Summary
Increasing demand of electricity and severer concerns to environment call for green energy sources as well as efficient energy conversion systems. SCO2 power cycles integrated with ...concentrating solar power (CSP) are capable of enhancing the competitiveness of thermal solar electricity. This article makes a comprehensive review of supercritical CO2 power cycles integrated with CSP. A detailed comparison of four typical CSP technologies is conducted, and the cost challenge of currently CSP technologies is pointed out. The thermophysical properties of sCO2 and the corresponding two real gas effects are analyzed elaborately to express the features of sCO2 power cycles. An extensive review of sCO2 layouts relevant for CSP including 12 single layouts and 1 combined layout is implemented logically. Strengths and weaknesses of sCO2 power cycles over traditional steam‐Rankine cycle generally adopted in current CSP plants are concluded, followed by metal material degration summary in CSP relevant temperature sCO2 environment, which shows that the nickel‐based alloy is a proper structural material candidate for sCO2‐CSP integration. Thermodynamic analyses of sCO2 power cycles when integrated with CSP are divided into three level of which design‐point analysis and off‐design modeling are conducted and compared, more researches into the off‐design point analysis, dynamic modeling, especially the transient behavior are suggested. Economic analysis of the integrated system is concluded and presents a considerable levelized cost of electricity reduction of 15.6% to 67.7% compared to that of state of art CSP. Taking the thermodynamic and economic analysis into consideration, target designs of sCO2 power cycles for CSP are summarized in three aspects. Finally, current theoretical and experimental researches of sCO2 power cycles integrated with CSP for market penetration are introduced. The strengths, weaknesses, and potential solutions to the gaps of three potential pathways (molten salt pathway, particle pathway, and gas phase pathway) to realize the integration of sCO2 power cycles in the next CSP generation plants up to 700°C are reviewed. In general, the integration of sCO2 power cycles with CSP technologies exhibits promising expectations for facilitating the competitiveness of thermal solar electricity.
Summary
This article proposes a dynamic analysis framework for a shell and plate particle‐SCO2 moving packed bed heat exchanger (MPBE) with straight SCO2 channel and four fins‐modified SCO2 channels, ...which contains a static model and a dynamic model. 13 cases covering an initial temperature deviation range of +50 ~ −100 K from design‐point case in solids inlet, an initial temperature deviation range of +50 ~ −100 K from design‐point case in SCO2 inlet, and a SCO2 inlet velocity range of 1.5 times to 0.05 times are introduced to measure the dynamic characteristics. Dynamic responses of five MPBE configurations to two types of inputs (step‐change perturbations and linear‐change perturbations) under 13 cases are analyzed. Relaxation times at each case are summarized and compared. A low threshold temperature exists for all five MPBE configurations to avoid oscillations in solids outlet temperature under step‐change perturbations in solids inlet temperature, and a low threshold SCO2 inlet temperature just appears in MPBE with straight SCO2 channels under step‐change perturbations in SCO2 inlet temperature. Improved MPBE configurations show apparent less relaxation time than MPBE with straight SCO2 channel. Among four fins‐modified MPBE configurations, MPBE channels modified with offset rectangular fins on SCO2 side exert the fastest dynamic characteristic in most cases.
Printed circuit heat exchanger has been recognized as a perspective mini-channel heat exchanger. Semi-circular cross-section is designed here due to the special manufacture technology. However, there ...is a lack of related investigations on condensation process in mini-channels with semi-circular cross-section. In this paper, corresponding three-dimensional numerical simulations are presented. The horizontal straight channels with radius of 1 mm and 1.222 mm, and length of 250 mm are considered as the geometric models. Constant mass flux (317 kg/m
2
s) and wall temperature (310 K) are applied as the boundary conditions. Heat transfer and flow characteristics are conducted by comparing to that in common circular channel with identical wetted perimeter. The effect of placing direction and surface wettability are present as well. The results show that the liquid film in the semi-circular channel is more uneven compared to that in the circular channel. The liquid is accumulated in corners because of surface tension, and this brings worse heat transfer to the semi-circular channel further. Besides, the placing direction has little influence on heat transfer with the variation range of heat transfer coefficient within 4% in the present conditions. The influence of surface wettability can be neglected for the larger tube.
