Energy alternatives: Hydropower

The world has a lot of dams — 45,000 large ones, according to the World Energy Council, and many more at small scales. Its hydroelectric power plants have a generating capacity of 800 gigawatts (for a guide to power, see ‘By the numbers’), and they currently supply almost one-fifth of the electricity consumed worldwide. As a source of electricity, dams are second only to fossil fuels, and generate 10 times more power than geothermal, solar and wind power combined. With a claimed full capacity of 18 gigawatts, the Three Gorges dam in China can generate more or less twice as much power as all the world's solar cells. An additional 120 gigawatts of capacity is under development.

One reason for hydropower's success is that it is a widespread resource — 160 countries use hydropower to some extent. In several countries hydropower is the largest contributor to grid electricity — it is not uncommon in developing countries for a large dam to be the main generating source. Nevertheless, it is in large industrialized nations that have big rivers that hydroelectricity is shown in its most dramatic aspect. Brazil, Canada, China, Russia and the United States currently produce more than half of the world's hydropower.

Cost: According to the International Hydropower Association (IHA), installation costs are usually in the range of US$1 million to more than $5 million per megawatt of capacity, depending on the site and size of the plant. Dams in lowlands and those with only a short drop between the water level and the turbine tend to be more expensive; large dams are cheaper per watt of capacity than small dams in similar settings. Annual operating costs are low — 0.8–2% of capital costs; electricity costs $0.03–0.10 per kilowatt-hour, which makes dams competitive with coal and gas.

Capacity: The absolute limit on hydropower is the rate at which water flows downhill through the world's rivers, turning potential energy into kinetic energy as it goes. The amount of power that could theoretically be generated if all the world's run-off were 'turbined' down to sea level is more than 10 terawatts. However, it is rare for 50% of a river's power to be exploitable, and in many cases the figure is below 30%.

Those figures still offer considerable opportunity for new capacity, according to the IHA. Europe currently sets a benchmark for hydropower use, with 75% of what is deemed feasible already exploited. For Africa to reach the same level, it would need to increase its hydropower capacity by a factor of 10 to more than 100 gigawatts. Asia, which already has the greatest installed capacity, also has the greatest growth potential. If it were to triple its generating capacity, thus harnessing a near-European fraction of its potential, it would double the world's overall hydroelectric capacity. The IHA says that capacity could triple worldwide with enough investment.

Advantages: The fact that hydroelectric systems require no fuel means that they also require no fuel-extracting infrastructure and no fuel transport. This means that a gigawatt of hydropower saves the world not just a gigawatt's worth of coal burned at a fossil-fuel plant, but also the carbon costs of mining and transporting that coal. As turning on a tap is easy, dams can respond almost instantaneously to changing electricity demand independent of the time of day or the weather. This ease of turn-on makes them a useful back-up to less reliable renewable sources. That said, variations in use according to need and season mean that dams produce about half of their rated power capacity.

Hydroelectric systems are unique among generating systems in that they can, if correctly engineered, store the energy generated elsewhere, pumping water uphill when energy is abundant. The reservoirs they create can also provide water for irrigation, a way to control floods and create amenities for recreational use.

Disadvantages: Not all regions have large hydropower resources — the Middle East, for example, is relatively deficient. And reservoirs take up a lot of space; today the area under man-made lakes is as large as two Italys. The large dams and reservoirs that account for most of that area and for more than 90% of hydro-generated electricity worldwide require lengthy and costly planning and construction, as well as the relocation of people from the reservoir area. In the past few decades, millions of people have been relocated in India and China. Dams have ecological effects on the ecosystems upstream and downstream, and present a barrier to migrating fish. Sediment build-up can shorten their operating life, and sediment trapped by the dam is denied to those downstream. Biomass that decomposes in reservoirs releases methane and carbon dioxide, and in some cases these emissions can be of a similar order of magnitude to those avoided by not burning fossil fuels. Climate change could itself limit the capacity of dams in some areas by altering the amount and pattern of annual run-off from sources such as the glaciers of Tibet.

Because hydro is a mature technology, there is little room for improvement in the efficiency of generation. Also, the more obvious and easy locations have been used, and so the remaining potential can be expected to be harder to exploit. Small (less than 10 megawatts) 'run-of-river' schemes that produce power from the natural flow of water — as millers have been doing for four millennia — are appealing, as they have naturally lower impacts. However, they are about five times more expensive and harder to scale than larger schemes.

Verdict: A cheap and mature technology, but with substantial environmental costs; roughly a terawatt of capacity could be added.