By ten metres down, the sea has taken most of the sunlight. What is left, after the water has absorbed and scattered the rest, is blue and green. For decades that remainder was considered a curiosity rather than a resource — no way to run anything, no reason to try. Previous studies of underwater solar had stopped at two metres or less.
Last week, researchers announced they had gone well past that. In a two-hour trial near the Weizhou Islands, off the southern coast of mainland China, their solar panels generated power at a depth of 10 metres — a depth long treated as impractical for the technology. The work was published in the science journal Joule, and it rests on the part of the spectrum the sea lets through: enough blue and green light, the researchers say, reaches those depths to make electricity worth harvesting.
The cells are built from perovskites, an alternative to the silicon in ordinary panels, and perovskites can be tuned to absorb particular wavelengths — in this case, the wavelengths that survive the water. In laboratory tests recreating conditions 10 metres underwater, the cells converted around 35 per cent of the sunlight that reached them into electricity.
Then came the sea itself. The team put larger versions — 115 square centimetres of panels — into the South China Sea. At two metres deep, the panels generated 1,416 milliwatt-hours over two hours, roughly half the capacity of a rechargeable AA battery. At 10 metres, they generated 324 milliwatt-hours over the same two hours. It is a small amount of power. It is also, the researchers say, far beyond what they expected. In simulated underwater conditions the panels showed almost no loss of performance after 1,160 hours, and on the strength of those tests the team estimates they could operate continuously at 10 metres below the surface for around five and a half years.
This work presents the first functional validation of submerged solar cells practically operating at a water depth of up to about 10 meters, greatly broadening their application scope
The words belong to study author Wen-Hua Zhang of China’s Yunnan University and Southwest United Graduate School in Kunming.
Solar power, once the property of rooftops and open fields, has lately been appearing wherever electricity and fuel are hard to come by. Earlier this year Dutch researchers unveiled what they called the world’s first solar-powered ambulance, at work in rural Kenya, bringing healthcare and equipment to communities without reliable power or fuel. In Norway’s Svalbard archipelago, solar has begun to run a remote former radio station reachable only by boat or helicopter, a site that once depended on diesel generators. In Switzerland, panels laid between railway tracks generate electricity without taking new land and can be lifted out when the tracks need maintenance.
Underwater solar answers a different inconvenience. Ocean sensors, cameras and communications equipment can operate far from any supply of power, but their batteries run down, and when they do, someone has to go and get them. The retrieval is the cost. A panel that feeds on the light that happens to be passing through could keep such equipment running far longer — years, if the laboratory estimates hold in open water.
That remains to be shown. The researchers themselves say their next step is to test the panels deeper, to establish how far beneath the waves solar power can go. The light thins with every metre. For now, ten metres is the number — the depth at which, this month, the sun was still good for something.