Carbon Dioxide Transcritical Power Cycle Research Articles

Introducing and performance evaluation of a novel Multi-Generation process for producing Power, fresh Water, oxygen and nitrogen integrated with liquefied natural gas regasification and carbon dioxide removal process

The objective of this paper is to develop and evaluate a new integrated multigenerational system comprised of cryogenic air separation, hydrate-based desalination, combined cycle and oxyfuel power plants, carbon dioxide amine absorption, and transcritical carbon dioxide power cycle, employing liquefied natural gas cold energy. This integrated multi-generation system, applicable for liquefied natural gas importer terminals with access to seawater, produces approximately 2,533 MW net output power and 1,985 kg/h fresh water. In addition, this integrated multi-generation process produces 16,735,861 kg/h high-purity liquid oxygen (100 mol%), 4,773,633 kg/h liquid nitrogen with 99.71 mol% purity, more than 31,724,465 kg/h high-purity gaseous nitrogen (99.9 mol%), and 90,903,230 kg/h low-purity gaseous nitrogen (83.38 mol%). About 5907.3 MW of produced power is generated by liquefied natural gas expander. The performance of this system is evaluated by energetic methods. The results of energy analysis show the specific energy consumptions of the air separation products are 0.551 kW/kg and 1.93 kW/kg for liquid oxygen and nitrogen, respectively. Moreover, in the hydrate-based desalination subsystem, the specific energy consumption is 0.5 kW/m3 for produced pure water. The combined cycle power plant includes two gas turbines, three steam turbines, and a three-pressure level heat recovery steam generation. The thermal efficiencies of gas turbine cycles (Bryton), steam turbine cycles (Rankine) and combined power cycles are 26.3 %, 29.6 %, and 36.2 %, respectively. The flue gases from power plants enter the carbon dioxide amine absorption subsystem. The separated carbon dioxide is used as a working fluid to produce 372.1 MW power in transcritical carbon dioxide cycle. The results of energy analysis for carbon dioxide amine absorption subsystem show that the specific energy consumption, the ratio of captured carbon dioxide per unit mass of liquefied natural gas and the carbon dioxide removal efficiency are 6.60 kW/kg, 33.91 kg/tone, and 90.06 %, respectively. Approximately 2,905,000 kg/h of liquid carbon dioxide is obtained as the byproduct of this multi-generation process. Additionally, sensitivity analysis is conducted to determine key parameters influencing the main energetic indexes. Finally, the amount of attainable regasified natural gas reaches 257,523 tone/h by applying this multi-generation system with total 86 % energy efficiency.

A transcritical carbon dioxide power cycle enhanced by ejector refrigeration for engine waste heat recovery: Comprehensive analysis and optimization

A novel approach to realizing and enhancing a transcritical carbon dioxide (CO2) power cycle can be achieved by simultaneously utilizing waste heat from an internal combustion engine at two different temperature levels. This paper introduces a combined power system integrating a transcritical CO2 power cycle and an ejector refrigeration cycle (tCO2-ERC) via internal condensation for engine waster heat recovery, where the ERC driven by jacket water provides cooling energy for the condensation of CO2 in the tCO2 subsystem driven by exhaust gas. Comprehensive analyses, including energy, exergy, and economic aspects, are conducted through parameter analysis and system optimization to explore the mechanisms underlying the performance improvement of the combined power system. The results of refrigerant screening demonstrate that implementing R717 in the ERC of the tCO2-ERC system yields better overall thermoeconomic performance. Exergy analysis demonstrates that the exergy losses from the exhaust and jacket water to the environment in the tCO2-ERC system are reduced to 17.62% and 30.27% of the total exergy input, respectively, which represents a reduction of 7.32%pt (percentage point) and 12.51%pt compared to the standalone reference CO2 power system. Multi-objective optimization results indicate that the optimized net power output and levelized cost of electricity of the tCO2-ERC system are 388.79 kW and 0.09023 $/(kW·h), respectively, which are 13.01% higher and 6.91% lower than those of the reference CO2 power cycle system. Furthermore, the net power output of the tCO2-ERC system exceeds that of the reference CO2 power system by 5.44% to 14.34% when the ambient temperature ranges from 10 ℃ to 30 °C. Additionally, the tCO2 subsystem consistently operates in a transcritical mode within an ambient temperature range of 10–30 °C, thereby expanding its application range.

