• Energy Research

Abstract In arid areas, dry cooling technology is a preferable alternate of wet cooling mainly owing to the scarcity of abundant water supply. However, the supercritical CO2 power cycle still offers considerable thermal performance even at higher ambient temperature using dry cooling. The novelty of this work is the exhaustive designing of dry cooler for supercritical CO2 cycles (recompression and partial cooling) in concentrating solar power application. Prior to the design of tower, a preliminary analysis is conducted in achieving the optimum main compressor inlet temperature (33 °C-recompression and 40 °C-partial cooling) at which the cycle delivers the maximal efficiency. The comparison is performed at same higher and lower pressure and for the partial cooling, the intermediate pressure is optimized. At relatively higher compressor inlet temperatures (above 50 °C), the partial cooling achieves higher efficiency while at lower temperatures (30–49 °C), the recompression shows superior performance. An iterative nodal method is used for the air-cooled finned tube heat exchanger units that takes account of the dramatic variation in thermodynamic properties of CO2 with the bulk temperature. Kroger’s detailed methodology of designing dry cooler is adapted with the implementation of nodal approach for CO2 property variation. A dry cooling tower with 52.45 m height is essential for the recompression cycle, whereas the partial cooling requires two towers of the height of 35.4 m and 38.7 m. A thermal assessment is carried out on the dry cooler under various cycle fluid inlet temperatures and ambient temperatures. During hot and humid ambient conditions, lower compressor inlet temperatures (up to 53.1 °C) are obtained with the recompression cycle compared to partial cooling (up to 64.5 °C). In extreme climate condition of 50 °C air temperature, the recompression cycle provides superior thermal efficiency (46.5% against 45.5%). For future commercialization of dry cooled sCO2 power plant, the recompression cycle is preferred due to its superior performance and lower capital cost for cooling tower design and solar field. The work demonstrates the impact of dry cooling tower design strategy in the context of cycle thermal assessment under various working condition.

The thermodynamic performance of a two-stage vapor compression cascade refrigeration system using hydrocarbon refrigerants is studied extensively in this study. The selection of hydrocarbon refrigerants for the cascade refrigeration system is conducted based on the molecular weight, freezing point, vaporization, density, global warming potential and ozone depletion potential. In the lower temperature circuit, Trans-2-butane (T2BUTENE) is utilized while Toluene (toluene), Cyclopentane (CYCLOPEN) and Cis-2-butane (C2BUTENE) are used on the higher temperature circuit. The performance of the system is evaluated considering three major operating temperature such as evaporator temperature, condensation temperature of lower temperature circuit (LTC), and condensation temperature of higher temperature circuit (HTC). Furthermore, comparisons between the presented work and the previous established work are conducted to show the improved performance of cascade refrigeration utilizing the hydrocarbon refrigerants. The results from the simulations suggest that highest COP and exergy efficiency is achieved when Trans-2-butane is employed in lower temperature circuit while Toluene is implemented on the higher temperature circuit. The results also suggest that the highest exergy destruction occurs at the condenser and the lowest can occur either at the lower circuit expansion valve or the evaporator for different refrigerant pairs. In addition, utilizing hydrocarbon refrigerants on cascade refrigeration systems can achieve a minimum 7.21 % higher COP than the recently employed refrigerants in previously established published works.

The thermal performance of a supercritical CO2 (sCO2) recompression cycle is expressively influenced by main compressor inlet temperature. Design of the cooling system is imperative since the compressor inlet temperature substantially influence the system performance. Due to nonlinear variation of both thermal and transport properties of the CO2 under critical condition, the cooling tower design and selection for the sCO2 cycle power plant is quite different from the power plants with steam cycle. The present work comprehensively investigates the effect of cooling system design on the optimal cycle performance under different operating condition. An iterative section method is applied while designing and optimizing the air-cooled heat exchanger bundles inside the tower. Prior to the design of natural draft dry cooling tower (NDDCT), an optimal operating condition is rectified at which the cycle efficiency is maximal. The tower performance is investigated by demonstrating unit height heat rejection and average heat rejection by each heat exchanger bundle. A detailed economic analysis of NDDCT is performed which takes account of capital cost, maintenance cost, annual cost, and specific investment cost. The thermo-economic assessment of the NDDCT is conducted by the influence of sCO2 inlet temperature inside the tower and variation of ambient air.

