Search Results (39)

This investigation introduces a conceptual configuration for an innovative combined cooling and power (CCP) arrangement that harnesses waste thermal energy from a supercritical carbon dioxide (sCO₂) power loop through integration with a cascading transcritical CO₂ cycle coupled with a lithium bromide–water vapor absorption chiller (tCO₂/LiBr-H₂O VAR). A systematic comparative assessment of thermodynamic behavior and exergoeconomic characteristics was executed across three configurations: the newly proposed CCP scheme, a hybrid sCO₂/tCO₂ power system, and a conventional standalone sCO₂ engine. Parametric sensitivity analyses were conducted to evaluate how various operating parameters influence overall system effectiveness, while particle swarm optimization (PSO) was employed to determine the optimal exergetic efficiency peaks and minimum unit product costs for each architecture. Findings demonstrate that the proposed CCP topology delivers exergy efficiency enhancements of 8.46% and 1.65% relative to the standalone sCO₂ configuration and the combined sCO₂/tCO₂ arrangement, respectively. Correspondingly, reductions in total product unit costs reach 2.80% and 0.80% for the same comparisons. These outcomes confirm that employing a cascading tCO₂/LiBr-H₂O VAR subsystem as the bottoming cycle represents a compelling solution for cooling and power production. Full article

This study presents a comparative thermodynamic assessment of four widely used CO₂ refrigeration configurations, namely, the basic cycle (BC), internal heat exchanger cycle (IHEX), mechanical subcooling cycle (MSC), and ejector cooling cycle (ECS), operating under both subcritical and transcritical conditions. The investigated systems were analyzed using validated numerical models developed in the Engineering Equation Solver (EES) under evaporating temperatures ranging from −30 °C to +5 °C and gas cooler temperatures ranging from 30 °C to 50 °C. For each operating condition, the refrigeration cycles were thermodynamically optimized in order to maximize the coefficient of performance (COP). The results indicate that an increasing gas cooler temperature significantly reduces the COP of all investigated systems, whereas an increasing evaporating temperature improves cycle performance. Among the investigated configurations, the MSC system exhibited the highest thermodynamic performance improvement, particularly under severe transcritical operating conditions characterized by high gas cooler temperatures and low evaporating temperatures. The ECS configuration also provided considerable performance enhancement by reducing throttling-related thermodynamic losses and compressor pressure ratio. In contrast, the IHEX configuration yielded comparatively moderate but relatively stable performance improvement with lower system complexity. In addition to the thermodynamic comparison, a simplified engineering-oriented practical assessment framework based on a relative cost index (RCI) was introduced to comparatively evaluate implementation complexity, control requirements, maintenance considerations, and relative investment burden of the investigated systems. The results indicate that, although the MSC configuration provides the highest thermodynamic performance, it is also associated with the highest implementation complexity and relative investment requirement, whereas the IHEX configuration offers a simpler and lower-cost alternative with moderate performance enhancement. The present study provides engineering-oriented comparative guidance regarding the thermodynamic performance, practical applicability, and operational suitability of advanced CO₂ refrigeration systems under varying climatic and operational conditions. Full article

Refrigeration and air-conditioning systems are vital to the global economy but contribute significantly to greenhouse gas emissions by using high-global-warming potential synthetic refrigerants. As regulatory frameworks like the Montreal Protocol, the Kigali Amendment, and the EU’s F-gas Regulations tighten, the industry faces a mandatory transition toward environmentally benign alternatives. This perspective paper evaluates the technological and environmental implications of replacing synthetic fluids with natural refrigerants, specifically ammonia, carbon dioxide, and hydrocarbons. A comparative assessment reveals that natural refrigerants offer superior thermodynamic efficiency, zero ozone depletion potential, and ultra-low global warming potential. While technologies like transcritical CO₂ and low-charge ammonia systems may involve higher initial capital costs, they increasingly achieve life cycle cost parity through improved energy performance and regulatory stability. The analysis further explores advanced cycle configurations, such as ejectors and expanders, which mitigate efficiency losses. The transition to natural refrigerants is presented as a technologically feasible and environmentally friendly strategy to mitigate the risk that rising cooling demands further accelerate climate change. Ultimately, natural refrigerants are expected to become the default global standard within the shortest feasible timeframe, with policy, industry, and research aligned to support and accelerate this transition. Full article

