查看更多>>摘要：Refrigeration systems in industrial food processing plants are large users of electric energy and often show high peak power consumption. Cold thermal energy storage (CTES) technology integrated into refrigeration systems can reduce the peak power requirement and achieve peak shifting by decoupling the supply and demand of the refrigeration load. This paper presents the design and performance of a CTES unit consisting of a pillow plate heat exchanger (PP-HEX) immersed into a low-temperature phase change material (PCM) as the storage medium. It is one of the first experimental investigations featuring a large-scale technical solution that allows for coupling the evaporation and condensation processes of the refrigeration system with the melting and solidification of a low-temperature PCM in the same heat exchanger. The charging and discharging performance of the plates-in-tank CTES unit was extensively tested using CO2 as the refrigerant and a commercial PCM with phase change temperature of -9.6 degrees C. The charging time was found to be mainly affected by the refrigerant evaporation temperature, while the discharging rate and discharged energy over the cycle was higher when increasing the refrigerant condensing temperature. Using a plate pitch of 30 mm resulted in the highest mean discharge rate and total discharged energy over the cycle with 9.79 kW and 17.04 kWh, respectively. The flexible CTES-PCM unit can be adapted to fit several refrigeration load characteristics and temperature levels by changing the PP-HEX geometry and type of PCM used as the storage medium.
查看更多>>摘要：At present, the cabin heating of aircraft mainly depends on engine bleed air, which results in large power loss from the engine. To address this problem, a new system coupling the vapor compression system and lubricating oil system to recover heat from the lubricating oil to heat the cabin is proposed based on the design concept of the integrated thermal management of aircraft. To investigate the feasibility and efficiency of the new system, an experimental system was established, and performance experiments were conducted under cold ground weather (5, 0 and -5 ?C), which was simulated in an environmental chamber. The lubricating oil temperature was kept at approximately 45 ?C, and heat was recovered from the lubricating oil at 0 and -5 ?C but not at 5 ?C. The experimental results and thermodynamic analyses indicate that when the lubricating oil system works rather than being closed, the heating time needed for the cabin temperature to reach 25 ?C is shortened from 494 s to 121 and 363 s, the compression ratio of the compressor is reduced from 7.65 to 2.81 and 3.73, the heating coefficient of performance is raised from 3.46 to 6.87 and 5.27, and the system exergy efficiency is improved from 22.9% to 33.2% and 32.8%. Therefore, the proposed system can be treated as feasible, efficient, and promising.
查看更多>>摘要：The formation of thermal hot spots is one of the most significant challenges in operating battery packs of electric vehicles (EVs). Aluminium (Al) is a common material for heat spreaders; however, its capability to dissipate heat in the event of a hot spot formation is limited by its thermal conductivity. The present work focuses on addressing this issue by coating the Al heat spreader with copper-graphene (Cu-Gr) nanocomposite to improve its heat dissipation characteristics. DC electrodeposition technique was employed for coating Cu- Gr nanocomposite material on Al plate, with an optimized graphene concentration of 0.5 g/L in the electrolyte bath, which showed maximum graphene loading in the coating. The Mach-Zehnder interferometric technique was employed to analyze the heat dissipation characteristics of Al heat spreaders, which is a novelty here. The real-time variation of heat dissipation and Nusselt number were obtained using Naylor's method. A comparative heat dissipation study was conducted among plain Al substrate, Cu film deposited Al substrate, and Cu-Gr nanocomposite film deposited Al substrate for three capacity ratings (C-rates), namely; 0.5 C, 1 C, and 3 C assuming a 5 Wh rated battery. The temperature difference between the center and the top of the Al heat spreader plate was the minimum for the composite coated plate. The drop in the temperature difference for the modified heat spreader was 66%, 55%, and 41% for 0.5 C, 1 C, and 3 C, respectively, compared to the bare Al heat spreader. The higher Nuavg values obtained from interferometric calculations for Cu-Gr composite coated heat spreaders indicate the uniform heat distribution along the surface than the Al heat spreader. The Cu-Gr composite coated Al substrate is a promising heat spreader candidate for maintaining uniform temperature in battery packs, extending battery life.
