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Introduction To Micro Hydropower

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Fig 1 - General layout of a hydropower plant

Fig 1 - General layout of a hydropower plant

A hydroelectric plant converts the potential energy of water into electricity by the use of flowing water.

This water flows in water streams with different slopes giving rise to different potential for creating heads (size of fall), varying from river to river.

The capacity (power) of a plant depends on the head (change in level) and flow as a result of the hydrology in the catchment area of a river.

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Medium and high head schemes:

This type of plant typically uses weirs to divert water to the intake. From there it is led to the turbines via a pressure pipe or penstock. An alternative to penstocks, which in many cases is more economic, relies on a canal with reduced gradient running alongside the river. The canal carries the water to the pressure intake, and then, in a short penstock, to the turbines.

Categories of heads of the streams:

Categories of heads of the streams

Low head schemes:

This kind of project is appropriate to river valleys, particularly in the lower reaches. Either the water is diverted to a power intake with a short penstock, or the head is created by a small dam, complete with integrated intake, powerhouse and fish ladder.

What are the main types of hydro schemes?

There are three main categories of hydroschemes, as described bellow by IEC (International Electrotechnical Commission):

  • Run-of-river hydro plants use the river flow as it occurs, the filling period of its reservoir being practically negligible. The majority of small hydropower plants are run-of-river plants because of the high construction cost of a reservoir.
  • Pondage hydro plants are plants in which the reservoir permits the storage of water over a period of a few weeks at most. In particular, a pondage hydro plant permits water to be stored during periods of low load to enable the turbine to operate during periods of high load on the same or following days. Some small hydropower plants fall into this type, especially high head ones with high installed capacities (> 1.000 kW).
  • Reservoir hydro plants are plants in which the filling period of the reservoir is longer than several weeks. It generally permits water to be stored during high water periods to enable the turbine to operate during later high load periods. As the operation of these plants requires the construction of very large basins, practically no small or micro hydropower plant is of this type.

What are the typical characteristics of small-sized hydro schemes?

  • Micro hydropower plants up to 100 kW
  • Mini hydropower plants up to 500 kW
  • Small hydropower plants up to 10,000 kW*

Micro-hydro power schemes normally only support investment in large reservoirs if these are built for other purposes in addition to hydropower (e.g. water abstraction systems, flood control, irrigation networks, recreation areas). Nevertheless, there are ingenious solutions for linking and fitting the turbine in waterways designed for other purposes, e.g. schemes integrated with an irrigation canal or a water abstraction system.

Below are a few examples of several possible applications of small, mini and micro hydropower plants.

Schemes integrated with an irrigation canal

EXAMPLE-1 The canal is enlarged to the extent required to accommodate the intake, the power station, the tailrace and the lateral bypass. The scheme should include a lateral bypass to ensure an adequate water supply for irrigation, should the turbine be shut down. This kind of scheme should be designed at the same time as the canal, because the widening of the canal in full operation is an expensive option.

EXAMPLE-2 The canal is slightly enlarged to include the intake and spillway. To reduce the width of the intake to minimum, an elongated spillway should be installed. From the intake, a penstock running along the canal brings the water under pressure to the turbine. The water, once through the turbine, is returned to the river via a short tailrace. As fish are generally not present in canals, fish passes are usually unnecessary.

Schemes integrated in a water supply system

Drinking water is supplied to a city by conveying the water from a headwater reservoir via a pressure pipe. Usually in this type of installation, the dissipation of energy at the lower end of the pipe at the entrance to the Water Treatment Plant is achieved through the use of special valves. The fitting of a turbine at the end of the pipe, to convert this otherwise lost energy to electricity, is an attractive option, provided that waterhammer, which would endanger the pipe, is avoided.
Waterhammer overpressures are especially critical when the turbine is fitted on an old pressure pipe.

Micro hydropower plants at sluice systems

The installation of a small hydropower plant in a sluice system along large rivers can be an interesting multi-purpose use of existing structures dedicated to other purposes. The exploitation for hydroelectric purposes of the head created by a sluice system allows the production of energy by a renewable energy source without further significant environmental impacts. An interesting and recent example of this application is given by a pilot project where a 26 kW turbine unit of four parallel 6.5kW propeller turbines has been inserted in an old sluice constructed for agricultural purposes at Niemieryczow in Poland.

