Thermodynamic performance evaluation of the CO2 parallel compression supermarket refrigeration system with a subcooler

Commercial refrigeration systems in supermarkets are intensive users of energy and CO₂ is recognized as the most promising refrigerant such systems. The CO₂ parallel compression system has been considered in medium- and low-temperature supermarket refrigeration systems, with auxiliary compressors to enhance system performance. This study is intended to investigate the effects of adding a subcooler in a CO₂ trans-critical supermarket refrigeration system with parallel compression (SPR system). Mathematical models based on mass and energy conservation are built. Optimization is performed for the gas cooler pressure and the receiver pressure for maximizing the coefficient of performance (COP). The performance of the parallel compression system with a subcooler is analyzed and compared to a baseline parallel compression system in different cities of China. A 6.8% average promotion at least in seasonal energy efficiency ratio (SEER) is predicted, and the energy consumption of the whole system decreases 7.1% totally in Haikou. Gas cooler, taking up 63.3% of the total exergy destruction rate, is the key component to increase efficiency through exergy analysis.     

Optimization and analysis of a novel hydrogen liquefaction process for circulating hydrogen refrigeration

In industrial engineering, hydrogen is usually transported and stored after being liquefied, which is an energy-intensive process. Aiming to liquefy hydrogen with high efficiency and low consumption, a novel hydrogen liquefaction process based on dual-path hydrogen refrigeration is proposed innovatively and simulated by Aspen HYSYS to determine the key parameters. Taking the specific energy consumption (SEC) as the objective function for the optimization by genetic algorithm (GA), optimum parameters could be obtained. Meanwhile, the single variable method is conducted to analyze the impact of key parameters on process characteristics. Under the premise of complete liquefaction, the SEC, coefficient of performance (COP) and exergy efficiencies (EXE) of the proposed system are 7.041 kWh/kg _LH2, 0.1834, 0.5413, respectively. Compared with the other three hydrogen liquefaction systems simulated under the same conditions, they are decreased by 22.16% and increased by 33.58% and 42.37%, respectively. The results show that the proposed system shows better performance under lower consumption.     

Incorporation of an organic Rankine cycle in a transcritical booster CO2 refrigeration system

The objective of this work is the investigation of an alternative refrigeration system, which combines a supermarket CO₂ booster system with an organic Rankine cycle. The waste heat after the high-pressure compressor is used in order to feed the organic Rankine cycle for electricity production. The working fluid in the organic Rankine cycle is selected to be the octamethyltrisiloxane (MDM), which has a low global warming potential, and so all the used working fluids in the system are environmentally friendly. The system is studied in the transcritical region for ambient temperatures from 27°C to 40°C. In every case, the system is optimized in order to minimize the overall electricity demand. According to the final results, the system coefficient of performance (COP) can be enhanced from 4.83% to 7.60%, while the electricity consumption can be reduced from 4.60% to 7.03% compared with the conventional booster system. Higher enhancements are found in cases with higher ambient temperatures. Furthermore, a preliminary financial study indicates that the examined idea is viable with the simple payback period to be 5.45 years. The present study is conducted by using a homemade model written in Engineering Equation Solver.     

A novel hydrogen liquefaction process configuration with combined mixed refrigerant systems

A novel large-scale plant for hydrogen liquefying is proposed and analyzed. The liquid hydrogen production rate of the proposed plant is 100 tons per day to provide the required LH₂ for a large urban area with 100,000–200,000 hydrogen vehicles supply. In the pre-cooling section of the process, a new mixed refrigerant (MR) refrigeration cycle, combined with a Joule–Brayton refrigeration cycle, precool gaseous hydrogen feed from 25 °C to the temperature ?198.2 °C. A new refrigeration system with six simple Linde–Hampson cascade cycles cools low-temperature gaseous hydrogen from ?198.2 °C to temperature ?252.2 °C. The process specific energy consumption (SEC) is $7.69{\text{kWh/kg}}_{L{H}_2}$ which minimum value is $2.89{\text{kWh/kg}}_{L{H}_2}$ in ideal conditions. The exergy efficiency of the system is 39.5%, which is considerably higher than the existing hydrogen liquefier plants around the world. However, assuming more efficiency values for the equipment can improve it. The energy analysis specifies that coefficient of performance (COP) of the process is 0.1710 which is a high quantity of its kind between other similar processes. Effect of various refrigerant components concentration, discharge pressure of the high pressure compressors of the pre-cooling section, and hydrogen feed pressure on the process COP, exergy efficiency, and SEC are investigated. After that, a new MR will be offered for the cryogenic section of the plant. The system improvements are considerable comparing to current hydrogen liquefying plants, therefore, the proposed conceptual system can be used for future hydrogen liquefaction plants design.     

