Heat-driven adsorption vacuum dehumidification system

Abstract

The present invention provides a heat-driven adsorption vacuum dehumidification system including a vapor adsorption apparatus having a water permeable hydrophilic membrane separating the apparatus into at least a feed section and a low-pressure or vacuum section (evaporator), and providing a water vapor pressure difference to extract moisture from the air flowing through the apparatus into the evaporator, followed by adsorption in an adsorption chamber, and subsequently desorbed when acted as a desorption chamber to form water vapor which is condensed in a condenser. Adsorption and desorption chambers inter-change periodically to form a complete system cycle. Heating of chamber/compartment can be from waste heat or a renewable source in the absence of any electricity supplied externally. Related method for using a heat-driven adsorption vacuum dehumidification system to remove moisture from the air is also provided. The present invention is superior to the adsorption chiller over a wide range of operating conditions.

Claims

1. A heat-driven adsorption vacuum dehumidification system comprising: a vapor adsorption apparatus comprising a water permeable hydrophilic membrane separating the apparatus into at least two sections comprising a feed section and an evaporator, the feed section being disposed at where process air flows through the apparatus and has a maximum contact area with a surface of the water permeable hydrophilic membrane; the evaporator being disposed distal to where the process air flows through the vapor adsorption apparatus and having a lower water vapor pressure than that of the feed section such that a water vapor pressure difference is established across the water permeable hydrophilic membrane; a two-bed adsorption-desorption section comprising at least an adsorption chamber and a desorption chamber, the adsorption chamber communicating with the evaporator and a cooling source, the desorption chamber communicating with a condenser and a hot source, respectively, the adsorption chamber comprises at least one adsorbent and supplied with a cooling agent from the cooling source to keep the water vapor pressure of the adsorbent below that of the evaporator so that moisture from the process air passing through the water permeable hydrophilic membrane migrates from the evaporator to the adsorption chamber; the desorption chamber communicating with the condenser and the hot source supplied with the heating agent in order to keep the vapor pressure of the adsorbent in the desorption chamber higher than that of the condenser so that the adsorbed water migrates from the desorption chamber into the condenser; the condenser communicating with the desorption chamber and the cooling source, respectively, and having been supplied with a cooling liquid to convert the water vapor migrated from the desorption chamber into a condensed water; and wherein the air after flowing through the vapor adsorption apparatus is dry and the system is free from electricity to establish and maintain the lower pressure in the evaporator.

2. The system of claim 1, further comprising an auxiliary cooling provision through the evaporator and a chilled liquid source by communicating with the condenser.

3. The system of claim 1, wherein the cooling agent source is a cooling water from a cooling water circulation collected from a cooling tower or nearby fresh water source.

4. The system of claim 1, wherein the heating agent comprises hot water being heated up by a renewable source comprising solar energy or waste heat.

5. The system of claim 1, wherein the adsorbent is selected from silica gel, activated carbon, zeolite, or MOF.

6. The system of claim 1, wherein the water permeable hydrophilic membrane is selected from a material with pores that only allow moisture from the process air to pass through along the vapor pressure difference.

7. The system of claim 1, wherein the evaporator of the vapor adsorption apparatus is kept at below 2 kPa.

8. The system of claim 1, wherein the temperature of water supplied to the adsorption chamber is above 60 degrees Celsius.

9. The system of claim 1, wherein the temperature of water supplied to the desorption chamber is below 35 degrees Celsius.

10. A method for using a heat-driven adsorption vacuum dehumidification system to remove moisture from process air, the method comprising: providing a water vapor pressure difference to extract moisture from the process air passing through a vapor adsorption apparatus incorporated with a water permeable hydrophilic membrane specific for water molecules to pass through from a feed side to an evaporator of the vapor adsorption apparatus in the absence of any electricity; cooling an adsorption chamber communicating with the evaporator of the vapor adsorption apparatus such that the moisture extracted from the feed side through the water permeable hydrophilic membrane into the evaporator is adsorbed on a surface of an adsorbent having been cooled to below a temperature when the water vapor pressure of the adsorbent is lower than that at the evaporator; heating a desorption chamber communicating with a condenser to reach a temperature that desorbs the water on the surface of the adsorbent from the desorption chamber or compartment into the condenser; cooling the condenser communicating with the desorption chamber to a temperature to condense the water vapor migrated from the desorption chamber into the condenser; wherein the hot source is supplied from a renewable energy source or waste source.

11. The method of claim 10, further comprising providing auxiliary cooling by transferring part of the condensed water from the condenser to the evaporator through an expansion valve to cool a chilled liquid source running through the evaporator.

12. The method of claim 10, wherein the cooling source is a cooling water from a cooling water circulation collected from a cooling tower or nearby fresh water source.

