India and the renewable energy challenge

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India has been witnessing an unprecedented surge in the installation of solar plants since the announcement of the National Solar Mission in 2010. With more than 25GW of installed capacity as on August 2018, the country is chasing the ambitious target of 100 GW Solar PV installations by the year 2022. With increasing installations on land and rooftops, the emerging focus area is to put the untapped vast area of waterbodies to good use. India has 91 major reservoirs with more than 75% of these reservoirs located in the sunny Southern, Western and Central regions.

Installation of solar PV plants on water bodies like lakes, man-made and natural reservoirs, industrial ponds and fish farms could be attractive due to the following reasons:

1. The scarcity of large land areas for installation which, in some cases has led to cultivable lands being sacrificed for installation of solar plants.

2. Increasing price trend of land.

3. Optimal use of evacuation infrastructure, e.g.: near hydel projects.

4. Reuse options for abandoned areas like mines, quarries etc.

5. Conservation of water by reduced evaporation.

6. Limiting algae growth through reduced sunlight penetration into water.

7. Improved generation from PV plants due to lower module temperatures.

8. Possible maintenance cost reduction of

plants e.g. Less frequent cleaning of panels and practically no vegetation growth control.

But, shifting of the installation to water throws open additional challenges in terms of cost, site complexities, safety, design life and equipment warranties, long-term effect on biodiversity etc. as outlined below.

Groundwork

As in the case of any installation, properly carried out groundwork goes a long way to ensuring successful implementation.

A floating plant consists of three main components – the floaters, pontoons or rafts, on which the modules are installed, the anchoring, or mooring system and the main equipment and balance of systems - modules, inverters, combiner boxes, cables etc..

The floaters must meet the buoyancy requirements to float on water while

supporting the installed equipment. This can be constructed from various materials like Ferro-cement, Fibre-reinforced plastic (FRP) etc., the most common being High-Density Polyethylene (HDPE). The cost of the floaters is proportional to the occupied surface. Hence, it is important to have a modular arrangement to achieve maximum generation with minimum cost, reduce logistics issues and optimize assembly at site.

Proper structural design will entail simulations to assess the dynamic stresses via wind tunnel tests or computational fluid dynamics (CFD) modeling to determine the forces expected on the floats, the connections, and anchoring system.

The system is generally fixed by anchoring to the bottom of the waterbody, mooring to the shore or using a combination of both. Water level variations are particularly important owing to seasonal variations in rain, size, and usage of the water body. The

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cost and complexity of the anchoring system increase with the depth of water, wind speed, water currents, water level variations and structure of basin edges.

It is also important to consider the water quality, e.g. salty or fresh, still or turbulent water, temperatures, water freezing during winter and the possibility of typhoons or similar extreme wind events.

A bathymetry survey along with water current measurements and analysis of soil sample from the waterbed is required as a minimum. Apart from this, long-term data is also relevant, e.g. water level variation of any dam will highly depend on the intake of water through catchment area and dam water management system which cannot be captured by short-term measurements.

It is also pertinent to consider the biodiversity at the site to decide on the type of floats and the extent of coverage of the surface.

Logistics could become particularly challenging depending on the site. Hence, the assessment of accessibility, area of the launching site and distance to grid evacuation points are important.

Energy generation

The conversion efficiency of a PV module is given by the ratio between the generated electrical power and the incident solar radiation intensity, according to the following expression:

nel= [Pmax / (S x Apv)] x 100%

where

• nel is the electrical efficiency (%),

• Pmax is the power generated by PV module (W),

• S is the solar radiation incident on the PV module (W/m²) and

• Apv is the PV module front surface exposed to the solar radiation (m²).

A typical PV module installed in India converts an average of 10-22% of the incident solar energy into electricity, depending upon the type of solar cells, type of installation and climatic conditions. The rest of the incident solar radiation is converted into heat, which significantly increases the temperature of the PV module. The power output of a module reduces with increased module temperature.

