As policymakers in the Mekong region try to meet both rapidly rising electricity demand and transition away from fossil fuels, hydropower remains central to the region’s power mix. There are 190 existing hydropower dams across the Mekong river basin in 2024, with an additional 19 projects under construction and 287 proposed dams. Though renewable, dams can significantly and negatively impact ecosystems and the natural environment, particularly when they have cumulative impacts.
Over 25% of projects under development or proposed will be in one sub-basin system: the Sekong, Srepok, and Sesan (3S), a transboundary system across Cambodia, Laos, and Vietnam (CLV), which remains crucial for fisheries and food security. However, hydropower is not the only energy option, as the 3S river basin has viable alternatives such as floating solar, wind, and pumped storage hydropower. In 2021, the IFC published a comprehensive cumulative impact assessment study for the Sekong Basin, which specifically recommended moving past a project-by-project approach to power generation buildout and instead coordinating development across all energy types—hydropower, wind, solar, and thermal energy—to strategically build out a diverse and resilient energy system for the Sekong River Basin and minimize unnecessary social and environmental impacts. Its recommendations are even more relevant today given new projects proposed and broader shifts in the regional energy plans and market. CLV governments should adopt a system-scale planning approach that incorporates these system-scale power principles and avoids badly sited dams to build a diverse, resilient, and secure energy mix that minimizes environmental and food security impacts.
While it’s unlikely all the proposed dams in the Mekong system will be built, there will be cumulative impacts from the dams which do move ahead. Different choices on dam siting, design, size, and operation can lead to differing environmental outcomes. For instance, a dam such as the 86 MW Sekong A will have disproportionate negative impacts on the entire Mekong system by cutting off pathways for fisheries migration while providing a small amount of electricity. This project is particularly important because the Sekong is the last free-flowing tributary of the river system in the lower Mekong. It underpins fish migration and supports the productivity of the world’s largest freshwater fishery. While construction on the Sekong A project appears to have slowed after the Mekong River Commission began a transboundary EIA process in 2023, it is arguably the single most damaging project currently under construction in the Mekong region.
There is still time for planners at the national and regional levels to adopt system-scale approach to better balance tradeoffs and meet energy needs by substituting solar, wind, and/or energy storage for particularly impactful hydropower projects like the Sekong A. Given the prevalence of hydropower in the region and its vulnerability to increasingly frequent and prolonged droughts, diversification should be attractive to grid managers. Floating solar (FPV) is a particularly attractive opportunity, given both high solar radiation in the 3S and because FPV avoids land competition costs, which can inhibit traditional solar buildout. While variability means solar power without storage is not a direct substitute for hydropower, diversification could have significant benefits. A 2022 study indicated that deployment of floating solar on only four reservoirs in Laos could close the gap between projected power supply and demand through 2030. And perhaps most importantly, solar complements hydropower, which loses capacity to produce power during the dry season due to lower water availability. Solar produces electricity most predictably during the dry season, helps reduce water losses to evaporation from reservoirs, and—if operated in conjunction with hydropower production—can conserve water and mitigate impacts of increasingly common and prolonged droughts.
The 3S river basins are an attractive area for FPV deployment given the ample amount of existing hydropower already in the basin. The Mekong Infrastructure tracker shows there are 160 reservoirs in these basins for hydroelectricity production (67), water supply (91), and mixed usage reservoirs (2) in the 3S system. Of these, 13 have reservoirs over 1,000 hectares that would be suitable for deployment of large-scale FPV, and 15 smaller reservoirs over 300 hectares that would be suitable for small-scale FPV. The Solargis global solar potential dataset shows that potential in the 3S ranges from 3.5 to 5 kWh/m2/day. This is comparable to radiation at the 45 MW FPV project at the Sirindhorn Dam in Thailand and the 60 MW Tengeh Reservoir FPV project in Singapore.
Investment will be more cost effective if FPV is built on a hydropower reservoir versus a water storage reservoir because it can take advantage of existing transmission lines. In terms of project size, lessons can be drawn from other FPV installations in Southeast Asia. The Tengeh FPV project in Singapore covers about 45 hectares or one-third of the reservoir’s surface area, which is relatively high coverage. In Thailand, the Electricity Generating Authority capped the Sirindhorn FPV project to less than 1% of the reservoir area (or only 121 hectares out of 23,000 ha in the reservoir) to limit impacts on the underwater environment. For dams in the 3S river basins with reservoirs over 1,000 hectares, even a 10% FPV coverage would generate significant additional power.
