Water constantly moves through a vast global cycle, in which it evaporates (due to the activity of the Sun) from oceans, seas and other water reservoirs, forms clouds, precipitates as rain or snow, then flows back to the ocean. The energy of this water cycle, which is driven by the sun, is tapped most efficiently with hydropower. The use of water to generate mechanical power is a very old practice. A flowing stream can make a paddle turn, but a waterfall can spin a blade fast enough to generate electricity. The real key in the magnitude of waterpower is the physical height difference achieved between source and sink - the distance through which the water falls.
Another methods of harnessing water's energy include utilisation of the temperature of ocean water in a thermal transfer process, waves and tidal power. The waves are a direct result of wind, which itself is cause by uneven heating of the ground and oceans by the Sun. Of the several types of hydropower, only the origin of the tides is not related to the Sun. The gravitational pull of the moon is responsible for the tides, which vary in magnitude by location according to latitude and geography.

When considered as a whole, the energy locked within Earth's water cycle and ocean waves is extremely large, but harnessing this energy has proved to be exceedingly difficult. There are many different ways to harness the energy in water. The most common way of capturing this energy is hydroelectric power, electricity created by falling water.

The principal advantages of using hydropower are its large renewable domestic resource base, the absence of polluting emissions during operation, its capability in some cases to respond quickly to utility load demands, and its very low operating costs. Hydroelectric projects also include beneficial effects such as recreation in reservoirs or in tail water below dams. Disadvantages can include high initial capital cost and potential site-specific and cumulative environmental impacts.

Simple water-wheels have been used already in ancient times to  relieve man of some forms of hard manual labour. Water power was probably first mentioned by the Greeks, around 4000 B.C. Greeks used hydro power to turn water wheels for grinding wheat into flour as well. Much later, but long before the advent of the steam engine, the art of building  large water-wheels and the use of considerable power capacities was highly developed. The use of this natural energy resource became  even easier and more widespread with the invention of the  water turbine in the early 1800's and hydro power was quickly adapted from mechanical uses, such as grist mill, to spinning a generator to produce electricity. The first small industries emerged soon after in many regions of Europe and North America,  powered by water turbines.

In later years, when cheap oil became available world-wide, interest in hydro power was lost to a great extent in many areas, but today  the situation is different again. Governments, policy-makers,  funding and lending agencies, institutions and  individuals take a growing interest. This led -and still does -to the reassessment of many projects once found not feasible; the  identification of new sites and potentials, and a number of other  activities related to hydro development.

Amongst renewable energy sources, hydroelectric power seems to be the most desirable for utilities and its economic feasibility has been successfully proven. Power stations with a capacity of up to 10 GW have been built and it is estimated that there are economic resources for 3,000 GW world-wide, compared to 10,000 GW world primary energy consumption. In Europe, however, most hydroelectric potential has been realised, with Norway deriving 98% of its energy consumption from water power and the West German government concluding that there are no more sites available for exploitation. World-wide it is estimated that about 10% of resources have been realised, with most potential remaining in Africa and Asia.

Consumption of hydro power in the world.

Present worlds total installed hydro power capacity is about 630 000 MW. The data are uncertain because the contributions from small hydro power plants and private systems are difficult to estimate, but it is assumed that these facilities can add just a few per cent to the total figure. The annual power production world-wide is 2200 TWh (billion kilowatt hours), which means that the power plants are running at 40 % of its rated power.

The largest hydroelectric complex in the world is on the Parana River, between Paraguay and Brazil. It is called the Itaipu Dam and its 18 turbines produce 12 600 MW of electricity. Hydro power is growing in many regions of the world. China and India pledged increases in large-scale hydroelectric development. In 1999 China completed its 3300 MW Ertan hydroelectric station which has six generating units, each with a capacity of 550 megawatts. Ertan is Asia's second tallest dam and China's largest electricity supplier.

The largest hydro power plants.

Hydroelectric projects currently under construction in China amount to some 32 000 MW of installed generating capacity. In India, 12 large-scale projects - adding up to 3700 MW of installed hydroelectric capacity - have been given government approval. All the projects are scheduled for completion by 2002. Construction on the world's largest hydroelectric project, the 18,2 GW Three Gorges Dam (China), entered Phase 2 of a three-phase process in 1998. Although construction on the dam was temporarily suspended in August 1988 because of the extensive flooding along the Yangtze, Phase 2 is still scheduled for completion in 2003, when the dam will start generating electricity. Phase 3 should end in 2009 with the beginning of full power generation. About USD 3.7 billion has already been spent on construction of Three Gorges Dam, including temporary diversion of the Yangtze and draining of the building site so that construction of the dam can continue. Upon completion, the project will extend 2 kilometres across the Yangtze and will be 200 meters tall, creating a 550 km long reservoir. The official Chinese estimate for the cost of the entire project is USD 25 billion. Three Gorges Dam has been the subject of much controversy. Environmental and social  problems related to this projects are enormous. Water pollution along the Yangtze will double as the dam traps more than 50 kinds of pollutants from mining operations, factories, and human settlements that used to be washed out to sea by the strong currents of the river. Heavy silt in the river will deposit at the upstream end of the dam and clog the major river channels of Chongqing. An estimated 1.1 million to 1.9 million people will have to be resettled before the reservoir is created; around 1,300 archaeological sites will have to be moved or flooded; and the habitats of several endangered species and rare plants will be jeopardised. In 1996, the U.S. Export-Import Bank declined to grant guarantees for U.S. companies  hoping to work on Three Gorges Dam, citing the potential environmental problems.

Construction is also underway on a pumped-storage station in Tibet at Yamzho Yumco Lake. The Tibetan station is being constructed at an elevation of 4 000 to 5,000 meters, the highest project in the world. In 1997, China announced plans to build a hydroelectric project along Tibet's Brahmapoutre river, near the Yalutsan mountain, which could generate a proposed 40 000 MWh per year.

Many countries of Central and South America rely heavily on hydroelectricity for electricity generation. In Brazil - which accounts for about 40 percent of the region's total installed capacity - 86 percent of the 59 000 MW of total installed capacity in 1996 consisted of  hydropower. Hydroelectric dams also account for 50 percent or more of the total installed generating capacity in Chile, Colombia, Paraguay, Peru, and Venezuela. Although many of the region's hydroelectric resources have been developed, there are still plans to add substantial capacity in near future. Brazil still has more hydroelectric projects under construction or planned for future installation than any other country in the Central and South America region. In September 1997, the final turbine was installed in the 3 000 MW  Xingó hydroelectric power facility on the São Francisco River at Piranhas. The USD 3.1 billion project accounts for 25 percent of the installed capacity in Northeast Brazil. Other large hydroelectric facilities currently under construction in Brazil include the 1450 MW Itá hydroelectric plant, which is scheduled  for completion in mid-2000, and the 1140 MW Machadinho hydroelectric plant, which is scheduled for completion in 2003; both facilities are located on the Uruguay River. Finally, there are also plans to expand the 12600 MW  Itaipu project held jointly between Brazil and Paraguay. The facility is to be expanded by 1400 MW at a cost of about USD 200 million.

ITAIPU - world largest hydro power plant.

More pictures from Itaipu

There are two main factors that determine the generating potential at any specific site: the amount of water flow per time unit and the vertical height that water can be made to fall (head). Head may be natural due to the topographical situation or may be created artificially by means of dams. Once developed, it remains  fairly constant. Water flow on the other hand is a direct result of the intensity, distribution and duration of rainfall, but is also a function of direct evaporation, transpiration, infiltration into  the ground, the area of the particular drainage basin, and the  field-moisture capacity of the soil. Runoff in rivers is a part of  the hydrologic cycle in which -powered by the sun - water  evaporates from the sea and moves through the atmosphere to land  were it precipitates, and then returns back to the sea by overland and subterranean routes.

Hydro power potential can be estimated with the help of river flows around the world. The results show that this total resource potential is 50 000 TWh per year – only a quarter of the world precipitation, but still over four times the annual output of all the world present power plants. Realistic resource potential which is based on local conditions of world rivers is in range 2 - 3 TW with an annual output of 10 000 - 20 000 TWh (UN 1992). But the important question remains : how much of hydro potential can we afford to use (see the chapter on environmental aspects).

A theoretical yearly production potential of 10.000 TWh of electrical energy means that the same amount of electrical energy produced in thermal plants with oil as  fuel would require approximately 40 million barrels of oil per day. If this is compared to the world consumption of petroleum  products, which amounted to around 80 million barrels per day in 1995. For developing countries, who together possess almost 60 % of the installable potential, the magnitude is striking.

Hydro power plants are very attractive for the investors. This is due to the relative low investment costs and competitive price of electricity produced. Moreover the life span o hydro facilities is considerably longer than for conventional fossil power plants. There are hydro power plants which run for almost 100 years.

The main reasons that hydro power plants are not build everywhere are that they are costly and require large bodies of water relatively close to inhabitants. According to the World Bank, “developing countries will need to raise an estimated USD 100 billion by the year 2000 for hydroelectric plants currently in the planning stage.” Another arising problems are the effects of dams on river ecosystems and social problems related to relocation of inhabitants.

A watercourse is an ecological system where changes within one component may create a series of spread-effects. For instance, changes in the water flow may affect the quality of the water and the production of fish downstream. Dam barriers may greatly change the living conditions for fish. In addition to the emergence of a major or completely new lake, the dam may divide upstream fish from downstream fish, and block their migration routes.

Environmental changes may be traced far downstream, at times even out into the sea. In the tropics there may be great seasonal variations as to the amount of precipitation, and in dry periods evaporation from lakes and reservoirs may be considerable. This may affect the water level of the reservoirs more dramatically than in temperate areas. The watercourse and its watershed mutually influence each other. The watercourse, for example, may affect the local climate and the ground-water level in surrounding areas. The sedimentation taking place in a reservoir can often lead to an increased erosion downstream, i.e. an increase in the total erosion. Changes in water flow and water level will also lead to changes in the transportation of sediments.
During the construction phase the transport of mud and sediments will be especially large downstream from the construction area. Excavation and tunnelling may lead to greatly reduced water quality and problems for those dependent on the water.

