N. M. Ortiz-RodriguezM. CondoriG. DuranO. Garcia-Valladares...
31页
查看更多>>摘要：The food industry accounts for approximately 30% of global energy consumption. In addition, food-processing activities contribute almost 26% of total greenhouse gas emissions. Dehydration is an energy-intensive unit operation, and most foods require drying, at least partially, at some stage of processing. Solar food drying can help sustainable energy supply and contributes to the solution of environmental degradation problems. This review focuses on medium and large-capacity solar drying systems, some challenges for their implementation in the agro-industrial sector, and the analysis of the integration schemes of solar thermal systems to the most common drying processes in the agroindustry. A systematic information search was carried out by analyzing four reviews on industrial solar drying and more than 50 articles on medium to large scale solar food drying (i.e., fresh product load greater than 90 kg or solar collection area greater than 30 m~2); from 1991 to date. The solar technology with thermal convection systems is mature for industrial food drying on an industrial and semi-industrial scale. There is a niche opportunity to produce industrial size systems for small and medium producers, which can incorporate added value to their production.
查看更多>>摘要：Vapor compression heat pump dryers (HPDs) are widely applied in food and industrial drying. Many efforts have been devoted to evaluating the HPD performance from the perspectives of energy, exergy, economy, etc. The current work offers a perspective of both energy efficiency and operational robustness to evaluate HPDs, intending to measure their design and off-design performance for various drying applications. Based on the proposed method, a case study on conveyor seaweed drying is presented. The preferable system(s) are found out among three candidates in general use, i.e., the basic heat pump dryer HPD_(basic), the HPD with air bypass HPD_(bypass), and the HPD with air bypass and subcooler HPD_(subcooler). The results show that, HPD_(basic) is not applicable for seaweed drying due to the required huge circulating air flow rate. HPD_(bypass) and HPD_(subcooler) are more preferred with better energy performance and operational robustness. Moisture extraction rate (MER) of 30.4 kg/h and specific moisture extraction rate (SMER) of 2.20 kg/kWh are reached in HPD_(bypass) at the design condition (T_(db) = 45℃, RH = 0.4), and are further increased to 35.8 kg/h and 2.58 kg/kWh in HPD_(subcooler). Meanwhile the off-design robustness indices are 0.14 and 0.22 for HPD_(bypass) and HPD_(subcooler), and can be enlarged to 0.19 and 0.33 after reselecting a compressor withstanding condensing temperature up to 58℃ (2℃ higher). Besides, HPD_(bypass) and HPD_(subcooler) can bear at least 30% air leakage in operation. Finally, operational robustness on other applications is also discussed for conveyor and batch dryers.
Sidra ZahidShafiq-Ur-Rehman QureshiWaqar Ahmed KhanUmer Zahid...
20页
查看更多>>摘要：Saline water desalination is attaining a prime focus to meet the challenges of water scarcity in the developing countries. Solar energy could be effectively utilized for desalinating the saline/sea water. In this respect, humidification dehumidification (HDH) cycle could prove as a promising solar driven desalination technology. In this paper, exergy analysis of single extraction air heated and water heated HDH desalination cycles are examined in detail. The top temperature and extraction rate parameters are considered crucial for any typical single extraction HDH cycle. Analysis of the effects of varying top cyclic temperature keeping fixed extraction/injection rate of 70% water/air stream is carried out. The top temperature is varied between 60℃-80℃ range. For the air heated air extraction cycle, total exergy destroyed is higher at lower top temperatures, resulting in lower exergetic efficiency. A similar trend is also followed by the air heated water extraction cycle for the top temperatures ranging up to 70℃. The total exergy destroyed registered an increase above 70℃ because of the significant rise in exergy destruction for the heater component. In the water heated cycle with water/air extraction, exergy destroyed decreases at lower top temperatures, thereby yielding higher exergetic efficiencies. For each HDH cycle configuration, correlation between the gain output ratio (GOR) and exergetic efficiency is additionally obtained. The results revealed that at the given top temperature, sub-component effectiveness, mass flow rate ratio and bottom temperature parameters, the GOR increases with the exergetic efficiency of the cycle.
