At a South Korean flood control dam, 47 MW of floating panels trace the national flag, and the array hands the grid to turbines below every sunset
The water at Imha Dam does not move the way reservoir water usually does.
It rises with the spring melt, drops through the dry season, and swings by several feet between the two, pulling against every anchor line fixed to the lakebed.
Into that restless surface, workers lowered roughly 100,000 solar panels onto pontoon floats, covering the reservoir in a shape most visitors recognize instantly.
The floating array sends solar power to the grid during daylight hours, then the complex switches to hydropower generation overnight to maintain steady output.
The finished installation traces the outline of the South Korean flag and the Mugunghwa, the country’s national flower, visible in full only from above.
What makes the combination so logical here, and what happens at sunset every day, is the real story.
How a flood control reservoir becomes a power station after dark
Imha Dam was built to do three jobs at once: hold back floodwater on the Banbyeoncheon River, supply water to a chain of industrial cities downstream, and spin turbines. That 50 MW hydropower plant sits at the base of the dam wall, drawing from the same water the reservoir holds through every season.
The combination the engineers designed around it is straightforward in concept but rare in practice. The floating array sends solar power to the grid during daylight hours, then the complex switches to hydropower generation overnight to maintain steady output. The result is a single piece of infrastructure that never goes dark, trading one source for the other as the sun drops.
That handoff matters because floating arrays, like all solar, generate nothing after dusk. Pairing them with a dispatchable water turbine turns an intermittent source into something closer to a baseload asset, at least for the hours when reservoir levels permit.
What the reservoir surface does to the hardware
A floating array on a multipurpose dam faces conditions that a field installation never encounters. Systems must account for extreme wind gusts and significant water level variations, sometimes exceeding 98 feet in hydroelectric dams, using elastic mooring lines or heavy gravity anchors.
At Imha, the panels sit on pontoons tethered to the lakebed, and every anchor must accommodate the seasonal swing of the surface without pulling a module under or snapping a cable. Fresh water is gentler than coastal saltwater, but the still, sheltered surface encourages algae growth on pontoons, and windblown dust accumulates on panels that no rain angle can fully rinse. Maintenance crews must reach every row by boat, making each cleaning visit far costlier than an equivalent pass on a ground mounted field.
Panel temperature, by contrast, runs lower than on land. The water surface beneath the floats cools the underside of the modules, and evaporation draws heat away from the pontoon deck, producing a measurable yield gain that partly offsets the access premium.
The numbers recorded when the switch flipped
The 47.2 MW Imha Dam floating solar plant is the largest floating PV facility alongside a multipurpose dam in South Korea. Construction began in July 2024, meaning the project moved from groundbreak to energisation in roughly 16 months, a compressed schedule for a floating array of this scale.
A completion ceremony was held in late September, with total project costs coming in at roughly $50 million. That figure covers the pontoon system, the panel supply, the mooring engineering and the grid connection, which ran through the dam’s existing switchyard rather than requiring new transmission infrastructure.
The combined hybrid system is expected to generate 61 GWh annually, enough to supply power to tens of thousands of households. Roughly 4,500 locals within a half mile radius of the plant are entitled to share in the profit from its power supply, a revenue sharing arrangement written into the original approvals.
The catch the water level swing creates for operators
The seasonal drawdown is the engineering problem that never fully goes away on a reservoir array. When the dam releases water during a dry period, the surface can fall faster than a gradual tide, and mooring lines tensioned for one water depth find themselves slack, or suddenly taut to the point of stress.
Operators must monitor water level forecasts and adjust anchor tension continuously. During heavy rainfall and flood control releases, boats cannot safely reach the array, so inspections and panel cleaning must wait. Biofouling on the pontoons, the slow build up of algae on submerged surfaces, can degrade buoyancy over time and requires periodic cleaning that is difficult to schedule around the dam’s operational demands.
Sang-jo Yoon, who led the green energy division overseeing the project, said the team expects the installation “to play a crucial role in achieving Korea’s carbon neutrality,” signalling that the government views the Imha model as a template rather than a one off experiment.
What Imha opens for the next generation of reservoir sites
Two national utilities have already signed a shareholders’ agreement to co-develop a second phase floating solar project at Hapcheon Dam in South Gyeongsang province. The Imha experience, and particularly its day to night handoff with the hydropower unit, is the operational model underpinning those expansion plans.
And the appetite for this combination is not limited to South Korea. Research published earlier this year found that Lake Powell could host almost 15 GW of floating solar using about 23 percent of its surface area, and Lake Mead could generate over 17 GW on 28 percent of its surface. Those figures are theoretical maximums, but they point to a global inventory of dam reservoirs that already have the grid connections and operational staff that new sites must build from scratch.
What Imha proved is narrower and more useful than any headline figure: that a multipurpose flood control dam can carry a floating solar field, run a clean day to night handover with the turbines below, share revenue with the nearest village, and complete the whole build in under a year and a half. Similar logic is already at work on Arkansas irrigation reservoirs and on southern hemisphere grids searching for the same pairing of existing infrastructure and available water surface. The flag shape on the Imha reservoir may be visible only from altitude, but the engineering argument it represents is becoming hard to miss from any angle.
Hugo is an engineer with strong technical expertise. Multilingual from an early age, his writing combines technical clarity with a strong interest in science and energy.
