Results 1 - 10 of 917
Results 1 - 10 of 917. Search took: 0.025 seconds
|Sort by: date | relevance|
[en] This preliminary study investigated data mining-based methods to assess and predict the performance of geothermal heat pump(GHP) system. Data mining is a key process of the knowledge discovery in database (KDD), which includes five steps: 1) Selection; 2) Pre-processing; 3) Transformation; 4) Analysis(data mining); and 5) Interpretation/Evaluation. We used two analysis models, categorical and numerical decision tree models to ascertain the patterns of performance(COP) and electrical consumption of the GHP system. Prior to applying the decision tree models, we statistically analyzed measurement database to determine the effect of sampling intervals on the system performance. Analysis results showed that 10-min sampling data for the performance analysis had highest accuracy of 97.7% over the actual dataset of the GHP system.
[en] Highlights: • A new numerical model for PCM based Latent Heat Storage System. • Numerical model is validated by experimental testing. • Influence of thermal conductivity on the thermal storage properties is analyzed. • Effect of phase transition temperature is discussed. - Abstract: The heat transfer properties of phase change materials (PCMs) are of importance for the efficiency assessment on the heat storage and release in solar thermal systems. Previous research results demonstrate that the increase of thermal conductivity of PCMs can enhance the thermal performance in solar thermal systems; however, the corresponding mechanism is not clear. To this end, this work investigates the influence of PCMs properties on storage performance of solar thermal systems. First, experimental testing was conducted to verify the effectiveness of a thermal simulation model in the heat storage and release process. Then, the proposed simulation model was used to investigate the performance of several commonly used PCMs in the process of melting and solidification. The influence of thermal conductivity and phase transition temperature on the thermal storage properties was analyzed. The analysis results demonstrated that the influence of phase transition temperature on the thermal system performance was greater than that of the thermal conductivity in short time, while the thermal conductivity contributed greater influence on the system performance in long time. The phase transition temperature hardly affected the total system efficiency if given enough heat transfer time. The findings in this work may provide a theoretical reference for the selection of heat storage materials.
[en] Highlights: • A TCS experimental setup was built to investigate temperature and mole fraction change details of the Ca(OH)2/CaO system. • Mass and heat transfer performance and cyclic reversibility of the Ca(OH)2/CaO system are analyzed. • Existing problems and its corresponding solution to improve cycle life of the system are concluded and raised. - Abstract: Thermochemical energy storage (TCS) stores and releases heat through a reversible chemical reaction. And since thermochemical material (TCM) is the most important part of an energy storage system, its properties directly affect the entire system. On account of a variety of advantages such as low cost, broad availability and suitable temperature range, thermochemical method based on reversible decomposition reaction has become a famous research. In this study, a TCS experimental setup was built to investigate thermal cycling stability of the Ca(OH)2/CaO system. Through successive 20 dehydration-hydration cycles, amount of stored thermal capacity and cyclic reversibility of the Ca(OH)2/CaO system are analyzed. After research and analysis, existing problems of the Ca(OH)2/CaO energy storage system including agglomeration and sintering, poor thermal conductivity, unevenness of heat release rate are concluded and raised. After an explanation of superior performance and existing problems occurred during the dehydration and hydration process, this research lays the foundation of applying the Ca(OH)2/CaO system to practical.
[en] The minority carrier’s recombination velocity at the junction and at the back surface is used for the modeling and determination of the optimum thickness of the base of a silicon solar cell in the static regime, under magnetic field and temperature influence. This study takes into account the Umklapp process and the Lorentz effect on the minority carriers photogenerated in the base. (Authors)
[en] Nuclear energy is categorized as the most economic and sustainable energy by IAEA. However, demand and needs for safety and reliability of nuclear industry are increasing after FUKUSHIMA. Especially, reliable data on the stability of nuclear fuel in a reactor and spent fuel is necessary for safety analysis of nuclear reactor and spent fuel. In this study standard reference data were developed and uploaded to the database of national standard reference center. The standard reference data produced in NFDC will be helpful for increasing reliability and stability evaluation of nuclear fuel and spent fuel
[en] Highlights: • A novel closed-loop pulsating heat pipe (CLPHP) with horizontal evaporator and condenser section, is proposed. • The evolution of small bubbles, vapor slugs, and long vapor plugs are observed. • The critical vertical height which can ensure the start-up of CLPHP is obtained. • The gravitational potential energy and heating power are the decisive factors that maintain the running of the CLPHP. - Abstract: Considering the requirement of special heat transfer position in practical application, a closed-loop pulsating heat pipe (CLPHP) with partial horizontal structure (horizontal evaporator and condenser configuration) is proposed, and the features of present CLPHP including vertical start-up height and thermal performance, are numerically investigated. The volume of fluid (VOF) model and the continum surface force (CSF) model are adopted in numerical process. The reliability of present numerical method is verified by comparing the results of vertical CLPHP using present method with those of available literature. The results indicate that the proposed CLPHP can start up and running normally, as long as the height difference between the evaporator and condenser section is more than a threshold which is defined as the critical vertical start-up height. Furthermore, the dimensionless critical vertical start-up height under the conditions of various heating power and filling ration is obtained. When the filling ratio of present CLPHP remains constant, it’s found that the thermal resistance reduces with the increase of the height difference between the evaporator and condenser section, which results from the greater gravity force effect. In addition, with the argument of heating power, the thermal performances of present CLPHP with different height difference are becoming the same gradually on account of the effect of latent heat and sensible heat transport.
