Transcritical Power Cycle Research Articles

Supercritical retrograde solubility in R-14 and the second law of thermodynamics

A closed transcritical power cycle is described that uses a positive excess enthalpy of solution reaction differential between the reaction in the cycle’s high-density liquid state and low-density expanded gas state to internally transfer heat energy. The heat energy input to satisfy the solution reaction near the cycle’s low temperature is returned as heat during supercritical retrograde solubility near the cycle’s high temperature before that heat energy affects gas expansion. The power cycle format is used to frame this heat energy transfer so that Carnot efficiency can be used to calculate the maximum second law allowed amount of heat energy that can be transferred. If the amount of heat transfer exceeds the determined minimum threshold, the cycle’s Q efficiency will surpass the cycle’s T efficiency. The process demonstrates that the maximum positive excess enthalpy differential allowed by the second law is irreconcilably low.

Thermos-economic performance of a novel CCHP system driven by low-grade thermal energy based on CO2 and organic fluids

A novel combined cooling, heating and power (CCHP) system is proposed based on mixture consisting of CO2 and organic fluids to meet the seasonal energy demand in buildings. Thermodynamic model was constructed based on the laws of thermodynamics, and seasonal operation modes were also established. Techno-economic performance was analyzed and the performance of mixture consisting of CO2 and organic fluids compared with CO2 and pure organic fluids. Results show that the temperature of the turbine inlet in trans-critical power cycle should be 10 °C lower than the heat source temperature, with the turbine inlet pressure being approximately 1.4 times as large as critical pressure. Turbine inlet temperature is negatively correlated to the payback period and levelized energy cost while turbine inlet pressure is positively correlated to the system economic performance. The carbon dioxide content is negatively correlated to net power output, thermal efficiency and exergy efficiency, while coefficient of performance will hit the bottom with the carbon dioxide content, except CO2/Propane. For the carbon dioxide content is between 5 % and 10 %, the system overall performance is also close to optimal value. In general, CO2/R134a (0.10/0.90) and CO2/R1234ze(E) (0.10/0.90) is recommendable.

Thermal Desalination Through Forward Osmosis Coupled With CO2-Mixture Power Cycles for CSP Applications

This work, performed in the framework of the H2020 EU project “DESOLINATION”, analyses the coupling between CSP plants using transcritical power cycles with CO2-mixtures and an innovative thermal desalination technique based on Forward Osmosis. Calculations are presented for a large scale CSP plant with central tower receiver and direct storage with solar salts in Dubai, adopting the mixtures CO2+SO2 and CO2+C6F6 in the power cycles. The heat rejected from the cycle condenser is recovered directly by the FO plant, where the draw solute is heated up from 40 °C to 76 °C, to allow for the regeneration of the draw solution used in the forward osmosis membrane. The thermo-responsive polymer adopted is PAGB2000, already considered in literature as a promising option. Results show a very effective synergy between the electricity and the freshwater production: high yearly solar to electric efficiencies are possible (around 19%), with a low freshwater specific thermal consumption (around 100 kWhth/m3). The proposed desalination method is more effective than a conventional MED system (with + 50% of yearly freshwater produced), while a larger solar field (+ 28% in surface area) is necessary for a PV+RO plant to produce annually both the energy and freshwater produced by the CSP+FO plants.

Experimental isochoric apparatus for bubble points determination: Application to CO2 binary mixtures as advanced working fluids

Carbon dioxide binary mixtures are increasingly considered as working fluids in transcritical power cycles, due to the capability to perform liquid-phase compression even at high environmental temperatures. However, a robust thermodynamic model is essential for optimal and reliable design conditions. It is widely recognised that fine-tuning the equation of state with experimental vapour-liquid equilibrium data of the mixture significantly enhances its reliability.In this work, a new apparatus dedicated to vapour-liquid equilibrium measurements of mixtures is presented. The proposed method consists of a constant-volume system, where bubble points are identified from the divergence of slope of the isochoric lines between the two-phase and liquid regions, in the temperature-pressure plane. The temperature and pressure limits of the apparatus are 503 K and 25 MPa.Bubble points of CO2 binary mixtures with hexafluorobenzene (C6F6) and n-pentane (C5H12) have been measured and compared with previous literature data for validation purposes. Then, the CO2 mixture with octafluorocyclobutane (c-C4F8) is experimentally studied, addressing a literature gap in bubble point data. The data are used to calibrate the thermodynamic model, leading to affordable design conditions of the power cycle compared to the non-optimised thermodynamics scenario, in a concentrated solar power tower plant.