Waste heat recovery of an UAV propulsion system based on PEM fuel cell by a novel transcritical CO2 - LNG hybrid cycle; thermodynamic and multiple linear regression analyses

ABSTRACT In this work, the performance of a proposed hybrid system consisting of a transcritical carbon dioxide-natural gas cycle for recovery of waste heat produced in a proton exchange membrane fuel cell has been evaluated. The effects of different parameters on proposed cycle performance have been investigated thermodynamically. This hybrid system can convert fuel cell’s released heat to electricity, augment production power, convert natural gas from the liquid phase to gas, and appropriate use of heat energy from consumed water. Results show that increasing the cell’s power by about 52% leads to a growth in overall efficiency of transcritical power cycle. Also, in a constant fuel cell power, the higher carbon dioxide mass flow rate tends to lower carbon dioxide turbine output power, and consequently, the efficiency of transcritical power cycle decreases. Changes in waste heat recovery efficiency among models with different values of CO2 mass flow rate increases very slightly as fuel cell power augments. For instance, at lowest examined fuel cell power (790 kW), growth in CO2 mass flow rate by about 26% causes a decline in cycle efficiency by about 6.35%. By increasing fuel cell thermal efficiency by 30%, improvement in overall cycle efficiency rises significantly by about 109.92%. On the other hand, augmentation in fuel cell thermal efficiency by about 30% leads to a very slight growth in improvement of the waste heat recovery efficiency by about 2.67%. Moreover, by increasing adiabatic efficiency of turbines by about 21.43%, improvement in waste heat recovery efficiency declines by about 10.75%. Increasing carbon dioxide turbine’s inlet temperature and reducing carbon dioxide mass flow rate have beneficial effects on efficiency of carbon dioxide transcritical power cycle. Eventually, two high-accuracy models have been presented for output power and efficiency of proposed hybrid cycle using multiple linear regression.

Transcritical carbon dioxide power cycle for waste heat recovery: A roadmap analysis from ideal cycle to real cycle with case implementation

Waste heat recovery (WHR) is an important measure to improve the utilization efficiency of primary energy and alleviate the scarcity of fossil fuels, and converting thermal energy into electrical energy by means of thermodynamic cycles is regarded as the best choice for WHR because of convenience in the use and transmission of electrical energy. Various thermodynamic cycles have been constantly proposed and proved to be applicable for WHR, which has led to a controversy about the most suitable cycles. Although some studies have proposed their own opinions by comparing the thermodynamic or economic performance of different cycles, this controversy has not been completely resolved due to the lack of research from the perspective of the ideal cycle. Therefore, a new ideal cycle that could solve difficulties hidden in the implementation of existing ideal cycles is proposed in this study. And a real cycle that includes over-expansion, recompression, and internal heat recovery is derived by gradually considering the real factors on the basis of the new ideal cycle. Then, to implement the above real cycle, a modified transcritical carbon dioxide power cycle is proposed to recover waste heat from the flue gas of 500 ℃. Thermodynamic analysis of the modified cycle is carried out, and results show that the maximum net work output and corresponding heat recovery rate of flue gas reach 109.99 kW and 81.5% under the considered conditions. Compared with the traditional transcritical carbon dioxide power cycle with recuperator, the net work output, heat recovery rate of flue gas, thermal efficiency, and exergy efficiency of the modified cycle improves by 33.6%–192.3%, 28.6%–39.5%, 3.9%–109.5%, and 11.4%–122.3%, respectively, under the same conditions. This indicates that the modified cycle performs better and the proposed ideal cycle can guide the research of WHR.

