Transcritical CO2 Research Articles

Thermo-economic analysis of a combined flash-binary geothermal cycle with a transcritical CO<SUB align="right">2 cycle to produce green hydrogen

Thermo-economic analysis of a combined flash-binary geothermal cycle with a transcritical CO<SUB align="right">2 cycle to produce green hydrogen

Exergy analysis and performance improvement of direct steam-generated solar-based transcritical CO<SUB align="right">2 Rankine cycle with energy storage

Exergy analysis and performance improvement of direct steam-generated solar-based transcritical CO<SUB align="right">2 Rankine cycle with energy storage

Thermo-economic analysis of a combined flash-binary geothermal cycle with a transcritical CO2 cycle to produce green hydrogen

Thermo-economic analysis of a combined flash-binary geothermal cycle with a transcritical CO2 cycle to produce green hydrogen

Energy, exergy and exergoeconomic analysis of a trans-critical CO2 cycle powered by a single flash geothermal cycle in with/without economizer working modes

The global utilization of RES, particularly geothermal energy, is rising and the inefficient nature of geothermal cycles necessitates recovering lost heat. This research proposes a combined power generation cycle that simulates integrating a trans-critical CO2 cycle with a single flash geothermal cycle, utilizing the engineering equation solver. The study contrasts the system?s performance between two operating states: ?Without Economizer? and ?With Economizer?. The investigation analyzes the impact of an economizer on key output parameters, including energy efficiency, exergy efficiency, and net power output. In the ?With Economizer? operating state, the net power output experiences a noticeable increase from 201.5 kW to 204.7 kW, resulting in a 1.58% enhancement in the performance of the ?With Economizer? system. The energy efficiency metric demonstrates a corresponding improvement, rising by 1.55% from 3.28% in the ?Without Economizer? system to 3.331% in the ?With Economizer? system, aligning with the principles of the First law of thermodynamics. Furthermore, the energy efficiency, expressed as a percentage of energy units, shows an increase from 16.3% in the ?Without Economizer? system to 16.56% in the ?With Economizer? system, representing a 1.595% improvement based on the Second law of thermodynamics or exergy. Regarding cost analysis, the study identifies the optimal separator pressure value for the system without an economizer, equivalent to 23. This configuration achieves a total cost rate of 01 $ per GJ. Conversely, in the system with an economizer, the optimal pressure value for the production cost rate is 322.4 kPa, resulting in a cost rate of 23.57 $ per GJ.

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

Research progress and applications of transcritical carbon dioxide heat pumps: A review

Heat pump technology is an energy-saving technology that can efficiently utilize low-grade energy. It has broad application prospects in building heating, industrial waste heat utilization, new energy and other fields. However, the refrigerants used in traditional heat pump systems have serious negative impacts on the environment, and there is an urgent need to find a safe, environmentally friendly, and efficient alternative refrigerant. As a natural refrigerant, CO2 has good physical and chemical properties and is very suitable as a working fluid in transcritical cycles, showing great advantages in the field of heat pump technology. At present, research on CO2 heat pumps has made certain progress, but there are few reviews of the research status and development trends of CO2 heat pumps in different applications. Therefore, this article systematically summarizes the latest research results of transcritical CO2 heat pumps in different application fields, pointing out the difficulties such as high pressure and low operating efficiency in system design and operation. It also summarizes the latest optimization research on system components, cycle structure, mixed refrigerants and control strategies. The results show that each optimization method can significantly improve system performance, among which mixed refrigerant is the simplest optimization method. Finally, the outlook for CO2 heat pump technology is put forward. With policy support and technological advancement, more comprehensive, energy-saving, and intelligent CO2 heat pump technology will continue to develop and innovate.

Theoretical investigation of a novel distributed compression cycle for CO2 trans-critical system

Efficient cooling of the trans-critical CO2 vapor compression cycle to improve system performance is a crucial research topic. This paper proposes an innovative approach called distribution compression (DC) to achieve the same effect as subcooling in a trans-critical CO2 thermodynamic cycle. In the DC system, the trans-critical CO2 at the gas cooler outlet is no longer further cooled but subjected to a secondary pressure boost and releases heat in ordinary heat sink conditions as in the gas cooler. Thermodynamics calculated the change in performance of the DC system under different working conditions with different secondary boost ratios. The optimal secondary boost ratio related to evaporation and gas cooler outlet temperatures under optimal discharge pressure based on the baseline system was obtained. The results show that compared with the baseline system, the DC system can effectively improve the system performance, and the maximum refrigeration COP increase is between 8.2 % ∼ 10.76 %, and the maximum heating COP increase is between 5.37 % ∼ 6.34 %. The cooling or heating capacity increase can reach up to about 26 %. The ideal secondary boost ratio in a DC system is not highly demanding, and the increased input power for the second input power for the secondary boost will not exceed 20 % relative to the baseline system. Comparing the COP of the DC system over current systems that only use a single subcooling technology still shows advantages. The DC system provides a new idea to improve the performance of the trans-critical CO2 vapor compression cycle.