There is growing commercial interest in FPV in the region. In 2024, the government of Laos approved a feasibility study for a 1,500MW FPV project on the Nam Ngum 1 reservoir just north of Vientiane. In 2021, the Nam Theun 2 company signed a project development agreement for 240 MW of FPV on its reservoir. In the 3S river basins, the Viet-Lao JSC in partnership with Convalt Energy proposed a 280 MW FPV project on the Xekaman 1 Dam in 2018. A 2018 report indicated that FPV on the Lower Sesan 2 Dam in Cambodia could potentially double the power capacity from 400 MW to 800 MW. This approach would also be consistent with the IFC’s Cumulative Impact Assessment for the Sekong, which prioritizes hydropower in tributaries and integrating FPV to make best use of existing assets and delays the need for dams on the Sekong mainstream.
If policymakers can coordinate on agreed priorities for projects and cross-border power trade, the region has significant potential to act as a truly green battery for Southeast Asia, where existing hydropower is complemented by further investment in floating solar PV and (as needed) accompanying energy storage projects. This could support system-scale analysis of which projects to prioritize, allowing for strategic avoidance of dams like the Lower Sekong A and others which have significant project impacts on river systems, reducing vulnerability to dry season and drought year productivity challenges, and allowing for targeted low-impact hydropower development to meet energy targets.
Recommendations:
- Pursue the recommendations highlighted in the IFC Cumulative Impact Assessment report, including development of a Sekong Basin power development master plan to strategically identify power production and demand targets, reduce social and environmental impacts through strategic design and selection of projects, optimize power generation across various and diverse assets. For example, drawing on the above analysis, the Sekong A dam could essentially be replaced by a similar size floating solar built on less than 3% of the reservoir for the existing Xekaman 1 Dam paired with backup energy storage.
- For purchasers in Thailand and Vietnam—both of which have existing MOUs with the government of Laos to increase imports—this could provide a cost-effective way to meet long-term clean energy and diversification targets. The national utilities in both should seek to review and reprioritize their approaches to power trade with Lao PDR to include the potential for FPV to help meet national import and clean energy targets.
- Cambodia recently exited the CLV Development Triangle Plan over domestic political pressures and concerns that it was benefiting neighboring economies rather than Cambodian citizens. Rethinking engagement through the 3S basin could provide an avenue for development which better meets Cambodia’s interests. On one hand, coordinating with neighbors to avoid dams like the Sekong A could help limit negative impact on Cambodia’s portion of the 3S. Reconsidering opportunities for floating solar inside Cambodia could also provide an opportunity for power trade, as the Lower Sesan 2 Dam would be a prime target for future FPV development given its immense reservoir size.
This is the second in a series on floating solar and new or emerging energy tech opportunities in the Mekong region in mainland Southeast Asia. This analysis is generously supported by the BRIDGE (Building River Dialogue and Governance) Project, which aims to build water governance capacities through learning, demonstration, leadership, and consensus building and is funded by the Swiss Agency for Development and Cooperation and implemented in Cambodia, Lao PDR, and Vietnam by IUCN.
Balancing Energy and Ecology: Floating Solar in the 3S River Basin
By Courtney Weatherby
Southeast Asia
As policymakers in the Mekong region try to meet both rapidly rising electricity demand and transition away from fossil fuels, hydropower remains central to the region’s power mix. There are 190 existing hydropower dams across the Mekong river basin in 2024, with an additional 19 projects under construction and 287 proposed dams. Though renewable, dams can significantly and negatively impact ecosystems and the natural environment, particularly when they have cumulative impacts.
Over 25% of projects under development or proposed will be in one sub-basin system: the Sekong, Srepok, and Sesan (3S), a transboundary system across Cambodia, Laos, and Vietnam (CLV), which remains crucial for fisheries and food security. However, hydropower is not the only energy option, as the 3S river basin has viable alternatives such as floating solar, wind, and pumped storage hydropower. In 2021, the IFC published a comprehensive cumulative impact assessment study for the Sekong Basin, which specifically recommended moving past a project-by-project approach to power generation buildout and instead coordinating development across all energy types—hydropower, wind, solar, and thermal energy—to strategically build out a diverse and resilient energy system for the Sekong River Basin and minimize unnecessary social and environmental impacts. Its recommendations are even more relevant today given new projects proposed and broader shifts in the regional energy plans and market. CLV governments should adopt a system-scale planning approach that incorporates these system-scale power principles and avoids badly sited dams to build a diverse, resilient, and secure energy mix that minimizes environmental and food security impacts.
