Principles of Hydropower Engineering (Part - 3) - Civil Engineering (CE) PDF Download

Types of hydroelectric projects 

Hydroelectric plants are classified commonly by their hydraulic characteristics, that is, with respect to the water flowing through the turbines that run the generators. Broadly, the following classifications. made are shown in Figure 6. 

Figure 6. Types of hydroelectric schemes
 (a)    Run-of-river without pondage (little or no storage)
 (b)    Run-of-river with pondage (storage suitable to balance diurnal variation
 in power generation)
 (c)    Storage schemes (reservoirs to store excess water of flood flows)
 (d)    Pump- storage schemes

1. Run-of-river s chemes These are hydropower plants that utilize the stream flow as it comes, without any storage being provided (Figure6a). Generally, these plants would be feasible only on such streams which have a minimum dry weather flow of such magnitude which makes it possible to generate electricity throughout the year. Since the flow would vary throughout the year, they would run during the monsoon flows and would otherwise remain shut during low flows. Of course, the economic feasibility of providing the extra units apart from the regular units have to be worked out. Further, the monsoon tailwater in rivers with flat slopes becomes higher, causing the plants to become inoperative. Run-of-river plants may also be provided with some storage (Figure6b) to take care of the variation of flow in the river as for snow-melt rivers, emerging from the glaciers of Himalayas. During off-peak hours of electricity demand, as in the night, some of the units may be closed and the water conserved in the storage space, which is again released during peak hours for power generation. A schematic cross sectional view of a typical run-of-river scheme is shown in Figure 7. 

FIGURE 7. Atypical run of-river hydroelectric station using a dam and an in-stream powerhouse

2. Storage s chemes Hydropower plants with storage are supplied with water from large storage reservoir (Figure 6c) that have been developed by constructing dams across rivers. Generally, the excess flow of the river during monsoon would be stored in the reservoir to be released gradually during periods of lean flow. Naturally, the assured flow for hydropower generation is more certain for the storage schemes than the run-of-river schemes. A typical schematic cross sectional view of a storage scheme power plant is shown in Figure 8. 

Figure 8. Atypical storage-type hydroelectric station with a power house build at the toe of the dam

3. Pumped-Storage schemes Hydropower schemes of the pumped-storage type are those which utilize the flow of water from a reservoir at higher potential to one at lower potential (Figure 6d). A typical schematic view of such a plant is shown in Figure 9. The upper reservoir (also called the head-water pond) and the lower reservoir (called the tail-water pond) may both be constructed by providing suitable structure across a river (Figure 10). During times of peak load, water is drawn from the head-water pond to run the reversible turbine-pump units in the turbine mode. The water released gets collected in the tail-water pond. During off-peak hours, the reversible units are supplied with the excess electricity available in the power grid which then pumps part of the water of the tail-water pond back into the head-water reservoir. The excess electricity in the grid is usually the generation of the thermal power plants which are in continuous running mode. However, during night, since the demand of electricity becomes drastically low and the thermal power plants can not switch off or start immediately, there a large amount of excess power is available at that time. 

FIGURE 9. General view of pumped stroage power station

Figure 10. Pump-storage scheme development with upper and lower pools in the same river

4. Tidal power development schemes These are hydropower plants which utilize the rise in water level of the sea due to a tide, as shown in Figure 11. During high tide, the water from the sea-side starts rising, and the turbines start generating power as the water flows into the bay. As the sea water starts falling during low tide the water from the basin flows back to the sea which can also be used to generate power provided another set of turbines in the opposite direction are installed. Turbines which generate electricity for either direction of flow may be installed to take advantage of the flows in both directions. 

FIGURE 11. Concept of a tidal power development scheme

According to the National Oceanographic and Atmospheric Administration, USA, the potential energy of tides (often referred to as Blue Oil) is estimated at 3*106 MW, of which one-third is dissipated in shallow seas. This implies that the exploitable energy available on sea coasts is of the order of 106 MW. Power can be generated where sufficiently large tides are available. According to experts it may be techno-economically possible to eventually develop 170,000MW at 30 sites worldwide. Globally, so far around 265 MW has been developed, although around 120,000MW are in the planning stage. 

Hydroelectric power plants are also sometimes classified according to the head of water causing the turbines to rotate. The Bureau of Indian Standards code IS: 4410(Part10)-1998 “Glossary of terms relating to river valley projects: Hydroelectric power station including water conductor system,” the following types of power plants may be defined:

1. Low head power plant: A power station that is operating under heads less than 30m (Figure12). 

Figure 12. Sectional view of a typical low head hydro power station

2. Medium head power plant: A power station operating under heads from 30m to 300m. Of course, the limits are not exactly defined and sometimes the upper limit for medium head power station may be taken as 200 to 250m. (Figure 13) 

Figure 13. Sectional view of a typical medium head hydropower station.

3. High head power station: A power station operating under heads above about 300m. A head of 200m/250m is considered as the limit between medium and high head power stations. (Figure 14). 

Figure 14. (a) Layout of the project showing an embankment dam creating a reservoir with a high head
 (b) Sectional view through the water conducting system for hydropower

IS: 4410(part10)-1998 also classifies hydropower plants according to their operating functions as follows:  

1. Base load power plant: A power station operating continuously at a constant or nearly constant power and which operates at relating high load factors. It caters to power demand at base of the load curve.  
2. Peak load power plant: A power station that is primarily designed for the purpose of operating to supply the peak load of a power system. This type of power station is also, therefore, termed as ‘Peaking station’. 

According to Mosonyi (1991), hydropower plants can also be classified according to plant capacity as follows: 

1. Midget plant:                     up to 100KW
2. Low-capacity plant           up to 1,000KW
3. Medium capacity plant     up to 10,000KW
4. High capacity plant           > 10,000KW  

In India, Micro-hydel plants with capacity less than 5000KW are being encouraged to tap small streams and canal falls. Of the larger hydropower stations in India, the following are at the top of the list: 

 A map showing the major hydroelectric power station in India as given in CBIP (1987) is shown in Figure 15. 

World-wide, there are a few hydropower plants with capacity greater than 10,000MW. These are given in the following list. 

On the other hand, China has over 88,000 small hydropower stations with a total installed capacity of 6929MW generating one-third of all the electricity consumed in rural areas. Hence, emphasis on micro-hydel development cannot be overlooked and similar developments can be done in the hilly regions of India where streams and small rivers may be tapped to provide power locally to the neighbouring rural community. 

Though there has been a study growth of hydropower development in India over the years (Figure16), the proportional contribution of hydropower to the country’s total energy production is rather small (Figure17).  

Figure 16. Total energy production and hydro power contribution for india

Figure 17. Installed capacity of india's energy producing units. This shows a hydro thermal mixratio of 29:71 approximately

In comparison, there are countries where hydropower production is the major source of electricity as given in the following table: