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HYdropower plants PERformance and flexiBle Operation towards Lean integration of new renewable Energies

The 20-20-20 strategic energy policy decided by the European Union and its accompanying Renewable Energy Directive is leading toward a dramatic electric energy system transition. The two major pillars of this transition are: i) a massive penetration of alternative renewable energies and ii) a broad deployment of energy efficiency initiatives and technologies.

In this context, hydropower already plays and will increasingly do so to, on one hand, contribute to renewable energy production and, on the other, provide for the highly dynamic energy storage requirements to enable a widely distributed injection of PV and wind energy into the transmission and distribution systems while preserving their stability through the provision of advanced system services.

As a result, the overarching objective of the project is the enhanced hydropower plant value by extending the flexibility of its operating range, while also improving its long-term availability. More specifically, the project aims to study the hydraulic, mechanical and electrical dynamics of several hydraulic machines configurations – fresh and seawater turbines and reversible pump-turbines as well – under an extended range of operations : from overload to deep part load. A two-pronged modelling approach will rely on numerical simulations as well as reduced-scale physical model tests. Upon suitable concurrence between simulations and reduced-scale physical models results, validation will take place on carefully selected physical hydropower plants properly equipped with monitoring systems. Finally, the benefits resulting from the extended control flexibility provided by a set of hydro units will be demonstrated through extensive simulation of the operational conditions of an electric power system with high share of highly variable sources.

To address this ambitious research plan, a consortium has been assembled featuring three leading hydraulic turbines, storage pumps, reversible pump-turbine and electric equipment manufacturers, SME, as well as worldrenowned academic institutions. Extensive tests both on both experimental rigs and real power plants will be performed in order to validate the obtained methodological and numerical results Hydropower plants key contribution to NRE integration. In the recent years due to tremendous development and integration of renewable energy resources (NRE) in Europe, hydraulic turbines and pump-turbines are key energy conversion technology to achieve both load balancing and primary and secondary power network control.

Hydropower plants key contribution to NRE integration

In the recent years due to tremendous development and integration of renewable energy resources (NRE) in Europe, hydraulic turbines and pump-turbines are key energy conversion technology to achieve both load balancing and primary and secondary power network control.

  

To illustrate the challenge of integrating NRE, we can refer to the time history of the 3.8 TWh generated in Germany by PV power plants and wind farms between June 15 and June 30, 2012, see Figure 1. The hourly change of the power generation leads to a minimum 10 GW storage capacity which can only be achieved by hydropower plants. The fast change of power generation by NRE is impacting directly the required operating range of hydro units going from overload down to part load, 20% to 30% of the maximum power and on the number of start-ups and changes between pumping and generating modes, as well. Moreover only pumped storage power plants are suitable for large scale electricity storage and fast control over an extended operating range with unrivalled pumping-generating cycle efficiency better than 80 %.

Figure 1 Selected time history of the NRE power generation in Germany and required storage capacity to achieve primary and secondary control of the power network. 
 

Extreme operating points like deep part load or overload lead the water turbine to experience complex two-phase flow phenomenon, cavitation, which is sources of dynamic loading of the turbine components as well as of the complete system including water piping, turbine structure, rotating train, generator, controllers and grid. Examples of the development of dynamic cavitation vortex structures in the draft tube cone for various operating regimes are shown in Figure 2 in the case of a reduced-scale physical model testing of a Francis turbine at the laboratory. Moreover, depending on the grid structure, the development of cavitation vortex can cause unstable operating condition preventing the hydropower plant to be made available to the TSO.

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Figure 2 Visualization of draft tube cavitation vortex rope during cavitation test of a reduced-scale physical model of a Francis Turbine for part load (up) and full load operations (down).

Predicting dynamic loads

Reduced-scale physical model testing of the turbines defined by the IEC 60193 standards are usually performed for predicting and assessing the behaviour of the hydraulic turbine when installed in the power plants. In particular these tests are made to ensure smooth turbine operation, as this may not be currently reliably predicted by numerical simulations. However, even though the model behaviour complies with the expectations, it is not sufficient for guarantee reliable behaviour of the generating units when installed in the hydropower plant. The generating unit behaviour can only be predicted, if mathematical models can capture the difference between the test rig hydraulic circuit and the hydropower plant hydraulic system as well as to scale the hydraulic excitation coming from the water turbine. In some cases the induced pressure pulsations may lead to unexpected active power fluctuations coming out of the generator (Figure 3).

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Figure 3 Relative peak to peak active power oscillation amplitude as a function of the active power output of a Francis turbine generating unit.

Therefore an extensive theoretical investigation has to follow the reduced-scale physical model testing for being able to transpose the reduced-scale physical model tests results such as active power, vibration, stress, pressure fluctuations to the real hydropower plant.