Harvesting solar energy in countries short on land space through use of floating photovoltaic systems, enabled by SABIC

The photovoltaic equivalent of a traditional power station is the solar farm. Made up of hundreds and even thousands of panels creating vast amounts of electricity for countless homes and businesses, solar farms inevitably require a lot of space. But in many highly populated Asian countries such as India and Singapore, the land on which solar farms would normally be located can be scarce or expensive – sometimes both.

One way out of this conundrum is to put the solar farms on water, through the use of panels supported on floating pontoons, all rigged together. These pontoons are hollow structures, made by blow molding plastics in a relatively low-cost process: imagine a whole network of waterbeds, but waterbeds made in strong, rigid plastics. Possibilities include natural lakes, man-made reservoirs, and disused mines and pits.

According to the World Bank (Where Sun Meets Water, Floating Solar Market Report, 2018), the possibility of adding floating solar capacity to existing hydropower plants is of particular interest, especially in the case of largeof large hydropower sites that can be flexibly operated. It says the solar capacity can be used to boost the hydro-energy yield and may also help to manage periods of low water availability by allowing the hydropower plant to operate in more cost-effective modes. “Floating solar may therefore be of particular interest where grids are weak, such as in Sub-Saharan Africa and parts of developing Asia,” it notes.

Not only can floating solar farms occupy otherwise unused space, but they can also be more efficient than solar farms on land, because the water keeps the photovoltaic (PV) panels cool and so improves their ability to generate electricity. Second, the panels help reduce evaporation from these expanses of water, which can be an important benefit when the water is being used for other purposes; as water becomes an increasingly precious resource, this benefit is only likely to increase. Furthermore, floating solar farms can improve water quality, by decreasing growth of algae.

FPV farms are generally more cost-efficient to maintain due to the higher energy production rate. Under the same weather condition, they could generate up to 12% more energy than land-based solar farms due to the cooling environment and taking the reflection off of the water surface.

Creating a cooler solar collaboration

Floating solar farms are already a reality. In fact, the first system was built in Japan in 2007 for test purposes, and the first commercial installation, rated at 175kW, was installed on a reservoir in California in 2008. Now, growth is picking up: the first plant larger than 10 MWp was installed in 2016, and bymid-2018, the total amount of energy around the world provided by floating PV (FPV) systems was 1.1 gigawatt-peak (GWp).

According to the World Bank, there are more than 400,000 square kilometers of man-made reservoirs in the world, suggesting that floating solar has a theoretical potential on a terawatt scale, purely from the perspective of the available surface area. “The most conservative estimate of floating solar’s overall global potential based on available man-made water surfaces exceeds 400 GW, which is equal to the 2017 cumulative installed PV capacity globally,” it says. After ground-mounted farms and building integrated PV systems (BIPV), floating solar farms are already the third-largest method of solar energy generation.

SABIC has developed grades of polyethylene and polypropylene, as well as compounds based on these polymers with add-on functional stabilization such as light and heat resistance, depending on the local climate conditions which are well suited for use in production of the vital pontoons that sit on the water under the blazing sun to support the panels over many years in use. These materials demonstrated high resistance to corrosion and weathering in the lab tests carried out according to international standards in various countries such as India, Japan and China, where their resistance to environmental stress cracking (ESCR) extended beyond 3000 hours, which outperformed the industry standard thatspecifies an ESCR requirement of 1000 hours. Furthermore, creep resistance is also very high: this means that parts will not stretch under constant stress, and so the pontoons will retain their integrity.

FPV systems installed to date typically make use of primary and secondary floats, which range in volume from around 50 liters to 300 liters. These floats are produced using very large extrusion-blow molding (EBM) machines. Processors require resins with the right flow properties (MFR) to run on their machines, with consistency across batches. End-user specifications may call for the floats to be resistant to ambient temperatures ranging from as low as -60° all the way up to +80°C. Apart from resistance to continuous exposure to the sun, they also obviously have to withstand constant contact with water, they need to be airtight, plus, they should not affect water quality (leaching).

Choosing the right materials

A grade of high-density polyethylene (HDPE) developed by SABIC specifically for FPV floats, SABIC® B5308 resins, is capable of meeting these processing and performance-in-use requirements. This grade has already been approved by several companies specializing in FPV systems. HDPE B5308 is a high molecular weight material with a so-called multi-modal molecular weight distribution that provides particular processing and performance characteristics. It has outstanding ESCR, excellent mechanical properties, a very good balance of toughness and stiffness (something not always easy to achieve in plastics), a long lifetime and easy blow molding process ability.

SABIC HDPE B5308 has a Melt Flow Rate (MFR, 21.6kg load) of around 8g/10 min. This is high enough to enable the molten polymer to be easily formed during blow molding, but low enough so that the melt strength of the parison (the tube of material extruded vertically before it is blown) is sufficient to stop it distorting during production. SABIC B5308 has a melting temperature of 132°C and a Vicat Softening Point (under 10N load) of 126°C. It has excellent ESCR properties: over 3000 hours according to ASTM 1693B. Rigidity is very good: tensile modulus is 1050 MPa (ISO527).

A bright future

SABIC fully expects that we will see many more FPV systems deployed as society grapples with increased pressures to create energy without harming the environment. SABIC has already worked on floating solar farm projects in India, Japan, and China. The company believes that its polymer solutions can be key to help further unlocking the potential of FPV technology.

- SABIC