1. Development and Advantages of Photovoltaic Power Station on Water

1.1 Development situation

On-water photovoltaic power stations refer to photovoltaic power stations built on water in ponds, reservoirs, lakes, etc. According to the water depth of the project, there are two types of construction forms: generally, the water depth is less than 3m, and it is installed with a piling frame in a high way (see Figure 1); floating installation system can be adopted for water depth over 3m (see Figure 2).
 

Fig. 1 Pile-supported high photovoltaic power station on water
 

Fig. 2 Floating photovoltaic power station on water
 
The pile-driving type is the main type of photovoltaic power station on the water in China. With the continuous maturity of floating technology on water, new materials, new technologies and new processes are constantly emerging, and the construction cost is constantly decreasing. In recent years, floating photovoltaic power generation projects have become a hit in the field of photovoltaic power generation, and the installed capacity has shown rapid growth.
 
1.2 Advantages of floating photovoltaic power station

① Water photovoltaic power station does not occupy scarce land resources such as cultivated land, forest land and general agricultural land, and increases the added value of water area.

② The water surface of ponds, lakes or coal mining subsidence areas generally has a wide surrounding environment and consistent component orientation, which is convenient for centralized layout and management. There is relatively little dust on the surface of the water area, and it is convenient to clean, which is beneficial to operation and maintenance and power generation.

③ The power generation can be increased by about 7% to 12%: the radiation on the inclined plane of a photovoltaic module is composed of direct solar radiation, scattered radiation and reflected radiation, while the reflectivity of the water surface is about 0.6%, which is much higher than that of the ground, grassland and other environments. After testing and evaluation, the total radiation on the inclined plane of the photovoltaic module is about 1.5% higher than that on the ground.

④ The temperature change in the water area is relatively small, and the cooling effect of the water body in summer can restrain the rise of the surface temperature of photovoltaic modules and increase the power generation.

⑤ The eastern region of China is rich in water resources, with a large demand for electricity and a strong power grid structure, which can generally be connected to the nearest grid for consumption.

⑥ Photovoltaic power stations on the water can effectively cover the water surface, reducing evaporation of the water body, inhibiting algae growth, and helping to control water pollution and improving water quality.
 
2. Technical characteristics of floating photovoltaic power station

2.1 Main technical features of pile-driving frame photovoltaic power station
 
Floating piled-up photovoltaic power plants are generally built-in waters with water depths less than 3m. The foundation adopts the combination of a PHC pipe pile and a hot-dip galvanized steel bracket. The height of the pile top is more than 0.4m higher than the flood water level. In order to facilitate the passage of ships, the lower end of the photovoltaic module is more than 1 meter away from the highest water level, and the module is installed at the best inclination angle. Generally, pile driving is carried out after drainage, dredging and air drying, or pile driving are carried out by ship directly. The main electrical equipment is arranged on both sides of the road or the shore, and the booster station is located on the shore. A cable tray is used, and the tray is fixed on the pipe pile foundation (see Figure 3).
 

Fig. 3 Electrical equipment is arranged on both sides of the road.
 
This kind of power station mostly adopts the construction mode of "complementary fishing and light". That is, the abundant pond water resources in aquaculture concentrated areas are used to develop and construct photovoltaic power generation projects, and the modes of power generation on water and aquaculture underwater are adopted to realize the complementary development of multiple industries. A photovoltaic power station on the water is more suitable for the cultivation of special fish that don't like light. In addition, photovoltaic power generation can directly provide electricity for cultivation, which reduces the cost.
 
