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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Doubts or questions? Read our [FAQ]

How to sizing a Kaplan turbine with HPP-Desing

 

What is a Kaplan turbine?

A Kaplan turbine is the most common type of propeller turbine, in which the angle of the blades and the gates to the flow can be adjusted. This type of turbine is most frequently used in the low- to moderate-head range and medium- to high- discharges.

 

When to use a Kaplan turbine

A Kaplan turbine can be used for heads ranging 2 to 50 m and discharges 1 to 100 m³/s (see the yellow line in the picture). Multiple turbines allow to increase the total discharge.

 

 

How to select a Kaplan turbine in HPP-design?

Just create a new sizing, enter a value of net head [H] and maximum discharge [Q] suitable for Kaplan turbines (e.g. H=20m, Q=10 m³/s), click “create sizing” and select the icon of the Kaplan turbine.

 

Why is not possible to select the Kaplan icon?

Check the values of net head [H] and maximum discharge [Q] you entered, probably they are out of the range of the Kaplan turbine ( H = 2 to 50 m and Q = 1 to 100 m³/s).

 

Why there are two numbers on the icon of Kaplan turbines?

The first refers to the number of available turbines without the gearbox while the second, if present, refers to the number of available turbines with the gearbox.

 

What is the “enable gearbox” switch?

When switched on it allows to use a gearbox between the turbine and the generator in order to better match the optimal speed of the turbine. The switch is enabled only for turbines with nominal power below a given value, when a low rotation speed is suggested for efficiency reasons. For high nominal powers a multiple pole generator is more effective and less expensive.

 

What is the “Suggested configuration”?

Is the optimal solution in terms of efficiency and it’s highlighted in green in the list.

 

Once I’ve selected the turbine on the list are there other possible options?

Yes, on the sizing-detail page you can choose the regulation strategy of the turbine and the type of generator.

 

How does the regulation strategy affects my turbine?

There are different possibilities concerning the regulation strategy:

·         Full Kaplan

·         Semi Kaplan rotor

·         Semi Kaplan stator

·         Helix

From the top to the bottom the equipment becomes cheaper while the efficiency at partial loads decreases, as you can see on the efficiency graph just below the sizing details.

 

How does the choice of generator affects my turbine?
For small powers only asynchronous generator are available, while for high power only synchronous are adopted. In between the user can choose a synchronous or asynchronous generator, the first one being capable of reactive power control and a bit more efficient, while more expensive than the second one.
 

HPP- Design will help you in this choice, providing accurate sizing data and energy calculation for every type of turbine!
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Doubts or questions? Read our [FAQ]

Hydraulic turbine range

 

This is the  range of application for the main hydraulic turbines. Have you seen how complex and overlapped it is? For a given head and discharge even three or more turbines could be suitable.

So how to choose the right one?

A good design should consider, alongside the nominal efficiency, the following aspects:

• Plant layout for the given location and layout related costs
• Annual average efficiency, matching the partial load efficiency curve with discharge and head variation over time
• Cost of the turbine and the mechanical equipment
• Operation and maintenance costs
• Water screen requirements as compared to the amount of debris carried by the river

By carefully considering all those aspects one can choose the optimal turbine and layout in order to maximize the energy and economic performance of the plant.

HPP- Design will help you in this choice, providing accurate sizing data and energy calculation for every type of turbine!
Need more information? You can follow us on Facebook, Twitter or send us an email.

Doubts or questions? Read our [FAQ]

 

 

New energy calculator in HPP-Design

A new HPP-Design feature is online: the Energy Calculator.

What can you do now?

It will be possible to calculate directly on line the production of an HPP.

We have reversed the concept of calculation: usually the flow of information, starting from a very thorough hydrogeological study, provides the calculation of a duration curve. Knowing the geodetic head and after some technical and economic considerations, we can define the turbine’s project data.


With HPP-Design, on the other hand, you can do the opposite: from the project data of the turbine, such as discharge and head, the tool will automatically set a hypothetical duration curve (you can choose between two pre-set curves) and then will calculate energy production if the hydroelectric plant has the chosen duration curve.
Simplifications introduced are:

• Number of input points. We decided to use only 12 points identified as monthly averages;
• Turbine efficiency is included (even with the flow variation), but not the generator’s and the transformer’s. At this stage efficiency of the generator and the transformer are equal to 1;
• The head is constant.

You can also change the default values to define the flow duration curve fitting your HPP.

HPP-Design continues to implement new functions. Our goal is the study and development of new deployment to help designers and developers.

