodynamic modelling of a simple recuperated cycle is employed to identify the optimal turbine pressure ratio and recuperator effectiveness that achieve the highest cycle efficiency for each assumed dopant molar fraction. A turbine design model is then used to define the turbine geometry based on optimal cycle conditions. It was found that doping CO2 with any of the three dopants (TiCl4, NOD, or C6F6) increases the cycle’s thermal efficiency. The greatest increase in efficiency is achieved with TiCl4 (up to 49.5%). The specific work, on the other hand, decreases with TiCl4 and C6F6, but increases with NOD. Moreover, unlike the other two dopants, NOD does not alleviate recuperator irreversibility. In terms of turbine design sensitivity, the addition of any of the three dopants increases the pressure ratio, temperature ratio, and expansion ratios across the turbine. The fluid’s density at turbine inlet increases with all dopants as well. Conversely, the speed of sound at turbine inlet decreases with all dopants, yet higher Mach numbers are expected in CO2/C6F6 turbines.
... In contrast to all previous studies on N 2 O 4 , recently published results [23,24] do not show a real advantage underlying the use of this reactive fluid. However, calculations have been performed with commercial software not enabling the expansion or compression of reactive fluids. ...
Article
Full-text available
Thermal engines, particularly closed power cycles, are currently a focus of many studies mainly because they represent the only way to exploit renewable thermal energy. To increase the exploitation of available thermal sources, this work investigates the higher potential offered by a complementary technology based on the use of reactive working fluids instead of inert fluids: the here-called “thermo-chemical” engine. Such a power cycle enables the simultaneous conversion of thermal and chemical energy into work. Based on a theoretical approach, this paper explores engine performance considering different stoichiometries and thermodynamic characteristics of reactive fluids and different operating conditions. It is shown that the use of specific equilibrated reactions occurring in the gaseous phase might lead to extremely powerful and highly efficient energy conversion systems in the whole current domain of the application of power cycles. Moreover, it is demonstrated that, unlike classical thermal machines, a thermo-chemical engine allows efficient and powerful exploitation of low-temperature heat sources and high-temperature cold sinks, which in general, characterize renewable thermal energy.
... There is just one activity, performed by University of Brescia and Politecnico di Milano [6,15], investigating the adoption of blending fluids capable of increasing the critical temperature and withstanding temperatures up to 700°C. In the framework of this collaboration, two promising fluids have been identified for blending: TiCl 4 and N 2 O 4 . ...
Conference Paper
The future of Concentrated Solar Power technology relies on significant cost reduction to be competitive against both fossil fuel power stations and renewable technologies as photovoltaics and wind. Most of the research activity on concentrated solar power focuses on supercritical CO2 cycles to increase the solar plant efficiency together with a cost reduction. Recently, several research groups have started investigating the blending of CO2 with small amounts of additives to boost the thermodynamic cycle performance. The SCARABEUS project aims at developing and demonstrating CO2 blends in concentrating solar power plant with maximum temperatures of 700°C, power cycle efficiency above 50% and cost of electricity below 96 €/MWh. The innovative fluid and newly developed components will be validated at a relevant scale (300 kWth) for 300 h in a CSP-like operating environment.
... Moreover, the application of sCO 2 blends in CSP plants have been previously studied [18] and have been shown to enhance efficiency of a Brayton power cycle by 3-4% compared to pure sCO 2 . Therefore, sCO 2 blends have been proposed instead of pure sCO 2 to increase the critical temperature of the working fluid [19,20]. The presented design methodology has been developed as part of a preliminary study related to the Horizon 2020 SCARABEUS project [21], and thus the methodology is capable of designing turbines intended for sCO 2 blends. ...
Article
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Supercritical carbon dioxide (sCO 2 ) power cycles are promising candidates for concentrated-solar power and waste-heat recovery applications, having advantages of compact turbomachinery and high cycle efficiencies at heat-source temperature in the range of 400 to 800 ∘ C. However, for distributed-scale systems (0.1–1.0 MW) the choice of turbomachinery type is unclear. Radial turbines are known to be an effective machine for micro-scale applications. Alternatively, feasible single-stage axial turbine designs could be achieved allowing for better heat transfer control and improved bearing life. Thus, the aim of this study is to investigate the design of a single-stage 100 kW sCO 2 axial turbine through the identification of optimal turbine design parameters from both mechanical and aerodynamic performance perspectives. For this purpose, a preliminary design tool has been developed and refined by accounting for passage losses using loss models that are widely used for the design of turbomachinery operating with fluids such as air or steam. The designs were assessed for a turbine that runs at inlet conditions of 923 K, 170 bar, expansion ratio of 3 and shaft speeds of 150k, 200k and 250k RPM respectively. It was found that feasible single-stage designs could be achieved if the turbine is designed with a high loading coefficient and low flow coefficient. Moreover, a turbine with the lowest degree of reaction, over a specified range from 0 to 0.5, was found to achieve the highest efficiency and highest inlet rotor angles.
Article
This paper proposes a methodology for the dynamic thermal analysis of solar tower external receivers and for the assessment of their lifetime. A dynamic thermal model is implemented in Modelica to assess the temperature distributions of receiver tubes and heat transfer fluid (HTF), considering real weather data with a 10-minute time resolution and PI controllers based mass flow rate control. Three representative days are simulated, and the resulting tubes temperature distributions are used to assess the elastic stresses distributions during the investigated days. Subsequently, the creep-fatigue lifetime of each receiver panel is evaluated accounting for material plasticity and creep-induced stress relaxation. The developed methodology is applied to compare three materials for solar receivers: Inconel 740H, Haynes 230, and Incoloy 800H. Results show that fatigue can be negligible with respect to creep for all investigated materials and the shortest lifetime is obtained for 800H, followed by H230, and 740H. The approximation of a receiver operation for 365 clear-sky days per year leads to errors around 30 % on the lifetime. For 740H and 800H, the damage accumulation during cloudy days with high DNI peaks is greater than during clear-sky days due to the remarkable creep damage originated during temperature spikes which follow clouds passages. H230 instead, accumulates more creep damage during clear-sky operation. This emphasizes the potential of the developed methodology to compare different HTF mass flow rate control strategies as well as receiver geometries and materials, HTFs, and heliostats aiming strategies, accounting for the trade-off between energy yield and receiver lifetime.