Micro hydropower plants on river stabilization ramps

This rather unusual application is very interesting from an environmental point of view. Ramps are often constructed to stabilize the river course, particularly on fast flowing mountain rivers. The artificial head created by the ramps or by a series of check dams can be exploited for hydroelectric production. Installed power is however generally small since the flows and heads are generally low. Nevertheless this application represents an opportunity to meet the twin objectives of river protection and the use of a renewable energy source for energy production at the same time.

Micro hydropower plants at bed load barriers

Bed load barriers create an artificial head in the watercourse which can be exploited for energy production.

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How to choose a site from a technical point of view?

Apart from the environmental issues, which will be discussed in detail in later chapters, a MHPP should consider the following three main issues if it is to be economically feasible.

Relevant aspects for site evaluation:

Relevant aspects for site evaluation

What does the power of an MHPP depend on?

The amount of electricity generated is the result of the head and the flow rate at a specific site. The power generated also depends on the turbine generator efficiency and pressure losses at the intake and penstock. Moreover, other constraints, such as environmental issues and fisheries, may oblige the developer to leave a minimum flow in the watercourse. It should also not be forgotten that the available energy depends on the day-to-day and year-to-year variations of the flow. The impact of these variations could be very significant, so careful measurements should be made.

How to estimate the power availability in a site:

How to estimate the power availability in a site

What parameters are used in selecting a hydro turbine?

Head, flow and power are the three main technical aspects in selecting a turbine. There are five main turbine types and each might be appropriate to certain physical conditions at each site.
Turbines can be grouped in two main categories:

Action turbines (or impulse turbines)

These only use the speed of the water, i.e. only use kinetic energy. This type of turbine is appropriate for high heads (75 meters to >1000 meters) and small flows.

The most popular such turbines are Pelton Wheels, which have a circular disc or runner with assembled vanes or double-hollow spoons. There are also other models like the Turgo side injection turbine, and the Ossberger or Banki Mitchell double propulsion turbines (these are further described in the text as “crossflow on Banki Mitchel turbine”).

Reaction turbines

This kind of turbine takes advantage of the water speed, and the pressure maintains the flow when contact takes place. The most frequently used are Francis and Kaplan turbines. Usually they have four basic elements: the casing or shell, a distributor, a pad, and the air intake tube.

There are two distinct groups: radial turbines (Francis type) are suitable for operating on sites with a medium head and flow and axial turbines (Kaplan and Propeller type) are appropriate for operation with low heads and high and low flows. Both action and reaction turbines may be used in MHPP.

What are the differences between the turbines?

Pelton turbine: is a typical high head turbine, which can also be used for medium heads, with power ranging from 5 kW to large sizes. This is an easy to use action-type turbine with a high efficiency curve and it has a good response to variations in flow.

Cross Flow or Banki Mitchell turbines: are mainly used at sites where there is low installed power. In general their overall efficiency (around 75-80%) is lower than conventional turbines. They have a good response to variations in flow, which makes them appropriate for work where there is a wide range of flows. They have the advantage of simplicity and ease of maintenance and repair. They are a tried and proven technology which can exploit sites that cannot otherwise be used economically and where, therefore, their limited efficiency is not relevant. They are suitable for low to high head sites from 1 m to 200 m head with flows over 100 l/s.

Francis Turbines: are single regulated turbines more appropriate to use with higher heads given their efficiency.

Propeller turbines: have the advantage of running at high speeds even at low heads. Kaplan Turbine are high efficiency propeller-type turbines, very advanced and consequently quite expensive in investment and maintenance. Their response to different ranges of flow conditions is very good.More is said about propeller turbines in the next chapter.

Pelton turbine

Pelton turbine

Cross Flow or Banki Mitchell turbines

Cross Flow or Banki Mitchell turbines

Francis Turbines

Francis Turbines

Propeller turbines (Kaplan Turbine)

Propeller turbines (Kaplan Turbine)

SOURCE: GUIDE LINES FOR MICRO HYDROPOWER DEVELOPMENT – European Commission

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Comments

One Response to “Introduction To Micro Hydropower”
  1. ecsanyi says:

    There is a sort of myth that micro hydro electricity is unreliable and low quality. What do you think?