Emerging technological trends aimed at increasing hydropower’s flexibility, efficiency and cost-effectiveness were discussed at a workshop organised by the European Commission Joint Research Centre in December 2017. The findings from this event were recently published in Renewable and Sustainable Energy Reviews, where existing technical challenges and knowledge gaps relevant to further hydropower development were analysed.
The authors Kougias et al say that when compared to other clean energy sources such as wind and solar, hydropower has achieved high levels of technological maturity.
“Accordingly,” they stated, “there exist fewer possibilities to identify and implement radical design concepts that revolutionise the way hydro operates. However, a significant potential for novel approaches in the planning, design and operation of a hydropower station still exists. This potential partially derives from the evolving role of hydropower in transforming electricity systems.”
The article analyses recent innovations in hydropower technology development. They say that any innovation aims at increasing hydro's efficiency, flexibility of operation, lifespan, and to reduce costs of installation, operation and maintenance. Indeed, technological progress will enable hydropower to respond to variabilities of electrical power systems, markets and climate.
The article describes recent R&D activities and divides the analysed technologies and concepts into six groups:
1.Techniques supporting the wide-range operation of hydraulic turbines:
The variable electrical energy production from renewable energy sources (RES), such as wind and solar, requires that hydraulic turbines operate at a wide range and variable conditions. Climate change mitigation strategies require a high share of RES which in turn requires that new hydropower technologies provide greater flexibility over an extended range of hydraulic conditions.
Some of the most salient flexibility technologies under development include: stabiliser fins, an adjustable diaphragm installed in the draft tube cone, J-grooves, air injection/admission, axial water injection with high/low velocity and low/high discharge, tangential water injection at a cone wall, and two-phase air-water injection along the axis.
2. Instabilities in Francis turbines of pumped hydro energy storage stations:
A better understanding of transient operating conditions of hydro turbines may lead to further improvements of hydraulic and mechanical designs, machine stability, and reliability. Findings would indicate problematic regions in terms of structural load or load changes. This would allow improving the turbine start procedure in order to reduce stress and increase lifespan. Further consideration of cavitation, in particular, would also be worthwhile since it seems to play an important role during the start-up and runaway processes.
Kruonis Pumped Storage Plant in Lithuania. Pumped storage is a mature technology that can still continue to evolve.
3. The digitalisation of hydropower operation:
The collection and processing of real-world data to adjust the actual working conditions of hydropower turbines can provide advanced grid supporting services without compromising station reliability and safety. The objective of the emerging technology presented in the article is to support hydropower plants in fulfilling future EPS requirements, by enabling fast frequency containment reserve, frequency restoration reserve and black start in emergencies. It is estimated that a total 42TWh could be added to present hydropower energy production by implementing hydropower digitalisation. Such an increase could lead to annual operational savings of US$5 billion and a significant reduction of greenhouse gas emissions.
4. Hydro generators with current-controlled rotors:
Modern power electronics with current-controlled power supplies provide new opportunities for the control of electrical machines. The idea of a segmented rotor could be combined with novel ideas on excitation to open-up possibilities for reduced investment cost and maintenance. Power electronic converters also open up new possibilities for electrical machines. The authors says that there is more to discover and it is expected that new ideas will enhance the coupling of electrical machines with power electronics.
5. Variable speed hydropower generation:
Variable speed hydropower generation is described as already having reached the highest possible technology readiness level. There are, however, only a few variable speed hydropower units (most of which are pumped storage units) in operation all over the world. The authors say that reasons for the slow pace of installation are due to that it usually takes a long time from the conception of a hydropower project to the commissioning; new developments often raise a wide range of environmental concerns; and that the extra revenue a variable speed hydropower plant can gain in the electricity and ancillary services markets is not always worth the extra cost necessary for the plant to operate at variable speed.
Despite the above points, the authors say that there are still some technological challenges left that, once overcome, might contribute to heighten the advantages of variable speed hydropower generation. They cite two important challenges as being:
- Enlarge the stable operating range of hydraulic machines in order to take full advantage of variable speed operation.
- Enhance the insulation system of converter-fed machines.
6. Novel concepts in hydroelectric energy storage:
The increasing penetration of variable renewable energy in the electrical power system is boosting innovation in energy storage, the authors say. Even though pumped storage is a mature storage technology, it continues to evolve to respond to the faster and more frequent mode transition requirements i.e. from pump to turbine and vice versa. In the authors’ opinion there are two state-of-the-art emerging pumped storage technologies which have significant potential. These are coordinated operation of fast energy storage systems and hydropower; and underwater pumped-hydro energy storage
7. Novel technologies in small-scale hydropower:
Small-scale hydropower plays an important role in mini-grid and rural electrification strategies. The authors say that there is unexploited potential with low head differences available in rivers, irrigation canals and at old mill sites.
Novel designs of gravity hydraulic machines (water wheels and Archimedes screws) and turbines (very low head turbines and hydrokinetic turbines), and advanced designs and operation strategies for pumps as turbines, are being introduced.
8. Fish-friendly hydropower technologies:
Contrary to the conventional turbines, low head gravity turbines (water wheels, hydrodynamic screws) are considered fish friendly. However, these turbines can be only employed at very low head sites and so two strategies have been developed for high head hydropower: fish passage facilities and fish-friendly turbines.
The authors say that recent scientific efforts have also focused on understanding the relationships between turbulent hydraulic environments and animal behaviour in the fishways to improve attraction, approach, entry and passage for multiple species. While in order to overcome the limitations of fish passages, recent R&D efforts have focused on the development of fish-friendly turbines for relatively higher head hydro stations. Accordingly, the Alden turbine and the Minimum Gap Runner turbine have been introduced. The Alden turbine specifically works with head differences of up to 25m.
Salmon ladder at Bonneville Dam on the Columbia River in the US.
Other scientific disciplines
In their article, Kougias et al discuss challenges, innovation trends and emerging hydropower technologies which mainly cover European research projects.
They believe that future technological advances in environmental sciences will have a direct impact on the way the ecological impact of existing and new hydropower is assessed. They say that defining advanced, more accurate methodologies to assess the environmental impacts, such as environmental flows, is an important step towards reaching an environmental friendlier operation of hydropower.
“A better understanding of hydro's interaction with the ecosystems achieved through research activities in the water and environment fields may help address some of the constraints and – at least partially– support the mitigation of environmental impacts,” Kougias et al say. “Such practices also extend to the construction and tunnelling fields. Advanced construction and drilling methods can also mitigate the environmental and social impacts of hydropower and particularly the part related to civil works (eg lifecycle GHG emissions).
At the same time, the authors say that the future evolution of the hydropower sector will need to take advantage of advances in climate sciences. The sector can benefit from advances in hydrology data collection and analysis of river basins. Breakthroughs in the estimations of water travel time, groundwater infiltration and evaporation can have a direct impact on hydropower scheduling. Climatic conditions and their possible changes are also crucial as future water availability is a decisive parameter for the economic viability of a hydropower station.
The authors conclude: “Advances in the detail, the refinement and the accuracy of climatic projections will also benefit hydropower. Reducing or partially removing uncertainties is very important, especially since hydropower is a capital-intensive technology that requires a large majority of the investment up front. It is, thus, clear that the future role of hydropower is not only dependent on the technological advancements that are covered in the present analysis. Knowledge breakthroughs in other scientific disciplines are also important and can potentially enable improved services. The availability of a portfolio of different, advanced technologies would certainly create solutions for an increased number of applications.”