Article
Compared to existing technologies, thermodynamic cycles based on supercritical carbon dioxide (sCO2) are leading to higher efficiencies and reduced component sizes. However, it is possible to further improve the performance of sCO2 power cycles by using mixtures of CO2 with suitable additives, as also discussed in the literature for different applications. This work investigates the potential to optimize the characteristics of sCO2 power cycles by selectively adding different substances in varying amounts to CO2. A new methodology is proposed: By using the reference equation of state for CO2 in combination with a multi-fluid mixture model, a theoretical screening of suitable additives was done. In the literature, studies were mainly limited to mixtures for which adjusted mixture models are available. By contrast, in this work the use of a predictive mixture model, which was recently developed at our institute, also allows to include fluids, for which no adjusted models are available. Applied to two thermodynamic cycles, changes in efficiency compared to the use of pure CO2 have been evaluated. Several promising mixture candidates have been identified. Additionally, individual effects on the cycle characteristics as well as shifts of the critical points have been investigated and are discussed.
Article
The potential contributions of this critical review are to provide a detailed complement of the status, barriers, and prospect of the supercritical carbon dioxide (S-CO2) cycle power technology, and give a clue to promote its application. The state-of-the-art and existing problems of the S-CO2 power technology are reviewed from the perspective of system analysis and component design. The emphasis is put on the application in next-generation high-temperature solar thermal power plants, next-generation compact nuclear reactor power plants, and coal-fired power plants to reveal the thermodynamic, economic, environmental, and flexible feasibility. The construction of the S-CO2 demonstration power station developed in recent years is also summarized to provide a comprehensive understanding of the development route. Finally, the potential of the S-CO2 cycle to establish a multi-generation system is proposed with a promising peak-shaving ability. Meanwhile, advice is stated to facilitate the further growth of this novel power conversion technology. This study is expected to help understand the recent development progress in S-CO2 power technology.
Article
This work introduces the use of three different types of HTFs (NaNO3-KNO3, NaCl-KCl-ZnCl2, LiF-NaF-KF) and three different energy cycles (sCO2 Recompression, sCO2 Partial cooling, and Rankine) in two different locations (Seville and Dubai) for the solar thermal tower power plant. Detailed simulations have been carried out to calculate the solar to electric conversion efficiency and optimization is carried out to achieve maximum performance. The results obtained showed that the performance of the plant that is based on NaCl-KCl-ZnCl2 as HTF is least with the sCO2 cycles and only suitability is with the Rankine cycle. Plants based on NaNO3-KNO3 as HTF have shown similar results and it also has more suitability with the Rankine cycle. The plant with LiF-NaF-KF as HTF performed better with both sCO2 Recompression and sCO2 Partial cooling cycle but it is not recommended with the Rankine cycle due to poor performance. For all three HTFs, the sCO2 Partial cooling cycle is the best performance cycle for the optimized plant. The placement of the Seville plant in Dubai and after optimization increased efficiency by 4.29%, capacity factor by 81.48%, and electricity by 81.41%. The power factor was increased by 30.8, and an additional 76.86 GW of electricity was generated.
Article
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This work presents the characteristics of a solar thermal tower power plant in two different places (Seville and Dubai) using three different HTFs (NaNO 3 -KNO 3 , KCl-MgCl 2 and Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 ) and three different power cycles (Rankine, sCO 2 Recompression and sCO 2 Partial cooling cycles). An indirect configuration is considered for the Gemasolar power plant. Detailed modelling is carried out for the conversion of incident power on the heliostat to the output electricity. Optimization of the cycle is carried out to determine the most promising cycle configuration for efficiency. The results showed that for the Gemasolar power plant configuration, the performance of the KCl-MgCl 2 based plant was poorest amongst all. NaNO 3 -KNO 3 based plant has shown good performance with the Rankine cycle but plant having Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 as HTF was best for all three cycles. Partial cooling was the best performing cycle at both locations with all three HTFs. Placing the Seville Plant in Dubai has improved the efficiency from 23.56% to 24.33%, a capacity factor improvement of 21 and 52 GW additional power is generated. The optimization of the plant in Dubai has shown further improvements. The efficiency is improved, the Capacity factor is increased by 31.2 and 77.8 GW of additional electricity is produced.
Article
This study provides an exploration on improving the performance of solar power tower (SPT) plant via integrating with the CO2-based mixture Brayton cycle, and proposes a comprehensive comparison method. The effects of the crucial parameters on the SPT system are firstly revealed. Then, a comprehensive evaluation method is proposed based on 3 performance metrics including exergy efficiency, specific work and temperature difference of the main-heater, which are used to assess the thermodynamic performance and the compatibility between the thermal storage system and power cycle. Finally, the trade-off relationships of these metrics are found. The optimal layout, additive and operating parameters are also pointed out based on the comparative results to meet different design requirements. The findings show that the effects of crucial parameters on the performance of SPT system are not monotonous and individual criterion cannot comprehensively evaluate the system performance. Using xenon as an additive can yield excellent performance with higher exergy efficiency and better compatibility between cycle and thermal storage system than the CO2 alone. While the ability to generate specific work is decreased to 105–112 kW·kg⁻¹ with xenon-added that compares to the inter-cooling S-CO2 cycle (121–130 kW·kg⁻¹). When the exergy efficiency is required higher than 33%, the inter-cooling CO2/xenon cycle is the primarily recommended layout for its large specific work and good compatibility between the thermal storage system and cycle. The findings provide a novel way to improve the efficiency of SPT system as well as the compatibility between the cycle and thermal storage system.
