Structure of the MSMD device

The solar cell harvests short wavelength sunlight to generate electricity via photovoltaic effect, which results in a high solar-to-electricity energy efficiency. Large amount of waste heat is simultaneously generated as a side effect during electricity generation from two pathways. The first one is the relaxation of short-wavelength sunlight excited electrons and the second pathway is photothermal conversion of the long-wavelength sunlight. The waste heat is regarded as a burden in conventional solar power plants and is directly dumped into ambient air as waste. In our design, the heat is considered as resource and is delicately utilized as energy source to power PV-MD device to produce clean water.

In this work, a commercial polycrystalline silicon solar cell from Sharp was adopted both as electricity generation component and photothermal component. A lab-made MSMD device was constructed on the backside of the solar cell for clean water production (Fig. 1a and Supplementary Fig. 1). In order to reduce heat loss into the ambient environment, the sides of the PV-MD device were sealed by polyurethane (PU) foam with low thermal conductivity (0.022~0.033 W m−1 K−1)33.

Fig. 1 Schematic illustration of the integrated photovoltaics-membrane distillation (PV-MD) devices. Operate in a dead-end mode (in this mode, the source water is wicked into the evaporation layer in the direction of the red arrow and the condensed water flows out from the condensation layer in the direction of the green arrow) and b cross-flow mode (in this mode, the source water flows to the evaporation layer in the direction of the red arrow and the condensed water flows out from the condensation layer in the direction of the green arrow) Full size image

Each stage of the MSMD device was composed of four separate layers: a top thermal conduction layer, a hydrophilic porous layer of water evaporation layer, a hydrophobic porous layer of MD membrane for vapor permeation, and a water vapor condensation layer. Aluminum nitride (AlN) plate was used as the thermal conduction layer because of its extremely high thermal conductivity (>160 W m−1 K−1) and its anti-corrosion property in salty water34. The hydrophobic porous layer was made of an electrospun porous polystyrene (PS) membrane. The water evaporation layer and condensation layer were of the same material, a commercial hydrophilic quartz glass fibrous (QGF) membrane with non-woven fabric structure.

In each stage of the MSMD device (Supplementary Fig. 2), the heat is conducted through the thermal conduction layer to the underlying hydrophilic porous layer. The source water inside the hydrophilic porous layer is thus heated up to produce water vapor. The water vapor passes through the hydrophobic porous membrane layer and ultimately condenses on the condensation layer to produce liquid clean water. The driving force for the water evaporation and vapor condensation is the vapor pressure difference caused by the temperature gradient between the evaporation and condensation layers. In each stage, the latent heat of water vapor, which is released during the condensation process, is utilized as the heat source to drive water evaporation in the next stage. The multistage design ensures the heat can be repeatedly reused to drive multiple water evaporation–condensation cycles. In a traditional solar still, the heat generated from the sunlight via photothermal effect only drives one water evaporation–condensation cycle, which sets up an upper theoretical ceiling of the clean water production rate, ~1.60 kg m2 h−1, under one-Sun condition in such a system. The multistage design makes possible to break the theoretical limit as demonstrated very recently by two groups27,28.

In this work, two source water flow modes, namely, dead-end mode and cross-flow mode, are designed for the MSMD device (Fig. 1). In the dead-end mode, the source water is passively wicked into the evaporation layer by hydrophilic quartz glass fibrous membrane strips via capillary effect. In this case, the concentration of salts and other non-volatile matters in the evaporation layer keeps increasing till reaching saturation in the end. A washing operation is indispensable to remove the salts accumulated inside the device for this mode, as reported in the previous works28. However, the passive water flow reduces the complexity of the device and gives a high water production rate in the early stage for this operation mode. In the cross-flow mode, the source water flows into the device driven by gravity or by a mechanical pump, and, it flows out of the device before reaching saturation. In this case, the outgoing water flow will take away a small amount of sensible heat, leading to a slight drop in clean water productivity in the early stage. However, it solves the salt accumulation problem and avoids the need for frequent cleaning and salt removal operation, which makes the device suitable for long-term operation.

In some experiments, a commercial spectrally selective absorber (SSA) (ETA@Al, Alanod Solar) was used to replace the PV panel for clean water production performance evaluation. This material can decrease the radiation heat loss during operation because it possesses a much smaller emissivity than PV panels, and that is why it was adopted in both of the previous works on solar membrane distillation28. We use the SSA-MD device to confirm that the multistage MD device we fabricated in this work is comparable with the state-of-the-art solar membrane distillation devices.

