1 Hydro-Thermal Scheduling (HTS) 1.0 Introduction From an overall systems view, the single most important attribute of hydroelectric plants is that there is no fuel cost, therefore production costs, relative to that of thermal plants, are very small. There are three basic types of hydroelectric plants: run-of-river, pumped storage, and reservoir systems. We will just introduce the first two in this section, and then the remainder of these notes will be dedicated to understanding reservoir systems. Run-of-river Here a dam is placed across a river to create a height differential between the upstream inlet and the downstream outlet, but without creating an expansive reservoir on the upstream side [1]. The turbine is rotated simply by the normal flow of the river. These plants run at a capacity associated with the natural river current. Figure 1 [2] illustrates a number of different run-of-the-river projects. Fig. 1 [2]2 Pump-storage This kind of hydro plant is a specialized reservoir-type plant which has capability to act as both a source and a sink of electric energy. In the source or generation mode, it supplies power to the grid using the kinetic energy of the water as it falls from higher-lake to lower-lake as would a typical reservoir plant. In the sink or pumping mode, it consumes power from the grid in order to pump water from the lower lake to the higher lake. Thus, electric energy from the grid is converted into potential energy of the water at the higher elevation. The original motivation for pumped storage plants was to valley-fill and peak-shave. - Valleys: During low-load periods, the plant is used in pumping mode, thus increasing overall system load. This is beneficial because a decreased number of thermal plants will need to be shut-down (avoiding shut-down and start-up costs), and for those remaining on-line, they can be used at higher, more efficient generation levels. - Peaks: During high-load periods, the plant is used in generating mode, thus decreasing the overall system load that must be met by thermal generation. This is beneficial because it avoids the need to start some of the expensive peaking plants. Figure 2 [3] illustrates a typical 24 cycle for a northwestern region of the US.3 Fig. 2 [3] Of course, the cycle of pumping and generating incurs a net loss. It is typical for the efficiency of a round-trip pump storage cycle to be about 70%; for every 100 MW used to pump water, only about 70 MW will be recovered by the grid. The cost of this loss is lessened by the fact that the energy is supplied by thermal plants operating at higher (and thus more efficient) loading levels because of the presence of the pumping. This cost is compensated by the savings incurred by avoiding shut-down and start up costs of the thermal plants during the valleys and by avoiding the start-up costs of the peaking plants during the peaks. Pump storage has become of even greater interest today because it offers a way to store energy that is available from renewable resources (wind and solar) during off-peak times so that they can then be used during on-peak times. Figure 3 [3] illustrates a situation in the BPA region (which is seeing significant wind growth) where the wind plants are frequently generating when load is low and not generating when load is high. Nuclear Fossil CTs Hydro P/S Gen P/S pump4 Fig. 3 [3] Figure 4 indicates the manner in which pumped storage could be used with wind over a 24 hour period. Fig. 4 [3] Pump storage also supplies regulation and load following to which renewables generally do not contribute. Figure 5 [4] illustrates a typical pump-storage set-up.5 Fig. 5 [4] One pump-storage plant of which I am familiar is called Helms pumped storage plant, commissioned in 1984. It consists of three units rated at 404 MW (1212 MW total) in the generating mode and 310 MW (930 MW total) in the pumping mode. Figure 6 [5] illustrates the overall setup of Helms which operates between Courtright and Wishon Lakes about 50 miles east of the city of Fresno California. Fig. 6 [5]6 Figure 7 [5] shows the powerhouse for Helms, where one can observe that it is underground (at a depth of 1000 ft!). Fig. 7 [5] Figure 8 [5] below shows the typical week-long cycles of Helms. Note that unit 2 is typically not used as a result of the fact that the region around Fresno has recently become transmission constrained. PG&E had to build new transmission to alleviate this problem.7 Fig. 8 [5] In addition to the ability to peak shave and valley fill, Helms is a highly flexible plant with operating flexibility characterized by the following attributes: - Dead stop to full generation in 8 minutes. - Dead stop to full pump in 20 minutes. - Ramp rate of 80 MW/min per unit (about 20% per minute!) This level of operational flexibility is highly desirable for systems that have high wind penetration levels. 2.0 US reservoir systems Reservoir systems are typically created where large water systems occur in highly mountainous terrain so that one or a series of cascading lakes, either natural or enlarged with dams, form reservoirs. Each lake has an associated penstock that runs down the mountainside and leads to one or more turbines. Fig. 9 [6] shows the 10 largest hydroelectric facilities in the US.8 Fig. 9: Ten largest hydroelectric facilities in the US The major US reservoir systems are in the states of Washington, Nevada, California, and Tennessee. The Colombia River system which flows from British Columbia to Washington State to Oregon has 14 reservoir dams ranging from 185 MW to 6809 MW (Grand Coulee Dam) for a total capacity of 24,149 MW (the overall watershed which also includes the Snake River includes more than this). Figure 9a shows a map of the Colombia River System [7].9 Fig. 10a [7] The Colorado River which flows through seven states (Wyoming, Colorado, Utah, New Mexico, Arizona, Nevada, and California) begins in the Rocky Mountains at an altitude of 9019 feet. Figure 10 below shows its course. It is one of the most diverted water systems in the US, with the major use of the river being to irrigate 4 million acres of agricultural land in the US and 500,000 acres in Mexico. In addition, it is the water supply for Los Angeles, Las Vegas, Phoenix,