Annex 5 - Hydropower Model Vakhsh

Transcription

1 Annex 5 - Hydropower Model Vakhsh 1. The Vakhsh Cascade The construction of dams on the Vakhsh River started in the late 1950s with the construction of the Perepadnaya diversion and power station. Until 2011 seven power stations have since been constructed, with the main purpose of generating hydropower. Some of the dams also have secondary purposes, such as feeding irrigation schemes. Power station Installed Capacity (MW) Nurek 3000 Baipaza 600 Sangtuda Sangtuda-2 (Under Construction) 220 Golovnaya 210 Perepadnaya 29.6 Central 18.0 Rogun (Feasibility Study Ongoing) 3600 Shurob (Planned) 600 Table 1: Power Stations on the Vakhsh Further extensions of the cascade are planned. Construction work on the Rogun dam and power station had already started in the late 1970 s and came to a halt with the end of the Soviet Union. Recently another feasibility study for the completion of the scheme has been commissioned. Further possibilities to expand the scheme into the Vakhsh s main tributaries Surhob and Obihingou, as can be seen in Figure 1. The power stations assessed in this report include Nurek, Baipaza, Sangtuda-1, Sangtuda-2 and Golovnaya. Not included in the assessment are the Perepadnaya and Central Power stations, which are on relatively small diversion weirs feeding irrigation canals and the Rogun HPP, for which no information could be obtained. Presently only the Nurek Reservoir provides seasonal storage for the Vakhsh cascade. The downstream power stations either operate as run-of-river plants or provide daily regulation only. 1

2 Figure 1: Vakhsh Cascade Schematic Diagram 2

3 2. Available Data 2.1. Power station data The power station data shown in the data sheets below was obtained from public sources. Data Sheet Nurek Installed Capacity MW 3000 Head Water Levels Flood Design Criteria Minimum Operating Level MOL m a s l Full Supply Level FSL m a s l Maximum Flood Level m a s l Dam Crest Elevation m a s l Extreme Design Flood (1:10000 years) Q 0.01% m 3 /s 5400 Spillway Capacity m 3 /s 4040 Max Powerhouse discharge m 3 /s 1360 Total maximum discharge capacity m 3 /s 5400 Turbine Data Generator Data Year of Commissioning 1972 Turbine Type Vertical Francis Number of Units 9.00 Rated Head Hr m Max Head Hmax m Min Head Hmin m Rated Power Pr MW Rated Discharge Qr m 3 /s Rated Efficiency % 93.92% Turbine Speed n RPM Runner Diameter D3 m 4.75 Assumed water to Wire Efficiency % 87.22% Generator Efficiency % 97.75% Generator Rating MVA Power Factor 0.85 Generator Power Limit MW Frequency Hz Generator Voltage kv 15.8 Spillway Data Spillway Type No of Bays Bay Width Sill Elevation m m a s l 3

4 TWL Rating Curve Q (m3/s) TWL (m) Mean TWL Data Sheet Baipaza Installed Capacity MW 600 Head Water Levels Flood Design Criteria Minimum Operating Level MOL m a s l Full Supply Level FSL m a s l Maximum Flood Level m a s l Dam Crest Elevation m a s l Extreme Design Flood (1:10000 years) Q 0.01% m 3 /s 5400 Spillway Capacity m 3 /s 3000 Max Powerhouse discharge m 3 /s 1236 Diversion Tunnel m 3 /s 1164 Total maximum discharge capacity m 3 /s 5400 Turbine Data Generator Data Year of Commissioning Turbine Type Vertical Francis Number of Units 4.00 Rated Head Hr m Max Head Hmax m Min Head Hmin m Rated Power Pr MW Rated Discharge Qr m 3 /s Rated Efficiency % 93.73% Turbine Speed n RPM Runner Diameter D3 m 6.20 Assumed water to Wire Efficiency % 86.00% Generator Efficiency % Generator Rating MVA Power Factor 85.00% Generator Power Limit MW Frequency Hz Generator Voltage kv

5 TWL Rating Curve Storage Capacity Q (m3/s) km3 Reservoir Volume Live Volume TWL (m) Data Sheet Sangtuda-1 Installed Capacity MW 670 Head Water Levels Flood Design Criteria Minimum Operating Level MOL m a s l Full Supply Level FSL m a s l Maximum Flood Level m a s l Dam Crest Elevation m a s l Extreme Design Flood (1:10000 years) Q 0.01% m 3 /s 5400 Spillway Capacity m 3 /s 4116 Max Powerhouse discharge m 3 /s 1284 Total maximum discharge capacity m 3 /s 5400 Turbine Data Generator Data Year of Commissioning 1989 Turbine Type Vertical Francis Number of Units 4.00 Rated Head Hr m Max Head Hmax m Min Head Hmin m Rated Power Pr MW Rated Discharge Qr m 3 /s Rated Efficiency % 93.00% Turbine Speed n RPM Runner Diameter D3 m 6.00 Assumed Water to Wire Efficiency % 88.86% Generator Efficiency % 97.50% Generator Rating MVA Power Factor 90.00% Generator Power Limit MW Frequency Hz

6 Generator Voltage kv 15.8 TWL Rating Curve Storage Capacity Q (m3/s) km3 Reservoir Volume 0.25 Live Volume TWL (m) Data Sheet Sangtuda-2 Installed Capacity MW 220 Head Water Levels Flood Design Criteria Minimum Operating Level MOL m a s l Full Supply Level FSL m a s l Maximum Flood Level m a s l Dam Crest Elevation m a s l Extreme Design Flood (1:10000 years) Q 0.01% m 3 /s Spillway Capacity m 3 /s Max Powerhouse discharge m 3 /s Total maximum discharge capacity m 3 /s Turbine Data Generator Data Year of Commissioning Turbine Type Vert Kaplan Number of Units 2.00 Rated Head Hr m Max Head Hmax m Min Head Hmin m Rated Power Pr MW Rated Discharge Qr m 3 /s Rated Efficiency % 93.00% Turbine Speed n RPM Runner Diameter D3 m Assumed Water to Wire Efficiency % 89.32% Generator Efficiency % 98.00% Generator Rating MVA Power Factor Generator Power Limit MW Frequency Hz 6

7 Generator Voltage kv TWL Rating Curve Storage Capacity Reservoir Volume Live Volume Q (m3/s) km3 TWL (m) Data Sheet Golovnaya Installed Capacity MW 210 Head Water Levels Flood Design Criteria Minimum Operating Level MOL m a s l Full Supply Level FSL m a s l Maximum Flood Level Dam Crest Elevation m a s l m a s l Extreme Design Flood (1:10000 years) Q 0.01% m 3 /s Spillway Capacity m 3 /s Max Powerhouse discharge m 3 /s Total maximum discharge capacity m 3 /s Turbine Data Generator Data Year of Commissioning Turbine Type Kaplan Number of Units 6.00 Rated Head Hr m Max Head Hmax m Min Head Hmin m Rated Power Pr MW 36.1 Rated Discharge Qr m 3 /s Rated Efficiency % 92.00% Turbine Speed n RPM Runner Diameter D3 m Assumed Water to Wire Efficiency % 85.00% Generator Efficiency % 97.00% Generator Rating MVA Power Factor Generator Power Limit MW Frequency Hz Generator Voltage kv 7

8 Elevation (m a s l) Annex 5 - Hydropower Model Vakhsh TWL Rating Curve Q (m3/s) TWL (m) Storage Capacity km3 Reservoir Volume Live Volume Sedimentation Sedimentation rates were estimated from bathymetric survey data from 1989, 1994 and The 2001 survey showed that the reservoir volume increased between 1994 and 2001, which does not appear plausible. Either the 2001 or 1994 survey results are probably flawed with Rogun 650 0,0 1,0 2,0 3,0 4,0 5,0 6,0 7,0 8,0 9,0 10,0 11,0 Revervoir Volume (km 3 ) Figure 2: Nurek Storage Curves The average annual loss of storage from 1972 to 2001 was approximately 70 Mm 3 /a. If sedimentation progressed at this rate, the reservoir capacity would theoretically be reduced to 1.8 km 3 by the end of the century. If the 2001 survey results are disregarded, the annual sedimentation rate would be approximately 115 Mm 3 /a and thus the siltation would progress even faster. 8

9 However, siltation is unlikely to be allowed to progress unimpeded until the end of the century as operations and possibly dam safety would be severely affected. If sediment builds up against the dam the operation of spillway tunnel (invert at 810 masl) and power intake (invert at 837 masl) would be compromised. Therefore sediment control measures need to be implemented in the medium term. Although other measures, such as sediment flushing tunnels or sediment retention dams are conceivable, the most obvious solution to the sedimentation problem would be the construction of the Rogun Dam, which would prevent bed load and most sediment from reaching Nurek. For the purpose of this study it is therefore assumed that sediment control measures will be implemented by Evaporation An evaporation estimate was obtained from BT s water balance for the Nurek reservoir. It was noted that the factors are identical as those used in BT s water balance for Kairakkum: month 10-6 m/s mm/d Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Table 2: Evaporation Data The annual evaporation is roughly 1180 mm. The evaporation over the surface area of the lake (approx 90 km 2 ) is therefore in the order of 0.1 km 3 or approximately 0.5 % of the average annual inflow. The model is therefore not sensitive to changes in reservoir surface evaporation. Nevertheless the 2080 predictions for Layhsh (see Annex 2) have been used for the model Historical Flow, Level and Energy Data Historical data was not provided, except for the 1994 water balance. Therefore the operating rules are only based on one year of observed values. Further the model could not be calibrated (see par. 4 below) 9

