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An introduction to Floating Solar: does it hold water?

Floating solar is an exciting development in the application of photo-voltaic technology. There are several advantages of floating solar over a land based system, but there are also some unique challenges in the implementation and design of floating PV systems. In this article we will give you a broad overview of this growing segment of the solar industry, highlighting the differences in performance, equipment, and design between land based solar plants and floating solar plants.

Floating Solar is also known as floating photo-voltaics, or FPV for short. This refers to any kind of solar pv array that is floating on a body of water. FPV is similar to land based pv in most respects. The key difference is that the panels are mounted on a buoyant structure that is anchored in place on top of a body of water, which calls for specialized equipment, and extra precautions must be taken for safety in the wiring and installation, and the maintenance.

Lets start with some interesting facts about FPV.

In 2014 the total installed capacity of FPV installations was only 11MW, but by the end of 2018 that figure had reached 1.3 GWp. That’s a healthy 1300% increase in a four year span. Below is a graph showing the new installations added per year and the cumulative installed capacity as well.

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ESMAP, https://www.esmap.org/where_sun_meets_water_handbook

The steep climb shows that the early installations have proven successful, and that they have provided valuable case studies and data which makes planning and implementing FPV easier.

Most of the global FPV installations are in China, where around 73% of the global installed capacity is located. The other installed capacity comes mostly from Japan, Korea, Taiwan, United Kingdom and Northern Europe with a small percentage elsewhere. Most of these installations are above 15MW in size.

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ESMAP, https://www.esmap.org/where_sun_meets_water_handbook

FPV has proven that it is a robust and economically viable application of PV generation. As more successful installations are seen and as prices fall and experience climbs, FPV is expected to continue rapid growth.

Advantages of FPV

Part of the reason that FPV is proving to be so successful is because there are several factors that contribute to better performance than equivalent land based PV systems. And when conditions can be optimized by proper site selection and thorough planning and design, the benefits are significant.

Floating solar systems are much easier to design and install where the water is calm. This means inland reservoirs, lakes and ponds are more attractive sites for FPV than on open ocean or rough water. Privately owned reservoirs and hydro dams are a particularly good sites for FPV.

Compared to land based PV systems, reservoirs and dams are attractive for FPV because the surface area of the water is generally unused space with no competing interests. Often there are no ‘land’ costs as there would be with land based pv systems. In privately owned reservoirs and dams, often the permits and other considerations for land based pv can be avoided or minimized.

And there is often no opportunity cost for using the space; with land based pv there is the consideration that the land can no longer be used for other developments, but the surface of reservoirs usually presents fewer options for other uses.

This allows land owners to make use of an area that otherwise would not be used.

Developing countries with a high population density are looking at large scale FPV in order to use their scarce land resources to generate solar power.

Additionally, there are certain challenges that land based pv systems face that can be avoided for floating pv: there is no land clearing required as needed in many large scale land based pv projects. There is no support foundation or cable trench excavation, and no road construction required within the plant.

And when comparing the power generated versus the space required, in regards to hydro power and the size of many inland reservoirs, the space occupied by the FPV system compared to the power generated is very low.

FPV allows for a greater panel density than on an equivalent landbased pv. This means the total footprint of the floating pv is slightly smaller than what it would be on land. Existing data has shown that floating pv requires about one hectare per MW for the panels, and a total area of about 1.7 hectares per MW when the anchoring and safety zone is accounted for. Compared to around 1.8 hectares for 1MW of land based pv.

Synergies with Hydropower

In an analysis comparing the reservoirs of hydro power dams around the world, in most hydro power the generating capacity of the power plant could be DOUBLED by installing FPV on 3-4% of the surface of the reservoir. Here is a table showing some examples of this:

One of the most intriguing aspects of FPV is the way that the solar PV can benefit from the hydro power generation and vice versa. A major limitation of solar power generation is that it is reliant upon the sun, so the power generation is limited to daytime, and subject to certain weather conditions and moment to moment changes in cloud cover.

The hydro dam can produce power with greater stability and greater control by the dam operators.

Combining floating solar PV power generation with existing hydro power sources means you can rely on the solar generation daytime and you can strategically generate the hydro power to stabilize the power supply to the grid, and also for nighttime generation. The water in the dam reservoir can be used more strategically with the additional generation of the FPV during the daytime. Combining the output of the hydro dam and the solar FPV can smooth the variability of the solar output. This combination presents a great opportunity to make better use of existing transmission assets. The connection costs of a floating PV system can be very low when connecting with existing transmission lines, which are often a large part of the costs of new power plants.

The floating solar will also provide shade for the reservoir which will reduce the rate of evaporation. This means more of the water will be available for the intended use, thereby allowing the water in the reservoir to be used more effectively, increasing the efficiency of the reservoir.

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ESMAP, https://www.esmap.org/where_sun_meets_water_handbook

There is also the possibility of using the floating pv as a way to pump water that has been released through the hydro dam back into the reservoir to be used again. This is a new application of floating solar that is still in its infancy.