Aiming to improve cooling and dehumidification performance in air conditioning systems and to meet the trend toward environmentally friendly refrigerants, this study proposes a coupled system that combines a CO₂ transcritical refrigeration cycle (CTRC) with a liquid desiccant dehumidification cycle. The system takes advantage of high-grade waste heat from the exothermic side of the CTRC to drive the regenerating process of the liquid desiccant dehumidification. A cooling evaporator is adopted to cool indoor air, while another evaporator (i.e., Evaporator II) is utilized to cool the concentrated solution, improving dehumidification capacity and enabling independent control of sensible and latent heat loads. Through thermodynamic modeling and the exergy analysis model, a mathematical model of the system is developed to examine how key parameters (such discharge pressure and the CO₂ mass flow rate ratio in Evaporator II (λ)) affect performance and to analyze exergy loss features. Results show that the system’s coefficient of performance (COP) and dehumidification coefficient of performance (COP_deh) initially rise and then fall with increasing CTRC discharge pressure, achieving an optimal pressure of around 10,500 kPa (COP up to 4.32) under a specific working condition, surpassing those of standalone CTRC systems. Properly increasing λ enhances dehumidification capacity and energy efficiency, with a low specific dehumidification energy (SDE) of 0.2033 kWh/kg, indicating high economic efficiency. Most exergy losses occur in the CO₂-solution heat exchanger and dehumidifier (over 60% of total losses). The system’s maximum exergy efficiency reaches 12.4%, leaving room for further improvements. This coupled system offers an efficient, eco-friendly way for air conditioning in high-humidity environments, combining cooling and dehumidification with the potential for energy recovery. Full article

This study introduces a novel transcritical CO₂ pumpless Rankine power generation cycle based on the thermal compression concept, utilizing low-temperature renewable sources. The investigated theoretical system consists of a 5 kW_e unit incorporating the aforementioned working cycle coupled with a 50 m² evacuated tube solar collector arrangement installed on the rooftop of a multifamily house in Athens, Greece. The proposed innovative configuration is parametrically analyzed for different hot water inlet temperature levels ranging from 70 to 120 °C and its efficiency is compared to the typical Organic Rankine Cycle (ORC) topology employing different conventional refrigerants. The energetic assessment is made using validated mathematical models developed in MATLAB integrating the CoolProp library. The derived results show that the investigated topology increases the performance figures compared to the baseline system for all the examined refrigerants, leading up to over 15% thermal efficiency enhancement for operation under low heat source temperatures. Finally, the year-round operation of the proposed system generates up to 5221 kWh/year for the building. Full article

As a natural refrigerant, CO₂ shows significant potential in sustainable thermal engineering due to its environmental safety and economic viability. While the transcritical CO₂ cycle demonstrates strong performance in heating, low-temperature applications, and integration with renewable energy sources, its widespread adoption is hindered by key challenges at the application level. These include: high sensitivity of system efficiency to operating conditions, which creates an “efficiency hump” and narrows the optimal operating window; increased component costs and technical challenges for key devices such as multi-channel valves due to high-pressure requirements; and complex system control with limited intelligent solutions currently integrated. Despite these challenges, the transcritical CO₂ cycle holds unique value in enabling synergistic energy conversion. Its ability to efficiently match and cascade different energy grades makes it particularly suitable for data center cooling, industrial combined cooling and heating, and solar–thermal hybrid systems, positioning it as an indispensable technology in future low-carbon energy systems. To fully realize its potential, development efforts must focus on high-value applications and key technological breakthroughs. Priority should be given to demonstrating its use in fields where it holds a distinct advantage, such as low-temperature refrigeration and high-temperature industrial heat pumps, to establish commercially viable models. Concurrently, core technologies—including adaptive intelligent control algorithms, high-efficiency expanders, and cost-effective pressure-resistant components—must be advanced. Supportive policies, encompassing energy efficiency standards, safety regulations, and fiscal incentives, will be essential to facilitate the transition from demonstration projects to widespread industrial adoption. Full article

This paper presents a novel, integrated supercritical CO₂ system for shale gas development, comprising a supercritical CO₂ shale gas extraction system, a gas turbine system, a supercritical CO₂ power generation system, and a transcritical CO₂ refrigeration system. A comprehensive thermodynamic and economic analysis is conducted for this integrated energy development system. To enhance system performance across multiple dimensions, three objective functions are proposed for optimization: exergy efficiency, levelized energy cost (LEC), and heat transfer area per unit power output (APR). First, the effects of key operating parameters—including the gas turbine pressure ratio, gas turbine inlet temperature, supercritical CO₂ pressure ratio, the temperature difference between flue gas and the supercritical CO₂ top cycle, and the temperature difference between flue gas and the supercritical CO₂ bottom cycle—on system performance were analyzed through parametric studies. Next, the optimal system parameters were determined using a multi-objective optimization method based on a genetic algorithm. The optimization results reveal that, when exergy efficiency and LEC are used as dual-objective functions, the system achieves an optimal exergy efficiency of 60.5% and an LEC of 6.3 cents/(kW·h). Furthermore, when exergy efficiency, APR, and LEC are considered as objective functions, the system attains an optimal exergy efficiency of 59.5%, an APR of 0.21 m²/kW, and an LEC of 6.3 cents/(kW·h). The compound shale gas development system proposed in this paper demonstrates excellent economic viability, environmental sustainability and operational efficiency. The research outcomes offer an innovative solution for the development of shale gas and contribute to the advancement of research on new energy systems. Full article

