Patent Application
20220390127
The present disclosure is directed to a desiccant coated fan blade. In one embodiment, a system includes one or more fans each with fan blades. The fan blades include a desiccant material on an outer surface. The desiccant material is operable to adsorb airborne moisture in an ambient airflow and desorb moisture in a heated airflow. The fan blades are operable to drive one or both of the ambient airflow and heated airflow via rotation of the fan.
In another embodiment, a method involves driving an ambient airflow with fan blades of a rotating fan. The fan blades include a desiccant material on an outer surface. The desiccant material is operable to adsorb airborne moisture in the ambient airflow. A heated airflow is also driven with the fan blades of the rotating fan. The desiccant material is operable to desorb moisture in the heated airflow.
In another embodiment, system includes a desorbing chamber that includes: a heater that emits heat into the desorbing chamber; a first fan that drives a heated airflow within the desorbing chamber; an entrance path providing ambient makeup air to the heated airflow; and an exit path through which humid heated air from the heated airflow exits the desorbing chamber. The first fan includes first fan blades with a first desiccant material on an outer surface of the first fan blades that desorb moisture in the heated airflow. The system also includes a cooling partition having a second surface onto which the humid heated air is directed. A second fan of the system drives ambient air to a first surface of the cooling partition. The second includes second fan blades with a second desiccant material on an outer surface of the second fan blades that adsorb moisture in the ambient airflow. A water collector of the system collects condensate resulting from the humid heated air being directed onto the second surface of the cooling partition.
These and other features and aspects of various embodiments may be understood in view of the following detailed discussion and accompanying drawings.
The discussion below makes reference to the following figures, wherein the same reference number may be used to identify the similar/same component in multiple figures. The drawings are not necessarily to scale.
The present disclosure relates to climate control systems such as heating, ventilation, and air conditioning (HVAC). In the commercial sector, HVAC systems are a major energy consumer, and many solutions have been proposed to increase the energy efficiency of these systems. Humidity control alone can consume roughly a quarter to one third of the energy requirements on an HVAC system, depending on climate (see, e.g., Airedale: “Vodacom case study,” www.airedale.com/web/About-Airedale/The-News-HVACNodacom.htm).
This disclosure relates to an efficient method of dehumidification and/or atmospheric water harvesting using a combination of desiccant materials and efficient integrated design. A fan with blades that includes a desiccant (e.g., a hierarchically porous, super-moisture absorbing material) on a surface of the blades is driven via a motor. The driving fan is used to circulate air, as in any HVAC system, while also optimizing convection at the surface of the moisture absorbing material, rather than directing air over a desiccant or cooling-coils. A desiccant material may use phase-changing polymers such as poly(N-isopropylacrylamide) combined with highly adsorbent materials such as Cl-doped polypyrrole is used which reduces the temperature required to regenerate the desiccant from >100 C to roughly 40 C, further increasing efficiency by allowing the material to be re-generated using low quality (waste) heat from other processes. Generally, low quality waste heat is heated air that is at a temperature <50 C, although may encompass higher temperatures, e.g., <60 C, <70 C, etc.
Existing dehumidifiers come in at least two forms, either in the form of mechanical or desiccant dehumidification. Mechanical dehumidification applies a vapor cycle similar to what is used in air-conditioners. Intake or recirculated air is over-cooled below its dew point by directing it over cooling-coils to condense the water vapor and reduce humidity. Afterward, the air is heated to its desired temperature before being circulated. This process is energy inefficient due to the over-cooling and heating cycles required to generate the dehumidified air. A desiccant dehumidifier uses a rotating wheel containing a regenerable desiccant to absorb moisture and reduce the humidity of incoming air. Hot air is passed through one part of the wheel as it rotates to regenerate the desiccant. Typical desiccant materials include nano-porous silica and zeolites which absorb moisture through capillary condensation. These materials need to be heated to temperatures exceeding 100 C in order to be regenerated. This heat is either generated through electrical heating or through waste heat sources.