Moving packed bed particle/supercritical carbon dioxide (SCO2) heat exchanger (MPBE) is a critical equipment to integrate particle thermal energy storage technology with SCO2 power cycle block in the ...next‐generation concentrated solar power plants. A predictive heat transfer model for designing and evaluation of shell and plate particle/SCO2 moving packed bed heat exchanger is presented, with radiation, pressure drop and SCO2 property variation taking into account. It is found that increasing particle diameter and solids bulk voidage will lower the overall heat transfer coefficient. Change of channel width imposes little effects on the overall heat transfer coefficient, while influences the total heat exchange greatly. MPBE with longer channel length allows smaller overall heat transfer coefficient. Due to approximate linear variation of specific heat and low pressure drop in the nominal boundary operating condition range, ignoring SCO2 Cp variation is valid, and the relative error due to adopting a Cp invariable evaluated at the bulk temperature and constant pressure is less than 1%. Convection resistance generally ranks first in the contribution to thermal resistance, followed by solids‐wall resistance and conduction resistance. Nevertheless, solids‐wall resistance dominates at larger particle diameter (≥550 μm). Radiation is an important contributor to heat transfer behavior, especially at high particle diameter and solids bulk voidage. The relative error caused by neglecting radiation can be up to 23.76%.
Precooling is widely adopted to weaken respiration and remove field heat of postharvest fruit and vegetables. Temperature and gas composition are two main factors that affect respiration. However, ...the interaction mechanism between them has not been clearly studied. In addition, for the mechanism of gas transport, existing models lack consideration of interaction of gas‐liquid two phases. This paper is dedicated to present the Marangoni effect during precooling of fruit and vegetables. The theory explanation and numerical method of the volume of fluid (VOF) model was firstly set up. Parameters investigation including temperature difference, bubble diameter, aspect ratio, and fluid type were then conducted to illustrate this phenomenon and reveal the mechanism. Finally, the synergetic regulation of temperature and gas composition based on Marangoni effect was proposed. It contributes not only maintain high quality of fruit and vegetables, but also reduce energy consumption, which will have a great significance on environment.
Practical applications
Marangoni effect induced by temperature difference would weaken the respiration during precooling, and thus reduce the decay of quality. It was found that low precooling temperature could enhance the Marangoni effect. In addition, for the bubble in the wide channel of fruits and vegetables with low concentration, the Marangoni effect is relatively strong. Furthermore, the bigger the bubble diameter is, the more obvious the Marangoni effect is. Thus, the most suitable regulation strategies of temperature and gas composition differ between cultivars and also between positions in the parenchyma tissue of the same object. In addition, gas composition could be controlled by changing the temperature, which indicated that precooling can also play the role of controlled atmosphere storage. Therefore, the synergetic regulation of temperature and gas composition has a beneficial effect on preservation of fruits and vegetables, which is far more than a single factor. Thus, it can not only maintain high quality of fruits and vegetables, but also reduce the energy consumption, which will have a profound significance on environment.
•Predictive static model to evaluate the thermal-hydraulic performance of a particle-SCO2 MPBE is proposed.•Mechanism to the poor heat transfer behavior in the straight SCO2 channel of a ...particle-SCO2 MPBE is demonstrated.•Four different fin configurations in the SCO2 channel side are presented, analyzed and compared.•Suggestions to the enhanced channel configurations in a particle-SCO2 MPBE are provided.
This article proposes a predictive static model to evaluate the thermal-hydraulic performance of a shell and plate particle-SCO2 moving packed bed heat exchanger, with consideration of pressure drop, radiation and property variations. The results reveal that convection resistance on the SCO2 straight channel side occupies a large part of total thermal resistance in most design and off-design cases. A three-dimensional model inheriting conclusions from the static model is raised, which demonstrates that extremely short entrance region in the x-y plane hinders the overall heat transfer behavior of the SCO2 straight channel. Deservedly, suppressions of the development of thermal layers in the y direction will contribute to improving the thermal-hydraulic performance of the shell and plate particle-SCO2 moving packed bed heat exchanger. Four different fin configurations on SCO2 channel side are presented, analyzed and compared via the static model at the design-point case. Offset rectangular fins configuration behaves the best heat transfer enhancement in total heat transfer rate and overall heat transfer coefficient when compared with the original channel, followed by zigzag fins configuration, offset airfoil fins configuration and s-shaped fins configuration. Zigzag fins-modified channel expresses the worst hydraulic behavior, of which the pressure drop is nearly four times that of original straight channel. The pressure drop of channel with s-shaped fins nearly equals to that of the unmodified channel. Introduction of fins on SCO2 channel side reduces the ratio of convection resistance as a result of apparently increased SCO2 convection coefficients.
