The optimal head and volume flow rate for hydropower are determined through axiomatic and analytical considerations by Pelz in 2011. Based on these results the Cordier-diagram and its asymptotes are discussed. The technological progress causes that nowadays machines of high specific speed and hence small specific diameter, like Kaplan turbines, can be used, delivering the same power as the antiquated machines of low specific speed, like water wheels, did during the last centuries. Therefore the power-specific investment costs decrease. Within the legal borders, which are given by the legislative organs and due to the responsibility towards the environment, systems of robust technologies with minimal impact on nature at the same time are required. Considering all this, the present paper proposes an innovative concept for a fish permeable hydraulic machine for small hydropower.

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