The groundwater level is important for the ecosystem‘s composition and development of plant and animal species. Groundwater is particularly important as a drinking-water source in most countries. The filling of a reservoir of hydro power plant and the flow of a watercourse are of great importance to the groundwater level and for the feeding of the groundwater reservoirs. A reservoir, together with the changes and variations of the water level caused by its operation, will change the groundwater level in surrounding areas. These areas may in turn influence the quality of the water and the sediment transport of the watercourse as a result of area run-off and erosion.

Whenever nutrients are trapped in a reservoir, the result may be excessive fertilisation - eutrophication - in the reservoir. It may lead to an increased growth of algae or large amounts of higher-order aquatic plants. A substantial production of organic matter in the reservoir, or the supply of external organic matter, may cause anaerobic conditions - lack of oxygen - in the deep-water layers.
On the whole, shallow lakes with a large surface area are most at risk, partly because the reserve of oxygen in the deep-water layers is limited in proportion to the productive area in the top layers. In deep, narrow lakes the oxygen content in the deep-water layers will be sufficient to recycle organic matter sinking down, provided there is a regular circulation of the waters. This is not always the case in the tropics. If the watercourse is initially rich in nutrients, the risk of eutrophication will increase.
Evaporation may cause a concentration of nutrients, leading to excessive fertilisation or eutrophication. Tropical soil normally has-a low humus content. This, combined with the great seasonal variations as to the amount of precipitation, and the fact that precipitation often comes in heavy showers, may cause considerable erosion. The transportation of eroded sediments will be halted and deposited in a reservoir. The reservoir's lifetime may in this way be reduced. Transport of sediments and nutrients tends to play a crucial role in the ecosystem of a watercourse. The population's utilization of nature and natural resources may be completely dependent on floods and waterborne sediments and nutrients.

A reservoir serves as a trap for nutritious elements and mud flowing in, possibly leading to a considerable reduction of the total transport of nutrients downstream. In addition, the annual variations in supply downstream may undergo changes. This may reduce the biological production all the way to the sea. There are grave examples of marine fishing being impaired in the wake of a major dam development.

The composition of fish species may be altered, since reproduction for some species may be hindered if the operation involves changes in the water level during the spawning period. Artificial reservoir tends to contain a less varied composition of species than a natural lake. Changes in the water flow and water-flow pattern may radically alter nutrient and spawning conditions downstream. The primary production as well as the direct accessibility of nutriment for fish will change. Changes made to the downstream floods, as a result of water control, may be decisive. At dam and turbine outlets a surfeit of gas may occur, principally of nitrogen, which can cause death among fish.

Some hydro power plants are equipped with fish ladders.

Submerging and water-flow changes, moreover, will lead to changes in the fauna and vegetation beyond the watercourse as such. Large reservoirs will exert a considerable direct impact on the flora and fauna of the hydro power plant area through submerging the area permanently or periodically. Animals may to some extent move to new habitats beyond the reservoir area, provided that suitable conditions are to be found. But normally the types and species of nature existing in areas being submerged must be considered as lost.
It is difficult to predict in general terms how changes beyond the submerged area will turn out. Local climatic changes and changes to the ground-water level may affect the flora and fauna. Valuable types and species of nature may be lost. A general activity increase in the area, such as traffic, noise etc., may also affect the fauna in a negative way. This especially pertains to the construction period.
Further, a reduced water flow or changed flow pattern downstream may influence the flora and fauna. The effects may be direct ones in that the flora and fauna react to the water flow, or the effects may be indirect owing to changes in the ground-water level and the transport of nutrients.

Large hydro power plants with dams require large reservoir and discharge areas. Many people have to be evacuated to make room for these areas. This could lead to a completely new situation for people who have lived in a relatively small, protected environment. Housing, land distribution, working conditions and way of life may change radically. The impacts will depend upon the size and location of the project. With major dam developments they can be serious.
Social consequences are likely to arise if the population concerned should be pressured into settling down in, or exploiting, more marginal and ecologically vulnerable areas than the ones they have traditionally utilized. These impacts may further aggravate their situation. Such indirect environmental effects can cause considerable ecological problems, with consequences for the entire project area.
Indigenous groups affected by hydropower development may be particularly deprived. Their principle socio-cultural conditions together with their traditional connection to land, water and other natural resources, tend to make them unadaptable to changes and new activities. The size of many hydropower projects and the rapid alterations in ecological conditions that may arise, usually allow little room for readjustment. The transfer of indigenous groups may endanger their entire cultural system. Such minorities are particularly exposed, as they tend to have little political influence and possibility of securing their own interests.
As a whole, the consequences of dam development can involve great damage to traditional ways of life and cultural expressions. Changes in terms of social, economic and religious organisation can create a series of indirect social impacts which are difficult to foresee during the planning of the project. Cultural landscapes, ancient monuments, holy places, burial grounds etc. are often areas and objects of great importance to a local population's cultural activities. Should such areas and objects be affected by a project, the cultural identity of the population might be at risk.

Large hydro power plants can increase the extent of water-related diseases. The reservoir may improve the living and breeding conditions of disease-causing organisms (pathogens) and their intermediate hosts. Among water-related diseases one could mention typhus, cholera, dysentery and several tapeworm and roundworm diseases. Several serious diseases have intermediate hosts linked to water. This applies to bilharzia, malaria, filariasis, sleeping sickness and yellow fever.
Reservoirs with large, stagnant waters and slow water-level variations offer favourable living conditions to pathogens. Vegetation in the reservoir also affords improved living conditions for several types of infection-carriers. The vegetation may provide infection-carriers with an increased supply of nutrients, improved conditions for breeding and protection in periods of a low water level. Moreover, the aquatic vegetation shields snails - which are carriers of bilharzia infection - from strong sunlight. In addition, research reveals that mosquito species carrying malaria and filariasis due to vegetation in dams. If the reservoir is employed both for irrigation and as the industrial and drinking water supply, there will be a risk of infection spread by pathogens living in the water. Such infection may spread over large areas.

A dam breach seldom occurs, but owing to the enormous consequences which it may involve, the impacts of a breach should be assessed. The risk of casualties and damaged property or technical installations must be considered the most serious consequences, but the impacts on the natural environment can also be considerable.
Statistically, the combination of a flood in the upstream watershed of the dam and faults in the spillway are the most frequent causes of accidents. Secondary causes are foundation errors or water seepage. At high water levels in the reservoirs, landslides of earth and rocks from the embankment above or inside the reservoir may cause flood waves so massive that water may spill over the total or partial width of the dam. If the dam is an embankment dam, this may lead to the dam itself being damaged. Special care should be taken if a major dam is planned in an area exposed to earthquakes.

In hydro power plants the kinetic energy of falling water is captured to generate electricity. A turbine and a generator convert the energy from the water to mechanical and then electrical energy. The turbines and generators are installed either in or adjacent to dams, or use pipelines (penstocks) to carry the pressured water below the dam or diversion structure to the powerhouse. The power capacity of a hydropower plant is primarily the function of two variables: (1) flow rate expressed in cubic meters per second (m3/s), and (2) the hydraulic head, which is the elevation difference the water falls in passing through the plant. Plant design may concentrate on either of these variables or both.
From  the energy conversion point of view, hydro power is a technology with very  high efficiencies, in most cases more than double that of conventional thermal power plants. This is due to the fact that a volume of  water that can be made to fall a vertical distance, represents  kinetic energy which can more easily be converted into the  mechanical rotary power needed to generate electricity, than  caloric energies. Equipment associated with hydropower is  well developed, relatively simple, and very reliable. Because no  heat (as e.g. in combustion) is involved, equipment has a long life and malfunctioning is rare. The service life of an hydroelectric plant is well in excess of 50 years. Many plants built in the twenties - the first heyday of hydroelectric power - are still in operation.
Since all essential operating conditions can be remotely monitored and adjusted by a central control facility, few operating personnel are required on site. Experience is considerable with the  operation of hydropower plants in output ranges from less than one  kW up to hundreds of MW for a single unit.

Hydropower technology can be categorized into two types: conventional and pumped storage. Another way of classification of hydro power plants is according to :
Rated power capacity (big or small)
Head of water (low, medium and high heads)
The type of turbine used (Kaplan, Francis, Pelton etc.)
The location and type of dam, reservoir.

Conventional hydropower plants use the available water energy from a river, stream, canal system, or reservoir to produce electrical energy. Conventional hydropower can be further divided between impoundment and diversion hydropower. Impoundment hydropower uses dam to store water. Water may be released either to meet changing electricity needs or to maintain a constant water level. Diversion hydropower channels a portion of the river through a canal or penstock, but  may require a dam. In conventional multipurpose reservoirs and run-of-river systems, hydropower production is just one of many competing purposes for which the water resources may be used. Competing water uses include irrigation, flood control, navigation, and municipal and industrial water supply.

Pumped storage hydro-electricity is a remarkably simple principle. To start with, two reservoirs at different altitudes are required. Water stored at height offers valuable potential energy. During periods of high electrical demand, the water is released to the lower reservoir to generate electricity. When the water is released, kinetic energy is created by the discharge through high-pressure shafts which direct the water through turbines connected to generator/motors. The turbines power the generators to create electricity. After the generation process is complete, water is pumped back to the upper reservoir for storage and readiness for the next cycle. The process usually takes place overnight when electricity demand is at its lowest.
While pumped storage facilities are net energy consumers, they are valued by a utility because they can be rapidly brought on-line to operate in a peak power production mode. This process benefits the utility by increasing the load factor and reducing the cycling of its base load units. In most cases, pumped storage plants run a full cycle every 24 hours.

Most conventional hydropower plants include following major components:
Dam. Controls the flow of water and increases the elevation to create the head. The reservoir that is formed is, in effect, stored energy.
Turbine. Turned by the force of water pushing against its blades.
Generator. Connects to the turbine and rotates to produce the electrical energy.
Transformer. Converts electricity from the generator to usable voltage levels.
Transmission lines. Conduct electricity from the hydropower plant to the electric distribution system.
In some hydro power plants also another component is present – penstock, which carries water from the water source or reservoir to the turbine in a power plant.

The oldest form of “water turbine” is the water-wheel. The natural  head difference in water level of a stream is utilised to drive it. In its conventional form the water-wheel is made of wood and is provided with buckets or vanes round the periphery. The water thrusts against these, causing the wheel to rotate. Traditional water wheels have been used for centuries, but these large and slow-moving wheels are not suitable for generating electricity. Water turbines used for electricity generation are made from metals, rotate at higher speeds, and are much easier to build and install. Over the years, many turbine designs have been developed to work best in different situations.

