Advanced carbon dioxide thermodynamic cycles for power production

I cicli termodinamici dell'anidride carbonica per la produzione di energia sono una nuova tecnologia in fase di ricerca e sviluppo in vari gruppi di ricerca in tutto il mondo. Le principali caratteristiche attrattive dei cicli ad anidride carbonica sono una maggiore efficienza del ciclo, un ingombro ridotto e la loro capacità di integrazione con diverse fonti di calore. Tuttavia, ci sono alcune sfide specifiche dell'applicazione di tali cicli di alimentazione, tra cui una minore efficacia di recupero del calore, un lavoro specifico per il ciclo inferiore e la necessità di schemi di ciclo complessi per il recupero del calore di scarto. In secondo luogo, la sensibilità dell'efficienza del ciclo all'aumento della temperatura minima del ciclo ne limita anche il funzionamento per condizioni in cui è disponibile solo un mezzo di raffreddamento freddo. Per affrontare le suddette sfide dei cicli energetici dell'anidride carbonica, questo lavoro propone nuovi cicli avanzati transcritici dell'anidride carbonica che funzionano con miscele binarie a base di CO2 come fluidi di lavoro per la produzione di energia. Il calcolo accurato delle proprietà termofisiche dei fluidi di lavoro è fondamentale per l'analisi termodinamica dei cicli di alimentazione utilizzando miscele binarie a base di CO2 come fluidi di lavoro. Di conseguenza, uno degli obiettivi primari di questa ricerca è quello di esaminare vari modelli di proprietà termodinamiche al fine di selezionarne uno in grado di calcolare proprietà termodinamiche accurate nell'intervallo di temperatura di interesse. Inoltre, i punti critici vapore-liquido in varie composizioni molari della miscela vengono calcolati combinando i criteri dei punti critici di Gibbs con Peng-Robinson EoS. Al fine di esplorare il potenziale termodinamico delle miscele di CO2 che lavorano fluidi su un'ampia gamma di temperature della sorgente di calore, vengono considerate due applicazioni: recupero del calore di scarto ad alta temperatura (Tmax=350℃) e energia solare concentrata (Tmax=550℃ e 700℃). Nel campo del recupero di calore ad alta temperatura, la miscela CO2-R134a (70% CO2 molare) nel ciclo di alimentazione transcritico mostra un'efficienza totale di 4 punti superiore rispetto al ciclo di alimentazione ad anidride carbonica (14,3% in caso di CO2-R134a e 10,8% in caso di CO2) con la stessa disposizione del ciclo e pressione massima di esercizio. Cinque miscele di CO2 sono considerate fluidi di lavoro per cicli energetici accoppiati a una torre solare concentrata (CSP) e il guadagno in efficienza del ciclo è studiato con riferimento al ciclo di ricompressione di sCO2 e al ciclo di recupero semplice di sCO2. Con un semplice layout del ciclo di recupero, il fluido di lavoro CO2-TiCl4 [80% CO2 molare] mostra un'efficienza del ciclo del 50,7%, che è vicina all'efficienza del ciclo ottimale del ciclo di ricompressione sCO2 (cioè 50,8%) alla temperatura massima del ciclo di 700 ℃ . Inoltre, quando la temperatura massima del ciclo è 550 ℃, CO2-TiCl4, CO2-C6F6, CO2-CF3I e CO2-SO2F2 determinano anche un guadagno nell'efficienza del ciclo rispetto al semplice ciclo di recupero sCO2. Considerando l'enorme potenziale delle miscele di CO2 nel migliorare l'efficienza termodinamica del ciclo, viene effettuata un'analisi dettagliata per un blocco di potenza da 100 MW integrato con una torre solare a concentrazione che adotta il fluido di lavoro della miscela CO2-SO2. L'analisi a diversa composizione molare della miscela suggerisce una miscela di CO2 molare all'85% come scelta ottimale a causa della maggiore efficienza del ciclo indipendentemente dal layout del ciclo. Sulla base dei vantaggi termodinamici ed economici, i cicli di alimentazione transcritici operanti con una miscela di CO2-SO2 risultano essere una scelta migliore per le centrali elettriche CSP.Carbon dioxide thermodynamic cycles for power production is a new technology under research and development in various research groups around the globe. The main attractive features of carbon dioxide cycles are higher cycle efficiency, lower size footprint and their integration capability with different heat sources. However, there are some application-specific challenges of such power cycles including lower heat recovery effectiveness, lower cycle specific work and necessity of complex cycle layouts for waste heat recovery. Secondly, the sensitivity of cycle efficiency to rising cycle minimum temperature also limits its operation for conditions where only cold cooling medium is available. To address the aforesaid challenges of carbon dioxide power cycles, this work proposes new transcritical advanced carbon dioxide cycles operating with CO2-based binary mixtures as working fluids for power production. The accurate calculation of thermophysical properties of working fluids is critical for thermodynamic analysis of power cycles using CO2-based binary mixtures as working fluids. As a result, one of the primary goals of this research is to examine various thermodynamic property models in order to select one that is capable of computing accurate thermodynamic properties in the temperature range of interest. Furthermore, vapor-liquid critical points at various molar compositions of mixture are calculated by combining Gibbs critical point criteria with Peng-Robinson EoS. In order to explore thermodynamic potential of CO2 mixtures working fluids over wide range of heat source temperatures, two applications are considered: high temperature waste heat recovery (Tmax=350℃) and concentrated solar power (Tmax=550℃ and 700℃). In the field of high temperature heat recovery, CO2-R134a mixture (70% molar CO2) in transcritical power cycle shows 4 points higher total efficiency compared to carbon dioxide power cycle (14.3% in case of CO2-R134a and 10.8% in case of CO2) with the same cycle layout and maximum operating pressure. Five CO2 mixtures are considered as working fluids for power cycles coupled with a concentrated solar power (CSP) tower, and the gain in cycle efficiency is studied with reference to sCO2 recompression cycle and sCO2 simple recuperative cycle. With a simple recuperative cycle layout, CO2-TiCl4 [80% molar CO2] working fluid shows cycle efficiency of 50.7%, which is close to the optimum cycle efficiency of the sCO2 recompression cycle (i.e. 50.8 percent) at cycle maximum temperature of 700℃. Moreover, when cycle maximum temperature is 550 ℃, CO2-TiCl4, CO2-C6F6, CO2-CF3I and CO2-SO2F2 also brings about gain in cycle efficiency compared to sCO2 simple recuperative cycle. Considering the huge potential of CO2 mixtures in improving thermodynamic efficiency of the cycle, a detailed analysis is carried out for a 100 MW power block integrated with concentrated solar power tower adopting CO2-SO2 mixture working fluid. Analysis at different molar composition of mixture suggests 85% molar CO2 mixture as optimum choice owing to higher cycle efficiency irrespective of cycle layout. Based on thermodynamic and economic advantages, transcritical power cycles operating with CO2-SO2 mixture turns out to be a better choice for CSP power plants

Evaluation and optimization of supercritical cycles using CO₂ based mixtures as working fluids:A thermodynamic study

This study focuses on the thermodynamic performance analysis and optimization of CO2-based binary fluid mixtures in supercritical thermodynamic power cycles exploiting high-temperature waste heat. Response surface method is used to establish relationships between cycle performances and significant cycle parameters. Multi-objective optimization is carried out to obtain optimal solutions with higher cycle specific work and higher cycle efficiency. The analysis reveals that increasing additive molar fraction of the considered mixtures improves cycle thermodynamic performance. Among considered mixtures, the CO2-R152a mixture exhibits a higher cycle specific work and a larger cycle efficiency. For instance, in the recompression cycle configuration, the CO2-R152a mixture achieves cycle specific work of 83.9 kJ/kg and corresponding cycle efficiency of 37.2% at the optimal conditions. Comparative analysis demonstrates improved cycle-specific work for CO2-based mixtures compared to supercritical pure CO2 power cycles. In the recompression cycle configuration, the CO2-R152a mixture shows an average increase of 12 kJ/kg in cycle specific work compared to the supercritical CO2 power cycle. The simple recuperated cycle configuration exhibits an average increase of 13 kJ/kg. The utilization of these mixtures results in a substantial gain in cycle specific work, thereby contributing to enhanced energy efficiency and sustainability in high-temperature waste heat recovery applications.</p

Thermal efficiency gains enabled by using supercritical CO2 mixtures in Concentrated Solar Power applications

Supercritical Carbon Dioxide (sCO2) power cycles have been proposed for Concentrated Solar Power (CSP) applications as a means to increase the performance and reduce the cost of state-of-the-art CSP systems. Nevertheless, the sensitivity of sCO2 systems to the usually hot ambient temperatures found in solar sites compromises the actual thermodynamic and economic gains that were originally anticipated by researchers of this innovative power cycle. In order to exploit the actual potential of sCO2 cycles, the utilization of dopants to shift the (pseudo)critical temperature of the working fluid to higher values is proposed here as a solution towards enabling exactly the same features of supercritical CO2 cycles even when ambient temperatures compromise the feasibility of the latter technology. To this end, this work explores the impact of adopting a CO2-based working mixture on the performance of a CSP power block, considering hexafluorobenzene (C6F6) and titanium tetrachloride (TiCl4) as possible dopants. Different cycle options and operating conditions are studied (250-300 bar and 550-700ºC) as well as molar fractions ranging between 10 and 25%. The results in this work confirm that CO2 blends with 15-25%(v) of the cited dopants enable efficiencies that are well in excess of 50% for minimum cycle temperatures as high as 50 or even 55ºC. It is also confirmed that, for these cycles, turbine inlet temperature and pressure hardly have any effect on the characteristics of the cycle that yields the best performance possible. In this regard, the last part of this work also shows that cycle layout should be an additional degree of freedom in the optimisation process inasmuch as the best performing layout changes depending on boundary conditions.Unión Europea SI-1900/10/201

Pure and Hydrocarbon Binary Mixtures as Possible Alternatives Working Fluids to the Usual Organic Rankine Cycles Biomass Conversion Systems

This study investigates the use of pure and hydrocarbons binary mixtures as potential alternatives working fluids in a usual biomass powered organic Rankine cycle (ORC). A typical biomass combined heat and power plant installed in Cremona (Italy) is considered as the benchmark. Eight pure hydrocarbons (linear and cyclic) and four binary mixtures of linear hydrocarbons were selected. The critical points of the binary mixtures at different composition were calculated using an in-house code developed in MATLAB⃝c (R2018b) environment. Based on the critical point of a working fluid, supercritical and subcritical cycle configurations of ORC were analysed. A detailed thermodynamic comparison with benchmark cycle was carried out in view of cycle efficiency, maximum operating pressure, size of the turbine and heat exchangers. The supercritical cycles showed 0.02 to 0.03 points lower efficiency, whereas, subcritical cycles showed comparable efficiencies than that of the benchmark cycle. The cycles operating with hydrocarbons (pure and mixtures) exhibited considerably lower volume flow ratios in turbine which indicates lower turbine size. Also, size parameter of regenerator is comparatively lower due to the lower molecular complexity of the hydrocarbons. A noticeable increase in turbine power output was observed with change in composition of the iso-octane/n-octane binary mixture at the same thermodynamic efficiency

Harnessing Ocean Thermal Energy from Offshore Locations in Pakistan Using an Organic Rankine Cycle

The temperature gradient of the top and bottom layers of sea water can be employed for power generation through ocean thermal energy conversion cycles (OTEC). In this thermodynamic study, an organic Rankine cycle is used to convert the ocean thermal energy of the Arabian sea into useful electrical power. A comparative investigation is carried out between R152a and R1234yf working fluids. Sensitivity analysis is performed at varying condenser saturation temperatures and evaporator saturation temperatures. For the R152a OTEC cycle, 2.8% thermal efficiency, 47.9% exergy efficiency and 9064 W turbine work are obtained at the optimum point. Similarly, for the R1234yf working fluid in the OTEC cycle, 2.8% thermal efficiency, 46.9% exergy efficiency and 4818 W turbine work are obtained at the optimum point

Exergetic optimization and comparison of combined gas turbine supercritical CO2 power cycles

