Hydrelio® floats are made of HDPE, and connection pins are made of polypropylene combined with fiberglass. The manufacturing process is blow molding for the floats, and injection for the pins.
Several additives are added to the raw material, such as UV protection and colouring agents.
1.2 UV protection
There are additives in the HDPE of floats, including UV protection agents. It is the same dose for any manufacturing batch, but based on this data we can easily calculate how long the floats will last with the protection of the UV agent. It is linked to the solar radiation of the location of the project.
Usually, it takes between 15 and 20 years before the UV starts to slowly damage the floats. Then it takes 5 to 10 more years before the floats start to be so damaged by the UV they need to be replaced.
Additionally, only a few parts of the floats are exposed to light.
A more detailed calculation can be provided regarding the site.
1.3 Metallic pieces – corrosion
Regarding the fixing system, the main parts are made of aluminium 6060, an alloy designed for usage in contact with food. It has very good corrosion resistance. We can provide either anodised or non-anodised pieces.
The fastening parts are in inox A4 which is mainly used in food industry due to its better corrosion resistance capacity.
1.4 Seal line
Whilst looking at the floats you may note a kind of seal line all around the float. However it is not a seal line: the float is manufactured in one shot. The line appears because the mould is composed of 2 pieces, but the float is blow moulded inside in one shot, therefore there is no particular weakest point there.
1.5 Tensile and dynamic binding tests
HYDRELIO® floats have been tested through tensile tests and dynamic binding tests (fatigue).
For the specification of the dynamic bending test, we considered the passage of a storm during 2 hours with wave’s period equal to 2 sec, which would happen four times in a year during the operating life of the plant (25 years). It corresponds to 360000 cycles with an angular movement between two adjacent floats of maximum 15°.
In order to compare to operation conditions, a swell with a wavelength equal to 6 meters and a wave height equal to 1 meter creates a maximum angle of 15° between two adjacent floats.
Compared to simple tensile test without dynamic bending before, we noticed that tensile strength of the floats had decreased of only 6 % with dynamic bending.
1.6 Connection pin/ ears strength
Connection pins and ears have been tested through tensile and dynamic binding tests.
The tests revealed that the failure appears on the ears, and not on the connection pins. Therefore the connection pins are stronger than the ears.
Connection pins have been tested separately and can withstand a load up to 2000 daN. However, the maximum load to consider for the ears is 600 daN.
1.7 Solar island design
The design of the solar islands is based on the requested electrical design details, such as the number of PV panels per string (depending on the inverter specifications).
Ciel & Terre® usually design the solar island to have 1 string = 1 row of PV panels, in order to avoid mutual shadings of PV panels within one same string.
The number of strings on the solar island is calculated regarding the selected inverter specifications. Usually Ciel & Terre® design the strings configuration using junction boxes and main boxes to collect the string cables on the solar island. This means there are only few submarine cables connecting the solar islands to the inverters. In this case, the junction boxes and/or main boxes are helpful, but otherwise Ciel & Terre® designs the string configuration based on usual boxes specifications.
1.8 Extra floats
Some extra floats, free of PV panels, are needed inside and outside the solar island:
They are needed inside the solar island to have space to install the junction or main boxes
They are need outside the solar island (the additional ring of floats) to improve buoyancy of the solar island.
An additional ring is needed because of the wind loads.
If anchoring at the bottom is selected, when wind is at maximum, the loads spread in the mooring lines pulling down the solar island into the water. To make sure that PV panels will never be in contact with water, and regarding the volume of the floats, an extra ring is needed.
If anchoring on the banks is selected, the extra ring is needed for the case when water level is at its lowest, to prevent mooring cables to scrape the PV panels.
1.9 Duopitch design
The usual configuration of the solar island is rows of PV panels oriented the same direction, with the alternation of rows of PV panels and rows of secondary floats for maintenance path.
But in specific cases a duopitch configuration can be proposed, in order to increase the installed capacity for a given usable area. In this case it is recommended to design with a back to back shape “/\”, in order to lower the wind loads. The “V” shape would contrariwise increase the wind loads.