STATEMENT OF PRIOR DISCLOSURE BY THE INVENTORS

Aspects of this technology are described in the article “The impact of thermodynamic balancing on performance of a desiccant-based humidification-dehumidification system to harvest freshwater from atmospheric air” published in Energy Conversion and Management, 2019, 199, available on Nov. 1, 2019, which is incorporated herein by reference in its entirety.

STATEMENT OF ACKNOWLEDGEMENT

This work was supported by the Research and Development Program, Research Pooling Initiative, Ministry of Education, Riyadh, Saudi Arabia.

BACKGROUND OF THE INVENTION

Technical Field

The present invention relates to a humidifier-dehumidifier atmospheric water generator system having a closed desiccant loop and a closed gas mixture loop and a method of generating freshwater using the atmospheric water generator system.

Description of the Related Art

The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.

Freshwater scarcity is one of the major issues that is facing humanity in many countries. With growing population, as well as agricultural and industrial use, freshwater needs are anticipated to significantly increase in coming decades[U. Nations, World Population Prospects The 2012 Revision, New York, 2013]. Water demand is expected to exceed the available water resources by 40% in 2030 [F R Rijsberman, Agric. Water Manag., 2006; 80:5-22]. Thus, water purification methods are critically important wherever the natural resources of drinking water are scarce [A K Venkatesan, et. al., Desalination, 2011; 272:120-127]. Desalination of saltwater is one technological solution to water purification used currently. The seawater and brackish water are the primary resources used in these technologies to obtain a satisfactory amount of freshwater [NAA Qasem, et. al., Desalination, 2018; 441]. Other resources such as water content in the humid air (especially, in coastal areas) could efficiently be used as a water resource to obtain sufficient amounts of freshwater [MA Ahmed, et. al., Desalination, 2018; 445:236-248]. Desiccant-based systems have been used for air conditioning purposes.

Recently, desiccant-based desalination systems have been proposed [MA Ahmed, et. al., Energy Conyers Manag, 2017; 148:161-173]. Such systems have been tested to be an energy-efficient technology to produce freshwater [R Qi, et. al., Energy Conyers Manag, 2015; 106:1387-1395]. These systems described by the authors of these proposals and tests as eco-friendly [M Mujahid Rafique, et. al., Renew Sustain Energy Rev, 2015; 45:145-159]. Modi and Shukla [K V Modi, D L Shukla, Energy Conyers Manag, 2018; 171:1598-1616] investigated a desiccant regeneration process for a hybrid solar-powered air conditioning/desalination system. A hybridization between desiccant dehumidification and absorption cooling system was investigated [B Su, et. al., Energy Conyers Manag, 2017; 153:515-525] for achieving both cooling and water production. Kabeel et al. [A E Kabeel, et. al., Energy Conyers Manag, 2017; 150:382-391] explored a hybridization of a desiccant cooling system with a humidification-dehumidification desalination (HDH) system to obtain freshwater along with a cooling effect. A new hybrid liquid desiccant cooling/desalination system has been suggested by modifying the traditional desiccant system by adding a condenser (additional component) between the system humidifier and regenerator. The system was able to produce about 86 kgh−1pure water as a by-product for a condenser effectiveness of 0.8. The cooling effect was the main product with a COP of 0.9.

A desiccant system to produce water from atmospheric air using solar energy was tested to produce 2.32 Lm−2per day [GE William, et. al., Energy, 2015; 90:1707-1720]. The most recent liquid desiccant-based system to produce freshwater from ambient humid air is a liquid desiccant-based HDH. The water production of this system was about 8 kgh−1. It (desiccant HDH) was proposed by adding one air dryer to the normal HDH system. The diluted (weak) desiccant is sprayed into the humidifier, in which it exchanges some water content with the circulated air. The humidified air from the humidifier losses this gained water content in the dehumidifier as freshwater, which is collected [MA Ahmed, et. al., Desalination 2018; 445:236-248].

The aforementioned studies focused on introducing some novel or hybrid desiccant-based systems. Moreover, because of mass and heat exchange between the desiccant and circulated air, thermodynamic balancing approach is viable to improve the performance of such systems by minimizing the entropy generation [GP Thiel, et. al., Appl Energy, 2014; 118:292-299]. In particular, thermodynamic balancing was successfully investigated and reported for substantial enhancement in the performance of HDH systems by applying mass extractions from the humidifier to be injected into the dehumidifier [K M Chehayeb, et. al., Int J Heat Mass Transf, 2014; 68:422-434]. Extraction of water or air from the humidifier into the dehumidifier and vice versa was also investigated [GP Narayan, et. al., Int J Therm Sci, 2010; 49:2057-2066]. Another way to achieve thermodynamic balancing is by continuously changing a water-to-air mass ratio to decrease the water and air temperature difference at the humidifier and dehumidifier terminals [H. Müller-Holst, Sol. Desalin. 21st Century, 2007, 215-225]. An HDH system having three stages was proposed [T. Schlickum, Device for separating a liquid from its dissolved matters, European Patent, EP 1770068 A2, 2007.] to have three water streams coupled in series with a separated air stream to be recirculated for each stage. Also, a four-stage cross flow HDH solar desalination system was investigated [Y Zhao, et. al., Desalination, 2019; 467:147-157]. Driven by forced convection, an HDH system was thermodynamically balanced by extraction of some streams between the dehumidifier and humidifier [T Brendel, Ruhr University Bochum; 2003.]. Two-stream extractions from the humidifier to be injected into the dehumidifier were also investigated [MA Younis, et. al., Desalination, 1993; 94:11-24].

