What is hydropower?

Put simply, hydropower is capturing energy from moving water. Of the renewable energy sources that generate electricity, hydropower is the most often used.

Understanding the water cycle is important to understanding hydropower. In the water cycle sunlight evaporates water which goes into the atmosphere as water vapour. As it rises away from the warm earth, some of the vapour condenses into the tiny droplets or ice particles we know as clouds. When the conditions are right, the droplets become big enough to fall as rain. Once the water is on the ground, gravity then starts it on its path downhill again. Some water soaks into the ground where it collects in underground streams or aquifers.

Renewable energy sources using water are:

• River based - River sources are primarily hydroelectric schemes of varying sizes, many of them associated with irrigation or flood mitigation schemes.
• Ocean based - Ocean energy sources include, wave energy, tidal energy and thermal energy.

River based energy

Water wheels and hydro-electric schemes harness the kinetic energy of falling or flowing water as it makes its way to sea level (although it may be collected in a lake or dam on the way).

Water wheels

The water wheel was developed more than 2000 years ago in two forms - horizontal and vertical:

• Vertical wheels - were first used to lift water and drain mining pits. It was driven treadmill-fashion by people climbing around its circumference. The step to recognising that flowing water could turn the wheel was then only a small one. It was quickly followed by building dams (mill ponds) and channels to control the flow of water. The vertical wheel needed a pair of gear wheels to turn the rotating force through 90 degrees but was much more powerful and efficient.
• Horizontal wheels - can drive millstones directly and were simple to build and repair.

The most powerful wheel was the 'overshot type'. In these, water ran down a chute to the top of the wheel, collecting in 'buckets' built into the frame. The weight of the water caused the wheel to turn.

Turbines

A wide range of turbines have evolved from the water wheel. In a turbine, a stream of water hits the blades causing it to spin. The velocity of the water is the critical factor and turbine installations usually include some means of speeding up the flow.

The power available in river based schemes depends on the quantity of water (flow) and the drop in elevation along the path of flow (head). The energy in the stream of water is converted to rotational energy by means of a water wheel or turbine. One litre per second falling about 150 metres can generate one kilowatt. 

Hydro-electricity

Hydro-electric power stations are usually associated with water storage schemes to ensure that a reliable supply of electricity can be generated whenever it's required. In fact these schemes address a basic problem with electricity: once it's generated, it has to be used immediately - it can't be stored. But the potential energy of water can be stored in dams or river systems and released to generate electricity as and when it's required.

For hydroelectric power to work, there must be a drop in height from one portion of a river to another. Usually a large dam is built to store water up a great height. At the base of the dam, you will find a turbine-generator: water from the dam is released through a gate onto the turbine causing it to spin. The mechanical power from the spinning turbine is transformed into electricity in the generator.

Small scale hydro

A number of smaller turbines have been developed for small scale or "microhydro" schemes. The water source may be from weirs in streams, or smaller creeks and rivers. Rivers flow almost constantly, small turbines from 100 W may be all that is needed for a dwelling by the river.

Pumped storage

The energy of falling water can turn the blades of a turbine and drive an electric generator to produce electricity. A supply of electricity can also power a motor to run a pump which can lift water back into 'storage' such as a dam.

This principle can be used in pumped storage schemes to store surplus electrical energy as potential energy. Water is released again through the hydro generator to produce electricity during peak periods. There are several pumped storage schemes in the eastern states of Australia.

Pumped storage schemes are viable economic propositions if they are cheaper to build and operate than alternative peak load sources, such as gas turbine plants. The savings over the project's lifetime must offset the capital cost and make allowances for the losses within the pumped storage cycle. Low cost off-peak electricity must be available for pumping, together with adequate water supplies and suitable locations for the reservoirs.

Ocean based energy

Wave energy

As wind blows across the surface of the ocean, it creates waves. In deep water waves oscillate up-and-down, while near the shore a surge of water results. Over the years, a number of schemes have been proposed to capture some of this energy:

• Floating-Pitching Devices - The vertical motion of deep water waves can be captured by mechanical devices such as Cockerell's rafts (which create a store of compressed air which drives a generator) or Salter's ducks (which impart a twisting motion to a mechanical drive).
• Oscillating Water Columns - As waves enter the column, they force the air in the column upwards past an air turbine, causing it to spin. As the wave recedes, the low pressure in the column, causes air to be sucked in from the top. This again may cause the turbine to spin, generating more power.
• Tapered Channel (TAPCHAN) - As waves crash along a rocky shoreline, water is splashed high in the air, against the cliffs. If a tapered channel is used to focus the wave towards a dam or enclosure, greater surges of water will crash against the entrance to the dam and splash over. In this way water can be collected a reasonable height and used to generate power in the same way as hydroelectricity.

The amount of power which can be generated from waves depends on their height and period. On a small scale, there are hundreds of wave-powered navigational buoys in use around the world. The oscillating wave motion drives an air turbine and generates about 60 watts.

On a larger scale, oscillating water columns have been and continue to be developed around the world to produce electricity for mains power production. Floating column developments may lead to off shore devices used to provide power for ships or fishing areas and wharfs.

Tidal energy

The tidal rise and fall of the oceans is caused by the varying gravitational pull of the sun and the moon. Larger and smaller tidal ranges occur from season to season and from year to year. The coastline and sea floor topography can also affect the tidal range which can be 15 metres or more.

Electric power can be generated using tidal energy by trapping water in an enclosed basin during high tide and allowing it to pass through turbines while the basin is emptying at low tide. Because of the relatively small difference between the high and low tide level, special turbines are required which can operate with low head and flow. There are also significant variations in both the level of the tide and when it occurs from day to day, making tidal power a relatively unreliable source.

Thermal energy

Covering nearly three-quarters of the earth's surface, the oceans are the largest collectors of solar energy on the planet. Surface temperatures of up to 26ºC can be reached in the tropics while hundreds of metres down, the water could be close to freezing.

Energy can be extracted from the temperature difference between the warmer and cooler layers in a process called Ocean Thermal Energy Conversion (OTEC). The heat energy is then used to generate electricity.

Variations have the OTEC plant on-shore, moored off-shore, or floating. Warm and cold water have to be piped to the on-shore plant and electricity cables have to be connected to the off-shore plant.

A closed cycle system for power generation seems to have the most potential. The warm surface water vaporises a working fluid such as ammonia or one of the fluorocarbon refrigerants and the vapour drives a turbine. The cold water pumped from the lower layers then condenses the vapour and the cycle continues. Because the temperature differences are so small, the heat exchangers required are enormous as are the volumes of water which must be pumped.