In contrast to existing desiccant dehumidifiers, embodiments described herein may use a set of fans being run in parallel. The fans may have blades comprising an electro-spun mat of a super moisture-sorbent material which readily absorbs atmospheric moisture down to 30% RH and has much lower cycling temperatures (˜38 C, thus requiring lower quality heat) and also produces liquid water rather than vapor. Each fan's mass can be monitored in time, and once a steady state is reached the fan is heated using waste heat from building utilities (e.g., via a system of valves), which may be readily implemented in facilities with large quantities of information and computer technology (ICT) equipment, such as data centers or telecommunications installations. Other sources of waste heat may be used, such as heat from electrical or hydraulic motors in a manufacturing facility.
The dehumification (or water harvesting) system shown in
This disclosure describes an efficient method of dehumidification and/or atmospheric water harvesting using a combination of novel desiccant materials and efficient integrated design. A set of fans with blades comprising highly moisture-absorbing material is driven via a motor. The driving fan is be used to circulate air (either drawing in humid intake air over dry fan blades or waste heat over moisture-saturated blades using a set of control valves). As opposed to typical HVAC desiccant wheel systems, the fan is the desiccant rather than using a fan to re-direct air over a desiccant wheel. Use of desiccant fan blades reduces inefficiencies inherent in a desiccant wheel arrangement.
The embodiments described herein employ a material that can be regenerated with lower quality heat (e.g., lower temperature) and can uptake more moisture per unit mass than a typical desiccant. The highly-moisture sorbent material uses phase-changing polymers, such as poly(N-isopropylacrylamide), which undergo a phase change becoming hydrophobic above 40° C. as a means to regenerate the desiccant (from >100° C. for typical desiccants). An adsorbent material, such as Cl-dope polypyrrole, is typically added to the phase-changing polymer to increase its vapor absorption capacity and kinetics. These materials can be electrospun into fibrous mats with hierarchical structures and/or combined with cross-linking materials to generate monoliths of varying porosity and pore size distribution.
The system utilizes fan blades which act as the adsorption (and desorption) surface of the system. There may be at least two sets of blades on a central shaft driven by a single motor. As the fans rotate the blades will both be exposed to fresh air and generate air flow. In addition to using lower quality heat to desorb the moisture, the system will produce water in liquid rather than vapor form, which can be collected via gravity and re-used in other processes.
A cross sectional diagram of a system according to an example embodiment is shown in
The desorption side 203 utilizes circular flow that is directed from a heated wall 214 to the cooling partition 210 with a small amount of makeup air 216 that enters through entrance path 217 and exits through exit path 218 as humid heated air 219. The makeup air 216 can be pre-warmed by the humid heated air 219 using counter-flow heat exchanger 223 to minimize the amount of exergy exiting the system.
Heat is emitted into the desorption side 203 from heater 220 on the heated wall 214. The heater 220 may include a heat exchanger, heat generator (e.g., resistive heater, combustion heater), or other heater known in the art. In one embodiment, the heater 220 may include one or more of photovoltaic (PV) cells and solar thermal absorption layers. The PV cells can also be used to drive the fan motor 224, e.g., for a portable water harvesting device that is designed for off-the grid use. In another embodiment, the heater 220 may include a heat exchanger driven by waste heat.
In yet another embodiment, the system may include an internal heater 230 integrated into the fan blades (shown on first fan 200 in
The heater 220 (other parts of the chamber on the desorption side 203) can be coated in an insulator 222. If the heater 220 is a PV cell, transparent aerogels can be used as an insulator, which are described in commonly-owned U.S. Pat. No. 10,421,253, issued Sep. 24, 2019. The thickness of the insulator 222 can be determined based on an energy size and mass balance for the system, as well as insulating factor of the materials used. The energy efficiency of this system may be dependent, among other things, on the energy used for fan rotation, and on inefficiencies in the fan drive motor 224, air flow (e.g., flow resistance through paths 217, 218), and heater 220.
When a cycle is completed, the blades of fan 200 will be dried out and the blades of fan 204 will be saturated with water. At this point, the fans 200, 204 can be switched from respective desorber to adsorber sides or vice versa, which may be considered a change from a first mode to a second mode. Where the system is small, e.g., a portable water-harvesting device, the blades and hub of each the fans 200, 204 can be an integral piece which can be easily removed and reattached to the drive shaft without requiring tools. In such an embodiment, the heater 220 and insulation 222 of the desorber side 203 can be readily opened to facilitate the mode change. Since only the fans 200, 204 need to be moved, the entire thermal mass of the hot desorption side remains.