Article
This paper discusses the adoption of CO2 mixtures for improving the thermal-to-power efficiency conversion in solar tower plants and reducing the Levelized Cost of Electricity. Two different fluids are considered for blending the CO2: N2O4 and TiCl4. The main advantage of the innovative mixtures relies in a higher critical temperature with respect to pure CO2, which allows condensing cycles even at relatively high ambient temperatures typical of solar plants locations. Thermodynamic results show that the innovative cycles can achieve conversion efficiencies as high as 43% and 50% at 550 °C and 700 °C maximum temperature respectively, outperforming the reference CO2 cycle by 2 points percent. In addition, the simpler lay-out and the liquid compression reduce the power block capital costs below 700 $/kW. Detailed solar plant annual simulation is performed to assess the overall solar to electricity efficiency which can be around 21% for the innovative fluid, corresponding to 10% increase with respect to state-of-the-art solar plant. The higher performance and lower costs lead to a Levelized Cost of Electricity reduction of 10% with respect to conventional steam cycle power block.
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This paper discusses the thermodynamic characteristics of the closed Brayton cycles in which the compression is placed near the critical point of the working fluid. Under these conditions, the specific volumes of the fluid during the compression are a fraction of the corresponding values under ideal gas conditions, and the cycle performances improve significantly, mainly at moderate top temperatures. As the heat is discharged at about the critical temperature, the choice of the correct working fluid is strictly correlated with the environmental temperature or with the temperature of potential heat users. To resort to mixtures greatly extend the choice of the right working fluid, allowing a continuous variation of the critical temperature. These cycles have a high power density, and the use of ordinary turbomachinery is accompanied by high capacities (tens of megawatts). In the low power range, microturbines or reciprocating engines are required. One important constraint on the choice of the right working fluid is its thermochemical stability that restricts the operative temperatures. Among the organic compounds, the maximum safe temperatures are limited to about 400 °C and, forecasting high temperature applications, it could be interesting to explore the potentiality of the inorganic compounds as secondary fluids in binary mixtures.
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Solar tower based plants are seen as a promising technology to reduce the cost of electricity from solar radiation. This paper assesses the design and overall yearly performances of two different solar tower concepts featuring two commercial plants running in Spain. The first plant investigated is based on Direct Steam Generation and a cavity receiver (PS-10 type). The second plant considers an external cylindrical receiver with molten salts as heat transfer fluid and storage system (Gemasolar type). About the optical assessment performed with DELSOL3, a calibration of heliostat aim points was performed to match available flux maps on the receiver. Moving to results, the PS-10 type has higher optical performances both nominal design and yearly average. This is due both to the field size and orientation which guarantee a higher efficiency and to the receiver concept itself. About power production, the molten salts allow higher temperature and consequently conversion efficiency than PS-10. The solar-to-electricity efficiency is equal to 18.7% vs. 16.4% of DSG cavity plant. The obtained results are strictly related to the set of assumptions made on each plant component: when available real plant data where used. The two solar tower plants results were also compared to corresponding commercial linear focus plants featuring the same power block concept. Gemasolar type shows a higher solar-to-electricity efficiency compared to a parabolic trough plant with storage (18.7% vs. 15.4%) because of the higher maximum temperatures and, consequently, power block efficiency. PS-10 is better than a linear Fresnel DSG (16.4% vs. 10.4%) because of the higher optical performances. (C) 2013 The Authors. Published by Elsevier Ltd.
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The effects of anthropogenic emissions of nitrous oxide (N(2)O), carbon dioxide (CO(2)), methane (CH(4)) and the halocarbons on stratospheric ozone (O(3)) over the twentieth and twenty-first centuries are isolated using a chemical model of the stratosphere. The future evolution of ozone will depend on each of these gases, with N(2)O and CO(2) probably playing the dominant roles as halocarbons return towards pre-industrial levels. There are nonlinear interactions between these gases that preclude unambiguously separating their effect on ozone. For example, the CH(4) increase during the twentieth century reduced the ozone losses owing to halocarbon increases, and the N(2)O chemical destruction of O(3) is buffered by CO(2) thermal effects in the middle stratosphere (by approx. 20% for the IPCC A1B/WMO A1 scenario over the time period 1900-2100). Nonetheless, N(2)O is expected to continue to be the largest anthropogenic emission of an O(3)-destroying compound in the foreseeable future. Reductions in anthropogenic N(2)O emissions provide a larger opportunity for reduction in future O(3) depletion than any of the remaining uncontrolled halocarbon emissions. It is also shown that 1980 levels of O(3) were affected by halocarbons, N(2)O, CO(2) and CH(4), and thus may not be a good choice of a benchmark of O(3) recovery.
Conference Paper
One of the option to make concentrated solar power competitive from economic point of view consists of adopting innovative power cycle which can efficiently exploit temperature up to 700°C. This work assesses the potentiality of pure N2O4 and mixed with CO2 when applied to solar tower plant using liquid as heat transfer fluid. N2O4 has a high critical temperature (158°C) which limits the compression work boosting the power block conversion efficiency. Results show that the best performance are achieved by the N2O4 mixture that obtains an overall solar-to-electric efficiency of about 26.3%. A 78%-22% CO2−N2O4 mixture achieves solar-to-electric efficiency of 25.3%. Both solutions seem promising with respect to the considered recompressed sCO2 cycle.
Article
This work assesses the performance of a solar tower power plant based on liquid sodium as heat transfer fluid and supercritical CO2 cycles. The adoption of liquid sodium as heat transfer fluid allows maximum temperatures up to 750 °C and higher heat fluxes on the receiver with respect to molten salts (both Solar Salts and KCl-MgCl2) also considered as reference. The assessment is carried out through detailed modeling of the solar to electricity conversion processes accounting for detail optical, thermal and power block models. Results at design conditions show that plants using sodium as HTF in the receiver can achieve overall efficiency above 25%, whereas the use of Solar Salts at 565 °C and KCl-MgCl2 at 750 °C reach 21.5% and 24% respectively. The higher efficiency is consequence of the higher thermal efficiency of sodium which is achieved increasing the concentration ratio. Considering a yearly analysis, the overall efficiency of sodium reduces to 20.5% and 19.3% in Seville and Las Vegas respectively which is 7–9% higher than using KCl-MgCl2 and 11% with respect to Solar Salts. Outcomes of this work are the importance of (i) coupling higher temperatures with higher allowable fluxes on the receiver and (ii) defining the system operating conditions on overall yearly efficiency rather than design point.
