5.2.1 General
The filling of the new reservoirs in the Nam Ngum River Basin will have a negative impact on the energy productions in the existing NN1 hydropower plant as well as in the NN2 plant, under construction.
When the new reservoirs are in operation, the seasonal variations of the incoming flows to the NN1 reservoir will be evened out, the wet-season flows will be lower while the dry-season flows will be higher. This results in a higher average water level in the reservoir, with reduced spillage and consequently a higher energy production in the NN1 hydropower plant.
In the following, the losses in energy production during the filling of the new reservoirs is studied for the existing NN1 and the future NN2 plants.
Thereafter, the increased energy productions are studied for the scenarios 1, 2 and 3.
The energy losses due to increased irrigation demands in the years 2013 and 2020 are also studied.
It should be observed that all main parameters for the hydropower plants used in this study are very uncertain. Most of the data have been collected from feasibility studies supplied by the GoL’s Department of Electricity. For some of the plants, the information is very limited. For most of the other plants, there are multiple studies available, often with contradictory data.
Nam Bak 1and 2 are in a planning stage, and has only been possible to source very limited data on these. Therefore, no energy-production estimations have been carried out for these plants. In the estimations of the impact of NB1+2 on NN1 during the filling period, as well as during normal operation, the two plants at Nam Bak have been substituted with one plant with an inflow of 29 m3/s and a reservoir with an active storage volume of 250 Mm3.
5.2.2 Basic Assumptions
5.2.2.1 Main parameters for the Hydropower Plants
Salient data for the hydropower plants and the reservoirs are presented in Tables 5.2.1 and 5.2.2 below.
Table 5.2.1: Hydropower Plant Data
Plant FSL (masl) TWL (masl) Gross Head (m)
Design flow (m3/s)
Nam Ngum 5 1 100.0 760 340 35.4
Nam Ngum 3 720.0 380 340 163
Nam Ngum 2 375.0 215 160 420
Nam Ngum 1 212.0 167 45 466.3
Nam Bak 1 630 ? ? 39.2
Nam Bak 2 1030 ? ? 16.0
Nam Lik 1 195.0 172 23 310
Nam Lik 2 310.0 240 70 178.8
Table 5.2.2: Reservoir Data
Reservoir FSL (masl)
MOL (masl)
Volume at FSL (Mm3)
Volume at MOL (Mm3)
Active volume (Mm3)
Incomi ng flow
(m3/s)
Incoming flow (Mm3/year)
Nam Ngum 5 1 100 1 060 314 63 251 20.2 636
Nam Ngum 3 720 660 1 316 337 979 92.1 2 904
Nam Ngum 2 375 345 6 774 3 780 2 994 169.4 5 341 Nam Ngum 1 212 196 7 030 2 330 4 700 363.4 11 457
Nam Bak 1 630 585 250 80 170 29.0 914
Nam Bak 2 1 030 1019 ? 65 ? 12.9 407
Nam Lik 1 195 191.5 61 30 31 195.4 6 162
Nam Lik 2 310 270 1 099 270 829 83.7 2 640
In the estimations of energy production, head losses in internal waterways have been included. For plants with long tunnels, these losses are considerable.
The information about waterways is, however, very limited. On the basis of tunnel lengths, tunnel areas and tunnel surfaces, we have estimated the head losses given below in Table 5.2.3.
Table 5.2.3: Estimated head losses in waterways Plant Head loss (m)
Nam Ngum 5 ~20
Nam Ngum 3 ~15
Nam Ngum 2 ~1.0
Nam Ngum 1 ~0.5
Nam Bak 1 ?
Nam Bak 2 ?
Nam Lik 1 ~0,5
Nam Lik 2 ~0.5
5.2.2.2 Flow reduction due to environmental flows
In the energy estimations, it has been assumed that there are no environmental flows at NN1, NN2 and NL1 since the stretches of dry river bed are very short at these plants.
At the other plants, 10% of the average monthly pre-regulation flows has been assumed as environmental flows. The choice of this figure is due to a lack of a better alternative. It has been impossible to ascertain information on standards applied for environmental flows in Lao PDR. However, future detailed project-level studies, e.g. EMPs, need to utilise more advanced methods in order to determine more appropriate ecological-flow requirements at each individual site, based on site- and plant-specific criteria.
Table 5.2.4 shows the environmental flows which have been utilised in the estimations.
Table 5.2.4: Environmental flows in m3/s used in the calculations.
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec NN5 0.2 0.2 0.2 0.3 1.2 3.2 5.6 6.1 4.6 1.6 0.6 0.3 NN3 2.4 2.0 1.7 1.9 4.3 10.8 21.8 27.9 19.1 9.6 5.1 3.3
NN2 0 0 0 0 0 0 0 0 0 0 0 0
NN1 0 0 0 0 0 0 0 0 0 0 0 0
NB2 - - -
NB1 0.1 0.1 0.2 0.2 1.6 5.3 8.5 9.0 6.6 2.0 0.6 0.2 NL2 2.0 1.7 1.5 1.5 4.2 9.7 16.2 23.5 24.1 8.8 4.2 2.6
NL1 0 0 0 0 0 0 0 0 0 0 0 0
5.2.3 Energy losses in NN1 and NN2 during the filling period of the reservoirs The energy losses in an existing hydropower plant, when reservoirs farther upstream are filled, depend mainly on the fact that water volumes used for filling up the dead-storage volumes in the new reservoirs are effectively lost for energy production.
