Southeast Asia's economic growth is heavily reliant on power systems utilizing readily available and inexpensive energy sources, primarily hydroelectricity and fossil fuels. This reliance raises concerns about CO2 emissions and the substantial socio-environmental consequences of extensive hydropower development, particularly in the biodiversity-rich Mekong River Basin. The Mekong River, home to the world's largest freshwater fishery, has seen significant hydropower development, with numerous dams built and many more planned. These dams profoundly impact hydrological regimes, block fish passage, reduce sediment and nutrient transport, and negatively affect riverine ecosystems and riparian communities. However, the increasing availability of regional grid interconnections and renewable energy sources, especially solar photovoltaic (PV), suggests potential alternatives to the current hydropower-centric approach. This study explores the feasibility and economic viability of these alternatives by integrating strategic dam planning with the deployment of decentralized renewable technologies, a problem often addressed in isolation. The research aims to determine if reducing hydropower dependence through solar PV and regional cooperation is a technically sound and economically reasonable path for meeting Southeast Asia's energy demands while protecting its environment.

Existing literature extensively documents the environmental impacts of large hydropower dams, highlighting their disruption of river ecosystems and negative effects on biodiversity, fisheries, and riparian communities. Studies focusing on the Mekong River Basin specifically showcase the extensive hydrological alterations caused by dam construction, including altered flow regimes, sediment transport disruption, and habitat fragmentation. Prior research has also examined the economic aspects of hydropower, analyzing its costs and benefits. However, the integration of renewable energy sources, particularly solar PV, as a viable alternative to large-scale hydropower projects in the region, has not been fully explored, particularly considering the potential for regional grid interconnections and coordinated planning. The lack of integrated approaches combining dam portfolio optimization with renewable energy expansion strategies is a key gap addressed by this paper.

The study employs a coupled modeling approach combining a hydrological-hydraulic model (VIC-Res) and a power system optimization model (urbs). VIC-Res simulates daily hydropower production for existing and planned dams across the Mekong and Chao Phraya basins, considering reservoir operations and hydrological processes. The model incorporates data on dam locations, design specifications, rule curves, and hydro-meteorological conditions. The model is calibrated and validated against observed discharge data from multiple gauging stations. For dams lacking detailed design information, a proximity search algorithm assigns hydropower profiles based on similar existing dams. The urbs model optimizes power system expansion, considering existing infrastructure, projected electricity demand, emission reduction targets, and techno-economic parameters. It minimizes the overall cost of expanding and operating the energy system, including investment, fuel, operation, and maintenance costs. The model has an hourly temporal resolution, simulating the energy system over several years, allowing for iterative capacity expansion plans based on model outputs from prior years. The spatial resolution utilizes provinces as model regions, incorporating detailed data on demand, renewable generation, and transmission constraints. The model assumes full regional cooperation among Thailand, Laos, and Cambodia, enabling cross-border electricity trading. The study examines the impacts of several different hydropower development portfolios, ranging from a business-as-usual scenario to scenarios where dam construction is halted or curtailed. In addition to cost analysis, the River Fragmentation Index (RFI) and River Regulation Index (RRI) quantify the ecological impact of different hydropower portfolios. The model also addresses seasonal variations and incorporates multiple climate scenarios to assess the robustness of the results.

The study's optimization results reveal multiple pathways to meet projected electricity demand and CO2 emission targets in Thailand, Laos, and Cambodia, with significantly reduced hydropower development. These alternatives range from completely halting new dam construction in the Lower Mekong to building 82% of the planned dams. The key enablers are the substantial expansion of solar PV capacity (reaching 68.2 GW by 2037) and effective regional coordination of electricity generation and transmission. While the alternative scenarios slightly increase the cumulative cost (up to 2.4%, approximately 10 billion USD over 2016-2037), this increase is potentially less than the estimated economic damage from hydropower's impact on fisheries. The spatial distribution of energy generation and demand shows a need for a significant upgrade of transmission infrastructure, particularly connecting hydropower sources in Laos and Cambodia with the demand centers in Thailand. Analysis using the River Fragmentation Index (RFI) and the River Regulation Index (RRI) demonstrates that alternative portfolios, with significantly reduced hydropower, would substantially limit the future fragmentation of the Mekong River compared to the business-as-usual scenario. The RRI analysis suggests that future flow alterations are primarily driven by large-scale dams in the Upper Mekong, independent of changes in Lower Mekong dam portfolios. The cost optimization modeling indicates a strong correlation between hydropower and coal generation. High hydropower shares allow for continued coal use due to the system's overall carbon-neutral generation capability, which is not as readily achieved with lower hydropower scenarios, instead necessitating greater use of gas-fired plants. Sensitivity analyses show that the capacity expansion plans are robust across varying hydrological conditions.

The findings challenge the conventional approach to hydropower development in the Mekong Basin. The study highlights the economic and technical feasibility of significantly reducing the planned hydropower expansion while still meeting energy demands and decarbonization targets. The slight increase in cost associated with the alternative scenarios is comparatively small against the potential environmental damage and other associated externalities linked to dam development, such as greenhouse gas emissions, thermal pollution, and community displacement. The capacity expansion plans, enabled by solar PV and regional cooperation, demonstrate that a transition from large centralized hydropower plants to decentralized renewable energy sources is technically achievable. The successful implementation of these plans requires robust regional collaboration and coordinated investment strategies. While the study focuses on the Lower Mekong, it highlights the need for broader regional cooperation, particularly concerning the influence of upstream dams in the Upper Mekong, highlighting the significance of joint infrastructure planning and data sharing initiatives between upstream and downstream countries.

This study demonstrates that Thailand, Laos, and Cambodia can achieve their energy and emission reduction goals with substantially less hydropower than currently planned. Integrating solar PV and regional grid coordination offers a viable and economically feasible alternative, mitigating significant environmental impacts. The findings advocate for a critical reassessment of hydropower development plans, emphasizing the need for broader regional collaboration and a transition to a more diverse and sustainable energy mix. Future research should explore the dynamic interaction between energy system planning and dam portfolio optimization, incorporating additional ecological factors and addressing the reliability implications of renewable energy integration.

The study's model relies on several assumptions which affect its predictive capability. First, it assumes complete regional cooperation for electricity trading between countries, which may not fully reflect the complex political dynamics that influence infrastructure development. Second, the detailed hydrological modeling used a representative year for hydropower output, potentially underestimating the impact of interannual hydro-climatic variability. Lastly, the socio-economic impacts beyond fisheries are not directly incorporated into the cost-benefit analysis.