Transcritical CO2 Rankine Cycle Research Articles

Performance assessment and multi-objective optimization of a novel transcritical CO2 Rankine cycle for engine waste heat recovery

In this study, a novel self-condensing transcritical CO2 Rankine cycle, which integrates the three-stage expansion process with an ejector cycle, is proposed to recover engine waste heat deeply and overcome the condensation issue caused by the relatively low critical temperature of CO2. Thermodynamic and economic mathematical models are developed, and the detailed parametric analysis is carried out to investigate the effect of main parameters on both thermodynamic and economic performances of the system. Thereafter, a multi-objective optimization is conducted to trade off the two different performances. Results show that the proposed system could operate under higher temperature cold source conditions with desirable performance. The increases of turbine1 inlet pressure and LT gas heater outlet temperature, the decreases of LT gas heater outlet pressure and turbine3 back pressure are beneficial to achieve better thermodynamic and economic performances. On the basis of multi-objective optimization, the maximum net power output is 70.04kW, which is a 10.20% improvement compared to the reference cycle. Meanwhile, the engine power output could be increased by 7.03% through adopting the novel system. Furthermore, the optimal exergy efficiency and unit net power cost are 37.02% and 0.1567$/kWh, respectively.

Comprehensive analysis and optimization for a novel combined heating and power system based on self-condensing transcritical CO2 Rankine cycle driven by geothermal energy from thermodynamic, exergoeconomic and exergoenvironmental aspects

In this paper, a novel combined heating and power (CHP) system is proposed to realize full-scale utilization of geothermal energy and efficient multi-generation, which not only performs preferable overall performance than previous homogeneous system, but also offers an effective energy cascade utilization approach for self-condensing transcritical CO2 (TCO2) Rankine cycle. Based on the established mathematical models, the performance comparison is conducted for proving the superiority of the novel CHP system. Then, an overall performance analysis is implemented to reveal the combined effects for six key parameters on system thermodynamic, exergoeconomic and exergoenvironmental performances. Furthermore, multi-objective optimization considering system overall performance is conducted. The results show that for the novel CHP system, the largest relative improvement rate of system exergy efficiency (ηexg) and declining rate of total unit product exergy cost (cP,total) versus the previous CHP system are 15.03 % and 18.89 %, respectively. The final optimization results of ηexg, cP,total and total unit product exergy environmental impact (bP,total) are determined as 51.10 %, 14.12 $/GJ and 9.00 mPts/GJ, respectively. This paper fulfills an elaborate performance analysis and optimization for the novel CHP system, which fills the research gap of efficient and promising CHP system based on self-condensing TCO2 Rankine cycle.

Thermodynamic, economic analysis and multi-objective optimization of an improved self-condensing transcritical CO2 Rankine cycle with two-stage ejector for low grade heat utilization

An improved self-condensing transcritical CO2 Rankine cycle (TCRC) with two-stage ejector on the basis of the preliminarily designed one with an ejector is proposed in this study, aiming to enable the condensation of CO2 using conventional cooling source and achieve desirable performance. Thermodynamic and economic mathematical models of the improved self-condensing TCRC are developed, and detailed parametric analysis is carried out to investigate the effect of main parameters on both thermodynamic and economic performances of the system. Then the multi-objective optimization is conducted to examine the potential for performance enhancement. Thereafter, a comparative analysis of the performances between the improved self-condensing TCRC and preliminary self-condensing TCRC is performed. Results show that the improved self-condensing TCRC with two-stage ejector outperforms the preliminary one with an ejector in both thermodynamic and economic aspects, achieving a 3.93% increase in maximum exergy efficiency and a 13.84% reduction in minimum unit net power cost through multi-objective optimization. Under optimal conditions, the exergy efficiency for the improved self-condensing TCRC is 45.48% with a unit net power cost of 0.1241$/kWh.