Real-time multi-variable optimization of the interstage subcooling vapor-injection based transcritical CO2 HPWH: An integrated model predictive control approach

The interstage subcooling vapor-injection technique has been introduced to transcritical CO2 heat pumps to alleviate performance degradation under conditions of low ambient temperature and high water-outlet temperature. This type of solution holds considerable appeal and finds utility across a wide range of applications. However, the efficacy and efficiency of such systems are contingent upon their management, which frequently requires the implementation of suitable control systems. The primary objective is to enhance the overall system operational performance, resulting in tangible advantages in terms of economic viability, energy conservation, and environmental sustainability. To achieve these objectives, one can utilize both explicit and implicit optimization methods ranging from Model Predictive Control (MPC) to Extremum Seeking Control (ESC), each with its strengths and weaknesses. This paper compares the performance of an ad hoc integrated MPC strategy and a multi-variable ESC one. Based on the results of the system operation employing the latter strategy, a significant linear correlation was discovered between the optimal medium pressure and the optimal discharge and suction pressure. Consequently, an intrinsic self-optimization strategy for the medium pressure was developed. Subsequently, a dynamic system identification was performed on the new system, incorporating this proposed strategy. To optimize the system Coefficient of Performance (COP) while maintaining the water outlet temperature, ensuring the thermal load constraint, the MPC strategy was employed. The main focus of the MPC was to determine the optimal Electronic Expansion Valve (EEV) opening degree and water pump settings. The integrated MPC strategy was evaluated and compared to the multi-variable ESC strategy under various conditions, including fixed design conditions, changing ambient conditions, and step-change water outlet temperature. The results clearly demonstrated the effectiveness and superiority of the integrated MPC.

A novel multi-generation system for sustainable power, heating, cooling, freshwater, and methane production: Thermodynamic, economic, and environmental analysis

Addressing environmental concerns and developing efficient systems pose significant challenges for researchers. This paper presents a groundbreaking multi-generation system designed to generate power, heating, cooling, freshwater, and methane. The system comprises a flash-binary power plant, a modified transcritical CO2 cycle, a proton exchange membrane electrolyzer, and a methanation unit. Integration of the modified transcritical CO2 cycle with the ejector refrigeration cycle, solar still desalination unit, and heating unit enhances its functionality. The proton exchange membrane electrolyzer supplies hydrogen to the methanation unit. The thermodynamic, economic, and environmental approaches are employed to analyze the system, and three scenarios are considered to assess the optimum state. Accordingly, the exergy efficiency, products' total cost, and exergoenvironmental impact rates are obtained at about 28.30%, 4.153 $/h, and 73.48 mPts/h, respectively. The gas heater unit has the highest exergy destruction rate and cost rate and exergoenvironmental impact rate by 37.6%, 36.4%, and 22.7% portions, respectively. The optimal state provides 33.25% exergy efficiency, 450.9 kW net power, 10.07 kg/h freshwater, and 2.149 kg/h methane. The cooling and heating production rates are attained 28.16 kW and 183.5 kW. The product's cost rate and exergoenvironmental impact rates are estimated at about 4.357 $/h and 76.64 mPts/h.

Energy and economic analysis of a sub-cooler based vapor injection transcritical CO2 heat pump for space heating

The Chinese government has considered the transcritical CO2 heat pump (TCHP) as a promising solution for space heating with the goal of “carbon peaking” and “carbon neutrality.” Considering the convenient implementation and the performance improvement, the vapor injection technique should be taken into consideration. This paper presents the experimental and simulation investigation of the SCVI TCHP prototype with energetic and economic analysis. Under the design condition, the SCVI technique could improve the peak COP by 5.02 % with a 10.11 % improvement in heating capacity. Compared to the direct electric heater solution, the annual operation cost was reduced by 11966CNY, 29.03 % higher than the Base TCHP. The payback period was decreased from 8 to 6.9 years due to the application of vapor injection. The effect of ambient temperature on the system performances was evaluated based on the experimental data. As the simulation progressed, a control strategy optimizing both the discharge and the medium pressure was developed, and the influence of inlet and outlet water temperature was studied.

