Research ArticleAPPLIED SCIENCES AND ENGINEERING

Multistage and passive cooling process driven by salinity difference

See allHide authors and affiliations

Science Advances  13 Mar 2020:
Vol. 6, no. 11, eaax5015
DOI: 10.1126/sciadv.aax5015
  • Fig. 1 Layout, schematics, and working principle of the passive cooling device.

    (A) Schematic of the working principle of a generic salinity-driven cooler: The salinity difference between two inlet solutions generates a net vapor flux from the evaporating (YE) to the condensing (YC) layers. These evaporation-condensation processes in multiple stages allow the removal of heat from the lower-temperature chamber and the transfer of heat into the higher-temperature external environment. This process eventually leads to a by-product, being the dilution of the solution in the condensers, which is here represented as an outlet flow with an intermediate salinity (Y*) with respect to the inlet solution at YC. (B) Schematic layout of the four-stage modular passive cooler discussed in this work. The actual experimental setup is reported in fig. S1. (C) Working principle of one stage of the passive cooler: Two solutions with different salinities are separated from each other by a hydrophobic membrane. The salt concentration difference creates an activity gradient (green triangle), which leads to a net vapor flux. The transfer of enthalpy of evaporation establishes a temperature gradient between the two solutions (red triangle), which is opposed to the gradient created by the activity difference. (D) Graphical representation of the assembly of one stage of the cooling device. A 3D-printed plastic frame (2, red) contains the cavity that forms the condenser, which is sealed by an aluminum plate cover (1, gray) and a hydrophobic membrane (3, brown). The hydrophilic layer (4, yellow), being the evaporator, is placed between the membrane and another aluminum plate.

  • Fig. 2 Experimental cooling performance.

    (A) Time evolution of the last-stage condenser (red), first-stage evaporator (blue), and ambient (green) temperatures during a typical laboratory test with the 3.1 mol/kg NaCl solution (25 W m−2 thermal load). (B) Experimental results (red dots) and modeling predictions (gray band) of the specific cooling capacity of the passive cooler with respect to the considered temperature difference across the device with the 3.1 mol/kg NaCl solution. See note S2 for details on the reported error bars of the experiments. Modeling predictions are obtained with Eqs. 3 to 24, and the reported band is enclosed between the minimum and maximum values obtained by varying the main input parameters of the model according to their bounds, namely, the activity, membrane thickness, and porosity (see table S1 for details).

  • Fig. 3 Cooling performance with different configurations.

    (A) Cooling performance of the four-stage passive cooler as predicted by the lumped parameter model for different aqueous solutions. While the evaporators contain distilled water in all cases, the condensers are supposed to contain seawater (0.6 mol/kg, blue band), a 3.1 mol/kg solution of NaCl (gray band), or a 3.1 mol/kg solution of CaCl2 (yellow band). (B) Modeling predictions for the four-stage passive cooler at different ambient temperatures (TA) but a fixed 3.1 mol/kg NaCl solution. According to Eq. 4, a higher TA implies greater vapor fluxes through the membrane, hence better cooling performance than the case that was experimentally assessed (solid red line, TA = 30°C). The increase in cooling capacity with the ambient temperature is mainly due to the exponential increase of vapor pressure with temperature, as predicted by Antoine’s law. (C) Modeling predictions for the passive cooler with different numbers of stages but a fixed 3.1 mol/kg NaCl solution and TA = 30°C. Increasing the number of stages substantially increases the maximum temperature difference achievable across the device owing to the reduction of the temperature across each membrane. (D) Modeling predictions for the four-stage passive cooler considering an additional air gap beneath the membrane, as predicted by the lumped parameter model. In detail, the performance of the cooling device is estimated by varying the thickness of the additional air gap (da) from 0 to 2 mm, with a 3.1 mol/kg NaCl solution and TA = 30°C. All the model parameters used in (A) to (D) are reported in table S1.

  • Fig. 4 Comparison with other passive cooling technologies.