查看更多>>摘要：In this study, a novel transient air dehumidifier that can meet the dehumidification demands at places without a constant energy supply and uses multi-stage desiccant plates was proposed. Based on the simulation methods, parametric and optimization studies were carried out to extend the effective working duration. First, the air handling processes, transient temperature and humidity distribution fields of a typical desiccant plate were investigated. The results indicated that there are three reasons for the generation of ineffective dehumidification sections. To reduce the proportion of the ineffective sections and to increase the effective working duration, parametric analyses were carried out on the desiccant plate, considering factors such as the initial water content (W-ini), inlet air humidity ratio (omega(a,in)), and length of the desiccant plate. The results show that, if the required outlet humidity ratio (omega*) is lower than 12-13 g/kg, W-ini = 0.1 kg/kg is preferred for omega(a,in) between 15 and 19 g/ kg. Subsequently, a dehumidifier with multi-stage desiccant plates (original dehumidifier) was designed, and the optimized stage number for different total thicknesses of desiccant plates was investigated to improve the dehumidification performance. When omega* is 10 g/kg, the recommended stage number is two. Finally, an advanced dehumidifier is introduced in which an ineffective desiccant plate can be replaced with a new desiccant plate. For the advanced dehumidifier, the proportion of ineffective sections can be effectively reduced by 14.6%-93.2%, and the effective working duration can be extended by 2.2%-34.8%, compared to the original dehumidifier that has the same total thickness as the desiccant plates.
查看更多>>摘要：Large-scale circulating fluidized bed (CFB) power generation units with high steam pressure and temperature have been put into operation in order to obtain high cycle efficiency and less emissions. To ensure the flexible operation of such CFB units, it is necessary to build a model for the design of coordinated control system (CCS). However, few studies focus on modeling of such CFB units. Thus, this work develops a mechanistic dynamic nonlinear model of a large-scale supercritical CFB unit, which is suitable for CCS design because of its relatively low complexity. Firstly, model structure is derived from mass and energy conservation laws, combined with analysis of operational characteristics of a 600 MW supercritical CFB unit. Then, unknown parameters and nonlinear functions in model are identified based on running data by using regression analysis and optimization algorithm. Validation results show that the model has satisfactory accuracy and it can capture essential dynamic characteristics of the unit and nonlinearity of CCS. More importantly, the model accuracy is further improved by proposed variable dynamic parameters and correction of coal quality. After this, transfer function matrix of CCS is derived from dynamic nonlinear model, which can be applied to controller design and simulation analysis.
查看更多>>摘要：Thermal management for electric vehicles is becoming ever more significant for ensuring prolonged driving range. Climatization of batteries to the optimal operating temperatures is crucial for their performance and lifetime, and therefore they need to be heated when operating in cold climate. This often results in reduced driving range. This work numerically investigates the potential of thermal encapsulation of large battery packs for electric truck applications. Vehicle-level simulations were performed under drive cycle conditions at different operating temperatures to study its influence on the battery performance. Parking-driving cycles at various ambient temperatures and a parametric study on the encapsulation characteristics were carried out to assess the energy consumption under each condition. The study shows that high thermal resistance of the insulation material significantly reduced the heat loss to the environment acclimatizing the battery pack close to near-optimal operating temperatures, which can result in potential energy savings of about 15% at -25 ? when operating after a 12-h parking period.