•A model conjugates heat transfer of condensation and transcritical flow is proposed.•Thermohydraulic characteristics of transcritical flow relies on condensation flow.•Heat deterioration exists in ...transcritical flow due to large heat flux in condensation.•The effect of buoyancy and flow acceleration of flow can be neglected.•Large irreversible loss exists in coupled transcritical-condensation heat transfer.
Printed Circuit Heat Exchanger (PCHE) has attracted researchers a lot those years due to its advantages of sturdy structure, compact volume and high thermal efficiency. Transcritical flow and condensation flow can be coupled as cold fluid and hot fluid respectively to exchange heat. However, large variation of thermophysical properties of transcritical process as well as large energy density of phase-change process bring unknown thermo-hydraulic characteristics when the two-process coupled with each other. In this study, double bank channel unit PCHE model was established to simulate the real case transcritical-condensation heat transfer. The effects of buoyancy and flow acceleration were calculated for transcritical process. Local information of heat transfer and flow characteristics for both hot side (condensation flow) and cold side (transcritical flow) were obtained and analyzed. The effect of mass flux on the two-side was then presented. At last, comparisons were conducted between the heat transfer and pressure drop characteristics of the double bank channel unit and that of the single bank channel unit with constant heat flux boundary conditions. Exergy analysis was also present to evaluate the irreversibility of coupled transcritical-condensation flow. The results showed that the effect of buoyancy and flow acceleration can be neglected under the present working conditions. As for the effect of mass flux, the variation of condensation flow mass flux would bring larger effect on heat transfer. Besides, the heat transfer performance of transcritical flow would be deteriorated when coupled with condensation flow due to a large heat flux segment caused by condensation, implying that the real thermal boundary was mainly constructed by condensation process. Besides, larger irreversible loss can be obtained when transcritical flow coupled with condensation flow compared to couple with single phase flow. The results would give a valuable reference for the study on conjugate heat transfer and the design on PCHE.
In the present paper, a multi-target temperature ejector-expansion subcooler vapor-injection refrigeration cycle is proposed in order to improve the performance of natural fluid CO2 refrigeration ...cycles. A thermodynamics model to simulate a two-phase ejector cycle integrating a vapor-injection is first established from the energetic and exergetic perspectives. The behavior of the new cycle is then analyzed, and the adjustment direction of the cooling capacity and the evaporating temperature is pointed out. Furthermore, the new cycle is also compared with the subcooler vapor-injection refrigeration cycle and the ejector-expansion refrigeration cycle. The obtained results show that the behavior of the evaporators is affected by the bypass mass ratio and the entrainment ratio, and the two parameter determines the direction of adjusting the cooling capacity distribution and the evaporating temperature. The COP of the new cycle is about 17.9–29.4% and 2.8–7.8% higher than that of subcooler vapor-injection refrigeration cycle and ejector-expansion refrigeration cycle; the exergy efficiency of new cycle is about 14–28.2% and 3.7–11.3% higher than that of subcooler vapor-injection refrigeration cycle and ejector-expansion refrigeration cycle; and it is also found that the performance of the new cycle is improved more significantly under low cooling temperature and high ambient temperature conditions.
•A MTES cycle for multi-target temperature refrigeration is proposed.•Energy and exergy analyses of MTES cycle are theoretically conducted and compared with the reference cycle.•The R and ω determine the cooling capacity distribution and evaporation temperature.•The MTES cycle could obtain three-temperature refrigeration simultaneously.•The MTES cycle has higher COP and exergy efficiency.
