查看更多>>摘要：Thermal energy storage/sink is imperative for any type of solar energy harnessing system/technology. However, the inevitable challenge remains in developing and optimizing a thermal energy storage (TES) system suitable for long cyclic operation. This work numerically analyses the melting characteristics of phase change material (PCM; RT42)-based latent thermal energy storage (LTES) as a two-dimensional rectangular enclosure. The consequence of utilizing internal-external extended surfaces (fins) with realistic convective boundary condition is studied for photovoltaic thermal (PVT) applications. The application of internal-external fins is intended for enhancing the charging (melting) duration according to the long cyclic solar operation and better dissipation of heat after complete melting. Three kind of fins-rectangular, triangular, and Y-type- are investigated for two orientations of storage/sink enclosures-upright (θ = 90°) and inclined (θ = 35°). Moreover, three fin configurations are analyzed-equal, decreasing and increasing-stepped-based on the arrangement of fins along the enclosure height. Two-dimensional, transient numerical simulations are conducted for governing PCM melting, contemplating the consequences of natural convection. The performance assessment of each storage unit (orientation/fin-type/fin-arrangement) is made based on parameters such as time enhancement ratio, suppression ratio, liquid fraction, temperature distribution, melt velocity magnitude, storage capacity/rate, and enhancement in Nusselt number. The melting is delayed for triangular and Y-fins enclosures as compared to that for rectangular fins. Equal and increasing-stepped Y-fin arrangements yielded the largest time enhancement ratios of 42.38% and 29.86% for inclined and upright enclosures, respectively. Moreover, Y-fins-based enclosure produced a larger average suppression ratio as compared to other types of fins. A 2.56, 2.67, and 2.64 times enhancement in Nusselt number is observed for increasing-stepped fin arrangement of rectangular, triangular, and Y-finned upright enclosures, respectively. However, for the inclined enclosure, a 2.32 times enhancement in Nusselt number is reported for increasing-stepped Y-fin.
查看更多>>摘要：Use of valley electricity for thermochemical energy storage that can be applied to building refrigeration is an important method for adjusting the natural fluctuations in the power grid and improving the energy efficiency of the system. This paper proposes an absorption thermochemical energy storage cycle of double compression coupled with two-stage generation to reduce the generation temperature and achieve a higher coefficient of performance and energy storage density. Based on the thermophysical properties of LiBr/H_2O and LiNO_3-[BMIM] NO_3/H_2O, the thermodynamic performance of this cycle under various working conditions was studied by MATLAB. The results showed that under the same working conditions, the temperature of the generator using LiNO_3-[BMIM]NO_3/H_2O was 34 K lower than that using LiBr/H_2O. The coefficient of performance of this new cycle using LiNO_3-[BMIM]NO_3/H_2O was obtained at 12.5, which was much greater than that using LiBr/H_2O at 7.9. Moreover, the former also achieved a larger energy storage density of 406 kJ/kg. The cycle using the LiNO_3-[BMIM]NO_3/H_2O working fluid not only exhibited better performance, but was also able to effectively use valley electricity and play the role of "peak cutting and valley filling".
Monica Caballero-EsparzaJuan Ramon Lizarraga-MorazanMartin Picon-Nunez
11页
查看更多>>摘要：The supply of low temperature heat to a process plant can be accomplished using different types of solar thermal technology such as Flat Plate Collectors (FPC), Compound Parabolic Collectors (CPC) or Parabolic Trough Collectors (PTC). These technologies differ in the surface area required, the initial investment cost and the number of hours in the day they can provide the required heat duty to replace the use of fossil fuel. Out of these technologies, the PTC proves to be the most convenient in the lifetime horizon despite the large initial costs either for low or for medium temperature applications. This is demonstrated using the net present value of the life cycle energy savings. It is shown that for a process with a heat duty of 700 kW, supply temperature of 130℃ and assuming the lowest solar radiation, the PTC requires a surface area of 3191.76 m~2 giving a life cycle energy saving of 1967089.32 USD and a payback of 3 years. For a case with 400 kW and supply temperature of 70℃, the PTC remains the best option with a surface area of 1768.42 m~2 and a life cycle energy saving of 1085939.35 USD and a payback of 4 years.
