Tag Archives: hydroelectric

Types of Dam

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A dam is a feature built to hold back water. They come in a number of different types.

Perhaps the most well-known type of dam is the arch dam, which holds back water in much the same way as an arch holds up a building – by distributing force perpendicularly. In the case of an arch dam, the forward force of the water is distributed sideways into the surrounding abutments (i.e. the “walls” on either side of the dam).

hoover-dam

The Hoover Dam, a concrete arch-gravity dam.

Gravity dams hold back water by simply being too big and heavy for the water to move. Gravity dams are often combined with arch dams to create arch-gravity dams.

wanapum-dam

The Wanapum Dam, an earth-filled gravity dam.

Gravity dams come in many types: mass concrete dams, which are exactly what they sound like – big lumps of concrete; hollow concrete dams, which are also exactly what they sound like; buttress dams, which are hollow dams that are supported by angled buttresses that are driven into the ground by the force of the water behind them; and embankment dams which are created by piling up and compacting soil, sand, clay or rock.

buttress-dam

The Barrage de Roselend, a concrete buttress dam.

embankment-dam

The Mica Dam, an earth-filled buttress dam, and the fifteenth tallest dam in the world.

 A barrage is a special type of dam that is not designed to create a reservoir, like other dams, but rather to regulate the amount of water flowing along a river.

barrage-dam

The Pioneer River barrage, which regulates water flow along the Pioneer river.

Barrages consist of a number of gates in between supporting piers that can be opened and closed as required. I had originally decided that the Thames Barrier was a barrage, but now I think it’s more of a temporary dam than a barrage: I think that the head (the difference in height either side of the dam) when the Thames Barrier is in use is too large for it to be considered a barrage.

Why is Quincy, Washington so popular with tech companies?

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Quincy, WA is a small town (population 6750) in the north-west of the US. So why have technology giants Microsoft, Yahoo!, Dell, Vantage Data Centers, Sabey and Intuit all chosen to build huge data centres there? Quincy is now home to nearly 190 000 square metres (more than two million square feet) of data centre.

The answer is very simple: electrical power.

The town of Quincy is close to the Columbia river, the fourth largest (by volume) river in the US. There are fourteen hydroelectric dams on the Columbia river, two of which, the Priest Rapids Dam and Wanapum Dam, provide electricity to Quincy.

The Priest Rapids Dam

The Wanapum Dam

Hydroelectric power is cheap,* but more importantly from the point of view of data centre operators, it is very reliable. The reliability of the electricity supply to Quincy is better than 99.99% which is very important for “mission-critical” always-on services like cloud computing. Using renewable non-polluting hydroelectric power also helps service providers demonstrate their green credentials. Server farms consume a huge amount of electricity – more for cooling than for processing – and using hydroelectric power helps to reduce their associated carbon footprints.

* The local utility company, Grant County Public Utility District, offered electricity at a discount rate to attract users to the area.

Kinetic hydropower

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Wind is moving around the world all the time, and we use that for generating electricity. But water is moving around the world all the time too – can’t we use that to generate electricity?

A wind turbine is a device that converts kinetic energy (of the wind) into electrical energy. If you solve all the right equations you find out that the power (P) of a wind turbine is the product of the density of the air travelling through it (ρ), the cross-sectional area of the blades (A) and the speed of the wind cubed (v). (This means that if you double the speed of the wind, the power produced goes up by a factor of eight; if you triple it the power goes up by a factor of 27.)

This equation applies to any situation in which we’re using a fan-type arrangement to “catch” a moving fluid and extract useful electrical energy from its kinetic energy, so it applies just as well to moving water as it does to moving air.

Kinetic hydropower is the fancy name for putting a wind turbine underwater. Although the water in a river or sea moves much slower than the wind, the fact that it is much denser (about a thousand times denser) means that it can still generate reasonable amounts of electrical power. The low rotational speed also helps to protect marine life, which isn’t the case for wind turbines and birds.

Kinetic hydropower systems are expensive and difficult to install, and have to very sturdy to withstand the larger forces that moving water imparts; but if the engineering challenges can be overcome they have significant advantages over wind turbines, particularly in respect of their intermittency – a river’s flow is far more reliable than wind’s.