For developing a sustainable power system, the key is to maximize the use of available resources with a minimal impact on the environment. One technique for achieving this is exhaust heat recovery. In this paper, three gas turbine exhaust heat recovery supercritical carbon dioxide combined power cycles are presented. They are combined gas turbine-recompression cycle, combined gas turbine-preheating cycle, and combined gas turbine-simple regenerative cycle. For all the cycles, thermodynamic models are developed and the influence of varying mass flow rates, compression ratio, and mass split/recompression percentages in different components of all three cycles are investigated. Using genetic algorithm, exergetic optimization is done to find the optimal configuration for each cycle. The reduction in CO2 emissions in presented cycles against fossil fuel power cycles is also assessed. Additionally, a comparison with a simple gas turbine (SGT) and an air bottoming combined cycle (ABC) is presented. The results indicate that owing to exhaust exergy recovery, there is a significant improvement in the energetic and exergetic performance of combined gas turbine-supercritical CO2 power cycles compared to that of SGT and ABC. The sum of exergy destruction and exergy loss in the combined cycles is lower as compared to the sum in SGT. The reduction in losses compared to SGT is 22.89% in the case of the combined gas turbine recompression cycle and 35.8% in the case of the combined gas turbine preheating cycle (CGTPHC). Moreover, the energetic and exergetic performances of the bottoming supercritical CO2 recompression cycles (BRECs) are better than those of the bottoming supercritical CO2 preheating cycle owing to lower exergy destruction in the components of BREC. As a result of comparative analysis based on the exergetic performance and environmental impact, the CGTPHC is selected as an appropriate option for gas turbine exhaust exergy recovery

Thermodynamic optimization and performance study of supercritical CO2 thermodynamic power cycles with dry cooling using response surface method

This paper deals with thermodynamic optimization of supercritical CO2 recompression and partial cooling cycles operating at cycle maximum temperature of 680°C and maximum pressure of 250 bar. The primary goal to investigate the effects of variation in heat sink temperature (ambient air temperature), mass split fraction (X), and cycle minimum pressure (Pmin) on the thermal efficiency of the power cycles. Response surface method (RSM) is adopted to create a second-order polynomial equation in order to develop the relationship between cycle thermal efficiency and selected decision variables and to find global optimum cycle efficiency. In addition, classification of most influencing cycle parameter is carried out using ANOVA approach. In the case of a recompression cycle, the results demonstrate that heat sink temperature has the greatest impact on thermal efficiency, owing to low p-value and high F-value, followed by mass split fraction and minimum pressure. In a partial cooling cycle, the minimum pressure has the most significant impact on cycle thermal efficiency, followed by the mass split fraction and heat sink temperature. The global optimum combination for the recompression cycle is at heat sink temperature of 20°C, the mass split fraction of 0.3182, and a minimum pressure of 89 bar to obtain the highest thermal efficiency of 0.4963. In addition, the global optimum combination for partial cooling cycle is at heat sink temperature of 32.8 °C, mass split fraction of 0.34, and minimum pressure of 76 bar, which results in an optimum thermal efficiency of 0.4708

Adoption of the CO2+SO2 mixture as working fluid for transcritical cycles: A thermodynamic assessment with optimized equation of state

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 5?degrees 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 sCO(2) 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?degrees 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 sCO(2) .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 sCO(2) cycle) and a reduction of the power block CAPEX from 1160 $/kWel to 1000$/kWel when compared to the sCO(2) 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 sCO(2) cycle), a decrease of power block CAPEX from 795 $/kWel to 718$/kWel and 70?degrees C of additional heat recovery from the hot source with respect to the analogous sCO(2) cycle

Exergetic, Economic and Exergo-Environmental Analysis of Bottoming Power Cycles Operating with CO 2 -Based Binary Mixture

This study focused on investigating the bottoming power cycles operating with CO2-based binary mixture, taking into account exergetic, economic and exergo-environmental impact indices. The main intent is to assess the benefits of employing a CO2-based mixture working fluid in closed Brayton bottoming power cycles in comparison with pure CO2 working fluid. Firstly, selection criteria for the choice of suitable additive compound for CO2-based binary mixture is delineated and the composition of the binary mixture is decided based on required cycle minimum temperature. The decided CO2-C7H8 binary mixture with a 0.9 mole fraction of CO2 is analyzed in two cycle configurations: Simple regenerative cycle (SRC) and Partial heating cycle (PHC). Comparative analysis among two configurations with selected working fluid are carried out. Thermodynamic analyses at varying cycle pressure ratio shows that cycle with CO2-C7H8 mixture shows maximum power output and exergy efficiency at rather higher cycle pressure ratio compared to pure CO2 power cycles. PHC with CO2-C7H8 mixture shows 28.68% increment in exergy efficiency with the levelized cost of electricity (LCOE) 21.62% higher than pure CO2 PHC. Whereas, SRC with CO2-C7H8 mixture shows 25.17% increment in exergy efficiency with LCOE 57.14% higher than pure CO2 SRC. Besides showing lower economic value, cycles with a CO2-C7H8 mixture saves larger CO2 emissions and also shows greater exergo-environmental impact improvement and plant sustainability index