    Comparison between the performance of the proposed device and different daytime radiative cooling technologies from the literature. The results reported refer to the works of Rephaeli et al. (36), Kecebas et al. (37), Mandal et al. (12), Zhai et al. (11), Fu et al. (38), Granqvist and Hjortsberg (16), Nilsson et al. (39), Bhatia et al. (40), and Raman et al. (10). The references labeled with an asterisk refer to modeling results. The maximum specific cooling capacity at a vanishing temperature difference for the proposed device and its uncertainties are estimated from the experimental results by linear fitting (least square method). The thermodynamic limit of daytime radiative cooling is computed at TA = 30°C (13).

  • Fig. 5 Stable operating cycle of the passive cooler.

    (A) Schematic of a possible coupling between the passive cooler and a thermal-driven device for salt water distillation. In the latter device, an external source of heat (e.g., solar) allows water to be distilled from salt water, specifically to regenerate the salinity gradient between the two solutions. Therefore, if the distilled water equals that consumed by the cooler, a stable and passive cooling cycle can be established. The heat dissipation from the top condenser of the cooler and the bottom condenser of the distiller can be ensured by either natural or forced convection. (B) Representation in the Clapeyron chart of a possible coupling between the passive cooler and a thermal-driven device for salt water distillation. For simplicity, the temperatures of evaporators and condensers are equal to those of the surrounding environments (TA and TF, respectively). The salt water contained in the condenser operates ideally at ambient temperature (TA) and constant concentration (YC) (point 1). The draw solution, diluted by the cooling device, is constantly regenerated by the solar distiller. A selective solar receiver passively heats the salt water (point 2, temperature TH), producing distilled water (YE) at ambient temperature (point 3). This can be used by the proposed device (point 4) to cool a chamber to a temperature lower than the environmental temperature. (C) Coupling between the cooling device and the passive solar distiller proposed by some of the authors (19). RS is the ratio between the number of stages of the distiller and those of the cooler; RA is the ratio between the solar collecting area of the distiller and the active area of the cooler. Potential regeneration capabilities of the salinity gradient in the cooler are evaluated considering that the condensers are fed by salt water, with NaCl concentration equal to 170 g liter−1.

  • Table 1 Cycle points for a complete solar cooling cycle.

    Actual Clapeyron cycle points referred to the case with RS = 1, RA = 1, and a specific cooling power of 100 W m−2 (see also Fig. 5).

    Y (g liter−1)T (°C)1T(K−1)pv(bar)ln(pv)
    1YC = 170TA = 30.0−3.29 × 10−33.74 × 10−38.22
    2YC = 170TH = 67.4−2.93 × 10−324.52 × 10−310.10
    3YE = 0TA = 30.0−3.29 × 10−34.15 × 10−38.33
    4YE = 0TF = 30.0−3.29 × 10−34.15 × 10−38.33

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/11/eaax5015/DC1

    Note S1. Membrane permeability measurement

    Note S2. Measurement uncertainty analysis

    Note S3. Solute transport through the condensing side

    Note S4. Exergy analysis of the passive cooler

    Note S5. Comparison with other passive cooling approaches

    Note S6. Details on the coupling between passive distiller and cooler

    Note S7. Coefficient of performance

    Note S8. Coupling with high-salinity brines produced by different desalination technologies

    Note S9. Cost analysis of the laboratory-scale prototypes

    Note S10. Considerations on environmental and life-cycle analysis issues of the passive cooler

    Note S11. Cooling performance of the device under the sun

    Note S12. Durability and corrosion of the passive cooler

    Note S13. Boundary effects on the temperature field in the evaporator

    Fig. S1. Experimental setup to measure cooling capacity.

    Fig. S2. Assembly of the passive cooling device.

    Fig. S3. Natural mass transport phenomena in the present passive cooler.

    Fig. S4. Distillate consumption by the passive cooler.

    Fig. S5. Experimental results and modeling predictions of membrane permeability.

    Fig. S6. Possible spacer for enhancing the cooler performance.

    Fig. S7. Distillate consumption by the passive cooler for different air gaps.