Article
This work discusses a preliminary thermodynamic assessment of three different supercritical CO2 (sCO2) power cycles applied to a high temperature solar tower system, with maximum temperatures up to 800 °C. The thermal power is transferred from the solar receiver to the power block through KCl-MgCl2 molten salts as heat transfer fluid, therefore an indirect cycle configuration is considered assuming a surrounded field as the one of Gemasolar plant. The most promising cycle configuration in terms of solar-to-electric efficiency is selected, optimizing the cycle turbine inlet temperature to achieve the best compromise between cycle and receiver performance: the highest efficiency at design conditions is achieved by the Recompression with Main Compression Intercooling (RMCI) configuration with a solar to electric efficiency of 24.5% and a maximum temperature of 750 °C. The yearly energy yield of the proposed power plant is estimated with a simplified approach and results in the range of 18.4%: the performance decay from design to average yearly conditions is mostly due to the optical and thermal efficiencies reduction (−10.8% and −16.4%, respectively).
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In this paper we investigate the thermal stability of three representative hydrocarbons used as working fluids in organic Rankine cycles (ORC): n-pentane, cyclo-pentane and toluene. The experimental used method is a “static” one, based on the recording of the pressure during the permanence of the fluid sample at constant temperature and on the measure of the differences in the vapour pressure in comparison with the reference values for the virgin pure fluid. The sample container and the circuit are in stainless steel.
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In this paper, Titanium tetrachloride (TiCl4) is analyzed/assessed and proposed as a new potential working fluid in Rankine Cycles. Besides its good thermodynamic properties, TiCl4 is in fact a fairly low cost, non-carcinogenic fluid, with zero Global Warming Potential (GWP) and Ozone Depleting Potential (ODP) and it is currently employed in high temperature industrial processes. It is however very reactive with humid air and water. A preliminary thermodynamic analysis confirms its possible application in power plants with maximum temperature up to 500 C, considerably higher than the ORC state-of-the-art technology, performing electrical efficiencies as high as 35–40%. This suggests the potential use of TiCl4 as an alternative fluid in ORCs allowing the exploitation of high temperature sources (up to 500 C), typically used in steam cycles. To assess the possibility of operating the cycle in such high temperature conditions, we carried out an experimental thermal stress analysis, showing that the fluid is remarkably stable at temperatures up to 500 C, even in presence of P91 and Cupronickel, two materials typically employed in the high temperature section of power cycles.
Article
This paper evaluates cost and performance tradeoffs of alternative supercritical carbon dioxide (s-CO2) closed-loop Brayton cycle configurations with a concentrated solar heat source. Alternative s-CO2 power cycle configurations include simple, recompression, cascaded, and partial cooling cycles. Results show that the simple closed-loop Brayton cycle yielded the lowest power-block component costs while allowing variable temperature differentials across the s-CO2 heating source, depending on the level of recuperation. Lower temperature differentials led to higher sensible storage costs, but cycle configurations with lower temperature differentials (higher recuperation) yielded higher cycle efficiencies and lower solar collector and receiver costs. The cycles with higher efficiencies (simple recuperated, recompression, and partial cooling) yielded the lowest overall solar and power-block component costs for a prescribed power output.
Conference Paper
With reference to a proposal made by Prof. Ackeret studies have been made on the composition of an optimal working medium for closed-cycle gas turbines. These studies are based on the assumption that this aim could be achieved by mixing helium with a gas of higher molecular weight, using neon, nitrogen, and carbon dioxide here. The influence of these gas mixtures on the circuit parameters and on the layout of the turbomachines and heat exchanging apparatus is shown. The calculations have been carried out giving consideration to the real gas behavior of the mentioned gas mixtures. In addition to this, the relationship of the costs of the turbomachinery and apparatus are shown in relation to a reference plant with pure helium as working agent. The basis for these studies is a process with the thermal efficiency, the upper and lower process temperatures and the turbine inlet pressure being the same for all gas mixtures. The results of the calculations are that, for certain gas compositions, cost advantages can be gained relative to the layout with pure helium. These advantages and the mixture compositions, for which they appear, are dependent on the admixed gases. For these calculations the cost relationship of turbomachines and heat exchangers to the total cost have been considered. Copyright © 1974 by ASME Country-Specific Mortality and Growth Failure in Infancy and Yound Children and Association With Material Stature Use interactive graphics and maps to view and sort country-specific infant and early dhildhood mortality and growth failure data and their association with maternal
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Nitrogen tetroxide is tested for its thermodynamic properties. The performance of gas and condensing cycles was evaluated. Material selection and corrosion problems were investigated. (A)
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The potential performance of carbon dioxide as working fluid is recognized to be similar to that of steam, which justifies thorough thermodynamic analysis of possible cycles. The substantially better results achievable with CO² with respect to other gases are due to the real gas behaviour in the vicinity of the Andrews curve. Simple cycles benefit from the reduced compression work, but their efficiency is compromised by significant losses caused by irreversible heat transfer. Their economy, however, is appreciably better than that of perfect gas cycles. More complex cycle arrangements, six of which are proposed and analyzed in detail, reduce heat transfer losses while maintaining the advantage of low compression work and raise cycle efficiency to values attained only by the best steam practice. Some of the cycles presented were conceived to give a good efficiency at moderate pressure which is of particular value in direct-cycle nuclear applications. The favourable influence on heat transfer coefficients of the combined variation with pressure of mechanical, thermal and transport properties, due to real gas effects, is illustrated. Technical aspects as turbo-machines dimensions and heat transfer surfaces needed for regeneration are also considered. Cooling water requirements are found to be not much more stringent than in steam stations. Copyright © 1969 by ASME Country-Specific Mortality and Growth Failure in Infancy and Yound Children and Association With Material Stature Use interactive graphics and maps to view and sort country-specific infant and early dhildhood mortality and growth failure data and their association with maternal
Article
This paper examines the potential of sodium receivers to increase the overall solar-to-electricity efficiency of central receiver solar power plants, also known as solar tower systems. It re-visits some of the key outcomes and conclusions from past sodium receiver experiments, in particular those at Sandia National Laboratories and Plataforma Solar de Almeria in the 1980s, and discusses some current development activities in the area. It also discusses research in sodium receivers with a liquid-vapour phase change (heat pipes and pool boilers), to explore whether technologies developed for dish-Stirling systems have applicability for solar tower systems. Lessons learnt from experience in the nuclear industry with liquid sodium systems are discussed in the context of safety risks.