Design and optimization of a cold energy and waste heat utilization system for LNG-powered ships with post-combustion carbon capture

The introduction of dual-carbon targets has accelerated LNG fuel adoption on vessels and driven the advancement of carbon capture technologies. This study?s aim is a 37000-deadweight tonnage liquified natural gas dual-fuel powered ship, for which chemical absorption carbon capture is applied, utilizing flue gas and liquified natural gas to supply the process?s heat and cold energy. Then a system with efficient utilization of energy and carbon capture for the LNG dual-fuel ship is designed, coupling the waste heat onboard with transcritical CO2 and organic Rankine cycle on the principle of energy cascade utilization. The system is simulated using Aspen HYSYS and the exergy analysis is carried out for this system. Then the working fluid is optimized for the system. After that, through the genetic algorithm, the system?s operating parameters are further optimized. Additionally, the system?s economic analysis is also performed. It is shown that the scheme?s exergy efficiency reaches 39.98%, and the expected cost-recovery cycle is 4.75 years.

Exergy Analysis and Performance Improvement of Direct Steam-Generated Solar-Based Transcritical CO2 Rankine Cycle with Energy Storage

Exergy Analysis and Performance Improvement of Direct Steam-Generated Solar-Based Transcritical CO2 Rankine Cycle with Energy Storage

The role of buoyancy and thermal acceleration in heat transfer deterioration and thermal oscillation for the CO2 around pseudo-critical region

Trans-critical CO2 Rankine cycle has great potential in converting low-grade heat into electricity, because of its good temperature glide matching between working medium and heat source. However, the drastic variation in thermo-physical properties of CO2 around the pseudo-critical region causes the abnormal flow and heat transfer behaviors. In this paper, the heat transfer deterioration and thermal oscillation in the heat addition of trans-critical CO2 cycle are investigated by experiments. The test section is heated uniformly by a DC power (0∼6 kW) and the minimum Reynolds number in the test section inlet is about 8000 (it is much larger than 2300) for the all cases. A shear stress reconstruction model is corrected by introducing the velocity profile of turbulence in the boundary layer to attempt to provide a quantitative criterion for deterioration and oscillation. The role of buoyancy and thermal acceleration in heat transfer deterioration and thermal oscillation is elucidated by connecting the shearing-stress reconstruction model and experimental results. It is found that, the thermal acceleration in the near-wall zone provides the key motivation to drive the heat transfer deterioration. The thermal oscillation is driven by the transition between partial and full re-laminarization flows in the local zone of heat transfer deterioration. When (Δτa/τw)<1&(Δτ/τw)tot>1 or 0.1<(Δτ/τw)tot<1, the turbulence is partially re-laminarized. When(Δτa/τw)>1&(Δτa/τw)>>(Δτbo/τw), the intense thermal acceleration causes the turbulence to be fully re-laminarized in the near-wall zone and it maintains the new laminar boundary . The turbulence is partially re-laminarized in the lower boundary and it is almost full re-laminarization in the upper boundary of the oscillation wall temperatures. The results confirm that the emergence of heat transfer deterioration and oscillation instability in mixed turbulent convective relate with the change of flow states caused by acceleration and buoyancy effects.

Advanced power-refrigeration-cycle integrated WHR system for marine natural gas engine base on multi-objective optimization

In the face of huge global energy consumption today, technological implementation of waste heat recovery for electrical generating and refrigeration is of crucial significance for energy conservation. The high-temperature exhaust from marine natural gas engines carries a substantial quantity of thermal energy, making it an attractive option for waste heat recovery. A unique cycle system involving a supercritical CO2 cycle subsystem, a trans-critical CO2 Rankine cycle subsystem, and a double effect absorption refrigeration cycle is developed to utilize the waste energy in the maritime natural gas engine. The parameter variation pattern of each subsystem is investigated, and a total of eight organic working fluids with varying fractions are introduced into the trans-critical CO2 system for further investigation. The thermal-economic outcomes of the suggested integrated system are maximized by implementing the multi-objective optimization process, and it is found that when R32 with a mass fraction of 0.6138 is used as an additive in the trans-critical CO2 system, it has the highest net output power (311.6 kW) and lowest LCOE (0.0298 $/kWh). The results indicate that the novel design is a viable and meaningful strategy for marine natural gas engines' waste heat recovery.

