Olivia C. da RosaAlexandre K. da SilvaLouis Gosselin
11页
查看更多>>摘要：Heat recuperators are commonly used in numerous industrial processes, including power generation plants. Additionally, with interest in power cycles operating with supercritical fluids, such as carbon dioxide (s-CO_2), a better understanding of heat exchangers operating with the hot and cold fluids near the pseudo critical points is needed. Therefore, this study models a printed circuit heat exchanger operating with s-CO_2 on the hot and cold sides using a finite volume formulation. The model considers the convective resistances associated with the s-CO_2 on both sides of the heat exchanger, as well as the conductive resistance due to the solid wall separating them. The recuperator's behavior was analyzed while varying its geometrical (diameter, length and number of channels) and operational (inlet pressure and temperature on both sides) parameters. Thermal and hydraulic figures of merit were considered, such as the heat exchanger effectiveness, thermal conductance and pressure drop. The results showed that similarly to geometrical parameters, the operational conditions also had a significant effect on the figure of merits considered, especially near the pseudo critical line, a region characterized by major property changes. In addition, a multi-objective optimization process was implemented returning a series of optimal solutions, which were presented as a Pareto curve. Among the results obtained, it is worth mentioning that an optimum hot s-CO_2 inlet temperature was obtained for the entire front at roughly 650 K, while there were a minimum number of channels and a cold s-CO_2 inlet pressure, close to 155,000 and 15 MPa, respectively, limiting the Pareto front, which were related to the maximum effectiveness.
查看更多>>摘要：As a burgeoning active thermal insulation technique, the pipe-embedded building system has attracted particular attention from architects and engineers. A novel pipe-embedded building system based on flat-plate heat pipe (FPHP) is proposed to solve the inherent issues like operation energy consumption and facade integration. The experimental investigation is firstly conducted on an FPHP prototype to explore its thermal response characteristics at different tilt angles and heat loads and then performed on an FPHP-embedded composite wall to explore the preliminary thermal performances. Results showed that the length of the condensation section with uniform temperature distribution accounted for at least 88.9% when the heat load was smaller than 3000 W/m~2, which verified the feasibility of using FPHP to complete the heat charging process. The tilt angle had a significant influence on the thermal resistances, startup behavior, and effective thermal conductivity of FPHP, and the optimal tilt angle could be reduced for load reduction mode, while this value could be increased for supplementary heating mode. When the heat source temperature is 25 and 35℃, the equivalent heat transfer coefficient value of the FPHP-embedded wall under test conditions is 0.19 and -0.08 W/m~2·℃, respectively. Meanwhile, the corresponding heat loss reduction rate could reach 62.7% and 116.4%, respectively. Overall, the results evidenced the effectiveness of FPHP-embedded building systems, which could also contribute to its system design, operation control, and further investigation.
M. V. N. Surendra GuptaHasan BaigE. AmeenAnanthanarayanan Veeraragavan...
15页
查看更多>>摘要：Thermophotovoltaic (TPV) systems offer an efficient way to directly convert thermal energy into electricity using the radiation emitted from a high temperature source. Compared to the conventional photovoltaic (PV) systems, TPVs have additional elements such as filters to tailor and shape the radiated energy impinging on the PV cell to improve the conversion efficiency. High contrast grating (HCG) structures are integrated within the combustor walls which act as a selective filter. The filter plays a major role by suppressing the sub-band gap radiation and reflecting it back to the combustion source. Typically, amorphous silicon (a-Si) periodic gratings with quartz as a substrate material are optimized for gallium antimonide (GaSb) PV cells. In this work, we have carried out detailed Multiphysics simulations to study the performance of GaSb, Si and InGaAsSb PV cells when operating in a micro combustor emitting non uniform radiation source operating in a temperature range of 500-2000 K. Results show that the proposed TPV system can have a power density of 250 mW/cm~2 using GaSb PV cell. Further, significantly more photons can be converted to useful power with a higher cut-off wavelength PV cell such as InGaAsSb. Therefore, the power density can be as high as 410 mW/cm~2. This study shows that the TPV systems employing low bandgap PV cells can have ＞ 20 times higher power density compared to conventional silicon PV cells.