Water turbines may be classified in different ways. One way of classification is according to the method of functioning (impulse or reaction turbine); another way is according to the design (shaft arrangement and feed of water). Water turbines may operate as turbines, as pump turbines or as a combination of both. They may be of the single regulated or double regulated type. Turbines may also be classified according to their specific speed.

Impulse turbines use a nozzle at the end of the pipeline that converts the water under pressure into a fast-moving jet. This jet is then directed at the turbine wheel (also called runner), which is designed to convert as much of the jet's kinetic energy possible into shaft power. Common impulse turbines are Pelton and cross-flow. In reaction turbines the energy of the water is converted from pressure to velocity within the guide vanes and the turbine wheel itself. Spinning of the turbine is a reaction to the action of the water squirting from the nozzles in the arms of the rotor. The typical example of reaction turbine is a  Francis turbine. The advantage of small hydro power reaction turbine is that it can use the full head available at a site. An impulse turbine must be mounted above tailwater level. The advantage of impulse turbine is that it is very simple and cheap and as the water flow varies , water flow to the turbine can be easily controlled by changing nozzle size. In contrast most small reaction turbines cannot be adjusted to accommodate variable  water flow.

Most hydraulic turbines consist of a shaft-mounted water-wheel or “runner” located within a water-passage which conducts water from a higher location (the reservoir upstream from a dam) to a lower one (the river below a dam). Some runners look very similar to a boat propeller, others have more complex shapes. The turbine runner is installed in a water passage that lets water from the reservoir flow pass the runner blades, which makes the turbine spin.

Almost all hydraulic turbine/generator units turn at a constant speed. The constant speed one type of turbine/generator operates at may be considerably different from the speed of another. The best speed for each type of turbine is set during design, and a generator is then designed that will produce usually alternating current at that speed. A device called a governor keeps each unit operating at its proper speed by operating flow-control gates in the water-passage. There are several types of turbine designs like Pelton, Kaplan, Francis or cross-flow turbine.

The principle of the old water-wheel is embodied in the modern Pelton turbine. This turbine has a similar look and physical principle like a classic water wheel. A Pelton turbine is used in cases where large heads of water are available (more than 40 m). The Pelton turbine is used for heads up to 2000 m. Below 250 m, mostly the Francis turbines are given preference. Today the maximum output lies at around 200 MW.

Together with crosflow turbines, Pelton turbines belong to the impulse type (or free-jet)  turbines, where the available head is converted to kinetic energy at atmospheric pressure and partial admission of flow into the runner. The free jet turbine was invented around 1880 by the American Pelton, after whom it got its name. The greatest improvement that Pelton made was to introduce symmetrical double cups. This shape is basically still valid today. The splitter ridge separates the jet into two equal halves, which are diverted sideways. The largest Pelton wheels have a diameter of more than 5 m and weigh more than 40.000 kg. The wheel must be placed above the tailrace water level, which means a loss of static head, but avoids watering of the runner. In order to avoid an unacceptable raise of pressure in the penstock, caused by the regulating of the turbine, jet deflectors are sometimes installed. The deflector diverts the jet, or part of it, from the runner.
Since then the turbine has been considerably improved in all respects and the output of power has increased. Power is extracted from the high velocity jet of water when it strikes the cups of the rotor (runner). There is a maximum of 40 cup-like paddles jointed in two half-cups each water is being squirted through nozzles onto the blades where it is deflected by 180° and thus gives almost all of its energy to the turbine. By the reversal almost all the kinetic energy is transferred into force of impulse at the outer diameter of the wheel. Because of the symmetry of the flow almost no axial force is created at the runner.

From the design point of view, adaptability exists for different flow and  head. Pelton turbines can be equipped with one, two, or more  nozzles for higher output. In manufacture, casting is commonly used for the rotor, materials being brass or steel. This  necessitates an appropriate industrial infrastructure. Pelton turbines require only very little maintenance.

In the great majority of cases (large and small water flow rates and heads) the type of turbine employed is the Francis or radial flow turbine. The significant difference in relation to the Pelton turbine is that Francis (and Kaplan) turbines are of the reaction type, where the runner is completely submerged in water, and both the pressure and the velocity of water decrease from inlet to outlet. The water first enters the volute, which is an annular  channel surrounding the runner, and then flows between the fixed guide vanes, which give the water the optimum direction of flow. It then enters the runner, which is totally submerged, changes the momentum of the water, which produces a reaction in the turbine. Water flows radially i.e., towards the centre. The runner is provided with curved vanes upon  which the water impinges. The guide vanes are so arranged that the energy of the water is largely converted into rotary motion and is  not consumed by eddies and other undesirable flow phenomena causing energy losses. The guide vanes are usually adjustable so as to  provide a degree of adaptability to variations in the water flow rate and in the load of the turbine.
The guide vanes in the Francis turbine are the elements that direct the flow of the water, just as the nozzle of the Pelton wheel does. The water is discharged through an outlet from the centre of the  turbine. In design and manufacture, Francis turbines are much more complex  than Pelton turbines, requiring a specific design for each head/flow condition to obtain optimum efficiency. Runner and housing are usually cast, on large units welded housings, or cast  in concrete at site, are common.
With a Francis turbine, downstream pressure can be above zero. Precautions must be taken against water hammer with this type of turbine. Under the emergency stop, the turbine overspeeds. One would think that more water is going through the turbine than before the trip occurred since the turbine is spinning faster. However, the turbine has been designed to work efficiently at the design speed, so less water actually flows through the turbine during overspeed. Pressure relief valves are added to prevent water hammer due to the abrupt change of flow. Besides limiting pressure rise, the pressure relief valve prevents the water hammer from stirring up sediment in the pipes.
With a big variety of designs, a large head range from about 30 m up to 700 m of head can be  covered. The most powerful Francis turbines have an output of up to 800 MW and use huge amounts of water.

For very low heads and high flow rates a different type of turbine, the Kaplan or Propeller turbine is  usually employed. In the Kaplan turbine the water flows through the propeller and sets the latter in rotation. In this turbine the area through the water flows is as big as it can be – the entire area swept by the blades. For this reason Kaplan turbines are suitable for very large volume flows and they have become usual where the head is only a few meters.
The water enters the turbine laterally, is deflected by the guide vanes, and  flows axially through the propeller. For this reason, these  machines are referred to as axial-flow turbines. They have the advantage over radial-flow turbines that it is technically simpler to vary the angle of the blades when the power demand changes what improves the efficiency of power production.
The flow rate of  the water through the turbine can be controlled by varying the distance between the guide vanes; the pitch of the propeller blades must then also be appropriately adjusted. Each setting of the guide vanes corresponds to one particular setting of the  propeller blades in order to obtain high efficiency. Important feature is that the blade speed is greater than the water speed – as much as twice as fast. This allows a rapid rate of rotation even with relatively low water speeds.
Kaplan turbines  come in a variety of designs. Their application is limited to heads from 1 m to about 30 m. Under such conditions, a relatively larger flow as compared to high head turbines is required for a given  output. These turbines therefore are comparatively larger.

The concept of the Cross-Flow turbine -although much less  well-known than the three big names Pelton, Francis and Kaplan -is not new. It was invented by an engineer named Michell who obtained  a patent for it in 1903. Quite independently, a Hungarian professor  with the name Donat Banki, re-invented the turbine again at the  university of Budapest. By 1920 it was quite well known in Europe,  through a series of publications. There is one single company who  produces this turbine since decades, the firm Ossberger in Bavaria, Germany. More than 7000 such turbines are installed world-wide,  most of them made by Ossberger.
The main characteristic of the Cross-Flow turbine is the water jet  of rectangular cross-section which passes twice through the rotor  blades -arranged at the periphery of the cylindrical rotor -  perpendicular to the rotor shaft. The water flows through the  blading first from the periphery towards the centre, and then, after crossing the open space inside the runner,  from the inside outwards. Energy conversion takes place twice;  first when water falls down on the blades upon entry, and then when water strikes the blades during  exit from the runner. The use of  two working stages provides no particular advantage except that it  is a very effective and simple means of discharging the water from the runner.
The machine is normally classified as an impulse turbine. This is  not strictly correct and is probably based on the fact that the original design was a true constant-pressure turbine. A sufficiently large gap was left between the nozzle and the runner,  so that the jet entered the runner without any static pressure. Modern designs are usually built with a nozzle that covers a bigger  arc of the runner periphery. With this measure, unit flow is  increased, permitting to keep turbine size smaller. These designs  work as impulse turbines only with small gate opening, when the  reduced flow does not completely fill the passages between blades  and the pressure inside the runner therefore is atmospheric. With  increased flow completely filling the passages between the blades,  there is a slight positive pressure; the turbine now works as a reaction machine.
Cross-Flow turbines may be applied over a head range from less than 2 m to more than 100 m (Ossberger has supplied turbines for heads  up to 250 m). A large variety of flow rates may be accommodated  with a constant diameter runner, by varying the inlet and runner  width. This makes it possible to reduce the need for  tooling, jigs and fixtures in manufacture considerably. Ratios of  rotor width/diameter, from 0.2 to 4.5 have been made. For wide  rotors, supporting discs welded to the shaft at equal intervals  prevent the blades from bending.
A valuable feature of the Cross-Flow turbine is its relatively flat  efficiency curve, which Ossberger are further improving by using a divided gate. This means that at reduced flow, efficiency is still  quite high, a consideration that may be more important than a higher optimum-point efficiency of other turbines. Due to low price and good control these turbines are, however, very successful in the area of small hydro-electric power plants.

Hydro power plants range in capacity between few hundred watts to more than 10.000 MW. Classification between big and small is quite common where usually all power plants with capacity larger than 10 MW are considered as big and all others as small. Classification among small hydro power is also possible and terms like micro or nano hydro with capacity less than 1 kW are also used in literature. Nevertheless it is worthwhile looking at the specific characteristics and basic differences between big and small power plants.