Experimental Study on a Small-Scale Transcritical CO2 Rankine Cycle for Solar Thermal Applications

The supercritical CO2 cycle is one of the next generation’s power generation cycle. Especially, the transcritical CO2 Rankine cycle (TCRC) is a suitable candidate for dispersed generation systems with small-scale solar thermal applications. Compared to other cycle studies applied in other fields such as nuclear energy, there are limited reports on renewable energy fields. Therefore, in this study, the TCRC for a small-scale solar thermal heat source is investigated by thermodynamic and experimental methods. The experimental facility was built with approximately 12 kW of thermal capacity and commissioned to evaluate its performance using a case study. The maximum temperature of the cycle is the primary optimization variable of the experiment, and it has a significant impact on the cycle thermal efficiency. Based on the experimental data, the trends of the cycle thermal efficiency and generated power are simulated assuming that the TCRC is operating with real insolation during the daytime. As a result of the simulation, maximum efficiencies of 6.41 and 6.03% are obtained from maximum solar radiation amounts of 758 and 674 W/m2 in May and June, respectively. At that time, the amounts of power generated were 726 W and 626 W, respectively.

Techno-economic and 4E comparisons of various thermodynamic power cycles for low-medium grade heat recovery

Various thermodynamic power cycles can be utilized to convert the low-to-medium temperature heat (available from many sources and has huge potential) to electricity and a detailed comparison between all of them is essential to identify the suitable one. Hence, to fulfill this research gap, six thermodynamic power cycles (Organic Rankine cycle with dry fluid, Organic Rankine cycle with wet fluid, Transcritical CO2 Rankine cycle, Kalina cycle, Organic flash cycle and Trilateral flash cycle) have been compared for various heat source and sink temperatures. Objective functions are power generation, efficiency, irreversibility, cost, profit, environmental benefit, techno-economic parameters, etc. Study shows that the trilateral flash cycle is best for medium heat source temperature and the CO2 cycle is best for low heat source temperature. The trilateral flash cycle shows an 11.5% higher exergy efficiency and 16.16% increment in annual profit at 160 °C heat source temperature as compared to the organic Rankine cycle with dry fluid. A decrease in condenser temperature is more advantageous than an increase. The study reveals that the CO2 Rankine cycle would be advantageous for low-grade heat sources while the trilateral flash cycle for medium-grade heat sources in terms of performance, cost, design, operation and environmental benefits.

Multi-variable investigation of an innovative multigeneration process based on geothermal energy and Allam power unit for yielding electric power, cooling, heating, and liquid CO2 with zero CO2 footprint

Multigeneration systems which use the produced carbon dioxide (CO2) are an appropriate solution for current global challenges in the energy sector, i.e., input fuel supply and environmental crisis. These techniques bring a situation for environmental protection in addition to sustainable production. This idea has been enriched in the current work using a clean and renewable energy source (geothermal energy) combined with an Allam power unit. Hence, this study presents a novel environmentally friendly multigeneration framework for the first time, where in addition to using a clean energy source, the produced CO2 is utilized as well. The components of this system include an Allam power unit, a geothermal power plant, a unit for producing cooling water, a geofluid Rankine cycle, a transcritical CO2 Rankine cycle, and a unit for generating power and heat. Electricity, heating, cooling, and liquid CO2 are other goods. The Aspen HYSYS program is used to model this system as it is studied from an exergy, energy, environmental, and economic perspective. The findings show that the suggested system's overall exergy and energy efficiencies are 50.33% and 53.41%, respectively. In addition, the Allam power unit is the most important irreversibility source, contributing to 37% of total exergy destruction. The levelized cost of energy is computed at 0.031 $/kWh. In addition, since this study uses an advanced design approach, its CO2 emission is zero. Thus, compared to earlier investigations, the suggested approach has been successful in enhancing the thermodynamic, economic, and environmental conclusions.

