Pumped storage


Mixing up Yixing

15 June 2004

David Hayes reports on China’s plans for the 1000MW Yixing project, the Jiangsu region’s first large hydro power scheme

DURING the past two decades China has implemented a huge power development programme to build the world’s second largest electricity industry after the US. At the end of the Ninth Five Year Plan (1996-2000) China’s installed electricity generation capacity stood at 319,000MW and the national power output had reached 1.37MTWh a year.

According to government figures, coal-fired power plants account for about 70% of China’s installed power generation capacity while hydroelectric plants account for a further 20%. However, hydro power resources are unequally distributed among China’s various provinces with the result that some provinces have become excessively dependent on coal-fired generation.

One province facing such a problem is Jiangsu, which with 75 million people and an average GDP growth of 20% during the 1990s, is one of China’s most populated and fastest developing provinces. In 2001 Jiangsu’s power grid supplied 92,300GWh of electricity generated by power stations totalling 22,300MW installed capacity, mostly coal-fired units. The local electricity supply was supplemented by small electricity imports of hydro and pumped storage power from neighbouring provinces.

Due to rising electricity demand from household and commercial consumers, Jiangsu’s power grid has faced increasing problems meeting peak load demand, particularly during summer months. According to government forecasts, even if electricity demand growth slows to 5% per annum compared with 9% on average since the mid-1980s, more installed electricity generating capacity is still needed, particularly in peak demand periods. This will help avoid power shortages from worsening that could not only affect the province’s economic growth rate but also lead to power price spikes once a competitive electricity market is introduced as planned.

One way of using off-peak coal-fired power generation to increase peak load power generation is to build pumped storage power plant facilities. This is one option that is being adopted in Jiangsu.

Pumped storage

As part of efforts to increase peak power supplies the World Bank is providing China with a US$145M loan towards the estimated $490M total cost of building the 1000MW Yixing pumped storage scheme, Jiangsu’s first large hydro power project. In addition to supplying peak load electricity, the pumped storage project is planned to assist the power sector reform process in Jiangsu province and corporate restructuring at Jiangsu Provincial Electric Power Company (JPEPC). The company is due to change from being a vertically integrated utility operating as a monopoly to become a grid company in a competitive power market.

The newly formed Jiangsu Pumped Storage Power Co Ltd (JPSPC) is building the Yixing pumped storage plant near Yixing city in the electricity load demand centre of Jiangsu in eastern China. The site is located 200km from Shanghai and 160km from Jiangsu’s provincial capital Nanjing.

JPSPC is 65% owned by JPEPC, which in mid-2003 had an installed capacity of about 24,000MW. China’s State Power holds 10% and Jiangsu International Trust & Investment Co 20%. The remaining 5% stake is held by Yixing Assets Operations Co.
The Jiangsu provincial power system is dominated by coal-fired thermal power plants, most of which are designed to operate base load generation facilities. JPEPC does not own large hydro or other power plants that can be used for additional peaking power generation.

The Yixing pumped storage scheme is designed to help cover part of the wide disparity between peak and off peak electricity demand which already has exceeded 6600MW on some occasions and regularly exceeds 4000MW on more than 275 days each year.

The site selected for the scheme is in an area of hills and flat plains in the Yixing Mount Tongguan region of the Taihu basin, where a number of rivers and small lakes connect with Lake Taihu. The Yixing pumped storage upper reservoir is being built at the source of a gully on the northeastern side of Mount Tongguan’s 524m high main peak and has a catchment area of only 0.21km2.

The lower reservoir located on the northeastern foot of Mount Tongguan will be built by heightening and expanding the existing Huiwu irrigation reservoir and has a catchment area of 1.87km2. If the natural water source proves insufficient the project developers plan to provide additional water by pumping from nearby rivers.

The geological strata in the area of the proposed upper reservoir, lower reservoir, water tunnels and underground power house consists primarily of fine grained sedimentary sandstone with some deposits of silt mudstone and granite.

The geological deposits in which the upper reservoir, water tunnels and underground power house will be built are located in mono inclined structures, while the lower reservoir will be located in a synclinal gully.

Following extensive site surveys, the topographic and geological conditions of the project site are considered satisfactory for constructing the proposed 1000MW pumped storage project. The power plant site is located 6km from the 500kV Yili substation to which a 500kV transmission line will be constructed to connect the Yixing pumped storage scheme with the East China power grid.

Project data

The construction design of Yixing’s upper reservoir is planned to take advantage of a natural depression on the upper, northern flank of Mount Tongguan’s main peak. The upper reservoir will be formed by a main reinforced concrete-faced, gravity saddle dam, and surrounding ridges. To increase stability, the rockfill dam will be roller compacted.
When completed the main dam will have a crest elevation of 474.2m, a crest length of 495m and a height of 47.2m above the concrete base slab. The area behind the main dam will be excavated to create the required maximum water storage capacity of 5.3Mm3 and a live storage capacity of 4.56Mm3. Before the reservoir is filled, a reinforced concrete anti-seepage blanket will be laid down to prevent reservoir leakage.

