A bright future ahead

Ocean Sun is a small technology company founded in 2016 in Norway, a country known for its hydropower traditions, maritime and offshore technology, but also mineral extraction and utilisation. This special mix of industries originates from the mountainous topology, deep and sheltered fjords and harsh offshore conditions in the North Sea. These features have been exploited for over a century to provide renewable hydropower to supply advanced metallurgical industries. More recently Norwegian engineers have directed their efforts towards decarbonisation of silicon oxide and silicon wafer production during the first solar PV boom. Around 15 years ago the company REC Solar mastered the entire value chain from quartz to solar grade silicon wafer, PV cells and solar modules, now accepted to be the cheapest renewable energy form in tropical countries.

Vessel designs for the Norwegian fishing and maritime industry have gradually adapted to variable conditions in both sheltered fjords and the high sea, exemplified by Viking ships, whalers, polar exploration, merchant shipping and deep-water operations for gas exploration and lately offshore wind farms. The Ocean Sun technology combines many of these technologies developed by Norwegian engineers to offer hydropower reservoir owners a new way of adding cheap renewable energy through floating solar plants on their reservoir surfaces. The proximity to a hydropower plant means that the intermittency problem of traditional solar plants can be balanced at source within the existing hydropower supply point to the grid, avoiding additional strain on the power system operator who is often struggling to balance variable output from large solar PV plants. No addition or modification to the grid is necessary although small solar plants may need a battery backup of some minutes’ duration to smoothen variations caused by passing cloud cover on the scale of a few minutes, thus avoiding many rapid changes in hydropower output.

Ocean Sun’s patented technology relies on standard silicon PV modules sealed and adapted for aquatic conditions resting on a flexible polymer membrane surrounded by circular floatation collars used by the aquaculture industry. Modular design means each ring can provide up to 0.7 MWp and mats of rings can be manufactured on a beach then tethered together to form a stable unit of up to many megawatts before being towed into position and moored permanently. The flexibility of the membrane means the unit behaves like oil on troubled water, preventing breaking of waves even in hurricane conditions. Moorings can be designed to tackle many metres variation in reservoir level, or even the occasional resting of the membranes on the lakebed.

The horizontal aspect of the solar panels makes this design best suited to tropical and semi-tropical reservoirs and here there are additional benefits. Since the back of the panels are in full contact with the thin membrane the conductive cooling of the underlying water is far more effective than convective air cooling of pontoon mounted alternatives. Researchers [2] have documented this effect as resulting in higher voltage which gives better energy yield than the same panel mounted on pontoons, land or rooftops. Secondly, the flat design almost eliminates the wind drag that otherwise would occur with tilted panels, enabling deployment also at sea or in big lakes. 

In contrast to floating PV designs based on floats or pontoons there is little wind resistance, and the latest pilot is designed to test the concept in exposed marine waters where pontoons would soon get destroyed. The circular polyethylene floatation rings developed for the fish farming industry are conveniently manufactured at site. These buoyancy rings and the large impermeable membrane provides an installation surface that also protects the PV panels from the moist corrosive saltwater environment. The polymer membranes also effectively stop evaporation of valuable fresh water from reservoirs, or conversely catch rainwater to provide fresh water. Surplus water on the membrane is evacuated using small bilge pumps. The ability to flood the surface of the system with clean water and wash out any airborne pollutants makes it possible to obtain a low level of soiling such as bird droppings.

The special combination of commonly available materials results in costs on a par with ground mounted PV plants. Each membrane can be packed into a single container and reduces transport cost to remote locations. Horizontal panels are optimal for locations near the equator such as in South America, Africa and the Middle East, Asia and the Indo-Pacific, but much interest has also been generated by Mediterranean countries. A new pilot will be constructed in Spain in January 2024. 

Favoured location

Locations where there is no available land for ground mounted PV are favoured and lush vegetation, rainforest, farmland, and recreational areas can be spared resulting in little conflict in licensing processes. Existing hydropower owners will often be able to experience simplified licence applications to the authorities since they use their own reservoir site and no new expansions of the grid are necessary. The floating solar plant can provide additional energy by ramping down the hydro output accordingly in daylight hours, thus saving water for the evening
peak load. This is much cheaper than building new hydro plants. 

A common myth is that the reservoir area will limit the size of PV plant, but this is never the case. In practice it is almost always the capacity of the existing hydro units, transformers and transmission line which determines the optimal size for combined operation. A suitable site near the transformer station with short cable connection is usually found and most of the reservoir is unaffected.

The first Ocean Sun system was constructed on the west coast of Norway by reusing an old floatation collar from a fish farmer. The first time we tested it we could walk and even run effortlessly across the 1mm thin membrane at sea. This was a big experience, similar to walking on a waterbed. Initially, it was thought we might need skis to distribute our weight more evenly but it was soon discovered that operators could move freely on the huge circular “sundeck”. Now we can walk safely on the panels during inspection and cleaning operations without causing damage, and are at no risk of falling into the reservoir.

The next system was a 100kWp micro grid installation for Leroy Seafood, powering an aquaculture feeding barge equipped with diesel generator and battery back-up. The system was tested for one year in the relatively exposed location, and survived storms and waves up to 3m.

A larger 250kWp system was installed on the Magat hydro power dam in the Philippines in 2019. This was the first hydro hybrid and the system could be benchmarked towards an air-cooled rooftop PV installation nearby. At a latitude of about 16 degrees north this system outperforms regular air-cooled panels by ca 10% higher yield. This pilot now has four years of successful operation and has survived several tropical cyclone episodes. The owner SNAP plans to expand it and has already installed a 20MW battery for smoothing combined hydro-solar operations.

After a troublesome start during the pandemic, Ocean Sun opened the 2MWp demonstrator with Statkraft in 2022 at the Banja reservoir in Albania. The system consists of four rings, 0.5 MWp each. It was erected over several weeks on a beach and towed into position, and has operated continually for over a year.

The latest pilot is a marine water test unit recently launched close to the port of Tazacorte, La Palma in the Canary Islands. This is the result of an EU-financed project named BOOST, in collaboration with Plocan, ITC, Inosea and Fred Olsen Renewables. It is a near shore installation in the Atlantic Sea and is exposed to open sea towards the south. Ocean Sun has also collaborated with the State Power Investment Corp for hybridisation with offshore wind turbines in the Yellow Sea in Shandong Province, China. This experimental system was relatively quickly taken ashore for further modifications.

The future

The future for floating solar looks promising and the hybridisation with hydropower plants is probably where the first multi-megawatt projects will come. The Ocean Sun solution is particularly suited in the tropics, warm climates and in areas exposed to high wind and waves. The catenary style anchoring of the circular floaters is suitable for reservoirs experiencing water level fluctuation of over 10m. Several of the pilots have received site-specific certification from third party maritime certification organisations, a prerequisite for insurance and general bankability of larger installations.  

References

• Bjørneklett, Offshore floating solar – a technical perspective, PV Tech Power Magazine, 16 (2018) 60
• Kjelstad et al, Cooling of floating photovoltaics and the importance of water temperature, Solar Energy 218 (2021) 544–551