Exploring performance map: theoretical analysis of subcritical and transcritical power cycles with wet and isentropic working fluids

This study examines the overall thermodynamic efficiency of several power cycles, including transcritical power cycle (TPC), trilateral flash cycle, organic Rankine cycle (ORC), partially evaporated ORC, and superheated ORC under both ideal and specific conditions. The research focuses on specific wet and isentropic working fluids, providing a performance map of the power system with identical configurations, internal machine efficiency, and vapor quality. Moreover, this study aims to determine the ideal working fluid to utilize given heat sources by TPC. The results indicate that the efficiency of TPC may reach a maximum and subsequently decrease under certain conditions. Moreover, some operating conditions for the thermal power system, such as temperature range and configurations, may not be recommended due to exceptionally low or negative effective thermodynamic cycle efficiencies. Furthermore, the study demonstrates that increasing the inlet temperature of the working fluid to an expander does not necessarily lead to higher power cycle efficiency, as observed in the case of TPC. However, the result also proves the potential of carbon dioxide (R744) as a promising working fluid when chosen for a TPC with x2,is = 1.00, still achieving positive effective efficiency under specific conditions.

Heat Recuperation for the Self-Condensing CO2 Transcritical Power Cycle

Heat Recuperation for the Self-Condensing CO2 Transcritical Power Cycle

Numerical investigation on thermal-hydraulic performance of variable cross section printed circuit heat exchanger

CO2 transcritical power cycle (CTPC) systems have attracted considerable research focus in the fields of thermoelectric conversion and waste heat recovery. The regenerator is a key component affecting the CTPC system's efficiency. To improve the comprehensive performance of the regenerator, extensive research has been conducted to optimize the regenerator flow channel design. However, the optimization of the traditional Z-channel printed circuit heat exchanger structure (ZPCHE) is limited to constant cross-sectional configurations along the flow direction, which can lead to low channel space utilization. To solve this problem, an efficient variable cross section Z-channel structure (UAPCHE) is proposed in this study. The structure is designed with different cross-sectional shapes along the flow direction to fit the flow path of the main fluid. UAPCHE achieves a coordinated optimization of the heat transfer (Nu), flow (dP), and compactness performance (Q/V) by increasing the effective utilization of the channel space and weakening the damage to the fluid boundary layer. The design principle of the UAPCHE is introduced, and based on this, the structural parameters of the UAPCHE are optimized to achieve the best comprehensive performance. The results show that, compared with the ZPCHE, Nu of UAPCHE can be increased by 16.79%, dP can be reduced by 19.48%, and Q/V can be increased by 22.65%.

Design of a 130 MW Axial Turbine Operating with a Supercritical Carbon Dioxide Mixture for the SCARABEUS Project

Supercritical carbon dioxide (sCO2) can be mixed with dopants such as titanium tetrachloride (TiCl4), hexafluoro-benzene (C6F6), and sulphur dioxide (SO2) to raise the critical temperature of the working fluid, allowing it to condense at ambient temperatures in dry solar field locations. The resulting transcritical power cycles have lower compression work and higher thermal efficiency. This paper presents the aerodynamic flow path design of a utility-scale axial turbine operating with an 80–20% molar mix of CO2 and SO2. The preliminary design is obtained using a mean line turbine design method based on the Aungier loss model, which considers both mechanical and rotor dynamic criteria. Furthermore, steady-state 3D computational fluid dynamic (CFD) simulations are set up using the k-ω SST turbulence model, and blade shape optimisation is carried out to improve the preliminary design while maintaining acceptable stress levels. It was found that increasing the number of stages from 4 to 14 increased the total-to-total efficiency by 6.3% due to the higher blade aspect ratio, which reduced the influence of secondary flow losses, as well as the smaller tip diameter, which minimised the tip clearance losses. The final turbine design had a total-to-total efficiency of 92.9%, as predicted by the CFD results, with a maximum stress of less than 260 MPa and a mass flow rate within 1% of the intended cycle’s mass flow rate. Optimum aerodynamic performance was achieved with a 14-stage design where the hub radius and the flow path length are 310 mm and 1800 mm, respectively. Off-design analysis showed that the turbine could operate down to 88% of the design reduced mass flow rate with a total-to-total efficiency of 80%.