A techno-economic assessment of sCO2 Brayton cycles for low-grade geothermal reservoirs

CO2-based power cycles have recently received much attention because they have several advantages over traditional power cycles in a wide variety of applications. For low-grade heat sources such as geothermal energy, the research has focused on the transcritical CO2 Rankine cycle (TCRC). However, the TCRC requires relatively low-temperature heat sinks to condense the CO2 and operate properly, preventing its deployment in many locations. A straightforward solution is to use a supercritical CO2 Brayton cycle (SCBC) instead. Nonetheless, in the context of binary cycle geothermal power plants, there is a lack of information about the feasibility of implementing SCBCs. Therefore, to elucidate the viability of employing SCBCs for power generation using geothermal heat, this paper presents an assessment of the thermodynamic and economic performances of four SCBC configurations. For this purpose, thermodynamic and economic models of the system, along with a thermal-hydraulic model of the heat exchangers, were developed. Net power output and net present value were used as technical and economic performance indicators, respectively. Subsequently, base case simulations were conducted, and multi-objective optimization using NSGA-II was performed. Results show that, using 20 kg/s of geothermal brine at 150 °C as the heat source and cooling water at 25 °C as the heat sink, an intercooled recuperated Brayton cycle (IRBC) is superior to the other considered configurations. It can achieve up to 932.07 kW of net power output at the optimal thermodynamic conditions and 4.45 × 103 USD of net present value at the optimal economic conditions. Besides, balancing the thermodynamic and economic performances, the IRBC attains the highest overall performance. In conclusion, using SCBCs in binary cycle geothermal power plants could be feasible, and the IRBC is a suitable layout.

Effect of Cascaded Phase Change Heat Storage on the Performance of Waste Heat Recovery Systems

The cascades phase change heat storage (CLTES) and trans-critical CO2 Rankine cycle were combined to study the effects of the number of phase change materials in series on heat storage on the performance of waste heat recovery systems in order to overcome the influence of heat source volatility. In terms of net work, 3-CLTES is 12.5% higher than 2-CLTES, but with 30.4% more heat exchange area. Compared with 4-CLTES, the net output work decreased by 2.6%, and the heat transfer area increased by 6.7%.

Güneş Enerjisi Kaynaklı Farklı Transkritik CO2 Rankine Çevrimlerinin Karşılaştırılması

In this study, different transcritical CO2 Rankine (tCO2-RC) cycles with vacuum tube solar collectors are examined. In this context, the transcritical CO2 Rankine cycle in different configurations, such as simple, regenerator, reheat, regenerator, and reheat, which is widely used in the literature, has been chosen. First, the energy and exergy efficiencies of the cycles were founded by performing thermodynamic analyzes of four various tCO2-RCs under certain operating parameters. Moreover, parametric studies were carried out according to the factors affecting the system performance, like turbine input temperature and input pressure of the turbine. Both analyzes and parametric studies were conducted utilizing the Engineering Equation Solver (EES) computer program. As outcomes of analyzes, the highest thermal efficiency was founded for the tCO2-RC with reheat and regenerator by 11.8%, and the lowest energy efficiency was calculated for the simple tCO2-RC and tCO2-RC with reheat by approximately 6.6%. While all tCO2-RCs' energy and exergy efficiencies increased with the rise of the turbine’s input pressure, the energy and exergy efficiency of all tCO2-RCs decreased with the rise of the pump’s input pressure.

Design and optimization of LNG-powered ship cold energy and waste heat integrated utilization system based on novel intermediate fluid vaporizer

Based on a novel integrated intermediate fluid vaporizer with liquefied natural gas (LNG) cold energy utilization proposed by the author's team, a full-power generation integrated system is constructed in this study. It is a two-stage cascade Rankine cycle with two-stage condensation and coupled with a trans-critical CO2 Rankine cycle. This study regards a river–sea direct transportation type of 25,000-ton LNG fuel-powered chemical ship as the application object and considers combined utilization of the medium-temperature waste heat of flue gas from the exhaust turbine power generation cycle outlet and the vaporization cold energy of LNG. Through the simulation and analysis of the scheme, circulating working fluid optimization and operation parameter optimization based on a genetic algorithm are conducted. The overall exergy efficiency of the optimized system reaches 45.1%, while the power generation of the system reaches 72.66 kW. Furthermore, economic analysis shows that the net annual income of the optimized system can reach 280,700 yuan, and the recovery cycle of initial investment cost is expected to be 6.11 years.