Solar absorptance of the SSA and photovoltaic cell

The UV-Vis-NIR absorption spectrum of the solar cell was collected and presented in Fig. 2. As seen, the solar cell possesses a high light absorption (>92%) in short wavelength range (<1000 nm) and a slightly lower absorption (70–80%) in long wavelength range. Since the solar spectrum is not uniformly distributed, solar absorptance (α), which is defined as a weighted fraction between absorbed radiation energy and incoming solar radiation energy, is calculated to estimate the solar energy capture ability of the solar cell by the following equation35:

$$\alpha = \frac{{\mathop {\smallint }

olimits_{300}^{2500} I(\lambda )A(\lambda )d\lambda }}{{\mathop {\smallint }

olimits_{300}^{2500} I\left( \lambda \right)d\lambda }}$$ (1)

Where I(λ) and A(λ) represent the light intensity and absorption of a material at different wavelength. The solar absorptance of the solar cell used in this work is calculated to be 0.87, indicating 87% solar energy is harvested by the solar cell. The thermal emissivity of the solar cell is evaluated to be 0.930 (Supplementary Fig. 3 and Supplementary Note 1). It has been reported that most commercial solar cells possess high light absorption and high emissivity because they are designed to capture as much sunlight as possible and dump the waste heat as fast as possible36,37. In comparison, the commercial SSA material shows efficient absorption (>95%) in short wavelength region (<1600 nm) and good reflectance in long wavelength region (>1600 nm), which is the characteristic of SSA type materials. The solar absorptance and emissivity of the SSA material are 0.94 and 0.123, respectively, which are similar to those reported in literatures38,39.

Fig. 2 UV-Vis-NIR spectra of the solar cell and the spectrally selective absorber (SSA) material. (The standard solar radiation spectra of air mass 1.5 global (AM 1.5G) is shown by the black line.) Full size image

Clean water and electricity production performance

The clean water production performance of a multistage SSA-MD device operated in dead-end mode was firstly evaluated in a lab-made setup (Fig. 3a), with pure water as the source water. The average water production rate, calculated from the slope of the mass change curve at the steady state (Fig. 3b), was 2.78 kg m−2 h−1 for 3-stage SSA-MD and was 3.25 kg m−2 h−1 for 5-stage SSA-MD (Supplementary Fig. 4), which is about 5 times the fresh water production rate of the state-of-the-art conventional solar stills. The multistage MD device fabricated in this work is comparable to the state-of-the-art multistage MD devices27,28.

Fig. 3 Water production evaluation of the multistage membrane distillation (MSMD) device. a Schematic representation of the experimental setup (① Solar simulator, ② computer ③, clean water collector, ④ photovoltaics/spectrally selective absorber-membrane distillation (PV-MD/SSA-MD), ⑤ source water container, ⑥ electrical balance). b The mass change rates of the collected water under one sun irradiation (starting from the red dash line) and dark (starting from the black dash line), and c water production rates of a three-stage dead-end PV-MD/SSA-MD devices, d temperature profile, e the mass change of the collected water and f the water production rate of each stage of the three-stage dead-end SSA-MD device Full size image

When the SSA material was replaced with the solar cell as photothermal component in the 3-stage PV-MD device and the PV-MD was not connected to an external circuit, i.e. the solar cell was used just as a photothermal material and the absorbed solar energy was converted to heat exclusively without any electricity output, the average water production rate was 1.96 kg m−2 h−1 (Fig. 3c), which is 29.5% lower than that of the 3-stage SSA-MD device. This significant decrease of clean water production for PV-MD can be attributed to its slightly lower solar energy harvesting and much more radiation heat loss, which will be discussed later.

For SSA-MD with 3-stage structure, after reaching the steady state, the temperature of the conduction layer from top to bottom was 61.8, 55.1, 47.5 and 38.4 °C (Fig. 3d). The corresponding temperature difference between the top surface of the water evaporation layer and bottom surface of the condensation layers in the 1st, 2nd and 3rd stage of the SSA-MD device was 6.7 °C, 7.6 °C and 9.1 °C, respectively. A MD stage working at higher temperature gives higher energy utilization efficiency as reported in numerous literatures40,41,42.

