Floating solar photovoltaic (FPV) installations open up new opportunities for scaling up solar generating capacity, especially in countries with high population density and competing uses for available land. They have certain advantages over land-based systems, including utilization of existing electricity transmission infrastructure at hydropower sites, close proximity to demand centers (in the case of water supply reservoirs), and improved energy yield thanks to the cooling effects of water and the decreased presence of dust. The exact magnitude of these performance advantages has yet to be confirmed by larger installations, across multiple geographies, and over time, but in many cases, they may outweigh any increase in capital cost. The possibility of adding FPV capacity to existing hydropower plants is of particular interest, especially in the case of large hydropower sites that can be flexibly operated. The solar capacity can be used to boost the energy yield of such assets and may also help to manage periods of low water availability by allowing the hydropower plant to operate in “peaking” rather than “baseload” mode. And the benefits go both ways: hydropower can smooth variable solar output by operating in a “load-following” mode. Floating solar may therefore be of particular interest where grids are weak, such as in Sub-Saharan Africa and parts of developing Asia .
Hydropower is a well-established technology that has played an important role in the global power system since the beginning of centralized power distribution systems. The oldest (but still operating) hydropower plants have been active since the end of the 19th century. Hydropower plants are operating throughout the planet, with presence in almost every country in the world. There is 701.1 GW of active hydropower plant, reservoir-based capacity installed worldwide and 138.7 GW of hydro pumped storage capacity . The main advantages of hybrid system:
Grid connectivity (with transmission lines, transformers, etc.) already present, make FPV more profitable;
Deploying PV systems on reservoir surfaces can save on the cost of land. The existing road access to the hydropower plant likely reduces construction and transportation costs, as well.
Every liter of water prevented from evaporation will produce additional hydropower energy;
Water resources and solar energy can compensate for each other when operated together as a hybrid. This is true not only over the diurnal cycle (using solar energy during the day and hydropower at night), but also across the seasons.
2. “Virtual battery” concept
Under a “virtual battery” configuration, during high irradiation time, the power generated by the FPV panels would be transmitted to the grid and used directly, while either the reservoir accumulates (when there is an inflow stream) or just holds water that can be then later used during times of low or absent solar irradiation. In this manner, the reservoir itself becomes a battery, where the “charge” is the water spared from being used or accumulated while the direct solar energy is being used. This is of course feasible due to the high flexibility of hydropower plant operation .
3. Global Potential
Based on the study  the FPV is capable of providing significantly more electricity (6270 TWh in total) than hydropower from reservoirs (2510 TWh in total) at a coverage rate of 25%, while providing balance to the FPV intermittent operation (Figure 1).
Figure 1: Potential electricity generated per year from (top) and potential capacity of (bottom) FPV covering 25% of the water surface of
Depending on the location and additional purposes of the reservoir, higher coverage ratios could be considered, thus providing even more capacity (and electricity), and increasing the rate of water conservation.
At the same time, batteries and other alternative energy storage technologies have still a strong role to be played. The main disadvantage of hybrid FPV-hydropower configurations is that they are geographically restricted to specific areas and strongly affected by seasons and weather patterns, and the “virtual battery” functionality is limited to the reservoir’s capacity. Furthermore, the availability does not necessarily match population centre (demand) locations. However, even more renewable electricity could be provided by such regions if hybrid FPV-hydropower plants were applied.
4. Case studies
The development of grid-connected hybrid systems that combine hydropower and floating photovoltaic (PV) technologies is still at an early stage. Only a small system of 220 kilowatt-peak (kWp) has been deployed in Portugal (Figure 2) (Trapani and Santafe 2015). But many projects, and of much greater magnitudes, are being discussed or developed across the world.
Figure 2: First-ever hydropower connected FPV operation, Montalegre, Portugal
The largest hybrid hydro-PV system involves ground mounted solar PV (Figure 3). This is the Longyangxia hydro connected PV power plant in Qinghai, China (Qi 2014), which is striking for its sheer magnitude and may be considered a role model for future hybrid systems, both floating and land-based. The Longyangxia hydropower plant was commissioned in 1989, with four turbines of 320 MW each, or 1,280 MW in total. It serves as the major load peaking and frequency regulation power plant in China’s northwest power grid. The associated Gonghe solar plant is 30 km away from the Longyangxia hydropower plant. Its initial phase was built and commissioned in 2013 with a nameplate capacity of 320 MWp. An additional 530 MWp was completed in 2015.
Figure 3: Satellite image of Longyangxia hybrid hydro/PV power plant (the role-model)
The PV power plant is directly connected through a reserved 330 kilovolt (kV) transmission line to the Longyangxia hydropower substation. The hybrid system is operated so that the energy generation of the hydro and PV components complement each other (Choi and Lee 2013). After the PV plant was added, the grid operator began to issue a higher power dispatch set point during the day. As expected, on a typical day the output from the hydro facility is now reduced, especially from 11 a.m. to 4 p.m., when PV generation is high. The saved energy is then requested by the operator to be used during early morning and late-night hours. Although the daily generation pattern of the hydropower has changed, the daily reservoir water balance could be maintained at the same level as before to also meet the water requirements of other downstream reservoirs. All power generated by the hybrid system is fully absorbed by the grid, without any curtailment. This system shows that hydro turbines can provide adequate response as demand and PV output varies .
Figure 4 compares the total system output and the hydro output before and after hybrid in a relatively dry year (Qi 2014). After the PV plant was added, the grid operator began to issue a higher power dispatch set point during daylight hours.
Figure 4: Power output, Longyangxia hydropower substation
5. Special considerations
Whether the hydropower plant owner/operator is allowed to add an FPV installation;
Whether the hydropower plant owner/operator is allowed to provide a concession to a third party to build, own, and operate an FPV plant;
Management of risks and liabilities related to hydropower plant operation and weather events that can affect the solar or hydropower plants;
Rules of dispatch coordination of the solar and the hydropower plants’ outputs;
Designing insurance policies that include liabilities for potential damage of hydropower plant.