Photovoltaics is now common on water because cooling increases output. It is used on a vast scale for on-grid solar farms but it has a place off grid too. See the IDTechEx report, Off Grid Zero-emission Electricity 2018-2038: New Markets, New Technology Roadmap.
Now we have a counterintuitive variant from the Green Energy Institute of Korea. For the first time in the world, they propose putting the photovoltaics under water for desalination. Desalination is becoming a huge need across the world for agriculture, human consumption and industry with at least ten cities now under threat of severe water shortages and increasingly large areas declared "water stressed". 97% of the world's water is salty: 2% of potable water is in the ice caps where it is falling into the salty sea. The remaining one percent is too much in demand. Just one example is a new dam on the Nile capable of triggering a war over water unless Ethiopia can agree a deal with Egypt and Sudan. Desalination plants traditionally make expensive water but they have to be put in anyway, partly because most places are poor at saline agriculture and water conservation. Sadly, most desalination plants cause considerable global warming by using fossil fuel electricity with their power stations creating up to 300 MW at a time, though that is slowly changing.
A salt farm adopting a solar evaporation method in Korea conventionally consists of two ponds, evaporation pond for increasing salinity and crystallization pond for harvesting salt. Harvesting clean water is an option. Water proof solar PV modules are installed in the beneath of shallow saltwater evaporation pond, two to 15 cm in depth. The front part of modules is only submerged in the water, but the rear part of modules where a junction box is placed is. The gaps between modules are sealed with specially to prevent the ingress of sea water.
The Institute points out that the merit of this system is harvesting salt and electricity at the same time. It says the power generation would be decreased due to the installation angle, zero degree and the weakened solar incident intensity by sea water reflection. However, it is believed that the loss of power generation can be overcome partially by the cooling effect of sea water. It terms these "Salt Farm Underwater Photovoltaic Systems".
In a study reported at ICREN Barcelona, April 28 2018, feasibility tests were conducted to confirm sea water effects on the performance of PV module. To do this, they fabricated water tanks where a single cell module of about 5 watts is installed at the bottom. The performance of module was examined by an AM 1.5 solar simulator at temperatures from 20- 60℃ and depth zero to 10 cm of water. The effects of temperature and depth of water on the performance of PV module were established and the properties of single- and multi- crystalline silicon solar cells in this situation were compared.
Raghu Das CEO of IDTechEx comments, "The results were positive and we encourage them to take this further. It could definitely have a place as desalination plants of all sizes are needed. It does not matter that it is unlikely to compete with reverse osmosis powered by photovoltaics and sometimes wind or water power in a zero emission future. This is a green technology that should have its niche in the industries of salt and clean water production." For more read the IDTechEx report, Desalination: Off Grid Zero Emission 2018-2028.
Top image: Koine Multimedia