查看更多>>摘要：Rectangular shell-tube thermal storage systems have a wide range of applications in both industrial waste heat recovery and solar power plants. Mastering the heat transfer mechanism and improving the heat transfer efficiency can considerably enhance the energy-utilization. In this study, we experimentally investigated the dynamic thermal behavior of a visualized rectangular shell-tube type phase-change heat storage device containing natural convection, using paraffin wax as the phase-change material. Subsequently, the corresponding numerical simulation was completed. The two results agreed well, within a maximum error of ± 5%. In addition, the theoretical parameters of paraffin wax were examined experimentally. The impact of inlet temperature, flow rate of the heat transfer fluid, and the length-to-diameter ratio of the heat flow tube on the efficiency of the system was studied. It was observed that a rise in inlet temperature from 353 K to 363 K increased the melting rate by 21.43%. When the inlet flow rate was increased from 2 L/min to 5 L/min, the melting rate was raised by 16%. However, reduction in tube length-to-diameter ratio from G = 14 to G = 12 shortened the melting time of the PCM by 6.64%. The equation for the liquid phase fraction of the PCM with dimensionless Ste and Ra numbers was fitted. The research results have a specific guiding significance for utilizing rectangular shell-tube energy storage systems.
查看更多>>摘要：The mid-temperature phase change materials have attracted wide attention in solar energy storage. In this study, erythritol and urea binary eutectic mixture was prepared and expanded graphite was used to reduce supercooling and enhance thermal conductivity of the binary material. The supercooling of binary eutectic mixture with 3 wt% expanded graphite was 7.43 K, which was 43.03 K less than pure binary mixture (50.46 K). The thermal conductivity of composite phase change material was 2.25 W·m~(-1)·K~(-1), which was approximately 3.73 times higher than the pure eutectic mixture. The test of corrosion results turned out that the new-made material had lower corrosion rate to common metal materials. X-ray diffraction and Fourier transform infrared spectroscopy tests exhibited that no chemical reaction was triggered during the preparation of the proposed composite. The energy consumption of the solar heating system with the proposed composite was numerically investigated by TRNSYS software. Compared with the traditional heating mode, composite phase change materials integrated solar heating system could save 5 tons of standard coal in a heating season for a building with heating area of 1125 m~2 in Tianjin, China. Therefore, it is proposed as a promising phase change material for solar energy storage.
Zbigniew RogalaRafal SiemaszBlazej BaranAdrian Kwiatkowski...
18页
查看更多>>摘要：The paper presents a preliminary experimental study of the precooled MR JT cryocooler, which was designed for LNG recondensation and potentially liquefaction purposes. The paper also includes the most vital aspects of the design: compressor selection and limitations, selection and size of expansion devices, effect of MR composition on the stability of the load temperature, and minimum approach during recuperation. In addition, a detailed description of the features of the test stand is provided. A series of experimental tests has been performed with varied heat load, suction pressure, discharge pressure, and precooling temperature. The cryocooler provided cooling power of 270 W at a temperature of -158℃, which is the largest MR JT cryocooler reported in the literature. Experiments indicate an optimum precooling temperature similar to that derived from the theoretical study (approximately -35℃).
Andrew N. AzizSaad MahmoudRaya Al-DadahMohamed A. Ismail...
