Renewable energy transition and small hydropower proliferation

Renewable energy sources are the oldest, more reliable, and environmentally friendly. Particularly, hydropower remains the most valuable energy source. The estimated potential of renewable energy sources is enormous. Theoretically, it exceeds the current world’s energy demand; therefore, these energy sources are expected to have a substantial share in the future global energy market.

Global flourishing efforts in cutting down Greenhouse Gases emissions (GHG), increasing awareness towards environmental protection, and high cost and limited non-renewable energy sources have led to increasing interest in renewable energy sources exploitation worldwide. Meeting such goals, particularly those set in the Paris agreement, which aims to limit global temperature rise below 2 ^oC, requires a drastic reduction of carbon dioxide emissions up to 85 % by 2035. Such accomplishments require low-carbon transitions. Therefore, future progress in the energy sector and global economic development will rely on exploiting renewable energy sources.

Indeed, shifting to a renewable energy-based economy has a twofold effect: reducing environmental impacts due to GHG emissions and improving energy coverage and security. In this regard, while many developed countries aim to increase the renewable share to decarbonize, shift to a more sustainable economy, and mitigate the consequences of climate change in some parts of the world, many developing countries are still striving to increase the basic electrification coverage. Most southern hemisphere countries, mainly in Africa, southern Asia, and central and south America, do not accomplish 100 % electrification coverage. Nearly 13 % of the world’s population does not have electricity access, and almost half live in sub-Saharan Africa. Access to affordable and reliable electricity is crucial to improving the quality of life while eradicating poverty by providing access to essential services such as clean water, food, health, and education.

Nowadays, humanity faces a major challenge in providing equal access to essential goods for all people. The most pressing humankind necessity pre-set in 17 Sustainable Development Goals (SDGs) includes 169 targets and guides development actions for the next ten years. SDG includes ‘’ensure access to affordable, reliable, sustainable, and modern energy for all’’ as goal number 7 (SDG 7), among others. SDG 7 has four targets, aiming mainly to guarantee universal access to energy, improve energy efficiency, increase the share of renewable energy in the global energy mix, and infrastructure and technology enhancement to provide sustainable energy. Last decade, thanks to hydropower development, the number of people receiving access to electricity has increased by nearly 118 million; nevertheless, the progress needs to be even faster to fulfill SDG 7 by 2030. In these aspects, hydropower development provides multiple benefits through numerous services, not only limited to energy storage and clean energy generation as per SDG 7, but also clean water (SDG6), resilient infrastructure (SDG 9), and climate change mitigation (SDG 13), among others.

Technological advancement, lower development, and operation cost of hydropower compared with other renewable energy sources make it one of the main contributors to global efforts to fight climate change and lead the transition to clean energy. Considered the backbone for renewable energy expansion in Europe and in many other countries that adhere to intergovernmental agreements, hydropower has a crucial role in meeting both the renewable energy targets and greenhouse gas emissions reduction targets

Small hydropower plants typology and status

The first hydropower design concepts were simple design and small (i.e., basically based on water wheels). Up to date, hydropower technology has evolved significantly, particularly regarding size, typology, exploitation, and operation mode. Hydropower is classified into two main categories: small hydropower plants (SHPs) and Large Hydropower Plants (LHPs). Each category then follows several sub-classifications based on different criteria that are not mutually exclusive. The term SHP is mainly related to the power capacity, whose definition varies worldwide; globally, it is less than 50 MW.

Moreover, all types of SHPs mainly use available natural flow discharge or induce slight (i.e., hourly, or daily) regulation compared to LHPs. The term SPHs implies and is commonly used interchangeably with Run-of-River (RoR). Therefore, hereafter these two terms will be used interchange with each other. In general, hydropower development has played an essential role in the global economy’s industrialization during the last century.

Hydropower is far more reliable than other renewable energy sources, e.g., solar and wind, in terms of responding to fluctuations in electricity demand by meeting both baseload and peak-load demand. Although SHPs were the first hydropower technology to be developed, LHPs, mainly because of their flexibility in operation and multipurpose capabilities (i.e., energy, flood control, irrigation, and water supply), gained much higher interest for almost 50 years (1920-1970), when thousands of dams were built around the world.

However, nowadays, mainly due to their substantial environmental impacts, the construction rate of LHPs has drastically decreased in most developed countries; even though, around the world, mainly in Asia, there are still more than 1200 LHPs under construction. In contrast, SHPs have gained more interest globally, particularly during the last decades. Most of them are RoR diversion weir schemes and operate as non-peaking. Global-scale figures show that the number of existing SHPs overpass that of LHPs.

Notably, from 2006 on, the trend of new SHPs remains distinctly high in almost all regions except Oceania. A closer look at the continent scale shows an enormous difference in the number of SHPs against LHPs, notably in Asia and Europe, followed by Latin America.