Article
This paper explores the potential applicability of the Supercritical (Feher) Thermodynamic Power Cycle to advanced ground nuclear power systems. The supercritical cycle is a closed cycle heat engine that operates entirely above the critical pressure of the working fluid. It is characterized by high thermal efficiency and compactness of the machinery. The cycle is highly regenerated and receives heat over a narrow temperature range. For the evaluation of the advantages of the power conversion concept, a 150-kwe power conversion module has been selected that employs a gas turbine driven high speed alternator, using carbon dioxide as the working fluid.
Article
Supercritical carbon dioxide cycles are a promising power conversion option for future nuclear reactors operating with a reactor outlet temperature in the range of 550 to 650°C. The recompression cycle version operating with ∼ 20-MPa turbine inlet pressure achieves similar cycle efficiencies as helium Brayton cycles operating at ∼ 250°C higher turbine inlet temperature. The simplicity and high efficiency of the recompression cycle makes it a prime option from among the family of supercritical carbon dioxide cycles. The elimination of the need for intercooling due to the small required compressor work (because of the high density close to the critical point) makes the recompression cycle even simpler than helium Brayton cycles, which require intercooling to achieve attractive efficiencies. The high operating pressure reduces the size of the plant components significantly, making it a promising power cycle for low-cost modularized electricity-generating nuclear systems. However, the real gas behavior that improves the cycle efficiency presents a challenge for part-load operation. The traditional inventory control used for helium Brayton cycles may not be feasible. Bypass control is thus the prime option for part-load operation, making the cycle less efficient than during base-load operation. Since nuclear power plants are operated almost exclusively in base load, this drawback is not a disqualifying blemish.
Article
Of the mechanisms to improve efficiency for solar-thermal power plants, one of the most effective ways to improve overall efficiency is through power cycle improvements. As increases in operating temperature continue to be pursued, supercritical CO2 Brayton cycles begin to look more attractive despite the development costs of this technology. Further, supercritical CO2 Brayton has application in many areas of power generation beyond that for solar energy alone. One challenge particular to solar-thermal power generation is the transient nature of the solar resource. This work illustrates the behavior of developmental Brayton turbomachinery in response to a fluctuating thermal input, much like the short-term transients experienced in solar environments. Thermal input to the cycle was cut by 50% and 100% for short durations while the system power and conditions were monitored. It has been shown that despite these fluctuations, the thermal mass in the system effectively enables the Brayton cycle to continue to run for short periods until the thermal input can recover. For systems where significant thermal energy storage is included in the plant design, these transients can be mitigated by storage; a comparison of short- and long-term storage approaches on system efficiency is provided. Also, included in this work is a data set for stable supercritical CO2 Brayton cycle operation that is used to benchmark computer modeling. With a benchmarked model, specific improvements to the cycle are interrogated to identify the resulting impact on cycle efficiency and loss mechanisms. Status of key issues remaining to be addressed for adoption of supercritical CO2 Brayton cycles in solar-thermal systems is provided in an effort to expose areas of necessary research.
Article
The thermodynamic performance of several condensation cycles employing carbon dioxide as working medium is analyzed and discussed. A balanced distribution of thermodynamic losses between mechanical components and heat exchangers attained through a compression performed partially in the liquid and partially in the gas phase yields cycle efficiencies which are among the highest achievable in present-day energy systems. At turbine inlet temperatures higher than 650 deg C single heating CO2 cycles exhibit a better efficiency than reheat steam cycles. This may prove of particular interest in connection with high temperature nuclear heat sources. However, the requirement of low temperature cooling water for a good cycle arrangement represents a geographical limitation to the widespread application of CO2 condensation cycles.
Article
Conventional machines use a gaseous working luid, but substantial improvement in specific output may be gawined with a partially reactive, condensing working fluid. The working fluid then consists of an inert gaseous carrier with a chemically reactive, condensing working fluid such as nitrogen tetroxide (N//2O//4). This may be liquid in the cold compression space and then evaporates and dissociates in the regenerative process to be in the elemental gaseous phase in the hot expansion space. The change of state of one component reduces the required compression work and has the effect of increasing the engine volume compression ratio with consequent benefit to the specific output. The results obtained using idealized theory show that an improvement may be gained in net cycle work of twice the output with a simple gaseous working fluid with no penalties in size, weight, or cost of the engine.
