相变储热式太阳能干燥系统连续干燥性能研究

    Continuous drying performance of phase change thermal storage solar drying system

    • 摘要: 为了解决传统太阳能干燥系统因太阳能不连续性和间断性导致的无法实现连续温度干燥的问题,该文设计了一种相变储热式太阳能干燥系统,旨在提高干燥效率、延长系统运行时间,并减少干燥室入口温度的波动。研究开发了两个TRNSYS模块(相变储热模块和干燥室模块),并结合TRNSYS其他标准模块搭建了相变储热式太阳能干燥系统仿真模型。以兰州市为例,模拟分析了系统在不同时间尺度(典型天气和7月1日至7月8日一周天气)下连续运行的动态特性及热性能。结果表明:在夏至日工况下,相比传统无蓄热太阳能干燥系统,相变储热式太阳能干燥系统干燥室平均入口温度提高10.34%,运行时间延长了3.7 h。在连续一周的运行中,系统干燥温度超过35 ℃的时间占比达到了72.7%,干燥室全天平均入口温度为57.3 ℃。特别是在7月2日8:00至7月3日8:00的运行过程中,干燥室入口温度超过35 ℃的时间占比达91.5%,夜间平均入口温度高达41.2 ℃,几乎实现了24 h的连续干燥能力。此外,相变材料的熔点对系统性能影响显著,低熔点材料在夜间干燥过程中表现出更佳的温度保持能力。该研究通过TRNSYS模拟相变储热式太阳能干燥系统在不同时间尺度和不同工况下的运行情况,揭示连续干燥下系统性能的动态变化规律,为进一步的太阳能干燥系统设计优化提供重要参考。

       

      Abstract: Drying can represent a critical process in both the industrial and daily life; It is often required to reduce the energy consumption during drying. Alternatively, solar drying can be expected to replace the traditional drying, due to the low carbon footprint and economic advantages. Among them, solar drying systems with the phase change materials (PCM) can also maintain the high drying efficiency, such as the simple structure and low cost. The operational time can also be significantly extended, compared with the solar-heat pump hybrid drying. Nevertheless, it is still lacking on the long-term continuous operation of these systems. In the present study, a numerical model was proposed for the PCM thermal storage unit in the drying chamber. The innovative concept of equivalent drying time was incorporated to facilitate the accurate prediction of drying curves under various operating conditions. A series of experiments were also carried out to validate the predictions. There was the close match between the prediction and measured data, with an average error of less than 5%. A dynamic simulation model of the PCM solar drying system was developed using the TRNSYS platform. The performance of system was evaluated at daily (a typical day of the summer solstice) and weekly (July 1 to July 8) time scales. The results indicate that the integrated PCM solar drying system was significantly enhanced the drying performance, compared with the traditional solar drying systems without thermal storage. Specifically, the average inlet temperature of the drying chamber increased by 10.34%, and the daily operating duration was extended by approximately three hours. Furthermore, the system was maintained a drying temperature above 35°C for 72.7% of the time during a continuous week of operation, with a daily average inlet temperature of 57.3°C. The nighttime inlet temperature reached 41.2°C under typical conditions. The duration above 35°C was accounted for 91.5%. The continuous drying was effectively achieved throughout the day. The simulation of performance was also performed on the typical days (spring and autumnal equinox). The significant improvements were found under the varying seasonal weather. The melting point of the PCM shared a considerable impact on the thermal stability of the system. The PCMs with the lower melting points demonstrated the superior thermal retention during nocturnal periods. There was a significant decrease in their temperature fluctuation coefficients, as the melting point decreased. The parameters were also optimized for the better performance of the system. A mechanistic model after numerical simulation was constructed to determine the dynamic patterns of PCM solar drying. Thereby, the finding can provide a theoretical foundation to enhance the energy efficiency. Rational selection of PCM and the optimization of thermal storage unit design can be expected to effectively solve the intermittency of solar resources towards all-weather operation. Furthermore, the heat transfer and thermal flow can be optimized for the high stability and drying efficiency. The finding can also provide the robust technical support for the scalable drying application of renewable energy.

       

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