13页
查看更多>>摘要：Evaporative cooling consumes fraction of the electricity required by the vapour compression refrigeration system but is only effective in dry weather conditions. A promising method for controlling the incoming air humidity is to use a desiccant wheel before the evaporative cooler. Currently, most desiccant cooling systems employ conventional materials like silica gel or zeolites which have low water uptake. Metal Organic Framework (MOF) materials are new class of meso-porous material with high water adsorption capabilities (～1.5 kg/kgads). Using numerical modelling and experimental testing, this paper investigates the use of four MOF materials namely CPO27(NI), MIL100(Fe), MIL-101(Cr), Aluminium Fumarate and silica gel on the performance of a stationary desiccant heat exchanger (honeycomb Aluminium structure coated with MOF desiccant material) in terms of the Coefficient of performance (COP) and moisture removal rate. Numerical results showed that Aluminium fumarate produced the highest COP of 0.65 with water removal rate of 12.65 g/kg dry air and MIL-101(Cr) produced the highest moisture removal of 15.99 g/kg dry air but with COP of 0.44 compared to silica gel and other MOF materials used. Experimental tests were carried out using honeycomb structure coated with Aluminium fumarate as the stationary desiccant wheel and results were compared to numerical modelling showing good agreement with maximum deviation of 13%. Experimental and modelling results showed that the rate of moisture removal increases with the increase of air inlet humidity and with the decrease of incoming air speed. Also, results showed that the highest moisture removal occur after the first 2 min from starting the dehumidification process highlighting the advantage of using MOF materials.
查看更多>>摘要：Due to the abundance and cleanliness, Solar energy has been used for desalination and demonstrated great potential to tackle the grand challenges of freshwater shortage. For more efficient solar evaporation, heat-localized solar evaporation was proposed in 2014, in which heat could be localized at the air-water interface. With the high energy efficiency and simple operation, heat-localized solar evaporation systems have drawn great interest recently, and much progress has been made in exploring new materials and structures for better evaporation performance. Especially, many three-dimensional evaporation structures have been proposed, which enhances the evaporation rate of such systems to be higher than 1.5 kg m~(-2) h~(-1). However, most works focused on improving the evaporation part of such 3-dimensional evaporation systems, while the research on the condensation part are rarely seen, which could be more important for freshwater production. Here, we propose a method of thermal redistribution to improve vapor condensation by increasing the average temperature of generated vapor. We firstly prepared the 3-dimensional triangle-shaped evaporation structures with thermal redistribution, which could be bent at different inclination angles to promote evaporation. The vapor condensation performances were compared between structures with and without thermal redistribution, and results show that thermal redistribution could increase the condensation rate by about 30%. This attributes to the increased average temperature of the generated vapor, while low-temperature vapor could hardly condense in practice. This work presents the method of thermal redistribution, which could be further used to increase freshwater production in heat-localized solar desalination systems.
查看更多>>摘要：Elastocaloric cooling has emerged as a promising alternative to traditional vapor-compression refrigeration, and miniaturization is essential for commercialization of this cooling technology. Here, a compact NiTi elastocaloric air cooler with low force bending actuation is presented. The phase transformation of bent NiTi sheets (with a maximum strain of 7.20% on the surface of tension side) is induced at a specific driving force of 5.64 N·g~(-1), which is at least two orders of magnitude lower than those of tensive and compressive modes. The cooler with single-row bent NiTi sheets and air duct is studied, achieving an output air flow of 5.5 K temperature reduction in the cooling cycle and a maximum specific cooling power of 0.137 W·g~(-1) in experiments. The cooler with multi-row NiTi sheets produces a maximum cooling power of 11.5 W at the current stage. These results show a development potential for the compact and miniature elastocaloric cooling device.
查看更多>>摘要：China General Nuclear Power Group (CGNPC) Delingha 50 MW parabolic trough solar thermal power plant is the first commercial trough solar plant in China, and its solar field consists of 190 parallel heat collecting loops. For large-scale trough solar plant, balancing the flow rate of heat absorbing medium in each loop and stabilizing the outlet temperature are the key technologies and difficult problems. Regarding the Delingha 50 MW solar field as the research object, this paper mainly focusing on the dynamic characteristics of flow and heat transfer in the solar field. The hydrodynamic calculation model and the heat transfer dynamic model are established on the realtime dynamic simulation platform STAR-90. With the actual operation data, the built solar field model is validated by comparing the simulation results. On this basis, the disturbance simulations of direct normal irradiance (DNI), heat transfer oil's mass flow and heat transfer oil's inlet temperature are carried out. The dynamic response curves of disturbance and the thermal inertia time constant of the loops are obtained. The conclusions lay a theoretical foundation for the formulation of outlet medium's temperature control strategy in the solar field of large-scale trough solar thermal power generation system.
NSTL
Elsevier