Combining the heat/mass exchanger with the fan helps to decrease system size and weight in several ways. By eliminating a separate heat/mass exchanger, inefficiencies in redirecting air flow are eliminated. The volume taken up by fan blades in a separated system is also effectively eliminated. These two factors allow for a larger diameter fan to be used. Larger diameter fans have higher efficiencies and rotate at slower speeds for the same air flow rate while requiring the same or reduced motor size.
In another embodiment, an air treatment system can be devised such that switching of the fans is not needed. This may be useful for larger systems, such as HVAC or large water harvesting systems. An example of such a system is shown in the diagram of
The ambient air inlets 306, 307 and ambient air outlets 308, 309 can be selectively opened and closed via respective valves 314-319 to reconfigure each chamber for desorbing or adsorbing. In this configuration, chamber 303 is configured as a desorbing chamber, with heater 321 activated, valve 315 slightly open to provide make-up air, and valve 319 open to allow heated and humid air to enter the cooling chamber 310. Chamber 302 is configure as an adsorbing chamber in this configuration, with valves 314 and 316 fully open to cause ambient air to be moved by fan 300 which adsorbs moisture. Fan 300 also cools cooling chamber wall 310a, which results in cooling chamber air condensing and leaving through liquid water channel 324.
By reversing the configuration of the valves 314-319 (e.g., valve 314 slightly open, valves 315, 317, and 318 open, and valve 319 closed), the chamber 303 can perform absorption and the chamber 302 can perform desorption. In such a case, wall 310b will be the cooling wall, heater 320 will be activated, and heater 321 will be deactivated. Note that the system may undergo a period of no activity (e.g., fans switched off, heaters deactivated, all valves closed) to allow the system to come to thermal equilibrium before changing modes.
The mode change described above can be triggered based on one or both of an adsorbing fan being saturated or a desorbing fan being desaturated. The system may include sensors to detect the saturation level of a fan. For example, the increased weight from higher sensor may be sensed using a torque sensor between the fan and a drive motor. Instead or in addition, a direct humidity sensor could be attached to a fan blade. Such sensor could transmit signals wirelessly or via a conductive rotating contact interface at the hub.
In order to achieve the structural requirements of the proposed design a mechanically robust composite of a polymer matrix containing the sorbent material can be used for the fan blades. Many methods have been explored to create composites of this type including incorporating crystalline fillers into a flexible polymer matrix (see, e.g., pubs.acs.org/doi/full/10.1021/acs.chemrev.9b00575); and covalently incorporating polymer backbones into active sorbents either postsynthetically (see, e.g., V. J. Pastore, T. R. Cook, J. Rzyev. Chem. Mater. 2018, 30, 8639-8649) or by synthesizing the MOF from a polymer ligand (see, e.g., Z. Zhang, H. T. H. Nguyen, S. A. Miller, A. M. Ploskonka, J. B. DeCoste, S. M. Cohen. J. Am. Chem. Soc. 2016, 138, 920-925, Z. Zhang, H. T. H. Nguyen, S. A. Miller, S. M. Cohen, Angew. Chem. Int. Ed. 2015, 54, 6152-6157).
One aspect in the design of the composite sorbent will be optimizing the tradeoff between large active area, low pressure drop, and mechanical robustness. This may be done with hierarchically porous systems, with porosity on multiple size scales so that surface area is high but flow impedance is low. Textiles, which have porosity on the scale of threads (100s of um to mm), individual fibers (100s of nm to 10s of um), and optionally pores in the fibers themselves (nms to 100s of nm) are one proposed solution. Polymer fibers may be easily and scalably fabricated via electrospinning. Hierarchically porous structures may also take advantage of aerogels, which have surface areas on the order of 100s of m2/g. A synthesis route for polymer aerogels has been developed (see, e.g., G. Iftime et al. Polymer Aerogel for Window Glazings, U.S. Pat. No. 10,421,253B2, Sep. 24, 2019) featuring surface areas >900 m2/g.
In
The fan blade 402 and hub 404 may be made from the same or different materials (in the latter case where the blade 402 and hub 404 are not formed integrally). The material may include metals such as Al, Mg, corrosion resistant steel, etc. The material may also or alternatively include polymers, composites (e.g., fiberglass, carbon fiber). The materials of the fan blade 402 and hub 404 should at least be resistant to the corrosive effects of water/humidity and have sufficient mechanical properties (e.g., strength, toughness) to withstand the predicted mechanical loading on the fan blade 402 (e.g., transverse forces caused by driving airflow, centrifugal forces due to rotation) and hub 404 (e.g., centrifugal forces due to rotation, stresses caused by imbalances in the blades 402).