10 3. Model Description Only the Nurek Reservoir provides significant storage capacity for the Vakhsh Cascade. A sequential stream flow model was selected as the appropriate methodology to estimate the effect of changes in environmental parameters to energy production. The model is essentially a reservoir water balance model, in which inflow, outflows and losses are accounted for. The general form of the water balance algorithm governing the operation can be expressed as: Where: V 2, V 1 : Storage volume at end and beginning of routing interval dt: Routing interval I: Inflow Q P : Power station discharge Q L : Leakage and other water requirements Q S : Spill E: Net evaporation losses During each time step, the reservoir level, surface area, average power and generated energy are calculated. The routing interval dt was chosen to be 1 month. Several assumptions and relations between the various parameters are introduced for the numerical model: 1) It is assumed that leakage through the dam and spillway amounts to 2 m 3 /s. Abstractions for irrigation were adopted from BT s 1994 water balance: month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec m3/s Table 3: Irrigation Abstractions Nurek 2) Reservoir volume and level are related through the storage curve (Figure 2). Volume is calculated as function of level and year so that the model takes gradual loss of storage through siltation into account. 3) Reservoir surface area is calculated from the original design data. 4) Evaporative losses from the reservoir are estimated by multiplying the reservoir surface area and the evaporation factor. The monthly evaporation factor (mm/day) was derived from BT s 1994 water balance. It is assumed that this factor includes rainfall gain on the reservoir surface. 5) Maximum turbine discharge was estimated from the data presented in par ) Net head is calculated as the difference between reservoir level and tailwater level. Friction loss in the power tunnels is assumed to be in the order of 2 m. 7) Power station output is calculated from net head, turbine discharge, number of units available, and an estimated water-to-wire efficiency. 8) The tailwater level is assumed to be approximately 645 masl. 10

11 9) Water discharged from Nurek (i.e. the sum of turbine and spillway discharges and leakage) is assumed to pass through the downstream power stations in the same time step, i.e. live storage of all downstream power stations is neglected. As no further information was available it had to be assumed that lateral inflows and irrigation discharges and evaporation losses between Nurek and Golovnaya are roughly equal. 10) For each power station, maximum turbine discharge, net head and efficiencies are estimated from the data presented in par The model as presented above is governed by a set of rules which can be modified to reflect current and future reservoir operation. For this project the following rules are used: 1) Reservoir operation follows a rule curve, i.e. it is assumed that turbine operation aims at controlling the reservoir level to a pre-set curve throughout the year. The level in 1994 is assumed to be representative of the reservoir rule curve. 2) If the reservoir level exceeds the full supply level (FSL) the volume in excess of FSL is spilled and the next time step of the simulation commences with the reservoir at FSL. As the equations used in the model are interrelated and follow non-linear relationships. Iterations are performed for each time step until the solution converges. As a check, the mass balance for the reservoir is calculated for each time step. 4. Data Analysis and Model Calibration As no historical level and energy generation data could be obtained, the model could not be calibrated. Instead the base case flow series was fed into the un-calibrated model and the average annual energy calculated. A comparison can then be made to published production figures (USSR Energomachexport, 1990; SNC-Lavalin International Inc., 2010; ADB, 2008). Nurek Baipaza Sangtuda-1 Sangtuda-2 Golovnaya Model Energomachexport (1990) SNC Lavalin (2010) * ADB (2008) Table 4: Vakhsh Cascade - Annual Generation (GWh/a) * The SNCL report assumes that Nurek is upgraded to 3200 MW. The energy data estimated by the hydropower model falls short of the original design data but matches the recently reported numbers reasonably well. 11

12 5. Climate Change/Hydrology Scenario Combinations 5.1. General Assumptions The following assumptions were made for the forecast scenarios: - As outlined above, the model is not sensitive to changes in evaporation. Increase of evaporation through the reservoir surface is therefore neglected. - Sedimentation progresses at 70 Mm 3 /a until 2020, where after sedimentation control measures are assumed to be effective. - Future upstream dam construction is not taken into account. - The monthly flow distribution follows the normalized pattern (Figure 3): Figure 3: Nurek - Assumed Inflow Distribution The assumption about future dam construction is probably the most critical one as the Rogun dam is not considered in the following scenarios. In order to assess the influence of Rogun on the energy production of the existing power stations of the Vakhsh cascade, access to the currently ongoing feasibility study would be required. 12

