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[en] Highlights: • Study of HCs and Freon as TPCT working fluids for renewable energy applications. • Experimental performances of TPCTs were studied with eight working fluids. • R245fa/R152a, R600a, and R1234ze were recommended as substitutes for R134a. • Suitability of typical HTC correlations were analysed for the TPCT working fluids. • A simplified Rohsenow correlation was developed to further improve accuracy. - Abstract: Two-phase closed thermosyphons (TPCTs) are simple, efficient, and low cost heat exchangers. They have been explored for use in the renewable energy resource utilization marker and low grade thermal energy heat recovery systems. Freon R134a has been extensively used in refrigeration systems and researched as a working fluid of TPCTs; however; it has high global warming potential and operating pressure. In this paper, an experimental investigation of the performance of TPCTs charged with eight working fluids: R134a, R601, R245fa, R600a, R1234ze, R152a, R245fa/R152a, and R601/R245fa have been carried out. The experimental results showed that R245fa/R152a offered the best performance in maximum heat transfer rate. R134a outperformed the other pure working fluids, while R600a and R1234ze had close performances to that of R134a. R245fa showed marginal improvement at higher operating temperatures. The predictions of six evaporation heat transfer coefficients (HTCs) correlations, including Imura, Shiraishi, Labuntsov, Kutateladze, Cooper, and Rohsenow were compared with the experimental results. In the five constant coefficients and powers correlations, the Shiraishi and Cooper correlations had superior accuracy. The coefficients and powers of the Rohsenow correlations fitted based on the experimental data, while they had the best accuracy. Nusselt and Hashimoto-Kaminaga correlations were chosen to predict the condensation HTCs. Both of them tend to over-predict the condensation HTCs in low heat fluxes while under-predicting in high heat fluxes. The experimental results had greater agreement with Hashimoto and Kaminaga correlations.
[en] The purpose of this study is to investigate the enhancement of polycaprolactone (PCL) on total nitrogen (TN) removal of coal pyrolysis wastewater (CPW) with low COD to nitrogen ratio by partial nitrification-denitrification bioprocess (PNDB) in one single reactor. With the innovative combination of PCL and PNDB, the TN removal efficiency in the experimental reactor (signed as R1) was 10.21% higher than control reactor (R2). Nitrite accumulation percentage (NAP) in R1 was 82.02%, which was 17.49% higher than R2 at the dissolved oxygen (DO) concentration of 0.9–1.5 mg/L, for the reason that the extra DO was consumed by PCL biodegradation at the aerobic period. Gel permeation chromatography (GPC) results demonstrated that organics with the molecular weight of 185 Da, which could serve as additional carbon sources for denitrifiers, were generated during the PCL hydrolysis process at the anoxic period. PCL was hydrolyzed by extracellular enzymes with the break of the ester bond which was confirmed by FT-IR spectrometer. Microbial community analysis revealed that Ferruginibacter was the dominant hydrolysis bacteria in R1. Nitrosomonas were the main ammonium-oxidizing bacteria (AOB) and Hyphomicrobium were the denitrifiers in this study.