Besides that, some other reasons which have led to an exponential increase of SHPs are related to a general perception of SHPs as more environmentally friendly than LHPs, short construction time, and opportunities for non-centralized management (i.e., off-grid transmission). The latter is becoming a convenient option to increase access to renewable energy in a timely and environmentally sustainable way, particularly for remote areas, despite the ongoing debate on rural electrification to boost local economic development.

The number of people who gained electricity access through off-grid options based on SHPs has doubled during the last decade. Simultaneously, the contribution of SHPs, mainly small diversion RoR hydropower, to global power generation increased by more than 10 % during the last decade. The number of newly constructed SHPs is estimated to be around 11 per every LHP. Thus, small diversion RoR hydropower is becoming an important integral part of a broader strategy to promote economic growth while at the same time contributing to GHG emissions reduction and supporting greater energy independence.

Almost 60 % of GHG emissions come from non-renewable-based energy sectors, so SHPs could substantially substitute fuel-based energy sectors in many regions. In general, most countries have access to abundant water resources and favorable conditions for developing SHPs, enabling them to generate clean electricity without polluting the environment. The small diversion RoR hydropower is particularly attractive for developing countries. There is enormous potential for developing such schemes in many of these countries. Also, hydropower schemes can be implemented much shorter than other types of SHPs and/or LHPs schemes. Nevertheless, the degree to which RoR hydropower development can contribute to renewable energy growth depends on its economic efficiency determined by the performance characteristics of power generation according to turbine type, installed capacity, environmental restrictions, and geographic location, among others.

RoR hydropower can generally be built in different terrain conditions, from lowland to upland rivers, or upgraded at existing dams and irrigation schemes. Nevertheless, as most RoR hydropower operates as non-peaking, their profitability relies on real-time flow availability and hydraulic head. Therefore, small RoR hydropower development requires detailed analysis concerning maximizing energy production, cost-effectiveness, and system optimization; the latter is particularly important in cascade schemes. Despite numerous social and economic benefits that the development of small diversion RoR hydropower plants provide, they are also associated with some environmental impacts, which depend on many factors; one of them is the upper limit of power capacity. Mainly in Europe, it is widely accepted that SHPs are considered those with a power capacity of up to 10 MW. Nevertheless, due to particularities in environmental legislation and renewable energy development agendas, the power capacity-based classification of SHPs remains quite diverse in each country.

Usually, the degree of ecological impacts induced due to the small diversion of RoR hydropower operation is closely related to the power capacity; higher power capacity requires diverting a higher proportion of flow discharge. The latter depends on the river’s mean annual discharge; if that value is considerably high, a small proportion of the flow discharge may give a significant power while not inducing substantial impacts on the riverine ecosystem. In many cases, it also implies extended diversion to gain a higher hydraulic head. Thus, flow discharge reduction in a considerable length of the diverted river results in numerous ecological impacts on the riverine ecosystems. As the global energy system is accelerating towards renewable-dominated and liberalized markets and the SHPs-based renewable energy sector is growing at a high rate, these circumstances are expected to intensify the conflicts between specific SDGs, i.e., between SDG 7 and SDG 14.

Thus, it is essential to develop rational trade-offs between the profitability gained through hydropower development and its ecological impacts on the aquatic ecosystem. Therefore, the construction of new small RoR hydropower or upgrading the existing ones, especially in small rivers characterized by fragile ecosystems, involves a trade-off between benefits gained through renewable energy generation, on the one hand, and healthy river ecosystem conservation, on the other hand.

 

Ecological impacts of small hydropower plants

Climate change is expected to put additional stress on the fluvial ecosystems due to the precipitation regime’s alteration. In general, damming impacts on alteration of natural flow regime and degradation of the riverine ecosystem is widely discussed. Nevertheless, it should be underscored that the amount of water used solely for energy production constitutes only around 15-20 % of global freshwater withdrawals and diversions; the rest is used for other purposes. In this regard, it is also widely accepted that LHPs, which operate at a different regulation-based time scale than SHPs, alter essential components and parameters of the natural flow regime. Their main effects may be a decrease in the magnitude of large floods and pulses, alteration of timing and frequency of different flow events, alteration of the duration of specific flow events, alteration of the rate of changes, and induction of sudden water level fluctuations or so-called hydropeaking effect. On the other side, the impacts induced by SHPs operation, particularly by the small diversion RoR hydropower plants, are relatively less investigated, perhaps because, in contrast to LHPs, impacts of SHPs are limited to a shorter length of river stretch, which experiences variable flow reduction over the year.

The most altered upstream-downstream biophysical parameters near the RoR hydropower site (i.e., diversion/retention structure); yellow-colored cells represent the most predominant and extensively reported impacts.

However, still, there is evidence that SHPs may substantially alter several components and parameters of the natural flow regime and ultimately degrade the riverine ecosystem to a considerable degree. Diversion RoR hydropower operation may significantly affect anadromous and resident salmonid fishes such as brown trout (Salmo trutta) and other cyprinid species such as barbel (Luciobarbus bocagei), whose habitats in medium and high-gradient mountainous rivers frequently overlap with a suitable location for RoR hydropower development.