The water production rate in the 1st, 2nd and 3rd stage of the 3 stage SSA-MD was 1.07, 0.89, and 0.75 kg·m−2·h−1, respectively (Fig. 3e, f). The water production rates of the 2nd and 3rd stage were equivalent to 83% and 84% of the 1st and 2nd stages, respectively, indicating a high latent heat recovery rate. It should be pointed out that this result does not mean that only ~83% of the latent heat was recycled by the next MD stage and the rest was lost. Actually, since the device was well sealed by the PU foam in all side surface, the heat loss through the side surface is negligible, and therefore the heat flux in all these three MD stages is almost the same. The decrease in clean water production rate is mainly because of the lower working temperature in the 2nd and 3rd stages, which led to a lower clean water production efficiency.

The water production performance of the 3-stage PV-MD was next further investigated by connecting the solar cell to an external circuit with different resistances. When the device was working under one-Sun illumination with pure water as source water, the temperature of the solar cell, which is slightly affected by the external resistance, was measured to be approximately 58 °C. Since the performance of the solar cell is affected by its working state temperature, the J–V curve of the solar cell at working state (58 °C) was measured under one-Sun illumination condition with simultaneous clean water and electricity production operation (Fig. 4a). Based on the J–V curve, the largest output power was 138 mW for this solar cell, which was achieved under an optimal load of 1.3 Ω with a current of 0.32 A and output voltage of 0.43 V. Although the effective working area of the MSMD device (4.0 cm × 4.0 cm) was 16 cm2, the effective working area for the solar cell was only 11.9 cm2 (Supplementary Fig. 5). The energy efficiency of the solar cell under this condition was calculated to be 11.6%.

Fig. 4 Electricity and water production evaluation of the photovoltaics-membrane distillation (PV-MD) device. a J–V curve of the solar cell under one Sun illumination (P max refers to the maximum power). b The mass change rate of the collected water and c clean water production rate at different loads of 3-stage PV-MD with dead-end mode; d the mass change rate of the collected water, e clean water production rate, and f electricity generation efficiency under different solar irradiation intensity of 3-stage PV-MD with dead-end mode Full size image

When the solar cell was connected to a resistance with its optimal load (1.3 Ω), the same PV-MD exhibited a water production rate of 1.79 kg m−2 h−1 (Fig. 4b, c), which is 8.7% lower than that without electricity output. When the resistance of the load was increased to 3.2 and 6.0 Ω, the output power was decreased to 84 and 50 mW, with an increase of output voltage to 0.52 and 0.53 V, respectively. The water production rates were 1.82 and 1.88 kg m−2 h−1 for these two cases, respectively (Fig. 4b, c). These results indicate that the water production rate is only slightly affected by the extraction of electricity from the system, which is expected. Overall, the device gave a high clean water productivity ( > 1.79 kg m−2 h−1) given that about 11% solar energy was extracted from the PV-MD device to produce electricity.

The clean water production performance of the 3-stage dead-end PV-MD device under solar illumination with different light intensity was also investigated and the results are presented in Fig. 4d–f. The average water production rates under 0.6, 0.8, 1.0, 1.2, and 1.4 Sun illumination were measured to be 0.92, 1.39, 1.82, 2.31, and 2.65 kg m−2 h−1, respectively (Fig. 4d, e). The relationship between the clean water production rate and solar irradiation intensity was linear (Supplementary Fig. 6) and the electricity generation efficiency of the solar cell was stable at around 11.1~11.6% under different solar irradiation. These results demonstrate that the PV-MD device possesses excellent clean water production and stable electricity generation performance under varying solar intensity.