Demonstration of a small‐scale power generator using supercritical CO2

AbstractThe supercritical CO2 (sCO2) power cycle could improve efficiencies for a wide range of thermal power plants. The sCO2 turbine generator plays an important role in the sCO2 power cycle by directly converting thermal energy into mechanical work and electric power. The operation of the generator encounters challenges, including high temperature, high pressure, high rotational speed, and other engineering problems, such as leakage. Experimental studies of sCO2 turbines are insufficient because of the significant difficulties in turbine manufacturing and system construction. Unlike most experimental investigations that primarily focus on 100 kW‐ or MW‐scale power generation systems, we consider, for the first time, a small‐scale power generator using sCO2. A partial admission axial turbine was designed and manufactured with a rated rotational speed of 40,000 rpm, and a CO2 transcritical power cycle test loop was constructed to validate the performance of our manufactured generator. A resistant gas was proposed in the constructed turbine expander to solve the leakage issue. Both dynamic and steady performances were investigated. The results indicated that a peak electric power of 11.55 kW was achieved at 29,369 rpm. The maximum total efficiency of the turbo‐generator was 58.98%, which was affected by both the turbine rotational speed and pressure ratio, according to the proposed performance map.

Feasibility assessment of trough concentrated solar power plants with transcritical power cycles based on carbon dioxide mixtures: A 4E analysis and systematic comparison

Medium and low-temperature solar thermal power generation, integrating parabolic trough collectors with transcritical carbon dioxide (CO2)-based mixture power cycles, is explored for enhanced solar energy utilization and improved efficiency. This study assesses the feasibility of parabolic trough concentrated solar power plants utilizing transcritical CO2-based mixtures, considering energy output, efficiency, economics, and environmental (4E) aspects. Seven candidate organic working fluids (R32, R134a, R152a, R1234yf, R161, R290, and R1270) are evaluated within a two-layer decision-making framework based on multi-objective optimization. Results indicate a significant performance improvement with an increased mass fraction of the organic working fluid. Particularly, systems employing CO2/R32 or CO2/R161 demonstrate superior overall performance compared to those with other CO2-based mixtures. Under the base case (at a hot tank temperature of 350 °C), the CO2/R32 (0.3/0.7 wt%) system achieves the maximum energy output (355.22 kW), while the CO2/R161 (0.3/0.7 wt%) system attains the highest efficiency with the lowest levelized cost of electricity and CO2 emissions (9.78%, 0.576 $/kW·h, and 0.0328 kg/kW·h, respectively). Exergy analysis emphasizes the predominant role of the solar collector in exergy destruction (approximately 70%), with the CO2/R161 (0.3/0.7 wt%) system yielding the highest exergy efficiency (14.96%). Across a hot tank temperature range of 250–350 °C and an organic fluid mass fraction of 0.3–0.7, the working fluids are generally ranked as CO2/R161, CO2/R32, CO2/R152a, CO2/R1270, CO2/R134a, CO2/R290, and CO2/R1234yf in terms of the 4E aspects via a multi-objective optimization decision-making framework.

Development of surrogate-optimization models for a novel transcritical power cycle integrated with a small modular reactor

In recent years, various types of surrogate optimization models have been proposed to reduce the computational time and to improve the emulation accuracy. In this study, by leveraging an ANN surrogate model developed earlier, a comprehensive and efficient optimization algorithm is conceived for the global optimal design of an integrated regenerative methanol transcritical cycle. It combines a unique converging/diverging classifier model into the surrogate model to form a surrogate-based model, which significantly improves the prediction accuracy of the objective function. Six binary classifiers are explored and the multi-layer feed-forward (MLF) neural network classifier is selected. In addition, within the five global optimizers being explored, the basin-hopping (BH) and dual-annealing (DA) are selected. The optimal surrogate-based model and global optimizers are then combined to form a unique surrogate-optimizer model. The surrogate-optimizer model is slightly outperformed by the physics-based model in terms of the optimization results, the time consumption of the surrogate-optimizer model during the optimization searching process is 99% less than that of the physics-based model. As the results, the surrogate-optimizer model is slightly outperformed by the physics-based model in terms of the optimization results, where the Levelized Cost of Energy (LCOE) of the Surrogate-DA and Surrogate-BH models are 77.912 and 78.876 $/MWh, respectively, compared to the 77.190 $/MWh of the Baseline model with fairly close penalties between them. In the meantime, the time consumption of the surrogate-optimizer model during the optimization searching process is 99% less than that of the physics-based model.

