March 10, 2008
Energy-Low Head Hydro
low head hydroelectric power plants
+ Hydropower, given the right site, can cost as little as a tenth of a PV system of comparable output
+ even a modest hydro output over 24 hours a day, rain or shine, will add up to a large cumulative total
+ Hydro systems get by with smaller battery banks because they only need to cover the occasional heavy power surge rather than four days of cloudy weather
Hydro turbines can be used in conjunction with any other renewable energy source, such as PV or wind, to charge a common battery bank. This is especially true in the West, where seasonal creeks with substantial drops only flow in the winter. This is when power needs are at their highest and PV input is at its lowest. Small hydro systems are well worth developing, even if used only a few months out of the year, if those months coincide with your highest power needs.
Use of existing sites:
Many small hydro electric sites were abandoned in the 1950's and 60's when the price of oil and coal was very low, and their environmental impacts unrealized.
HOW IT WORKS
Here are the basic components of a conventional hydropower plant:
• Dam - Most hydropower plants rely on a dam that holds back water, creating a large reservoir. Often, this reservoir is used as a recreational lake.
• Intake - Gates on the dam open and gravity pulls the water through the penstock, a pipeline that leads to the turbine. Water builds up pressure as it flows through this pipe.
• Turbine - The water strikes and turns the large blades of a turbine, which is attached to a generator above it by way of a shaft. The most common type of turbine for hydropower plants is the Francis Turbine, which looks like a big disc with curved blades. A turbine can weigh as much as 172 tons and turn at a rate of 90 revolutions per minute (rpm), according to the Foundation for Water & Energy Education (FWEE).
• Generators - As the turbine blades turn, so do a series of magnets inside the generator. Giant magnets rotate past copper coils, producing alternating current (AC) by moving electrons. (You'll learn more about how the generator works later.)
• Transformer - The transformer inside the powerhouse takes the AC and converts it to higher-voltage current.
• Power lines - Out of every power plant come four wires: the three phases of power being produced simultaneously plus a neutral or ground common to all three.
• Outflow - Used water is carried through pipelines, called tailraces, and re-enters the river downstream
Hydro-electric power plants can generally be divided into two categories:
+ most common
+ utilize a dam to store water at an increased elevation
+ use of a dam to impound water also provides the capability of storing water during rainy periods and releasing it during dry periods
+ consistent and reliable production of electricity, able to meet demand
+ high head plants with storage are very valuable to electric utilities because they can be quickly adjusted to meet the electrical demand on a distribution system.
+ utilize heads of only a few meters or less
+ may utilize a low dam or weir to channel water, or no dam and simply use the "run of the river"
+ run of the river generating stations cannot store water, thus their electric output varies with seasonal flows of water in a river
+ a large volume of water must pass through a low head hydro plant's turbines in order to produce a useful amount of power
Hydropower plants harness water's energy and use simple mechanics to convert that energy into electricity. Hydropower plants are actually based on a rather simple concept—water flowing through a dam turns a turbine, which turns a generator.
Hydro power is better than burning coal, oil or natural gas to produce electricity, as it does not contribute to global warming or acid rain. Similarly, hydro-electric power plants do not result in the risks of radioactive contamination associated with nuclear power plants.
Small scale and low head hydro capacity will probably increase in the future as research on low head turbines, and standardized turbine production, lowers the costs of hydro-electric power at sites with low heads
Low head hydro is more beneficial than high head hydro because:
With high head hydro comes flooding of vast areas of land, much of it previously forested or used for agriculture. The size of reservoirs created can be extremely large. The La Grande project in the James Bay region of Quebec has already submerged over 10,000 square kilometers of land; and if future plans are carried out, the eventual area of flooding in northern Quebec will be larger than the country of Switzerland. Reservoirs can be used for ensuring adequate water supplies, providing irrigation, and recreation; but in several cases they have flooded the homelands of native peoples, whose way of life has then been destroyed. Many rare ecosystems are also threatened by hydro-electric development.
A few recent studies of large reservoirs created behind hydro dams have suggested that decaying vegetation, submerged by flooding, may give off quantities of greenhouse gases equivalent to those from other sources of electricity. If this turns out to be true, hydro-electric facilities such as the James Bay project in Quebec that flood large areas of land might be significant contributors to global warming.
+ high head hydro floods vast area of land which may produce greenhouse gases that contribute to global warming
+ high head hydro disrupts some fragile ecosystems
+ Run of the river (or low head) hydro plants without dams and reservoirs would not be a source of these greenhouse gases.