2.2 Main technical characteristics of floating photovoltaic power station 

A floating photovoltaic power station means that photovoltaic modules, inverters and other power generation equipment float on the water surface to generate electricity by means of floating bodies and floating platforms. It is suitable for waters with water depth greater than 3m and stable water bodies not affected by typhoons. The floating foundation mainly has three structural types: floating pipe+bracket, buoy+bracket and integrated buoy. Considering the influence of big wind and waves, it is advisable that the assembly installation inclination should not exceed 20°, and the mainstream design and installation inclination in the industry is generally 10° to 18°. Therefore, the floating photovoltaic power station is more suitable to be built in areas with small latitudes, and the radiation loss of the inclined plane is not obvious compared with the optimal inclination angle. The floating body is fixed by the anchoring system, and a reasonable fixing method is designed according to the offshore distance, water depth, etc..  When it is close to the shore, fix the floating body on the shore with ropes or braces (see Figure 4). When it is far from the shore and the water depth is large, it can be fixed by a concrete anchor block plus tension spring (see Figure 5). The anchoring system should be able to adapt to the change of water level, and it is generally designed to be adjustable within 5m above and below the water level.


Fig. 4 Brace anchorage
 

Fig. 5 Concrete anchor block+tension spring
 
Cable and cable bridges are laid along the pontoon, and sufficient cable reserve is considered according to the change of water level. Box-type inverters and transformers can be placed on the concrete floating platform, and the power distribution room in the switch station can also be placed on the concrete floating platform as required to make a fully floating photovoltaic power station. There are two construction methods of floating photovoltaic power stations: splicing pontoons on the shore and installing equipment on the water; operating the platform on the shore, and entering the water after the components are installed. Through practical experience, the latter one is more convenient and faster.
 
3. Selection of main equipment and materials

3.1 Pile footing

The type of pile footing should be selected according to the engineering properties, geological conditions of reservoir water level, construction conditions, influence of construction on reservoir environment and comprehensive economic benefits. The type of pipe pile commonly used in the industry is PHC-300 (70) A prestressed concrete pipe pile. The calculation of pipe pile is divided into the calculation of pile strength, stiffness, vertical bearing capacity and horizontal bearing capacity, and the vertical compressive static load and horizontal static load test of a single pile should be carried out on site. After pile driving, the pile head should be de-rusted and anticorrosive.
 
3.2 Floating foundation

The floating material must have the characteristics of low density, corrosion resistance, environmental stress cracking resistance (wind and waves, frost heaving environment), water vapor permeability resistance, ultraviolet resistance without pollution to the water environment. At present, the commonly used raw materials are HDPE, that is, high-density polyethylene.
 
The material can meet the above basic characteristics. There are three types of floating foundations: floating pipe+bracket, buoy+bracket and integrated buoy (see Figures 6, 7 and 8), and their advantages and disadvantages are compared.
 

Fig. 6 Floating pipe+bracket
 

Fig. 7 Buoy+Support
 

Fig. 8 Integrated buoy
 
Comparison of advantages and disadvantages of three types of floating foundation
 
Floating pipe+bracket
Advantages: The floating pipe is convenient to transport and install with a firm structure. The components can be installed at the best inclination angle.
Disadvantages: The support base is placed underwater, which is not good for anticorrosion. The overall aesthetics is insufficient, and the east-west maintenance channel is not convenient to set.
 
Buoy+support
Advantages: The buoy is uniformly stressed, and the components can be installed at the best inclination angle.
Disadvantages: When the buoy is connected, it has poor wind and wave capacity. The steel consumption can be larger.
 
Integrated buoy
Advantages: less steel consumption, convenient installation and maintenance, and good overall aesthetics
Disadvantages: The installation inclination of the assembly is limited by the floating body.
 
To sum up, the structural form of integrated buoy gains more and more recognition because of its advantages and low cost.
 
3.3 Photovoltaic module

Under the water environment, higher requirements are put forward for the performance of photovoltaic modules. The main risk factors and countermeasures of modules are as follows:

① In a humid environment, modules are prone to generate PID effect——choose double-glass modules with strong anti-PID effect.

② The electrical insulation performance of components in humid environments declines —— more rigorous packaging materials are used, and the components pass the double 85 test.

③ The shaking of the big wind and waves is easy to cause the cracked cell——use monocrystalline silicon with better mechanical strength, and use reinforced glass as the backplate material. In order to reduce the risk of components caused by the environment as much as possible, it is recommended to select double-glass single-crystal photovoltaic modules with good PID resistance, low water permeability and better acid-alkali aging resistance.
 