McGovern et al. [RK McGovern, et. al., Appl Energy, 2013; 102:1081-1090] found a 300% enhancement in the seawater HDH performance by just applying a single extraction between the humidifier and dehumidifier. They also showed that the ratio of water production to the inlet saline water increased from 7% to 11%. About 54% increase in the HDH performance was experimentally reported for a single air extraction [GP Narayan, et. al., Int J Heat Mass Transf, 2013; 58:740-748].

The idea of thermodynamic balancing is to reduce entropy production, resulting in a tremendous improvement in the HDH performance [KH Mistry, et. al., Int J Therm Sci, 2010; 49:1837-1847]. A multi-stage HDH system was considered as a thermodynamically balanced system [M Zamen, et. al., Chem Eng Trans, 2011; 25:1091-1096]. In this system, a new performance index denoted by “temperature pinch” was suggested as the difference between the water and air temperatures at each stage terminals. For a one-stage system (containing one humidifier and one dehumidifier), the “temperature pinch” method is applied to minimize the temperature difference of the air and saline water at the inlet and outlet of the humidifier and dehumidifier. For a multi-stage system, the approach aims at minimizing the temperature difference at the inlet and outlet of each stage. Thus, for an infinite number of stages, the temperature difference of air and saline water reaches zero along with the whole humidifying and dehumidifying processes. The “temperature pinch” method focuses on heat exchange between the air and saline water (or other solutions). However, the real process includes heat and mass exchanges. Thus, it was suggested that the temperature pinch is not sufficient to approach the fully thermodynamic balanced system.

For this reason, another index was proposed, that is “enthalpy pinch”, to efficiently replace the “temperature pinch” [GP Narayan, et. al., Int J Heat Mass Transf, 2013; 57:756-770] by considering both the heat and mass exchanges [GP Thiel, et. al., Int J Heat Mass Transf, 2012; 55:5133-5147]. The enthalpy pinch was used to design conventional and modified closed-air open-water HDH systems [SM Elmutasim, et. al., Desalination, 2018; 435:114-127]. The implementation of enthalpy pinch was assessed as a robust method for HDH design and optimization [KM Chehayeb, et. al., Desalination, 2015; 369:125-139]. It was implemented for zero and multiple extractions. At zero enthalpy pinch, the system is approaching the reversibility limit. Similar to seawater desalination, thermodynamic balancing was carried out for desiccant-based HDH systems.

The enthalpy versus temperature plot is usually used to study the thermodynamic balancing principle. This graphical method was so called “the pinch technology” [S Hou, et. al., Desalination, 2005; 183:143-149; and S Hou, Desalination, 2008; 222:572-578]. The evaluation of temperature—enthalpy profiles in HDH systems having some extractions was explored [JA Miller, et. al., Desalination, 2013; 313:87-96]. One-stage [X Huang, et. al., Desalination, 2019; 455:19-33], two-stage [X Huang, et. al., Energy Conyers Manag, 2019; 197:111872] and up to six-stage [H Kang, et. al., Desalination, 2016; 385:158-166] HDH systems have recently modeled using this graphical method. It is also implemented for HDH coupled with heat pump [WF He, et. al., Energy Conyers Manag, 2019; 194:11-21]. The first experimental verification of the graphical method and enthalpy pinch was carried out by Narayan et al. [GP Narayan, et. al., Int J Heat Mass Transf, 2013; 58:740-748].

It is important to note that the aforementioned enthalpy pinch literature has mainly focused on the water-based HDH systems to evaluate the system performance using the thermodynamic balancing concept. The desiccant-based HDH was, however, has not been investigated using this concept.

In view of the foregoing, one objective of the present invention is to provide a desiccant-based HDH system with a closed desiccant loop and a closed air loop. Said HDH system comprises a plurality of air dryers that contact the desiccant with the ambient air. The closed air loop passes air from a heater to the humidifier and back and forth between the humidifier and dehumidifier 2 to 6 times before being returned to the heater. A second objective of the present invention is to provide a method for extracting freshwater from ambient air using the HDH system.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect, the present disclosure relates to a closed-air, closed-desiccant (CACD) humidifier-dehumidifier atmospheric water generator system, comprising:

an air dryer system comprisinga desiccant delivery line,a desiccant return line,a strong desiccant,a weak desiccant,a plurality of air dryers connected in series, each comprising a dryer desiccant inlet, a dryer desiccant outlet, an air inlet, an air outlet, and a contacting chamber;

