[en] The serious charge recombination originates from the thermal instability of perovskite/ZnO and the low electron injection efficiency of ZnO. It is the important issue to be improved in ZnO-based perovskite solar cell (PSC). In this paper, the core-shell structure of ZnO@TiO₂ nanorod arrays (NRs) is designed as electron transport layer (ETL) for PSC. A novel synthesis of PSC based on ZnO@TiO₂ NRs in ambient atmosphere was proposed. The photoelectric conversion efficiency (PCE) of the core-shell device is 50.46% higher than that of common ZnO nanorod device. This is due to the improved interface contact between nanorods and perovskite layer, and the suppression of charge recombination. The PCE of the TiO₂ modified device shows still more than 83% after 168 h, compared to that of the pristine one which decreased to less than 50%. This is due to TiO₂ modification which can serve as a buffer layer to avoid direct contact between perovskite films and ZnO NRs, and inhibits the decomposition of perovskite film on ZnO NRs. Both theoretical calculation and Raman test result show that the interaction between CH₃NH₃PbI₃ and TiO₂ is mainly the bonding between I atoms of PbI₂ slabs and Ti atoms of the TiO₂ surface at PbI₂/TiO₂ interface. The mechanism of carrier transport and recombination in the PSC based on ZnO and ZnO@TiO₂ NRs was also discussed. These results highlight the potential of ZnO@TiO₂ NRs as ETL for all-solid-state PSC with high efficiency and good stability.

[en] Highlights: • Perovskite film modified by PVP/PEG mixture was prepared by a two-step method in air. • The cell with 1.5 wt%PVP/0.5 wt%PEG shows 30% PCE increasement compared with pure one. • The cell with 1.5 wt% PVP/0.5 wt%PEG retains 76% of its initial PCE value for 30 days. • The interactions between CH₃NH₃PbI₃ and PVP/PEG improve film quality and stability. • The bondings between perovskite and PVP/PEG increase cell efficiency and stability. -- Abstract: The key to commercialization of organic‐inorganic lead halide perovskite solar cells (PSCs) is to achieve high photoelectric conversion efficiency (PCE) and long-term stability under ambient condition. In this paper, polymer mixture of polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG) with appropriate mass ratio are incorporated into PbI₂ precursor solution in the preparation process of perovskite film via two-step sequential deposition method in air. The PCE of polymer mixture modified PSC based on ZnO@TiO₂ nanorod arrays is better than those of the PSC with single polymerPVP or PEGand the pristine one, and the PCE of 1.5 wt%PVP/0.5 wt%PEG modified device increased by 30% compared with that of the unmodified one, which is mainly a consequence of the better quality of perovskite film caused by the synergistic effect including the strong attraction between the C=O in PVP and Pb²⁺ in perovskite, and the intense hydrogen bond between O atoms from PEG and H atoms in CH₃NH₃⁺. Moreover, polymer mixture modified PSC also represents better air stability compared with the device with single polymer or the pristine onewhich is largely attributed to the exceptional hygroscopicity of PVP/PEG mixture, the synergistic stabilization of perovskite structure and inhibition of ion migration by the bondings between polymer mixture and perovskite. Furthermore, the mechanism of the interaction between PVP/PEG mixture and CH₃NH₃PbI₃ to increase the efficiency and stability of the PSC was also studied. Our work offers an effective additive to simultaneously improve the efficiency and stability of the PSCs in high humidity environment for further commercial application.

