Dear Colleagues,

The current development of renewable energy sources forces a change in the role of the hydropower sector in electric power systems. This is often a huge technical challenge for existing and new constructed hydropower plants and the scale of growth of intermittent energy sources changes the issues facing them.

The formal and legal rules as well as the specificity of technology used render the hydropower plants less competitive than desired in relation to other Renewable Energy Sources (RESs). Therefore, it is necessary to redefine the goals of hydropower. Currently, more and more opportunities for the development of hydropower are sought in the provision of a wide range of system services. The most important ones include energy storage services and regulatory services, as well as system recovery after black-out, and hydropower plants, in particular reservoir and pumped-storage facilities, are especially equipped to provide these services.

Hydropower plants equipped with reservoirs constitute energy storage with a large capacity and long discharging time (hours, days and even seasons of the year). This feature combined with the longevity of hydrotechnical structures and the significant lifetimes of hydrounits and the flow systems of their turbines in comparison with the production equipment of other RES installations provide a buffer for electric power systems, ensuring their flexibility, which enables better adaptation to the conditions in the field of electricity supply and demand which change over time. Energy storage with the use of hydropower plants is an excellent regulatory tool in the system, which in cooperation with other energy storage technologies (e.g., batteries) is the basic element of security of energy supply. Other storage technologies with very fast reaction times should not be perceived in terms of competition but should be used as valuable supplements to the energy storage system. It should be noted that the storage capacity of hydropower plant reservoirs constitutes over 93% of the total storage capacity in the global energy industry and this storage technology is being further developed worldwide. A significant increase in the efficiency of energy storage can be achieved by creating hybrid storage solutions using hydropower plant reservoirs with storages based on other technologies (e.g., battery technologies).

Regulatory services and system recovery after black-out constitute another very important task carried out by hydropower plants for electric power systems with a considerable number of sources with generation levels difficult to forecast (PV, wind turbines, etc.). The implementation of distributed energy systems with local balancing areas reduces the probability of system black-out; however, this is only done on the condition that facilities with high regulatory capabilities are installed in these systems. Reservoir hydropower plants with pumping units or classic pumped-storage power plants cooperating with batteries and accumulator or flywheel energy storage are perfect for this purpose.

Hydropower plants also play important roles as elements of the water management system. Water resources, which are increasingly scarce today, are one of the most important natural resources in the world. The absolute necessity of water retention in water reservoirs entails significant costs, which, thanks to the energetic use of the collected water, can be largely reduced.

Today, when we are convinced of environmental degradation largely caused by human activity, it is obvious that the priority of any project that uses any natural resources should be to manage them in accordance with the principles of sustainable development. In the case of water resources, this requires the use of solutions that optimally use the available hydropower potential, mainly through the design and construction of machines optimally suited to the expected operating conditions and the use of appropriate operating procedures that enable the maximization of this potential, taking into account environmental protection.

Adapting the current facilities to new conditions requires, first of all, the adaptation of machines and axillary devices to the cooperation of hydropower plants with electric power systems. Keeping up with new challenges requires innovative technological solutions, taking into account not only energy aspects, but also the issues of structural durability, which is significantly influenced by variable loads—i.e. pressure pulsations and cavitation in flow systems, vibrations of structural elements of hydrounits, noise and others.

We hope that the articles collected in the Special Issue "New Insights into Hydropower and Hydraulic Machinery" will enable the dissemination of contemporary, advanced scientific and technical achievements and a wide discussion on the possibilities of using them for the development of hydropower, both today and in the future. They will present new approaches and initiatives as well as innovative technical solutions allowing for the efficient use of water resources, contributing to their sustainable management. We hope that the papers collected in this issue of the Applied Sciences Journal will focus on the challenges related to the design, construction and operation of hydropower machines and devices in connection with the current trends in the hydropower sector. We believe that the published papers will contribute to the acceleration of the development of hydropower and its wider use to ensure energy security and protection of water resources.

Dr. Mariusz Lewandowski
Prof. Dr. Adam Adamkowski
Guest Editors

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• water reservoirs
• penstocks
• hydraulic machines
• pumps as turbines (PAT)
• discharge measurements
• efficiency measurements
• optimal operation
• dynamic diagnostics
• mechanical vibrations
• hydraulic transients
• cavitation
• abrasive erosion
• corrosion
• new material in hydropower
• fish-friendly solutions