PVT integrated hydrogen production with small-scale transcritical power cycle

The objective of the present research is to examine the performance of an integrated hydrogen generation system based on solar power. The system is comprised of photovoltaic-thermal (PVT) panels, a transcritical Rankine cycle (tRC), and a hydrogen generation unit, namely proton exchange membrane electrolysis (PEME) process. Carbon dioxide (CO2), which is an environmentally friendly fluid, is used as the working fluid of tRC, where it is directly heated up in the PVT collector. To perform the performance assessment of the system, first, a dynamic simulation of the PVT is conducted based on data from August 17 of Izmir. After, energy and exergy analyses of the integrated system are carried out, followed by parametric analyses. According to the results, the maximum power obtained from the PVT is found to be 2.6 kW, the net energy generated from tRC is calculated as 0.8 kW, while the maximum H2 generation rate is 16 g/h.

Off-design of a CO2-based mixture transcritical cycle for CSP applications: Analysis at part load and variable ambient temperature

This work focuses on the off-design analysis of a simple recuperative transcritical power cycle working with the CO2 + C6F6 mixture as working fluid. The cycle is air-cooled and proposed for a state-of-the-art concentrated solar plant with solar salts as heat transfer fluid in a hot region, with a cycle minimum and maximum temperature of 51 °C and 550 °C at design conditions. The design of each cycle heat exchanger (primary, recuperator and condenser) is carried out in MATLAB with referenced models and the turbine designed in CFD, providing performance maps adopted by the cycle operating in sliding pressure. The off-design of the cycle is developed with a routine simulating the thermodynamic conditions of the cycle at variable ambient temperature and thermal inputs down to 40 % of the nominal value. The results show that the cycle can efficiently run in a wide range of part load conditions and ambient temperatures, from around 0 °C to over 40 °C, with net electric cycle efficiencies from 45 % to 36 %: according to the control philosophy proposed, the condenser fans are fixed at design speed, while the cycle operates in sliding pressure, when is possible. The results evidence the flexibility and good performances of the proposed system in various operating conditions.

Dynamic performance analysis of transcritical power cycle with preheater and regenerator using CO2-based binary zeotropic mixtures

The CO2 transcritical Rankine cycle is considered as an ideal solution in the application of waste heat recovery due to its safety, environmental friendliness and compactness characteristics. In this work, for the heat recovery from internal combustion engine, a dynamic model of an improved CO2 transcritical Rankine cycle system containing both a preheater and regenerator with CO2-based binary zeotropic mixtures as working fluid is developed. The performance of CO2 transcritical power cycle with preheater and regenerator was compared and analyzed for mixture working fluids with different proportions under the disturbance of heat source conditions and system input parameters, which can provide references for system control strategy to ensure efficient and stable operating conditions when selecting control variables. The results show that there is a certain pump speed to obtain the optimal value of the net output work and thermal efficiency of the system. And the net output power and thermal efficiency of system with CO2-based binary zeotropic mixtures at optimum point are higher 6. 34 kW and 2.19% respectively than pure CO2 as working fluid by adjusting pump speed.

Performance prediction and design of CO2 mixtures with the PR-VDW model and molecular groups for the transcritical power cycle

Nowadays, CO2 mixture has been widely considered as the working fluid of power cycle. However, how to select appropriate fluid from the numerous organic fluids has to be solved, so as to achieve the high-efficient power cycle. Thus, in this work, PR-VDW and GCM-PR-VDW models are respectively developed to predict the properties of CO2 mixture. Under the given operating conditions, the prediction deviations of cycle performance are obtained and the influences of cycle temperature, pressure and mixing ratio on prediction accuracy are explored. The results show that PR-VDW model has high prediction accuracy for the thermal properties and cycle performance of CO2 mixtures. The prediction deviations of cycle efficiency and net work are 11.38% and 6.88%, respectively. For the GCM-PR-VDW model, cycle efficiency and net work have deviations 15.65% and 8.65%, respectively. On this basis, with the maximum output power as the optimization objective, the optimal organic working fluid and its ratio in CO2 mixture are determined by employing GCM-PR-VDW model. Under the given heat source conditions, it is found that CO2+R32 (0.23/0.78, molar ratio) is the optimal mixture for the transcritical power cycle, and the corresponding cycle efficiency and net work are 16.48% and 1148.04 kW, respectively.