While it’s unlikely all the proposed dams in the Mekong system will be built, there will be cumulative impacts from the dams which do move ahead. Different choices on dam siting, design, size, and operation can lead to differing environmental outcomes. For instance, a dam such as the 86 MW Sekong A will have disproportionate negative impacts on the entire Mekong system by cutting off pathways for fisheries migration while providing a small amount of electricity. This project is particularly important because the Sekong is the last free-flowing tributary of the river system in the lower Mekong. It underpins fish migration and supports the productivity of the world’s largest freshwater fishery. While construction on the Sekong A project appears to have slowed after the Mekong River Commission began a transboundary EIA process in 2023, it is arguably the single most damaging project currently under construction in the Mekong region.
There is still time for planners at the national and regional levels to adopt system-scale approach to better balance tradeoffs and meet energy needs by substituting solar, wind, and/or energy storage for particularly impactful hydropower projects like the Sekong A. Given the prevalence of hydropower in the region and its vulnerability to increasingly frequent and prolonged droughts, diversification should be attractive to grid managers. Floating solar (FPV) is a particularly attractive opportunity, given both high solar radiation in the 3S and because FPV avoids land competition costs, which can inhibit traditional solar buildout. While variability means solar power without storage is not a direct substitute for hydropower, diversification could have significant benefits. A 2022 study indicated that deployment of floating solar on only four reservoirs in Laos could close the gap between projected power supply and demand through 2030. And perhaps most importantly, solar complements hydropower, which loses capacity to produce power during the dry season due to lower water availability. Solar produces electricity most predictably during the dry season, helps reduce water losses to evaporation from reservoirs, and—if operated in conjunction with hydropower production—can conserve water and mitigate impacts of increasingly common and prolonged droughts.
The 3S river basins are an attractive area for FPV deployment given the ample amount of existing hydropower already in the basin. The Mekong Infrastructure tracker shows there are 160 reservoirs in these basins for hydroelectricity production (67), water supply (91), and mixed usage reservoirs (2) in the 3S system. Of these, 13 have reservoirs over 1,000 hectares that would be suitable for deployment of large-scale FPV, and 15 smaller reservoirs over 300 hectares that would be suitable for small-scale FPV. The Solargis global solar potential dataset shows that potential in the 3S ranges from 3.5 to 5 kWh/m2/day. This is comparable to radiation at the 45 MW FPV project at the Sirindhorn Dam in Thailand and the 60 MW Tengeh Reservoir FPV project in Singapore.
Investment will be more cost effective if FPV is built on a hydropower reservoir versus a water storage reservoir because it can take advantage of existing transmission lines. In terms of project size, lessons can be drawn from other FPV installations in Southeast Asia. The Tengeh FPV project in Singapore covers about 45 hectares or one-third of the reservoir’s surface area, which is relatively high coverage. In Thailand, the Electricity Generating Authority capped the Sirindhorn FPV project to less than 1% of the reservoir area (or only 121 hectares out of 23,000 ha in the reservoir) to limit impacts on the underwater environment. For dams in the 3S river basins with reservoirs over 1,000 hectares, even a 10% FPV coverage would generate significant additional power.
There is growing commercial interest in FPV in the region. In 2024, the government of Laos approved a feasibility study for a 1,500MW FPV project on the Nam Ngum 1 reservoir just north of Vientiane. In 2021, the Nam Theun 2 company signed a project development agreement for 240 MW of FPV on its reservoir. In the 3S river basins, the Viet-Lao JSC in partnership with Convalt Energy proposed a 280 MW FPV project on the Xekaman 1 Dam in 2018. A 2018 report indicated that FPV on the Lower Sesan 2 Dam in Cambodia could potentially double the power capacity from 400 MW to 800 MW. This approach would also be consistent with the IFC’s Cumulative Impact Assessment for the Sekong, which prioritizes hydropower in tributaries and integrating FPV to make best use of existing assets and delays the need for dams on the Sekong mainstream.
If policymakers can coordinate on agreed priorities for projects and cross-border power trade, the region has significant potential to act as a truly green battery for Southeast Asia, where existing hydropower is complemented by further investment in floating solar PV and (as needed) accompanying energy storage projects. This could support system-scale analysis of which projects to prioritize, allowing for strategic avoidance of dams like the Lower Sekong A and others which have significant project impacts on river systems, reducing vulnerability to dry season and drought year productivity challenges, and allowing for targeted low-impact hydropower development to meet energy targets.
Recommendations:
This is the second in a series on floating solar and new or emerging energy tech opportunities in the Mekong region in mainland Southeast Asia. This analysis is generously supported by the BRIDGE (Building River Dialogue and Governance) Project, which aims to build water governance capacities through learning, demonstration, leadership, and consensus building and is funded by the Swiss Agency for Development and Cooperation and implemented in Cambodia, Lao PDR, and Vietnam by IUCN.
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