2.3 Turbine selection
Types of turbine
Hydropower turbines use water pressure to rotate its blades and generate energy. Selecting the appropriate type of turbine depends primarily on available head and less so on available flow rate. The three primary types of turbines are: the Pelton turbine, for high heads; the Francis turbine, for low to medium heads; and the Kaplan turbine for a wide range of heads (see Figure 2.3a below). Several other types of turbines exist on the market, described below.
Figure 2.3a. Three main types of water turbines: (A) Pelton wheel; (B) Francis turbine; (C) Kaplan turbine. (The Encyclopedia of Alternative Energy)
Click on the following links to learn more about the different types of turbines.
The Pelton wheel is an impulse water turbine, similar to the Turgo turbine. The geometry of the paddles efficiently extracts almost all of the energy from the falling water, which exits the wheel with very little speed. The Pelton wheels operate most efficiently at sites with relatively high hydraulic head at low flow rates. Conversely, efficiency decreases under conditions of low-pressure and high flow (Wikipedia – Pelton wheel). 3D animation of Pelton wheel
Francis turbines are the most common water turbine in use today, and operate in a wide head range of 10 to 650 meters. They combine radial and axial concepts in an inward-flow reaction turbine. Francis turbines are almost always mounted with the shaft vertical, and they extract energy from the flow as moves through the turbine changing from high pressure to low pressure. In order to contain the water pressure (or suction) reaction turbines must be contained in airtight housing (Wikipedia – Francis turbines).
Kaplan turbines, like Francis turbines, are inward flow reaction turbines. However, Kaplan turbines preform more efficiently than Francis turbines in high-flow, low-head applications (10-70m). Kaplan turbines are widely used throughout the world in the lowest head hydro sites. They are designed to operate at the highest possible efficiency, typically over 90%. While Kaplan turbines are very expensive to design, manufacture and install, they operate for decades (Wikipedia – Kaplan turbine).
Turgo turbines are impulse water turbine, similar to the Pelton wheel. They work with medium head plants (~50-250m), overlapping the head range of Francis, Pelton, and to a small extent Kaplan turbines. They offer some advantages over these other turbines. The runner on the Turgo is less expensive than a Pelton wheel and it doesn’t require airtight housing like the Francis (see Figure __). Purchasing and installing Turgo generators tends to cost less than Pelton wheels, since Turgo runners can handle a greater flow than the same diameter Pelton wheel. For the same power, the Tugo runner is one half the diameter of the Pelton runner. Turgo turbines achieve efficiencies of about 87% efficiency in practice (Wikipedia – Turgo turbine).
In a Cross-flow turbine water passes through the turbine transversely across the turbine blades, unlike most turbines with axial or radial flows. The cross-flow turbine is well suited for locations with a low head but high flow, and is most commonly used in mini and micro hydropower units of less than 2,000 kW. The cross-flow turbine has a flat efficiency curve under varying load, which is particularly useful for small run of the river plants subject to variable flow. However, the peak efficiency of a cross-flow turbine is somewhat less than a Kaplan, Francis or Pelton turbine. Cross-flow turbines are also relatively cheap and the simple construction, mechanical system, and allow for easy maintenance and repairs by local mechanics (Wikipedia – Cross-flow turbine).
Folsom turbine selection
Recall from the previous activity that Folsom powerplant possesses up to 332 ft (101m) of head when the reservoir is full, and only 292 ft (89m) of head during the flood season. Let's assume that the plant facilities require 60 MW for daily operations, while more energy is required for additional pumping in the summer. Using this information, along with what you've learned about different turbines, what type of turbine(s) might you suggest for Folsom dam? Choose up to three turbines that meet the operational criteria at Folsom
Folsom turbine capacity
While the hydraulic head dictates the type of turbine, the available flow rate tends to determine the appropriate turbine capacity. When constructed, Folsom powerhouse contained three 54 MW Francis turbines, for a total nameplate capacity of 162 MW. However, the actual power produced depends on the hydraulic head and discharge, as you've seen. In addition, turbines are rated for a specific head and flow rate, and the efficiency of the turbine drops for flows and head further away from those for which the turbine is rated. The turbines in Folsom dam are rated at 74,000 hp for a speed of 163.6 rpm under a rated head of 300 ft.