Monday, November 23, 2015

Efficiency of hydroelectric turbines and HPP-Design

Many hydroelectric plants are evaluated according to the turbine’s performance, and many tenders are won (or lost…) for a 0,1% of efficiency. But how important is the efficiency in a hydroelectric power plant?
Talking about large size hydroelectric plants, performance is a crucial aspect, or possibly the only one. An efficiency increase of just 0,1% in a 100MW turbine causes an increase in the annual production at about 500 MWh / year, with a revenue increase of approximately $ 25,000 / year and $ 500,000 over 20 years of operation. For this reason, in large-scale hydropower plants advanced design solutions of the turbine are preferred, in order to achieve the maximum efficiency.
On the other side, for small hydropower plants, the achievement of the maximum efficiency is no longer aimed, since an efficiency increase does not always justify the increased costs of design and production of the turbine. With an efficiency increase of 0.1% in a 100 kW turbine, the power plant production increases at about 5000 kWh / year, with a revenue increase of about $ 25 / year and $ 500 over 20 years of operation. It is hence obvious that the economic sustainability of the most complex design and construction solutions for turbines is not justified by the achieved revenues.
Moreover, another extremely important aspect must be taken into account. Even disregarding the economic aspect (which is not at all negligible) and adopting advanced design and construction solutions, efficiency values ​​comparable with those of large-scale hydropower plants cannot be achieved anyway. The reason is related to internal machine losses and to the so-called “scaling factor". Without going into details about fluid mechanics and turbo-machinery design, it is possible to explain this concept in a simplified way.
The hydraulic losses linked to the friction strains between fluid and wet surfaces (impeller and volute), depend on the fluid velocity and on the friction factor f (see the figure below - Moody diagram) which in its turn depends on the characteristics of the flow field (laminar, turbulent or fully developed turbulent expressed by the Reynolds number - Re in the figure below), and on the relative roughness (ε / d in the figure below), defined as the ratio between the absolute roughness of the wet surface ε and the turbine diameter d. The roughness of the surface depends on the manufacturing process, which does not significantly vary along with the machine size. As a consequence, small machines are characterized by greater values of the relative roughness ε / d and therefore by greater values of the friction factor and of the linked hydraulic losses.
For this reason, in small size plants, advanced design and construction solutions not only are unprofitable, but wouldn’t even allow to achieve efficiency values ​​comparable with those of large-scale plants. In literature some correlations have been proposed for evaluating the variation in maximum achievable efficiency between machines having different sizes, but these correlations should be applied with due caution, taking into account the turbine geometry and expected characteristics of the flow field inside the turbine. 
HPP-Design considers all these aspects related to fluid mechanics and turbomachinery design. The obtained efficiency value can be used to calculate the plant production or to establish a baseline for the technical specifications of a tender, but does not replace the efficiency value provided and guaranteed by turbine manufacturers in the power plant construction phase. HPP-Design will give you a reasonable and accurate reference value to build your own project.

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Thanks to prof. Giovanna Cavazzini of the University of Padua for her contribution.

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