a humidifier-dehumidifier (HDH) system comprisinga desiccant bypass line connected to the desiccant delivery line of the air dryer system and the desiccant return line of the air dryer system,a dehumidifier comprisinga dehumidifier desiccant inlet connected to the desiccant delivery line of the air dryer system,a dehumidifying chamber,a humidified air cooling apparatus connected to the dehumidifier desiccant inlet and placed inside of the dehumidifying chamber,a series of 2 to 6 dehumidifier humidified gas mixture inlets spaced along a dimension of the dehumidifier,a series of 2 to 6 dehumidifier dehumidified gas mixture outlets separated from the series of 2 to 6 dehumidifier humidified gas mixture inlets and spaced along a dimension of the dehumidifier,a dehumidifier desiccant outlet,a gas mixture, anda freshwater outlet;a heater connected to the dehumidifier desiccant outlet;a humidifier comprisinga humidifier desiccant inlet connected to the heater,a humidifying chamber connected to the humidifier desiccant inlet,a series of 2 to 6 humidifier dehumidified gas mixture inlets spaced along a dimension of the humidifier and connected to the dehumidifier dehumidified gas mixture outlets, anda series of 2 to 6 humidifier humidified gas mixture outlets separated from the series of 2 to 6 humidifier dehumidified gas mixture inlets and spaced along a dimension of the humidifier and connected to the dehumidifier humidified gas inlets, anda humidifier desiccant outlet connected to the humidifying chamber and the desiccant return line of the air dryer system.

In some embodiments, the strong desiccant is an aqueous solution comprising lithium chloride at a concentration of 0.341 to 0.40 kg lithium chloride per kg of solution.

In some embodiments, the weak desiccant is an aqueous solution comprising lithium chloride at a concentration of 0.25 to 0.340 kg lithium chloride per kg of solution.

In some embodiments, the air dryer system comprises 2 to 20 individual pack-bed dryers configured in series such as to pass multiple aliquots of ambient air over a single aliquot of desiccant.

In some embodiments, the system further comprises a heat exchanger placed along the return line between the desiccant pathway outlet of the HDH system and the air dryer system such that the heat exchanger cools the strong desiccant as it flows from the HDH system to the air drier system.

In some embodiments, the strong desiccant is cooled to a temperature of 20 to 40° C.

In some embodiments, the heater heats the weak desiccant to a temperature of 70 to 90° C. before the weak desiccant passes into the humidifier.

In some embodiments, the system has an adjustable ratio of the desiccant mass flowrate to the air mass flowrate of 3.0 to 16.

In some embodiments, the adjustable ratio of the desiccant mass flowrate to the air mass flowrate is adjusted by changing the desiccant mass flowrate, the air mass flowrate, or both.

In some embodiments, the system has a gained output ratio, defined as the ratio of the product of a mass of freshwater generated by the system times a vaporization latent heat of the freshwater to an amount of heat used to heat the weak desiccant, of 6 to 110.

The present disclosure also relates to a method of extracting freshwater from ambient air comprising:

passing a flow of ambient air over an air dryer system comprising a strong desiccant to produce dry air and a weak desiccant;

heating the weak desiccant to produce a heated desiccant;

cycling a gas mixture, the cycling comprisinghumidifying a dehumidified gas mixture using a humidifier comprising the heated desiccant to produce a humidified gas mixture and to regenerate the strong desiccant;dehumidifying the humidified gas mixture using a dehumidifier by cycling the humidified gas mixture from the humidifier to the dehumidifier to produce freshwater and a dehumidified gas mixture;

collecting the freshwater; and

returning the strong desiccant to the air dryer system; wherein

the cycling of the humidified gas mixture from the humidifier to the dehumidifier and the dehumidified gas mixture from the dehumidifier to the humidifier does not mix the humidified gas mixture with the dehumidified gas mixture and does not mix the humidified gas mixture or the dehumidified gas mixture with ambient air.

In some embodiments, the cycling is performed 2 to 6 times.

In some embodiments, the strong desiccant is an aqueous solution comprising lithium chloride at a concentration of 0.341 to 0.40 kg lithium chloride per kg of solution.

In some embodiments, the weak desiccant is an aqueous solution comprising lithium chloride at a concentration of 0.25 to 0.340 kg lithium chloride per kg of solution.

In some embodiments, the weak desiccant is heated to 70 to 90° C.

In some embodiments, the method further comprises cooling the strong desiccant before returning the strong desiccant to the air dryer system.

In some embodiments, the strong desiccant is cooled to a temperature of 20 to 40° C.

In some embodiments, the ratio of the desiccant mass flowrate to the air mass flowrate is adjustable and is of 3.0 to 16.

In some embodiments, the adjustable ratio of the desiccant mass flowrate to the air mass flowrate is adjusted by changing the desiccant mass flowrate, the air mass flowrate, or both.

In some embodiments, the method has a gained output ratio, defined as the ratio of the product of a mass of freshwater generated by the system times a vaporization latent heat of the freshwater to an amount of heat used to heat the weak desiccant, of 6 to 110.