Technologies for utilizing waste heat for power generation have attracted significant attention in recent years due to their potential to enhance energy efficiency and reduce greenhouse gas emissions. This research focuses on the comparative and optimization analysis of three supercritical carbon dioxide (sCO2) Rankine cycles (simple, cascade, and split) for gas turbine waste heat recuperation. The study begins with parametric analysis, investigating the significant effects of key variables, including turbine inlet temperature, condenser inlet temperature, and pinch point temperature, on the thermal performance of advanced sCO2 power cycles. To identify the most efficient cycle configuration, a multi-objective optimization approach is employed. This approach combines a Genetic Algorithm with machine learning regression models (Random Forest, XGBoost, Artificial Neural Network, Ridge Regression, and K-Nearest Neighbors) to predict cycle performance using a dataset extracted from cycle simulations. The decision-making process for determining the optimal cycle configuration is facilitated by the TOPSIS (technique for order of preference by similarity to the ideal solution) method. The study's major findings reveal that the split cycle outperforms the simple and cascade configurations in terms of power generation across various operating conditions. The optimized split cycle not only demonstrates superior power output but also exhibits enhanced net power output, heat recovery, system and exergy efficiency of 7.99 MW, 76.17 %, 26.86 % and 57.96 %, respectively, making it a promising choice for waste heat recovery applications. This research has the potential to contribute to the advancement and widespread adoption of waste heat recovery in energy technologies boosting system efficiency and economic feasibility. It provides a new perspective for future research, contributing to the improvement of energy generation infrastructure.

Supercritical CO2 (sCO2) stands out for concentrating solar power (CSP) due to its superior thermophysical and chemical properties, promising higher cycle efficiency compared to superheated or supercritical steam. Leveraging the waste heat from sCO2 cycles through the organic Rankine cycle (ORC) as a low-grade energy source enhances overall thermal efficiency. This research explores advanced sCO2 power cycles and introduces a novel approach by integrating machine learning and genetic algorithms for optimizing cycle performance. Utilizing a thermodynamic model-derived dataset, various machine learning algorithms, including Random Forest, XGBoost, and Artificial Neural Network are employed for prediction, evaluation and optimization. This innovative integration enables a comprehensive understanding of the complex dynamics of sCO2 power cycles. Subsequently, the study employs multi-objective optimization for the systematic evaluation and optimization of the combined power cycles, incorporating multiple bottoming cycles to maximize efficiency. The findings not only showcase the superiority of the unified sCO2ORC cycle but also emphasize the impact of integrating advanced computational methods in achieving optimal performance. The sCO2 cycle is explored in recompression, partial cooling, and main compression intercooling configurations. Recompression cycles utilize a single cooling system, while partial cooling and main compression intercooling layouts integrate a pair of ORCs at two precoolers. The ORC cycle enhances the recompression cycle through heat recuperation, extracting enhanced power originating from the bottom cycle. Critical parameters are taken into consideration to carry out optimization and performance evaluations. such as cycle temperatures, recuperator effectiveness, bottoming cycle pressure ratio, and condensation temperature. Results indicate that the combined partial cooling sCO2ORC cycle yields the optimal net output power of 7994.541 kW and thermal efficiency of 53.365 %. The findings highlight the capacity to propel and promote the utilization of waste heat recovery in energy technologies through enhanced and optimized power cycle designs, contributing to their advancement and widespread adoption.

Researchers and power plant engineers have all taken an interest in Concentrating Solar Power (CSP) of its capacity to generate large amounts of energy while overcoming the sporadic nature of solar energy. Using CSP as a renewable energy source increases the electrical grid's reliability and has a good impact on the environment and human health. CSP storing energy is a versatile renewable resource that can respond swiftly to demand and system operator demands. Thermal Energy Storage (TES), in combination with CSP, enables power stations to store solar energy and then redistribute electricity as required to adjust for fluctuations in renewable energy output. In this article, the development and potential prospects of different CSP technologies are reviewed and compared with various TES systems. Energy systems benefit significantly from the addition of TES, which not only removes inconsistencies in supply and order but also improves the efficiency and dependability of such systems. Future CSP researchers will benefit from this paper's thorough overview of the technology, its potential prospect, and its research status. The fundamentals of various technologies on energy storage and the computation of their storage capabilities are enlightening. Water tanks, underground, and packed-bed techniques of heat storage are briefly discussed. Given the finite availability and depletion of fossil fuels, as well as the foreign currency spent on imported oil, CSP technology can provide a solution for a future energy source that is both sustainable and affordable. It has been determined if CSP technology can be utilized in developing countries and, if so, which CSP plant would be most suitable. Future policies and instruments to aid in the promotion, development, and long-term usage of CSP technologies may be developed based on the findings of this study.

