Imagine a serene Alpine lake, ringed by forested slopes, its surface dotted with floating solar panels that quietly convert sunlight into electricity. Now imagine that same lake storing water for one of the most responsive energy systems we have: pumped-storage hydropower. The result is more than a picturesque thought experiment — it’s a model for how to stabilise renewable energy while protecting water ecosystems.
That’s the vision behind a new study from researchers at the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) [47.4°N, 8.4°E] and the University of Bologna [44.5°N, 11.3°E], who modelled what would happen if Switzerland’s historic Etzelwerk hydropower plant were “hybridised” with a floating solar installation.
The results are both technically promising and ecologically sensitive — and they offer a roadmap for regions like Norway, British Columbia, and even Scotland, where mountain reservoirs already exist, and energy needs are shifting toward clean, reliable sources.
The Power of Hybrid Thinking
Pumped-storage hydropower is the quiet backbone of many energy systems. When excess electricity is available, water is pumped uphill into a reservoir. When electricity is needed, it’s released downhill through turbines. It’s one of the few large-scale ways to store energy.
But like any storage system, it has limits: it depends on water availability, requires large volumes, and has seasonal constraints. Enter floating solar.
Floating photovoltaic (FPV) panels, placed on a reservoir surface, don’t compete for land, benefit from cooling by the water (which improves efficiency), and — as this study found — can produce up to 20% more annual energy when paired with a hydropower plant.
A Lake, a Grid, and a Railway
The case study focuses on Lake Sihl, a reservoir in the Swiss Alps built in the 1930s to regulate the River Sihl and power the Etzelwerk plant, which supplies electricity to the Swiss Federal Railways. The plant can generate up to 120 MW using Pelton turbines — and store excess grid electricity by pumping water from Lake Zurich back up to Lake Sihl, 480 metres above.
The researchers imagined adding a floating solar array covering 10% of the lake’s surface — a conservative estimate that avoids disrupting aquatic ecosystems. Their simulations, spanning 38 years of data at hourly resolution, showed that:
- Annual energy production increased by 20%
- Reliability improved, reducing shortfalls in supply by nearly 70%
- Less energy was needed from external sources, especially during pumping
- And critically, the system could release up to 50% more water downstream in summer, helping sustain rivers at risk of drying out in hot months
Beyond Switzerland
Switzerland’s pre-Alpine terrain is especially suited to this kind of hybridisation — but so are many parts of the world. Norway has abundant high-altitude reservoirs and is already exploring battery alternatives. British Columbia has historic dams in mountainous terrain, and Eastern Canada is building more variable renewables into its grid.
What’s important is the combination:
- A height difference for water storage (hydropower)
- A calm water surface suitable for solar floating panels
- And energy demand patterns that require flexibility — such as charging EVs, electrifying railways, or balancing wind farms
In these environments, floating solar becomes not just a power source but a complement: letting water stay in the reservoir when the sun is shining, and only releasing it when needed. This protects ecosystems, saves water, and keeps electricity flowing smoothly.
The Environmental Upside
One of the most compelling findings wasn’t about energy at all — it was about ecological flow. In the hybrid model, solar power lets the system spare water that would otherwise be used for electricity. That “saved” water can be released strategically into downstream rivers, helping maintain minimum streamflows and reduce heat stress in aquatic habitats.
This is particularly relevant in summer, when rivers can become dangerously low and warm, threatening biodiversity. With climate extremes rising, this ability to “power and pour” — generate electricity while keeping rivers alive — could make hybrid plants indispensable.
A Model for the Mountains
While solar panels on lakes might sound futuristic, they’re already being deployed around the world — from the Netherlands to Thailand. What this study adds is a rigorous model of how such systems can work in mountainous terrain, with real constraints on water use, flood protection, and biodiversity.
The key lesson? Clean energy is not just about generation — it’s about integration. By combining old hydropower with new solar, countries can make the most of existing infrastructure while responding to modern challenges.
And for mountain regions across Northern Europe and Canada, the message is clear: your lakes, your legacy dams, your high places — they might be the perfect platforms for the next energy transition.
Source
Hybridization of an Alpine pumped-storage hydropower plant with floating solar photovoltaics: a study from the water resource perspective, Renewable Energy, 2025-05-19
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