Run-of-the-river hydroelectricity (ROR) is a type of hydroelectric generation plant whereby little or no water storage is provided. Run-of-the-river power plants may have no water storage at all or a limited amount of storage, in which case the storage reservoir is referred to as pondage. A plant without pondage has no water storage and is, therefore, subject to seasonal river flows. Thus, the plant will operate as an intermittent energy source while a plant with pondage can regulate the water flow at all times and can serve as a peaking power plant or base load power plant.
Run-of-the-river or ROR hydroelectricity is considered ideal for streams or rivers that can sustain a minimum flow or those regulated by a lake or reservoir upstream. A small dam is usually built to create a headpond ensuring that there is enough water entering the penstock pipes that lead to the turbines which are at a lower elevation. Projects with pondage, as opposed to those without pondage, can store water for daily load demands. In general, projects divert some or most of a river's flow (up to 95% of mean annual discharge) through a pipe and/or tunnel leading to electricity-generating turbines, then return the water back to the river downstream.
ROR projects are dramatically different in design and appearance from conventional hydroelectric projects. Traditional hydro dams store enormous quantities of water in reservoirs, sometimes flooding large tracts of land. In contrast, run-of-river projects do not have most of the disadvantages associated with dams and reservoirs, which is why they are often considered environmentally friendly.
The use of the term "run-of-the-river" for power projects varies around the world. Some may consider a project ROR if power is produced with no water storage while limited storage is considered ROR by others. Developers may mislabel a project ROR to soothe public perception about its environmental or social effects. The Bureau of Indian Standards describes run-of-the-river hydroelectricity as:
A power station utilizing the run of the river flows for generation of power with sufficient pondage for supplying water for meeting diurnal or weekly fluctuations of demand. In such stations, the normal course of the river is not materially altered.
Many of the larger ROR projects have been designed to a scale and generating capacity rivaling some traditional hydro dams. For example, the Beauharnois Hydroelectric Generating Station in Quebec is rated at 1,853 MW and a 2006 proposal in British Columbia, Canada has been designed to generate 1027 megawatts capacity. Some run of the river projects are downstream of other dams and reservoirs. The run of the river project didn't build the reservoir, but does take advantage of the water supplied by it. An example would be the 1995, 1,436 MW La Grande-1 generating station, previous upstream dams and reservoirs are part of the 1980s James Bay Project.
When developed with care to footprint size and location, ROR hydro projects can create sustainable energy minimizing impacts to the surrounding environment and nearby communities. Advantages include:
Cleaner power, fewer greenhouse gases
Like all hydro-electric power, run-of-the-river hydro harnesses the natural potential energy of water, eliminating the need to burn coal or natural gas to generate the electricity needed by consumers and industry.
Substantial flooding of the upper part of the river is not required for run-of-river projects as a large reservoir is not required. As a result, people living at or near the river don't need to be relocated and natural habitats and productive farmlands are not wiped out.
Run-of-the-River power is considered an "unfirm" source of power: a run-of-the-river project has little or no capacity for energy storage and hence can't co-ordinate the output of electricity generation to match consumer demand. It thus generates much more power during times when seasonal river flows are high (i.e., spring freshet), and depending on location, much less during drier summer months or frozen winter months.
Availability of sites
The potential power at a site is a result of the head and flow of water. By damming a river, the head is available to generate power at the face of the dam. Where a dam may create a reservoir hundreds of kilometers long, in run of the river the head is usually delivered by a canal, pipe or tunnel constructed upstream of the power house. Due to the cost of upstream construction, a steep drop in the river is desirable.
Small, well-sited ROR projects can be developed with minimal environmental impacts. Larger projects have more environmental concerns. For example, Plutonic Power Corp.'s canceled Bute Inlet Hydroelectric Project in BC would have seen three clusters of run-of-river projects with 17 river diversions; as proposed, this run-of-river project would divert over 90 kilometres of streams and rivers into tunnels and pipelines, requiring 443 km of new transmission line, 267 km of permanent roads, and 142 bridges to be built in wilderness areas.
British Columbia's mountainous terrain and wealth of big rivers have made it a global testing ground for run-of-river technology. As of March 2010, there were 628 applications pending for new water licences solely for the purposes of power generation – representing more than 750 potential points of river diversion.
- Diverting large amounts of river water reduces river flows, affecting water velocity and depth, minimizing habitat quality for fish and aquatic organisms; reduced flows can lead to excessively warm water for salmon and other fish in summer.
- New access roads and transmission lines can cause extensive habitat fragmentation for many species, allowing the introduction of invasive species and increases in undesirable human activities, like illegal hunting.
- The lack of reservoir storage may result in intermittent operation, reducing the project's viability.
- Belo Monte Dam, 11,233 megawatts (15,064,000 hp), Pará, Brazil
- Chief Joseph Dam, 2,620 megawatts (3,510,000 hp)
- Beauharnois Hydroelectric Power Station, 1,903 megawatts (2,552,000 hp)
- Bonneville Dam, 1,092 megawatts (1,464,000 hp)
- Satluj Jal Vldyut Nigam Ltd, Satluj River, Shimla, India, 1,500 megawatts (2,000,000 hp)
- Ghazi-Barotha Hydropower Project on River Indus in Pakistan, 1,450 megawatts (1,940,000 hp)
- La Grande-1 generating station, 1,436 megawatts (1,926,000 hp)
- Kohala Hydropower Project, Jhelum River, Muzaffarabad, Pakistan, 1,100 megawatts (1,500,000 hp)
- Neelum–Jhelum Hydropower Plant, Jhelum River, Muzaffarabad, Azad Kashmir, Pakistan, 969 megawatts (1,299,000 hp)
- Baglihar Hydroelectric Power Projecton Chenab River in India, 900 megawatts (1,200,000 hp)
- Carillon Generating Station, Quebec, Canada, 752 megawatts (1,008,000 hp)
- Upper Tamakoshi Project, Nepal, 456 MW
- Nyagak Hydroelectric Power Station on Nyagak River in Zombo District, Uganda, 3.5 megawatts (4,700 hp)
- East Toba/Montrose Hydro Project, British Columbia, Canada, 196 megawatts (263,000 hp)
- Forrest Kerr Hydro Project, British Columbia, Canada, 195 megawatts (261,000 hp)
- Patrind Hydropower Plant, Kunhar River, Pakistan, 150 megawatts (200,000 hp)
- Upper Toba Valley, British Columbia, Canada, 123 megawatts (165,000 hp)
- Upper Kotmale Hydropower Project (UKHP), Talawakele, Sri Lanka, 150 megawatts (200,000 hp)
- Sechelt Creek Generating Station, British Columbia, Canada, 16 megawatts (21,000 hp)
- Environmental concerns with electricity generation
- Environmental impacts of dams
- Micro hydro
- Pico hydro
- Pumped-storage hydroelectricity
- Small hydro
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