How to sizing an Archimedean Screw turbine with HPP-Design

What is an Archimedean Screw Generator (ASG)?

An ASG is a positive displacement machine, which consists of a rotor in the shape of an Archimedean screw rotating in a semicircular trough. By filling the buckets of the screw, incoming water provides a tangential force, thus a torque, on the shaft of the turbine.

Thanks to its robust design, cheap construction, good efficiency, and tolerance to floating object transit, the ASG is a good solution for low heads and medium discharges.

When to use an ASG?

An ASG can be used for heads ranging from 1 to 5 m and discharges from 0.5 to 7 m³/s, as depicted by the red line in the picture below. Multiple ASGs are commonly used in side-by-side arrangement in order to increase the plant discharge, up to 30 m³/s (green area in the picture below).

How to select an Archimedean Screw Generator in HPP-design?

Just create a new sizing, enter a value of net head [H] and maximum discharge [Q] suitable for ASGs (e.g. H=3m, Q=5 m³/s), click “create sizing” and select the icon of the Archimedean Screw.

Why it is not possible to select the ASG icon?

Check the values of net head [H] and maximum discharge [Q] you entered, probably they are out of the range of the ASG (H = 1-5 m and Q = 0.5-30 m³/s)

Why there is more than one possible solution when I select ASG icon?

Because the required discharge can be achieved with different numbers of generators. More generators require more space and the cost of the plant is normally higher, nevertheless the single generator will be smaller and easier to transport, plant efficiency higher at partial loads and the maintenance easier.

What is the “Suggested configuration”?

Is the solution that allows minimizing the number of generator in the plant, thus the width and the total cost.

Why is there a limit in the maximum discharge of the single Archimedean screw?

Because of construction and transportation limits. Moreover, the lower the available head, the lower the maxim discharge per turbine due to aspect ratio constraints.

Once I have selected the turbine on the list, are there other possible options?

Yes, on the sizing-detail page, you can choose between a fixed-speed and a variable-speed regulation of the turbine. You will see a change in the part load efficiency on the graph.

Which turbine should I choose if both Kaplan and ASG are available?

What are pro and cons of an ASG as compared to a Kaplan turbine?

For heads between 2 and 7m, both a Kaplan turbine and an ASG are available. The first one has a slightly higher efficiency and smaller dimensions. On the other side, ASGs are frequently cheaper (lower CAPEX and OPEX), easier to inspect and allow the transit of debris without the needing of an automatic trash rack.

Through HPP-Design you can compare multiple solutions, check the features of each machine and ask for a quotation!

What is the expected efficiency of an ASG?

Despite its simple construction, an ASG is able to achieve hydraulic efficiencies over 80%. Main losses are related to hydraulic frictions, turbulence at intake and discharge section and water leakages between the screw and the through. Provided that water velocity in the screw is one order of magnitude lower then in reaction turbines, friction losses and kinetic energy loss at discharge are low.

How to regulate an ASG?

An ASG is rather different from traditional turbines, such as Kaplan, Francis or Pelton, since it is a positive displacement machine. Thus, it is regulated without the need of adjustable blades or gates. The screw self-adapt to the decreasing flowrate through a lower filling of the buckets and a lower water level at intake section.

A speed regulation through an inverter system allows achieving higher efficiencies at partial loads by keeping a constant level at intake section. 

Is an ASG reliable?

Although ASG is a relative newcomer to the hydro world, having only arrived on the scene over the last 25 years, they have been around for many centuries as pumps where tens-of-thousands unit have been installed worldwide, particularly in sewage treatment works. The same manufacturers that dominate the pump market are now the main suppliers into the hydropower market as well, providing reliable machines with very low operative costs.

HPP- Design will help you in this choice, providing accurate sizing data and energy calculation for every type of turbine!

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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.

For information, contact us or send an email, you can also read our FAQ. HPP-Design develops day by day and I advise you to register for our newsletter to be up to date on new releases.

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