Collaborative Research: Enhanced Adsorption Cooling with Monolithic Nanoporous Adsorbents

采用单片纳米多孔吸附剂的高性能吸附冷却系统吸附冷却是蒸汽压缩空调的一种替代技术。它由低品位的热能、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作流体。吸附床是吸附冷却系统的核心，在该系统中，工作流体被吸附/解吸，以补偿传统蒸汽压缩循环所需的工作。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。制冷剂通过加热吸附剂材料并在冷凝器中冷凝来解吸，同时在蒸发器中蒸发并通过冷却吸附剂材料来吸附。床的快速热响应是导致高性能的关键因素。由于颗粒与颗粒之间的接触不良以及颗粒与冷却表面的接触不良，填充床的导热性较低，因此通常用于此类系统。本研究将在换热器的铜翅片上生长一层厚度可观的单片纳米多孔吸附剂，构建一种新型床层。整体体具有内部蒸汽通道，降低了蒸汽扩散阻力，其导热系数预计比填充床高许多倍。所提出的整体可用于高效紧凑的吸附冷却装置。在研究的同时，高年级学生将接受设计环保冷却系统的培训。一个完全不同的和潜在的变革吸附冷却系统将被开发。该系统是基于单片纳米多孔二氧化硅或铜。我们的研究计划将包括：制备单块体，建立实验装置，模拟传热传质过程，设计和建造使用纳米多孔单块体的新床，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单体。硅胶的表面将覆盖一层液体石蜡，以便在高温下以温和的方式进行该过程，以防止单体开裂。垂直的柱子将被放置以形成蒸汽路径。在形成巨石后，柱子可以被移除。其次，将建立一个实验测量装置，以监测在所需压力和温度下吸附剂质量随时间的变化。测量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，传热传质过程将在纳米单体产生的微孔中进行求解。该模型将依赖于解决孔隙中的流动以及蒸汽在固相中的吸附和扩散过程。在两相之间的界面处，将应用质量和能量平衡。接下来，为了监测所开发的单体在吸附冷却系统中的性能，将通过在热交换器上生长较厚的单体层来构建新的吸附床，以增加吸附吸收率。将测试不同的设计配置，以得出最佳设计。将计算比冷功率（SCP）和性能系数（COP）来评价新床的性能。在冷却系统运行过程中，单片纳米多孔材料可能会受到来自液体的巨大表面张力，这可能会破坏纳米结构。因此，通过原子力显微镜（AFM）测量涂覆在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).