Thermal assessment and optimization of process fluids in transcritical organic and transcritical CO2 Rankine cycle for waste energy recuperating system

• Thermodynamic modeling of basic TORC and CTRC is conducted. • Evaluation of optimum operating conditions for the working fluids. • Energy and exergy analysis performed with optimized input parameters. As natural resources are depleting gradually, scientists are inclining toward the quest for an alternate source of energy and searching for efficient ways to mitigate wastage of energy. The potential of the organic Rankine cycle (ORC) in the application of recovering energy from medium and low-temperature heat sources has been explored by researchers for a long time. Over the years numerous optimizations and modifications have appeared to extract usable power from sources that are fundamentally discharging waste energy to the environment. In this study, a thermodynamic model of basic transcritical Organic Rankine Cycle (TORC) and carbon dioxide-based transcritical Rankine cycle are modeled using a validated MATLAB code, scrutinizing the optimum condition of eleven working fluids. This study prioritizes environment-friendly working fluids and the fluids with lower ODP and GWP indices have been selected for experimentation. These working fluids have been tested setting optimized input parameters and a comparative study of working fluids has been done using exergy destruction distribution. According to the second law analysis, at low turbine inlet temperature, R32 works better than other process fluids. Whereas, at higher temperatures, carbon dioxide delivers better performance compared to other working fluids.

Energy, Exergy, Economic, Exergoeconomic, and Exergoenvironmental (5E) Analyses and Optimization of a Novel Three‐Stage Cascade System Based on Liquefied Natural Gas Cold Energy

Herein, a novel three‐stage cascade power/cooling cogeneration system that includes the gas turbine (GT) cycle, supercritical CO2 Brayton cycle (S‐CO2), transcritical CO2 Rankine cycle (T‐CO2), and organic Rankine cycle is proposed. The system is simulated by connecting MATLAB and Aspen HYSYS via Active‐X control. It is indicated in the results that the system exhibits power generation, primary energy utilization, and exergy efficiencies of 59.43%, 81.66%, and 53.94%, respectively. The total product cost and environmental cost per unit exergy are 23.80 and 32.66 $GJ−1, respectively. Monoethanolamine is used to absorb CO2 from the exhaust gas, and cold energy from liquefied natural gas is utilized to capture carbon, with a capture rate of 96%, to realize low‐carbon‐emission characteristics. Additionally, the performance of the established system is verified using three basic cycles for comparison. It is revealed in the results that the designed system exhibits the highest thermodynamic performance and net present value among the investigated cycles. Furthermore, multi‐objective optimization of the systems is performed using the second‐generation non‐dominated sorting genetic algorithm algorithm. Although the GT/S‐CO2/T‐CO2 cycle is found to be more cost‐effective than the proposed system at generation efficiencies less than 55%, the established system is deemed the best option among the cycles at power‐generation efficiencies greater than 55%.

Thermodynamic analysis and evaluation of a novel trans-critical CO2 power system incorporating partial condensation, sub-cooling, and recompression

Trans-critical CO2 Rankine cycle (TRC) is a promising technology that converts heat into power generation with its safe and environment-friendly characteristics. However, the low critical temperature of CO2 restricts the practical application of TRC, which requires a relatively low temperature heat sink to ensure the CO2 condensation. In addition, the heat capacity mismatch between hot and cold CO2 streams in the recuperator restricts the performance improvement of traditional recuperative trans-critical CO2 Rankine cycle (RTRC). In this study, a novel partial condensation trans-critical CO2 recompression cycle (PCRC) is proposed to simultaneously address the issues of condensing difficulty and low performance. Thermodynamic analysis and optimization models for both PCRC and RTRC are formulated and a solution algorithm is developed. Case studies are explored to validate the superiority of PCRC over traditional RTRC in thermal efficiency and operation flexibility. Influence of key parameters on the PCRC performance is also investigated to reveal the improvement mechanism over RTRC. Furthermore, sensitivity analysis is carried out on the upper operation pressure, heat source parameter, and heat sink parameter of the PCRC to determine its operational flexibility.

