The Science — What Causes Water to Move?

Water is constantly moving, and much of that movement can be traced to the sun and a process known as the water cycle.

The sun causes water to evaporate from the earth’s surface. Cooler temperatures in the earth’s atmosphere cause the water to condense, forming clouds. Precipitation from the clouds falls back to the earth, feeding the rivers. Small-scale hydro installations make use of the running water in the rivers to power turbines and generate electricity.

Wave power can also be traced back to the sun. Waves are caused by wind, which, as we discussed in the wind power section, is caused by the sun heating the air.

Tidal power is one of the few energy sources that cannot be traced back to the sun’s radiant energy, although that does not mean that the sun has no effect on the earth’s tides. The gravitational forces of the moon and sun, as well as the earth’s rotation, cause the tides.

The moon, although smaller than the sun, is closer to the earth and therefore has a greater impact on tides. The gravitational force of the moon causes the oceans to bulge along an axis pointing directly at the moon. A ‘spring’ tide is caused when both the sun and moon are pulling from the same direction. A ‘neap’ tide occurs when they are pulling at a 90-degree angle from each other, which pulls water in different directions.

For more information on the water cycle and tides, see the US Geological Survey website and the Energy Systems Research Unit.

The Technology — How is Waterpower Harnessed?

Both traditional hydro and tidal installations use flowing water to power turbines in a similar way to wind.

Whether the water is being stored in a reservoir behind a large dam, or through a smaller run-of-the-river installation, the moving water in rivers tends to be used in the same way to produce electricity. The only difference is the electricity output.

In general, the water is channelled through a tunnel known as a penstock. The tunnel is dug on a downward slope to ensure the water will travel with enough speed and force to turn the turbine. The turbine wheel is situated within the tunnel and the kinetic energy from the water is what causes it to turn.

A generator sits on top of the turbine and is connected by a shaft. The shaft turns a series of large electromagnets called the rotor inside a tightly wound copper wire coil called the stator within the generator. The magnetic field between the coil and the magnets creates an electric current. The power is then transferred from the generator, through power lines onto the electricity grid.

Tidal power is harnessed in a similar way to river power. A dam is built across a river estuary to control the flow of water as the tides go in and out. The water flows through tunnels in the dam and pushes a turbine.

One of the challenges with tidal technology currently is that it only works while the tides are flowing, about ten hours per day. Another problem is that tidal estuaries are environmentally sensitive areas, and the barrages or dams disrupt the balance. They are therefore not widely used and, like large-scale dams, they are not considered to be a truly sustainable renewable energy option.

Wave power is a little more challenging to harness than the power from rivers and tides. The generator has to be able to take a beating from the crashing waves during rough seas, and it is not very useful during calm spells. The generator is usually attached to the shoreline, like the Land Installed Marine Powered Energy Transformer (Limpet) generator in Scotland. The inside of the installation is hollow and full of air. When the wave arrives, it forces the air out of the inner chamber through a hole. A turbine is located within the hole, and the air drives the turbine. The turbine is attached to the generator in a similar way to the generators used in river and tidal installations.

For more information on how waterpower plants work, see How Stuff Works website, Ontario Waterpower Association website, and information on Limpet on the Greenpeace website.

The Applications — What can Water Power Do?

Water, like wind, has been used for thousands of years to grind grains and power sawmills. Mills were set up on riverbanks where the water would drive the mill wheel, creating mechanical energy. The first hydroelectric generator in Canada was installed near Ottawa in 1882, and Ottawa was the first city in North America to sign a contract to light its streets with electric power.

All of these early installations were small in scale and it wasn’t until after World War II that advances in technology and larger installations began to leave these smaller installations behind. Many sites were decommissioned because small-scale units and their potential sites became progressively less economical to maintain and operate by public utilities.

Waterpower provided all of Ontario’s electricity until just over 50 years ago, and although waterpower no longer makes up the majority of the electricity generated in the province, there are still over 200 installations in operation. There is also some considerable potential for community or First Nations development of sites in Ontario.

Furthermore, as the environmental effects of large-scale hydro dams become more understood, and as the number of suitable large-scale sites are exhausted, there has been renewed interest in small-scale hydro. For more information on waterpower in Ontario and small-scale applications, see the Ontario Waterpower Association and CanREN.

A small-scale hydroelectric facility uses existing structures or geological formations and requires only minor engineering works. This both reduces the cost of development and the effect of the installation on the environment. Small installations still require a sizable flow of water and an adequate head of water (the height of the drop of water). Small hydroelectric plants can be developed at existing dams and have been constructed in connection with water level control of rivers, lakes and irrigation schemes. 

The CanREN website classifies small hydro installations into three categories based on size. Micro installations produce less than 100 kW of electricity, or only enough to power a couple of homes. Mini installations produce between 100 and 1000 kW (1 MW) of electricity, which is about enough to power a small factory or community. Small installations range between 1 MW and 30 MW of capacity and will usually supply power to the electricity grid.

Interesting advances in tidal power are being made. While there are only a few suitable sites for tidal generators at this time, new underwater turbine technology may make harnessing tidal and even river power easier and less environmentally harmful.

The new technology involves placing turbines similar to wind turbines underwater. The water pushes the blades, creating mechanical energy, which is then converted to electricity. These tidal technologies do not completely block the tidal estuary the way traditional dams do, and so are more sensitive to the environment. Furthermore, the websites advertise that the rotors turn slow enough that they are easily avoided by fish and other sea life. For more information on this emerging technology, visit the Blue Energy, SwanTurbines or the Marine Current Turbines website.

Benefits of Small Hydropower

As mentioned briefly above, not all hydropower installations are considered to be renewable energy. Large-scale hydro dams and tidal barrages may produce a large amount of emission free electricity, but they cause all sorts of other environmental problems.

For example, tidal barrages create a barrier to fish, change how the silt settles on the bottom of the estuary, prevent pollutants from washing out to sea (resulting in their accumulation in the estuary), and lets out the water more slowly than nature does, with the result that sometimes tidal pools where birds used to fish at low tide no longer materialize.

Small-scale hydropower has many advantages over larger installations, and some of the negative impacts can be minimized through design and appropriate operating and construction practices.

For example, some small dams use fish ladders (like the one shown in the photograph on the right) to allow migrating fish to continue upstream (or down) past the dam.

Small-scale hydroelectric developments do not take up much space, they rarely cause significant shoreline flooding and they do not require river diversions. Furthermore, they have all the general environmental benefits associated with waterpower as a renewable resource. Hydro installations do not emit greenhouse gases, the fuel is free, and the process to generate electricity does not produce waste.

Small-scale installations can also be owned by communities. Such installations benefit communities by providing electricity in remote locations and other economic benefits for the communities they are in. Like other forms of renewable energy, small-scale installations also contribute to a more distributed electricity system by generating power where it is needed rather than transporting it great distances.