2.0 Hydropower production

Overview of hydropower production

Dams produce hydropower by releasing water through the penstocks and using the power of that falling water to turn a turbine (see Figure 2.0a).

Figure 2.0a. Folsom powerhouse components

 

The amount of power generated depends primarily on the discharge through the turbines and the hydraulic head, or height between the reservoir surface and turbines:

P = ηρ gQH

where, P is power (MW), η  is turbine efficiency, ρ is water density (lbs/ft3, kg/m3) Q is discharge (ft3/s, m3/s), g is gravity (32 ft/s2, 9.81 m/s2), H is hydraulic head (ft,  m). Using the English units for ρ, g, Q, and H, and including a conversion factor for P to MW, yields the simplified equation:

P = (η QH) / (1.181 * 10^4)

We will use this equation and English units in the following activities.

Types of powerplants (base vs. peak)

Hydropower plants can generally be classified as either base-load plants or peaking plants, with some plants serving both purposes. Base loading plants output a steady flow of power to meet electrical demands that exists throughout the day and throughout the year. Alternatively, peaking plants output energy that fluctuates on a daily and/or seasonal basis to meet prime hour power demands (typically during weekdays). Hydropower plants posses the capability to quickly reach full power in response to increasing electrical demands, making them ideal for meeting peak demands. Put more simply, hydropower is easier to turn on and off than other power supplies, such as coal or natural gas. Under reliable water supply conditions, hydropower may also contribute to meeting base power demands. The graph below show shows example discharge regimes for producing peaking and baseload power.

Figure 2.0b. Example discharge regime from a peaking and baseload hydropower dam

 

 

References

 

 

Activities

2.1 Power, discharge, head relationship

Historical hydropower development in the U.S.

In the early days of large dam construction in the US, hydropower was considered a byproduct of water development. However, the need for hydropower soared as energy demands increased during World War II. The 1944 Flood Control Act empowered the USACE to sell the power produced at federal projects, leading the way for a multipurpose approach to dams. In the early 20th century, hydropower represented about 75 percent of all the electricity consumed in the West and Pacific Northwest, and about one third of the total United States' electrical energy.

Power, discharge, head relationship

Hydropower generation curves for different reservoir elevations and hydropower releases provide a useful resource for hydropower planning. Recall the simplified equation for hydropower generation:

 P = (η QH) / (1.181 * 10^4)

where, P is power (MW), η  is turbine efficiency (%), Q is discharge (ft3/s), and H is head (ft). For this example let us assume (incorrectly) that the efficiency factor is constant, while discharge and head vary over the course of a day, yielding different power output. The following activity allows you to explore how energy output varies with hydraulic head and discharge.

Folsom powerplant power, discharge head relationship (historic)

As mentioned previously, Folsom Dam was initially authorized for flood management objectives and reauthorized in 1949 for power production, after construction began. The plant provides peaking power to first meet the requirements of the project facilities (including power to pump residential supply) and then the remaining energy is marketed to northern California power customers.

 

 

The flood control objectives dictated the height of Folsom Dam at 340 ft. Planners tried to maximize the head available for power production and place the turbines at base of the dam (134 ft. elevation) such that at the gross pool elevation (466 ft) minus the head equals 332 ft (= 466 - 134 ft). Let ε = 0.85, signifying an 85% efficiency rating, typical of an older powerplant. Use this information to write an equation for the relationship between energy and discharge when the reservoir is full.

P= ( Q  x   x   ) / (   x 10^  )

As mentioned previously, Folsom Dam was initially authorized for flood management objectives and reauthorized in 1949 for power production, after construction began. The plant provides peaking power to first meet the requirements of the project facilities (including power to pump residential supply) and then the remaining energy is marketed to northern California power customers.

However, during the flood season, the operators manual for Folsom dam requires the reservoir to remain at the flood conservation elevation of 426 ft during non-emergencies. Try again assuming the reservoir is at the flood conservation pool elevation, rather than the maximum pool elevation. Remember, the turbines are located at 134ft, so the head is the difference between the pool and turbine elevations.

P= ( Q  x   x   ) / (   x 10^  )

Now try to enter some reservoir elevations at intermediate stages to get a better idea of the relationship between power, discharge and head. 

P= ( Q  x   x   ) / (   x 10^  )

Present day

Today, hydropower accounts for about seven percent of US energy consumption, although it still plays a major role in some basins.

