Flexible hydro operation for lean integration of new renewables

Hydropower helps incorporate the growing share of intermittent renewable sources into the energy mix

By Dr Arthur Favrel, Mech. Eng. MSc. Université Claude Bernard de Lyon, France, EPFL LMH post doctoral assistant and Prof. François Avellan, Eng. Dr Université d’Aix Marseille 2, Director of EPFL LMH, Convenor of IEC TC 4 Maintenance Team 32

In 2015, global generation of electricity was 24 255 TWh. Hydropower accounted for around 16% of the total, making it the main renewable energy (RE) source for electricity generation. It will also play a key role in the future integration of power generated by new RE sources and in balancing its impact on the grid.

Hydroelectric unit Hydroelectric unit at Xiangjiaba hydro power plant (Photo: Alstom)

Steady growth of electricity production

• During the last 3 decades, annual electricity production worldwide has grown by an average of 3,14%

• Conventional thermal electricity generation represents two-thirds of the total

• Electricity generation is strongly correlated with the economy as was demonstrated by the large drop following the 2008 financial crisis

• Hydropower makes a significant contribution, at around 16% of the total, making it the main RE source

World total primary energy supply (TPES) by fuel from 1971 to 2015 (Graphic: IEA)

The contribution made by New Renewable Energies (NREs) was considerable in the last decade, particularly in Europe where it accounted for a 14% share of total generation. Electricity supply worldwide is still heavily reliant on conventional thermal power plants, mainly coal-fired. Coal alone represented nearly 40% of total electricity generation in 2015.

In Europe, the development of NRE capacity in the last decade has superseded that of other types of fuels due to the public energy policies adopted by several European Union states. 2015 saw the installation of almost 20 GW of NRE power plant, predominantly wind power. Annual generation of more than 10 GW by conventional thermal power plants in 2002, 2005 and 2008 is due to new constructions of large gas-fired combined-cycle power plants.

As for hydropower, it is estimated that by 2050, a total capacity of 1 000 GW will be installed, mainly in Africa, Asia and South America, representing a massive potential contract volume for suppliers of hydro equipment. It goes without saying that, in this highly competitive area, it’s vital to maintain a significant and consistent Research & Development (R&D) effort in order to keep ahead of the competition in terms of installing and refurbishing hydropower plants.

When modernizing existing hydropower plants, adding one unit with modern technology is usually an economically viable and technically interesting option. It is likely that plants accounting for a total capacity of 1 000 GW will be refurbished in the next few decades.

New challenges for hydro: enhancing the integration of NREs in electric power systems

In recent years, environmental and geopolitical concerns have led to the establishment of energy policies promoting the development of alternative RE sources such as wind and solar power, so as to reduce drastically the emission of greenhouse gases and dependence on the producers of fossil fuels. This has meant that global electrical energy production from RE sources has risen constantly over the last two decades. It has been particularly notable in the case of solar and wind power, the total output production of which has multiplied by 57 and 10, respectively, in the last decade.

In Europe, the 20-20-20 strategic energy policy adopted by the European Union with its accompanying Renewable Energy Directive is leading towards a dramatic transition in terms of its electrical energy system. The two major pillars of this transition are:

• a massive penetration of alternative renewable energies

• a broad deployment of energy efficiency initiatives and technologies

With the ratification of the Paris Agreement on climate change (UNFCCC COP21), this energy policy will be reinforced further in the future. For instance, a proposal was published by the European Commission in November 2016 to ensure that renewable energies accounted for at least 27% of total energy consumption in the EU by 2030.

The intermittent nature of NRE production seriously affects the energy balance between production and consumption, as well as the stability of the electrical grid. Typical examples are wind and solar energy sources, which are highly dependent on weather conditions. In order to guarantee NRE integrates smoothly with the existing power system, the electrical grid must have sufficient storage capacity as well as primary and secondary grid control capability to allow the energy balancing process to succeed.

Hydropower plants are key to NRE integration

In this context, hydropower already plays an important role. This will increase in order to:

• contribute to renewable energy production, and

• provide for the highly dynamic energy storage requirements that enable an injection of solar and wind energy to be distributed widely within the transmission and distribution systems while preserving their stability through the provision of advanced system services

In the period between 15 and 30 June 2012, 38 TWh of energy was generated by photovoltaic (PV) power plants and wind farms in Germany. The continuous changeability in power generation leads to a minimum requirement of 10 GW of storage capacity.

In this context, the Transmission System Operator (TSO) needs to be able to balance energy generation and consumption rapidly to achieve a lean integration of NRE sources within the power network.

Both gas-fired and hydropower plants are capable of generating power flexibly to accommodate peaks and other regulation ancillary services on a large scale. However, unlike gas-fired facilities, only hydropower plants can exploit a renewable primary energy source with negligible emission of greenhouse gases. In addition, only pumped storage power plants are suitable for large-scale electricity storage and fast control over an extended operating range with an unrivalled pumping-generating cycle efficiency of more than 80%.