Performance investigation of solar-assisted supercritical compressed carbon dioxide energy storage systems

Energy storage technology plays a vital role in realizing large-scale grid connection of renewable energy. Compared with compressed air energy storage system, supercritical compressed carbon dioxide energy storage (SC-CCES) system has the advantages of small size and high energy storage density. In this paper, two solar-assisted supercritical compressed carbon dioxide energy storage (SASC-CCES) systems are proposed. One is coupled with simple regenerative compression cycle (SR-SASC-CCES), the other with recompression cycle (RC-SASC-CCES). Thermodynamic and economic analyses are carried out. The results show that the energy efficiency of the system is higher than that of other organic working fluids when the transcritical CO2 is used as the working fluid of the bottom cycle due to the residual heat available in the energy storage system of the recompression cycle. Compared with the baseline SASC-CCES system, the SR-SASC-CCES system and RC-SASC-CCES system are more conducive to improving the solar energy utilization rate, and their photoelectric conversion efficiencies are 23.56 % and 28.77 %, respectively, 2.63 % and 7.84 % higher than that of the baseline SASC-CCES system. Increasing the inlet temperature of turbine 1, energy storage pressure and energy release pressure can effectively increase the energy efficiency, solar-electricity conversion efficiency, exergy efficiency and energy storage per unit volume of these two systems. In addition, the RC-SASC-CCES system has better economic performance, whose dynamic investment recovery period is about 6.8 years. The 20-year net present value (NPV) is 23,456.16 k$, 7956.69 k$ higher than that of the SR-SASC-CCES system.

Sustainable Power Generation Through Solar-Driven Integration of Brayton and Transcritical CO2 Cycles: A Comprehensive 3E (Energy, Exergy, and Exergoenvironmental) Evaluation.

Solar power tower technology has strong potential among the other concentration solar power techniques for large power generation. Therefore, it is necessary to make a new and efficient power conversion system for utilizing the solar power tower system. In present research, a novel combined cycle is proposed to generate power for the application of the solar power tower. The pre-compression configuration of the Brayton cycle is used as a topping cycle in which helium is taken as the working fluid. The transcritical CO2 cycle is used as bottoming cycle for using the waste heat. The proposed system is investigated based on exergy, energy, and exergoenvironmental point of view using computational technique engineering equation solver. Also, the parametric analysis is carried out to check the impact of the different variables on the system performance. It is concluded that the overall plant's optimized thermal and exergy efficiencies are obtained as 31.59% and 33.12%, respectively, at 800°C optimum temperature of combined cycle and 850Wm-2 of direct normal irradiation and 2.278 of compressor pressure ratio. However, exergetic stability factor and exergoenvironmental impact index are observed as 0.5952 and 0.6801 respectively. The present proposed system performs better than the previous studies with fewer components.

Driving range evaluation of electric vehicle with transcritical CO2 thermal management system under different battery temperature controls

Improper battery temperature will lead to reduced battery discharge efficiency and electric vehicle driving range. Endeavors to find an efficient and precise battery temperature control method for the transcritical CO2 thermal management system of electric vehicles, two evaporation temperature control methods for battery cooling were proposed. First, the effects of the control methods on battery temperature, optimal discharge pressure and coefficient of performance (COP) were simulated under typical operating conditions. Subsequently, a comparative simulation was conducted on the battery temperatures and COP of the two control methods under full cooling conditions. The results revealed that equal evaporation temperature control induced a reduction in battery temperature to 11 °C (suboptimal levels) under conditions of low ambient temperature and vehicle speed. Conversely, while unequal evaporation temperature control could maintain the battery temperature within the optimal range of 20–45 °C, it would result in a maximum COP drop of more than 20% under certain conditions. Furthermore, the effects of the two control methods on driving range under different ambient temperatures and vehicle speeds were studied. The reasults indicated that the ultimate influence of uneuqal evaporation temperature control on the quarterly average driving range was virtually negligible, peaking at 1.35%. Given the dual-objective constraints pertaining to the driving range and the battery cooling performance, it was concluded that uneuqal evaporation temperature control was the optimal choice for direct battery cooling.

