Hydropower (Large-scale)

While water resources are valued for sustaining human health and food production, the energy contained in moving water such as rivers or tides can also be harnessed to create energy through hydropower or mechanical uses. Hydropower schemes can either be small scale or large-scale, depending on the local conditions and the energy demand. Globally, 1.4 billion people lack access to electricity, with an additional 1 billion having only intermittent access (UNDP 2012). Where water resources permit, large-scale hydropower is an option to produce large quantities of renewable, inexpensive energy to meet growing energy demands in the face of rising costs of fossil fuels. Today, it is a widely used technology that is applied to produce power in many different ways.

Regardless of size, the principle behind hydropower generation is the same: that the power of moving water can be harnessed to produce energy. The key elements for hydropower generation are “head” and “flow.” “Head” refers to the height of the gradient over which the water falls, while “flow” refers to the volume of water per unit time. To maximise energy production, both head and flow should be high. In other words, greater amounts of energy are generated when a large volume of water flows quickly over a steep gradient. Hydropower plants generate electricity by directing water through a turbine, which in turn drives an electric generator. The large quantity of electricity produced in large-scale hydropower is usually fed into an electricity grid.

Adapted from SINGH (2009)

Hydropower plants are classified according to their energy production capacity, expressed in megawatts. Large-scale hydropower plants can produce well over 100 MW, while small hydropower plants generally produce less than 10 MW. The main difference between the six different sizes ranges of hydropower plants is determined by the amount of civil infrastructure work involve during the construction. Whereas a large facility producing a great amount of electricity involves damming huge rivers and building powerhouses, smaller hydropower schemes utilise the difference in altitude, small flows or the decline in the pipes from water infrastructure to provide power for small communities.

Adapted from US DEPARTMENT OF INTERIOR (2005)

Large-scale hydropower is a form of renewable energy generation derived from flowing water used to drive large water turbines. In order to generate large amounts of hydroelectricity for cities, lakes, reservoirs and dams are needed to store and regulate water for later release for power generation, irrigation, domestic (see PPT) or industrial (see PPT) use. Since large-scale hydropower facilities can easily be turned on and off, hydropower is more reliable than most other energy sources for meeting electricity peak demands throughout the day.

There are several types of large-scale hydropower plants, which can be classified as:

• Conventional hydroelectric dams
• Pumped-storage
• Run-of-the-river
• Tidal

Conventional Hydroelectric Dams

Adapted from US DEPARTMENT OF INTERIOR (2005)

In order to generate electricity, a large-scale hydropower (LHP) plant uses the kinetic (moving) energy of water in lakes or reservoirs. The water flows through a penstock, which channels the water to the turbine. When flowing water turns a turbine, its kinetic energy is converted to mechanical (machine) energy. The turbine turns the generator rotor, which then converts this mechanical energy into electricity. After the water has flowed through the turbine blades, the water is released for such purposes as irrigation, domestic and industrial use or to generate electricity again in another hydroelectric power plant located downstream. 

Often, dams are located in isolated locations and therefore vast networks of transmission lines and facilities are used to bring electricity to homes, schools, factories and offices. Due to the long distances involved in transmitting this electricity, there are often large losses within the distribution network.

Pumped-Storage

Pumped storage is a method of keeping water in reserve for peak period power demands. During the night, water is pumped from a low lake to a storage pool above the power plant at a time when customer demand for energy is low, and electricity is cheaper. The water is then released through the turbine-generators at times when demand is high and a heavy load is place on the system (adapted from US DEPARTMENT OF INTERIOR (2005)).

Run-of-the-River

Another type of large-scale hydropower plant is the run-of-the-river hydroelectricity. Run-of-river systems do not rely on large storage reservoirs, but rather divert river water to drive turbines, and then discharge the water back into the river system. These hydropower schemes are most often used for micro and small scale hydropower, but can also be large-scale projects (adapted from US DEPARTMENT OF ENERGY (2005)).

Tidal

Tidal power is a method of large-scale hydropower that uses the motion of the ocean’s tides to generate electricity. There are two types of tidal power: tidal range and tidal stream. Tidal range systems trap huge volumes of water in a large basin and when then drains through a tidal estuary to drive water turbines built into a dam. Tidal streams are built in narrow channels around headlands. The energy produced by fast flowing tidal currents can be harnessed by using underwater turbines that are fixed on the seabed or estuary floor (adapted from RWE (2012)).