3.4 Cable

Cable laying of the photovoltaic power station on the water can be done by bridge frame or along the top surface of the floating body. Although it is not directly immersed in water, it is difficult for conventional cables to avoid water immersion in a high humidity environment and turbulent water surfaces. After the cable enters the water, under the action of the electric field, the water tree phenomenon will occur. The generation of water trees will increase the loss of insulation medium, reducing the insulation resistance and insulation breakdown voltage at the same time, accelerating the aging speed, and even leading to the breakdown of cable insulation and shortening the service life of cables. Waterproof cables are recommended for the cables of photovoltaic power stations on water. The waterproof cables are longitudinally wrapped with an aluminum-plastic composite tape layer as a waterproof layer inside the outer sheath or inner lining layer to realize the waterproof function.
 
3.5 Junction box, inverter booster equipment

In the high humidity environment, the electrical equipment is most prone to the phenomena such as box corrosion, connector corrosion, PCB corrosion. The electrical equipment selected for a photovoltaic power station on water must have good corrosion resistance, salt fog resistance, and insulation and pressure resistance. The protection level of the junction box or cascade inverter must be IP65 or above. Because the junction box or cascade inverter bracket is installed and fixed on the floating body, the floating power station is easy to contact with water vapor, so IP67 is recommended. The booster transformer has an IP65 protection level, and the centralized inverter has an exhaust system, but the protection level of the whole transformer cannot be lower than IP54, so the protection level of key components in the cabinet is improved. The inverter should have a reliable anti-PID function. Water photovoltaic power stations have high requirements for the performance of each piece of electrical equipment. Many equipment manufacturers have developed special equipment or components for the special operating environment of water photovoltaic projects, and pay attention to selecting special equipment for water photovoltaic power stations.
 
4. Problems that need to be improved in floating photovoltaic power stations
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At present, the design life of photovoltaic power stations is 25 years or even longer. As an important supporting platform of the surface photovoltaic power station, the floating body system is an important link related to the normal operation of the whole photovoltaic power station. The adaptability of floating materials to extreme weather and great changes in water level needs further study. The commonly used HDPE floating materials are flammable, whose UV resistance and toughness need to be improved. Also, their service life needs to be verified.
 
The initial investment cost is relatively high, compared with the ground power station. The investment cost of the piled-up photovoltaic power station is about 4% higher per watt, and that of the floating photovoltaic power station is about 12% higher.
 
Practical problems faced by complementary fishing and light projects: inconvenient fishing, inconvenient feeding management. Generally, fishing ponds need regular desilting and disinfection, but it is difficult to desilting after piling. Besides, there are not many projects that truly integrate farming with power generation. The influence of the construction of the hydropower station on the ecological environment of the pond and fishery needs to be further considered. At the same time, the operation and maintenance are difficult and costly. The operation and maintenance of photovoltaic power stations on water mainly depend on manual patrol inspection, which is difficult, inefficient and risky for personnel safety. Intelligent operation and maintenance and fast & accurate fault handling ability need to be improved. At present, there are no targeted and mature standards and specifications for the design, construction and acceptance of water photovoltaic power plants at home and abroad, and the construction of the project mainly depends on its own subjective experience. The standardization process needs to be perfected.
 
5. Conclusion
China is rich in water resources, and there are many lakes and reservoirs. The development of floating PV power plants can relieve the constraints of land factors and broaden the application of photovoltaic power generation. Water photovoltaic power plants have obvious advantages, but at the same time, there are many problems that need to be solved and improved, especially in the stage of exploration and application. There is still a certain distance from large-scale development. The long-term reliability of components in a humid environment, bearing capacity and service life of floating platform needs to be further verified. However, with the development of industry and the continuous innovation and progress of new materials, new technologies and new processes, the problems encountered in the exploration stage will inevitably be reasonably solved. We believe that in the future, photovoltaic power plants on the water will get more and more development and application.