Research on Principle and System of Atmosphere Water Harvesting Unit Based on Active Carbon Fiber Composite Material
Author:Wang Jia Yun
Supervisor:wang ru zhu
In order to alleviate the shortage of fresh water resources in the island and inland arid regions,solar driven sorption Air Water Harvesting(AWH)is a prospective way to supply fresh water as a small decentralized system there.This technique based device captures water vapor in the air with selective water sorbent(SWS)at night,then the solar collector desorbs the sorbent,and water vapor is released,which can be condensed to liquid water in condenser.Compared with the traditional electric driven condensing AWH in the wet area with a dew point of 15℃ or above,the solar-driven sorption AWH improves the dew point of the circulating air through sorption cycle,achieving high-efficiency fresh water production in arid regions with a dew point below 5℃ without additional power consumption.However,the existing AWH device can not achieve a large amount of water intake in less energy consumption and shorter cycle time due to the rough cycle design,inefficient adsorbent,bad heat and mass transfer capacity of the sorbent bed,and unreasonable unit structure..In view of the shortcomings in the current research,an efficient semi-open intermittent AWH cycle is designed in this paper,which is open sorption phase at night and closed desorption phase during daytime.Each working component is independent,achieving high-efficiency water harvesting at 60℃ desorption temperature(70℃ hot air temperature).Based on this cycle,a high efficient ASLI composite sorbent is developed,and the adsorption bed with excellent heat and mass transfer capacity is designed based on ASLI sorbent.In order to verify the high efficiency of materials and systems,10kg AWH device was designed to complete the humid sorption processes and the desorption processes by the electric heater and the solar air collectors.In order to develop the large-scale sorption AWH device and study its universal operation law,the authors constructed a new type of hot air-driven AWH system,which contains energy storage part,and the daily water production can reach 50kg under typical Shanghai summer climate.The variation criteria of the universal air state in AWH system were constructed and verified by experiments in dry and wet conditions.Based on the actual working conditions of different regions in the world,the water intake potential of this AWH system in the typical climate of the world is measured,which provides a theoretical basis for the scale and practical promotion of the device.The main research contents of this paper are as follows:(1)A set of semi-open high-efficiency cycle was constructed to solve the shortcomings of the current AWH device.At the same time,combined with the actual operating conditions,the theoretical framework of the AWH is established and the wet air states are analyzed.From the variation cycle diagram of the state of the sorber outlet air,the water harvesting mass can be increased by increasing absolute moisture content of the sorber inlet air in adsorption phase,or decresing the absolute moisture content of the desorber inlet air,or increasing the desorption temperature in desorption phase.The adsorbent bed,condenser,heater,and the fan are separated so that the components are coupled to each other and operate efficiently.The advantages of this cycle is that no electrical input,simple cycle,high efficiency,and the easy maintenance.(2)Activated carbon fiber-based composite adsorbents are developed,and the silica gel and expanded vulcanized graphite matrix are compared,the properties of two kinds of hygroscopic salts of LiCl and CaCl2 were studied.The above composite adsorbent was intensively studied on the aspects of micro characteristics,equilibrium adsorption performance and desorption property.After impregnating the salt solution,the BET specific surface area and pore volume of the composite adsorbent are decreased,and the adsorption performance is enhanced.consolidated activated carbon fiber composited with LiCl(ASLI)has the best adsorption and desorption properties,which can achieve large circulating water withdrawal in 15-25℃ adsorption environment and 65℃-70℃ desorption condition.The results show that under 30%salt concentration,the sufficient amount of adsorption can be ensured and the carryover problem can be avoided within the target adsorption time.Equilibrium adsorption equations of the ASLI material are fitted based on Polanyi theory to establish a theoretical basis for the expansion research of the AWH device.(3)Based on the novel ASLI high-efficiency composite adsorbent,a honeycomb adsorption bed structure with great heat and mass transfer capability was developed.The single-layer bed structure is 0.4×0.4×0.2m~3.The air flow process is simulated by CFX to verify the low flow resistance of the flow channels.At a flow rate of 2m/s,the simulated flow resistance of the single sorber layer is 8.8 Pa.The measured flow loss using a differential pressure transmitter is 1 mm water column,i.e.,9.8 Pa.After optimizing the size of the flow channel,the sorber is manufactured,and a small 10 kg AWH device is built.Under the high humidity sorption condition and the different desorption temperature driven by electric heater,the adsorption and desorption properties are studied to verify the feasibility of the sorbent operating in the entire AWH system.The device was then connected to an actual solar air collector for experiments to test the water harvesting performance of the unit driven by solar energy.The experimental results show that when the desorption temperature is constant,the higher the inlet sorption relative humidity,and the more the water intake;while the adsorption condition is constant,the higher the regeneration temperature,the more the water collected.(4)For the demand of sorption-based large-scale AWH system application,a 50kg automated AWH unit with energy storage was prepared.The experiments are carried out under dry desert conditions and humid island conditions,and the inherent efficiency of the unit is obtained.The properties of materials and units are simulated in typical conditions around the world to obtain their water intake potential.At the same time,combined with the solar radiation intensity of typical regions of the world,the required solar air collector area is calculated,which provides theoretical guidance for the practical promotion of the scalable AWH unit in various places.