Experimental study on the pressure drop characteristics of a supercritical CO2/R134a mixture in a rectangular microchannel

The proper addition of additives to pure CO2 can improve the performance of CO2 as a working fluid. By working fluid, we mean a fluid used for the CO2-based transcritical power cycle characterized by the addition of R134a to CO2. Hence, an experimental investigation of the pressure drop characteristics of a supercritical CO2/R134a mixture in a rectangular microchannel was conducted. For this purpose, an accurate and stable CO2/R134a mixture thermal-hydraulic experimental system was designed and built. The experimental results show that the pressure drop of the CO2/R134a mixture is lower than that of pure CO2 under the same conditions. The proportion of friction resistance to the total pressure drop decreases with an increase in R134a composition. Finally, a frictional resistance correlation that fully considers the effects of CO2/R134a mixture components and thermophysical property variations is proposed, which can guide the design of mixture heat transfer devices.

A theoretical expression on performances of transcritical power cycle with the pseudocritical temperature prediction of working fluid

As a representative energy conversion technology of medium and low grade heat source, organic Rankine cycle (ORC) has attracted much attention. Compared with the cycle performance calculation from REFPROP, a theoretical expression can conveniently and quickly predict the cycle performance. Although there exist various theoretical equations to predict the performances of subcritical ORC, few expressions have been proposed for the transcritical cycle. Thus, in this study, based on the pseudocritical temperature of working fluid, a theoretical equation is proposed to predict the performance of the transcritical power cycle. For the required pseudocritical temperature, an artificial neural network (ANN) model is developed to obtain the reduced value (Tpr) from the reduced pressure (Pr), molar mass (M) and critical compressibility factor (Zc), based on the generated 30,670 data of 93 working fluids. From the obtained results, it can be concluded that the developed ANN has the absolute average relative deviation (AARD) 0.44% for the prediction of pseudocritical temperature. For the cycle performance prediction of the proposed expression, when the cycle condition is closer to the critical region of working fluids, the prediction accuracy is lower. Compared with the dry and wet fluids, the isentropic fluid generally has lower deviations of thermal efficiency and net work. Furthermore, for any working fluid, deviations of cycle performances are lower than 10% in the typical ranges of cycle conditions. Meanwhile, the average AARDs of cycle efficiency and net work are 4.93% and 6.36%, respectively. Based on the developed theoretical expression, cycle performance of transcritical cycle can be accurately and quickly predicted for any working fluid.

Computer-aided molecular design of CO2-based mixture working fluid harvesting engine waste heat

The CO2 transcritical power cycle is an important method for recovering waste heat. The cycle performance can be further improved by using CO2-based mixtures instead of pure CO2. To avoid the empirical screening of CO2 mixtures, a computer-aided molecular design method, which is based on the group contribution method and perturbed chain-statistical associating fluid theory equations of state, is used for the first time in this study to design CO2-based mixtures for power cycles. The calculation method for group to system thermodynamic performance is established and verified. A molecular identification model is used to obtain all compounds based on the group library, calculate their thermodynamic properties for ranking, and find the best CO2-based mixture in the global range. The performance and cost of the expander are also briefly discussed. Taking the simple transcritical power cycle as an example, the results show that when the mole fraction of CO2 is greater than 0.70, the optimal CO2-based mixture, CH3-C≡CH, can increase the system efficiency and net output work by 16.58% (9.07%) and 17.88% (17.87 kW), respectively, compared to those of pure CO2. The peak system efficiency is 9.83% at an evaporation pressure of 24 MPa. Shorter-chain additives can improve thermodynamic performance, which is demonstrated by extracting the contribution of each group to the performance of the system using statistical methods. This method enables the ranking of CO2 mixtures on a global scale and provides some rules. It is also suggested that further economic analysis is necessary.

Optimum operations and performance comparison of CO2-propane and CO2-R152a mixture-based transcritical power cycles recovering diesel power plant waste heat

Optimum operations and performance comparison of CO2-propane and CO2-R152a mixture-based transcritical power cycles recovering diesel power plant waste heat

Analysis of CO2-based trans-critical power cycle in waste heat recovery

AbstractCarbon-dioxide-based trans-critical power cycle is a novel technology for waste heat recovery. This technology can handle the high-temperature exhaust gas and can be built in a compact size, which is an important feature for the auxiliary equipment for an internal combustion engine. To obtain the best output, four configurations were constructed: the basic system; one with preheater, another with regenerator and a fourth with preheater and regenerator. Special features of supercritical CO2 make these cycles able to recover more energy than the traditional organic Rankine cycle. According to this study, heat regeneration increases thermal efficiency while preheating influences the net power output. Thus, it is beneficial to add both regenerator and preheater to the basic cycle.