A device that can generate electricity while desalinating seawater has been developed by researchers in Saudi Arabia and China, who claim that their new system is highly efficient at performing both tasks. The device uses waste heat from the solar cell for desalination, thereby cooling the solar cell. It also produces no concentrated brine as waste, cutting its potential environmental impact.
In many parts of the world, climate change and population growth are putting huge demands on freshwater supplies. In some coastal regions, desalination – removing the salt from brackish water or seawater to turn it into fresh water – is increasingly being used to meet demand. Indeed, there are now around 16,000 desalination plants around the world producing about 95 million cubic metres of freshwater every day .
However, current desalination systems can be expensive and energy hungry, producing significant carbon emissions. The process can also produce highly concentrated salt water, or brine, as well as freshwater. This brine can also contain toxic chemicals introduced during the desalination process and if not disposed of properly, it can have negative environmental impacts.
Efficiency trade-off
Climate change is also driving demand for renewable energy, like solar power. Simultaneous electricity and freshwater production using the waste heat from solar cells for desalination has been touted as a way to cut the energy required for desalination. However, this has typically resulted in a trade-off between efficient electricity generation and efficient desalination.
Now, Wenbin Wang at King Abdullah University of Science and Technology in Saudi Arabia and colleagues claim to have developed a new device called a PV-membrane distillation-evaporative crystallizer (PME) that combines efficient desalination and electricity generation.
PME consists of a solar panel on top of a multistage membrane distillation (MSMD) component. The MSMD uses waste heat from the solar cell to drive water evaporation, and is designed to collect and reuse latent heat from vapour condensation in each distillation stage to drive evaporation in the next stage.
Cooler running
In laboratory tests simulating solar illumination at an ambient temperature of 24 °C, the temperature of the solar cell on the PME was around 14 °C cooler than an identical solar cell not mounted on a MSMD component. The led to almost 8% more electricity production, compared to the bare solar cell. In the same test, the PME produced fresh water from seawater at a rate of about 2.4 kg/m2h, which is almost double that previously reported for a combined solar and desalination device.
“The high desalination performance of this design is attributed to the recycling of the latent heat of vapour condensation,” Wang told Physics World, adding that previous devices have not recycled latent heat.
Each of the five stages of the MSMD consists of four parts: a thermal conduction layer, an evaporation layer, a hydrophobic membrane, and a condensation layer. The conduction layer transports heat from the solar cell or previous distillation stage to the evaporation layer. Seawater flows into the evaporation layer and, driven by the heat, some water evaporates. The water vapour then passes through a porous hydrophobic membrane and condenses in the condensation layer as freshwater.
Evaporative crystallizer
The MSMD device sits on an evaporative crystallizer that uses latent heat from the last distillation phase to evaporate off the liquid from the final concentrated brine that is produced alongside the freshwater, leaving behind only solid salt.
Salt-free drinkable water comes at a cost
Evaporation and condensation through the system is governed by the vapour pressure gradient between the evaporation layer and condensation layer in each stage. A theoretical model suggests that the hydrophobic membranes are key to achieving simultaneous PV cooling and a high rate of water production.
“The key development with this device is the utilization of the hydrophobic membrane with a low thickness and high porosity, which is guided by our theoretical model,” Wang says. “Previous work mainly utilized the hydrophobic membrane with a high thickness to reduce the thermal conduction loss and our theoretical model found that the reducing the thickness of hydrophobic membrane can achieve a high desalination performance and low solar cell temperature simultaneously.”
“We are currently scaling up this device and planning to build a photovoltaic farm that combines electricity generation and seawater desalination,” Wang says.
The research is described in Joule.