Owing to water deficit and environmental concerns, the compatibility of dry cooling technology with supercritical CO2 (sCO2) power cycle in concentrated solar power (CSP) offers superior thermal performance even at higher climate temperatures. One of the key challenges associated with sCO2 power cycles is the cooling of working fluid near the critical temperature and pressure. The nonlinear and sharp variation of sCO2 properties with bulk temperature poses a unique heat exchanger design problem not encountered with the constant property fluids. A reliable and well-designed cooling system is required for the efficient and smooth operation of a thermal power plant. In arid areas, natural draft dry cooling towers (NDDCT) are preferred over the wet coolers due to the insufficient water supply, environmental concern and the high maintenance and operation cost related to wet cooling.At the initial stage of this project, a validated MATLAB code has been developed to design NDCCT for a 25 MW solar thermal power plant. Both direct and indirect layout of cooling systems have been designed for the power plant and their performance has been compared. For the direct cooling system, the air-cooled heat exchanger unit is designed whereas for the indirect cooling system, optimum size of the shell and tube heat exchanger is selected addressing the retainment of shell side heat transfer correction factor within a reasonable limit. The nonlinear property variation of sCO2 near the critical condition is reasonably well captured by the present code with the nodal approach applied in both shell and tube and air-cooled heat exchanger units. The draft equation of the tower is solved by including various air-flow resistances at different parts of the tower. Later, the NDDCT has been coupled with the sCO2 recompression and partial cooling cycle for concentrated solar power application. Prior to the design of NDDCT, preliminary analysis of the power cycles (recompression and partial cooling cycle) is carried out in order to achieve the optimum main compressor inlet temperature at which the cycle delivers the highest thermal efficiency. A thermal performance assessment has been carried out on the NDDCTs under various cycle fluid inlet temperatures and ambient temperatures. During the high ambient temperature period, the lower compressor inlet temperature is achieved with the recompression cycle.The thermal assessment of the recompression cycle has been investigated by calculating several performance indicators (cycle efficiency, exergy efficiency, cooling efficiency, and irreversibility analysis). A comprehensive cost analysis of NDDCT is also presented. In addition, the seasonal effect on the performance of a dry-cooled sCO2 recompression cycle has been performed in Alice Spring, NT, Australia by using the daily meteorological data. The seasonal and annual variation of the plant performance is presented by evaluating various performance indicators using the historical temperature data. Finally, the solar field (heliostat filed and central receiver) is coupled with the power block to investigate the dynamic attributes of the dry cooled sCO2 recompression cycle in terms of net power generation at different climate conditions. The supplementary bypass arrangement prior to the dry cooling tower ensures the system operating at the design point compressor inlet temperature in the occasion of low ambient temperatures. The matured molten salt thermal energy storage is equipped with the solar tower to supplement dispatchable energy production during nighttime. The influence of the variant solar irradiation and fluctuation of air temperature on the cooling potential of NDDCT and net power generation has been investigated. The major findings of the thesis are summarised below.(i) A one-dimensional validated MATLAB code is developed to design a NDDCT with detailed specification of finned tube heat exchanger bundles for a 25 MW power plant. The tower height, the tower inlet, and outlet diameters and the number of heat exchanger bundles are evaluated after accomplishing the design requirements. The cooling tower performance is evaluated in isolation of the power block.(ii) Both direct and indirect cooling system configurations are designed for the sCO2 power plant and their performance is compared. The nonlinear property variation of sCO2 near the critical condition is reasonably well captured by the present code by implementing the nodal approach in both shell and tube and air-cooled heat exchanger units. During the high ambient temperature period, the direct dry cooling system shows superior cooling performance in terms of lower sCO2 outlet temperature compared to the indirect cooling system. (iii) The cooling tower model has been coupled with the sCO2 power block and a comprehensive thermal assessment of dry cooled sCO2 power cycles (recompression and partial cooling cycle) has been performed. Prior to the design of the tower, a preliminary analysis is conducted in achieving the optimum main compressor inlet temperature at which the cycle delivers the maximal efficiency. The work demonstrates the impact of dry cooling tower design strategy in the context of cycle thermal assessment under various working conditions.(iv) The effect of cooling system design on the performance of the sCO2 recompression cycle is conducted with an optimized geometry of the cooling tower. The tower performance for various capacities of the power plant is investigated by demonstrating unit height heat rejection and average heat rejection by each heat exchanger bundle. A detailed economic analysis of NDDCT is performed which takes account of capital cost, maintenance cost, annual cost, and specific investment cost.(v) The seasonal effect on the off-design performance of NDDCT has been performed in Alice Spring, NT, Australia by using the daily meteorological data. The weekly and monthly seasonal variations of the plant performance in summer, autumn, winter, and spring are performed based on the mean maximum and minimum temperature data. The fluctuation of heat input to the cycle, heat rejected by the cycle and air mass flow rate in NDDCT are also demonstrated.(vi) Finally, the dynamic attributes of a dry-cooled novel sCO2 cycle have been demonstrated with additional bypass equipped with a central receiver and thermal energy storage. Based on the annual climate temperature profile, two sets of ambient air temperatures are selected to design the cooling system. This requires the rectification of two sets of design point main compressor inlet temperature/tower exit temperature at optimum turbine exhaust pressure in advance of the integration and design of the dry cooling tower. The key parameters of the solar system (cold tank temperature and molten salt split ratio) and power block (MCIT, pressure ratio, cycle mass, and bypass fraction) are optimized to maximize the power generation at any circumstances. The year-round plant performance supplemented with thermal energy storage for both cases is assessed with the historical air temperature data.The ultimate goal of this thesis is to comprehensively investigate the influence of dry cooling technology on the context of thermal performance of the sCO2 power cycle in CSP application under variant climate conditions. The implementation of dry cooling is imperative under prescribed weather conditions. Unlike the steam cycle, the cooling system design requires special consideration since the working fluid is sCO2. For long-term future employment of sCO2 power plant on a large scale, the design methodology and working mechanism of dry cooling are extremely helpful to power plant engineers, especially in arid areas. ​​​​​​​

