Review of experimental research on supercritical and transcritical thermodynamic cycles designed for heat recovery application

Supercritical operation is considered a main technique to achieve higher cycle efficiency in various thermodynamic systems. The present paper is a review of experimental investigations on supercritical operation considering both heat-to-upgraded heat and heat-to-power systems. Experimental works are reported and subsequently analyzed. Main findings can be summarized as: steam Rankine cycles does not show much studies in the literature, transcritical organic Rankine cycles are intensely investigated and few plants are already online, carbon dioxide is considered as a promising fluid for closed Brayton and Rankine cycles but its unique properties call for a new thinking in designing cycle components. Transcritical heat pumps are extensively used in domestic and industrial applications, but supercritical heat pumps with a working fluid other than CO2 are scarce. To increase the adoption rate of supercritical thermodynamic systems further research is needed on the heat transfer behavior and the optimal design of compressors and expanders with special attention to the mechanical integrity

Exergetic and Economic Evaluation of a Transcritical Heat-Driven Compression Refrigeration System with CO2 as the Working Fluid under Hot Climatic Conditions

The purpose of this research is to evaluate a transcritical heat-driven compression refrigeration machine with CO2 as the working fluid from thermodynamic and economic viewpoints. Particular attention was paid to air-conditioning applications under hot climatic conditions. The system was simulated by Aspen HYSYS® (AspenTech, Bedford, MA, USA) and optimized by automation based on a genetic algorithm for achieving the highest exergetic efficiency. In the case of producing only refrigeration, the scenario with the ambient temperature of 35 °C and the evaporation temperature of 5 °C showed the best performance with 4.7% exergetic efficiency, while the exergetic efficiency can be improved to 22% by operating the system at the ambient temperature of 45 °C and the evaporation temperature of 5 °C if the available heating capacity within the gas cooler is utilized (cogeneration operation conditions). Besides, an economic analysis based on the total revenue requirement method was given in detail

Experimental determination of the optimum working conditions of a transcritical CO2 refrigeration plant with integrated mechanical subcooling

Subcooling methods for transcritical CO2 plants are being studied in order to improve their behavior. Among them, the Integrated Mechanical Subcooling system is one of the most promising owing that performs with high efficiency and it is a total-CO2 system. This work presents the experimental determination of the optimum working conditions of a transcritical CO2 plant working with an integrated mechanical subcooling system. The plant was tested at different pressure and subcooling conditions in order to optimize the COP of the plant and determine the optimal conditions for three ambient temperatures 25.0 °C, 30.4 °C and 35.1 °C and evaporation levels between −15.6 °C and −4.1 °C. Optimum operating conditions were determined and two correlations are proposed to determine the optimal pressure and subcooling as function the gas-cooler outlet temperature and the evaporation level

Improvements in CO2 Booster Architectures with Different Economizer Arrangements.

CO2 transcritical booster architectures are widely analyzed to be applied in centralized commercial refrigeration plants in consonance with the irrevocable phase-out of HFCs. Most of these analyses show the limitations of CO2 cycles in terms of energy e ciency, especially in warm countries. From the literature, several improvements have been proposed to raise the booster e ciency in high ambient temperatures. The use of economizers is an interesting technique to reduce the temperature after the gas cooler and to improve the energy e ciency of transcritical CO2 cycles. The economizer cools down the high pressure’s line of CO2 by evaporating the same refrigerant extracted from another point of the facility. Depending on the extraction point, some configurations are possible. In this work, di erent booster architectures with economizers have been analyzed and compared. From the results, the combination of the economizer with the additional compressor allows obtaining energy savings of up to 8.5% in warm countries and up to 4% in cold countries with regard to the flash-by-pass arrangement and reduce the volumetric displacement required of the MT compressors by up to 37%

Transcritical Carbon Dioxide Charge-Discharge Energy Storage with Integration of Solar Energy