Big Hydropower
Big hydropower stations are of a nature that requires a good  infrastructure such as roads (during construction) and access to a big market, resulting in long high-tension grid systems and an  extensive distribution system. It serves a great number of  individual consumers and supplies power to electricity-intensive  large industry.
Big plants are usually owned and operated by big companies or state enterprises. The skill requirements in management, administration, operation and maintenance are considerable. Unit cost of energy generation is  relatively low. This is due to a decrease in specific investment cost  with rising plant size, and the probability of higher load factors with a larger number of consumers. A problem is peak demand; big  numbers of consumers tend to have their maximum individual demand  during the same time-interval, which results in a largely  uncontrollable peak of demand that must be met with increased  capacity, such as standby installations and high cost  pumped-storage.
From the engineering point of view, big hydro power calls for  sophisticated technology in manufacturing electro-mechanical  equipment, and high standards of feasibility studies, planning and  civil construction activities, because the risks involved are  great. Long-term flow data are a necessity and gestation periods are long. It is possible to apply computer design technology and  highly specialised fabrication technology to achieve very high  performance efficiencies that may reach 96 % in the case of turbines. Needless to say, this process brings about very  high cost, which however may be justified because of the large  scale, where equipment cost is generally a relatively small  fraction of total cost.Big-scale hydropower stations require careful environmental considerations. Artificial lakes may change an entire landscape and inundate sizeable areas of arable land. Positive aspects are flood  controlling capability and the creation of new recreational sites (boating, fishing, camping) although it is obvious that the benefits for recreation do not rise in proportion with size.

large centralised power demand; large-scale industry, cities,  urban areas
international, national and regional grid-systems
big corporations or state enterprises employing highly-skilled  and well paid staff
depends on long term assessment of potential, long planning and construction periods involving sophisticated technology
depending on potential it can make a sizeable contribution to a nation's commercial energy requirements

Small and micro or nano hydropower schemes combine the advantages of  large hydro on the one hand an decentralized power supply, on the other. They do not have many of the  disadvantages, such as costly transmissions and environmental  issues in the case of large hydro, and dependence on imported fuel  and the need for highly skilled maintenance in the case of fossil fuelled plants. Moreover, the harnessing of small hydro-resources, being of a decentralised nature, lends itself to decentralised utilization,  local implementation and management, making rural development  possible mainly based on self-reliance and the use of natural, local resources.

There are in fact many thousands of small hydro plants in operation today all over the world. Modern hydraulic turbine technology is very highly developed with the a history of more than 150 years. Sophisticated design and manufacturing technology have evolved in industrialised countries over conventional technology  the last 40 years. The aim is to achieve higher and higher conversion efficiencies, which makes sense in large schemes where 1 percent more or less may mean several MW of capacity. As far as costs are concerned, such sophisticated technology tends to be very expensive. Again, it is in the big schemes where economic viability is possible. Small installations for which the sophisticated  technology of large hydro is often scaled down indiscriminately, have higher capital cost per unit of installed capacity.  On the other hand environmental impacts due to small hydro stations are generally negligible or are controllable because of their size. Often they are non-existent.

Small hydro power plants are in large majority connected to the electricity grids. Most of them are of the “run-of-river” type, meaning simply that they do not have any sizeable  reservoir (i.e. water not stored behind the dam) and produce electricity when the water provided by the river flow is available but generation ceases when the river dries-up and the flow falls below a predetermined amount. Power can be supplied by a small (or micro) hydro power plant in two ways. In a battery-based system, power is generated at a level equal to the average demand and stored in batteries. Batteries can supply power as needed at levels much higher than that generated and during times of low demand the excess can be stored. If enough energy is available from the water, an alternating current (AC) direct system can generate power. This system typically requires much higher power level than the battery-based system. Small hydropower in developing countries, on the other hand, implies decentralisation. Energy produced is usually supplied to relatively few consumers nearby, mostly with a low-tension distribution network only.

Small hydro schemes have different configurations according to the head. High head schemes are typical of mountain areas, and due to the fact that for the same power they need a lower flow, they are usually cheaper. Low heads schemes are typical of the valleys and do not need feeder canal. Of the numerous factors which affect the capital cost, site selection and basic lay-out are among the first to be considered. Adequate head and flow are necessary requirements for hydro generation.

Most hydro power systems require a pipeline to feed water to the turbine. The exception is a propeller machine with an open intake. The water must pass first through a simple filter to block debris that may clog or damage the turbine. The intake is usually placed off to the side of the main water flow to protect it from the direct force of the water and debris during high flow.

High safety standards in construction works are often not necessary, even the rupture of a small dam would not usually threaten human life, and the risks are smaller anyway if initial costs are kept down. This makes it possible to  use mainly local materials and local construction techniques, with  a high degree of local labour participation.
Small hydro systems can require more maintenance than comparable wind or photovoltaic  systems. It is important to keep debris out of the turbine. This is done by reliable screening and construction of  a settling basin. In the turbine itself, only the bearings and brushes will require regular maintenance and replacement.

Hydropower plants are  characterised by high initial capital-investment (according to World Bank total costs are between USD  1800  and USD  8800 per kW for heads from 2,3 to 13,5 m and USD  1000 to USD  3000 for heads between 27 and 350 meters.) and low operation  and maintenance cost. The investment costs include:
Construction (dam, channel, machine house),
Parts for electricity generation (turbine, generator, transformer, power lines),
Other  (engineering, ground property, commissioning).

Usually equipment for low head and low output becomes very costly and equipment cost ranges from 40 to 50 % of total cost in conventional hydro installations. As far as costs of civil construction-components are concerned, no  standard cost unit can be given. Dams, canals and intakes will  obviously cost a very different share of the total for different sites. Much depends on the topography and the geology, and also on  the construction method applied and the materials used.  Just to mention some examples the total cost of new small hydro power plants in Germany was 10-16 DM/W (5-9 ECU/W) and are divided in most cases 35% (construction) - 50% (electricity parts) - 15% (other). There are of course some differences between countries e.g. costs of 8 kW turbine (Banki type with regulation) in Czech republic is 4000 USD , equivalent to 3500 ECU or 0,45 ECU/W.
The high investment costs is the largest barrier in development of small hydro power schemes. Despite this obstacle and long pay-back times (7-10 years in some countries e.g. Slovakia) small hydro power plants are often cost-effective because of their long life-time (often more than 70 years) and low maintenance costs. As a general rule, total costs of operation and maintenance without major replacements account for approximately 3 to 4% of capital costs for small and micro-hydropower installations.

Decentralised, small power demand; small industry, individual  farms and enterprises, rural communities.
Low tension distribution networks and eventually sub-regional  micro-grid systems.
Individual, co-operative or communal ownership with semi-skilled  labour requirements and co-operative administration.
Short gestation period with local materials and skills applicable depending on potential, it can make a considerable impact on the  quality of rural life.
Its flexibility regarding adaptation to quick load variations makes it a favoured component in any integrated power system.
Plants can last for very long time. Some are more than 70 years old and still in operation. Plants commissioned recently may show even longer life span and thus can serve consumers over several generations without polluting the atmosphere.
Investment in small hydro power have proved to be safe and secure over several decades.

In developing countries the domain where small hydropower can potentially have an important  impact on development is in domestic lighting and in providing  stationary motive power for such diverse productive uses as  water-pumping, wood and metal working, grain milling, textile fibre spinning and weaving. While much of the discussion is concerned  with the generation of electricity, it must be recognised that the  same source of power can perform mechanical tasks directly via  gears and belt drives, very often more economically.
Emphasis is on the use of currently available know-how, using  simple equipment that can be made locally, and the use of local  construction materials and techniques. The aim is to reduce capital  costs as far as possible. Rather than scaling down large-scale  technology, this may lead to a more appropriate upgrading of local  technology for larger schemes at a later stage.

The construction of small hydropower stations has been a very  meaningful in the past 25 years. Besides the development of large resources,  much emphasis was given to small-scale developments resulting in an estimated 100.000 stations around the vast countryside with installed capacity approaching 10.000 MW.
The first large-scale campaign to establish many small waterworks started in 1956. An ambitious plan called for the construction of 1000 small stations of a multi-purpose character, combining  irrigation, flood control and power generation, in one year, reaching a total capacity of 30 MW. Although industrial capability permitted construction of large  turbines, and the range under which small hydropower falls in China was extended to 12 MW, this indicates that construction of very small units continued. In fact, a range of miniature turbine-generators with outputs from 0,6 to 12 kW was developed,  suitable for scattered mountain villages with small hydropower  resources.
The development activities in this field were entirely relying on  local resources -materials, skill and labour - and the results achieved are from this perspective even more impressive. Hydropower development in China faces some major natural obstacles. The regional distribution of resources is very uneven and concentrated in regions that are thinly populated. Flow variations  in many rivers are considerable. The maximum recorded flood flow in the Huang Ho river was 88 times larger than the minimum discharge and in smaller rivers this ratio is likely to be  much higher.

Microhydro systems are defined as hydroelectric systems that produce less than 1000 Watts. At the high end, microhydro systems produce enough power to run three electrically efficient households. No other form of renewable energy is so reliable or powerful for what it costs. Micro hydro system means that the site has either very little fall or very small flow of water, but probably not both. At sites with lower flow rates, systems are usually tied to a battery bank and configured to produce direct current. With larger hydro resources, systems may be configured to produce alternating current without the use of a battery bank. These systems must be able to directly power peak loads. In some case excess power produced is transferred to an alternate load such as a hot water heater.

A hydropower turbine appropriate for household use can be bought for about USD 1000. These simple units are about the size of a breadbox and use a rewired automobile alternator to produce direct current. The direct current is used to charge batteries, then converted to AC power with an inverter.

A typical micro hydro installation diverts a small portion of stream flow across a screen into a water storage e.g. 200 litre drum. The drum acts as a settling basin and the screen collects debris from the water which may clog the intake to the turbine. The water flows from the drum to the turbine through PVC piping (usually 5 to 10 centimetres in diameter), and then returns to the stream. Additional costs for piping, controls, batteries, and wiring vary depending on the particular application, but range from USD 1000 to USD 5000.

Micro hydro turbines come in two basic forms. One uses an alternator, just like an automobile. The other (nano hydro systems) uses a permanent magnet (permag) generator/motor. The alternator based machines are for larger systems producing from 100 to 1000 watts, while the permag units are best suited to systems producing under 80 Watts.
Larger systems use shunt diversion for regulation. This prevents overspeeding of the turbine and premature wear of parts. Smaller systems use regulation schemes that unload the alternator when power is not needed. In all cases, these controls need to be user adjustable. Micro hydro systems are easy to fit with batteries. The turbine produces constant power all the time. The battery acts as a “flywheel” to smooth out the inevitable peaks of consumption. Micro hydros refill the batteries almost immediately after even a little power is consumed from the battery. These systems are “shallow-cycling” and ordinary batteries will last a long time. Usually spending money on good pipe and an efficient turbine is more effective than spending it on batteries. In a microhydro system the length and diameter of the pipe must be specified to suit the situation and the turbine. Using long runs of small diameter pipe will make even the finest turbine ineffective.