[en] Recent investigations show that a power system is a highly nonlinear system and can exhibit chaotic behaviour leading to a voltage collapse, which severely threatens the secure and stable operation of the power system. Based on the finite-time stability theory, two control strategies are presented to achieve finite-time chaos control. In addition, the problem of how to stabilize an unstable nonzero equilibrium point in a finite time is solved by coordinate transformation for the first time. Numerical simulations are presented to demonstrate the effectiveness and the robustness of the proposed scheme. The research in this paper may help to maintain the secure operation of power systems. (general)
[en] Zinc oxide (ZnO) phosphors with highly efficient green emission have been prepared by calcining ZnS with NH4Br as additive in air atmosphere. The luminescent properties of as-prepared ZnO phosphors were characterized by X-ray photoelectron spectroscopy and photoluminescence. Our results reveal that the green emission is ascribed to a transition of a photo-generated electron from the localized defect centers (Vo+) to a deeply trapped hole (VZn-) within the band gap. The addition of NH4Br enhances the luminescent emission of ZnO by promoting the formation of vacancies of both oxygen and zinc
[en] Highlights: • A novel refrigerant subcooling method is experimentally studied for the first time. • Condensation heat is used to drive an integrated cycle to subcool the refrigerant. • COP and exergy efficiency are improved by 18.6% and 27.9% respectively. • Effects of key parameters on the system performance are disclosed. • Economic analysis shows payback period varies from 2.4 to 3.2 years. - Abstract: Refrigerant subcooling could increase the refrigerating capacity and potentially improve the performance of refrigeration systems. In this paper, a novel subcooling method is experimentally studied for the first time in a hybrid vapor compression refrigeration system. In this system, condensation heat (~40 °C) is used to drive an integrated subcooling cycle to subcool the refrigerant leaving the condenser, which significantly increases the system performance. Changes in system performance are measured as functions of the following variables: the mass flow rates of the dehumidification air, ambient air, dehumidification solution, regeneration solution, and spraying water. Comparisons are made between the proposed system and the traditional water-cooled chiller. The proposed system can achieve a larger degree of subcooling (15–20 °C); what’s more, it shows much higher performances than the traditional water-cooled chiller: COP and exergy efficiency of the chiller are improved by 18.6% and 27.9%, respectively. Performance of the integrated subcooling cycle is also evaluated; it has a low COP, with the maximum value of 0.13, due to the low-grade condensation heat; however, it has a pretty high exergy efficiency, with the maximum value of 0.28, which indicates the effective use of the low-grade heat. In addition, an economic analysis of the integrated subcooling cycle is made with a project life cycle of 15 years; the payback period varies from 2.4 to 3.2 years based on different electricity tariffs, and the savings to investment ratio is between 1.3 and 2.1, which indicates that the project is profitable.
[en] Highlights: • A novel compact STF with internally extruded pin–fin flow channel is proposed. • Thermal performance of the STF is characterized in different operation modes. • This STF is with simple structure, low cost and high feasibility in building design. • Such STF can achieve better thermal performance as conventional ones. - Abstract: The fully building integrated Solar Thermal Facade (STF) systems can become potential solutions for aesthetics architectural design, as well as for the enhancement of energy efficiency and reduction of operational cost in the contemporary built environment. As a result, this article introduces a novel compact STF with internally extruded pin–fin flow channel that is particularly suitable for the building integration. A dedicated simulation model was developed on basis of the heat transfer and the flow mechanics. A prototype of this STF was fabricated and then it was tested under a series of controlled environmental conditions. The experimental validation illustrated a good agreement with the simulation results, indicating the established model was able to predict the STF’s thermal performance at a reasonable accuracy (i.e. mean deviation of less than 5.46%). The impacts of several operational parameters, i.e. equivalent solar radiation, air temperature, air velocity, water mass flow rate and inlet water temperature, on the STF thermal performance were then discussed respectively. Given the baseline testing condition, the collector efficiency factor F′ is almost 0.9930, leading to a relatively high nominal thermal efficiency at about 63.21%, which demonstrates such STF, with simpler structure, lower cost and higher feasibility in architectural design, can achieve an equivalent or better thermal performance than recent bionic STF or the conventional ones. It is also concluded that the thermal efficiency varies proportionally with solar radiation, air temperature, and mass flow rate of water, but oppositely to air velocity and inlet water temperature. A sharp decreasing trend of this STF’s thermal efficiency against the (T_i_n − T_a)/I was observed under the given operational conditions, which indicates current STF design is only suitable for pool heating, domestic hot water and radiant space heating in areas/climates with warm ambient air temperature and sufficient solar radiation. The overall research results are beneficial for further design, optimization and application of such STF in various solar driven systems, including the provision of hot water, space heating/cooling, increased ventilation, or even electricity in buildings. Such STF technology has the potential to boost the building energy efficiency and literally turn the envelope into an independent energy plant, creating the possibility of solar-thermal technologies deployment in high-rise buildings.