Hybrid hydropower-connected floating solar PV plants: impact of the downstream water release constraint

Solar and wind power continue to dominate the renewable energy expansion, jointly accounting for more than 90% of the new capacity installed in 2019. Hydropower, however, still accounts for 47% of the 2,537 GW of global renewable power in operation. Solar power continued to lead the yearly expansion, for the fourth year in a row, with an annual increase of +20% while hydropower capacity increased by +1%. However, the inherent intermittency and stochastic nature of solar PV is a well-known obstacle to the further large-scale integration of the technology in existing power systems. Large-scale reservoir hydropower offers a cost-competitive, mature and dispatchable alternative that can provide both production flexibility and storage. Nonetheless, the costs of large hydropower are highly site-specific and new capacity development has been more and more constrained by substantial environmental and social impacts in many places worldwide. Solar power and hydropower resources have been identified to be quite complementary and hybrid plants could have many flexibility benefits in addition to the increase of renewable energy production. In this context, floating solar PV (FPV) on hydropower reservoirs is emerging as a relevant solution to accommodate both energy sources at the same location.

Adding FPV to an existing hydropower plant, aiming at hybridizing the output, might impact its reservoir operations and water-related constraints need to be carefully considered. Solar PV can contribute to saving water on mid- to long-term scheduling considering that solar energy generation corresponds in some extent to non-turbined water, i.e. saved energy. Besides, on the short-term time scale, one of the main benefits is that hydropower could, in some extent, compensate for the variability of PV generation by its rapidly adjustable output. In practice, a utility-scale solar PV plant could lose several MW of generation in seconds, if a large cloud passes, for example. To avoid consequences on the power grid, this energy loss would need to be translated almost immediately (according to available capacity and ramp rates capabilities) to hydropower generation, meaning substantial (and potentially more frequent) surges in released water downstream.

The presentation investigates these opportunities and challenges linked to reservoir operations of hybrid hydropower-connected floating solar PV plants and provide inputs on optimal solutions.