Hydro Power Basics

Micro Hydropower

 

How it works

 

Hydropower is based on simple concepts. Moving water turns a turbine, the turbine spins a generator, and electricity is produced. Many other components may be in a system, but it all begins with the energy already within the moving water.

Water power is the combination of head and flow. Both must be present to produce electricity. In a typical hydro system water is diverted from a stream into a pipeline, where it is directed downhill and through the turbine (flow). The vertical drop (head) creates pressure at the bottom end of the pipeline. The pressurized water emerging from the end of the pipe creates the force that drives the turbine. More flow or more head produces more electricity. Electrical power output will always be slightly less than water power input due to turbine and system inefficiencies.

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Head is water pressure, which is created by the difference in elevation between the water intake and the turbine. Head can be expressed as vertical distance ( meters). Net head is the pressure available at the turbine when water is flowing, which will always be less than the pressure when the water is turned off (static head), due to the friction between the water and the pipe. Pipeline diameter has an effect on net head.

Flow is water quantity, and is expressed as "volume per time," such as cubic feet per second (cfs), or liters per minute (lpm). Design flow is the maximum flow for which your hydro system is designed. It will likely be less than the maximum flow of your stream (especially during the rainy season), more than your minimum flow, and a compromise between potential electrical output and system cost.

When is hydropower micro?

The definition of micro hydropower varies in different countries and can even include systems with a capacity of a few megawatts.  In some cases up to  a rated capacity of 300 kW is considered as Microhydro  because this is about the maximum size for most stand alone hydro systems not connected to the grid, and suitable for "run-of-the-river" installations. 

But, In general Micro hydro is a term used for hydroelectric power installations that typically produce 10 to 100 kW of power . They are often used in water rich areas as a Remote Area Power Supply (RAPS).

 

Classification of Hydropower by size

Large hydro	More than 100 MW and usually feeding into a large electricity grid
Medium-hydro	15 - 100 MW - usually feeding a grid
Small-hydro	1 - 15 MW - usually feeding into a grid
Mini-hydro	Above 100 kW, but below 1 MW; either stand alone schemes or more often feeding into the grid
Micro-hydro	From 5kW up to 100 kW; usually provided power for a small community or rural industry in remote areas away from the grid.
Pico-hydro	From a few hundred watts up to 5kW

 

Micro hydro is perhaps the most mature of the modern small-scale decentralized energy supply technologies used in developing countries. There are thought to be tens of thousands of plant in the “micro” range operating successfully in China[1], and significant numbers are operated in wide ranging countries such as Nepal, Sri Lanka, Pakistan, Vietnam and Peru. This experience shows that in certain circumstances micro hydro can be profitable in financial terms, while at others, even unprofitable plant can exhibit such strong positive impacts on the lives of poor people. 

Components of a Micro Hydro system

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Suitable conditions for micro-hydro power

The best geographical areas for exploiting small-scale hydro power are those where there are steep rivers flowing all year round, for example, the hill areas of countries with high year-round rainfall, or the great mountain ranges and their foothills, like the Andes and the Himalayas. Islands with moist marine climates, such as the Caribbean Islands, the Philippines and Indonesia are also suitable. Low-head turbines have been developed for small-scale exploitation of rivers where there is a small head but sufficient flow to provide adequate power.

To assess the suitability of a potential site, the hydrology of the site needs to be known and a site survey carried out, to determine actual flow and head data. Hydrological information can be obtained from the meteorology or irrigation department usually run by the national government. This data gives a good overall picture of annual rain patterns and likely fluctuations in precipitation and, therefore, flow patterns. The site survey gives more detailed information of the site conditions to allow power calculation to be done and design work to begin. Flow data should be gathered over a period of at least one full year where possible, so as to ascertain the fluctuation in river flow over the various seasons. There are many methods for carrying out flow and head measurements and these can be found in the relevant texts.

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Measuring Head & Flow

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Before designing a hydro system or estimating how much electricity it will produce, four essential measurements should be taken:

• Head (the vertical distance between the intake and turbine)

• Flow (how much water comes down the stream)

• Pipeline (penstock) length

• Electrical transmission line length (from turbine to home or battery bank)

Head and flow are the two most important facts of a  hydro site.  This will determine everything about the hydro system—pipeline size, turbine type, rotational speed, and generator size. Even rough cost estimates will be impossible until  head and flow are measured. Accuracy is important when measuring head and flow. Inaccurate measurements can result in a hydro system designed to the wrong specifications, and one that produces less electricity at a greater expense.

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 Calculation o Hydro Power

Power is measured in watts or Kilowatts.

I kW = 1000W

Flow: 1 m³/s = 1000 l/s

Gross head = Head of the water

Net head : Deducted the energy loss from forebay through penstock to hydro turbine, a little smaller than gross head.