13. The method of claim 10, wherein the renewable source comprises solar energy or waste heat.

14. The method of claim 10, wherein the adsorbent is selected from silica gel, activated carbon, zeolite, or MOF.

15. The method of claim 10, wherein the water permeable hydrophilic membrane is selected from a material with pores that only allow moisture from the process air to pass through along the vapor pressure difference.

16. The method of claim 10, wherein the evaporator of the vapor adsorption apparatus is kept at below 2 kPa.

17. The method of claim 10, wherein the temperature of water supplied to the adsorption chamber or compartment is above 60 degrees Celsius.

18. The method of claim 10, wherein the temperature of water supplied to the desorption chamber or compartment is below 35 degrees Celsius.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention are described in more details hereinafter with reference to the drawings, in which:

(2) FIG. 1 schematically depicts a conventional vacuum dehumidification method;

(3) FIG. 2 schematically depicts an adsorption vacuum dehumidification system according to an embodiment of the present invention;

(4) FIG. 3 shows the change in average cooling capacity (CAP) with the temperatures of cooling water under different temperatures of hot water supplied to the system according to an embodiment of the present invention;

(5) FIG. 4 shows the change in COP with the temperatures of cooling water under different temperatures of hot water supplied to the system according to an embodiment of the present invention;

(6) FIG. 5 shows the change in thermal power input (Q.sub.heat) with the temperatures of cooling water under different temperatures of hot water supplied to the system according to an embodiment of the present invention;

(7) FIG. 6 shows the change in performance improvement index of COP (PII.sub.COP) with the temperatures of cooling water under different temperatures of hot water supplied to the system according to an embodiment of the present invention;

(8) FIG. 7 shows the change in performance improvement index of CAP (PII.sub.CAP) with the temperatures of cooling water under different temperatures of hot water supplied to the system according to an embodiment of the present invention;

(9) FIG. 8 shows the change in CAP and COP of the present system with different air temperature fed to the feed side of the vapor absorption and adsorption apparatus of the system according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(10) In the following description, systems, devices, methods of dehumidifying process air, and the likes are set forth as preferred examples. It will be apparent to those skilled in the art that modifications, including additions and/or substitutions may be made without departing from the scope and spirit of the invention. Specific details may be omitted so as not to obscure the invention; however, the disclosure is written to enable one skilled in the art to practice the teachings herein without undue experimentation.

(11) It should be apparent to practitioner skilled in the art that the foregoing and subsequent examples of the system and method are only for the purposes of illustration of working principle of the present invention. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed.

(12) Turning to FIG. 2, a preferred embodiment of the present system includes a vapor adsorption apparatus (201) having at least a feed side, a low-pressure or vacuum side, and between the feed side and the low-pressure/vacuum side is separated by a water permeable hydrophilic membrane (201a). It is defined that the water vapor pressure on the feed side is higher than that on the low-pressure or vacuum side of the apparatus to establish a water vapor pressure difference across the water permeable hydrophilic membrane (201a). To establish the water vapor pressure difference, the vapor adsorption apparatus (201) communicates with an adsorption chamber (202a) of a two-bed adsorption-desorption section (202) of the system where the adsorption chamber (202a) incorporates an adsorbent, a relatively cool water supply, and a heat exchange to direct the heat absorbed by the adsorbent away from the adsorption chamber (202a). In one embodiment, the adsorbent is selected from silica gel. The cool water supply helps keep the water vapor pressure of the adsorbent inside the adsorption chamber (202a) sufficiently low so that the migrated water vapor can adsorb on the surface of the adsorbent.

(13) With the heat from a heat source through a water circulation communicating with the desorption chamber (202b), the vapor pressure of the adsorbent inside the desorption chamber (202b) is increased to a sufficiently high level in which the water vapor desorbs from the surface of the adsorbent, and the water vapor is transferred to a condenser (203) which communicates with a cooling water supply such that water vapor from the desorption chamber (202b) is converted into condensate which is water.

(14) Should auxiliary cooling be required, part of the condensate from the condenser is transferred to the evaporator through an expansion valve to cool a chilled liquid source running through the evaporator.

(15) The adsorption and desorption processes inside the adsorption (202a) and desorption (202b) chambers are transient, and the mass transfer rates will decrease with time. Consequently, the roles of the adsorption (202a) and desorption (202b) chambers inter-change periodically as well as the supply of cool and hot water in a complete system cycle. In other words, the system operates intermittently.

(16) Because VD is an isothermal process, only mass transfer across the water permeable hydrophilic membrane (201a) is considered. The only heat transfer in the low-pressure or vacuum section is due to the water vapor migrated from the process air and subsequently extracted to the adsorption chamber.