    Fig. S8. Qualitative thermodynamic cycle of a passive solar cooling cycle.

    Fig. S9. Coupling between the passive cooler and distiller to implement a stable cooling cycle.

    Fig. S10. Mirror screening of the cooler exposed to the sun.

    Fig. S11. SIMSCAPE implementation of the mirror screening of the cooler exposed to the sun.

    Fig. S12. Lumped model for the heat transfer in the passive cooler.

    Fig. S13. Temperature profiles across the cooling stages in case of a 1-, 4-, and 10-stage configuration device.

    Fig. S14. Results of the finite element model for the hydrophilic layer (evaporator).

    Fig. S15. Passive cooler operating with high-salinity brines produced by different desalination technologies.

    Table S1. Uncertainties in the theoretical model.

    Table S2. Exergy performance of the passive cooler with different number of stages.

    Table S3. Estimated costs for the prototype of passive cooler.

    Table S4. Estimated costs for the prototype of passive distiller considered here to implement a solar cooling cycle.

    Table S5. Parameters considered for the simulations of the cooler performance under the sun.

    Dataset S1. Experimental raw data of tests with NaCl.

    Dataset S2. Experimental raw data of tests with CaCl2.

    References (4175)

  • Supplementary Materials

    The PDF file includes:

    • Note S1. Membrane permeability measurement
    • Note S2. Measurement uncertainty analysis
    • Note S3. Solute transport through the condensing side
    • Note S4. Exergy analysis of the passive cooler
    • Note S5. Comparison with other passive cooling approaches
    • Note S6. Details on the coupling between passive distiller and cooler
    • Note S7. Coefficient of performance
    • Note S8. Coupling with high-salinity brines produced by different desalination technologies
    • Note S9. Cost analysis of the laboratory-scale prototypes
    • Note S10. Considerations on environmental and life-cycle analysis issues of the passive cooler
    • Note S11. Cooling performance of the device under the sun
    • Note S12. Durability and corrosion of the passive cooler
    • Note S13. Boundary effects on the temperature field in the evaporator
    • Fig. S1. Experimental setup to measure cooling capacity.
    • Fig. S2. Assembly of the passive cooling device.
    • Fig. S3. Natural mass transport phenomena in the present passive cooler.
    • Fig. S4. Distillate consumption by the passive cooler.
    • Fig. S5. Experimental results and modeling predictions of membrane permeability.
    • Fig. S6. Possible spacer for enhancing the cooler performance.
    • Fig. S7. Distillate consumption by the passive cooler for different air gaps.
    • Fig. S8. Qualitative thermodynamic cycle of a passive solar cooling cycle.
    • Fig. S9. Coupling between the passive cooler and distiller to implement a stable cooling cycle.
    • Fig. S10. Mirror screening of the cooler exposed to the sun.
    • Fig. S11. SIMSCAPE implementation of the mirror screening of the cooler exposed to the sun.
    • Fig. S12. Lumped model for the heat transfer in the passive cooler.
    • Fig. S13. Temperature profiles across the cooling stages in case of a 1-, 4-, and 10-stage configuration device.
    • Fig. S14. Results of the finite element model for the hydrophilic layer (evaporator).
    • Fig. S15. Passive cooler operating with high-salinity brines produced by different desalination technologies.
    • Table S1. Uncertainties in the theoretical model.
    • Table S2. Exergy performance of the passive cooler with different number of stages.
    • Table S3. Estimated costs for the prototype of passive cooler.
    • Table S4. Estimated costs for the prototype of passive distiller considered here to implement a solar cooling cycle.
    • Table S5. Parameters considered for the simulations of the cooler performance under the sun.
    • References (4175)

    Download PDF

    Other Supplementary Material for this manuscript includes the following:

    • Dataset S1 (Microsoft Excel format). Experimental raw data of tests with NaCl.
    • Dataset S2 (Microsoft Excel format). Experimental raw data of tests with CaCl2.

    Files in this Data Supplement:

Stay Connected to Science Advances

Navigate This Article