The blade 402 is shown coated with a desiccant material 406 that is operable to adsorb airborne moisture in an ambient airflow and desorb moisture in a heated airflow. The desiccant material 406 may, for example, include at least one of a phase-changing polymer, silica gel or zeolite adsorbent, metal-organic framework, ionic liquid, and halogen-doped nanoparticles. In another embodiment, the desiccant material 406 may include quaternary salt polymers, which have excellent water adsorbing properties. For example, poly(diallyldimethyl ammonium chloride) (PDADMAC) was used in a recent dehumidification project in synergistic combination with a phase-changing polymer, where the PDADMAC absorbs moisture from the air and the phase changing polymer takes up the moisture and swells. As seen in the detail view on the right side of
Note that the portion 410 has higher porosity than portion 411, and this can be repeated with fewer or more portions of different porosity. Other material structures may also be introduced into the coating of desiccant material 406, such as fibers, nanowires, vias, etc., as illustrated by nanowire mesh 412 These structures may be formed of a material with high thermal conductivity, such as Ag, Cu, Au, Al, SiC, graphene, etc. These thermally conductive structures can reduce thermal gradients within the fan, such as within and along the coating of desiccant material 406.
In order to form the desiccant material 406 coating, controlled polymer aerogel synthesis and electrospinning can be used to produce hierarchically porous systems. The pore sizes can span a wide range of size scales, providing high surface area with low flow resistance. Metal-organic framework (MOF) particles can be added as inclusions, as has been demonstrated for ZIF-8 in PVP (see, e.g., R. Ostermann et al., Chem. Commun. 2011, 47, 442-444). Hydrophilicity of the porous support matrix can be enhanced through the incorporation of comonomers such as β-cyclodextrin (see, e.g., A. Alsbaiee, B. J. Smith, L. Xiao, Y. Ling, D. E. Helbling, W. R. Dichtel, Nature, 2016, 529, 190-194) or through blending with hydrophilic polymers of intrinsic microporosity (see, e.g., R. A. Kirk, M. Putintseva, A. Volkov, P. M. Budd. BMC Chem. Eng. 2019, 1, 18), as needed. Other additives such as Ag nanowires or sacrificial materials can optionally increase surface area and conductivity (see, e.g., J. Xue et al., Chem. Rev., 2019, 119, 5298-5415).
In
One characteristic that may be optimized in a system as described herein is electrical efficiency. The efficiency of the electrical system can determine the fan operation characteristics along with the back pressure imposed by the adsorption/desorption subsystems. In some embodiments, the supply air can flow in channels past the surface of the polymer films. This leads to convection-diffusion type problem where Peclet number determines the transport rates. Composite material properties can be estimated using experimentally obtained pure MOF and composite material sorption characteristics. Commercially available software can provide computer simulations of 3D airflow and conjugated heat and mass transfer models using Reynolds averaged Navier-Stokes equations coupled with energy and species equation using.
In
Unless otherwise indicated, all numbers expressing feature sizes, amounts, and physical properties used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the foregoing specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by those skilled in the art utilizing the teachings disclosed herein. The use of numerical ranges by endpoints includes all numbers within that range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5) and any range within that range.
The various embodiments described above may be implemented using circuitry, firmware, and/or software modules that interact to provide particular results. One of skill in the arts can readily implement such described functionality, either at a modular level or as a whole, using knowledge generally known in the art. For example, the flowcharts and control diagrams illustrated herein may be used to create computer-readable instructions/code for execution by a processor. Such instructions may be stored on a non-transitory computer-readable medium and transferred to the processor for execution as is known in the art. The structures and procedures shown above are only a representative example of embodiments that can be used to provide the functions described hereinabove.
The foregoing description of the example embodiments has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the embodiments to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. Any or all features of the disclosed embodiments can be applied individually or in any combination are not meant to be limiting, but purely illustrative. It is intended that the scope of the invention be limited not with this detailed description, but rather determined by the claims appended hereto.