New and improved energy storage technologies are required to overcome non-dispatchability, which is the main challenge for the successful integration of large shares of renewable energy within energy supply systems. Energy storage is proposed to tackle daily variations on the demand side, i.e., storing low-price energy during off-peak or valley periods for utilization during peak periods. Regarding electrical energy storage, several technologies are available with different potentials for scalability, density, and cost. A recent approach for grid-scale applications is based on transcritical carbon dioxide charge and discharge cycles in combination with thermal energy storage systems. This alternative to pumped-hydro and compressed air energy storage has been discussed in scientific literature, where different configurations have been proposed and their efficiency and costs calculated. The potential of the concept has been demonstrated to be an economical alternative, including hybrid concepts with solar thermal storage. Even at low temperatures, the addition of solar energy has proved to be cost effective. This paper explores the effect of introducing solar-based high temperature heat on the performance of different configurations of “Transcritical carbon dioxide ‒ thermal energy storage system” cycles. A base-cycle with 8-hour discharge time is compared with different layouts. Discussions include details on the models, parametric analyses -including solar technology alternatives-, and simulation results. Round trip efficiency of the base case, without solar support and at pressure ratio of 9.4, is 52%. When solar input is considered, the efficiency is above 60%, increasing the turbine inlet temperature to 950 K. Estimated levelized cost of electricity values are in the range of pumped hydro and compressed air energy storage, 90-140 USD/MWh in agreement with other works on this thermal storage technology. The global analysis shows clear advantages for advancing in the study and definition of this technology for exploitation of synergies at different power ranges, integrated with mid/high temperature solar power plants and with smaller-scale renewable installations.Unión Europea. Fondo Europeo de Desarrollo Regional SOE1 / P3 / P0429E

Performance of a transcritical carbon dioxide heat pump for simultaneous refrigeration and water heating : a thesis presented in partial fulfilment of the requirements for the degree of Master of Technology at Massey University

Many industrial processes require both refrigeration to less than 0°C and water heating to greater than 60°C. Traditional independent refrigeration and boiler systems have relatively poor energy efficiency, whilst conventional heat pumps can provide both cooling and heating but are limited in terms of the temperature lift that can be achieved. A novel heat pump using CO 2 as the refrigerant in a transcritical cycle has been proposed as a new technology that can overcome these disadvantages. The use of CO 2 as a refrigerant has many advantages. It is environmentally benign, safe, and has good thermodynamic properties, especially compared with fluorocarbons. The transcritical cycle involves evaporation of CO 2 at constant temperature and pressure below the critical point to provide refrigeration, while cooling of the CO 2 occurs at temperatures and pressures above the critical point to provide heating of water. The objective of this project was to design and construct a prototype transcritical CO 2 heat pump to simultaneously provide refrigeration and water heating, and to test its performance over a wide range of operating conditions. The prototype CO 2 heat pump had a nominal cooling capacity of 90 kW at -6°C and nominal water heating capacity of 127 kW from 10°C to 90°C. The prototype was designed to operate with a suction pressure of 30 bar and discharge pressure of 130 bar. The major components were a gas cooler, recuperator, flooded evaporator, low pressure separator/receiver, compressor, expansion valve, connecting piping and a control system. All components were standard high pressure equipment used by the natural gas processing industry. The gas cooler had a reasonably unique design to ensure close to pure counter-current heat exchange between the cooling CO 2 gas and the water being heated, both of which had relatively low flowrates. The compressor used was an open crankcase, reciprocating type with special gas seals on the piston rod to prevent CO 2 leakage. Refrigeration capacity (suction pressure) was controlled by varying the compressor speed. Water heating capacity was controlled by both using the expansion valve to control the CO 2 discharge pressure and varying the water flowrate through the gas cooler. The main problem encountered during commissioning of the prototype was CO 2 leakage through the compressor piston rod seals. Alternative sealing systems were tried, but the leakage remained an on-going problem that prevented prolonged operation of the prototype, such as would be necessary in industrial applications. Performance of the prototype was determined by energy balances based on measurements of CO 2 and water flowrate and temperature when it operated at steady-state. The energy balances generally agreed to within 6%. Trials were performed with suction pressures from 29.6 to 35.5 bar, discharge pressures from 80 to 130 bar, with hot water outlet temperatures from 65°C to 90°C, and evaporator water inlet temperatures from 11°C to 21°C. When heating water to 90°C and providing refrigeration at 1°C (35.5 bar suction pressure), the maximum overall Coefficient of Performance (COP) achieved was 5.4 at a discharge pressure of 114 bar. Below this optimum discharge pressure, the COP declined due to gas cooler heat transfer limitations (lower compressor discharge temperature led to lower temperature difference in the gas cooler and high CO 2 outlet temperature). Above the optimum, the decline in thermodynamic and compressor efficiency as pressure ratio increased caused the COP to decrease. The maximum heating and cooling capacities were about 13% less than the design values. This was attributed to the lower than expected volumetric efficiency of the compressor. The performance of the heat exchangers were generally close to the design values when allowances for lower than design water flowrates were taken into account. As expected, when suction pressure was reduced to 29.6 bar (-6°C), there was up to a 10% decrease in optimum COP as well as reduced heating and cooling capacity. When heating water to 65°C rather than 90°C, the optimum COP was about 20% higher. When suction pressure or hot water outlet temperature was decreased, the optimum discharge pressure became slightly lower due to the gas cooler heat transfer being less of a limitation on overall system performance. Addition of oil to the CO 2 did not reduce the CO 2 leakage sufficiently to allow long-term operation without recharging, and had minimum impact on the performance of the gas cooler, recuperator and compressor. However, oil fouling caused a significant drop in heat transfer performance of the evaporator. The measured prototype performance agreed well with process simulations of the equipment and with results for similar laboratory scale equipment reported in the literature. Therefore, simulations could be used to optimise component and system design with a reasonable level of confidence. It was shown that the biggest increase in COP could be achieved by improving compressor isentropic efficiency rather than increased heat exchanger size. Overall, the concept of the transcritical CO 2 heat pump for simultaneous refrigeration and water heating was proven and the required energy efficiency was sufficiently high that the heat pump is likely to be economically competitive with traditional heating and cooling systems. Further work should concentrate on improving compressor design to eliminate CO 2 leakage and to improve both isentropic and volumetric efficiency