Nevertheless, RoR hydropower operation mainly alters specific components and parameters of the flow regime. Its impact becomes more prominent during specific periods of the year. Namely, it decreases the magnitudes and alters the timing and duration of low flows, resulting in loss of connectivity, water temperature increase, alteration of other physicochemical parameters, and overall degradation of aquatic habitat conditions. Understanding to which extent the natural flow regime characteristics can be altered without compromising the ecosystem is an essential step towards ecological assessments of rivers and sustainable development of hydraulic infrastructure, particularly small diversion RoR hydropower plants. Despite the enormous impacts of anthropogenic pressures on the fluvial ecosystem, decelerating and reversing dramatic freshwater biodiversity losses and ecosystems is still possible. Several efforts have been made regarding protection. Sustainable management of water resources and aquatic ecosystems, e.g., the European Union has developed an ambitious water policy, the Water Framework Directive (WFD) 2000/60/EC, to reduce the anthropogenic pressures significantly and achieve ‘’good’’ ecological status for all European rivers and water bodies.

 

Fluvial ecosystem conservation and hydrological alteration

The fluvial ecosystems are fundamental and interconnected to many aspects of the natural environment, health, and humankind’s activities. They provide plenty of natural resources such as clean water and fish and several other services, i.e., cultural, ecological, recreation, scientific, transportation, irrigation, waste assimilation, and energy production. Freshwaters constitute about 0.01% of the water on Earth; nevertheless, this small fraction of global water supports more than 6 % (i.e., nearly 100,000) of all designated species. Their conservation and sustainable management are crucial to humankind’s high interests. Conservation of the fluvial ecosystem health largely relies on maintaining a suitable flow regime, which is the primary driver of numerous biological, chemical, and physical processes in the aquatic habitat.

To ensure the well-functioning of the fluvial ecosystem, hydrological parameters, such as magnitude, duration, timing, frequency, and rate of changes, are essential for ensuring suitable habitat conditions at the biota’s specific life-stage. Namely, ensuring adequate magnitude of specific flow events influences several ecological processes, such as providing optimal soil moisture, available water, and food for aquatic biota; duration is essential for structuring the aquatic ecosystem and creation of suitable conditions for plant colonization; timing is essential to ensure the compatibility and predictability within life cycles of the aquatic biota; frequency ensures appropriate organic matter and nutrient exchange between the main river and flood plain, influencing bedload transport disturbance and channel sediment heterogeneity. Finally, the rate of change controls drought stress on plants and the stranding of organisms in the floodplain area.

The native riverine species evolve in response to the overall flow regime components and parameters. The natural flow regime dynamics greatly impact the specific habitat units such as riffles, pools, and runs, which are dwelling niches for dozens of freshwater aquatic species. Therefore, the hydraulic regime through which these units interact triggers the biota’s dynamic in the aquatic ecosystem.

On the other side, several hydromorphological processes, such as sedimentation and erosion, occur due to the natural flow regime dynamic, influencing the hydraulic regime considerably. The alteration of the quantity and natural dynamic of water flowing in a river that provides a habitat for biota may significantly influence water quality (e.g., temperature, oxygen availability, and pH), food web, and many geomorphic processes that shape river channels and floodplain dynamics. Water surface elevation, velocity, and wetted perimeter are the main hydraulic parameters controlling flow patterns, habitat availability, and suitability. Reducing these parameters due to the depletion of the natural flow discharges leads to habitat availability loss. It, therefore, puts the existence of aquatic species at risk. In this context, unsustainable development and/or management of hydraulic infrastructures have significantly altered vital components and parameters of natural flow regimes and aquatic habitats in many rivers worldwide.

 

Reference

Kuriqi, A., & Jurasz, J. (2022). Small hydropower plants proliferation and fluvial ecosystem conservation nexus. In Complementarity of Variable Renewable Energy Sources (pp. 503-527). Academic Press.

Kuriqi, A., Pinheiro, A. N., Sordo-Ward, A., Bejarano, M. D., & Garrote, L. (2021). Ecological impacts of run-of-river hydropower plants—Current status and future prospects on the brink of energy transition. Renewable and Sustainable Energy Reviews, 142, 110833.

Kuriqi, A., Pinheiro, A. N., Sordo-Ward, A., & Garrote, L. (2019). Flow regime aspects in determining environmental flows and maximising energy production at run-of-river hydropower plants. Applied Energy, 256, 113980.

Kuriqi, A., Pinheiro, A. N., Sordo-Ward, A., & Garrote, L. (2019). Influence of hydrologically based environmental flow methods on flow alteration and energy production in a run-of-river hydropower plant. Journal of Cleaner Production, 232, 1028-1042.

Kuriqi, A., Pinheiro, A. N., Sordo-Ward, A., & Garrote, L. (2020). Water-energy-ecosystem nexus: Balancing competing interests at a run-of-river hydropower plant coupling a hydrologic–ecohydraulic approach. Energy Conversion and Management, 223, 113267.