Hydroelectric: Technology Overview

Hydroelectric power is generated by capturing the kinetic energy of water as it moves from one elevation to a lower elevation by passing it through a turbine.  Often, the water is raised to a higher potential energy by blocking its natural flow with a dam.  The amount of kinetic energy captured by a turbine is dependent on the head (distance the water is falling) and the flow rate of the water. Another method of capturing the kinetic energy is to divert the water out of the natural waterway, through a penstock and back to the waterway.  This allows for hydroelectric generation without the impact of damming the waterway.  The existing worldwide installed capacity for hydroelectric power is far and away the largest source of renewable energy at 740,000 MW (IEA, 2002).

Applications

Hydroelectric projects are divided into a number of categories based upon their size.  Micro hydro projects are up to 100 kW in size.  Systems between 100 kW and 1.5 MW are classified as mini hydro projects.  Small hydro systems are between 1.5 and 30 MW.  Medium hydro is up to 100 MW, and large hydro projects are greater than 100 MW in size and are good resources for baseload power generation because they have the ability to store a large amount of potential energy behind the dam and release it consistently throughout the year.  Small hydro projects, generally do not have large storage reservoirs and are not dependable as dispatchable resources. 

Resource Availability

Hydroelectric resource can generally be defined as any flow of water that can be used to capture the kinetic energy of its water.  Projects that store large amounts of water behind a dam regulate the release of the water through turbines over time and generate electricity regardless of the season.  These facilities are generally base-loaded.  Pumped storage hydro plants pump water from a lower reservoir to a reservoir at a higher elevation where it is stored for release during peak electrical demand periods.  Run of the river projects do not impound the water, but instead divert a part or all of the current through a turbine to generate electricity.  This technique is used at Niagara Falls to take advantage of the natural potential energy of the waterfall.  Power generation at these projects varies with seasonal flows.  In general, the energy producing potential at any one site is dependent upon the flow rate of the water as well as the hydraulic head.

Environmental Impacts

The damming of rivers for small and large scale hydro applications may result in significant environmental impacts.  The first issue involves the migration of fish and disruption of spawning habits.  One of the few viable abatements of this issue is construction of “fish ladders” to aid the fish in bypassing the dam when they swim upstream to spawn.

The second issue involves flooding existing valleys that often contain wilderness areas, residential areas, or archeologically significant remains.  Related to this point, there are also concerns about the consequences of disrupting the natural flow of water downstream and disrupting the natural course of nature.

In a more positive light, resulting reservoirs from dams are valuable recreation areas and dams assist in the efforts of flood control, thereby preventing economic hardship to local agriculture and municipalities.

Many environmental groups object to the broad definition of hydroelectric resources as renewable.  Numerous classification systems for hydro have developed in attempt to distinguish “renewable” projects.  For the most part this distinction is based on size, although “low-impact,” low-head, and run-of-river plants are also often labeled renewable.