Yixing’s lower reservoir will be created by increasing the height of the existing clay core rockfill dam behind which the Huiwu irrigation reservoir presently is contained within a natural gully on Mount Tongguan’s lower northeastern flank. The lower reservoir dam will have a crest elevation of 83.4m, a crest length of 484m and a maximum height of 50.4m.

The total storage area of the lower reservoir after the overburden and accumulated sediment is excavated will be 5.2Mm3.

Yixing pumped storage scheme involves constructing one pair of head race tunnels and one pair of tail race tunnels. The tail race tunnels will both measure 1782m in length and 7.2m in diameter. Each tunnel will be finished with a reinforced concrete lining.
Both headrace tunnels will be 1214m in length and will range from 4.8-6m in diameter. Both vertical shift headrace tunnels will be built with full face steel lining to prevent any collapse of the surrounding rock mass.

The underground power house will be built in moderately thick debris sandstone. The overhead rock thickness will range from 268m to 368m. The main caverns will consist of the main and auxiliary power houses. Each will be excavated to measure 151.2m in length, 22m wide and 51.7m high. These will hold four 250MW turbine generators, each turbine having a 353m rated head.

The dimensions of the main transformer chamber after excavation will be 134.6m long, 17.5m wide and 19.7m high. The chamber will hold Yixing’s four 300MVA, three phase, 500kV power transformers.

The turbine units are planned to operate at a rated head of 353m. The sizes of the upper and lower dam reservoirs are designed to allow the scheme to operate at full power output for four hours daily. Average annual power generation is forecasted at 1491GWh, which involves a plant factor of 17%. With a planned overall efficiency of 76%, the pumping energy required is 1959GWh.

Among other operating conditions for Yixing, the average run off from the water catchment area has been calculated at 0.043m3/sec, which is insufficient to cover maximum forecasted evaporation and water leakage losses. To make up the shortage in supply, water from nearby Three Jiu lake will be pumped into the lower reservoir for initial impounding and make-up during operation.

The two Yixing reservoirs feature a considerable range of draw down – 36m in the upper reservoir and 21m in the lower reservoir, most of which can be used on a daily basis.

Among the main features of the Yixing scheme, the upper dam will have a normal storage water level of 471.5m while the lower reservoir normal storage water level will be 78.9m. The operating reservoir storage will be 4.56Mm3.

The Yixing project’s rated water flow in turbine mode will be a combined total of 323m3/sec when all four turbines are in operation (4 x 80.78m3/sec).

Meanwhile, constructing Yixing pumped storage scheme will involve excavating 10.09Mm3 of material from the open excavation sites and 980,000m3 from the power cavern excavations. Some 3.3Mm3 of excavated material will be used as back fill.
Among construction materials to be used, the Yixing project planners have calculated that some 656,000m3 of concrete will be used to construct the Yixing upper and lower dam reservoirs, underground caverns and tunnel system. Concrete reinforcement will include the use of 17,000t of steel bars.

In compliance with World Bank procurement guidelines, all turbine generators, transformers and other heavy electrical equipment are being procured by international competitive bidding. The World Bank loan covers the cost of heavy electrical equipment procurement and consultancy services including project design, bidding, installation commissioning, preparation for staff training and plant operation, plus consultancy costs to support JPEPC in implementing power sector reform in Jiangsu. Domestic bank loans will be used to finance the civil works and plant installation.

Environmental assessment

Lengthy project preparation prior to starting work on the Yixing scheme has involved a full environmental assessment which has strictly adhered to Chinese regulations and standards as well as the World Bank’s environmental safeguard policies. Field work has included biophysical surveys to monitor water quality, identify vegetation resources and potential wildlife habitat and land use.

Because the project involves involuntary resettlement, a resettlement action plan was prepared for the project. In fact, most inhabitants in the Yixing area expressed wide support for the project.

Water surveys found that the water quality of Lake Tuanjiu is relatively poor as a result of domestic and industrial waste water discharge, but the discharge is expected to decline once a number of new waste treatment plants, now under construction, are completed. However, water in the existing Huiwu irrigation reservoir which will become the Yixing scheme’s lower reservoir is of high quality the year round and contains a low concentration of suspended sediment. Huiwu is used for fish farming various species of carp which is expected to continue after the lower reservoir is enlarged.

The upper and lower reservoir areas of the Yixing scheme are vegetated by commercial plantation forests of pine, fir and bamboo along with commercial tea gardens. According to research by Yixing Forestry and Agricultural Bureau there are no rare plants, parks or nature reserves in the area. Similarly no rare or endangered fauna live in the area any longer. Aquatic ecosystems of the project area and adjacent waterways already have been influenced by centuries of human influence and do not contain listed varieties of aquatic organisms.

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