查看更多>>摘要：An experimental study was conducted on trigger conditions of thermal stratification (TS) and fluid oscillation (FO) parameters during steam-air mixture horizontal jets at low mass flux. Two pipes with different lengths and a sparger were used as nozzle. Temperature distributions in the pool and nozzle were monitored continuously through thermocouples. In the experiments through pipe, only slight TS was observed in the case of short pipe. In the experiments through sparger, TS was observed in all pure steam jets and steam-air mixture jets with air mass fraction below 2%. Comparison between present experiments and previous experiments of vertical jets showed that horizontal jets were less prone to TS under the same conditions. Compared with short pipe, the experiments through long pipe had larger FO amplitude. The experiments through sparger had smaller probability and amplitude while larger frequency.
查看更多>>摘要：Adsorption desalination systems (ADs) driven by low-grade energy are regarded as sustainable alternatives to address water shortages due to their low energy consumption. Adsorbents is the key materials for the performance of ADs. In this work, a numerical study was performed to assess the water production performance of ADs based on metal-organic frameworks (MOFs) and other adsorbents under different working conditions. It is found that the trends in specific daily water production (SDWP) and performance ratio (PR) with hot and chilled water temperature are closely correlated with the adsorption characteristics, i.e., water uptake and the shape of adsorption isotherms. Moreover, considering both SDWP and PR, MOFs outperform other adsorbents owing to their high water transfer amount per cycle under various conditions. Among 12 adsorbents, DUT-67, Al-fumarate and MIL-100(Fe) are the top performers at chilled water temperature of 10℃, 20℃ and 30℃ respectively with average SDWP (PR) of 3.31 m~3/ton/day (0.33), 8.05 m~3/ton/day (0.56) and 13.46 m~3/ton/day (0.71), increased by 43.8% (32.0%), 65.3% (40%), and 102.7% (44%) compared to traditional silica gel. This work provides helpful insights into the correlation between desalination performance and water adsorption characteristics of adsorbents under varying working conditions, which favors choosing and designing of potential adsorbents for ADs.
查看更多>>摘要：The operating temperature of a solar panel affects its electrical efficiency. As the temperature increases, the solar cell's ability to generate electricity decreases so cooling is required to improve its performance. In this study, a novel design of photovoltaic phase change materials (PV-PCMs) system is established. It consists of a separate convex/concave dimpled aluminum plate and multiple PCMs that act as a heat sink. In order to achieve longer thermal management of PVs, the PCMs were arranged along the heat flow direction according to the melting temperatures. The thermal and electrical performance of the PV-PCMs system was numerically analyzed at different inclination angles. The system was tested using different numbers of dimples (smooth, 6, 8, and 10 dimples) and multiple PCMs with different thicknesses (single PCM, two-PCMs: 10-10 mm, two-PCMs: 15-5 mm) to obtain the optimal design. The numerical model and the literature data agreed very well. The (two-PCMs: 15-5 mm) arrangement with 8 dimples provides the longest uniform operating temperature duration and the highest PV electrical efficiency. The thermal management durations were 40, 30, and 25 min, during which the percentage decrease in PV cell temperature was 7.14, 4.65, and 2.22% at an inclination angle of 90°, 60°, and 30°, respectively, compared to the smooth wall with a single PCM. The novel design is recommended for future modeling of the PV-PCMs system.
查看更多>>摘要：Latent heat thermal energy storage has been recommended as an effective technology for the thermal management system of space exploration for its excellent ability to store thermal energy. However, the low thermal conductivity of phase change material seriously weakens the heat charging and discharging rates of the system. This paper introduces electrohydrodynamic, a popular active heat transfer enhancement technology, to enhance the phase change material melting in a shell-tube latent heat storage system under microgravity using the lattice Boltzmann method. Different from some previous works, this work mainly focuses on the combined effects of the electric Rayleigh number T and different gravity conditions on the melting performance. Results indicate that the electrohydrodynamic technique always shows good performance in accelerating the melting process, and especially, it is possible to save up to 90% of the charging time in some cases. In addition, the effect of eccentricity on the charging process is investigated, and it is found that although the concentric annulus is always the optimal configuration under no-gravity conditions, the optimal eccentric position of the internal tube largely depends on the electric Rayleigh number if one takes into account the gravity effects. Moreover, the effect of radius ratio Γ on the heat storage efficiency is also evaluated under no-gravity condition. The present work illustrates that electrohydrodynamic is an effective method for enhancing phase change material melting even under microgravity.
