Collaborative Research: Enhanced Adsorption Cooling with Monolithic Nanoporous Adsorbents

采用整体式纳米孔吸附剂的高性能吸附制冷系统吸附制冷是蒸汽压缩空调的替代技术。它由低档热能、太阳能或工业过程或汽车发动机产生的废热提供动力。它使用像水这样的环保制冷剂作为工质。吸附床是吸附冷却系统的核心，在该系统中，工质被吸附/解吸，以补偿传统的蒸汽压缩循环所需的功。为了产生冷却，吸附循环主要经历两个过程：加热-解吸-冷凝和冷却-吸附-蒸发。制冷剂在蒸发器中蒸发时，通过加热吸附材料并在冷凝器中冷凝来解吸，并通过冷却吸附材料来吸附。床的快速热响应是实现高性能的关键因素。填充床由于颗粒间的接触不良和颗粒与冷却面间的接触不良而导致导热系数低，常用于此类系统。在本研究中，将通过在换热器的铜翅片上生长具有一定厚度的整体式纳米多孔吸附层来构建一种创新的床。该整体柱具有内部蒸汽通道，以减少蒸汽扩散阻力，其导热系数预计将比填充床中的高出许多倍。所提出的整体可用于高效紧凑型吸附制冷机组。在研究的同时，高年级学生将接受设计环境友好型冷却系统的培训。将开发一种截然不同且具有潜在变革性的吸附制冷系统。该系统是基于整体式纳米多孔二氧化硅或铜。我们的研究计划将包括：制备整体，建立实验装置，模拟传热和传质过程，设计和建造使用纳米孔整体的新型床，以及测量表面应力。首先，通过完全去除纳米结构硅胶的残留溶剂来制备整体。硅胶的表面将覆盖一层液体石蜡，这样这一过程将在高温下以温和的方式进行，以防止整体开裂。将放置垂直柱子以形成蒸汽路径。在形成单体后，可以移除柱子。其次，将建立一个实验测量装置，以监测在期望的压力和温度下，吸附剂质量随时间的变化。测量的数据将用于确定质量扩散系数、活化能和吸附热。在建模部分，将求解纳米整体中产生的微孔内的传热传质过程。该模型将依赖于求解气孔内的流动以及水蒸气在固相中的吸附和扩散过程。在两相之间的交界处，将应用质量和能量平衡。下一步，为了监测所开发的整体吸附制冷系统的性能，将通过在换热器上生长一层厚的整体吸附床层来构建新的吸附床，从而增加吸附吸收。将对不同的设计配置进行测试，以得出最佳设计。通过计算比制冷功率(SCP)和性能系数(COP)来评价新型床层的性能。在冷却系统运行过程中，整体式纳米多孔材料可能会受到来自液体的巨大表面张力的影响，这可能会破坏纳米结构。因此，将通过测量覆盖在原子力显微镜(AFM)微悬臂梁上的一层材料的表面应力来测试所开发材料的机械稳定性。

High Performance Adsorption Cooling Systems Using Monolithic Nanoporous AdsorbentsAdsorption cooling is an alternative technology to vapor compression air conditioning. It is powered by low-grade heat, solar energy, or waste heat from industrial processes or automotive engines. It uses environmentally friendly refrigerants like water as the working fluid. The sorption bed is the core of an adsorption cooling system in which the working fluid is adsorbed/desorbed to compensate for the work needed in a conventional vapor compression cycle. To produce cooling, adsorption cycle undergoes two main processes: heating-desorption-condensation and cooling-adsorption-evaporation. The refrigerant is desorbed by heating the adsorbent material and condensing in the condenser while it vaporizes in the evaporator and is adsorbed by cooling the adsorbent material. Fast thermal response of the bed is the key factor that leads to high performance. Packed beds, which suffer from low thermal conductivity due to the poor particle-to-particle contact and poor particle-to-cooling surface contact, are regularly used in such systems. In this research, an innovative bed will be constructed by growing a monolithic nanoporous adsorbent layer with considerable thickness on copper fins of heat exchangers. The monolith has internal vapor channels to reduce vapor diffusion resistance and its thermal conductivity is expected to be many folds higher than those in packed beds. The proposed monoliths can be used in highly efficient and compact adsorption cooling units. In parallel to the research, senior students will be trained in designing environmental friendly cooling systems. A radically different and potentially transformative adsorption cooling system will be developed. The system is based on monolithic nanoporous silica or copper. Our research plan will include: preparing monoliths, building an experimental set-up, modeling the heat and mass transfer processes, designing and building new bed using nanoporous monoliths, and measuring surface stresses. First, monoliths will be prepared via complete removal of residue solvent of nanostructured silica gel. The surface of the silica gel will be covered with a layer of liquid paraffin so that the process will be carried out at high temperature and in a mild way to prevent the monolith from cracking. Vertical pillars will be placed to form the vapor paths. After forming the monoliths, the pillars can be removed. Second, an experimental measurement set-up will be built to monitor the change of the adsorbent mass as a function of time at a desired pressure and temperature. The measured data will be used to determine mass diffusion coefficient, activation energy, and heat of adsorption. In the modeling part, the heat and mass transfer processes will be solved in the micropores generated in the nano-monoliths. The model will rely on solving the flow in the pores as well as the adsorption and diffusion processes of vapor in the solid phase. At the interface between the two phases mass and energy balances will be applied. Next, to monitor the performance of the developed monoliths in adsorption cooling system, a new adsorbing bed will be constructed by growing a thick monolith layer on a heat exchanger, which increases adsorption uptake. Different design configurations will be tested to come up with the best design. Specific cooling power (SCP) and coefficient of performance (COP) will be calculated to evaluate the new bed performance. During the operation of the cooling system, the monolithic nanoporous material may experience large surface tension from liquid that may damage the nano structures. So the mechanical stability of the developed material will be tested by measuring the surface stresses of a layer of the material coated on a micro cantilever of Atomic Force Microscope (AFM).