Performance analysis of carbon dioxide based combined power cycle for concentrating solar power

A detailed analysis of a combined power block system using carbon dioxide integrated with a thermal energy storage system and solar field is presented. The combined power block system consists of a topping supercritical carbon dioxide recompression Brayton cycle and its waste heat is recovered by coupling a bottoming transcritical carbon dioxide power cycle. Transcritical carbon dioxide power cycle is selected owing to its temperature glide merit and better heat transfer characteristics than conventional waste heat recovery cycle systems using other working fluids. A molten salt that can operate at relatively higher temperature than nitrate solar salt is selected as a storage medium and as heat transfer fluid between thermal energy storage system and external heat exchanger of power block. A complete mathematical model for the heliostat field, solar receiver, thermal energy storage, and power block system has been developed. The discretized heat exchanger approach has been implemented to accurately capture the properties of working fluid and to provide more refined results. Theoretical analysis of the system includes first and second law efficiencies and power output. The research findings suggest that the integrated combined cycle configuration is a promising alternative for efficient energy conversion and the established framework provides a profoundly potential way to utilize concentrated solar power.

Dynamic Performance Comparison of CO2 Mixture Transcritical Power Cycle Systems with Variable Configurations for Engine Waste Heat Recovery

Carbon dioxide transcritical power cycle (CTPC) is suitable for engine waste heat recovery owing to its advantages, such as compact construction and high decomposition temperature. In addition, the addition of refrigerant can further improve the performance of pure carbon dioxide (CO2). Because there are limited studies considering the dynamic performance of CTPC systems with CO2 mixture as the working fluid (CMTPC), let alone the dynamic performance comparison of different structures of the CMTPC system, the object of the current work was to compare the dynamic performance, including the off-design performance and dynamic response speed, of four kinds of CMTPC systems, as well as their sensitivity to system input parameters. The dynamic models of four CMTPC systems were established and validated against experimental data, which includes basic CMTPC (B-CMTPC), CMTPC with a preheater (P-CMTPC), CMTPC with a recuperator (R-CMTPC), and CMTPC with both a recuperator and preheater (PR-CMTPC). Based on the dynamic models, the off-design performance and dynamic response speed of four CMTPC systems were compared by changing the engine load. The fluctuation amplitude and response time of a R-CTPC system are the maximum under off-design conditions. Moreover, the sensitivity analysis demonstrates that different output parameters of four CMTPC systems have differing sensitivity to input parameters. It is necessary to pay attention to the more sensitive input parameters under the specific working condition to avoid system damage or unsafe operation.

Exergoeconomic analysis and optimization of a transcritical CO2 power cycle driven by solar energy based on nanofluid with liquefied natural gas as its heat sink

The thermodynamic and economic analysis of a transcritical carbon dioxide power cycle is coupled with solar thermal subsystem and LNG subsystem. Solar thermal subsystem consists of parabolic trough collectors and a thermal storage tank, and the Copper-Therminol VP1 nanofluid is used in this subsystem. In most hours of the day, the solar subsystem based on Copper-Therminol VP1 nanofluid has a better thermal performance, which includes the output temperature of the collector and temperature of the storage tank, compared to the solar subsystem based on Therminol VP1. LNG subsystem is employed as a heat sink of the transcritical CO2 power subsystem, as well as the power generation by LNG turbine. The exergoeconomic analysis is performed to evaluate the effects of the key parameters, including turbine inlet temperatures and pressures, condensate pressure, CO2 mass flow rate and LNG pressure, on the exergy efficiency and product cost rate. In addition, parameter optimization is conducted via genetic algorithm. TOPSIS decision making technique is employed to select optimum point. System is capable of producing power with exergy efficiency of 8.53%, and product cost rate is equal to 2.09 million dollars per year, under optimum conditions. The values of exergoeconomic variables for each component of the system are calculated. The results represent that solar collector, evaporator, condenser, CO2 turbine and LNG turbine have the highest total cost rate of exergy destruction and investment $$ (\dot{C}_{\text{d}} + \dot{Z}) $$, respectively. The highest amount of investment cost rate $$ \dot{Z} $$ occurs in solar collector. The condenser has the lowest value of exergoeconomic factor which indicates that the costs associated with condenser come from exergy destruction rate.