As a result of increased power demand, Folsom's three generators were upgraded in 1972 from a nameplate capacity of 162 MW to 198 MW of power. In addition, current construction will raise the dam approximately 7 ft, providing more head for power supply.

 

 

 

 

References

http://www.calwater.ca.gov/Admin_Record/C-073130.pdf

2.2 Meeting Power Demand

2.2 Meeting power demand

The figure and table below show the how power demand at Folsom varies over the course of a year. You should notice that power demand is highest during summer months (May - July) and the reservoir is also most full (highest head) during these same months.

Figure 2.2a. Average monthly power production (MW) and reservoir head (ft) at Folsom Dam from 2000-2012 (Source CDEC).

Using the power equation below calculate the average discharge needed to meet the power demand in every month. Fill in the missing variables in each cell of the table below. After checking that you've entered the correct values, the table will calculate the requisite discharge and display a graph of monthly power demand, discharge and head.

Q = (P * 1.181 * 10^4)/(η H)

 

Table__. Average of 2000-2012 Folsom power output and reservoir head

Month Average power demand (MW) Ave. monthly demand (MW-hr) Ave. res. elevation (ft) Ave. head (ft)
Discharge Equation(ft3/s)
Discharge (ft3/s)
Jan 47 34,900 403 290 X 1.181 X 10^4) /
( 0.85 X )
0
Feb 52 35,100 409 296 X 1.181 X 10^4) /
( 0.85 X )
0
Mar 59 44,200 422 309 X 1.181 X 10^4) /
( 0.85 X )
0
Apr 74 53,400 435 322 X 1.181 X 10^4) /
( 0.85 X )
0
May 92 68,400 445 331 X 1.181 X 10^4) /
( 0.85 X )
0
June 94 67,700 446 333 X 1.181 X 10^4) /
( 0.85 X )
0
July 84 62,100 436 323 X 1.181 X 10^4) /
( 0.85 X )
0
Aug 57 42,700 422 309 X 1.181 X 10^4) /
( 0.85 X )
0
Sept 43 31,200 416 302 X 1.181 X 10^4) /
( 0.85 X )
0
Oct 41 30,300 408 294 X 1.181 X 10^4) /
( 0.85 X )
0
Nov 37 26,500 400 287 X 1.181 X 10^4) /
( 0.85 X )
0
Dec 45 33,300 399 285 X 1.181 X 10^4) /
( 0.85 X )
0

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)

Three main types of turbines

 

Click on the following links to learn more about the different types of turbines.

 

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 



 

 

2.4 Addressing current and future hydropower challenges

Facing future power challenges

Today, hydropower accounts for about seven percent of overall US energy consumption, however three states - Washington, California, and Oregon - possess more than half of the nation's hydropower capacity. In the Western United States, hydropower represents about 40 percent of the total energy capacity in the region.

As a result of increased power demand since Folsom's construction, the initial three generators were upgraded in 1972 from a nameplate capacity of 162 MW to 198 MW of power. In addition, current construction will raise the dam approximately 7 ft, providing more head for power supply. By 2050, California’s population is expected to grow from the 2005 level of 37 million to 55 million, and as a result the state will need roughly twice as much energy in 2050 as we use today (http://ccst.us/publications/2011/2011energy.pdf).

If you examine the simplified equation for hydropower generation, you should see that a doubling of power output would require a doubling of the discharge, Q (assuming that head, H, remains constant):

 2 * P = (η 2* QH) / (1.181 * 10^4)

Figure 2.4a Current and projected doubling of future power demand, with associated outflow necessary to meet that demand (assuming H remains constant).

 

Click on the button below the graph to display the average monthly inflow into Folsom dam. Compare the inflow (dotted purple line) with the current level of outflow (solid blue line) and outflow necessary to meet a doubling of demand (dashed blue line). What do you think about the capability of Folsom to meet the projected doubling of power demand?

 

Increasing hydropower capacity

Only about 3 percent of the roughly 79,000 dams in the United States have hydropower plants and can generate electricity. One approach for meeting projected future power demands involves adding hydropower capacity to existing dams. A 2011 report by the Department of the Interior shows that the department could generate up to one million megawatt hours of electricity annually by adding capacity at 70 of its existing facilities.