Optimization of a hydrogen liquefaction process utilizing mixed refrigeration considering stages of ortho-para hydrogen conversion

The ortho-to-para hydrogen conversion (OPC) is an integral part of the hydrogen liquefaction process. The optimal stages and conversion temperatures of OPC are important to reduce the specific energy consumption (SEC) of the process. However, a limited number of studies directly discuss this aspect of the effect of the OPC stages on the SEC. Therefore, based on an efficient refrigeration liquefaction cycle, this paper analyzes the effect of different OPC stages on the SEC of the efficient refrigeration cycle. Additionally, the impact of different combinations of conversion temperatures for the same OPC stages on the SEC is investigated. Results are obtained using a process model developed in HYSYS V10 and the particle swarm optimization algorithm in MATLAB. The results show the impact of OPC stages on the SEC, and that the increase in the stages of OPC can reduce the SEC, but the reduction tends to be slow. The SEC of the five-stage OPC process decreases by 16.39% compared to the SEC of the one-stage. Furthermore, the reduction of SEC for the process with the same stages of OPC can be achieved by setting the optimal combinations of conversion temperatures, which can reduce the SEC by up to 10.63%. The results of this study demonstrate that optimizing the OPC stages is important to reduce the SEC and will provide valuable information for the design of OPC for hydrogen liquefaction processes.     

太阳能喷射/压缩复合制冷循环性能研究

研究了一种太阳能喷射/压缩复合制冷循环,由太阳能集热子系统、喷射制冷子系统及压缩制冷子系统组成,系统充分利用热电两种能源以及两种制冷方法各自的优点,优化喷射制冷子系统工作性能的同时,改善压缩式子系统的工作条件,从而提高复合制冷循环性能的同时节约高品位电能。采用性能较好的高蒸发温度式喷射制冷带走压缩机排气余热具有实际意义。通过数值模拟的手段分析系统性能及其主要影响因素,并优化工作条件。研究表明,与相同工作条件下的单压缩制冷循环相比,复合制冷循环工作日全天候运行时电力性能系数提升约为31.5%,节电优势显著。存在一个最佳的喷射子系统蒸发温度使得复合制冷循环性能系数达到运行工况的最大值。     

Novel integrated helium extraction and natural gas liquefaction process configurations using absorption refrigeration and waste heat

Conceptual design and modeling of novel-integrated process configurations for helium extraction and natural gas liquefaction is investigated. Mixed fluid cascade (MFC) refrigeration system is considered for providing the needed refrigeration in the natural liquefaction section. Using an absorption refrigeration system as the precooling cycle is investigated in one of the introduced processes. Integrated flash and distillation method is used for helium extraction. Purity of the extracted crude helium is 50% (mole). Process streams operational condition and specifications of the devices are presented and explained. Composite curves of the heat exchangers demonstrate that thermal design has been done properly. Ratio of the power consumption to the produced liquefied natural gas (LNG) of the MFC process is 0.265 kWh per kg LNG and applying absorption refrigeration system instead of the pre-cooling cycle decreases it to 0.1849 kWh per kg LNG. For the modified process with absorption refrigeration system helium extraction rate and power consumption ratio are 0.951 and 132.9 (kWh/[kgmole Helium]) respectively. Exergy method is applied on the under consideration processes. The results show that the compressors have the highest rate of exergy destruction among the other process equipment. An extensive economic analysis is done on the proposed processes. The results show that prime cost of the product (US$/kg LNG) for MFC and modified MFC processes are 0.1939 and 0.2069, respectively. Finally, a sensitivity analysis is done based on the economic factors such as electrical energy price and prime cost of the product.     

