Experimental design

Figure S7 shows the schematic of the experimental setup used in our study. The setup includes three flow loops: (i) the freshwater loop in the dehumidifier (dashed line), (ii) the air loop through the heater and the bubble column to achieve a specific humidity and temperature and finally along the dehumidifier (dotted line), and (iii) the closed water loop between the water heater and the bubble column for controlling the water temperature in the bubble column (dash-dotted line).

The dehumidifier consists of a 0.6-m-long vertical cylinder with a diameter of 2.5 inch and includes a square array of 52 (or 96) cotton threads with a 7-mm (or 5-mm) pitch. The cotton threads with a diameter of 0.76 mm were fixed to the bottom acrylic sheet to ensure their verticality. The liquid reservoir was located at the top. To introduce water onto the cotton threads, stainless steel nozzles with an inner diameter of 1.2 mm were used in our dehumidifier design and throughout this study, except in Fig. 1 for which we used a nozzle with an inner diameter of 0.8 mm. The inlet plenum at the bottom was designed to create a uniform flow of humidified air stream. Distilled water at 20°C (monitored using two thermocouples inside the reservoir) was pumped to the top reservoir from a main reservoir (FWR-1). A pump and a flow meter were used to control the flow rate. Water flows down the cotton threads after exiting the nozzles and absorbs the water vapor from the countercurrent humidified air stream. It was then collected at the bottom reservoir (FWR-2). A weight scale was placed under the freshwater reservoir (FWR-2), with a resolution of 0.1 g to measure the flow rate of freshwater at the outlet of dehumidifier.

We used a bubble column as a humidifier to supply air saturated with water to the dehumidifier. Filtered compressed air at 20°C flows into the bottom chamber of a bubble column, which has the cross section of 150 mm by 150 mm and height of 250 mm. The sieve plate has a square array of 25 by 25 holes with a diameter of 1 mm. Heated water circulates through the bubble column using a heating/pumping unit to ensure the uniform temperature distribution of water inside the bubble column. By adjusting the height of the heated water in the bubble column, a desired output air condition was obtained. The humidified air flows through two plastic tubes with an inner diameter of 25 mm to reach the air inlet of the dehumidifier.

Four sets of four microthermocouples with a tip diameter of 250 μm were mounted at four axial locations (0.0, 0.1, 0.4, and 0.7 m from the liquid nozzle) and four positions in each axial location (two for air temperature and two for water temperature), as shown in fig. S7 (locations 1 to 4). A differential pressure transducer (P) was used to measure air stream pressure drop along the 0.5-m-long mid-section of the dehumidifier. Two humidity sensors were also mounted at the air inlet and outlet of the dehumidifier to measure the relative humidity of air.