The hydro power in a stream or a river can be calculated as follows:

Hydro power ( kW ) = Net head ( m ) x Flow ( m³/s ) x Gravity ( 9.81 m/s² )*

 

( 9.81 is acceleration due to gravity which can be assumed to be constant )

 

For example, If the available flow is 0.15 cubic meters per second and the net head is 4.7 metres, the hydro power is= 4.7 x 0.15 x 9.81 = 6.9 kW

If the flow in litres per second ( l/s ) is used, then the power will be given in watts instead of kilowatts.

To estimate the electrical power produced by a generator, the efficiency of the system must be taken into consideration. Efficiency is the word used to describe how well the power is converted from one form to another. A turbine that has an efficiency of 70 % will convert 70 % of the hydraulic power into mechanical power ( 30% being lost ). The system efficiency is the combined efficiency of all the processes together. The system efficiency for electricity generation using micro hydro is typically between 50% and 60%.

 

Electrical power = Hydro power X System efficiency

 

I.e. as a rough estimate, if there is found to be 6.9 kW of hydro power in a small river, the electrical power is = 6.9 x 50% = 6.9 x 0.5 = 3.45 kW

(the theoretical power must be multiplied by 0.50 for a more realistic figure).

If a machine  is operated under conditions other than full-load or full-flow then other significant inefficiencies must be considered. Part flow and part load characteristics of the equipment needs to be known to assess the performance under these conditions. It is always preferable to run all equipment at the rated design flow and load conditions, but it is not always practical or possible where river flow fluctuates throughout the year or where daily load patterns vary considerably.

Depending on the end use requirements of the generated power, the output from the turbine shaft can be used directly as mechanical power or the turbine can be connected to an electrical generator to produce electricity. For many rural industrial applications shaft power is suitable (for food processing such as milling or oil extraction, sawmill, carpentry workshop, small scale mining equipment, etc.), but many applications require conversion to electrical power. For domestic applications electricity is preferred. This can be provided either:

• directly to the home via a small electrical distribution system or,

 • can be supplied by means of batteries which are returned periodically to the power house for recharging - this system is common where the cost of direct electrification is prohibitive due to scattered housing (and hence an expensive distribution system),

Where a generator is used alternating current (a.c.) electricity is normally produced. Singlephase power is satisfactory on small installations up to 20kW, but beyond this, 3-phase power is used to reduce transmission losses and to be suitable for larger electric motors. An a.c. power supply must be maintained at a constant 50 or 60 cycles/second for the reliable operation of any electrical equipment using the supply. This frequency is determined by the speed of the turbine which must be very accurately governed.

The economics  of microhydro systems

Normally, small-scale hydro installations in rural areas of developing countries can offer considerable financial benefits to the communities served, particularly where careful planning identifies income-generating uses for the power.

The major cost of a scheme is for site preparation and the capital cost of equipment. In general, unit cost decreases with a larger plant and with high heads of water. It could be argued that small-scale hydro technology does not bring with it the advantages of 'economy of scale', but many costs normally associated with larger hydro schemes have been 'designed out' or 'planned out' of micro hydro systems to bring the unit cost in line with bigger schemes. This includes such innovations as:

• using run-of-the-river schemes where possible - this does away with the cost of an expensive dam for water storage • locally manufactured equipment where possible and appropriate

• use of HDPE (plastic) penstocks where appropriate

 • electronic load controller - allows the power plant to be left unattended, thereby reducing labour costs, and introduce useful by-products such as battery charging or water heating as dump loads for surplus power; also does away with bulky and expensive mechanical control gear

 • using existing infrastructure, for example, a canal which serves an irrigation scheme

• siting of power close to village to avoid expensive high voltage distribution equipment such as transformers

• using pumps as turbines (PAT) - in some circumstances standard pumps can be used 'in reverse' as turbines; this reduces costs, delivery time, and makes for simple installation and maintenance

• using motors as generators - as with the PAT idea, motors can be run 'in reverse' and used as generators; pumps are usually purchased with a motor fitted and the whole unit can be used as a turbine/generator set

• use of local materials for the civil works

• use of community labour • good planning for a high plant factor (see above) and well balanced load pattern (energy demand fluctuation throughout the day)

• low-cost connections for domestic users (see following chapter on this topic)

• self-cleaning intake screens - this is a recent innovation which is fitted to the intake weir and prevents stones and silt from entering the headrace canal; this does away with the need for overspill and desilting structures along the headrace canal and also means that, in many cases, the canal can be replaced by a low-pressure conduit buried beneath the ground - this technology is, at present, still in its early stages of dissemination

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