Analysis and assessment of a novel hydrogen liquefaction process

In the present study, a novel supercritical hydrogen liquefaction process based on helium cooled hydrogen liquefaction cycles to produce liquid hydrogen is thermodynamically analyzed and assessed. The exergy analysis approach is used to study the exergy destruction rates in each component and the process efficiency. The energy and exergy efficiencies of liquefaction process are found to be 70.12% and 57.13%, respectively. In addition, to investigate the process efficiency more comprehensively to see how it is affected by varying process parameters and operating conditions, some parametric studies are undertaken to examine the impacts of different design variables on the energy efficiency, exergy efficiency and exergy destruction rates of the hydrogen liquefaction process. The results show that the increases in the cycle pressure of hydrogen and helium result in increasing hydrogen liquefaction process exergy efficiency and providing a smaller pinch point temperature difference of catalyst beds related with the heat transfer surface area and more efficiently process.     

CO2双级压缩制冷热泵循环系统的研究

关于CO2双级压缩制冷热泵循环系统的研究是目前一些生活服务行业最大的制冷热泵的循环需求,例如在餐厅、宾馆、理发店等地方对于循环系统的寻求.研究了关于CO2双级压缩制冷热泵循环系统的工作原理,及该系统今后的发展方向.     

制冷压缩热为烘干线热源的实验与研究

对以制冷压缩热为烘干线热源进行了可行性研究与实验,为更有效地利用制冷机压缩热进行了有益的探索。同时探讨了以制冷压缩热作为烘干线热源的烘干线的设计方法。     

Comparative analysis of thermodynamic performance of CO2 cascade refrigeration system assisted with expander and mechanical subcooling

In order to discuss the feasibility of using R744/R744 cascade refrigeration system (CRS) instead of R744/R717 CRS, six configurations of R744/R744 CRS assisted with expander and mechanical subcooling system (MS) are analyzed. Based on the thermodynamic analysis, the results show that the high pressure, the condensing temperature of the low‐temperature cycle (LTC), and the degree of subcooling of LTC and the high‐temperature cycle (HTC) are three important operating parameters with an optimum value corresponding to the maximum coefficient of performance (COP). Compared with other CRSs, CRS with HTC throttling valve and MS of HTC (CTS_H) and CRS with HTC expander and MS of HTC (CES_H) show an excellent performance. CES_H has the highest COP and is improved by an average of 13.8% compared with the COP of R744/R717 CRS. The COP of CTS_H is improved by an average of 4.2% compared with the COP of R744/R717 CRS. In conclusion, it is an efficient way to improve performance that CRS combines MS in HTC and HTC expander. And it is possible R744/R744 CRS instead of R744/R717 CRS.     

CO2跨临界双级压缩/喷射制冷循环热力学分析

建立了同时采用双级压缩和利用喷射器代替节流阀的CO2跨临界双级压缩/喷射制冷循环模型,在系统稳定运行的条件下,分析了高压压力、气体冷却器出口温度、蒸发温度和高、低压压缩机吸气过热度对循环性能的影响,并与CO2跨临界单级压缩/喷射制冷循环和双级压缩制冷循环进行了比较.结果表明:在给定条件下,双级压缩/喷射循环的性能系数明显优于其他两种循环;随着气体冷却器出口温度的升高和蒸发温度的降低,循环的性能系数分别降低了54.9%和43.2%,并且其下降速度大于双级循环的性能系数下降速度;高、低压压缩机吸气过热度升高均导致双级压缩/喷射循环性能系数降低.     

Thermodynamic analysis of a novel energy storage system based on compressed CO2 fluid

Because of rapidly growing renewable power capacity, energy storage system is in urgent need to cope with the reliability and stability challenges. CO₂ has already been selected as the working fluid, including thermo‐electrical energy storage or electrothermal energy storage systems and compressed CO₂ energy storage (CCES) systems. In this paper, a CCES system based on Brayton cycle with hot water as the heat storage medium is proposed and analyzed. Thermodynamic model of the system is established for energy and exergy analysis. Sensitivity analysis is then conducted to reveal effects of different parameters on system performances and pursue optimization potential. At a typical transcritical operation condition, round trip efficiency is 60% with energy density of 2.6 kWh/m³. And for the typical supercritical operation condition, the round trip efficiency can reach 71% with energy density of 23 kWh/m³. High round trip efficiency and energy density, which is comparable with those of compressed air energy storage systems, thermo‐electrical energy storage (electrothermal energy storage) systems, and other CCES systems, lead to promising prospect of the proposed system. Copyright © 2017 John Wiley & Sons, Ltd.     