查看更多>>摘要：The performance of open refrigerated display cabinet (ORDC) is largely affected by operational parameters and ambient air parameters, especially for that with two air curtains. Therefore, this paper aims to find a suitable operational range (volume flow and volume flow ratio) to satisfy the different ambient conditions for ORDC with two air curtains. Based on the validated 3D model, the average temperature, the thermal entrainment factor (TEF), and the energy consumption are employed as performance indicators. Results showed the ambient parameters had obviously influence on its performance. The increase of air velocity was harmful to the ORDC operation. By changing the stability of air curtain, the air flow direction also had influence on the performance. For the operational parameters, though the increase of volume flow could decrease the average temperature and TEF, it would largely increase the energy consumption. Therefore, the optimal volume flow ranged from 0.08 m(3)/s to 0.1 m(3)/s and volume flow ratio ranged from 1 to 2 were suggested to meet all different ambient air flow direction, during which the lowest average temperature and TEF can be obtained.
查看更多>>摘要：A performance evaluation method based on the Pareto frontier for different types of enhanced microchannel heat sinks has been proposed and applied to find the best comprehensive performance. The concept of Design Optimization Area (DOA) developed in our previous work is used for comprehensive evaluation and a microchannel heat sink within DOA means that the pumping power P-p and the total thermal resistance R-t could be both reduced compared with user demands. The comprehensive evaluation method consists of three steps: (1) conducting a multi-objective optimization of various microchannel structures to obtain the Pareto frontiers; (2) determining DOA above those Pareto frontiers based on user demands; (3) selecting the best one with the largest DOA. The immediacy and practicability of the method are verified through comparative studies. Detailed comparisons through this method between a hybrid microchannel heat sink and other three enhanced microchannel structures have been conducted. The results indicate that the hybrid microchannel has superior performance and it enables a lower P-p under the same R-t. The proposed evaluation method can also be applied to evaluate the comprehensive performance of various heat exchangers by changing the optimization objectives according to the limits or priorities of users.
查看更多>>摘要：Critical heat flux (CHF) phenomenon plays an important role during quenching phase when loss of coolant accident (LOCA) occurs in plate-type fuel reactor core. Heat transfer capability can be greatly enhanced with larger CHF value so that the cladding integrity can be maintained during quenching process. However, the related research on transient CHF during quenching process in narrow rectangular channel is extremely limited in literature, not to speak of combined quenching. Considered that quenching CHF mechanism may be totally different from that during heating process, experimental and mechanical researches were conducted comprehensively on CHF behavior during combined/bottom quenching in this article. Conclusion can be obtained that during combined quenching, bottom injection portion plays a dominant role because major portion of top injection may be expelled due to counter current flow limitation (CCFL) in narrow rectangular channel. Besides, boundary conditions such as injection methods, inlet subcooling, pressure and heating power have strong effect on CHF behavior. Based on previous visualization results, mechanism of quenching CHF can be attributed to dryout of liquid sublayer beneath coalesced bubble, and the proposed CHF model matches well with experimental data (RMS error 22.24%). The relevant findings are important for safety operation of plate-type fuel reactor and also can be applied in other industrial scenes where quenching phenomenon are encountered.
查看更多>>摘要：Determining a proper trade-off between energy consumption and indoor thermal comfort is important for HVAC system control. Deep Q-learning (DQN) based multi-objective optimal control strategy is designed for temperature setpoint real-time reset to balance the energy consumption and indoor air temperature. In addition, this study develops an EnergyPlus-Python co-simulation testbed to evaluate DQN control strategy in a simulation environment. A case study experiment is conducted to evaluate the performance of DQN control strategy for real-time reset of supply air temperature and chilled supply water temperature setpoint in a multi-zone building VAV system. The developed EnergyPlus-Python co-simulation testbed is used to train and test the DQN control strategy for performance analysis. The applied DQN strategy leans to update control actions (i.e. temperature setpoint) through interaction with the simulation environment. Simulation results show that the DQN control strategy is effective in finding a proper trade-off between the energy consumption of HVAC system and indoor air temperature. Meanwhile, the DQN control strategy can find a proper temperature setpoint reset sequence in smaller training episodes, and the control actions can be stable after ten DQN training episodes. This study provides a preliminary direction of deep reinforcement learning control strategy for temperature setpoint realtime reset in multi-zone building HVAC systems.
NSTL
Elsevier