(17) The heat power input 6 ({dot over (Q)}.sub.heat) and the cooling load ({dot over (Q)}.sub.cool) of the system are given:
{dot over (Q)}.sub.heat={dot over (m)}.sub.hwc.sub.p,w(T.sub.hw,i−T.sub.hwo)  (1)
{dot over (Q)}.sub.cool={dot over (m)}.sub.da(h.sub.ai−h.sub.ao)  (2)

(18) The average cooling capacity over a complete dehumidification cycle and the overall system COP can be determined by equations (3) and (4), respectively:

(19) $\begin{array}{cc}\mathrm{CAP}=\frac{{\int }_o^{t_{\mathrm{cycle}}}{\stackrel{.}{Q}}_{{\mathrm{cool}}^{\mathrm{dt}}}}{t_{\mathrm{cycle}}}& \left(3\right)\end{array}$$\begin{array}{cc}\mathrm{COP}=\frac{{\int }_o^{t_{\mathrm{cycle}}}{\stackrel{.}{Q}}_{{\mathrm{cool}}^{\mathrm{dt}}}}{{\int }_o^{t_{\mathrm{cycle}}}{\stackrel{.}{Q}}_{{\mathrm{heat}}^{\mathrm{dt}}}}& \left(4\right)\end{array}$

(20) To compare the performance of the present system with an existing AdC, design parameters from Chan et al. (2015) are taken as references for the adsorption cycle. Regarding the membrane, parameters from Bukshaisha and Fronk (2019) are adopted. The total membrane area is taken as 5 m.sup.2.

(21) To compare the performance between the AdC and the present system, a performance improvement index (PII) is determined by:

(22) $\begin{array}{cc}{\mathrm{PII}}_{CAP}=\frac{{\mathrm{CAP}}_{AdVD}}{CA{P}_{AdC}}-1& \left(5\right)\end{array}$$\begin{array}{cc}{\mathrm{PII}}_{COP}=\frac{CO{P}_{AdVD}}{CO{P}_{AdC}}-1& \left(6\right)\end{array}$

(23) Before comparison, according to Chan et al. (2015), a complete dehumidification cycle should include different combinations of operation modes, i.e., pre-heating/cooling (PHC), adsorption/desorption (AdDe), heat and mass recovery (HMR). A basic combination includes AdDe plus HMR; thus, a test cycle can be AdDe>>HMR>>AdDe>>HMR, or so forth. CAP and COP of AdC under different cooling/hot water temperatures supplied to the corresponding chamber/section can therefore be obtained from the test cycle sequence/combination. An AdC model validated based on these parameters and values can be used as a baseline to compare with the CAP and COP of the present system. For both AdC and the present system, feed rate of the air flow is set as 0.1 m.sup.3/s at an entry conditions of 33 degrees Celsius and 67% relative humidity.

Example 1—Comparison of Performances Between AdC and the Present System in Terms of Different Combination/Sequence of Operating Conditions

(24) Table 1 shows the effect of different combination/sequence of operational conditions on CAP and COP of AdC and the present system.

(25) TABLE-US-00001 TABLE 1 Cycle AdC Present Invention Sequence CAP/COP CAP / COP PII.sub.CAP/PII.sub.COP AdDe 0.593/0.205 1.016/0.317 0.713/0.546 PHC + AdDe 0.596/0.214 1.009/0.326 0.693/0.523 AdDe + HMR 0.612/0.269 1.105/0.425 0.806/0.580 PHC + 0.615/0.279 1.077/0.431 0.751/0.545 AdDe + HMR

(26) From Table 1, the overall CAP and COP of the present invention are better than those of AdC (at least about 69.3% and 52.3% enhancement in CAP and COP, respectively, over AdC). COP of the present invention is also comparable to that of conventional VD using electrical vacuum dehumidification such as that by Bui et al. (2017) with the inclusion of HMR mode. The present invention is also coil-free in the evaporation step and cooling step as compared to conventional chilled water-based air-conditioning system. Thus, the present invention is better in terms of energy performance, in particular, in primary energy consumption.

(27) From Table 1, it is observed that the inclusion of HMR increased both PII.sub.CAP and PII.sub.COP, suggesting that HMR mode could benefit the performance of the present invention. Regarding the use of PHC, although there was a small improvement in COP, CAP was actually decreased. Therefore, whether PHC can actually increase the performance of the present invention is not apparent from the results of Table 1. In the subsequent sensitivity test, cycle sequence of AdDE>>HMR will be used.