[en] Highlights: • Subcooled boiling first occurs in the inner side of helical coils under the studied test conditions. • ONB can appear in helical coils when mean wall temperature is lower than saturation temperature. • Effects of various parameters on wall temperature and HTC are studied in helical coils. • New correlations have been proposed for ONB and HTC of subcooled boiling in helical coils. - Abstract: Helical coils have been widely used in a variety of applications, such as heat recovery processes, power plants, cryogenic systems, etc, due to the practical importance of high efficiency heat transfer, compactness in structure, ease of manufacture and arrangement. Experiment investigation of heat transfer characteristics of subcooled flow boiling in helical coils with different inner diameters and coil diameters was performed in the present paper. The rise angles of these helical coils were all 6 degrees. The system pressure was in the range of 1.8 MPa and 7.8 MPa, mass flux ranged between 300 kg/(m2·s) and 1100 kg/(m2·s), and heat flux varied from 100 kW/m2 to 450 kW/m2. The experimental results showed that the onset of subcooled boiling was significantly influenced by heat flux and system pressure. A new correlation to predict the onset of subcooled boiling was proposed, correlating experimental results within ±20%. The effects of heat flux, mass flux and system pressure on heat transfer behavior in subcooled boiling region were discussed. A new correlation of subcooled boiling heat transfer coefficient in helical coils was developed, correlating experimental results within ±20%.
[en] In this work, we deal with a three-dimensional modeling study of a silicon solar cell in steady state, at the temperature T, under monochromatic illumination of wavelength and placed in a magnetic field B. The use of the maximum diffusion law as a function of the optimal temperature made it possible to choose the values of the magnetic field B in the study of the effective diffusion coefficient Dk,j. The effect of both, the magnetic field B and the temperature T on the excess minority carrier density (x, y, z) is analyzed through the determination of the eigenvalues (Ck, Cj), for different grain sizes (gx, gy) and there combination velocity at the grain boundaries Sg. (Authors)
[en] Highlights: • DHC thermal mass and heat losses in energy supply are studied. • An Energy Management System is proposed to provide optimal supply strategies. • DHC thermal mass is used as energy storage increasing power plant benefits. • Heat losses are reduced by the use of an optimal supply temperature. - Abstract: Optimal supply strategies are a promising and inexpensive way to improve energy efficiency and to reduce expenditure in district network energy supply. These strategies aim to reduce heat losses by modifying the supply temperature within its working boundaries. Through supply temperature adjustment and network’s thermal mass it is possible to modify the energy stored in the network and to use it as an active energy storage. In this work, scenarios with variable and fixed supply temperature are compared during four weeks representing the different seasons. The simulations are carried out using an energy management system. Results of both scenarios for a district heating and cooling network placed in the Mediterranean region are presented and discussed.
[en] Subcooled boiling flow is the optimal choice for engine cooling system design, as no essential changes in its architecture are required while it is still possible to take advantage of the highest rates of heat transfer associated with nucleate boiling. The Chen's model is widely used for heat transfer coefficient (HTC) prediction in practical application conditions. However, direct validation based on experimental data upon the phenomenon interpreted by ‘suppression factor’ in the boiling component of Chen’s model is rarely shown in previous literatures. The present work conducted an experimental study on heat transfer from the bottom side of a rectangular heated block heated by the heating device to the upper side cooled by an internally flowing fluid in a rectangular channel. Tests were operated under representative pressure and temperature conditions with heat transfer coefficient (HTC) obtained in almost all the practical application velocity of flow. Results show that in the fully developed boiling region (FDB) the subcooled boiling heat transfer coefficient decreases as the flow velocity speeds up, which provides direct evidence for the validity of Chen's model. However, the wall heat transfer coefficient predicted by Chen's model doesn’t fit well with the present experimental data. Regarding this, a modified heat transfer model based on Chen style model for subcooled boiling flow is proposed. The HTC predicted by the modified model coincides well with the experimental data for all considered flow conditions in this paper.