[en] Highlights: • A one-dimensional model for PCHEs is proposed and validated against 3-D CFD simulations. • The one-dimensional methodology is used to model a 630 kW PCHE. • Off-design performance maps of the heat exchanger are presented. • Transient analyses are carried out to study the dynamic response of the device. -- Abstract: The paper presents a modelling methodology for Printed Circuit Heat Exchangers (PCHEs) in supercritical CO₂ (sCO₂) power systems. The PCHE model can be embedded in models of the full sCO₂ power unit for optimisation, transient simulation and control purposes. In particular, the purpose of the study is to assess the potential and limitations of lower order models in predicting the overall heat transfer performance of PCHEs. The heat transfer processes in the channels of the PCHE recuperator are modelled in 1-D and 3-D using commercial software platforms. The results show that predictions from the two modelling approaches are in good agreement, confirming that the 1-D approach can be used with confidence for fast simulation and analysis of PCHEs. Using the 1-D approach, the model was validated against manufacturer’s data for a 630 kW PCHE recuperator, and subsequently used to simulate the performance of the heat exchanger at design and off-design operating conditions. Performance maps produced from the simulations, enable visualization of the influence of operating conditions on the heat transfer performance and pressure drops in the heat exchanger. Dynamic simulations under transient operating conditions show that the thermal expansion of the working fluid caused by a fast reduction in density and increase in pressure in the system, can be a concern, requiring careful management of the start-up process to avoid sudden changes in temperature and thermal stresses.

[en] The effect of geometric parameters on water flow and heat transfer characteristics in microchannel heat sink with triangular reentrant cavities is numerically investigated. A three-dimensional laminar flow model, consisting of Navier-Stokes equations and energy conservation equation, with the conjugate heat transfer between the silicon base and water taken into consideration is solved numerically. In order to find the optimum geometric parameters, four variables, representing the distance and geometry of the triangular reentrant cavity, are designed. It is found that the vortices in the triangular reentrant cavities lead to chaotic advection and can greatly enhance the convective fluid mixing. The thermal and hydraulic boundary layers are interrupted and the repeated developing flow enhances heat transfer in the constant cross-section segment. Furthermore, the effects of the four design variables on heat transfer augmentation and pressure drop penalty are investigated depending on different Reynolds numbers by using the simulated annealing method. Based on the thermal enhancement factor performance maps, the optimal geometric parameters are obtained in principle. - Research highlights: → The microchannels with different triangular reentrant cavities are numerically investigated. → The heat transfer enhancement attributes to fluid mixing and redeveloped thermal boundary layers. → The optimal distance and geometry of the triangular reentrant cavity are obtained in principle.

[en] Highlights: • The transient analysis method for PTES system is proposed. • The cyclic transient of 10 MW/4 h Joule-Brayton PTES is studied. • Both the round-trip efficiency and delivery stability of the PTES are discussed. • Helium has the overwhelming advantage above argon as the working gas. • Impact of particle sizes and length to diameter ratio of packed bed was analyzed. -- Abstract: Pumped heat electricity storage (PHES) has the advantages of a high energy density and high efficiency and is especially suitable for large-scale energy storage. The performance of PHES has attracted much attention which has been studied mostly based on steady thermodynamics, whereas the transient characteristic of the real energy storage process of PHES cannot be presented. In this paper, a transient analysis method for the PHES system coupling dynamics, heat transfer, and thermodynamics is proposed. Judging with the round trip efficiency and the stability of delivery power, the energy storage behavior of a 10 MW/4 h PHES system is studied with argon and helium as the working gas. The influencing factors such as the pressure ratio, polytropic efficiency, particle diameters, structure of thermal energy storage reservoirs are also analyzed. The results obtained indicate that, mainly owing to a small resistance loss, helium with a round-trip efficiency of 56.9% has an overwhelming advantage over argon with an efficiency of 39.3%. Furthermore, the increases in the pressure ratio and isentropic efficiencies improve the energy storage performance considerably. There also exit optimal values of the delivery compression ratio, particle sizes, length-to-diameter ratios of the reservoirs, and discharging durations corresponding to the maximum round-trip efficiency and preferable discharging power stability. The above can provide a basis for the optimal design and operation of the Joule–Brayton based PHES.

