• Energy Research

Abstract The theoretical analysis of the field synergy is tried to apply to desiccant coated heat exchangers (DCHEs) to clarify its coupled heat and moisture transfer characteristics and the way to enhance the characteristics. In this paper, the heat and moisture transfer model of DFHE is established and verified by comparison with experimental data. Three field synergy angles between temperature gradient, velocity vectorand moisture gradient are introduced and calculated for different types of DFHEs with different air velocities. Through the analysis, the field synergy method is proved available for analysing the coupled heat and mass transfer process of DCHE. Different heat and mass transfer performance index of different structure sizes of DFHEs are analysed. By comparing the sensible heating capacity, dehumidification capacity, Re and field synergy angles of them, the optimization design direction of structure size is obtained. The DFHE with higher ratio between total transfer area and total volume is proved to display better performances. So by optimazing the types of DCHE, DMHE is introduced, and the superiority of DMHE is proved. And the reason why DMHE presents higher heat and mass transfer capacity is illustrated.

Abstract The theoretical analysis of the field synergy is tried to apply to desiccant coated heat exchangers (DCHEs) to clarify its coupled heat and moisture transfer characteristics and the way to enhance the characteristics. In this paper, the heat and moisture transfer model of DFHE is established and verified by comparison with experimental data. Three field synergy angles between temperature gradient, velocity vectorand moisture gradient are introduced and calculated for different types of DFHEs with different air velocities. Through the analysis, the field synergy method is proved available for analysing the coupled heat and mass transfer process of DCHE. Different heat and mass transfer performance index of different structure sizes of DFHEs are analysed. By comparing the sensible heating capacity, dehumidification capacity, Re and field synergy angles of them, the optimization design direction of structure size is obtained. The DFHE with higher ratio between total transfer area and total volume is proved to display better performances. So by optimazing the types of DCHE, DMHE is introduced, and the superiority of DMHE is proved. And the reason why DMHE presents higher heat and mass transfer capacity is illustrated.

Abstract Numerous novel devices have already utilized desiccant coating to implement various functions, such as atmospheric water harvesting, energy storage, and thermal/humidity management. The mathematical models employed in former literature, for instance the finite-element or finite-volume models usually suffer significant drawbacks such as extreme complexity and time-consuming. To solve this problem, this paper outlines a general strategy to develop dynamic compact thermal models of the quasi-one-dimensional sorption-based systems in the virtue of the thermal resistance-capacitance network. To be specific, the mass transfer process induced by the adsorption/desorption process is equivalent to a heat/cooling source, therefore the coupled heat and mass transfer kinetics in the corresponding device can be evaluated simultaneously. Two comprehensive experimental prototypes in our previous literature are employed to validate the effectiveness of the proposed model. The results show that our model can forecast the transient thermal behavior of the devices accurately within a few seconds. The dynamic deviation of the system output, for instance the material temperature and outlet air humidity, between simulation and experiment is within 7%. Furthermore, a parametric study is conducted based on the proposed model to analyze the influence of key parameters on system performance, showing great potential for guiding the system design and optimization.

Abstract Numerous novel devices have already utilized desiccant coating to implement various functions, such as atmospheric water harvesting, energy storage, and thermal/humidity management. The mathematical models employed in former literature, for instance the finite-element or finite-volume models usually suffer significant drawbacks such as extreme complexity and time-consuming. To solve this problem, this paper outlines a general strategy to develop dynamic compact thermal models of the quasi-one-dimensional sorption-based systems in the virtue of the thermal resistance-capacitance network. To be specific, the mass transfer process induced by the adsorption/desorption process is equivalent to a heat/cooling source, therefore the coupled heat and mass transfer kinetics in the corresponding device can be evaluated simultaneously. Two comprehensive experimental prototypes in our previous literature are employed to validate the effectiveness of the proposed model. The results show that our model can forecast the transient thermal behavior of the devices accurately within a few seconds. The dynamic deviation of the system output, for instance the material temperature and outlet air humidity, between simulation and experiment is within 7%. Furthermore, a parametric study is conducted based on the proposed model to analyze the influence of key parameters on system performance, showing great potential for guiding the system design and optimization.

Summary Humidity management is essential and widely needed in various fields. Traditional moisture removal technologies are mostly based on cooling condensation or desiccant dehumidification. These methods come with inherent defects of energy efficiency and are bulky and complex for small-space humidity-control applications. Here, we demonstrate a full-solid-state humidity pump using the advantages of commercial thermoelectric coolers and silica gel materials. Silica gel is coated on the surface of heat sinks to minimize contact thermal resistance between the material and heat source while maintaining enough heat and mass transfer area between the desiccant and the air. The humidity pump device exhibits 0.61 g W−1 h−1 humidity transfer efficiency, corresponding to a humidity transfer rate of 28.38 g h−1. The proposed device neither uses refrigerant nor introduces liquid water. It opens up the possibility of use in localized humidity control applications with higher efficiency and a wider ambient temperature range than traditional technologies.

