Metal-organic frameworks (MOFs) for energy-efficient indoor moisture control: Synthesis, characterization and applications

Carbon neutral actions have dramatically changed the structure of the current energy market in the balance of carbon emissions and removals. In the past decades, building energy consumption has increased yearly and accounted for more than one-third of primary energy, contributing to a considerable part of total carbon emissions (40%). The built environment control in an energy-efficient way will determine the volume of the overall building energy consumption. Traditional control methods have been developed over the years but are still energy-intensive. However, in recent years, desiccant-based built environment control gained much more interest due to their flexible system structures and low energy consumption. The main drawback of this system lies in the poor performance of the selected desiccant, such as silica gel and zeolite, thus ongoing research on novel materials is indispensable.
Metal-organic frameworks (MOFs) are novel nanocrystal materials that have varieties of tunable structures and enable customized structure design based on the specific application, and represent the cutting edge of the available porous materials. Most MOFs have a strong adsorption affinity on water vapor uptake and mild regeneration conditions. Therefore, the overall scope contributes to investigating the potential applications of novel materials such as MOFs to the built environment control. With the pursuit of energy-efficient moisture control, a series of discoveries have been concluded. The present thesis includes three parts: i) the investigation of the moisture buffering performance on MOFs; ii) the dynamic performance of MOF coatings in actual conditions; iii) the fabrication and optimization of a novel humidity pump using MOFs.
In the first study, the moisture buffering performance of MOFs has been quantified by using experimental and numerical approaches. The experiments include the measurements of the practical moisture buffer value (MBVprac) and the determination of moisture properties of MOFs to calculate the MBVideal. Variation in the mass change of MOFs was continuously recorded as the MOF samples were attached to a balance. The measured results showed that MIL-100(Fe) has the maximum MBV compared with MIL-160(Al) and Al Fum, which is 10 times higher than traditional materials, such as laminated wood. Afterward, the numerical calculation was conducted to investigate the effect of moisture buffering performance using MIL-100(Fe) on the building energy consumption, which indicated that MIL-100(Fe) has excellent moisture buffering performance under dry, semi-dry, and temperate climates. Moreover, the mathematical deduction on moisture buffering theories with different boundary conditions shows excellent potential for predicting the indoor moisture variation. Based on Fick’s law, the relationship of moisture flux under different boundary conditions (square wave and harmonic wave) can be obtained and further modified by the MBV tests. The results show that this MBV model can predict indoor moisture variation well when referred to the HAMT model.
The sorption dynamics of MOFs has been experimentally and numerically investigated. The equilibrium uptake and kinetics of MOFs have been measured to obtain the corresponding isothermal model and linear drive force model, all of which have high R-squared. Based on these models, a dynamic sorption model has been built to predict the dynamic sorption performance of MOF coatings, and the measured results matched well with the simulated ones, indicating the high fidelity of the model. The parametric studies were then performed on the dynamic sorption model, and the results demonstrated that different factors, such as operation variables (i.e., cycle time, time ratio, regeneration conditions), material properties (i.e., crystal size, diffusivity, porosity), and the coating thickness could affect the net volumetric uptake. Different MOFs have different degrees of impact on the dynamic sorption performance under different surrounding air conditions.
The final work includes the fabrication, experiments, and simulation of a MOF-based humidity pump, aiming to achieve fast moisture regulation for a specific space. A MIL-100(Fe) based humidity pump was running with twice as much the dehumidification capacity and dehumidification coefficient of performance (DCOP) as a silica gel-based one, and the operation variables, including cycle time, supply power, and air velocity, can significantly affect the practical dehumidification performance and energy consumption. Moreover, this device was reported to have a fast humidity response when it comes to a high humidity condition. According to the symmetry of the unit channel, a CFD model was then proposed to disclose the correlation between flow field and heat and mass transfer, which is validated by a MIL-160(Al) based humidity pump. The simulated results have shown that the changes in the geometry of the unit channel and airflow state will change the flow field and then the dehumidification performance and the energy consumed. Subsequently, a lumped model on the MOF-coated humidity pump is constructed and validated to predict the overall performance of the device on the dehumidification performance, and the results indicated that this model could provide reasonable accuracy in predicting the outlet air conditions. The humidity pump proposed here provides a new dehumidification path and gives new insight into the application of MOFs on the built environment control.