I. J. Canela-SanchezD. Juarez-RomeroJ. A. HuicocheaR. F. Escobar-Jimenez...
13页
查看更多>>摘要：This work presents the development of a physical model for an absorption heat transformer with falling film heat exchangers. The absorption heat transformers are devices for increasing moderate temperatures of heat sources to more useful levels to be used in some processes as water purification. The process units are modeled in lumped form to reduce the model complexity. The phenomena that appear in the falling film heat transfer are analyzed. The laminar flow with partial wetting describes the behavior of the units. The falling film reaches full wetting efficiency at the film breakdown onset Reynolds number, which is considered a parameter. Sensitivity analysis is applied to the AHT model to identify the sensible variables that are affected by the film breakdown onset Reynolds number of each unit, then these parameters are estimated. The film breakdown onset Reynolds number estimated for the units are from 8.90 to 141.28, which depend on the unit geometries, the fluid properties, and the physical phenomenon. The evaporator lumped model of the process of this work is compared with a detailed evaporator model specific regarding each round of coil presented in previous research. From this comparison, it is recommended to model the first tube of the evaporator individually while the other tubes can be modeled in lumped form. The model results are compared with experimental results to validate the model. The comparison shows low errors from 0.22 to 6.13%, except for the pressure of the evaporator-absorber chamber with an error of 13.83%. Finally, some tests at different generator temperatures varying the cooling water temperature from 33.8℃ to 31.8℃ decrease the pressure by approximately 0.53 kPa and increase the flow of refrigerant by 0.00011 kg/s. The establishment of the model and the estimation of the unknown parameters would contribute to the development of accurate dynamic models for absorption heat transformers to implement strategies to improve control process.
Nazar GrinisinTibor BesenicDarko KozaracMarija Zivic...
10页
查看更多>>摘要：Flue gas desulfurization technology for sulfur dioxide removal from exhausts of combustion processes is becoming more widespread as allowed emission levels keep getting lowered. Spray scrubbers have gained traction in coastal and maritime applications, where seawater can be used as a scrubbing liquid. Detailed numerical models are required for accurate replication of processes in these applications, with complex physical and chemical phenomena considered. Among them, mass transfer modeling between phases has proven to be particularly important for obtaining accurate simulation results. Present work investigates a new model for calculating liquid side mass transfer coefficient in falling droplets, which considers additional parameters such as surface renewal rate, Reynolds number, and droplet diameter, compared to simplified and more common approaches. Initial modelling was performed on a single droplet level, followed by the implementation in the computational fluid dynamics framework and expansion to the entire spray. Desulfurization efficiencies for real cases modelled with the new approach were compared with the experimental data and the previously used penetration theory model results. The newly implemented model investigated the influence of operational parameters such as water and gas flow, sulfur dioxide concentration, droplet size, and distribution on desulfurization efficiency. The results obtained by the new model showed the expected trend of increased efficiency with the water flow increase, as well as greater sensitivity to operational conditions in different cases. Furthermore, compared to the previously used model, the present work more accurately replicated removal efficiencies in simulations of seawater spray scrubber applications, which is beneficial in designing new and more efficient equipment.
查看更多>>摘要：This paper investigates turbulent fluid flow and heat transfer over a porous medium in a channel using pore-scale large eddy simulation. Special attention is placed on the exchange of heat and flow between the porous and nonporous regions through the interface between the two regions. For this purpose, two different porous systems made of a packed bed of spheres and rectangular rods are analysed and the results are compared against a solid block case of the same size. Flow visualization shows that a significant portion of the fluid entering the porous blocks leaks from the porous region to the non-porous region through the porous-fluid interface. To discuss the effects of this flow leakage on the flow features and heat transfer, discussions are made regarding velocity, pressure, and temperature fields, as well as coherent structures, and turbulence production. The flow pattern inside the porous region indicates that the flow leakage clogs the pore channels inside the porous medium which induces a significant reduction in the streamwise momentum of the pore flow. In addition, coherent structures show that flow leakage leads to the creation of counter-rotating vortex pairs of fluid flow within and above the porous block that results in the formation of organized hairpin structures. Finally, the comparison of turbulence production for the porous and solid cases together with the onset growth of the Kelvin-Helmholtz instability on the porous-fluid interface show a reduction in turbulent kinetic energy above the leading edge of porous blocks. This observation implies that for the porous cases the transition to turbulence is postponed to the downstream of the porous block and it is not achieved as fast as the solid block.
查看更多>>摘要：Solar receiver, as the core device of the dish concentrated solar thermal power generation system, is becoming a research focus and has attracted much attention. High temperature heat pipe, which works by following the principle of vapor-liquid circulation, has the ability to flatten the temperature of the hot spot, and maintain excellent heat transfer performance with the illumination of concentrated facula with inhomogeneous ultra-high energy flux density; Through the structural design, the compressed air flows outside of the high temperature heat pipe, and thus, compared with flowing in the pipe, the heat transfer coefficient is increased and the flow resistance is reduced. In view of the above advantages, we designed a dish concentrated solar receiver with finned high temperature heat pipe as the core heat transfer device, prepared a high temperature heat pipe with a large length-diameter ratio, and conducted a quantitative performance study comprehensively and systematically. The present study verified the safety and stability of the high temperature heat pipe with the illumination of concentrated facula with inhomogeneous ultra-high energy flux density, theoretically analyzed the process of frozen start-up and heat transfer limits of high temperature heat pipe, explored the reasonable frozen start-up mode, carried out the parameter optimization and consecutive long-time operation stability test, and quantitatively verified the enhanced heat transfer capacity of finned high temperature heat pipe through the experiment. The results show that the HTHP prepared by the same process can operate safely and stably with the illumination of concentrated facula at the average energy flux density of 2.3 × 10~6 W/m~2, and maintain a high temperature above 700℃ as well as excellent temperature uniformity, which fully verifies the feasibility and superiority of HTHP as the core heat transfer device of the dish concentrated solar receiver. In addition, these results can provide quantitative experimental data and theoretical reference for the design and development of the dish concentrated solar receiver.
