The type of turbine that can be used in a micro hydro installation depends on different factors such as, head of water, the volume of flow, and such factors as availability of local maintenance and transport of equipment to the site.
A turbine converts energy from water falling into a rotating shaft power; the selection of a hydro turbine depends on the site characteristics and the head and flow available. The desired running speed of the generator or other devices in the turbine also plays a vital role in the selection process. However other conditions such as weather the turbine is expected to produce power under part-flow conditions could also be considered. All turbines have a power-speed characteristic that will tend to run most efficiently at a particular speed, head and flow combination.
Impulse Turbine and Reaction Turbine
A turbines design speed is largely determined by the head with which it operates. Turbines can either be classified as impulse turbines or reaction turbines. In the impulse turbine, the turbine runner operates in air and is turned by one or multiple jets of water which make contact with the runner blades. On the other side in a reaction turbine, the turbine runner is fully immersed in water and is enclosed in a pressure casing, the runner blades are angled so that pressure differences across them create lift forces, like those on aircraft wings, which cause the runner to rotate.
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The rotating element, also known as the ‘runner’ of a reaction turbine are fully immersed in water and enclosed in a pressure casing. The runner blades are profiled in a mechanism that pressure differences across them imposes lift forces that make the runners to rotate , like aircraft wings. In contrast, an impulse turbine runner operates in air driven by jets of water. This makes the water to remain in atmospheric pressure before and after making contact with the runner blades. In this case a nozzle converts the pressurised low velocity water into a high speed jet. The runner blades deflect the jet so as to maximise the change of momentum of the water and thus maximising the force on the blades.
Impulse are generally more suitable for micro hydro applications compared with reaction turbines because of the following advantages:
- They have a greater tolerance to sand and other particles in water.
- There is a better availability of spare parts
- There is no pressure seal around the shaft
- They are easier to fabricate and maintain
- They have a better part-flow efficiency.
However with the advantages, the main disadvantage for the impulse turbines is that they are mostly unsuitable for low-head sites because of their low specific speeds too great an increase in speed would be required of the transmission to enable coupling to a standard alternator. The crossflow, Turgo and multi-jet Pelton are suitable at medium heads.
It consists of a set of specially shaped buckets mounted on a periphery of a circular disc. . It is turned by jets of water which are discharged from one or more nozzles and strike the buckets. The buckets are split into two halves so that the central area does not act as a dead spot incapable of deflecting water away from the oncoming jet. The cutaway on the lower lip allows the following bucket to move further before cutting off the jet propelling the bucket ahead of it and also permits a smoother entrance of the bucket into the jet. The Pelton bucket is designed to deflect the jet through 165 degrees (not 180 degrees) which is the maximum angle possible without the return jet interfering with the following bucket for the oncoming jet.
In large scale hydro installation Pelton turbines are normally only considered for heads above 150 m, but for micro-hydro applications Pelton turbines can be used effectively at heads down to about 20 m. Pelton turbines are not used at lower heads because their rotational speeds becomes very slow and the runner required is very large and unwieldy. If runner size and low speed do not pose a problem for a particular installation, then a Pelton turbine can be used efficiently with fairly low heads.
This is an impulse machine similar to a Pelton turbine but which was designed to have a higher specific speed. In this case the jets aimed to strike the plane of the runner on one side and exits on the other. Therefore the flow rate is not limited by the discharged fluid interfering with the incoming jet (as is the case with Pelton turbines). As a consequence, a Turgo turbine can have a smaller diameter runner than a Pelton for an equivalent power. With smaller faster spinning runners, it is more likely to be possible to connect Turgo turbines directly to the generator rather than having to go via a costly speed-increasing transmission.
Like the Pelton, the Turgo is efficient over a wide range of speeds and shares the general characteristics of impulse turbines listed for the Pelton, including the fact that it can be mounted either horizontally or vertically. A Turgo runner is more difficult to make than a Pelton and the vanes of the runner are more fragile than Pelton buckets.
Also called a Michell-Banki turbine a crossflow turbine has a drum-shaped runner consisting of two parallel discs connected together near their rims by a series of curved blades. A crossflow turbine always has its runner shaft horizontal (unlike Pelton and Turgo turbines which can have either horizontal or vertical shaft orientation).
In operation a rectangular nozzle directs the jet onto the full length of the runner. The water strikes the blades and imparts most of its kinetic energy. It then passes through the runner and strikes the blades again on exit, impacting a smaller amount of energy before leaving the turbine. Although strictly classed as an impulse turbine, hydro dynamic pressure forces are also involved and a mixed flow definition would be more accurate.