This fast change of power generation by NREs has a direct effect on the operating range required of hydro units going from overload down to part load, 20% to 30% of maximum power, as well as on the number of start-ups and changes between pumping and generating modes.

Extension of the operating range of hydropower stations from deep part load (i.e. with very low discharge) to overload (i.e. with very high discharge), as well as fast and frequent mode transitions for pumped storage hydropower units, are increasingly required to enhance a lean integration of NRE sources within the power network.

Predicting dynamic loads

Extreme operating points like deep part load or overload lead the hydro turbine to experience complex two-phase flow phenomena such as cavitation. These represent the source of dynamic loading of 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 of a Francis turbine for various operating regimes are shown below. The development of a cavitation vortex can cause the existence of an unstable operating condition, characterized by pressure pulsations, additional mechanical loading on the machine components and, in some cases, unexpected active power fluctuations coming out of the generator, preventing the hydropower plant being made available to the TSO.


HYPERBOLE Research Project to enhance hydropower plant value

The HYPERBOLE research project consortium was assembled to take on the challenge of enhancing the capability of hydropower plants to function over a larger operating range and with a faster response time. It is made up of three leading electric equipment manufacturers of hydraulic turbines, storage pumps and reversible pump-turbines as well as small and medium-sized enterprises (SMEs) and world-renowned academic institutions. The project, funded by the European Commission (ERC/FP7-ENERGY-2013-1-Grant 608532), and led by the Laboratory for Hydraulic Machines of the Swiss Federal Institute of Technology in Lausanne (LMH-EPFL), has reached important milestones and objectives for enhancing the capability of hydropower stations as required.

The overarching objective of the HYPERBOLE project was to enhance hydropower plant value by extending the operating range of machines while also improving the long-term availability of components. More specifically, the project aimed to study the hydraulic, mechanical and electrical dynamics of several hydraulic machine configurations under an extended range of operations: from overload to deep part load. A two-pronged modelling approach relied on numerical simulations as well as reduced-scale physical model tests. Once there was sufficient concurrence between the two sets of results, validation took place on a carefully selected physical hydropower plant, properly equipped with monitoring systems. The benefits resulting from the extended control flexibility provided by a set of hydro units were demonstrated through extensive simulation of the operational conditions of an electric power network with a large variety of sources.

Towards a revision of IEC standards for assessing hydropower plant stability?

One of the main milestones of the HYPERBOLE project is the development of a complete methodology for predicting and assessing the dynamic behaviour of hydropower stations operating outside their rated operating ranges and in transient conditions.

Reduced-scale physical model tests for hydraulic turbines, storage pumps and pump-turbines (defined by IEC 60193:1999) are often performed for predicting and assessing the behaviour of prototype machines installed in power plants.

Notably, these tests are performed to ensure smooth operation, as numerical simulations may not offer reliable predictions. However, while a model that is similar to the prototype should behave similarly, this is no guarantee that the generating units will perform reliably when installed in the hydropower plant. The generating unit behaviour can only be predicted if mathematical models can capture the differences between the test rig hydraulic circuit and the hydropower plant hydraulic system and how these will affect the scaled-up hydraulic excitation coming from the machine.

A complete methodology, presented in the figure below, has been developed to transpose the reduced-scale physical model test results – such as active power, vibration, stress and pressure fluctuations – to the real hydropower plant. This will enable hydropower plant stability to be assessed fully over a wide operating range and might lead in the future to the revision of the scale-up relating to oscillating phenomena in the IEC 60193 standard for industrial model testing.

Methodology developed for assessing stability of hydraulic turbine or pump-turbine unit in a power plant

Outlook for hydropower

The Xiangjiaba hydropower plant, which holds the world record for power per unit, paves the way to the appearance of a 1 GW turbine. Absolute control of stability over an extended operating range is essential, since the smallest fluctuation in the power output can cause critical perturbations in the connected grid, especially with the long transmission lines to be found in Asia, Africa and Latin America or where there is less inertia, as is seen in Europe. This means that harnessing the dynamic behaviour of hydropower power plants will require drastic improvements in knowledge of machine internal hydrodynamics and fluid-structure interactions as well as of system stability. The detailed knowledge of the dynamic loads experienced by hydro-electrical equipment during the transient operation of conventional or pumped storage hydropower plants will necessarily contribute to the development of improved components with a longer life expectancy.

Future R&D activities will be more and more oriented to the disruptive concept of Turbine Digital Twin. This will prove to be the only viable approach to expanding the flexibility and fast response time of hydropower plants while keeping or even enhancing plant reliability and safety.

Hydroelectric unit Hydroelectric unit at Xiangjiaba hydro power plant (Photo: Alstom)
cavitation vortex Visualization of a draft tube cavitation vortex rope of a reduced-scale physical model of a Francis turbine for part load (left) and full load operations (right)
Methodology developed for assessing stability of hydraulic turbine or pump-turbine unit in a power plant. Methodology developed for assessing stability of hydraulic turbine or pump-turbine unit in a power plant