In recent years, advanced CO₂ cycles, including supercritical CO₂ power cycles, transcritical CO₂ power cycles and refrigeration cycles, have demonstrated significant potential for application across a broad spectrum of energy conversion processes, owing to their high efficiency and compact components that are environmentally benign and non-polluting. This study presents a comprehensive review of the dynamic performance and control strategies of these advanced CO₂ cycles. It details the selection of system configurations and various control strategies, detailing the principles behind different control strategies, their applicable scopes, and their respective advantages. Furthermore, this study conducts a comparison between the joint control strategy and single control strategies for CO₂ cycles, demonstrating the superiority of the joint control strategy in CO₂ cycles. It then delves into the potential of novel control technologies for CO₂ cycles, using model-based control technology powered by artificial intelligence as a case study. This study also offers an extensive overview of control theory, methodology, scope of application, and the pros and cons of various control strategies, with examples including extreme value-seeking control, model predictive control (MPC) based on an artificial neural network model, and MPC based on particle swarm optimization. Finally, it explores the application of AI-controlled CO₂ cycles in new energy vehicles, solar power generation, aerospace, and other fields. It also provides an outlook on the development direction of CO₂ cycle control strategies in light of the evolving trends in the energy sector and advancements in AI methodologies. Full article

This review focuses on the advancements of CO₂ transcritical cycle technology in building indoor environmental regulation, particularly in combined heating and cooling applications. The paper highlights the energy efficiency and environmental benefits of CO₂ as a natural refrigerant, which has zero ozone depletion potential (ODP) and very low global warming potential (GWP). It provides a comprehensive overview of recent optimization strategies, including distributed compression, the integration of ejectors and expanders, and the design improvements of key components such as gas coolers, compressors, and throttling valves. Through optimization strategies such as dual-system cycles, this technology can achieve a COP improvement of 15.3–46.96% in heating scenarios; meanwhile, with the help of distributed compression technology, its cooling capacity can be enhanced by up to 26.5%. The review also examines various operating conditions such as discharge pressure and subcooling, which significantly affect system performance. The paper concludes by identifying the current challenges in the application of CO₂ systems, such as high initial costs and system stability under extreme conditions, and suggests future research directions to overcome these limitations and improve the practical application of CO₂ transcritical cycles in the building industry. Overall, it is concluded that the development of expander-compressors holds great potential for achieving better performance and represents a promising direction for future advancements in this field. Full article

Heat pumps are recognized as a key tool in the energy transition toward a carbon-neutral society, enabling the electrification of the heating sector at least for low- and medium-temperature heat demands. In recent years, natural refrigerants have been reconsidered due to their low environmental impact: among them, CO₂ is a safe option without an impact on the ozone layer and low global warming potential compared to synthetic fluids. However, as a consequence of its thermophysical properties, its thermodynamic cycle is transcritical and is particularly suitable for specific end-user temperature profiles. This paper analyzes in a systematic and thorough way the most significant modifications to the reference cycle that have been proposed in the literature to improve the performance, finding how the optimal configurations change with a change in the rated operating conditions (inlet temperature and temperature glide of the heat demand, and ambient temperature). Exergy analysis explains why there is an optimal gas cooler pressure and why its trend with the average temperature is split into two distinct regions, clearly recognizable in all cycle layouts. The maximum coefficient of performance (COP) of the reference cycle varies in the 1.52–3.74 range, with a second-law efficiency of 6.4–36.1%, for an optimal gas cooler pressure of up to 15.45 MPa, depending on the ambient temperature and end-user temperature profile. The most effective modification is the cycle with an ejector and internal heat exchanger, which raises the COP to 1.84–4.40 (second-law efficiency 8.7–45.56%). The presented results provide an extensive guide to understanding the behavior of a transcritical CO₂ cycle and predict its performance in heat pump applications. Full article

