Site Selection & Configuration for Pumped Storage Power Plants

Sri Lanka is currently developing coal fired power plants. Currently one coal power plant is in operation with an installed capacity of 3 units each of 300 MW. The Long Term Generation Plan 2013-2032, indicates that there will be 14 new coal power generating units to be introduced to the power system in future. In Sri Lanka, the electricity demand rapidly varies with time. The daily load curve shows a minimum of 1,000 MW at off-peak, and 2,150 MW at peak. Such rapid variations cause base load coal power plants to be operated with lower efficiency during low demand hours. Such situations create an opportunity for storage mechanisms. Pumped storage power plants (PSPPs) is one of such storage power plant that could be deployed in Sri Lanka. The country’s natural geography is suitable to facilitate nearly 5,000MW of PSPPs and some attractive sites have already been identified. Most importantly, some of them can be designed as the Pumped Storage Power Plant Complexes (PSPPC). A common large upper pond or lower pond can be utilized by two or more lower ponds or upper ponds forms a PSPPC. The economic viability of having PSPPs in Sri Lanka can improve with such PSPP complexes. This paper discusses the basic design principles of proposed PSPPs and PSPPCs in Sri Lanka.


Introduction
Not only that country's economy depends on electricity supply; Sri Lanka has reached the national electrification ratio of 99.5% by now. The maximum demand is around 2164MW while the installed capacity is 3362MW (Statistical Digest 2013, CEB).Sri Lanka has to run expensive thermal power in the peak demand time. At present, Sri Lanka has 900MW coal power plants but cannot run them efficiently at full load during late night off-peak time and some hours during the day, too.
In year 2013, peak demand is around 2,150MW while off peak is around 1,000MW. This off-peak demand is maintained by the utility by giving special off-peak tariffs for medium and large customer. The off-peak tariff is Rs. 5.90 while peak tariff is Rs. 23.50 for Industrial-3 customers. That means peak tariff is four times the off-peak tariff; both figures do not reflect the actual cost electricity. By reducing the peaks of the load curve using PSPP, Sri Lanka may be able to produce cheap electricity, while giving actual cost reflective, lower tariff to customers.  Table 1 shows the criteria used for determining the sites of PSPPs in a reconnaissance study, and their expected status. For example, the selected site should be suitable for the construction of a power plant of more than 250 MW in capacity and should run for six hours during the period of higher demand each day.

Selecting Sites for PSPPs
The suitable sites were searched using the 1:50,000 scale topographical maps. Basically, natural location for the reservoir and dam with adequate reservoir volume with minimum environment & social effects (avoiding forests, natural parks, historical & cultural heritage sites, areas with high population density) and the locations which has L/H ratio less than 12 were considered. Detailed Analysis comprising basic design and the calculation, was done for the selected sites, using 1:10,000 scale topographical maps. The following This paper discusses the details of locations and design parameters of remaining PSPP sites.

Temporary Fixing of Maximum Plant Discharge
The maximum plant discharge is obtained by the following equation;

Determination of Storage Capacity of Pond
The peak duration hour is set at 6 hours to obtain the active storage capacity. The sedimentation level is determined by estimating sedimentation for 100 years. The sediment volume is estimated as follows provided that qs is assumed to be 200m 3 /km 2 /year.  (1) Low Water Level The low water level (LWL) is set at a position of about twice the inner diameter (D) of the headrace tunnel above the sedimentation level as shown in Figure 5, to prevent intrusion of air into the tunnel. High Water Level The high water level (HWL) is determined by using the storage capacity curves shown in Figure 6, Figure 7,

Preparation of Waterway Profile
(1) Setting the elevation of turbine center The turbine center is set at the elevation corresponding to the draft head below the low water level of the lower pond as described in the following formula. Draft Head is obtained from the relation between the maximum pumping head and draft head as shown in Figure 9.

Puna-Kotmale Pumped Storage Power Plant
The project is to be situated on the Puna Oya and Kotmale Oya which are connected to Kotmale Oya reservoir in the N' Eliya district, Central Region of Sri Lanka. The principal features of the Project will be;

Dambagastalawa Pumped Storage Power Plant
The project is to be situated on the Dambagastalawa Oya, near Ambewela, Pattipola fields in the Nuwara Eliya district, Central Region of the Sri Lanka. The principal features of the Project will be;

Agra Pumped Storage Power Plant
The project is to be situated on the Agra Oya, near Pattipola field in the Nuwara Eliya district, Central Region of the Sri Lanka.
The principal features of the Project will be;