13 5.2. Scenarios Baseline The baseline scenario assumes a long-term average inflow of 607 m 3 /s. Figure 4: Base Case - Inflow and Energy Energy calculated for the cascade (Nurek, Baipaza, Sangtuda-1, Sangtuda-2 and Golovnaya) is approximately GWh/a. Firm capacity was calculated as shown in Figure 5 below. As firm capacity is calculated as the minimum power over a 30 year interval the calculations are very sensitive to operating rules and forecast monthly and annual flow factors. While In reality a different set of operating rules is likely to be applied for exceptionally dry years, this is not considered in the model. It is therefore likely that the model significantly underestimates the firm capacity. Nevertheless, the calculated trends are still useful to compare the effects of different climate change scenarios. 13

14 Figure 5: Firm Capacity 14

15 5.2.2 Climate Change/Hydrology Scenario Combinations Figure 6: WBM Hot- Dry Figure 7: WGM Hot-Dry 15

16 Figure 8: WBM Central Figure 9: WBM Warm Wet 16

17 Figure 10: REG Hot-Dry Figure 11: REG Central 17

18 Figure 12: REG Warm-Wet Figure 13: SRM Hot Dry 18

19 Figure 14: SRM Central Figure 15: SRM Warm Wet 19

20 Figure 16: Climate Change / Hydrology Scenario Combinations Energy Trend Summary Figure 17: Climate Change / Hydrology Scenario Combinations - Firm Capacity Trend 20

21 6. Optimization 6.1. Effects of Rogun on the Vakhsh Cascade As demonstrated above, the main environmental factors affecting energy production will be sedimentation and potentially reduced inflows. The construction of the Rogun Scheme and/or other large dams will alleviate the sedimentation for Nurek. Further monthly peak flows would be attenuated by the Rogun reservoir, which in turn would reduce spillage at Nurek and the other downstream power stations. As no information about the planned operation of Rogun could be obtained, it was assumed that the monthly flow pattern will be damped as shown in Figure 18. As Rogun would do most of the annual regulation, operating rules for Nurek would be changed so that the Nurek Reservoir would not be drawn down as it seems to be the case presently. Figure 18: Assumed Nurek Inflow after Construction of Rogun 21

22 Figure 19: Baseline Inflow, Rogun & Nurek Operation at FSL With these measures an increase in scheme output of approximately 5% appears feasible. This conclusion would be valid for the near-baseline scenarios (all REG scenarios and WBM central). Installing additional units or increasing installed capacity would not significantly increase energy productions for the base case or any of the near baseline scenarios. Other benefits of increasing capacity may still make up-rating worthwhile, if done as part of a major refurbishment. Figure 20: Baseline scenarios 22

23 6.2. Other optimization opportunities Apart from the construction of the Rogun Dam, other measures could be considered to increase energy production and firm capacity. Installing additional generating capacity could be attractive for scenarios which predict a significant increase in discharge. Further optimization would be possible by considering the effect of the construction of the Rogun Dam and other dams upstream. Figure 21: SRM Central - Rogun and Uprating Figure 22: Effects of Climate Change/Hydrology Scenario Combinations, Rogun and Upgrading Nurek on Firm Capacity 23

24 Firm capacity would significantly benefit from the construction of the Rogun dam. Increasing installed capacity would have no effect on firm capacity Other measures could include changing the operating rules; however the current operating rules would have to be made available for a further assessment Summary As most of the requested data was not provided, the above is based on assumptions and uncalibrated models. Results must therefore be viewed with caution. Nevertheless some general statements can be made: The construction of the Rogun dam would be beneficial for annual energy and firm capacity. Should the Rogun Dam not be built, then other sedimentation controls need to be investigated and implement towards mid-century. Increasing the installed capacity may be beneficial for annual energy production if one of the scenarios that predict increased runoff materializes. 7. Bibliography ADB. (2008). Proposed Asian Development Fund Grant Republic of Tajikistan: Nurek 500 kv Switchyard Reconstruction Project. ADB. SNC-Lavalin International Inc. (2010). CENTRAL ASIA - SOUTH ASIA ELECTRICITY TRANSMISSION AND TRADE (CASA-1000) PROJECT FEASIBILITY STUDY UPDATE. USSR Energomachexport. (1990). Nurekskaya Hydroelectric Power Station. Moscow: Vneshtorgizdat. 24