The internet, not unsurprisingly, is a Pandora's box of much interesting information on almost any subject one can think of - not all of it reliable. Googler beware! Almost every hydro-electric power station on the surface of the globe seems to have a web site. With painstaking data tabulation from a selection of these sites it has been possible to observe that from initial planning to eventual commissioning almost all hydro-electric power stations, no matter where located, or how large or small, conform to an average gestation time of about 10-15 years. This means that with an approximately 20 year window until 2030 any large new hydro-electric power station in excess of 1 GW that has any likelihood of coming on-stream and thereby helping to replace fossil fuel usage, will have to be already substantially into the planning and approval stage of development at this point in time (2008). The World Energy Report  suggests that worldwide there are 77 large hydroelectric schemes (> 1 GW) at the approved or building stage with the potential to bring new renewable power into service by 2030. There are many much smaller schemes but their aggregated power is relatively insignificant in global terms. We can therefore conclude that the additional capacity which the new hydro-electric stations will bring to the generation mix by 2030 could amount to 124 GW. This is a 15% increase on current capacity. In 20 years therefore a potential total power available to the consumer from hydro-electric generation, allowing for grid and transformer losses is likely to be of the order of 840 GW, a small but significant proportion (4%) of the required ~ 20 TW.
When turbine, generator, transformer, and transmission losses for the hydroelectric system are aggregated, it is salutary to observe that in 2030, of the ~ 1.2 TW of power locked up in the streaming waters of the hydro-electric dams of the world only 0.84 TW reaches the users. A massive 360 GW disappears in heating the electrical power industry's real estate. While it is not possible to make electrical systems 100% efficient an improvement on current standards would certainly not be too difficult. In the past efficiency has never really been a pressing issue with engineers because fossil fuels were considered to be plentiful and cheap, and now renewables are often mistakenly considered to be 'free'. Each new hydro-station, although carbon clean once built, has its environmental costs. They are anything but environmentally friendly at the construction stage, if fossil fuel powered machinery is employed, while large schemes destroy farm land and disrupt the local ecology. Dams in tropical and sub-tropical regions of the world are claimed to release large volumes of methane created by decaying vegetation drowned when the reservoir was formed. So the ecological impact of medium and large dams is not insignificant. For example, stagnant water is retained in the artificial lake behind the dam and has the tendency to be under-oxygenated. The fish that live in the impoverished water that comes out of the turbines are not impressed. On the other hand, when the water from the top of the dam is suddenly released it is heavily enriched with oxygen and contains tiny air bubbles. The fish don't appreciate this either. It is not easy to keep the little blighters happy!
Improvements in efficiency could mean fewer power stations and less environmental damage. A very large proportion of the hydro-equipment in operation today will need to be modernised by 2030. This modernisation should be driven by the need to achieve efficiency improvements. Just a 1% increase in the efficiency of hydro-power stations world wide would yield a 0.01 x 1200 GW = 12 GW reduction in electricity wastage. This is equivalent to 6-10 major new hydro-schemes, for 'free'!
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