Article
There is a common interest in the distributed power generation: generally for the combined production of electrical and thermal energy and often, although not necessarily, in association with renewable energies as heat sources for the prime mover. For example, in the field of distributed concentrated solar power generation of small size, the gas engine technology now seems to be prevailing (Stirling engines operating at maximum temperatures of 600–800􏰀C, with peak net efficiencies at 20–30% and power up to several kilowatts are commonly considered). Organic Rankine engines, fed by biomass, in the power range of about 1 MW are actually a standard. From a strictly thermodynamic point of view, the binary cycle technology, accomplished by alkaline metal Rankine cycle as the topping cycle and a Rankine cycle with organic fluid as the bottoming cycle, could be an advantageous alternative. By their very nature, Rankine cycles have good thermodynamic qualities and, potentially, their thermodynamic performance, for the same maximum and minimum temperatures, could be better than that of a gas cycles. This paper discusses the possibility of adopting binary cycles with a power level in the order of tens of kilowatts. Following an overview of the characteristics of alkaline metals and a look at the possible organic fluids that can be employed in Rankine engines at high temperature (400 􏰀C), assuming a limit condensation pressure of 0.05 bar, the thermodynamic efficiency of binary cycles was evaluated and the preliminary sizing of turbines was discussed. The results (e.g. a net cycle efficiency of around 0.46, with maximum temperature of 800–850􏰀C) appear encouraging, even though setting up the systems may be far from easy. For instance, there are difficulties due to the extremely high volumetric expansion ratios of bottoming cycles (400–600, an order of magnitude larger than those of the topping cycles with alkaline metals that we considered), which are moreover associated with a very low minimum pressure and elevated number of revolutions of the turbomachinery (50,000–200,000 r/min). Without doubt, the design tends to be easier as the power levels increase and the minimum condensation pressure for the bottoming cycle rises. Although the authors know of no activity in progress on binary cycles at present, the interesting prospects suggest the topic deserves further study and research.
Article
We have used in this work the crossover soft-SAFT (soft-Statistical Associating Fluid Theory) equation of state to model nitrogen dioxide/dinitrogen tetroxide (NO2/N2O4), carbon dioxide (CO2) and their mixtures. The prediction of the vapor–liquid equilibrium of this mixture is of utmost importance to correctly assess the NO2 monomer amount that is the oxidizing agent of vegetal macromolecules in the CO2 + NO2/N2O4 reacting medium under supercritical conditions. The quadrupolar effect was explicitly considered when modeling carbon dioxide, enabling to obtain an excellent description of the vapor–liquid equilibria diagrams. NO2 was modeled as a self-associating molecule with a single association site to account for the strong associating character of the NO2 molecule. Again, the vapor–liquid equilibrium of NO2 was correctly modeled. The molecular parameters were tested by accurately predicting the very few available experimental data outside the phase equilibrium. Soft-SAFT was also able to predict the degree of dimerization of NO2 (mimicking the real NO2/N2O4 situation), in good agreement with experimental data. Finally, CO2 and NO2 pure compound parameters were used to predict the vapor–liquid coexistence of the CO2 + NO2/N2O4 mixture at different temperatures. Experimental pressure–CO2 mass fraction isotherms recently measured were well described using a unique binary parameter, independent of the temperature, proving that the soft-SAFT model is able to capture the non-ideal behavior of the mixture.
Article
The supercritical carbon dioxide Brayton cycle (S-CO2 cycle) has attracted much attention as an alternative to the Rankine cycle for sodium-cooled fast reactors (SFRs). The higher cycle efficiency of the S-CO2 cycle results from the considerably decreased compressor work because the compressor behaves as a pump in the proximity of the CO2 vapor–liquid critical point. In order to fully utilize this feature, the main compressor inlet condition should be controlled to be close to the critical point of CO2. This indicates that the critical point of CO2 is a constraint on the minimum cycle condition for S-CO2 cycles. Modifying the CO2 critical point by mixing additive gases could be considered as a method of enhancing the performance and broadening the applicability of the S-CO2 cycle. Due to the drastic fluctuations of the thermo-physical properties of fluids near the critical point, an in-house cycle analysis code using the NIST REFPROP database was implemented. Several gases were selected as potential additives considering their thermal stability and chemical interaction with sodium in the temperature range of interest and the availability of the mixture property database: xenon, krypton, hydrogen sulfide, and cyclohexane. The performances of the optimized CO2-containing binary mixture cycles with simple recuperated and recompression layouts were compared with the reference S-CO2, CO2–Ar, CO2–N2, and CO2–O2 cycles. For the decreased critical temperatures, the CO2–Xe and CO2–Kr mixtures had an increase in the total cycle efficiency. At the increased critical temperatures, the performances of CO2–H2S and CO2–cyclohexane with the recompression layout were superior to the S-CO2 cycle when the compressor inlet temperature was above the critical temperature of CO2.
Article
Measurements of the critical pressure pc, densityρc , and temperature Tcof the pure substanceCH2F2 as well as {xCO2 + (1 − x)SF6}, {xSF6 + (1 − x)CH2F2}, and {xCHF3 + (1 − x)CH2F2} were performed by observing the critical opalescence in a static equilibrium cell. When possible, the experimental data were compared with values given in the literature. The experimental results for the binary mixtures are described by the Peng–Robinson and the Trebble–Bishnoi–Salim equations of state, using only the experimental critical data of the pure components and those of a mixture with x ≈ 0.5. The equations of state are used to describe the critical loci, the dependence of the supercritical density on temperature and composition, and the excess volume of the binary systems investigated.
Article
Closed Brayton gas and supercritical cycles operating with mixtures of carbon dioxide and hydrocarbons, in particular mixture with a low content (5-15%) of benzene, were studied. Totally supercritical cycles and condensation cycles were looked at and a comparison was made with pure carbon dioxide cycles with a minimal temperature around 40 − 50 ◦C (typical minimum temperatures of air cooled radiators). First and second law cycle efficiencies were considered and analyzed. Critical points calculations of several mixtures were per- formed by means of an accurate model for the thermodynamic properties and compared with experimental data from literature. For the cycle calculations a simpler model with classical mixing rules was used because the results were in sufficient agreement. The cycles operating with mixtures showed lower maximum pressures and higher cycle efficiencies compared to the pure carbon dioxide cycles. Taken into account the high GWP (Global Warming Potential) of fluorinated fluids and the high flammability and high volume expansion ratios of comparable Rankine toluene cycles, mixtures of carbon dioxides and hydrocarbons exposed promising features.
