Gravity-based solar cell cooler
A passive cooling device for solar cells has been designed to work using only gravity.
The atmospheric water harvesting (AWH) device comes from researchers at King Abdullah University of Science and Technology (KAUST) in Saudi Arabia, where solar energy makes up more than 80% of its renewable energy output.
The device combines radiative cooling while harvesting water from the air using no electricity. It relies on cheap, readily available materials, says the team behind it.
The solution is based on their previous technology, which has a vertical double-sided architecture. That system was originally designed to reflect thermal heat back to the sky to prevent the solar cells from overheating.
Their previous research shows that the double-sided architecture can enhance the power output of solar panels by up to 16.8% through effective temperature regulation.
Professor Qiaoqiang Gan at KAUST explains that the subsequent AWH device incorporates this structure with two aluminium mirrors and a vertically aligned emitter. Capturing water adds another layer to the technology.
The researchers explain how AWH systems are often not very efficient as 'water droplets tend to remain pinned to the surface [of the device], necessitating active condensate collection', shares Shakeel Ahmad, postdoctoral researcher in Gan’s group.
By adding a lubricant made of polydimethylsiloxane (PDMS) and silicone oil on the condenser side, the KAUST researchers can collect water using gravity. 'Our coating effectively eliminated pinning, enabling true passive water collection driven by water [gravity],' says Ahmad. 'Since this system operates entirely on passive radiative cooling, it doesn’t consume any electricity.'
By integrating a lubricant surface coating with vertical solar panels, Gan says, 'radiative cooling can be further optimised, improving cooling performance during the day. This enhanced cooling not only boosts solar panel efficiency, but also supports atmospheric water harvesting at night'.
Gan suggests this is the first passive condensation on a lubricated surface. The collected water can then be reused for irrigation, washing or cooling the buildings on which the cells are fitted.
The lubricated surface is made by infusing a lubricant into the bulk matrix of a 3D-crosslinked, PDMS elastomer network. This reservoir allows the lubricant to migrate to the surface, ensuring a continuously replenished slippery interface.
The system is prepared by mixing a Sylgard 184 silicone base and curing agent in a 10:1 weight ratio using a vacuum mixer. The mixture is applied to the condenser side of the substrate and cured at 60°C for two hours.
After curing, the PDMS coating is submerged in low-viscosity silicone oil, causing the PDMS to swell.
The scientists expect the lubricated surface coating to last for around 750 hours (three-to-four months) of active condensation, assuming a nightly operation of six-to-eight hours.
The KAUST team has tested the 30cm prototype six times over a year in the town of Thuwal, about 100km north of Jeddah. They find it almost doubles the rate of water collection compared to alternative AWH technologies.
In controlled conditions of 20°C and 80% humidity, using a single mirror, the device was able to achieve a condensation rate of 51g.m-2h-1, which Gan says is around 87% of the theoretical upper limit. This equates to collecting around 0.4L of water per 1m2 per night.
As well as not using any electricity, the system does not rely on any mechanical parts like fans, reducing maintenance over traditional systems.
Gan suggests the water could be made potable by expanding the condenser with filtration systems.