Performance evaluation of transcritical CO2 desiccant heat pumps for electric vehicles

Transcritical CO2 heat pump systems are eco-friendly and have excellent heating performance, which makes them suitable for electric vehicles. The recirculation mode can save energy but requires a dehumidification function to ensure driving safety. Therefore, this paper proposes a transcritical CO2 desiccant heat pump, which differs from the typical transcritical CO2 heat pump system by incorporating an indoor evaporator and a full-pass throttle valve. The flow area of the full-pass throttle valve can be adjusted to obtain the appropriate dehumidification rate. In addition, this study suggested using the desiccant heat pump coefficient of performance (DHCOP) to evaluate the overall energy efficiency of the heat pump system. We have investigated the dehumidification rate and optimal discharge pressure under different operating conditions. The results show that even under the harsh operating conditions at -10 °C, the HDCOP of the transcritical CO2 desiccant heat pump can still exceed 2.1, and reach 3.12 at a moderate condition of Ta=5 °C. The transcritical CO2 heat pump desiccant system demonstrates significant energy-saving potential.

Development of a surrogate model of a trans-critical CO2 heat pump for use in operations optimization using an artificial neural network

Conventional physics-based models can demand substantial computational resources when employed for operational optimization. To allow faster system simulations that can be employed for operational optimization, a surrogate model of the CO2 heat pump has been developed using an artificial neural network (ANN). The ANN model takes in six (6) inputs: evaporator water-side mass flow, its temperature, gas cooler water-side mass flow, its temperature, set-point output temperature, and high-side heat pump pressure. The model’s outputs comprise the electrical energy needed to run the heat pump, the heat from the gas coolers, the temperature of the heat pump-heated fluid, and the outlet temperature of the heat pump’s evaporator. Data used for training, validating, and testing the ANN model were generated by running a calibrated Modelica model of the CO2 heat pump for various combinations of input parameters obtained from Latin hypercube sampling. The ANN model developed includes an input layer with 6 inputs, 2 hidden dense layers, each with 30 neurons, and an output layer for 4 outputs (6-30-30-3). The ReLU activation function was implemented on each hidden layer and no regularizations were imposed. The Adam optimizer was used with a learning rate of 0.001 specified. Early stopping (patience = 2000) was implemented to ensure that the training data was not overfitted. A maximum of 30000 epochs was specified. The resulting Mean Square Error (MSE) obtained for the training, validation, and testing data sets were 1.38x10−5, 2.05x10−5, and 3.65x10−5, respectively. When tested against one-week operational runs generated by Modelica, the Root Mean Square Errors (RMSEs) for coefficient of performance (COP)s for spring, summer, autumn, and winter operations obtained were 0.232, 0.346, 0.089 and 0.076, respectively. The resulting surrogate ANN model can be integrated into the system model as a functional mock-up unit within Modelica to facilitate faster simulations for operational optimization.

Effect of spindle position on the performance and two-phase characteristics of controllable ejector for transcritical CO[formula omitted] refrigeration system

Spindle-controlled variable-geometry ejectors can adjust the system capacity to meet varying load requirements in transcritical-CO2 refrigeration systems. The present study numerically explores the effect of spindle position on the performance and flow characteristics of a controllable two-phase CO2 ejector. Spindle-controlled ejectors achieved entrainment ratios higher than that of fixed geometry ejector. Motive and suction flows exhibited high sensitivity to the spindle position near the critical operating range because of the transition between operating modes. The two-phase and shock characteristics associated with ejector flow and the variations observed for different spindle positions are discussed. Critical discharge pressures and entrainment ratios are estimated for various spindle positions. The critical compression and entrainment ratios correlate with the respective normalized spindle position in the ejector. The impact of the effective nozzle throat area on the flow entrainment range and achievable pressure lift is investigated. The results showed that the critical entrainment ratio bears a common linear relationship with the critical compression ratio for all positions of the spindle. The study relates the most significant performance and geometric parameters of the spindle-controlled two-phase CO2 ejector.

Integration of large-scale heat pumps to assist sustainable water desalination and district cooling

Climate change intensifies stress on global potable water supply. Over 2 billion people still lack access to safe potable water, hindering social and economic development in arid regions. As a result, cost-effective desalination is important. The two main desalination technologies are thermal-based and membrane processes. The membrane-based desalination process Reverse Osmosis is gaining more and more popularity. The reason is that thermal-based processes often rely on non-renewable heat sources, while high temperature heat pumps offer a sustainable alternative for heat supply. Thus, this study investigates the integration of a transcritical CO2 heat pump into thermal-based desalination processes. The CO2 process operates at up to 180 °C, with an eco-friendly working fluid. The heat pump provides both potable water and potentially cooling energy. Six configurations were considered, showing comparable energy demands to RO. Moreover, considering cooling duty, the integrated system outperforms RO. A commercially available 19 MWel transcritical CO2-based heat pump system can produce up to 64.000 m3/d of potable water and 75 MW of cooling energy. This highlights the potential of heat pumps as sustainable solutions for addressing water scarcity and providing cooling.

