The world is investing in renewable sources of energy, spurred by falling photovoltaic prices.
One of the major constrain being land for ground mounted power plants. Alternate spaces are being explored, spaces above and along irrigation canals, lakes and marshlands, deserts. Solar power plants atop irrigation canals have already been built in Gujarat. A conventional method of bridging the canals with heavy structures, that use a lot of steel have been used. Modules are mounted on these structures. This design is more expensive than ground mounted systems. That is a big deterrent in implementing this in a big way.
Taking inspiration from suspension bridges, a module mounting structure has been designed. This design is relevant for covering irrigation canals in particular as they are usually less than 20 meters wide. This system is also relevant to land based arrays in rocky areas and undulating land which is expensive to level. Covering canals also reduces water evaporation from the channels underneath. Modules are cooler and perform better.
In this concept design, an array of 20 PV modules in one row is considered, spanning 20 meters. The maximum wind speed of 190 KMPH is taken for the purpose of calculation. The other important issues are vibrations, swinging, sag, uplift, dynamic loads, flexing, rattling in gusty and windy conditions.
The design concept is based on a structure with wire ropes spanning the canal and interlocked to provide a stable platform for mounting modules. This design incorporates methods to lock all six axes (XYZ) for increased stability.
The system consists of three wire ropes anchored at two ends and stretched across two support columns on either sides of the canal. The cable ends have tensioners and turnbuckles to pre-load the ropes to counter the forces of wind.
To prevent swaying of the ropes, all the ropes are clamped together with braces. These braces also provide fixing arrangement for PV modules.
Each row in turn are linked to the other with braces. This results in a grid or weave like structure. swaying and sag also will be arrested to a large extent due to the reinforcement provided.
The Sag in the middle is arrested with a suspension cables spaced every two or three rows. Similar in design to a suspension bridge. The vertical wire ropes support the structure from sagging in wind load.
Another anchor is provided at the center to prevent uplift movement.
The three sets of three of wire ropes in the X direction are anchored to the ground on each side of the canal. These wire ropes are in turn locked at 400 points in the Y direction with the braces or tie members resulting in a big interlocked grid.
Locking the Z direction is achieved by suspension cables every third row. Vertical cables hold the structure up. Wind load pushing the modules and the wire ropes in the downward direction is arrested by these suspension cables.
To prevent upward movement, a set of anchor cables in the middle of the canal are incorporated.
Walkways for inspection and cleaning also can be provided between rows, using Galvalume coated sheets.
Panels mounted in three rows.
Front bottom view locking braces and module mounting stands.
Rear view and rear module mounting stands.
Tensioners and cable
Wind Load Calculation.
q = ρV2/2
q is the pressure corresponding to velocity V, ρ is air density, approximately 1.25kg/m3, V is velocity = 190km/hr = 52.8m/s
q = 1.25*(52.8)2/2 = 1742N/m2
q' = 0.5q = 871N/m2
q' is the pressure on an inclined surface of 30o with wind direction as shown in the diagram.
Normal Force, F = A*q' where A is area of panel
Fx = F*sin30 = 0.5F and Fz = F*cos30 = 0.867F
Area of a panel is considered as 2 SQM, therefore, the Force on each Panel will be 871*2 = 1742N = 177 KGF per Panel.
Sizing of Cable for a 20 panel array 30-degree tilt (usual tilt angle is from 0 o to 15 Degrees)
Panels in one row = 20 :: Wind Load = 3500 KGF :: Weight of panels = 20 * 20 = 400 Kg
Max static load on the 20 panel array = 3900 Kg
As the span is over 20 meters, three wire ropes are proposed to be used.
9.5 mm Wire rope has a safe tensile loading of 2.2 MT and a breaking strength of 6.5 MT
The combined capacity of 3 ropes = 6.6 tonnes, well within the safety limit.
Material requirements for a 20 x 3 Panel 3 rope construction supporting 18 to 20 kWp panels.
9.5 mm Wire Rope 350 meters about 150 Kg
End support structure for angle and anchoring approx 600 Kg Structural Steel
Module mounting stands 90 Kg.
Braces between wire ropes 150 Nos, approx. 450 Kg.
Concrete Anchors / Piles , Tension Mechanism, turnbuckles, eye bolts, fasteners and rivets.
The structural steel, wire ropes, braces / tie beam and fasteners for a 60 panel structure (18 kW) is about 1500 Kg (i.e. structure costs 8 to 10 per watt)
Compare this design with some of the designs of canal structures installed. These use a huge amount of steel and expensive.
In conclusion, a wire rope based solar structure is feasible and technically and commercially viable. A 5 Crore per megawatt Canal Mounted Solar power plant is within reach.
Power can be used for lift irrigation, pumped storage and also connected to the grid.
Further studies need to be carried out to optimize the design and material requirements. Top institutes like IIT’s and IISC can help in this endeavor.
Author: Mr. Sanjay Godbole
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