Example 2—Variations of COP and CAP Under Different Operating Conditions

(28) FIGS. 3 and 4 show the variations of CAP and COP against different temperatures of the cooling water/hot water supplied to the adsorption/desorption chamber of the present invention. In FIG. 3, CAP of the present system was decreased with an increase in cooling water temperature or with a decrease in hot water temperature, which is similar to the trend in conventional AdC. However, it is observed that variation of COP of the present system at higher hot water temperature, e.g., at 85° C., resulted in an increasing trend against an increasing cooling water temperature, which is different from a comparative test result of AdC. The increasing trend was not obvious when the cooling water temperature was equal to or below 30° C. However, when the cooling water temperature was over 30° C., it is observed that the higher the hot water temperature, the more likely the COP is increased. This phenomenon was not observed in hot water temperatures below 85° C. It is even observed that hot water temperature equal to or below 65° C. resulted in a more rapid decrease in COP when the cooling water temperature is over 30° C. These observations are mainly due to an open circuit at the refrigerant side of the present system. That is, pressure at the condenser has no impact on the cooling capacity of the present system, but only the pressure in the low-pressure or vacuum section of the vapor adsorption apparatus accounts for these changes. Subsequent test for variation of thermal power input with different hot water temperatures when cooling water temperature is increased.

Example 3—Variation of Thermal Power Input Under Different Operating Conditions

(29) FIG. 5 shows that {dot over (Q)}.sub.heat under different hot water temperatures decreases in a fairly constant rate when the cooling water temperature increases, the pattern of which are different from those in FIG. 3 (CAP was decreased more sharply when the hot water temperature was getting lower while the cooling temperature was increasing). From FIG. 4, COP was generally lower at higher hot water temperature and lower cooling water temperature, even though the resulting cooling capacity was higher. An optimal combination of the hot water temperature and cooling water temperature is likely to be determined according to a desirable compromise between CAP and COP of the present system.

Example 3—Variation of PII Under Different Operating Conditions

(30) FIGS. 6 and 7 show the changes of PII.sub.CAP and PII.sub.COP against different temperatures of the cooling water/hot water supplied to the adsorption/desorption chamber of the present invention. The present invention out-performs the AdC more substantially when the cooling water temperature increases and/or the hot water temperature decreases. This characteristic is particularly beneficial when applied to a solar cooling system as the system performance does not reduce much when the solar energy is not sufficient.

Example 4—Variation of COP and CAP with Different Feed Air Temperature

(31) From FIG. 8, it shows that the temperature of air flowing through the feed side of the vapor adsorption apparatus has limited effect on the COP and cooling capacity of the present system, provided that feed rate remains constant (at 0.1 m.sup.3/s on an area of about 5 m.sup.2 in this example). Compared to the conventional use of AdC plus a cooling coil in which the cooling capacity decreases considerably at a lower air temperature, the merit of the present invention over the AdC is apparent.

(32) The foregoing description of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations will be apparent to the practitioner skilled in the art.

(33) The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications that are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalence.

INDUSTRIAL APPLICABILITY

(34) The present invention provides an environmental friendly adsorption vacuum dehumidifier which can be applied in a wide range of air-conditioning systems which conventionally requires a more energy-consuming and high-emission mechanism to extract moisture from the fed air into the system where the water absorption/adsorption refrigeration cycle takes place.

REFERENCES

(35) The following references are described herein: 1. Fong, K F, Chow, T T, Lee, C K, Lin, Z, Chan, L S (2010). Comparative study of different solar cooling systems for buildings in subtropical city. Solar Energy 84(2) 227-44. 2. Qu, M, Abdelaziz, O, Gao, Z, Yin, H (2018). Isothermal membrane-based air dehumidification: A comprehensive review. Renewable and Sustainable Energy Reviews 82 4060-9. 3. Rahimi-Ahar, Z, Sadegh Hatamipour, M, Ghalavand, Y, Palizvan, A (2020). Comprehensive study on vacuum humidification-dehumidification (VHDH) desalination. Applied Thermal Engineering 169 Article no. 114944. 4. Scovazzo, P, MacNeill, R (2019). Membrane module design, construction, and testing for vacuum sweep dehumidification (VSD): Part I, prototype development and module design. Journal of Membrane Science 576 96-107. 5. Scovazzo, P, Scovazzo, A J (2013). Isothermal dehumidification or gas drying using vacuum sweep dehumidification. Applied Thermal Engineering 50(1) 225-33. 6. Bukshaisha, A A, Fronk, B M (2019). Simulation of membrane heat pump system performance for space cooling. International Journal of Refrigeration 99 371-81. 7. Bui, D T, Ja, M K, Gordon, J M, Ng, K C, Chua, K J (2017a). A thermodynamic perspective to study energy performance of vacuum-based membrane dehumidification. Energy 132 106-15. 8. Bui, T D, Wong, Y, Islam, M R, Chua, K J (2017b). On the theoretical and experimental energy efficiency analyses of a vacuum-based dehumidification membrane. Journal of Membrane Science 539 76-87. 9. Chan K C, Tso, C Y, Chao, C Y H, Wu, C L (2015). Experiment verified simulation study of the operating sequences on the performance of adsorption cooling system. Building Simulation 8(3) 255-69.