[en] Highlights: • Review of sCO₂ technologies with emphasis on components, materials and applications. • Classification and overview of thermodynamic aspects of sCO₂ cycles. • Challenges for sCO₂ turbomachinery, heat exchangers and control systems outlined. • Key applications summarised with table of predicted levelised costs of electricity. Thermal-power cycles operating with supercritical carbon dioxide (sCO${}_2$) could have a significant role in future power generation systems with applications including fossil fuel, nuclear power, concentrated-solar power, and waste-heat recovery. The use of sCO${}_2$ as a working fluid offers potential benefits including high thermal efficiencies using heat-source temperatures ranging between approximately $350{}^{\circ }\mathrm{C}$ and $800{}^{\circ }\mathrm{C}$, a simple and compact physical footprint, and good operational flexibility, which could realise lower levelised costs of electricity compared to existing technologies. However, there remain technical challenges to overcome that relate to the design and operation of the turbomachinery components and heat exchangers, material selection considering the high operating temperatures and pressures, in addition to characterising the behaviour of supercritical CO${}_2$. Moreover, the sensitivity of the cycle to the ambient conditions, alongside the variable nature of heat availability in target applications, introduce challenges related to the optimal operation and control. The aim of this paper is to provide a review of the current state-of-the-art of sCO${}_2$ power generation systems, with a focus on technical and operational issues. Following an overview of the historical background and thermodynamic aspects, emphasis is placed on discussing the current research and development status in the areas of turbomachinery, heat exchangers, materials and control system design, with priority given to experimental prototypes. Developments and current challenges within the key application areas are summarised and future research trends are identified.

[en] Highlights: • The mass flow rate ratio relation of outflow-to-inflow of packed bed was developed. • Proportion of unbalanced flow is 0.62% for hot reservoir and 0.26% for cold. • Impact of pressure ratio, porosity and TES material heat capacity was analyzed. • Proportion of unbalanced mass flow is 0.36% in pumped heat electricity storage. • A self-balance PTES system was proposed to void buffer vessel. -- Abstract: As the most suitable thermal energy storage manner for the Joule-Brayton based Pumped Thermal Electricity Storage (PTES), packed beds thermal energy storage has the natural feature that a steep thermal front propagates with great difference of temperature and density, which lead to an unbalanced mass flow rate of packed bed reservoirs and the PTES close loop. In this paper, the expression of thermal front propagation and unbalanced mass flow rates between the inflow and the outflow of packed beds is developed and validated experimentally. The result indicates that there are 0.62%, 0.26% and 0.36% unbalanced mass flow for the hot reservoir, the cold reservoir and the PTES close loop respectively. The sensitivities of the factors such as pressure ratio, heat capacity of TES material and porosity on the unbalanced mass flow rate and the round–trip efficiency of PTES system considering mass flow rate is discussed. Furthermore, a feasible and self-balancing PTES system without buffer vessel is proposed, with a round-trip efficiency 0.12% higher than the buffer vessel balancing PTES system.

[en] Highlights: • Thermophysical properties of N/MPCS in mini/microchannels are analyzed. • Non-dimensional numbers during N/MPCS in mini/microchannels are summarized. • Heat transfer performances of N/MPCS in mini/microchannels are discussed. • Hydrodynamic characteristics of N/MPCS in mini/microchannels are analyzed. • Heat transfer and pressure drop correlations of N/MPCS are presented. - Abstract: Mini/microchannel heat sinks are currently widely used in a variety of thermal and energy applications with the advantages of compactness, light weight and higher heat transfer performance. In order to further improve the performance of such heat sink, many recent studies have introduced the nano/microencapsulated phase change slurry (N/MPCS) as the working fluid due to their high storage capacity during phase change. This paper concerns the channel with hydraulic diameter from 10 μm to 3 mm, covering the range of microchannel and minichannel. Firstly, the existed review works relate to mini/microchannel heat sinks are summarized, with topics covering manufacturing processes and geometric designs, thermal and hydrodynamic performance with different working fluids, and their typical and potential applications. Then, the N/MPCS used in mini/microchannels from experimental and numerical simulation works are discussed, with focuses placed on the base fluid, core and shell materials, and thermophysical properties of slurry. Next, the local, average and overall heat transfer and hydrodynamic characteristics of mini/microchannel heat sinks with N/MPCS flowing inside are reviewed and analyzed, considering different flow conditions, material and dimension of test section, and composition and fraction of such slurry. Finally, the proposed heat transfer and pressure drop correlations in this research field are evaluated. The purpose of this review article is to provide exhaustive and comprehensive study of recent published works in this new area and supply useful information for the design of compact heat exchangers and thermal storage systems with N/MPCS as working fluid.