查看更多>>摘要：Due to the two key stages involving the enhanced geothermal system (EGS) development: the HDR fracturing stage and the EGS heat mining stage after fracturing, it is necessary to conduct the integrated research of heat mining and EGS fracturing for the accurate prediction of EGS productivity. Based on the THM-D coupling, we carry out the field-scale fracture propagation of HDR, and then launch the research on heat mining evaluation based on the obtained fracture morphology. Firstly, the fracture morphology and the corresponding heat mining performance obtained by hydraulic and SCO_2 fracturing are compared by synchronous fracturing of doublet-well. Subsequently, the impact of fracturing procedure, duration, velocity and in-situ stress on fracture propagation of SCO_2 fracturing and the EGS heat mining performance after fracturing is studied. The results show that the SCO_2 fracturing can reduce the fracture connection time by 77.01% and increase the damage area of 16.1% compared with hydraulic fracturing. Compared with asynchronous fracturing, the synchronous fracturing can save the fracturing time by 51.2% and increase the fracture surface area by 5.99%. The increase of 10 MPa in horizontal stress difference enhances the fracture surface area by 1.37%, the corresponding heat mining rate increase by 7.01%.
查看更多>>摘要：In this study, static numerical simulation was used to evaluate the performance of a water-cooled variable refrigerant flow (VRF) system for geometry of plate-type outdoor heat exchanger. In general, heat exchanger is designed based on full-load operating condition. Since most of operation for VRF system is part-load operation, part-load operating characteristics should be considered to optimize entire system. The geometry characteristics are divided into chevron angle and vertical length. Numerical models of cooling and heating modes were developed for the VRF system, system performance was represented by integrated energy efficiency ratio (IEER) for the cooling system and coefficient of performance (COP) for the heating system by AHRI standard 1230. Regardless of operating mode, 20° of chevron angle among 20°, 35° and 45° is the optimal angle of outdoor heat exchanger. The energy efficiency was proportional to the vertical length of the plate heat exchanger and inversely proportional to the chevron angle. In the cooling mode, the energy efficiency ratio (EER) calculation converged at a shorter vertical length of the heat exchanger with a decrease in operating loads. The optimal vertical length is 1.7 m with only considering full-load, and 1.2 m with considering part-load operation. In the heating mode, the tendency of energy efficiency was similar to that of the cooling mode. The optimal vertical length is 1 m which is 0.2 m shorter than the optimal length in cooling mode.
查看更多>>摘要：This paper tackles the issue of frost formation in non-hygroscopic rotary heat exchangers used for energy recovery from exhaust air under high-speed rotor conditions by means of numerical simulations and experimental approaches. On the basis of some idealized assumptions, a frost growth submodel is presented to predict the behavior of the rotary heat exchanger under frosting conditions. Frost formation is modeled by considering the mass diffusion of water vapor through the frost layer, taking into account supersaturation phenomena. Calculations were carried out using a three-zone model based on the modified ε-NTU method. The local heat transfer coefficient and the NTU calculation method resulting from the influence of the heat exchanger entrance region were also applied. The obtained correlations for the temperature effectiveness agree with the simulation data within uncertainty bounds. The results of the numerical simulations allowed us to determine the outdoor air conditions that initiated the frost accumulation phenomenon inside the thermal wheel for two values of return air relative humidity: RH_(2i) = 20% and RH_(2i) = 40%. In both cases, the threshold temperature for unsafe operating conditions increases with increasing relative humidity of the outdoor air. Under 'frost accumulation' operating conditions, the frost growth rate is approximately five times higher at RH_(2i) = 40% than at RH_(2i) = 20%. In this regard, the need to implement frost protection techniques increases significantly with an increase in relative humidity of return airflow. Further analysis conducted for the operation of a thermal wheel's operation under frosting conditions revealed that a latent heat flux contributed to the local frost density should not be neglected in a compact heat exchanger's model. Interestingly, the operation time of the rotary heat exchanger, and hence the growth of the thickness of the frost layer, has a significant influence on a local heat transfer coefficient α, however, it does not affect the Number of Transfer Units (NTU) visibly.
NSTL
Elsevier