The generation from the modules installed on waterbodies is expected to be generally higher than classic installations due to the cooling effect of water and wind and reduced shading due to the open and flat environment. It is also worth noting that albedo from water surface could be lower due to change in water reflectivity at

different incidence angles and light absorption property of water. The effect of evaporative cooling on the modules depends largely on the size of water body, type of floats employed and the extent of water surface coverage beneath the modules. Overall improvements from 10% to 25% [1,2] in generation as compared to land-based PV systems have been reported for some early floating installations, but as variation may be large, site specific gain, if any, would require a detailed assessment.

The long-term degradation and Potential Induced Degradation (PID) of modules installed on water are less known, primarily due to lack of data. Depending on the location, the generation could also be negatively impacted by bird droppings and fog or mist over the waterbody in the morning and evening, especially during winter months.

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Materials and components

The choice of materials should be done responsibly keeping in mind design life, ecological sustainability and cost. To avoid leaching into the water, non-toxic, UV resistant and chemically stable materials are desirable. They should also be completely recyclable after decommissioning of the plant.

Corrosion is another important issue to be addressed as the components are constantly in an environment which accelerates the conditions for electrochemical reactions. Marine grade cables should be installed for carrying the power, especially if they are intended to be submerged.

Warranties of the components, especially solar modules, and inverters must be given due consideration. Higher ingress protection (IP) ratings and conformal coatings could be adopted. It would also be interesting to understand the effects of the dynamic forces and vibrations that the equipment would be subjected to, in normal operation.

There is an immediate need for standards to be formulated for floating applications. The adequacy of the current test methods of components is also to be reassessed.

Construction, safety, and maintenance

With floating installations, the installation

timelines are generally expected to be faster compared to ground-mounted systems. The floats are designed to have a modular design which can be assembled on land and then deployed into the water without the use of complex machinery. The modular design also gives the dimensional flexibility and mechanical versatility to adapt to the actual site conditions and better quality control during production.

But each site could pose unique challenges, for e.g. difficult access to the launching site, soft soil around a new reservoir, a small area of the launching site, undulated water bed, high water level variations etc. Safety of personnel is also a concern, most probable being drowning and electrocution.

Since the floating concept is fairly new, there is a lack of standards to address issues such as grounding. Marine grounding methods could be explored. The increased corrosion of the electrodes also needs to be suitably factored and managed.

An environmental impact assessment needs to be conducted to assess the changes to the natural ecosystem at the site and water quality. The impact of a sudden lack of sunlight on biodiversity in these water bodies is often raised as a potential problem.

Overall, the maintenance costs could be

influenced by the following:

1. Reduced water consumption and frequency of cleaning.

2. Practically no vegetation control. However, water plants and weeds could pose issues at some sites.

3. Reduced civil maintenance costs. However, the maintenance of the floats, connections and anchoring system are added.

4. Reduced security needs, but increased safety needs.

5. More frequent inspections and periodic maintenance of equipment could be necessary to identify system degradation in terms of corrosion, material fatigue, aging etc.

6. Close monitoring for loose mechanical and electrical connections which can cause DC arcing and electrical fires due to the constant movement of the floating platform.

7. The impact of improper maintenance on the overall availability of the plant could be higher.

Closing Remarks

Renewable energy is no doubt the environment-friendly answer to a heavily coal-dependent world. According to a study by GTM Research, there is a 1.3GW pipeline of floating PV projects globally slated for completion by 2019 [3].

There is an urgent need to capitalize on the experience gained from the existing installations and translate them in the form of standards and guidelines to be followed by the ‘floatovoltaics’ segment. While tendering strategies such as reverse auction will help drive the cost competitiveness of the systems, the risk of quality compromised sub-standard installations could jeopardize the purpose.

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References:

[1] Choi, Y.-K. and N.-H. Lee. Empirical Research on the efficiency of Floating PV systems compared with Overland PV Systems. Conference proceeding of CES-CUBE, 2013.

[2] Trapani, K.; Redón-Santafé, M. (2015). A review of floating photovoltaic installations: 2007-2013. Progress in Photovoltaics. 23(4):524-532. doi:10.1002/pip.2466.

[3] Ben Gallagher (March 2018). GTM Research: Solar Technology Snapshot: Floating PV Systems: Is This A Real Thing?

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