Thermodynamic and economic seasonal analysis of a transcritical CO2 supermarket with HVAC supply through ice thermal energy storage (ITES)

A real case of a supermarket where a CO2 refrigerating plant also supplies heating, air conditioning and hot water is considered. Ice thermal energy storage (ITES) is used both as latent storage in summer and as sensible thermal energy storage (TES) in winter to partially cover the space cooling/heating load of the supermarket. In particular, it allows to reduce peaks in the electrical power use, when the refrigeration and HVAC systems are running at full power together with ovens and heaters for meals. A thermodynamic analysis, including a detailed theoretical model of the formation and melting of ice on the coils, is carried out to predict the behaviour of ITES during the charging and the discharging phases. A daily energy analysis for both a winter and a summer typical day, and an annual analysis are carried out for the whole system. In summer, two cases are evaluated, i.e. supplying the whole AC demand in the morning or partially covering the AC demand to reduce the design capacity of the reversible heat pump. In all cases, the use of ITES aimed at shaving electrical peaks leads to a higher electrical energy use, also on an annual basis. However, the cost analysis reveals significant benefits, including a reduction in the required capacity of the reversible heat pump, better exploitation of tariffs and the avoidance of installing an electrical transformer in a dedicated room. This results in savings €58,699 over 10 years €47,888 over 15 years, making the choice of ITES more economically advantageous within the typical lifetime of these systems.

An innovative study on a geothermal based multigeneration plant with transcritical CO2 cycle: Thermodynamic evaluation and multi-objective optimization

Renewable energy-sourced integrated plants are among the novel and interesting technologies for production of the clean and effective beneficial multiple commodities. Also, these technologies are one of the main motivations for overcoming environmental troubles. The newly designed geothermal-based multigeneration plant deals with clean and sustainable power, hydrogen, cooling, and heating generation and then comprehensive thermodynamic and economic analyses. This system is driven by a geothermal source that includes a transcritical CO2 based Rankine cycle, an ejector cooling cycle, a hydrogen generation and compression unit, and a domestic hot water preparation unit. Here, a comprehensive mathematical modeling is discussed that are energy and exergy efficiency methods along with economic cost evaluation. Furthermore, to define the optimal working conditions, a multiobjective optimization method is fulfilled based on the relationship between the developed multigeneration system's cost and exergy efficiency. Additionally, a comprehensive parametric assessment is implemented to describe how effects of various indicators on the developed model’s efficiency. In outlook of these outcomes, it is crystal clear that the quantity of clean power and hydrogen generation rate is 1283 kW and 0.009374 kgs−1, respectively. The energy and exergy efficiencies of the developed MG cycle are figured as 27.60% and 22.56%, at 152 ℃ geothermal source temperature. the optimal plant total cost rate is computed as 80.48 $h−1. Finally, it is concluded that this proposed system's performance and cost rates are compatible and applicable.

Transcritical-transcritical cascade CO2 heat pump cycles for high-temperature heating: A numerical evaluation

Industrial heat pumps are becoming increasingly popular as a low-emissions technology due to the ability to leverage renewable electricity at high efficiencies. While heat pumps are already well-established for process temperatures below 100 °C, high-temperature heat pumps that can produce greater temperatures are limited by high pressure and temperature requirements that surpass the performance of conventional heat pump compressor technologies. Alongside this, new heat pump cycles which are being developed need to be compatible with low GWP and low ODP refrigerants. This paper presents a numerical evaluation on the thermodynamic performance of high-temperature heat pumps for supplying 200 °C sink temperatures with large temperature glides. Four new transcritical–transcritical cascade CO2 heat pump systems are modelled in this study which aim to limit the pressure requirements required to achieve high sink temperatures. An air heating case study for milk powder spray drying is presented. To achieve a COP of 3, all cycles had a similar maximum air outlet temperature between 110 °C and 113 °C. The transcritical-transcritical cascade cycle with internal heat exchangers on both the top and the bottom cycles achieved the lowest compressor discharge pressure (13.47 MPa maximum) for this case. The same cycle also achieved the highest COP of 2.22 when air outlet temperature was fixed at 200 °C (i.e., complete replacement of external heating). For such a heat pump to be economically feasible for heating air to 200 °C, an electricity-to-reference price ratio equal to or below 2.06 is required.