One targeted application of PV-MD is to generate electricity and at the same time produce clean water from various source water with impaired quality, such as seawater, brackish water, contaminated surface water, and groundwater. When 3.5% NaCl aqueous solution was used as a seawater surrogate, the clean water production rate was 1.77 kg m−2 h−1 in open circuit state and 1.71 kg m−2 h−1 in the optimal load state (1.3 Ω). These two values are both lower than those recorded when pure water was used as source water (Fig. 4), which should be attributed to the decrease of the saturation vapor pressure of the salt water35. For the devices operated at dead-end mode, the salt concentration of the source water in the evaporation layer would gradually increase during operation, leading to a slight decrease in clean water production rate (Supplementary Fig. 7). The concentrated source water inside the device can be sucked out of the device by a dry paper via capillary effect. Although not all the NaCl salt was removed in this way, the performance of the device could be nearly fully recovered in the next operation cycle. Figure 5a shows the clean water production rate of the dead-end device measured in five operation cycles. In cycle 1, 3, and 5, the solar cell was not connected to external circuit, while in cycle 2 and 4, the solar cell was connected to external circuit (Fig. 5a). The result clearly demonstrates that this device can be regenerated from salt accumulation state with fully recovered performance. The concentration of Na+ in the collected condensate water in each cycle was always lower than 7 ppm, which is only 0.02% of the source water and much lower than the World Health Organization (WHO) drinking water standard (Fig. 5b). In another experiment, PV-MD with dead-end mode was used to produce clean water from a heavy metal-contaminated seawater. The PV-MD device exhibited a clean water production rate of 1.69 kg m−2 h−1 under one-Sun illumination (Supplementary Fig. 7). The concentrations of the ions in the source water and clean water product were measured and shown in Fig. 5c. For the collected clean water, the concentrations of Na+, Ca2+, and Mg2+ decreased to be lower than 4 ppm while the concentrations of Pb3+ and Cu2+ decreased to almost zero and 0.02 ppm, respectively. All of the ion concentrations are below the WHO drinking water standards43. These results convincingly indicate a perfect desalination performance via the membrane distillation process.

Fig. 5 Reusability evaluation of the photovoltaics-membrane distillation (PV-MD) device. a Water production rate in different cycles under open circuit state (blue column) and optimal stage (red column) via the 3-stage photovoltaics-membrane distillation (PV-MD) device with dead-end mode for salt water desalination, b water salinity of the source water and desalinated water collected in every cycle (The red line is the World Health Organization’s (WHO) guidelines for drinking-water quality). c Ion concentrations of the heavy-metal contaminated source water and desalinated water by PV-MD device Full size image

In a PV-MD device operated at dead-end mode, the salts from the source water will continuously accumulate inside the evaporation layer during operation as mentioned above, which may cause failure and damage if salt crystals block the pores of the MD membrane. Although the salt can be cleaned out of the device by frequent regeneration operation as discussed earlier, it deems not practical for long-term operation and large-scale application. Therefore, we further designed a 3-stage PV-MD device that can be operated at cross-flow mode to solve the salt accumulation problem (Fig. 1b). In this device, a source water flow layer (recycle layer) was added at the bottom part to recycle the heat for the purpose of pre-heating the source water before it enters into the evaporation layer. When the water outlet of this 3-stage cross-flow type PV-MD device was blocked, i.e., it was operated in a dead-end mode with no water flowing out of the device, the clean water production rate was 2.09 kg·m−2 h−1 (Supplementary Fig. 8) with pure water as source water, which is 7% higher than that recorded on the dead-end type device under the otherwise same conditions (1.96 kg·m−2 h−1) (Fig. 1a). This result suggests that adding a source water flow layer at the bottom to recycle the heat can improve the clean water productivity.

When the water outlet of the 3-stage cross-flow type PV-MD device was opened and the flow rate of the source water was controlled to be 5 g h−1, which is about two times the water production rate in the dead-end condition, the clean water production rate was slightly decreased to 1.93 kg m−2 h−1. This can be explained by the fact that some sensible heat was carried away by the outgoing water flow at the outlet. When the flow rate of the source water was increased to 6 and 7 g h−1, the clean water production rates were further decreased to 1.83 and 1.76 kg;m−2 h−1, respectively. These results indicate that the clean water production rate was only slightly affected by the flow rate of the source water because the outgoing water contains only small amount of sensible heat.

The seawater desalination performance of the 3-stage PV-MD device with cross-flow mode was then evaluated and is presented in Supplementary Fig. 8. The flow rate of the source water was controlled at 5 g h−1 to avoid continuous salt accumulation inside the device and the device exhibited a very stable clean water production rate of 1.65 kg m−2 h−1 under one-Sun illumination in a 3-day continuous test. In this case, a continuous concentrated source water stream steadily flowed out of the device, keeping the salt concentration at a steady state inside the device. The salt concentration of the source and concentrated seawater was 3.8 wt% and 8.7 wt%, respectively. Although the clean water production rate was slightly lower when the device was operated under this condition, comparing to dead-end mode, its long-term clean water production stability outweighs its slightly reduced rate. Field tests of a large PV-MD device were conducted and the details can be found in Supplementary Fig. 9 and Supplementary Note 2.