Article
Previous studies have been made on the composition of an optimal working fluid for closed cycle gas turbines. Those studies were based on the assumption that the objective could be achieved by mixing helium with a gas of higher molecular weight. The results of those calculations, based on heat transfer data of pure gases and air, indicated the possibility of significant cost advantages of gas mixtures relative to pure helium. Recent heat transfer measurements of low Prandtl number gas mixtures indicate that existing scaling laws for normal Prandtl number (approximately 0.7) gases do not adequately represent the heat transfer characteristic of these gas mixtures. In this paper the influence of these gas mixture heat transfer data on closed cycle gas turbine coolers, recuperators, and heat source exchangers is presented. The characteristic dimensions (flow area, surface area, and length) of these heat exchangers utilizing pure gases and gas mixtures are compared. Bibtex entry for this abstract Preferred format for this abstract (see Preferences) Find Similar Abstracts: Use: Authors Title Keywords (in text query field) Abstract Text Return: Query Results Return items starting with number Query Form Database: Astronomy Physics arXiv e-prints
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An experimental apparatus for assessing the thermal stability threshold of refrigerant working fluids is described and results for R-134a (1,1,1,2-tetrafluoroethane), R141b (1,1-dichloro-1-fluoroethane), R-13I1 (trifluoromethyl iodide), R-7146 (sulphur hexafluoride), R-125 (pentafluoroethane) are presented. The information is a concern for the design of refrigeration systems, high temperature heat pumps and Organic Rankine Cycles (ORC), for which the above refrigerants are proposed. The method aims to identify a maximum temperature for plant operation in contact with stainless steel and involves the evaluation of four indicators: (1) pressure variation while the fluid is maintained at set temperature; (2) saturation pressure comparison after heat treatment; (3) chemical analysis; and (4) vessel visual inspection after the test session. The highest temperatures at which no evident degradation occured are: 368°C for R-134a; 102°C for R-13I1; 90°C for R-141b; 204°C for R-7146; and 396°C for R-125.
Article
Organizing a closed Brayton cycle in such a way that the compression process is performed in the vicinity of the critical point where specific volumes are a fraction of those of an ideal gas yields performance indices particularly attractive, mainly at moderate top temperatures. Cycle thermodynamic analysis requires the development of adequate methods for the computation of thermodynamic properties above the vapour saturation curve about the critical point. Working fluids suitable for the proposed cycle can be found in the class of organics, in particular among the newly developed, zero ozone depletion potential, chlorine-free compounds. The numerous technical and environmental requirements which a fluid must meet for practical use combined with the peculiar thermodynamic restraints limit the number of suitable fluids. Mixing two substances of different critical temperatures yields an indefinite number of fluids with tailor-made thermodynamic properties. One such mixture 0.93 HFC23 + 0.07 HFC125 (molar fraction), having tcr = 30°C, at tmax = 400°C, pmax = 150 bar, gives an efficiency above 27 per cent with heat rejection temperatures between 89 and 33°C. With a different mixture composition with a 50°C critical temperature, at the same tmax and pmax, an efficiency of 25.1 per cent is attained in a combined heat and power generation cycle with heat available in the range 53-103°C. An experimental programme to test the thermal stability of organic fluids showed that top temperatures of 380-450°C are achievable with some commercially available fluoro-substituted hydrocarbons. In view of practical applications a conversion unit based on a reciprocating engine could handle without problems the pressures and temperatures involved. The use of turbomachinery would lead to power plant of large capacity for the usual rotor dimensions or to micro-turbines at high rotating speed in the low power range.
Article
Characteristic requirements of a closed-cycle gas turbine (CCGT) working fluid were identified and the effects of their thermodynamic and transport properties on the CCGT cycle performance, required heat exchanger surface area and metal operating temperature, cycle operating pressure levels, and the turbomachinery design were investigated. Material compatibility, thermal and chemical stability, safety, cost, and availability of the working fluid were also considered in the study. This paper also discusses CCGT working fluids utilizing mixtures of two or more pure gases. Some mixtures of gases exhibit pronounced synergetic effects on their characteristic properties including viscosity, thermal conductivity and Prandtl number, resulting in desirable heat transfer properties and high molecular weights. Typical examples of such synergetic gas mixture are helium-xenon and helium-carbon dioxide.
Article
Experimental bubble pressure, as well as liquid density of (CO2 + NO2/N2O4) mixtures are reported at temperatures ranging from (298 to 328.45) K. Experiments were carried out using a SITEC high-pressure variable volume cell. Transition pressures were obtained by the synthetic method and liquid density was deduced from measurement of the cell volume. Correlation of experimental results was carried out without considering chemical equilibrium of NO2/N2O4 system. (Liquid + vapour) equilibrium was found to be accurately modelled using the Peng–Robinson equation of state with classical quadratic mixing rules and with a binary interaction coefficient kij equal to zero. Nevertheless, modelling of liquid density values was unsatisfactory with this approach.
Article
The results of an analysis to estimate the performance that could be obtained by using a chemically reacting gas (nitrogen tetroxide) as the working fluid in a closed Brayton cycle are presented. Compared with data for helium as the working fluid, these results indicate efficiency improvements from 4 to 90 percent, depending on turbine inlet temperature, pressures, and gas residence time in heat-transfer equipment. This concept could be attractive for solar thermal power applications.
Article
The memorandum summarizes the available information on the compatibility of liquid rocket propellants with prominent materials of construction. Fuels and oxidizers of current interest are discussed. The corrosion data which are presented will apply to storing, handling, and control equipment outside of missiles and to missile components excluding combustion chamber. The compatibility of materials with reaction products in combustion chambers, nozzles, etc., is not considered. Included in the summary are data for many nonmetallic materials. The memorandum is subdivided into sections according to the propellant. Each material of construction is rated for a given medium as belonging to one of four classes, based primarily upon corrosion resistance. Consideration also is given to such factors as catalytic decomposition and sensitivity to impact.
Article
Oxides of nitrogen play an important role in the radical chemistry of the atmosphere and in the production and destruction of tropospheric and stratospheric ozone. Ozone is a principal agent in forming the OH radical which attacks inert gases in the troposphere; the acidity of precipitation is in part the result of HNO3 and H2SO4 formed by reactions involving the OH radical. Fast removal processes for oxides of nitrogen in the lower troposphere are described. Long-distance transport of oxides of nitrogen in peroxy-acetyl nitrate also receives attention. In the stratosphere, the participation of NO and NO2 in reactions that influence the equilibrium of photochemical systems may render the total ozone abundance insensitive to additions of oxides of nitrogen.