Utilizing waste heat to drive thermodynamic systems is imperative for improving energy efficiency, thereby improving sustainability. A combined cooling and power systems (CCP) utilizes heat from a temperature source to deliver both power and cooling. However, CCP systems utilizing LNG cold energy suffers from low second law efficiency due to significant temperature differences. To address this, an "Advanced Power and Cooling with LNG Utilization (ACPLU)" system is proposed, integrating a cascaded transcritical carbon dioxide (TCO2)-LNG cycle with an Organic Rankine cycle (ORC) for improved power generation and an absorption refrigeration system (ARS) for simultaneous cooling. This study evaluates the second law efficiency, net work output, and exergy destruction performance through a sensitivity analysis, optimizing variables such as heat source temperature, superheater temperature difference, ORC and CO2 turbine inlet and condenser pressures, evaporator temperature, and pinch point temperatures of heat exchangers and generator. Compared to previous studies on CCP systems, the ACPLU shows a superior performance, with a second law efficiency of 27.3 % and a net work output of 11.76 MW. Cyclopentane as an ORC working fluid resulted in the highest second law efficiency of 29.06 % and net work output of 12.27 MW. Parametric analysis suggested that heat source temperature significantly impacts net power output. The exergy analysis concluded that a high-pressure ratio and good thermal match between the heat exchangers enhance overall performance. Utilizing artificial neural network (ANN) to produce a multiple-input-multiple-output (MIMO) objective function and performing multi-objective optimization (MOO) using genetic algorithm (GA), an improved second law efficiency and net power output by 28.11 % and 14.16 MW respectively, with pentane as the working fluid, is demonstrated. An average cost rate of 9.121 $/GJ was observed through a thermo-economic analysis. The ACPLU system is promising for medium temperature waste heat recovery, such as, pharmaceutical manufacturing plants.

Concentrating solar power (CSP) applications can benefit from the superior thermophysical and chemical properties of supercritical CO2 (sCO2), which offers greater cycle efficiency in contrast to supercritical or superheated steam cycles. The transcritical CO2 (TCO2 cycle) excels in low-grade waste heat recovery (WHR), while the organic Rankine cycle (ORC) is also a potential option for WHR from sCO2 cycles. This study focuses on the thermodynamic assessment and optimization of combined power cycles, incorporating multiple bottoming cycles, to enhance overall thermal efficiency. The configurations employed include recompression, partial cooling, and main compression intercooling within the sCO2 cycle. Waste heat recovery is achieved using both TCO2 and ORC. The parametric analysis explores key variables such as turbine inlet temperature, main compressor inlet temperature, recuperator effectiveness, pressure ratios in the bottoming cycles, and condensation temperatures of the bottoming cycles. Furthermore, this research conducts performance analysis of the proposed combined cycle configurations. The results underscore the substantial enhancement of combined cycle performance with regard to thermal and exergy efficiency. A rise in the highest temperature that can occur in the sCO2 cycle leads to improved thermal efficiency. The Main Compressor Inlet Temperature (MCIT) exhibits an efficiency increase of up to 40°C–45°C, attributed to pseudocritical effects, followed by a decline. Among the various configurations, the combined partial cooling cycle generates the highest net output power. The study finds that the combined main compression model consistently outperforms other layouts, yielding a 2-2.5% increase in efficiency. Optimization of the intermediate pressure value for the partial cooling (PC) and main compression (MC) cycles at 10 MPa further contributes to enhanced performance. This study not only presents the optimized operating conditions for each combined cycle but also offers a comparative analysis of these models under different boundary conditions. The findings from this research significantly advance the utilization of waste heat and the development of clean power generation.