What sets nano hydro systems apart from other hydro generators is the use of permanent magnet generators for the power source. The advantage to this is that no power is fed back into the machine to electrically generate a magnetic field, as is the case with most alternators, so all of what is produced will feed the batteries. The disadvantage of a permag set-up is that the maximum output is limited by the inherent strength of the magnets. Normally that's not a problem in a nano hydro situation because usually flow and head of water are too small for a larger, more powerful system anyway.

Most micro and nano hydro systems are battery-based. They require far less water than AC systems and are usually less expensive. Because the energy is stored in batteries, the generator can be shut down without interrupting the power delivered to the loads. Since only the average load needs to be generated in this system, the pipeline, turbine, generator and other components can be much smaller than those in AC system. For conversion of DC battery power to AC output (type of power needed by most of home appliances) inverters are used. The input voltage to the batteries in battery-based system usually ranges from 12 to 48 Volts DC. If the transmission distance is not long then 12 V system is used. For longer transmission distances higher voltage is used.

Alternating current (AC) hydro power systems are those used by utilities, but it can also be used on a home power scale under the appropriate conditions. In home power scale system power is not sent to the utility grid, but is directly used by a homeowners appliances (load). AC system does not need batteries. This means that the generator must be capable of supplying the continuous demand, including the peak load. The most difficult load is the short-lasting power surge drawn by motors in refrigerators, washing machines and some other appliances. Usually in typical AC system, an electronic controller is keeping voltage and frequency within prescribed limits. The output from hydro power plan can not be stored and any unused power is sent to a “shunt” load, which can be e.g. a hot water heater. There is almost always enough excess power from this type of system to heat domestic hot water and provide space heating as well.

High costs of equipment and civil works, or more generally, the capital-intensive nature of small hydropower plants, has long been a major constraint. However, in many situations it is necessary not only to achieve a better relation of costs compared to other energies, but to reduce them in absolute terms. This is possible to some degree by standardising equipment, but the scope for using such standardised equipment remains limited since no two sites are exactly the same. Efforts at cost reduction through indigenous manufacture are more promising, largely due to much lower labour costs. To make this possible, standards of design, performance and sometimes reliability must be lowered and all unnecessary sophistication avoided. The same is true in civil construction work, where local materials and techniques should be  used to the largest possible extent.

In developing countries and especially in rural areas, it is generally recognized that small hydropower may play a significant role. However, high initial investment costs of small hydropower plants have restricted rapid development of this energy potential in many countries. The use of standard pumps as turbines (PAT) may often be an alternative with a considerable economic advantage and might therefore contribute to a broader application of micro-hydropower. Direct drive of machinery, electricity generation (in parallel to a large grid or isolated) or combinations of these are possible just as with a conventional turbine. The only difference is that a PAT cannot make use of the available water as efficiently as a turbine due to its lack of hydraulic controls.

Pumps (rotational fluid machines) are completely reversible and can run effectively as a turbine. Standard pumps not intentionally designed to operate as turbines are now more and more used in small and micro-hydropower schemes due to their advantages mentioned above. However, performance in both modes are not identical although the theory of ideal fluids would predict the same. Without exception, the optimum flow and head in the turbine mode is greater than in pumping mode. The main reason for this difference is related to the hydraulic losses of the machine.
Applications of PAT range from direct drive of machinery in agro-processing factories and small industries (flour mills, oil expellers, rice hullers, saw mills, wood and metal workshops) to electricity generation both in stand-alone and grid-linked stations.
In most instances, no design changes or modifications need to be made for a pump operating as a turbine provided that selection has taken into account the higher operating head and power output of the machine in turbine mode and consequently, nominal turbine speed has been taken well below maximum permissible pump speed. However, a design review is also required to check any adverse effects occurring from the reverse rotation in turbine mode.

Advantages of PAT
the investment costs of PATs may be less than 50% of those of a comparable turbine (especially for small units below 50 kW). This might be an important issue for projects with limited budgets and loan possibilities
construction: the absence of a flow control device, usually felt as a drawback, is at the same time an advantage since the pump construction is usually simple and sturdy
availability: due to their widespread application (irrigation, industry, water supply), standard pumps are readily available (short delivery times) and manufacturers and their representatives are world-wide present
spare parts: spare parts are readily available since major pump manufacturers offer after- sales services almost throughout the world
maintenance: no special equipment and skills are required.

No hydraulic control device: therefore, a control valve must be incorporated in the penstock line (additional costs) to start and stop the PAT. If the valve is used to accommodate to seasonal variations of flow, the hydraulic losses of the installation will increase sharply.
Lower efficiency at part load: a conventional turbine has an effective hydraulic control (adjustable guide vanes, nozzles or runner blades) to adjust the machine to the available flow or the required output. If PATs are operated at other than the design flow, i.e. below their best efficiency point  a relatively rapid drop of efficiency will occur.

The disadvantages of PATs can be reduced to a minimum if the PAT is very carefully selected and only applied where justified. Poor performance due to an inappropriately selected machine or application will lead to a reduction of gains. Summed up over the entire lifetime of the machine, this reduced output might by far offset the cost advantage of the PAT (lower investment costs) in comparison to a conventional turbine.

Pumps are usually operated with constant speed, head and flow. A pump is therefore designed for one particular of operation (duty point) and does not require a regulating device (guide vane). Ideally, the duty point coincides with the maximum efficiency of the pump.
Turbines  operate under variable head and flow conditions. In an small hydro power plant, flow must be adjustable to either accommodate to seasonal variations of the available water or to adjust power output according to the demand of the consumers. Adjustable guide vanes and/or runner blades (or nozzles controlled by a streamlined valve) regulate the flow.

Virtually any type of pump may be used as turbine. However, the main advantage of a PAT, i.e. lower costs than a conventional turbine, is very pronounced for standard centrifugal and mixed flow pumps whereas axial flow pumps are less advantageous in that respect. The vast field of different pump designs and power ranges provides a suitable PAT for almost any application with heads from about 10 m up to several hundred meters. Large flows may be accommodated with double-flow pumps. Even submersible pumps may be used as PATs which, when integrated in the water course or pipe system, are completely hidden away underground, an important factor for the conservation of the environment. Efficiencies of pumps used as turbines may be the same as in pump mode but are more often several percent (3 - 5%) lower.

Direct drive of machinery, electricity generation (in parallel to a large grid or isolated system) or combinations of these are possible just as with a conventional turbine. Although the PATs cover a wide range of the small hydropower domain, they cannot replace conventional turbines everywhere. Since PATs have no hydraulic control device such as guide vanes, they are usually unsuitable to accommodate variable flow conditions. Throttling flow by means of a control valve in the penstock is inefficient and only applicable over a small range.

The lack of a hydraulic control device of a PAT has long been seen as a disadvantage also in terms of constancy of PAT speed under variable load. Grid-linked electricity generation or direct drive of machinery are either constant load applications or do not require precise speed control. These applications are therefore very suitable for PATs. Stand alone electricity generation on the other hand requires some form of governing to keep voltage and frequency within acceptable limits under changing load. The use of PATs in free-standing electricity generation is, however, not excluded due to the recent development of electronic load controllers which provide effective governing in conjunction with both induction and synchronous generators. Electronic load controllers keep the load on the PAT constant by switching in ballast loads whenever the electricity demand of the consumers drops.

Hydro ram is not an animal but a self-driven pump first installed at the turn of the century when they were popular with farmers who had natural water courses on their land. With the coming of grid electricity and mains water, many rams were left to rot and rust in the post-war period. Nevertheless this device is a useful source of cheap energy even today. Ram pumps do not produce electricity but the mechanical work for pumping water to higher elevations. They use a downhill water pressure to pump a portion of that water  higher uphill to a holding tank. No other source of power is needed. The hydro rams are complete in themselves and designed to work with the minimum of attention, and to suit all the ordinary conditions.
The hydro ram has proved to be one of the most reliable devices used for water pumping. Many over 100 years old are still in use, and it remains one of the few really practical and efficient uses of renewable energy today. Hydro rams are relatively cheap, will last almost indefinitely and with no moving metal parts and its simplicity require only minimum of maintenance. If the two essentials are provided – a supply of water (spring or stream, as little as 4 litres per minute will suffice) and the ability to provide a “fall” for that water – the hydro ram can reduce or even eliminate costly water bills. Typical uses of hydro rams include :
Village water supplies
Water pumping and circulation in industry
Water circulation for heat pumps
Water circulation for solar panels

How a Ram Pump Works
All ram pumps work on the principle of momentum which is controlled by a cycle set up by the interaction of two valves in the pump. The water, being admitted into the drive pipe, flows through it by gravitation until it reaches the ram, passes through the ram and through the pulse valve into the waste drain. As the water flows, its velocity increases until the pulse valve is no longer able to pass the volume of water flowing, and on this point being reached the pulse valve is suddenly closed. The outlet thus being closed, the flow of water suddenly stops. This produces a concussion of more or less severity in the body of the ram, according to the height and distance from which the water is flowing, and a result of this concussion is that a portion of the water in the body of the ram is forced upwards through the delivery valve into the air cylinder. At the same time the recoil allows the pulse valve to return to its original position. The outlet being thus reopened, the water which was brought to rest by the closing of the pulse valve recommences to flow through the ram till it acquires the necessary velocity to raise the pulse valve a second time , closing the outlet, producing a concussion, and forcing more water into the air chamber through the delivery valve. This series of events occurs from 40 to 90 times per minute, according to the size of the hydro ram, fall of water driving ram, etc. The ram will continue working automatically for months, the pulse valve rubber and the delivery valve rubber being the only moving parts.
The water, which is forced into the air chamber, finds its way from it through a pipe, known as the rising main or delivery pipe, to the place where it is required for use, a continuous flow being maintained so long as the ram remains working.
The fall of water necessary to work a ram may be as low as 0,5 meter and with such a fall, water may be raised to 10 to 15 meters. With higher falls, such as from 2 to 10 meters and over, water can be raised to upwards of 100 meters in height and more than 1 kilometre in distance.
The installation is extremely simple. All that is required – water at the point of by constructing a pool. From this running downwards on an even gradient to the point of location of the ram itself, runs the drain pipe which has to be heavy gauge galvanised steel or cast iron pipe and of appropriate length which is dependent upon the height to which the water is to be pumped. Although it is not essential that this pipe should be buried, it is preferable in order to avoid interference from wild life and unauthorised persons. The ram chamber itself can vary considerably but all that is required is a concrete base which securely hold the ram in place. Hydro rams are working unaffected by the temperature changes (especially low temperatures which might cause a conventional system to freeze unless some form of heat is provided.)

