Modern concepts of low-head small hydropower plant.

Due to the ever-increasing demand for electricity and the high exploitation of water resources, especially in Central and Western Europe, resources with low hydropower potential are increasingly being used. These sites are characterized by a very low head (up to 5 meters) and are often placed in urban areas. Aside from required effective technical-economic design (the goal is to minimize payback period) it is essential to consider the type of surrounding development and design the power plant so that it would not have disruptive impression and minimize the impact on the aesthetic and ecological character of the locality such as noise minimization, suitable design of the fish pass enabling fish migration, etc. This article will discuss the design of the hydropower plant in urban area, which is located in close proximity to the historic city. The first concepts were to place the power plant on the right ban of the river, away from the historic city. A variant with two modern propeller axial-type vertical turbines was designed. These low specific speed long-bladed turbines are much more fish-friendly than conventional Kaplan turbines. The design of powerplant also originally included a new fish ladder enabling fish to migrate upstream and downstream. This variant was the most suitable of all assessed variants, both from the technical-economic point of view (investment versus energy production) and from the point of view of minimizing the disruptive attribute of the site (the power plant would be located on the right bank and except the aerial view the power plant would be practically invisible). The historical survey of the right bank has shown the existence of well-preserved historical architectural elements and therefore, for reasons of monument protection, the location of the power plant on the right bank is not possible. For this reason, a variant proposal for the location of a power plant with a similar installed capacity in the left weir block was proposed. This would be the concept of a classic overspilled power plant with bulb turbines. An even more modern concept corresponds to a submerged, "invisible" hydropower plant with submerged turbine-generator units without any need to build a standard "dry machine room". The strongest aspect of this concept is the invisibility of the power plant, which is an advantage in terms of the absence of disruptive attribute of the character of the site (located in close vicinity of the historic city). Other advantages are keeping high flood capacity of the weir profile with submerged hydropower plant and maintaining the existing fish pass on the left bank. The only disadvantage of this solution is the need to remove the entire left weir block and associated financial demands and technical solution of the construction site in the riverbed. It is important to emphasize that this solution will not disturb the character of the river in the long run. [ABSTRACT FROM AUTHOR]