Thermodynamic, Economic Analysis, and Multiobjective Optimization of a Novel Transcritical CO2 Rankine Cycle with a Vortex Tube

The transcritical CO2 (TCO2) Rankine cycle has been receiving more and more attention for the lower irreversible losses and the excellent thermophysical characters of CO2. However, it is hard to condense CO2 to liquid due to its low critical temperature (about 31°C) under ambient conditions. In this paper, a novel transcritical CO2 Rankine cycle called TCO2 is proposed, in which a vortex tube is added to condense CO2 under ambient conditions. A mathematical model is established to analyze the parametric effects on thermodynamic and economic performance based on specific equipment investment cost; the model is verified with experimental data. Nondominated sorting genetic algorithm II is used to achieve multiobjective system optimization for obtaining optimum cycle performance. Parametric analysis results show that an increase turbine inlet temperature and vortex tube outlet pressure can increase exergy efficiency. A decrease in turbine inlet pressure and turbine inlet temperature can reduce the TCO2 cycle equipment investment cost. In addition, multiobjective optimization results indicate that a conflict exists between TCO2 cycle thermodynamic and economic performance based on specific equipment investment cost.

Thermo-Economic Analysis and Three-Objective Optimization of a Novel Solar-Based Power, Cooling, Hydrogen, and Freshwater Production System Using Trans-Critical Co2 Rankine Cycle, Lng Cold Energy, and Stirling Engine

Thermo-Economic Analysis and Three-Objective Optimization of a Novel Solar-Based Power, Cooling, Hydrogen, and Freshwater Production System Using Trans-Critical Co2 Rankine Cycle, Lng Cold Energy, and Stirling Engine

LNG regasification and electricity production for port energy communities: Economic profitability and thermodynamic performance

The integration of electricity and gas systems is a key priority for the energy transition. Alongside electricity, low-carbon energy vectors, such as Liquified Natural Gas (LNG) or hydrogen, are required in future energy systems. Hence, regasification systems, often located nearby port areas, will be strategic assets promoting environmental, social, and economic benefits.This study investigates and compares three integrated configurations exploiting LNG cold exergy for the efficient production of electricity in a port area . In all three configurations, electricity is produced by a gas turbine fuelled with regasified natural gas and different combinations of two bottoming cycles, consisting of a transcritical CO2 Rankine cycle and a natural gas direct expansion cycle. These configurations are compared with a traditional regasification and combined cycle power plant. The study aims at identifying the design of the system that maximizes the second law efficiency and the Net Present Value (NPV) for various relative sizes between gas turbine and regasification capacity, including analyses on Internal Rate of Return (IRR). A sensitivity analysis on the installation of turbine inlet air-cooling taking advantage of LNG cold exergy is also performed. The optimization is performed through Particle Swarm Optimization (PSO) algorithm and Aspen Hysys.Results show that the mutual size between the gas turbine and regasification unit is an important variable that requires careful analysis, as the second law efficiency can vary between 49% and 42% depending on the case. Electric efficiency of the proposed configurations can approach 65%, thus resulting greater than the traditional combined cycle in most of the test conditions. Economic optimizations confirm that the NPV of two configurations is often 10–15 M€ greater than the corresponding value of the base case system; however, only a single configuration can increase the IRR by 1–2% in comparison to the case with separate units. These results can guide designers and developers in the definition of the most cost-effective solution of any integrated energy systems including regasification, similar to a port system, which are expected to be key facilities in the future energy scenario.