Many people have access to some form of running water and do not know how much power, if any, can be produced from it. Almost any house site has solar electric potential (photovoltaic). Many sites also have some wind power available. But water power depends on more than the presence of water alone. A lake or well has no power potential. The water must be flowing. In construction of small or micro hydro system many factors determine the feasibility of such a system. These include:
the amount of power available from the stream, and if it is sufficient to meet power requirements;
legal restrictions-local or state, on the development of the hydroelectric site, and the use of the water;
the availability of turbines and generators of the type or capacity required;
the cost of developing the site and operating the system; and
the rate a utility will pay for electricity you generate (if you connect to their system).

Principal question is: do I have a site suitable for hydroelectric power production? To answer that question, we have to examine four factors.
The distance of head or vertical fall that the water source develops.
The amount of water available for generation.
The length of pipe needed to go from the water source to the hydropower plant.
The distance from the hydropower plant to the electrical load, whether that be storage batteries, or in the case of AC generation, the appliances themselves.

Given these four factors, we can determine not only if hydroelectric power generation is feasible, but which diameter of pipe is needed, which type of the available hydroplants to use, and approximate output and costs.

The first step in assessing the feasibility of any hydroelectric system is to determine the amount of power that you can obtain from the stream at your site. The power available at any place is primarily a product of the flow and “head.” Flow is amount of water flowing through the turbine and is typically measured in cubic meters per second – m3/s or in cubic feet per second – cfs or gallons per minute – GPM are used.
Head is a measure of the pressure of falling water available at turbine expressed in meter water column. This pressure is a function of the vertical distance that water drops and the characteristics of the channel, or pipe, through which it flows. It must be distinguished between gross head, which is the difference of elevation between the water surface of the forebay and the tail race and net head, which is the actual pressure available at the turbine. To obtain net head, allowances must be made for losses in the penstock and draft tube. Gross head can be determined by a topographical survey using levels and tape measures. Head is expressed in meters (or in feet in the USA). High flow and/or head means more available power. The higher the head the better, because less water is necessary to produce a given amount of power, and smaller, more efficient, and less costly turbines and piping can be used.
Hydroelectric sites are broadly categorized as “low” or “high” head. Low head typically refers to a change in elevation of less than 3 meters. A vertical drop of less than 0,6 meters will probably make a hydroelectric system unfeasible. A high flow rate can compensate for low head, but a larger, and more costly turbine will be necessary. It may be difficult to find a turbine that will operate efficiently under very low heads and low flow.

Small hydro turbines can be configured to operate efficiently at sites with a wide range of head and flow rates. In case of micro hydro systems with batteries the greater predictability of hydro resources can help reduce the size of other system components like battery banks. Battery banks for PV systems are usually sized to provide five days of cloudy-day power, while small hydro systems usually need only one or two days of storage. It is responsible to assess a hydro resource during both wet and dry seasons. It is the responsibility of anyone who uses a hydro resource to evaluate the effects that water diversion may have on the ecology of the waterway and understand any applicable regulatory or legal restrictions. A rule of thumb used by some hydro builders is to divert 10 percent or less of the stream's minimum flow. Note that use, access to, control, or diversion of water flows is highly regulated in many countries. So is any physical alteration of a stream channel or bank that may effect water quality or wildlife habitat, regardless of whether or not the stream is on private property.

Determining Head
When determining head (fall), you must consider both gross or “static” head, and net or “dynamic” head. Gross head is the vertical distance between the top of the penstock (the piping that conveys water, under pressure, to the turbine) and the point where the water discharges from the turbine. Net head is gross head minus the pressure or head losses due to friction and turbulence in the penstock. These head losses depend on the type, diameter, and length of the penstock piping, and the number of bends or elbows. You can use gross head to approximate power availability and determine general feasibility, but you must use net head to calculate the actual power available. There are several ways to determine gross head. The most accurate technique is to have a professional survey the site. If you know that you have an elevation drop of several dozens meters, a less expensive, but less accurate technique is to use an aircraft altimeter. In some countries it is possible to buy, borrow, or rent an altimeter from a small airport or flying club. You will have to account for the effects of barometric pressure and calibrate the altimeter as necessary. Another option is to use the “hose/tube” method described below.
Whatever method you use, you will need to determine the vertical distance between the point where water will enter the penstock and the point where water will discharge from the turbine. Always be safety-conscious when working near or in a stream, especially in narrow or steep stream channels and fast flowing water. Never work alone. Never wade into water in which you cannot see the bottom and without first testing the depth with a stick.
To perform the “hose/tube” method you will need an assistant, 6 to 9 meter length of small-diameter garden hose or other flexible tubing, a funnel, and a yardstick or measuring tape. Begin by stretching the hose or tubing down the stream channel from the point that you have decided is the most practical elevation for the penstock intake. Have your helper hold the upstream end of the hose, with the funnel in it, under the water as near the surface as possible. While he/she does this, lift the downstream end until water stops flowing from it. Measure the vertical distance between your end of the tube and the surface of the water. This is the gross head for the section of stream between you and your helper. Have your assistant move to where you are and place the funnel at the same point where you took your measurement. Then walk downstream, and repeat the procedure. Continue taking measurements until you reach the point where you plan to site the turbine. The sum of these measurements will give you a rough approximation of the gross head for your site. Note that, due to the force of the water into the upstream end the hose, water may continue to move through the hose after both ends of the hose are actually level. You may subtract few centimetres from each measurement to account for this. It is best to be conservative in these preliminary head measurements.

Determining Water Flow
Environmental and climatic factors, as well as human activities in the watershed, determine the amount and characteristics of stream flow on a day-to-day and seasonal basis. A storage reservoir can control flow, but unless a dam already exists, building one can greatly increase cost and legal complications. You may be able to obtain stream flow data from the local offices, from the local engineer, or local water supply or flood control authorities. If you cannot obtain existing flow data for your stream, you will need to do a site survey. Generally, unless you are considering a storage reservoir, you should use the lowest average flow of the year as the basis of the system design. Alternatively, you can use the average flow during the period of highest expected electricity demand. This may or may not coincide with lowest flows. There may be legal restrictions on the amount of water that you can divert from a stream at certain times of the year. In such a case, you will have to use this amount of available flow as the basis of design.

Measuring flow is a little more difficult. This should probably be done in more than one place too. This is because most streams pick up water as they go. Therefore choosing the best spot for your system requires careful consideration of several things. There are several ways to measure flow; here are two. In both cases, the brook water must all pass through either a pipe or a weir. A common method for measuring flow on very small streams is the “bucket” method. This involves damming the stream with logs or boards to divert the stream flow into a bucket or container. This method is the easiest way of measuring flow for streams with up to 5 litres per second or so, which accounts for most small hydro sites by far. You'll need to construct a temporary dam of sorts at the water source. Than fit a short length of pipe large enough to handle all the water you plan to use for generation into the dam. Using a bucket of known capacity and a stopwatch you will have to estimate the time - how long it takes to fill the bucket. Repeat several times to determine that your technique is accurate. The rate that the container fills is the flow rate. For example, 20 litre bucket that fills in one minute is a flow rate of 20 litres per minute.

You can also try the following method to roughly estimate the flow in streams where it is impractical to attempt the bucket method. This method involves wading across the stream channel. Do not try this method if the stream is fast-flowing and over your calves! You could lose your footing, be swept downstream, and possibly drown. Never wade into any stream in which you cannot see the bottom! Always check the depth and character of the stream bed with your stick before you take a step. To perform this method, you will need an assistant, a tape measure, a yardstick or calibrated measuring rod, a weighted float (a plastic bottle half filled with water to give a better estimate of flow velocity), a stopwatch, and some graph paper. Begin by calculating the cross-sectional area of the stream bed during the time of lowest water flow. To do so, select a stretch of the stream with the straightest channel and most uniform depth and width as possible. At the narrowest point of this stretch, measure the width of the stream. Then, with the yardstick, walk across the stream and measure the water depth at 30 centimetres increments across the stream. Be sure to keep the measuring stick as vertical as possible. You may want to stretch a string or rope across the stream with the increments marked on it to assist in this process. Plot these depths on a piece of graph paper. This will give you a cross-sectional profile of the stream. Determine the area of each block or section of the stream by calculating the areas of the rectangles and triangles in each section. (Area of a rectangle = length x width; area of a triangle = ½ base x height). Add the areas of all the blocks together for the total area.

Next, determine the flow velocity. From the point where you measured the width, mark a point at least 10 meters upstream, and release the weighted float in the middle of the stream. Carefully record the time it takes the float to pass between the two points. Make sure that the float does not hit or drag on the bottom of the stream. If it does, use a smaller float. Divide the distance between the two points by the float time in seconds to get flow velocity in meters per second. Repeat this procedure several times to get an average value. The more times you do so, the more accurate your estimate will be. If the float gets hung up or “stalls,” start over, or this will throw the average off. Multiply the average velocity by the cross-sectional area of the stream. Multiply this value by a factor that accounts for the roughness of the stream channel (0.8 for a sandy stream bed, 0.7 for a bed with small to medium sized stones, and 0.6 for a bed with many large stones). The result will be the flow rate in cubic meters per second.

Keep in mind that this value will be the flow at the time of measurement. You should repeat the procedure several times during the low flow season to more accurately estimate the average low water flow. You do not have to measure the water depth each time. You can simply measure the water depth above, or below, the water level when you first measured the stream, and calculate the area of greater or less water, and add or subtract this from the baseline area. Alternatively, you may be able to install a gauge (made from a calibrated rod or post) on the bank so that you can easily read the water depth and calculate the cross-sectional area of the stream. You will need to repeat the flow velocity procedure each time, however.
You may be able to correlate your survey data with long term precipitation data for your area, or flow data from nearby rivers, to get an estimate of long-term, seasonal low, high, and average flows for your stream. Remember that, no matter what the volume of the flow is at any one time, you may be able to legally divert only a certain amount or percentage of the flow. Also try to determine if there any plans for development or changes in land use upstream from your site. Activities such as logging can greatly alter stream flows.