Performance

One thing that is quite clear is that floating solar can generate solar power more efficiently than equivalent land based solar.

One reason for the improved performance of FPV systems is that the performance of solar panels generally declines as panel temperature increases. Solar panels perform more efficiently at moderate temperatures, and perform less efficiently when they become very hot. The bodies of water beneath the floating pv panels keeps the ambient temperature low, thereby cooling the panels and allowing for better performance.

Reservoirs generally have wide open spaces which allows more wind than in some land based pv sites, further contributing to the cooling effect on the panels.

This cooling effect means the panels of floating PV perform better than the same system on land which will generally be a few degrees warmer.

The wide open spaces also mean there are generally fewer obstructions and shadows on the panels, and long hours of sunlight, which increases the overall production and decreases the string mismatch on the pv array over time. The solar irradiation is more uniform and the illumination time can be longer, resulting in improved performance.

The wide open space of the reservoir also means there is often less soiling than with an equivalent land based pv system. Floating on a reservoir far from the dust and dirt, the floating modules tend to stay cleaner than solar modules on land which are subject to gusts of wind blowing dirt and sand onto their surfaces. When you consider the performance loss due to soiling of land based pv systems, and the maintenance costs of continuous cleaning of the land based panels, the FPV has a significant advantage in this area.

Floating PV tends to have less dust soiling but more bird droppings. Some birds have been known to make nests on the floating structures, and the increased bird droppings can cause hot spots on the panels and reduce the performance more frequently than most land based pv. However the increased bird droppings is not comparable to the increased dust cover on land.

Studies have shown that all of these factors mean that FPV can perform between 10 to 15% more efficiently than an equivalent land based pv system.

Costs

Though FPV is similar to land based PV in most respects, there are certain items that represent an additional cost over land based PV.

The installation costs of FPV are usually higher than in a land based PV system, partially because this is a relatively new application of solar pv that may require some specialized equipment and more niche installation knowledge. Boats are required to pull the floating arrays into position on the reservoir, specialized training is required in some cases, there are anchoring considerations and additional analysis of the shore and waterbed required and other considerations that we will look at later in this article.

But just as we have seen with traditional solar systems, the costs of installing FPV systems are expected to continue to fall as the technology progresses.

BoQ

Here is a diagram of a typical floating solar system:

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Author’s compilation and ESMAP, https://www.esmap.org/where_sun_meets_water_handbook

As you can see from the diagram above, most of the FPV system is the same as a land based system. There are the PV panels, the DC connection to the inverters, the AC combiner and connection to a transformer and then to the grid or transmission lines.

The main equipment in and FPV system that is different from land based PV is

1.       The floating pontoon structures,

2.      The anchors and mooring lines for the floating arrays,

3.      And the extra protection that must be afforded to the cables to protect them from water and erosion over time. Notice the snaking cable design which allows for slack in case of movement of the floating structures and variations in the water levels.

Standard PV modules can be used for FPV, although care should be taken to ensure that the movement of the water does not put stress on the panels. The panels will experience higher humidity than the equivalent land based PV site, and there is more potential corrosion from the water.

The inverters can be placed on land or floating near the array. Putting the inverters close to the panels is a good way to reduce cable losses and is generally recommended. This means the inverters should be sealed to IP-66 rating, which means they are water resistant against high pressure water jets, and they will be safe in a floating application. The inverters should also have multiple MPPTs to reduce string mismatch and improve performance. They should also have Type II surge arrestors on the AC and the DC side, and proper ground fault protection for improved safety. And there is a greater likelihood of Potential Induced Degradation (PID) affecting the panels. PID is a phenomenon that will cause significant losses in the generating capacity of your panels. The effects of PID are increased in hot and humid environments, and the floating panels will be exposed to more humidity than an equivalent land based pv system. In FPV systems it is more critical to have an ant-PID safety feature in your equipment. This can be a feature built in to some inverters, or a third party solution as an extra piece of equipment.

Pontoons

The buoyant structures that support the pv panels and the inverters come in a variety of designs and types. This is a fast growing sector of the solar manufacturing industry.

Generally, floats are made of high-density polyethylene. The mounting structures can be made of aluminum of stainless steel. The structures should be galvanized to increase corrosion resistance. This is important if the water has high levels of salinity.

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Author’s compilation and ESMAP, https://www.esmap.org/where_sun_meets_water_handbook

Anchors and mooring lines

The floating platforms must be held in place by an anchoring and mooring system. This prevents the floating platforms from drifting with the wind and the water currents and potentially damaging the equipment.

Wind can cause the entire floating array to drift and exert large forces on the anchoring structures. Therefore, wind load calculations are an important part of the anchoring and mooring systems.

The anchors are actually the most important feature of the design of an FPV system. There are significant implications for the project costs of the anchoring system based on the environmental constraints and considerations.

The design of the anchors and mooring system varies from site to site, and depends on such factors as the wind speed and direction, the type of floating pontoon structure, the tilt and mounting structure of the panels, the water depth, the composition of the waterbed, the variability of the water level of the reservoir and other factors.