Summary Humidity management is essential and widely needed in various fields. Traditional moisture removal technologies are mostly based on cooling condensation or desiccant dehumidification. These methods come with inherent defects of energy efficiency and are bulky and complex for small-space humidity-control applications. Here, we demonstrate a full-solid-state humidity pump using the advantages of commercial thermoelectric coolers and silica gel materials. Silica gel is coated on the surface of heat sinks to minimize contact thermal resistance between the material and heat source while maintaining enough heat and mass transfer area between the desiccant and the air. The humidity pump device exhibits 0.61 g W−1 h−1 humidity transfer efficiency, corresponding to a humidity transfer rate of 28.38 g h−1. The proposed device neither uses refrigerant nor introduces liquid water. It opens up the possibility of use in localized humidity control applications with higher efficiency and a wider ambient temperature range than traditional technologies.

Abstract Atmospheric water harvesting (AWH) is now anticipated as a prospective solution to the current shortage of fresh water. The prevailing cooling-based (CB) strategy is proven inefficient in arid region where fresh water supply is more urgent. Based on the humidity enrichment nature of nano-porous sorbents, sorption-based AWH is proposed to fill this niche. Preliminary studies in this field focus mainly on system implementation under low-humidity conditions, rather than energy-conservation design and optimization due to the lack of relevant guidelines, leading to several energy-intensive prototypes and a misconception that the sorption-based technology is inherently energy-consuming. To break former stereotype, this work proposes a robust method of energy assessment, identifies clear scope of application for AWH technologies, as well as outlines materials and operational parameters choice guidelines. It is concluded that active sorption-based AWHs are not only the supplement of the cooling-based AWHs under arid weather, but also the efficient substitution except in tropical coastal region. Unexpectedly, the employment of adsorption cooling source can largely reduce the energy burden and weaken the demand for materials, while the one in condensation side imposes subtle influence. Considering the diurnal or seasonal weather variation in most areas, traditional sorbents with moderate stepwise position or linear isotherms might outperform the prevailing materials with early stepwise position. These findings provide new insights into the design of AWH technologies.

Abstract Atmospheric water harvesting (AWH) is now anticipated as a prospective solution to the current shortage of fresh water. The prevailing cooling-based (CB) strategy is proven inefficient in arid region where fresh water supply is more urgent. Based on the humidity enrichment nature of nano-porous sorbents, sorption-based AWH is proposed to fill this niche. Preliminary studies in this field focus mainly on system implementation under low-humidity conditions, rather than energy-conservation design and optimization due to the lack of relevant guidelines, leading to several energy-intensive prototypes and a misconception that the sorption-based technology is inherently energy-consuming. To break former stereotype, this work proposes a robust method of energy assessment, identifies clear scope of application for AWH technologies, as well as outlines materials and operational parameters choice guidelines. It is concluded that active sorption-based AWHs are not only the supplement of the cooling-based AWHs under arid weather, but also the efficient substitution except in tropical coastal region. Unexpectedly, the employment of adsorption cooling source can largely reduce the energy burden and weaken the demand for materials, while the one in condensation side imposes subtle influence. Considering the diurnal or seasonal weather variation in most areas, traditional sorbents with moderate stepwise position or linear isotherms might outperform the prevailing materials with early stepwise position. These findings provide new insights into the design of AWH technologies.

Abstract Desiccant coated heat exchangers (DCHEs), utilizing an inner cooling source to remove sorption heat, are promising alternatives for evaporators and condensers (DCEs/DCCs) in vapor compression (VC) heat pumps. A mathematical model is necessary to facilitate the design, analysis and performance prediction of the component and the relevant systems. In this study, a three-dimensional model of DCEs/DCCs is proposed, accounting for the two-phase phenomena, the periodical switchover, the solid-side resistance, the fluid transport in multiple directions and the coupled heat and mass transfer. Study reveals that, high latent load (inlet humidity) reduces the sensible heat handling capacity of the DCEs, while the dehumidification capacity is almost independent of its sensible heat load. DCEs possess a satisfying effect of dehumidification above the dew point, thus it is unnecessary to employ low evaporation temperature. Meanwhile, the adsorption-desorption temperature difference of 30 °C seems to be the optimal value for the commonly adopted evaporation temperatures (10 °C-20 °C). For a specific coating thickness, there is a critical switchover period where the best performance of dehumidification is obtained. Switchover period shorter than the critical time should be avoided, and elongating the switchover cycle helps balance the ability of the system to handle the sensible and latent heat loads at the same time.