In real fluid flows, losses occur due to the resistance of the pipe walls and the fittings to this flow and lead to an irreversible transformation of the energy of the flowing fluid into heat. Two forms of losses can be distinguished: losses due to friction and local losses.
Losses due to friction originate in the shear stresses between adjacent layers of water gliding along each other at different speed. The very thin layer of water adhering the pipe wall does obviously not move while the velocity of every concentric layer increases to reach maximum velocity at the centre-line of the pipe. If the fluid particles move along smooth layers, the flow is called laminar or viscous and shear stresses between the layers dominate. In engineering practice however, the flow in a pipeline is usually turbulent, i.e. the particles move in irregular paths and changing velocities. It is important to use pipelines of sufficient diameter to minimise friction losses from the moving water. When possible the pipeline should be buried. This stabilises  the pipe.
Local losses occur at changes of cross sections, at valves and at bends. These losses are sometimes referred to as minor losses since in long pipelines their effect may be small in relation to the friction loss.

Determining Power
At most sites, what is called run of river is the best mode of operation. This means that power is produced at a constant rate according to the amount of water available. Usually the power is generated as electricity and can be eventually stored in batteries. The power can take other forms: shaft power for a saw, pump, grinder, etc. Both head and flow are necessary to produce power. Even a few litres per second can be useful if there is sufficient head. Since power = Head x Flow, the more you have of either, the more power is available.
To calculate available power, head losses due to friction of flow in conduits and the conversion efficiency of machines employed must also be considered. The simple formula for potential power output is following:

Power (kW) = Head (metres) x Flow (m3/second) x Gravity (9,81) x Efficiency (0,6)

Head = Net head = Gross head -losses (m)
Here the overall efficiency was set at 60%.

For small outputs of interest here, and as a first approximation, the formula can be simplified:

Power(kW)= Head (m) x Flow (litres/second)/200

Here the overall efficiency of 50 % is implied. The “rule of thumb” calculation is therefore on the  conservative side.

For the US units a simple rule of thumb to estimate your power is :

Power (Watts) = Head (feet) x Flow (gpm) /10

Keep in mind this is power that is produced 24 hours a day. So 100 W in hydro power plant is equivalent to a PV system of 400-500 watts if the sun shines every day. Of course, the water may not run year round either.
The efficiencies (including turbine and generator efficiencies), which were chosen in above mentioned equations between 50-60 %, depend on make and operating conditions (head and flow). Generally, low head, low speed water wheels are less efficient than high head, high speed turbines. The overall efficiency of a system can range between 40% and 70%. A well-designed system will achieve an average efficiency of 75%. Turbine manufacturers should be able to provide a close estimate of potential power output for their turbine, given the head and flow conditions at your site. There will also be “line” losses in any power lines used to transmit the electricity from the generator to the site of use.

A turbine/generator that produces 500 watts continuously (12 kWh per day), and includes batteries for power storage, will be sufficient to meet the power requirements of a small house for lighting, entertainment, a refrigerator, and other kitchen appliances. Remember that using energy conservatively in energy-efficient appliances can reduce energy requirements significantly.

Estimation of annual electricity production E:

E (kWh) = Power (kW) x Time (hours)

Where time is estimated number of operational hours in the year. Mostly it is supposed to be 5000 hours.
Hand Rule
In a typical small hydro power plant every litre per second (0.001 m3/s) of water falling down from 1 meter height can produce 20 - 30 kWh of electricity per year.

Conversion Factors
Here are some of the conversion factors you may need when evaluating a hydro power site:
1 cubic foot (cf) = 7,48 gallons;
1 cubic foot per second (cfs) = 448,8 gallons per minute (gpm);
1 inch = 2,54 centimetres; 1 foot = 0,3048 meters;
1 meter = 3,28 feet; 1 cf = 0,028 cubic meters (cm); 1 m3 = 35,3 cf;
1 gallon = 3,785 litres; 1 cf = 28.31 litres; 1 cfs = 1698,7 litres per minute;
1 cubic meter per second (m3 /s) = 15842 gpm;
1 pound per square inch (psi) of pressure = 2,31 feet (head) of water;
1 pound (lb) = 0,454 kilograms (kg);
1 kg = 2,205 lbs;
1 kilowatt (kW) = 1,34 horsepower (hp); 1 hp = 746 Watts.

The oceans have long been recognized as a potential source of energy. The ocean's motion carries energy in the form of tides, currents, and waves. In principle, some of this energy could be used to perform work and to produce electricity.

Tidal energy differs from all other energy sources in that the energy is extracted from the potential and kinetic energies of the earth-moon-sun system. The well known ocean tides result from this interaction, producing variations in ocean water levels along the shores of all continents. As the water level fluctuates twice daily through this range, it alternately fills and empties natural basins along the shoreline, suggesting that the currents flowing in and out of these basins could be used to drive water turbines connected to generators and thus to produce electricity. The higher the tides, the more electricity can be generated from a given site, and the lower the cost of electricity produced. The technology employing this energy source is very similar to that of low head hydropower.

World-wide, approximately 3000 GW of energy is continuously available from the action of tides. Experts estimated that only 2% (60 GW), what is about 50 times less than the world's potential of hydroelectric power capacity, can potentially be recovered from tides for electricity generation. Currently, only in places with large tidal range (greater than 5 meters) can tidal power be extracted economically.
In some places of the world tidal energy is quite attractive. For coastal areas, usually at the entrances to large estuaries, resonance can occur, leading to far greater than average tidal ranges which could relatively conveniently be blocked off. Such circumstances are found e.g. in Canada, with a mean tidal range of 10,8 metres or in the Severn Estuary in Britain with a mean range of 8,8 metres, making large scale projects at both these locations economical.

Over the past forty years, there has been constant interest in harnessing tidal power. Initially, this interest focused on estuaries, where large volumes of water pass through narrow channels generating high current velocities. Engineers felt that blocking estuaries with a barrage and forcing water through turbines would be an effective way to generate electricity. From an engineering point of view they were right. But, increasingly the environmental costs of such a design became clear.

There are three commercial-scale tidal power plants (barrages) in operation: a 240 MW plant which was completed on the estuary of the La Rance River near St. Malo, France in 1967, a  1MW plant on the White Sea in Russia completed in 1969 and a 16 MW plant in Nova Scotia, Canada. The environmental problems have prevented further development of the barrage technology.

Tidal power plant in La Rance.

Tidal power plant at La Rance River has turbines that can also serve as pumps; thus, the installation can function as a pumped hydro storage facility to even out the loads on a large electricity generating and distribution system. In this way water pumped into the basin during times of low power demand increases the head on the turbines at other times. Tidal range there is up to 13,4 meters. The dam's width is 760 meters. At high tide, the dam traps Atlantic waters in the bay. At low tide, the water flows back to the sea. En route, it passes through 24 turbines connected to generators that produce 240 megawatts of power. This provides enough electricity for a city of 300.000. In 1997, they began installing turbines that can spin on both the incoming and outgoing tides.

Tidal power is a proven technology: it has been used for centuries in small mill-type applications where natural conditions make it possible. Tidal energy can be converted into electrical energy in several ways. Conventional systems such as barrages (or low dams) store water in inlets from high tides for release through hydraulic turbines during lower tides. The newest technology which converts tidal or coastal currents to power seems to be very promising because it is less environmentally destructive.

The usual technique (referred to as “barrage” technology) is to dam a tidally-effected estuary or inlet, allowing the tidal flow to build up on the ocean side of the dam and then generating power during the few hour high tide period. In this way it is functioning in La Rance. After the water level reaches maximum high tide, gate valves are closed and the water is impounded and awaits low tide when it is released and produces power. The gates can be opened or closed in sequence with the tides permitting water flow only when there is sufficient head to power the turbines. The basic technology of power production is similar to that for low head hydro power plants what means that the head drives the water through the turbine generators. The main difference, apart from the salt water environment, is that the turbines in tidal barrages have to deal with regularly varying heads of water. The turbines are designed so that the flow of water both into and out of the basin produces electricity. Because of the intermittent nature of this flow, the effective duty factor of such an installation is less than 100%. A tidal power station produces only about one third as much electrical energy as would a hydroelectric power plant of the same peak capacity operating continuously. Tidal barrages are effectively fences which completely block a estuary channel.

The barrage does not easily scale up to modern commercial levels of output capacity. By increasing the size of the pond one increases the four major negative environmental impacts of the barrage technology: navigation is blocked, fish migration is impeded and fish are killed by passing through the turbines, the location and nature of the intertidal zone are changed, and the tidal regime is changed downstream. Reduced tidal range would destroy much of the habitat used by wading birds, fish (such as salmon) would be unable to travel upstream to breed, and sediment trapped behind the barrage could quickly reduce the volume of the estuary. It seems that while there are few environmental impacts associated with a smaller tidal facility, (i.e., no siltation, no negative impacts to water tables, fisheries or fish migration), larger operations could potentially limit fish and mammal passage and change tidal ranges, thereby effecting salt water intrusion into local tributary streams and impacting salmon spawning.

By the early 1990s, interest in estuarine-derived tidal power had declined, and scientists and engineers began to look at the potential of coastal currents which can be harnessed by tidal turbines. Instead of using costly barrages and low head turbines located in estuaries, it may be possible to harness the kinetic energy of the tides in fast tidal currents or streams at suitable sites, using relatively simple techniques - tidal turbines. As tides ebb and flow, currents are often generated in coastal waters (quite often in areas far-removed from bays and estuaries). In many places the shape of the seabed forces water to flow through narrow channels, or around headlands (much like the wind howls through narrow valleys and around hills). However, sea water has a much higher density than air (832 times). Thus, currents running at velocities of 5 - 8 knots (9,25 km/h – 16,7 km/h) have the same energy potential as a windmill site with windspeeds of 390 km/hr! In addition, unlike the wind rushing through a valley or over hilltops, tidally-generated coastal currents are predictable. The tide comes in and out every twelve hours, resulting in currents which reach peak velocity four times every day.