Article
In this paper an analysis of the use of dissociating gases in Brayton Cycle Space Power Systems has been presented. It has been shown that the development of higher efficiency cycles is necessary for minimizing isotope costs which have a dominant influence on total cycle economics. A dissociating gas Brayton cycle has been optimized for maximum efficiency and minimum radiator surface area. Results show that 40% higher thermal efficiencies and 25% reduction in radiator area can be achieved with a dissociating gas cycle when compared with a nondissociating gas cycle.
Article
Thermodynamic properties of the chemically reactive system N2O4 ⇌ 2NO2 ⇌ 2NO + O2 have been evaluated over a pressure range of 0.005 to 200.0 atm. and a temperature range of 200° to 900°K. by making use of Lennard-Jones potential. In these calculations, the dissociation of nitrogen tetroxide to nitrogen dioxide, nitric oxide and oxygen, and the effect of pressure on the equilibrium constants for the system of reactions N2O4 ⇌ 2NO2 and 2NO2 ⇌ 2NO + O2 have been taken into account.
Article
The progress in predicting critical transitions in fluid mixtures is reviewed. The critical state provides a valuable insight into the general phase behavior of a fluid and is closely linked with the nature and strength of intermolecular interaction. Calculations of critical equilibria have been confined mainly to binary mixtures. The prediction of binary gas-liquid critical properties was initially limited to empirical correlations. These techniques have been superseded by rigorous calculations of the critical conditions using realistic models of the fluid or equations of state. All of the known types of critical phenomena exhibited by binary mixtures can be, at least, qualitatively calculated. If an optimal combining rule parameter is allowed, continuous gas-liquid properties can be calculated accurately for a wide variety of mixtures. Similarly, the pressure and composition dependence of upper critical solution phenomena can be accurately predicted. Progress has been achieved in predicting discontinuous critical transitions in polar and nonpolar binary mixtures. There is increasing interest in calculating the critical properties of ternary and multicomponent mixtures. Although the techniques applied to binary mixtures often can be directly extended to ternary mixture calculations, calculated critical properties of ternary mixtures indicate that their behavior cannot be considered as a simple extension of binary mixture phenomena. Consequently, ternary critical calculations are likely to provide a superior insight into the phase behavior of multicomponent fluids.
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The thermodynamic behavior of the carbon dioxide + nitrogen dioxide (CO2 + NO2) mixture was investigated using a Monte Carlo molecular simulation approach. This system is a particularly challenging one because nitrogen dioxide exists as a mixture of monomers (NO2) and dimers (N2O4) under certain pressure and temperature conditions. The chemical equilibrium between N2O4 and 2NO2 and the vapor-liquid equilibrium of CO2 + NO2/N2O4 mixtures were simulated using simultaneously the reaction ensemble and the Gibbs ensemble Monte Carlo (RxMC and GEMC) methods. Rigid all atoms molecular potentials bearing point charges were proposed to model both NO2 and N2O4 species. Liquid-vapor coexistence properties of the reacting NO2/N2O4 system were first investigated. The calculated vapor pressures and coexisting densities were compared to experimental values, leading to an average deviation of 10% for vapor pressures and 6% for liquid densities. The critical region was also addressed successfully using the subcritical Monte Carlo simulation results and some appropriate scaling laws. Predictions of CO2 + NO2/N2O4 phase diagrams at 300, 313, and 330 K were then proposed. Derivative properties calculations were also performed in the reaction ensemble at constant pressure and temperature for both NO2/N2O4 and CO2 + NO2/N2O4 systems. The calculated heat capacities show a maximum in the temperature range where N2O4 dissociation occurs, in agreement with available experimental data.
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A system analysis and preliminary design were conducted for an organic Rankine-cycle system to bottom the high-temperature waste heat of an adiabatic diesel engine. The bottoming cycle is a compact package that includes a cylindrical air cooled condenser regenerator module and other unique features. The bottoming cycle output is 56 horsepower at design point conditions when compounding the reference 317 horsepower turbocharged diesel engine with a resulting brake specific fuel consumption of 0.268 lb/hp-hr for the compound engine. The bottoming cycle when applied to a turbocompound diesel delivers a compound engine brake specific fuel consumption of 0.258 lb/hp-hr. This system for heavy duty transport applications uses the organic working fluid RC-1, which is a mixture of 60 mole percent pentafluorobenzene and 40 mole percent hexafluorobenzene. The thermal stability of the RC-1 organic fluid was tested in a dynamic fluid test loop that simulates the operation of Rankine-cycle. More than 1600 hours of operation were completed with results showing that the RC-1 is thermally stable up to 900 F.
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The Kyoto Protocol seeks to limit emissions of various greenhouse gases but excludes short-lived species and their precursors even though they cause a significant climate forcing. We explore the difficulties that are faced when designing metrics to compare the climate impact of emissions of oxides of nitrogen (NOx) with other emissions. There are two dimensions to this difficulty. The first concerns the definition of a metric that satisfactorily accounts for its climate impact. NOx emissions increase tropospheric ozone, but this increase and the resulting climate forcing depend strongly on the location of the emissions, with low-latitude emissions having a larger impact. NOx emissions also decrease methane concentrations, causing a global-mean radiative forcing similar in size but opposite in sign to the ozone forcing. The second dimension of difficulty concerns the intermodel differences in the values of computed metrics. We explore the use of indicators that could lead to metrics that, instead of using global-mean inputs, are computed locally and then averaged globally. These local metrics may depend less on cancellation in the global mean; the possibilities presented here seem more robust to model uncertainty, although their applicability depends on the poorly known relationship between local climate change and its societal/ecological impact. If it becomes a political imperative to include NOx emissions in future climate agreements, policy makers will be faced with difficult choices in selecting an appropriate metric. • climate change • climate metrics • Kyoto Protocol
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