Dynamic simulation model of trans-critical carbon dioxide heat pump application for boosting low temperature distribution networks in dwellings

This research investigates the role of new hybrid energy system applications for developing a new plant refurbishment strategy to deploy small scale smart energy systems. This work deals with a dynamic simulation of trans-critical carbon dioxide heat pump application for boosting low temperature distribution networks to share heat for dwellings. Heat pumps provide high temperature heat to use the traditional emission systems. The new plant layout consists of an air source heat pump, four trans-critical carbon dioxide heat pumps (CO2-HPs), photovoltaic arrays, and a combined heat and power (CHP) for both domestic hot water production and electricity to partially drive the heat pumps. Furthermore, electric storage devices adoption has been evaluated. That layout has been compared to the traditional one based on separated generation systems using several energy performance indicators. Additionally, a sensitivity analysis on the primary energy saving, primary fossil energy consumptions, renewable energy fraction and renewable heat, with changes in building power to heat ratios, has been carried out. Obtained results highlighted that using the hybrid system with storage device it is possible to get a saving of 50% approximately. Consequently, CO2-HPs and hybrid systems adoption could be a viable option to achieve Near Zero Energy Building (NZEB) qualification

Thermodynamic analysis of a novel fossil-fuel–free energy storage system with a trans-critical carbon dioxide cycle and heat pump

This paper presents and analyzes a novel fossil-fuel–free trans-critical energy storage system that uses CO2 as the working fluid in a closed loop shuttled between two saline aquifers or caverns at different depths: one a low-pressure reservoir and the other a high-pressure reservoir. Thermal energy storage and a heat pump are adopted to eliminate the need for external natural gas for heating the CO2 entering the energy recovery turbines. We carefully analyze the energy storage and recovery processes to reveal the actual efficiency of the system. We also highlight thermodynamic and sensitivity analyses of the performance of this fossil-fuel–free trans-critical energy storage system based on a steady-state mathematical method. It is found that the fossil-fuel–free trans-critical CO2 energy storage system has good comprehensive thermodynamic performance. The exergy efficiency, round-trip efficiency, and energy storage efficiency are 67.89%, 66%, and 58.41%, and the energy generated of per unit storage volume is 2.12 kW·h/m3, and the main contribution to exergy destruction is the turbine reheater, from which we can quantify how performance can be improved. Moreover, with a higher energy storage and recovery pressure and lower pressure in the low-pressure reservoir, this novel system shows promising performance

Control optimisatlion of CO(2) cycles for medium temperature retail food refrigeration systems

This is the post-print version of the final paper published in International Journal of Refrigeration. The published article is available from the link below. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. Copyright @ 2009 Elsevier B.V.This paper describes a detailed procedure into the investigation of optimised control strategies for CO2 cycles in medium temperature retail food refrigeration systems. To achieve this objective, an integrated model was developed composing of a detailed condenser/gas cooler model, a simplified compressor model, an isenthalpic expansion process and constant evaporating temperature and superheating. The CO2 system can operate subcritically or transcritically depending on the ambient temperature. For a transcritical operation, a prediction can be made for optimised refrigerant discharge pressures from thermodynamic cycle calculations. When the system operates in the subcritical cycle, a floating discharge pressure control strategy is employed and the effect of different transitional ambient temperatures separating subcritical and transcritical cycles on system performance is investigated. The control strategy assumes variable compressor speed and adjustable air flow for the gas cooler/condenser to be modulated to achieve the constant cooling load requirement at different ambient conditions.DEFR

Vortex Tube Refrigeration Systems And Methods

Briefly described, embodiments of this disclosure, among others, include vortex vapor compression refrigeration (VCR) systems and methods of cooling.Georgia Tech Research Corporatio