Tidal turbines are the chief competition to the tidal barrages but the idea is as yet relatively underdeveloped. Looking like an underwater wind turbine they offer a number of advantages over the tidal barrages. They are less disruptive to wildlife, allow small boats to continue to use the area, and have much lower material requirements than the dam. Tidal turbines function well where coastal currents run at 2-3 m/s (slower currents tend to be uneconomic while larger ones put a lot of stress on the equipment). In such currents a turbine 20m in diameter will generate as much energy as a 60m diameter windmill. The advantages of the tidal turbine is that it is neither seen, nor heard. The whole assembly (apart from the transformer) is below the waterline.

There are many sites around the world where tidal turbines would be effective. Coastal currents are strongest at the margins of the worlds larger oceans. A review of likely tidal power sites in the late 1980s estimated the energy resource was in excess of 330.000 MW. South East Asia is one area where it is likely such currents could be exploited for energy. In particular, the Chinese and Japanese coasts, and the large number of straits between the islands of the Philippines are suitable for development of power generation from coastal currents. In all of these regions underwater turbine farms can be developed. The ideal site is close to shore, in water depths of about 30m where at the best sites currents could generate more than 10 megawatts of energy per square kilometre. The European Union has already identified 106 sites which would be suitable for the turbines, 42 of them around the UK. The first tidal turbines will be deployed off the Southwest coast of England. It will be 12-15 m in diameter, and is expected to generate 300 kW (enough to power a small village). It is estimated that the cost of energy from these early turbines will be USD  0,10/kWh. This is more expensive than conventional sources of energy (coal, gas), but significantly lower than what many island communities already pay for energy. As the technology matures further, prices will probably continue to drop.

A large part of the major influx of energy to this planet, solar energy, is converted by natural processes, i.e. through wind generation, to energy associated with waves. Waves are generated by the wind as it blows across the ocean surface. The energy thus contained is significant, in favoured latitudes with values of around 70 MW/km of wave frontage.
Ocean waves represent a considerable renewable energy resource. They travel great distances without significant losses and so act as an efficient energy transport mechanism across thousands of kilometres. Waves generated by a storm in mid-Atlantic will travel all the way to the coast of Europe without significant loss of energy. All of the energy is concentrated near the water surface with little wave action below 50 metres depth. This makes wave power a highly concentrated energy source with much smaller hourly and day-to-day variations than other renewable resources such as wind or solar.

Since in principle hundreds of kilometres lined with generating stations are conceivable, wave energy could contribute significantly to world energy supply if an economic way of extracting this energy could be found. The highest concentration of wave power can be found in the areas of the strongest winds, i.e. between latitudes 40 deg. and 60 deg. in both the northern and southern hemispheres on the eastern sides of the oceans. Countries like the United Kingdom are thus the world's most favoured locations for the extraction of wave power.

Typically ocean wave devices capture the energy of waves and convert their energy to electricity. Wave energy devices include hydro-piezoelectric, oscillating water columns, wave run up (tapered channel) and sea clams. Particularly ‘sea clams' involve wave action forcing air between blades located on the perimeter of a circular barge structure. The air is then run through air turbines which rotate at a shaft connected to an electrical generator.

Europe, and in particular the United Kingdom, are looking at wave power. A recent review by the UK government has shown that there are now types of wave power devices which can produce electricity at a cost of under USD 0,10/kWh, the point at which production of electricity becomes economically viable. The most efficient of the devices, the “Salter ”Duck can produce electricity for less than USD 0,05/kWh. The “Salter ”Duck was developed in the 1970s by Professor Stephen Salter at the University of Edinburgh in Scotland and generates electricity by bobbing up and down with the waves. Although it can produce energy extremely efficiently it was effectively killed off in the mid 1980s when a European Union report miscalculated the cost of the electricity it produced by a factor of 10. In the last few years, the error has been realised, and interest in the Duck is becoming intense.

The “Clam” is another device which, Like the “Salter ”Duck can make energy from sea swell. The Clam is an arrangement of six airbags mounted around a hollow circular spine. As waves impact on the structure air is forced between the six bags via the hollow spine which is equipped with self-rectifying turbines. Even allowing for cabling to shore, it is calculated that the Clam can produce energy for around USD 0,06 /kWh.

Both the Duck and the Clam generate energy from waves at sea. This is useful for generating energy for offshore structures and low-lying islands. However, where islands offer suitable sites, cliff-mounted oscillating water column (OWC) generators have a number of advantages, not the least of which is the fact that generators and all cabling are shore-based, making maintenance of the former and replacement of the latter much simpler. The OWC works on a simple principle. As a wave pours into the main chamber, air is forced up a funnel which houses a turbine. As the wave retreats, air is sucked down into the main chamber again, spinning the turbine in the opposite direction
OWC machines have already been tested at a number of sites, including Japan and Norway. The UK is on the verge of deploying Osprey II, a second generation OWC. There is particular interest in OWC systems because of the large amount (7,000 MW) of shoreline wave energy available for exploitation. Costs for OWC machine-generated electricity is likely to start at USD 0,10 /kWh. The first-generation system, based on the island of Islay takes advantage of a natural rock gully to drive a 180 kW turbine attached to an electricity generator. Built by researchers from the Queen's University of Belfast the system supplies electricity to the local grid, which is connected to the mainland national grid by submarine cable However, both OWC-systems and ocean-wave systems suffer from trying to harness violent forces. The first Norwegian OWC was ripped off a cliff-face during a storm, the Islay station is completely submerged under storm conditions.

There have been several proposals to harness ocean waves to generate electricity or to make other useful products such as fresh water or hydrogen. To date none of these has been successfully commercialised. Ocean Power Delivery Ltd. is developing a novel offshore wave energy converter called Pelamis. The company has successfully bid for a contract to install a pair of 375kW prototype devices off Islay, Scotland, under the 1999 Scottish Renewables Obligation. The device has an annual capacity factor of 38% at the site chosen. It is approximately 130metres long and 3,5metres in diameter. It is scheduled to be installed early in 2002 and will generate over 2,5 million kWh's of electricity per year, enough to provide power for 150-200 homes.

The Pelamis device is a semi-submerged, articulated structure composed of cylindrical sections linked by hinged joints. The wave induced motion of these joints is resisted by hydraulic rams which pump high pressure oil through hydraulic motors via smoothing accumulators. The hydraulic motors drive electrical generators to produce electricity. Power from all the joints is fed down a single cable to a junction on the sea bed. Several devices can be connected together and linked to shore through a single seabed cable. A novel joint configuration is used to induce a tuneable, cross-coupled resonant response which greatly increases power capture in small seas. Control of the restraint applied to the joints allows this resonant response to be ‘turned-up' in small seas where capture efficiency must be maximised or ‘turned-down' to limit loads and motions in survival conditions.
The complete device is flexibly moored so as to swing head-on to the incoming waves and derives its ‘reference' from spanning successive wave crests.
The Pelamis device has a number of important advantages over other existing or proposed Wave Energy Converters, these include:
Tuneable response allows power capture to be maximised in small seas while limiting loads and motions in extreme conditions
The head on aspect to severe waves presents the minimum resistance to the high velocities in extreme wave crests
The finite length of the device is optimised to extract power from shorter wavelengths and is unable to reference against the long waves associated with storm conditions
The small diameter leads to local submergence or emergence in large waves limiting the forces and moments in the structure
The flexible mooring system has a range of motion able to accommodate the largest waves

The Marine Science & Technology Centre of Japan launched the world's largest offshore floating wave power device in July 1998, and the full-scale prototype will be tested until the year 2000.
This floating device, called the Mighty Whale, converts wave energy to electricity. The device measures 50 metres long by 30 metres wide, and uses waves in the Pacific Ocean to drive three air turbines (one with a rated output of 50 kW + 10 kW and two of 30 kW) on board the platform, to generate 120 kW of electricity.

After being towed to its mooring about 1.5 km from the mouth of Gokasho Bay, the Mighty Whale was anchored to the bottom of the sea (about 40 m deep) with six mooring lines; four lines on the seaward side and two on the lee side. Mooring lines are designed to withstand typhoon winds, and the unit is designed to handle waves of 8 m. The Mighty Whale converts wave energy to electricity by using oscillating columns of water to drive air turbines. Waves flowing in and out of the air chambers at the ‘mouth' of the Mighty Whale make  the water level in the chambers rise and fall. The water forces air into and out of the chambers through nozzles on the tops of the chambers. The resulting high-speed air-flows rotate air turbines which drive the generators. The Mighty Whale can be remotely controlled from on-shore. In the demonstration prototype, the energy produced is mostly used by the instruments carried on board; any surplus is used to charge a storage battery or, when this is fully charged, is used by a loading resistor. A safety valve protects the air turbines from stormy weather by shutting off the flow of air if the rotation speed of the turbines exceeds a predetermined level. So that it can be used in the future to improve water quality, the prototype is also equipped with an air compressor to provide aeration.
Because it has absorbed and converted most of the energy in the wave, the Mighty Whale also creates calm sea space behind it, and this feature can be utilised; for example, to make areas suitable for fish farming and water sports. The structure of the Mighty Whale itself can be used as a weather monitoring station, a temporary mooring for small vessels or a recreational fishing platform.

At the present time both tide and wave energy are suffering from orientation problems, in the sense that neither method is strictly economical on a large scale in comparison with conventional power sources. In addition, neither will produce electricity at a steady rate and thus not necessarily at times of peak demand. Wave power stations suffer even more from these problems, their rate of production being unreliable. In Norway development of wave power was taken a step further, concentrating on small applications on remote islands and the like, and for quite a while a small power plant (500 kW) operated successfully in Toftestallen until it was swept away by the sea.

The disadvantages of wave power stations compared to maybe their closest rival - wind power - are obvious: A wave power unit will probably not have much more than three times the output of a single wind turbine, but the construction costs are likely to be far higher due to mooring problems, the bulkiness and comparative complexity of the whole structure and the water-based location. It will take some time - and far more investment into renewable energy sources - before the only comparative bonus, the fact that they use up and deface less land, will prevail over economic considerations.

And while wave energy is used successfully in very small scale applications, such as powering lighthouses or navigation buoys, its short term prospects as a major contributor to large scale energy production seems to be economically almost ruled out. So until the cost of maintaining the present rate of carbon dioxide emission is taken into account when building new power stations and a policy is adopted that depends less rigorously on market forces, the likelihood of tidal or wave power playing a major part in the energy supply of western industrialised nations even in the medium term future is small.