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Doubts or questions? Read our [FAQ]

Energy calculator in HPP-Design

E' online la nuova funzione di HPP-Design: Energy Calculator.

Cosa puoi fare ora?

Finalmente è possibile calcolare in maniera molto semplice la produzione di un impianto idroelettrico direttamente online. 

Abbiamo ribaltato il concetto di calcolo: solitamente il flusso di informazioni prevede, a valle di uno studio idrogeologico molto approfondito, il calcolo di una curva di durata. Noto il salto geodetico e dopo alcune considerazioni di tipo tecnico ed economico, si arriva alla definizione dei dati di progetto della turbina.

Con HPP-Design, invece, potrai fare il contrario: partendo dai dati di progetto della turbina, cioè salto e portata, il tool imposterà automaticamente una curva di durata ipotetica (è possibile scegliere fra due curve pre-impostate) e verrà calcolata la produzione di energia se l'impianto idroelettrico avesse la curva di durata proposta. 

Le semplificazioni introdotte sono:

• Numero di punti inseriti. Abbiamo deciso di usare solo 12 punti identificati come le medie mensili. Un valore che normalmente si fa presto ad avere a disposizione;
• E' compreso il rendimento della turbina al variare della portata e non quello del generatore e del trasformatore. Si considerano (in questa fase) rendimento del generatore e del trasformatore pari a 1;
• Si considera (in questa fase) il salto costante. Non è compresa una variazione di salto.

Inoltre è possibile modificare i valori preimpostati andando a definire la propria curva di durata aderente all'impianto scelto.

HPP-Design è in continua crescita. Il nostro obiettivo è lo studio e lo sviluppo di nuove implementazione per aiutare i progettisti, gli sviluppatori e, in generale, gli operatori del settore.

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How to choose a hydroelectric turbine, a specific speed question!

The two main data needed to design a turbine are the maximum flow rate and the net head associated to this flow. However, these two values do not allow to uniquely define the type of turbine that must be designed; the designer is required to choose a third fundamental parameter: the number of revolutions.

Then it will be possible to define what we call the turbine specific speed.

But what is the specific speed? Leaving aside the theory about mechanical similarities, let's briefly explain what it is.

The specific speed of a turbine is defined as:

Where

    Q    maximum discharge     [m^3/s]

    H    net head  [m]

    g     gravity  (normal 9.806[m/s^2])

angular velocity of turbine [rad/s] calculated as:  

This parameter relates to each other the characteristic data of the turbine, and has the peculiarity of identifying families of geometrically similar machines (and then to scale them), having very different powers and sizes, but with one fundamental common characteristic: they are part of the only family of turbines that can process the available head and the discharge with the highest performance possible.

With simple calculations you can use the characteristic data of the turbine (H, Q and n) to find a value of specific speed  that uniquely identifies the most efficient type of turbine and its main dimensions. All this thanks to the experience gained in the design of more than a century of hydropower construction and excellent theoretical studies that have enabled the development of the theory of similarity at the basis of the specific speed calculations.

This does not mean that all turbines manufacturers build the same equipment once given the same typical number. After choosing the type of turbine and the main dimensions defined, each manufacturer has developed its own geometry and parameters that differentiate the turbines in terms of operation, reliability, cost and efficiency. Over the years, various design schools have proposed many definitions of the specific speed. The UNI-ISO has tried to standardize them into a single parameter, but in fact the definitions remain different.

In HPP-Design we use the parameter k, seen above, and nq

In the picture, every value of typical number of machine k (or nq) corresponds to a very specific type of turbine, exception made for some overlapping areas in which there is no univocal choice and the experience has shown that they can adapt well to different types of turbines (Pelton / Francis and Francis / Kaplan).

The question that arises is: how is this value really used?

Known the head and discharge, a preliminary study is made to define the appropriate number of revolution, taking into account some possible rotation speeds, and then calculating the relative specific speed which corresponds to a well-defined design solution. The possible solutions are then compared, in terms of performance, cavitation behaviour, main dimensions, etc.  and finally the choice is made for the solution that best suits the specific project. Once the rotation speed is fixed, starting from the specific speed it is possible to choose the type of turbine and start the detailed design.

Hpp-design is the tool that helps the designer to make a preliminary choice by comparing these elements directly into the chosen page. Try it out!

For information contact us here or send us an email here, you can also read the FAQs. Hpp-design is constantly updating and I recommend you to register to our newsletter here to keep up to date on new releases.