Potential of carbon dioxide transcritical power cycle waste-heat recovery systems for heavy-duty truck engines

Carbon dioxide transcritical power cycle (CTPC) systems are considered a new and particularly interesting technology for waste-heat recovery. In heavy-duty truck engine applications, challenges arise from the highly transient nature of the available heat sources. This paper presents an integrated model of CTPC systems recovering heat from a truck diesel engine, developed in GT-SUITE software and calibrated against experimental data, considers the likely fuel consumption improvements and identifies directions for further improvement. The transient performance of four different CTPC systems is predicted over a heavy-heavy duty driving cycle with a control structure comprising a mode switch module and two PID controllers implemented to realize stable, safe and optimal operation. Three operating modes are defined: startup mode, power mode, and stop mode. The results demonstrate that CTPC systems are robust and able to operate safely even when the heat sources are highly transient, indicating a promising potential for the deployment of this technology in such applications. Furthermore, a system layout with both a preheater and a recuperator appears as the most promising, allowing a 2.3% improvement in brake thermal efficiency over the whole driving cycle by utilizing 48.9% of the exhaust and 72.8% of the coolant energy, even when the pump and turbine efficiencies are as low as 50%. Finally, factor analysis suggests that important directions aimed at improving the performance and facilitating CTPC system integration with vehicle engines are: (1) ensuring long-duration operation in power mode, e.g., by employment in long-haul trucks; and (2) enhancing pump and turbine performance.

Effects of external perturbations on dynamic performance of carbon dioxide transcritical power cycles for truck engine waste heat recovery

Carbon dioxide transcritical power cycle (CTPC) systems are dynamically tested on a constructed test bench using an expansion valve. Effects of external perturbations of pump speed, expansion valve opening and engine conditions on dynamic performance of the CTPC systems are investigated with step changes. Dynamic characteristics are identified with rise time, settling time and overshoot. Results show that both a basic CTPC system and a CTPC system with a recuperator (R-CTPC) behave as a second-order underdamped system with the perturbations of pump speed and expansion valve opening. Overshoot or undershoot tends to appear in pressures when pump speed changes, while overshoot or undershoot occurs more noticeably in temperatures when expansion valve opening varies. Although the CTPC systems respond slowly with the perturbations of engine conditions, they have the ability to operate safely and produce power continuously. Therefore, the CTPC systems are robust when facing narrow fluctuations of heat sources while swift when required to make adjustments, showing great potential for truck engine waste heat recovery. Overall, current research gives a full understanding of the dynamic performance of the CTPC systems, which will provide references for dynamic model validation and possible control strategy identification.

Thermodynamic analysis of an integrated transcritical carbon dioxide power cycle for concentrated solar power systems

This paper investigates the thermodynamic performance, through energy and exergy efficiencies, of a conceptual design of a reheat transcritical carbon dioxide (T-CO2) power cycle for concentrated solar power (CSP) plants. Herein, a parabolic trough collector (PTC) solar field is used to harvest solar energy and provide the thermal energy to the T-CO2 power cycle. Thermal energy storage (TES) is also integrated to overcome the intermittent nature of solar energy and maintain stable thermal energy supply to the power cycle. Furthermore, the T-CO2 power cycle is integrated with an absorption refrigeration system (ARS) to enhance the cycle efficiency and production stability by sustaining low condensation temperature at various weather conditions. A parametric study through energy and exergy analyses is conducted considering the performance of each subsystem independently, and that of the overall integrated CSP. The energy and exergy efficiencies, thermal losses, and exergy destruction rates are evaluated under the different design and operating conditions for the T-CO2 power cycle and the ARS. For example, the effects of variations in the maximum cycle temperature and pressure on both the power cycle’s energy and exergy efficiencies and integrated system efficiencies are investigated. In addition, the impacts of variations in these parameters on the integrated CSP energy and exergy efficiencies are examined. The T-CO2 power cycle achieved energy and exergy efficiencies of 34% and 82%, respectively. The integrated CSP (solar-to-electric) energy and exergy efficiencies are about 20% and 55%, respectively.

