Small Scale Hydro Power

Small Scale Hydro Power Hydro power is perhaps the most long-established renewable energy technology. The first water wheels - used in irrigation - were developed in the far-east over two-thousand years ago. Later, waterwheels were applied to milling (hence watermills); by the time of the Industrial Revolution the use of waterwheels was extensive: working the bellows in foundries and forges, driving the machinery of factories. Mechanical outputs ranged from a few hundred watts to a few tens of kilowatts. Towards the middle of the 19th Century the first water turbines were developed. The turbine was distinct from the wheel in that it was smaller, more compact, more efficient, and ran at a higher speed. The latter qualities made the water turbine particularly suited to use in electricity generation. Technology Continuous improvements in small turbine and generator technology mean that 'Micro' (under 100kW), and 'Small-scale'(up to 5 MW), hydro-schemes are an increasingly attractive means of producing electricity. Useful power may be produced from anything upwards of a small stream; the likely range is from a few hundred watts (possibly for use with batteries) for domestic schemes, to a minimum of 25 kW for commercial schemes. The power available in a body of water depends on: the amount of water flowing per second, the flow rate; the height through which the water falls, the head; and the force of gravity. The actual electrical power output of a scheme will depend on how efficiently it converts the power of the water into electrical power (maximum efficiencies of perhaps 80% are possible, but for small schemes it is more realistic to aim for 50%). P = H x Q x g x e Where - P is power measured in kW - H is the head measured in metres, m - Q is the flow rate measured in m3/s - g is the gravitational constant (~9.8m/s2) - e is the efficiency factor (usually ~0.5, i.e. 50%) (With very small schemes it may be convenient to measure flow in litres per second - if this measure is used the resulting power will be in watts, W, as opposed to kW).

Components of a typical scheme Given the expected power output, a typical micro-hydro scheme will be constructed from a variety of components:

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An intake, often incorporated into a weir, to divert the flow from the water course

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A penstock pipe to convey the water from the intake - must be of sufficient diameter to minimise 'head-loss' A powerhouse, in which the turbine and generator-set convert the power of the water into electricity An outflow through which the water is released back to the river or stream Underground cables, or overhead lines to transmit the electricity to its point of use - must be of a sufficient size to minimise 'voltage-drop'.

Turbines The turbine unit consists of a wheel, or runner, connected to a shaft. The shaft spins as a result of the water acting upon the runner and drives a generator which converts the rotational power into electricity. To suit a variety of head and flow conditions there are many different types of turbine; they fall into two basic categories: Impulse Turbines A high-speed jet of water hits a series of specially shaped cups on the runner. The cups capture neally all of the jet's energy and the water falls down into the outflow. Reaction Turbines Here the water is not converted into a jet; instead, it flows over the blades of the runner. This causes a pressure differential across the blades, causing the runner to spin - similarly to the movement of a windmill. Reaction runners often resemble ship's propellers. Environmental Aspects of using the Technology Unlike large-scale hydro schemes, some of which have proved environmentally disastrous, small-scale systems cause far less disturbance. With proper management any ecological effects are negligible. It is important to consider other users of the watercourse: a sufficient level of flow should be maintained along the section over which the water is abstracted; a safe passage for fish may be required - possibly through the provision of fishladders. Before installing a hydro scheme, a number of consents and licences may be required from the relevant authorities. In most cases the Environment Agency will charge a small one-off fee for the abstraction of the water; and Planning Permission may be required by the Local Authority. Small-scale hydro power has many environmental benefits:

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It displaces generation by fossil-fuels or nuclear power, thereby reducing the Earth's pollution burden It is quiet and visually unobtrusive, therefore not suffering some of the problems often associated with other renewables, such as windpower The embodied energy (that used in the manufacture of the components) of a scheme is typically equalled by the energy generated by the scheme within 9 months of commissioning.

Costs Involved As a rule, the capital cost per kW of installed capacity falls in proportion to the size of the scheme, varying from around £1,000 to£3,000. In addition to 'economies-of-scale', the costs will be a function of: the type of components used - e.g. new or second-hand; whether or not any existing infrastructure is available; the degree to which external contractors are to be used; the reliability of the resource - is the flow constant throughout the year?

UK Potential for Small-Hydro Power If the small-scale hydro-electric power from all the streams and rivers in the UK could be tapped it would be possible to produce 10,000GWh (1GWh = 1,000,000kWh) per year - enough to meet just over 3% of our total electrical needs. The economic potential is somewhat lower; estimates range from 500GWh to 2000GWh per year. At present, only a fraction of this is being exploited.