Small-scale hydro power plants range up to an unit output of 15 MW for Axial and Kaplan turbines and an output of 30 MW for Francis and Pelton turbines while the mini compact hydro installations include Kaplan and Axial units (20 kW – 1,500 kW), Francis units up to 3,000 kW as well as Pelton units up to 5,000 kW.
Compact Hydro stands for “water to wire” solutions based on pre-designed modular components and offers a single source of supply for the entire electromechanical equipment, as well as workshop-tested units, reduced dimensions for transportation and short installation times. For Mini Compact Hydro the level of standardization and parametrization was deeper expanded, keeping the same level of performances and quality, for lower investment costs and simplifying operation and maintenance of small hydropower plants.
Features for small and mini hydropower plants
Compact Hydro services include engineering, design, model testing, purchasing, manufacturing, assembly, project management, site management, installation, commissioning, testing, and staff training for all mechanical and electrical equipment:
- Hydraulic turbines
- Digital speed governors and HPUs
- Shut-off valves
- Synchronous and asynchronous generators
- Automation, control, protection, excitation and synchronization equipment
- Electrical power system
All guarantees concerning efficiency, output and resistance to cavitations are backed up by model tests in the ANDRITZ Hydro laboratories for three to six bladed runners, double or single regulated, horizontal or vertical axis.
The axial flow turbines are based on a modular design concept with the same key components used for different turbine types.
- Mini compact from 20 to 1,500 kW and heads from 2 to 26 m
Small hydro from about 300 kW up to 15,000 kW and head up to 45 m
- Belt driven bulb units for very low head application with a head up to 5 m and outputs up to 600 kW
- Bevel gear bulb units for head up to 12 m and outputs up to 2,600 kW
- ECO bulb units for a net head up to 20 m and outputs up to 10,000 kW
- Bulb units for a net head up to 15 m and outputs up to 15,000 kW
- PIT units for a net head up to 12 m and outputs up to 10,000 kW
- Compact Axial Turbine units (CAT) units for a net head up to 45 m and outputs up to 15,000 kW
- S-Type units for a net head up to 35 m and outputs up to 10,000 kW
Schattenhalb 3 hydropower plant, Switzerland
The features of the Francis turbines ensure a wide application field according to specific needs. Guarantees of efficiency, output and resistance to cavitation for single and double discharge runners are based on model tests from the ANDRITZ Hydro laboratories.
Francis turbines propose the highest efficiency. As far as the standard concept is applicable, the runner is directly fitted on the generator shaft, which ensures a compact construction and low maintenance.
For smaller units the turbine-generator aggregate is delivered as a complete assembly package ready for immediate installation.
- Mini hydro from 20 to 3,000 kW and heads from 10 to 150 m
- Small hydro from about 500 kW up to 30 MW and heads up to 300 m
- Horizontal or vertical axis
- Single or double discharge runners
- Spiral intake or open flume
Pelton units up to 30 MW
Guarantees of efficiency and output are based on model tests from the ANDRITZ Hydro laboratories for one to three nozzles in horizontal and three to six nozzles in vertical arrangement.
The flow control of the Pelton turbine, via operation of nozzles and deflectors, provides a high efficiency along the flow range of operation as well as limited overpressure effect over the penstock.
Over the years, the Compact Hydro Division has also developed a strong expertise in special applications for hydraulic turbines, like the energy recovery in Reverse Osmosis desalination process or in the drinking and waste water systems.
- Mini hydro from 100 to 5,000 kW and heads from 60 to 800 m
- Small hydro up to 30 MW and heads from 50 to 1,000 m
- Horizontal, 1 to 3 nozzles
- Vertical, 1 to 6 nozzles
Stave hydropower plant in Canada
- Synchronous or asynchronous type
- Horizontal or vertical shaft
- Air or water cooled
- Brushless excitation
Electrical equipment includes solutions for unit control, protection systems, and cabling and power transmission
To meet the requirements of the market in conjunction with the Compact Hydro turbines, an overall concept has been implemented which comprises both the mechanical and electrical equipment of the powerhouse.
Following the modular concept of the mechanical equipment, we also implement the same approach with the balance of plant:
- Generators with AVR
- Control-protection-measuring system
- Digital turbine governor
- Supervisory Control and Data Acquisition (SCADA)
- AC-DC distribution
- Auxiliary transformer
- LV and MV-switchgear
- Transmission line
This means that the Compact Hydro division is in a position to tender both the hydro-mechanical equipment, as well as the total or partial electrical equipment up to a unit output of 30 MW.