by: Michael Hoexter
The oldest, most well-established and most controversial renewable source of electricity is hydropower. Hydroelectric power uses the kinetic energy of water as it descends from one elevation to a lower elevation to drive turbines that in turn provide the rotational energy for dynamos. Technically, tidal and wave power are also hydroelectricity (’hydro’ = Greek for water) though they are not usually grouped together with the now fairly common technology of using river power.
As I discussed in the posts about wind and fossil fuels, dynamos produce electricity by rotating a conductor (usually a bundle of wires) through a magnetic field. The kinetic energy of falling water is then the provider of the rotational momentum, the torque, for turning dynamos that in fossil fuel power plants is provided by the pressure of hot gases and steam and in wind turbines is provided by the pressure of the wind against the rotors of the turbine.
Hydroelectric plants already generate a substantial portion of the world’s electricity, estimated for the year 2006 at 19% and rising. They are also by far the largest renewable source of electricity at this time, weighing in at 64% of the total renewable portfolio of the planet. The expansion of hydroelectric power from fresh water sources is limited by geography and the balancing of environmental and human concerns. Despite these limitations, some estimate the world’s potential for hydroelectric power generation to be 3 times the current installed base. Europe and the US are estimated to have exploited half their hydro potential but in Asia (11%), former Soviet Union (7%) and Africa (4%), hydroelectric might be considered to be under-exploited.
Hydroelectric plants generate no emissions from their operation, though as they are often large construction projects, some embedded carbon emissions are contained in their construction. Also, when organic matter is submerged in the reservoirs created by the dam associated with most larger hydroelectric plants, substantial amounts of methane can be released as these plants decay in the water. Clearing of the plant matter (mostly trees) prior to the flooding of the reservoir or afterwards (more expensive) can reduce methane emissions though regrowth and re-submersion of plant matter at the edges of reservoirs will remain a source of methane.
The Pros and Cons of Hydro
If hydroelectric plants are far cleaner than fossil fuel electric generation, the building of dams and disrupting a natural landscape are part and parcel of enjoying the energy output of hydroelectricity. The soon-to-be largest power plant in the world, the Three Gorges Dam in China, while generating 22.5 GW of carbon-dioxide free energy at its future peak, has displaced 1.3 million people and destroyed a unique natural and cultural landscape. Beyond the Three Gorges project, China leads the world in planned hydroelectric projects with an estimated hydroelectric capacity of 70 GW by the middle of the next decade.
A hydroelectric project can function as either baseline or peak power, as the flow of water can be routed through or around the turbines as needed and the project is presumably sited on a stretch of river with a fairly reliable flow under usual weather conditions. Though damming the river incurs some environmental damage and potential for greenhouse gas emissions from uncleared land under the reservoir, pairing a dam with the hydro turbines will boost the project’s capacity factor and reliability. All large-scale hydro-projects are paired with dams though smaller hydro may not use dams to control river flow. With a hydro-project one expects a capacity factor of over 50% (close to year-round water availability).
A hydro-electric dam or for that matter any dam will disrupt the natural flow of a river and create a barrier for fish species to ascend and descend the river as they have evolved to do over the millennia. Irrigation on land near the river may be effected by the regulation of water flow that is a part of any dam project. Rivers are “multi-use” eco-systems and dam building by its nature declares certain uses for the human species a higher priority than other human and non-human uses.
The early environmental movement was almost always an opponent or at least skeptical of dam projects, be they for controlling flooding, storing and distributing freshwater, and/or generating electricity. With climate change ascending the list of priorities of the environmental movement, a balance between the preservationist impulses of traditional environmentalism and the new anti-climate change movement has not yet been struck. With each hydroelectric project, the potential damages and merits will need to be balanced and adjudicated prior to and after the building of these projects, hopefully with the help of a well functioning and impartial legal system.
Hydroelectric power is also vulnerable to climate variation and climate change as drought can limit water supply and electricity production. In addition, while a remote possibility under normal operation and with modern construction techniques, catastrophic failure and/or military attack, can endanger downstream populations especially when dealing with large scale dams, which happen to be the most efficient in terms of cost and electricity generated.
Emerging Trends in Hydro
The technology involved in generating electricity from hydro is mature but there have been innovations in the sizing of hydro projects as well as the use of innovative low-cost technologies on smaller hydro projects. With lower capital costs and lesser environmental effects, smaller hydro projects are used increasingly in developing and rapidly developing nations as the anchor for the beginnings of a grid in areas that have had intermittent or no electric power. In philosophy, micro hydro projects attempt not to dam or disrupt river flows, allowing for ongoing multiple uses of the river by multiple species.
Hydroelectric projects can be built to almost any size depending on the water resources available as well as political, economic and environmental factors. Pico hydro is classified as hydro projects with under 5kW of peak power, enough to power small appliances and lighting for a number of households with low demand, ideal for remote villages or single households in the developed world. Micro hydro are larger scale projects with up to 100kW capacity suitable for entire villages in the developed world or powering larger facilities in the developed world. Mini hydro, as you would expect, is on a larger scale with facilities that range in size up to 2MW, enough to power a small town, especially with expectable greater than 50% capacity factor of a hydro project vs. other renewables. The largest dam projects, of which there can only be a limited number, are measured in 100s of megawatts and, even, as above with Three Gorges, gigawatts.
As climate change becomes to far outweigh other considerations, a fuller exploitation of the hydroelectric potential of most countries would seem to be in order, in which case hydroelectricity will be able to substitute for more fossil fuel plants or cover projected future electric demand. In some areas, all electric demand might be met by hydro-projects, though a grid powered exclusively through one means would be vulnerable to climate variation and drought.
While the philosophy of hydro-electric project planners in the 20th century focused on the big damming projects with their attendant disruption of the river and natural landscape, a flexible, distributed-power approach to designing and implementing hydro-projects seems to be the trend in the early 21st century. These approaches are greener in terms of the local impact of the project and can be built in ways that do not disrupt riverine habitats to the degree that environmentalists have feared when the word “hydro” has been uttered in the past. Rather than exclude hydro-electric projects from consideration of the greenest technologies, careful design of mini- and micro-hydro in both the developed and developing world may go some way to reducing the use of fossil fuels and for that matter nuclear power. If we consider ourselves in a planetary emergency, we should, in addition, not shy away from large hydro projects that avoid methane emissions from their reservoirs and reduce pressure on electricity producers to turn to fossil fuels.