CO2跨临界循环与传统制冷循环的热力学分析

以热力学第二定律和卡诺定理为基础，提出了一种新的热力学分析方法-当量温度法，并以此为基准对各种制冷循环进行了分析。与传统分析方法相比，当量温度法可以得出更加公正、合理的结论。     

CO2减排机理及与燃煤电厂集成特性的研究

推导了CO2吸收工艺系统再生能耗计算公式,分析了吸收剂性质对再生能耗的影响,计算得到不同质量分数下吸收剂的再生能耗;提出几种基于槽式太阳能辅助燃煤发电技术的CO2减排集成方案,以N600-24.2/566/566型发电机组为例,以集成系统热耗率、发电标准煤耗率和热效率作为经济性指标,采用热平衡法对不同集成方案机组的热经济性指标进行了计算和比较.结果表明:在加入太阳能热量和机组主蒸汽质量流量不变的情况下,集成方案5在设计辐照强度下的热耗率和发电标准煤耗率均最低,是最经济的集成方案.     

Study on a solar-driven ejection absorption refrigeration cycle

This paper deals with a solar-driven ejection absorption refrigeration (EAR) cycle with reabsorption of the strong solution and pressure boost of the weak solution. The physical model is described and the corresponding thermodynamic calculation is performed with the working pair NH₃–LiNO₃. It is demonstrated that the EAR cycle has obvious advantages as compared with the conventional absorption refrigeration cycle: (1) the controllable high absorption pressure allows for substantially high coefficients of performance by the action of a liquid–gas ejector in which the low-pressure refrigerant vapour is injected and pressurized as a result of the ejection of high-pressure solution; (2) internal steady operation can be realized for refrigeration cycles driven by unsteady heat sources, especially for solar energy, by adjusting the power input consumed by solution pumps under the condition of economical and reasonable utilization of electric energy. © 1998 John Wiley & Sons, Ltd.     

封存CO2不如植树造林

叙述了工业社会以来碳增了100pm,中国万元GDP能耗仍可能下降,提出了回收厨房垃圾再利用和以煤变油是非常好的节能途径,全世界只有中国的森林面积在增加,封存C02不如植树造林。     

Review on the design and optimization of hydrogen liquefaction processes

The key technologies of liquefied hydrogen have been developing rapidly due to its prospective energy exchange effectiveness, zero emissions, and long distance and economic transportation. However, hydrogen liquefaction is one of the most energy-intensive industrial processes. A small reduction in energy consumption and an improvement in efficiency may decrease the operating cost of the entire process. In this paper, the detailed progress of design and optimization for hydrogen liquefaction in recent years are summarized. Then, based on the refrigeration cycles, the hydrogen liquefaction processes are divided into two parts, namely precooled liquefaction process and cascade liquefaction process. Among the existing technologies, the SEC of most hydrogen liquefaction processes is limited in the range of 5–8 kWh/ {\mathrm{k}\mathrm{g}}_{{\mathrm{L}\mathrm{H}}_2} : liquid hydrogen). The exergy efficiencies of processes are around 40% to 60%. Finally, several future improvements for hydrogen liquefaction process design and optimization are proposed. The mixed refrigerants (MRs) as the working fluids of the process and the combination of the traditional hydrogen liquefaction process with the renewable energy technology will be the great prospects for development in near future.     

Performance analysis and optimization of a hydrogen liquefaction system assisted by geothermal absorption precooling refrigeration cycle

The present paper deals with the hydrogen liquefaction with absorption precooling cycle assisted by geothermal water is modeled and analyzed. Uses geothermal heat in an absorption refrigeration process to precool the hydrogen gas is liquefied in a liquefaction cycle. High-temperature geothermal water using the absorption refrigeration cycle is used to decrease electricity work consumption in the gas liquefaction cycle. The thermoeconomic optimization procedure is applied using the genetic algorithm method to the hydrogen liquefaction system. The objective is to minimize the unit cost of hydrogen liquefaction of the composed system. Based on optimization calculations, hydrogen gas can be cooled down to ?30 °C in the precooling cycle. This allows the exergetic cost of hydrogen gas to be reduced to be 20.16 $/GJ (2.42 $/kg LH₂). The optimized exergetic cost of liquefied hydrogen is 4.905 $/GJ (1.349 $/kg LH₂), respectively.