Most FPV systems are anchored to the waterbed, while some FPV systems are anchored to the shore. Common types of anchors when anchoring to the waterbed are dead weights such as concrete blocks. Helical anchors can sometimes be installed and deployed by screwing into the floor of the reservoir, similar to large hangar bolts used in land based pv systems. To deploy these anchors in an FPV application requires specialized barges and trained divers.

Waterbed anchors can be direct to the floating pontoons, or sometimes used with floating buoys.

The mooring lines that go from the anchor to the floating structure are usually made of galvanized steel wires, chains, wire rope, synthetic fiber rope or a combination of these materials. They often have a spring to allow for variations in water level.

Here are some examples of different anchoring and mooring structures. The first image is a system anchored to the waterbed with a dead weight anchor and moored directly to the floating structure. The second image is a system anchored to the waterbed in a similar fashion, but the mooring system makes use of floating buoys before attaching to the floating pv structure. The third image shows an FPV system that has been anchored to the shore of the reservoir. And the fourth shows how spring boxes can be placed on the mooring lines to adjust to different water levels while holding the floating structures securely.

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Cables and accessories

Cable lengths and cable routes must be planned in advance and calculated with care. There must be enough slack to account for the movement of the floating structures due to the tides, wind, waves, and water currents, and also enough slack to account for variations in the water levels.

Cables can float on the water or they can be under the water, but floating on the surface is much cheaper and generally recommended. The floating cables can obstruct boats, so this must be considered in the design stage.

For grounding and lightning protection systems, grounding to the waterbed is widely practiced. All of the non-current-carrying exposed metal (such as module frames and mounting structures) must be properly grounded.

The water composition must be known so that the proper non corrosive insulators and protection can be used on the cables.

Feasibility and Prefeasibility

In light of all of the design and equipment considerations, we can say that for a floating PV system, the pre-feasibility study and feasibility study have added importance to the success of the FPV system. FPV designs have more variation than their land based counterparts. There are more factors to consider in the feasibility study for a floating PV system than an equivalent land based pv system, and in an FPV system these variables have a greater impact on the equipment and the lifetime performance of the solar system.

The main considerations for assessing the suitability for an FPC installation include many of the same factors in assessing a land based PV, and a few unique factors to include. Here is an approximate list of what should be considered in a prefeasibility or feasibility study for floating PV:

  • Solar resource
  • Local climate conditions
  • Available water surface area and shape
  • Bathymetry (depth and topography of the water bed)
  • Water level, wave amplitudes, and wind speeds
  • Subsurface soil conditions
  • Shading, soiling, and other site conditions
  • Environmental considerations
  • Grid access, substation location, and power availability
  • Access rights, permits, and regulations.

Author’s compilation and ESMAP, https://www.esmap.org/where_sun_meets_water_handbook

As mentioned earlier wind is an important consideration and will help determine the types of floats, anchors and mooring systems to use. Attention should be paid to prevailing direction, average speed as well as speed of gusts. Also the likely hood of severe storms should be considered. Floating structures have shown great resilience to high wind speed storms as long as this is planned for from the beginning.

Water level over time, and seasonal changes in water level is very important. As well as the composition of the waterbed and the shore, and the topography of the water bed and shore, as these will help determine the anchors mooring systems to use. In general FPV is best suited for sites where the depth is 15 meters or less and the water variation is minimal.

The wind and water level and other factors will also contribute to the size and frequencies of waves which must be accounted for.

A water analysis should be done to know the chemical composition of the water and the likelihood of corrosion and extra wear on the FPV equipment.

The slope and strength of the shore is important for planning where the floating pontoons can be assembled and launched into the water. Sometimes special assembly floats are needed when the shore is not attractive for this purpose. Proximity to a main road for equipment delivery is also good to identify and plan.

And there should also be special attention paid to the aquatic life that may be affected by an FPV system. According to an ESMAP report, generally a floating pv structure has a neutral or positive impact on the natural environment as the shade from the floating structures can cool the water and prevent overgrowth of algae, and when done correctly there are generally few adverse affects on the environment.

Many GIS specialist companies are turning more attention to floating solar installations and can use satellite imagery and geospatial data to greatly assist with the prefeasibility studies.

Conclusion

In conclusion, FPV is a viable and fast growing application of Solar pv technology. The floating aspect affords several benefits to the performance of the solar equipment. Prefeasibility studies are more important and have more variables to consider when planning and designing the systems.

There are additional costs for FPV in the equipment and the installation when compared to an equivalent land based PV system. However when conditions are optimized the FPV presents very attractive ROI,

Some optimal conditions include if the proposed FPV system can take advantage of existing transmission assets—possibly in partnership with existing hydro power stations, land cost and permitting are can be reduced greatly with private ownership of the body of water, and if there is convenient access and ample launching space for the floating equipment.

In these cases FPV is a great option, and although this sector of the solar industry is still in its infancy, you can be sure to see steep increases in the growth of FPV in the coming years.