Multi-objective optimization of the carbon dioxide transcritical power cycle with various configurations for engine waste heat recovery

In this paper, a systematic multi-objective optimization methodology is presented for the carbon dioxide transcritical power cycle with various configurations used in engine waste heat recovery to generate more power efficiently and economically. The parametric optimization is performed for the maximum net power output and exergy efficiency, as well as the minimum electricity production cost by using the genetic algorithm. The comparison of the optimization results shows the thermodynamic performance can be most enhanced by simultaneously adding the preheater and regenerator based on the basic configuration, and the highest net power output and exergy efficiency are 25.89kW and 40.95%, respectively. Meanwhile, the best economic performance corresponding to the lowest electricity production cost of 0.560$/kW·h is achieved with simply applying an additional regenerator. Moreover, a thorough decision making is conducted for a further screening of the obtained optimal solutions. A most preferred Pareto optimal solution or a representative subset of the Pareto optimal solutions is obtained according to additional subjective preferences while a referential optimal solution is also provided on the condition of no additional preference.

Introducing a novel air separation process based on cold energy recovery of LNG integrated with coal gasification, transcritical carbon dioxide power cycle and cryogenic CO2 capture

An integrated coal gasification process with a novel double-column cryogenic air separation unit (ASU) based on the LNG (liquefied natural gas) cold energy recovery is proposed and analyzed. The process consists of a trans-critical carbon dioxide power generation cycle, a shift converter unit and cryogenic CO2 capturing system. ASU is used to produce high purity oxygen (99.99%) and nitrogen (99.99%). The specific power demand per flow rate of pure oxygen is 0.11 kWh/kg. Due to effective integration between two distillation columns in the ASU, the latent heat of the condenser in high pressure column is exchanged with reboiler of the low pressure column. The outlet LNG stream from ASU is utilized as cold source of the condenser in the trans-critical CO2 power cycle. The results show energy saving in the ASU and trans-critical power generation are 2301.6 kW and 14,217.6 kW respectively. The produced high purity gaseous oxygen is sent to the coal gasification unit to participate in the gasifier reactions. In this process, 99.83% of carbon dioxide with 99.80% purity, is captured and the required power is about 0.10 kWh/kg CO2.

Thermodynamic analysis and parametric optimization of CDTPC-ARC based on cascade use of waste heat of heavy-duty internal combustion engines (ICEs)

This paper proposes a novel carbon dioxide transcritical power cycle combined with an absorption refrigeration cycle (CDTPC-ARC) to recover waste heat from jacket water and exhaust gas of heavy-duty ICEs. Firstly, a detailed study is conducted to investigate the effects of key operating parameters on the performance of CDTPC-ARC. Secondly, a comparative study on the proposed CDTPC-ARC and the existing CDTPC-DT (consisting of CDTPC with regenerator and CDTPC without regenerator) under the same waste heat conditions shows that CDTPC-ARC can produce approximately constant net power output (Ẇnet) when Tcw=18–36°C, while CDTPC-DT can’t operate in the transcritical power cycle when Tcw⩾26.1°C. Finally, parametric optimization is performed to achieve the maximum Ẇnet of the combined cycles by using genetic algorithms (GA). Results show that CDTPC-ARC can produce more Ẇnet and have a larger recycle efficiency compared with CDTPC-DT when Tcw>20°C. When Tcw=25°C, Ẇnet,op of CDTPC-ARC is 14.6–20.2% higher than that of CDTPC-DT and the corresponding ηrec is 9.8–25.5% higher than that of CDTPC-DT. In addition, economic performance comparison between the two cycles is performed. The results show that CDTPC-ARC has better economic performance.

