Solar Hydropower Integration. Solar energy and hydropower are the two oldest and most abundant renewable energy sources on earth — yet their relationship is more complex than most energy commentary acknowledges. As solar installations accelerate globally, reaching costs that make it the cheapest electricity source in history, hydropower’s role in national grids is being fundamentally reshaped. In some markets solar is threatening the financial viability of planned hydel projects. In others it is making existing hydel infrastructure more valuable than ever. Understanding the engineering reality of solar and hydropower grid integration — how each affects the other, where they conflict and where they complement — is one of the most important questions in global energy development right now.