While large-scale hydropower can provide large quantities of reliable, relatively inexpensive electricity, there are significant environmental and social consequences, namely:

• Ecosystem damage and loss of land. The damming of rivers and creation of reservoirs strongly disrupts stream ecosystem habitats and breeding cycles of fish species. As a reservoir is created, large areas of land are flooded, representing a loss of this land for other purposes such as settlement, farming, or as a natural ecosystem.
• Siltation and flow shortage. As river water flows, it naturally carries silt, which is normally transported downstream and deposited once water velocity slows. However, when rivers are dammed, this process is disrupted and silt is deposited in the reservoir instead of downstream. This can cause reservoirs to fill up with sediment and lose their ability to control floods, or can cause dams to break, flooding huge areas of land.
• Methane emissions. As reservoirs are created, large areas of land are flooded. Once underwater, organic material contained in soil or decaying plants begins to degrade anaerobically, causing methane production, which is a potent greenhouse gas (see climate change). In instances where the reservoir is large compared to the energy generation of the power plant, hydropower plants can produce more greenhouse gases than fossil fuel power plants. Clearing of forests before flooding and minimising the area of flooding can reduce methane emissions (LIMA et al. 2008). 
• Displacement of people. The large amount of flooding in creating a reservoir necessitates the displacement of any people who work or live near the site of the hydropower plant. In February 2008, an estimated 40-80 million people were displaced due to dam construction, causing loss of homes as well as loss of livelihood (INTERNATIONAL RIVERS 2008). To learn more about political and social conflicts between water users, read about water conflicts.
• Failure risks. If dams are poorly constructed or are the site of natural disasters, they can break and cause dam failure. This releases an enormous quantity of stored water, causing catastrophic damage to downstream settlements and infrastructure. With respect to damage and loss of life, dam failures have been among the largest man-made disasters in history.
• Water loss via evaporation. Reservoirs create large pools of water with a large surface area. This leads to high amounts of evaporation, where water is lost from the reservoir into the atmosphere before it has gone through turbines to generate energy. This evaporation leads to huge water losses. This means that, in areas where water is scarce, large-scale hydropower may be a very inefficient use of water resources.

All of the above concerns should be taken into consideration when designing and implementing new hydropower plants. Dams built with mitigation measures and adequate site selection techniques can provide benefits to local populations, whereas those build without proper mitigation techniques, compensation measures and site selection evaluations criteria will have considerable adverse environmental effects. For example, the 500–megawatt Pehuenche Hydroelectric Project in Chile flooded only about 400 hectares of land (with minimal damage to forest or wildlife resources) and has had no water quality problems. By contrast, the Brokopondo Dam in Suriname inundated about 160,000 hectares of biologically valuable tropical rainforest and is known for serious water quality and aquatic weed problems, while providing relatively little electric generating capacity (only 30 megawatts) (LEDEC AND QUINTERO 2003).

The cost of large-scale hydropower plants is imposed by the natural setting, the capacity of the hydropower plant, and size of urban centres. It is estimated that 65-75% of total costs are related to civil engineering, 15-20% for meeting environmental regulations, and the remaining 10% for the turbine, generator, and control systems. The total cost of a hydropower scheme depends on the head of the system, and can range from $8000-$13000 USD per kW for a low-head system, and $4500-$9500 UDS per kW for a medium-head system (LOCAL GOVERNMENT ASSOCIATION 2012).

Despite the high cost of installation, large hydroelectric power plants produce electricity at low costs, especially to large urban centres (LEDEC AND QUINTERO 2003). Considering fuel, operation, and maintenance, large-scale hydropower is much less expensive than many other energy sources, including nuclear, fossil fuels, and gas turbines. On average, hydropower in the U.S. produces energy for less than $0.01 per kilowatt-hour (US DEPARTMENT OF THE INTERIOR 2005). Because of this inexpensive energy generation, in spite of the high amounts of environmental remediation that are often associated with dams and reservoirs, large-scale hydropower can still be a cost effective energy source compared to fossil fuels (WILLIAMS & PORTER 2006).

Large-scale power plants are expensive to build but they have very low maintenance costs. However, the operation and maintenance has to be done by highly skilled professionals in order to ensure the safety of dams. This is mainly because most of operations has been optimised with the use of computer technologies (adapted from US DEPARTMENT OF INTERIOR (2005)).