Due to its environmental benefits, CO₂ shows great potential in refrigeration systems. However, a basic CO₂ transcritical (BCT) refrigeration system used for airconditioning in buildings might generate massive indirect carbon emissions for its low COP. In this study, a novel CO₂ transcritical/two-stage absorption (CTTA) hybrid refrigeration system is broadly investigated, and both energy efficiency and life cycle climate performance (LCCP) are specifically engaged. The theoretical model shows that optimal parameters for the generator inlet temperature (T_G2), intermediate temperature (T_m), and discharge pressure (P_c), exist to achieve maximum COP_tol. Using the LCCP method, the carbon emissions of the CTTA system are compared to six typical refrigeration systems by using refrigerants, including R134a, R1234yf and R1234ze(E) etc. The LCCP value of the CTTA system is 3768 kg CO_2e/kW, which is 53.6% less than the BCT system and equivalent to the R134a system. Moreover, its LCCP value could be 3.4% less than the R1234ze(E) system if the COP of the CO₂ subsystem is further improved. In summary, the CTTA system achieves ultra-low carbon emissions, which provides a potential alternative to air conditioning systems in buildings that can be considered alongside R1234yf and R1234ze(E) systems. Full article

The negative impact of actual refrigerants on the environment urged conduct to search for new refrigerants with low global warming potential (GWP). CO₂ is one of the promising alternatives; however, its thermodynamic properties impose a transcritical cycle that has a low energy efficiency. In the present study, a simulation program was developed to investigate the performances of an ejector refrigeration cycle working with three CO₂-based mixtures: (CO₂+R290), (CO₂+R1234yf), and (CO₂+R600a) according to subcritical mode and (CO₂+R116) under transcritical mode. The addition of other pure compounds to CO₂ displaces the critical point and modifies the phase equilibrium lines resulting in a reduction in operating pressures and an increase in the energy efficiency of the refrigeration cycle. Simulation results showed that the Suction Nozzle Pressure Drop (SNPD) has a significant impact on the performance of the cycle and has no effect on the entrainment ratio of the ejector. Moreover, it was found that there is an optimal SNPD which gives maximum COP and pressure recovery. It was also noticed that the maximum performance of the refrigeration cycle with a subcritical ejector depends on the evaporation and the condenser temperatures. Full article

In order to achieve efficient utilization of solar energy resources, this study combines the trans-critical organic Rankine cycle (ORC) power cycle (TORC) with the trans-critical CO₂ refrigeration cycle (TCO₂). Additionally, a comprehensive three-level index decision evaluation system is developed based on system safety and environmental protection, thermodynamics, and techno-economic performance. The evaluation focuses on typical medium- and high-temperature solar energy applications and considers six organic working gases. The evaluation results demonstrate that the R600 + CO₂ solution outperformed the others. This solution achieved a maximum net output power (P_net) of 1531.31 kW and 2306.43 kW, a maximum coefficient of performance (COP) of 3.16, a predicted payback period of 2.651 years and 2.033 years, and a benefit–investment ratio of 4.533 and 5.773. Full article

To reduce CO${}_2$ emissions in the industrial sector, high-temperature heat pumps are a key technology. This work presents an approach to design such an industrial heat pump system capable of supplying 200 ${}^{°}$C sink temperature and a capacity of approximately 1 MW. Today’s market-available heat pumps using displacement compressors are not suitable for reaching that high sink temperatures as they need lubricating oil, which is not temperature resistant enough. As a consequence, in this study a transcritical heat pump cycle using a two-stage oil-free radial compressor is investigated. Based on preliminary studies, R1233zd(E) is chosen as a refrigerant. The procedure couples 1D thermodynamic cycle simulations with a radial compressor mean-line design model. A preliminary geometry for a compressor with and without inlet guide vanes is presented, and compressor maps including the compressors behaviour in off-design are calculated. The compressor design is then imported into a 1D simulation to analysis the performance of the heat pump in the whole operating range. In the analysis, the application of a fixed inlet is evaluated, and an improvement of approximately 21% and 16% of the isentropic efficiency is achieved. The thermodynamic simulations showed a maximum COP of approximately 2.8 and a possible operating range of 0.5 to 1.3 MW thermal power. Furthermore, a techno-economical analysis by means of a deep-fryer use case showed reasonable payback times of between 2 and 10 years, depending on the electricity to gas price ratio. Full article

Refrigerant substitution is an urgent need in the context of reducing carbon emissions and slowing global warming. CO₂ is now being proposed as a promising solution based on its excellent properties and system performance, especially in low-temperature environments. This paper presents an overview of recent advances in system configuration and operation characteristics to improve the performance of transcritical CO₂ heat pump and refrigeration systems. The paper first introduces the basic research background, system cycle, and thermodynamic characteristics. Secondly, CO₂ cycle improvements with single modifications and modification combinations are reviewed. Then, some important operation characteristics and control methods are discussed. Additionally, the paper provides a detailed description of the development of transcritical CO₂ heat pump and refrigeration systems in the vehicle field. At the end of this review, conclusions and opportunities for future work in this field are presented. Full article