查看更多>>摘要：The load cycling range enlargement of thermal power plants is essential to ensure the power grid stability, which can facilitate the penetration of large-scale renewable power. If a part of reheat steam is extracted from the intermediate pressure turbine inlet to the thermal energy storage system, the minimum power load of the coal-fired power plant can be decreased. Moreover, constructing a thermal energy storage system extracting heat from the reheat steam is relatively easy because no modification on the boiler system is required. The reheat steam carries some sensible heat and much latent heat, and how to store the heat efficiently and cost-effectively is investigated in this study. Three thermal energy storage configurations are introduced: configuration A, as a basic configuration, one-stage sensible heat storage; configuration B applied two-stage sensible heat storage; and configuration C applied sensible and latent hybrid heat storage. Thermodynamic and economic performance of three thermal energy storage systems is evaluated and compared. The results show that integrating the thermal energy storage allows the minimum power load to be reduced from 30% to 17.64% of the rated load. Configuration B achieves the largest equivalent round-trip efficiency (67.54%), which is higher 2.56% than that of configuration A, and configuration C exhibits the lowest total cost of the equipment and storage materials (14.75 million USD) and levelized cost of delivery (143.98 USD/MWh), which is 5.94 million USD and 12.69 USD/MWh lower than those of configuration A.
Hang-Suin YangMuhammad Aon AliKarumudi Venkata Ravi TejaYi-Feng Yen...
12页
查看更多>>摘要：A Stirling engine is an external combustion engine that can be operated using different types of heat sources. Stirling engines can be applied in many renewable energy fields. The purpose of this study was to optimize the design of a kW-class beta-type Stirling engine with a rhombic drive mechanism for a concentrated solar power system. In this study, an energy method and a modified non-ideal adiabatic model were used to predict the shaft power and operating speed of the proposed engine. The simulation results obtained using the model were validated by the experimental results. The experimental results revealed that the instantaneous maximum power of the proposed engine could reach 1312 W at 1390 rpm under a helium pressure of 6 bar and heating temperature of 650℃. On the basis of the experimental results, the local maximum shaft power was obtained using the quadratic interpolation method. Therefore, the effect on shaft power of the engine speed could be ignored, thereby reducing the calculation time. The effects of the charged pressure and design parameters of the heat exchanger on local maximum shaft power were studied. Multivariable optimization of the prototype engine was conducted based on the results of the quadratic interpolation method. Both the simplified conjugate gradient method and the variable step size conjugate gradient method were adopted as optimization algorithms. The results indicated that the number of iterations required in the variable step size conjugate gradient method to approach the optimum point was approximately one-third that required in the simplified conjugate gradient method. The results demonstrate that the maximum shaft power of the prototype engine can be further increased from 1137 W to 1503 W. Thermal efficiency and mechanical efficiency can also be increased to 13.5% and 94.3%, respectively.
查看更多>>摘要：The present study built a type of wall composed of phase change materials (PCMs), micro-channel heat pipe, and a radiative plate, the combination of which makes use of the radiative cooling (RC) technology of the radiative plate that ultimately releases the heat of PCMs at night. First, the new wall (RC-PCM) testing rig was built and tested, after which the experimental and numerical results were compared. Furthermore, to compare to the thermal performance of the RC-PCM wall, Brick and PCM walls of the same size as the RC-PCM wall were established as reference walls. Second, the thermal evaluation of RC-PCM wall was investigated using a numerical method. Finally, the RC-PCM wall thermal performance was investigated by varying the PCM position of the load-bearing wall, from which the numerical results of the two were compared. The research results indicated that: (1) the interior surface temperature of the RC-PCM wall was lower than that of the Brick and PCM walls, where PCM availability was 7% higher than that of the PCM wall. (2) The indoor thermal comfort of PCM on the indoor side is higher than that of PCM on the outdoor side. (3) The RC-PCM-in (PCM layer is located inside load-bearing wall) showed certain advantages in the interior surface temperature and PCM availability as compared to the RC-PCM wall. In conclusion, RC-PCM wall exhibited good potential in engineering applications.
NSTL
Elsevier