Exergoeconomic analysis and multi objective optimization of performance of a Carbon dioxide power cycle driven by geothermal energy with liquefied natural gas as its heat sink

In this study a transcritical Carbon dioxide power cycle has been coupled to a liquefied natural gas to work either as the cold source and to further enhance to generate electricity. The detailed thermodynamic analysis is performed in order to investigate the effect of key parameters on the cycle performance. Also, heat exchangers are measured to find the heat transfer surface area for economic evaluation. To investigate the aforementioned cycle and for optimization purposes, an exergoeconomic analysis is done to know the important components with respect to exergoeconomic criterion. The exergoeconomic analysis reveals that Carbon dioxide turbine and condenser have the highest rate of sum cost rate associated with capital investment and the cost of exergy destruction and special attention should be paid to these components. The parametric analysis shows that there is an optimum turbine inlet pressure which brings about the highest exergy efficiency and lowest product cost rate. Moreover, the condensate pressure has the highest effect on system exergy efficiency compared to others. With the help of multi-objective optimization, the cumulative effects of these variables are investigated on the system to maximize the exergetic efficiency and to minimize the product cost rate of the system. Results show that the system is capable of producing power with exergy efficiency and product cost rate equal to 20.5% and 263592.15$/year, respectively, according to technique for order of preference by similarity to ideal solution decision making technique. Also, the system exergy efficiency of 22.1% and 295001.26$/year product cost rate is achieved through linear programming techniques for multidimensional analysis of preference technique and 23.97% exergy efficiency and 370378.758$/year product cost rate is given with FUZZY decision making technique.

Investigation of novel integrated air separation processes, cold energy recovery of liquefied natural gas and carbon dioxide power cycle

Cryogenic air separation unit (ASU) consume high amount of energy to produce oxygen and nitrogen with high purity. Liquefied natural gas (LNG) regasification process provides low temperature cryogenic refrigeration source which can be used in ASU with high efficiency. In this study two cryogenic air separation processes for production of high purity nitrogen and oxygen with low energy consumption are proposed and analyzed. The first proposed process is a two-column configuration which power consumption per oxygen production is 16.6% lower, compared to a convectional cryogenic air separation process. In the second one, liquefied natural gas cold energy is used for pre-cooling the feed air. This greatly reduces the energy consumption of the compressors located before the columns by more than 55.6% compared to the first proposed process, without much change in purity of the products. In the second process, energy saving in the air separation unit and power generation cycle is 2715 kW and 17,810 kW respectively. Both processes use an integrated heat exchanger which is both condenser of the high pressure column and reboiler of the low pressure column. With this integration latent heat of the pure nitrogen and pure oxygen can be exchanged in a two-column process configuration. The second proposed process is a hybrid with a carbon dioxide trans-critical power cycle. The liquefied natural gas cold energy is used for cooling the power cycle condenser. Energy and exergy analysis are carried out on the air separation unit and power generation cycle.

Theoretical analysis of a transcritical power cycle for power generation from waste energy at low temperature heat source

The present paper reports the results obtained on a carbon dioxide transcritical power cycle using an energy and exergy analysis. The procedure consisted of modifying the inlet pressure to the turbine from 66bar, by means of the software HYSYS®, maintaining constant each evaluated turbine inlet temperature (60, 90, 120 and 150°C) until the net work was approximately zero. As a result, an increase up to 25% for the exergy efficiency, and up to 300% for the energy efficiency are obtained when the inlet temperature to the turbine is risen from 60 to 150°C. Consequently, the analysis shows the viability of implementing this process as alternative energy, because of the possibility to recovery energy from waste heat from industrial processes.

