Energie hidraulică 

    Hydraulic power is the ability of a physical system (the water) to make a mechanical work at the shift from one state to another given state (flow). Due to the water circuit in nature sustained by the energy of the Sun, it is considered a form of renewable energy. 
    Hydraulic energy is a mechanical energy, consisting of water’s potential energy given by the difference in level between the accumulation lake and the power plant, respectively of the kinetic energy of moving water. Exploitation of this energy it’s made in hydro power plants, which converts potential energy of the water into kinetic energy, which then traps by means of hydraulic turbines that operate electric generators which converts it into electricity.
    The kinetic energy of the waves and tides are also considered forms of hydraulic energy.
    Hydraulic energy has been used since antiquity In India, where they used hydraulic wheels on water mills. In the Roman Empire water operated mills that produced flour, were also used to power sawmills for cutting wood and stone. The strength of a torrent of water released from a reservoir was used in ore extraction, a method that even Pliny the Elder had described. The method has been widely used in the Middle Ages in Great Britain and even later to extract lead and tin ore. The method was evolved in hydraulic mining, used during the gold rush in California.
    In China and in the Far East, hydraulic wheels with cups were used for crop irrigation. Around the year of 1830, during the peak of the channels, hydraulic power was used to tow barges up and down steep slopes. The mechanical energy needed for various industries has resulted in their location near the water falls.
    These days we use hydraulic energy to produce electricity, which is produced in this case, with relatively low costs and the produced energy can be used relatively far from sources.
    Modes of exploitation of the hydraulic energy
    Hydraulic Wheel
    Hydraulic wheel energy uses rivers to produce mechanical work directly.
    At low flow rates by exploiting the potential energy of the water. For this purpose there are used are fitted cups and the supply of water is at the top of the wheel, filling water cups. The weight of the cups of water is the force acting the wheel. In this case, the fall corresponds to the difference between the level where water is admitted into the cups, thus evacuated and bigger if the wheel’s diameter is bigger.  

    At high flow rates it’s exploited the kinetic energy of the water. For this purpose wheels that have mounted blades are used and the supply of water is at the bottom of the wheel, the water pushing the palettes. To achieve bigger moments, the radius of the wheel should be big. Often, to speed water flow in the direction of the wheel, before they place an overflow weir, which raises the water level (fall) and convert this fall’s potential energy into suplimentary kinetic energy, the discharge speed adding to the velocity normal flow of the river.  


Hydro Powerplants 
    A hydroelectric facility uses the river in the form of dams, to produce electricity. Exploitation of hydropower potential depends on both the fall and the water flow available. The greater the collapse and the flow are,the bigger the quantity of electricity produced. Hydraulic energy is captured with turbines.  
    Romania's hydro power potential was exploited in 1994 at the rate of approx. 40%. Hydroelectric powerplants had an installed power of 5.8 GW, representing approximately 40% of installed power in Romania. Actual production of hydro powerplants in 1994 was nearly 13 TWh, representing about 24% of the total electricity produced. Actual power exceeds 6 GW installed and production is about 20 TWh per year. The share of electricity produced on the basis of hydraulic energy is about 22 - 33%.
      Hydraulic Micro and Pico Powerplants
      Hydraulic micro powerplant is a hydroelectric powerplants that has the installed power of 5 - 100 kW, and a hydraulic pico power plant has an installed power of 1 - 5 kW. A pico power plant can supply with energy a group of several houses, and a micro power plant a small settlement. 
    Since power consumption variations are large, to stabilize the functioning of the power plant batteries can be used, which are loaded at times of low consumption and ensures the consumption in peak periods. Given that the low voltage current generated by the micro powerplant can not be conveniently transported to remote locations, batteries must be placed near the turbine. It takes all components of a classical hydroelectric powerplant - less the dam - ie the catchment, the supply pipes, turbine, generator, batteries, regulators, inverters, which raises the voltage to 230 V, therefore the cost of such a facility is not small and the solution is advisable only for remote areas, which do not have electric lines.
    Micro powerplants can be installed on relatively small rivers, but due to seasonal fluctuations in river flow, without the dam, the river flow should be considerably greater than that taken for the micro powerplant. For a power of 1 kW, a drop of 100 m and a flow of 1 l/s is needed. In practice, because the yield of conversion requires a flow nearly double, the usual yield is just over 50%.
      Tidedriven Powerplants
    Tidedriven powerplants recover the tide. In tidal areas, tide appears twice daily, causing the lifting and lowering of the water level. There are two ways of expoiting the tidal energy:
- Powerplants without a dam, using only kinetic energy of water, similar to the way that wind mills operate wind power.
- Powerplants with a dam, which exploit the potential energy of the water, obtained by the increase of the water level due to the tides.
      As the tide of the Black Sea is just of a few centimeters, Romania has no potential for such powerplants.
      Powerplants that recover wave energy
    For wave energy recovery, similar schemes to those of tidedriven powerplants with a dam can be used, but due to the short wave period, these schemes are not effective. 

    A floater object running on a wave executes a motion with elliptical trajectory. The simplest form of harnessing this movement for the recovery of wave energy, is through articulated pontoons. A modern construction is the pelamis type [8] , that consists of several articulated cylinders, which, because of the movement of the waves, have relative moves which act some pistons. Pistons pump under pressure oil through hydraulic motors which operate electrical generators. 

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