Hydropower Guide
History of Hydropower
Figure 1 - Example of Traditional Water Wheel.
Energy exists within any flowing water course, natural or manmade, and this energy can be extracted using hydropower technology. This concept of transferring this energy or 'power, from flowing water has been a longstanding tradition dating back to the earliest water wheels (used for milling, mining, water supply and irrigation) over 2,000 years, to the Roman, Greek and Han dynasties. With significant technological advancements in the 20th century, the evolution of hydropower escalated as a source of electricity led to a new scale of utilisation for this valuable energy resource. Today, hydropower remains one of the most efficient forms of energy production, with in excess of 90% efficiency typical of most well designed turbine installations.
Energy in Water
The energy extracted from hydropower facilities is derived from the potential energy, which is transformed into kinetic energy and is a function of two parameters; head and flow. The head is the energy per unit weight (or unit mass) of water. This static head is proportional to the difference in height through which the water falls. The volumetric flow of the water is also proportional to the amount of energy that can be extracted from a water source.
Turbines
There are two main types of turbines; Reaction and Impulse turbines. The selection of a turbine to match head and flow conditions is an important factor when installing a turbine at a hydropower site. The following sections give a summary of the two main turbine types;
Reaction Turbine
Reaction turbines are the most common type of turbines. They generate power from exploiting oncoming flows, as the turbine propeller blades absorb the energy from the moving water. The main types of reaction turbines include; the Francis turbine, the Propeller/Kaplan turbine, the Tyson, Gorlov and China turbines. Also, the development of reverse pumps or pump as turbines (PaT's) are used as reaction turbines. Low flow and high head conditions are the most suitable for reaction turbines and the typical turbine is used for enclosed water flows.
Figure 2 - Selection of Reaction Turbines.
Impulse Turbine
Figure 3 - A range of Impulse Turbines.
Impulse turbines are typically more suitable for micro-hydropower installations e.g. run-off-river applications. This type of turbine has several benefits in comparison to the reaction turbine; it tolerates particles in flow, better access to parts, easier to fabricate and a better part-flow efficiency. The disadvantage of the impulse turbine is its unsuitability for low head sites. The Pelton, Turgo, Crossflow and Archimedes are the most well known types of impulse turbines and work under the principle of a jet of water acting upon buckets or runners on a wheel, which is rotated due to the force of the moving water.
Calculating the Power Potential
The potential power available at a hydroelectric facility can be esimtated based on the hydraulic head and flow rate in the water course. The power available from falling water can be calculated from the flow rate and density of water, the height of fall, and the local acceleration due to gravity. In SI units, the power is:
- P - power (kW)
- η - turbine efficiency
- ρ - density of water (kg/m^3)
- Q - flow (m^3/s)
- g- acceleration due to gravity (m/s^2)
- h - head of water (m)
e.g. Power is calculated for a turbine that is 85% efficient, e.g. water density of 1000 kg/m^3; a flow rate of 80 m^3/s; gravity of 9.81 m/s^2; and a net head of 70 m. In SI units:
Power (MW) = (0.85 x 1000 x 80 x 9.81 x 70)/10^6 => which gives 46.7 MW
Turbine Selection
As previously stated, the selection of a suitable turbine is based on flow and head characteristics for a water course. The following plot is used as a guideline to decide on the most suitable turbine type for a specific site.