Low temperature heat source for power generation: Exhaustive analysis of a carbon dioxide transcritical power cycle

The main results of a theoretical work on the use of a low temperature heat source for power generation through a carbon dioxide transcritical power cycle are reported in this paper. The procedure for analyzing the behaviour of the proposed cycle consisted in modifying the input pressure to the turbine from 66 bar, maintained constant each evaluated temperature (60 °C, 90 °C, 120 °C and 150 °C) until the net work was approximately zero. As a result, the maximum exergy efficiency was 50%, while the energy efficiencies obtained were 9.8%, 7.3%, 4.9% and 2.4% and the net specific work was 18.2 kJ/kg, 12.8 kJ/kg, 7.8 kJ/kg and 3.5 kJ/kg, respectively. Furthermore, the effect of the addition of an internal heat exchanger, which obviously supposed an increase in the efficiency, was analyzed. The analysis of the proposed system shows the viability of implementing this type of process as an energy alternative and/or strengthener of non-conventional energy sources in non-provided zones, or for increasing the energy efficiency in the industry.

Analysis of a carbon dioxide transcritical power cycle using a low temperature source

A detailed analysis of a carbon dioxide transcritical power cycle using an industrial low-grade stream of process gases as its heat source is presented. The methodology is divided in four steps: energy analysis, exergy analysis, finite size thermodynamics and calculation of the heat exchangers’ surface. The results have been calculated for fixed temperature and mass flow rate of the heat source, fixed maximum and minimum temperatures in the cycle and a fixed sink temperature by varying the high pressure of the cycle and its net power output. The main results show the existence of an optimum high pressure for each of the four steps; in the first two steps, the optimum pressure maximises the thermal or exergetic efficiency while in the last two steps it minimises the product UA or the heat exchangers’ surface. These high pressures are very similar for the energy and exergy analyses. The last two steps also have nearly identical optimizing high pressures that are significantly lower that the ones for the first two steps. In addition, the results show that the augmentation of the net power output produced from the limited energy source has no influence on the results of the energy analysis, decreases the exergetic efficiency and increases the heat exchangers’ surface. Changing the net power output has no significant impact on the high pressures optimizing each of the four steps.

A comparative study of the carbon dioxide transcritical power cycle compared with an organic rankine cycle with R123 as working fluid in waste heat recovery

The organic rankine cycle (ORC) as a bottoming cycle 1 The expression “bottoming cycle” refers to the power cycle that uses waste industrial heat for power generation by supplementing heat from any fossil fuel. 1 to convert low-grade waste heat into useful work has been widely investigated for many years. The CO 2 transcritical power cycle, on the other hand, is scarcely treated in the open literature. A CO 2 transcritical power cycle (CO 2 TPC) shows a higher potential than an ORC when taking the behavior of the heat source and the heat transfer between heat source and working fluid in the main heat exchanger into account. This is mainly due to better temperature glide matching between heat source and working fluid. The CO 2 cycle also shows no pinch limitation in the heat exchanger. This study treats the performance of the CO 2 transcritical power cycle utilizing energy from low-grade waste heat to produce useful work in comparison to an ORC using R123 as working fluid. Due to the temperature gradients for the heat source and heat sink the thermodynamic mean temperature has been used as a reference temperature when comparing both cycles. The thermodynamic models have been developed in EES 2 EES – Engineering equation solver. The thermodynamic properties for carbon dioxide in EES are calculated by the fundamental equation of state developed by R. Span and W. Wagner, A new equation of state for carbon dioxide covering the fluid region form the triple-point temperature to 1100 K at pressures up to 800 MPa, J. Phys. Chem. Ref. Data, Vol. 25, No. 6, 1996. http://www.fchart.com/ees/ees.shtml. 2 The relative efficiencies have been calculated for both cycles. The results obtained show that when utilizing the low-grade waste heat with the same thermodynamic mean heat rejection temperature, a transcritical carbon dioxide power system gives a slightly higher power output than the organic rankine cycle.