In order to minimise the impact on downstream hydropower and irrigation, it is assumed that during the first season, each reservoir is filled up to the Minimum Operation Level (MOL).
Thereafter, incoming flows are dispatched intermittently. Not until the following season, are the reservoirs filled up to their Full Supply Levels (FSL), during the wet season.
The filling of the reservoirs in the Nam Lik catchment has no influence on the hydropower plants in the Nam Ngum river, only on the irrigation interests in the Vientiane plains.
At NN5, NN3, NB1 and NB2, it is considered that 10% of the incoming flows shall be dispatched as environmental flows during the filling periods.
It is evident from Table 5.2.2 that all reservoirs can be filled up to MOL in less than one year and, except for the NN2 reservoir, they can also be filled up to FSL in one year.
The reservoir elevation in NN1 varies over the year. Consequently, considerations should be taken to the time of the year when the filling of each reservoir starts, when estimating the impact on generation on any plant by the filling of upstream-located reservoirs. Alas, the same considerations should be taken at NN2 when filling NN3 and NN5.
At the time of writing, it is not known at what time of the year the filling of each reservoir will start and end. The seasonal variation of the energy prices is low. For these reasons, the
most important thing to know is the energy loss in GWh due to the filling periods. The value of the energy loss is almost constant over the year.
The following assumptions have been made:
• The dead-storage volume is the water volume, which will be lost for hydropower- electric energy production in downstream-located plants;
• If the dead-storage volume of a reservoir not had been impounded, it would have been discharged at average water levels in downstream-located plants;
• It is assumed that the reduced inflow is taken into account in the daily operation of downstream-located plants during the filling periods, i.e. there shall not be any unnecessary draw-down of the reservoirs.
The average water levels in NN1 and NN2 reservoirs have been estimated by computerised reservoir simulations.
Table 5.2.5 shows the energy losses that have been estimated for the NN1 hydropower plant during the filling periods, assuming an average net head of 38.7 metres at NN1.
Table 5.2.5: Energy losses at NN1 during filling periods
Plant In operation (Year)
Dead storage vol (Mm3)
Energy loss NN1 (GWh)
NN2 2010 3 780 359
NN5 2011 63 6
NN3 2013 337 32
NB1 2013 80 8
NB2 2015 65 6
Total 2010 – 2015 412
Table 5.2.6 shows the energy losses that have been estimated for the NN2 hydropower plant during the filling periods, assuming an average net head of 146.2 metres at NN2.
Table 5.2.6: Energy losses at NN2 during filling periods
Plant In operation (Year)
Dead storage vol.
(Mm3)
Energy loss NN2 (GWh)
NN5 2011 63 22
NN3 2013 337 121
Total 2010 – 2015 143
The greatest potential energy losses in NN1 and NN2 occur if the losses are assumed to occur when the reservoirs are at FSL at NN1 and NN2. The energy losses will increase to 473 GWh and 157 GWh respectively. However, this worst-case scenario is almost certainly a considerable overestimations of the real energy losses for both plants.
5.2.4 Energy Production in the scenarios 5.2.4.1 General
The energy production in the NNRB has been estimated for the following cases:
• Present situation 2007, i.e. unregulated inflows to NN1 + diversions from Nam Xong and Nam Leuk;
• Scenario1, regulated inflows to NN1 from NN2 + diversions from Nam Xong and Nam Leuk;
• Scenario 2, regulated inflows to NN1 from NN2 and NN3+diversions from Nam Xong and Nam Leuk and regulated inflows to NN2 from NN3;
• Scenario 3, regulated inflows to NN1 from NN2, NN3, NN5, NB1 and NB2 + diversions from Nam Xong and Nam Leuk, regulated inflows to NN2 from NN3 and NN5, as well as regulated inflows to NN3 from NN5 and to NL1 from NL2.
In accordance with the above, energy losses due to increased irrigation demands in the years 2013 and 2020 are also included.
The simulations have been modelled using data for the hydrological period 1987-2006.
Appendix D shows the results of the simulations in more detail.
5.2.4.2 Total Energy Production in the scenarios
Table 5.2.7 shows the energy production figures which have been estimated for the scenarios.
Environmental flows, evaporation losses and losses due to irrigation demands, have been taken into consideration.
It should be noted that the energy production from Nb1 and Nb2 are not included in Table 5.2.7 below, due to poor input data.
Table 5.2.7: Energy production in the NNRB, exclusive of NB1 and NB2
Case NN5 NN3 NN2 NN1 NL2 NL1 Total
NN1 2007 - - - 987.17 - - 987.17
NN1 2013 986.92 986.92
NN1 2020 986.65 986.65
Scenario 1a, 2013 - - 1 835.84 1 062.28 - - 2 898.12 Scenario 1b, 2020 - - 1 834.96 1 061.96 - - 2 896.92 Scenario 2a, 2013 - 1 769.22 1 871.39 1 068.08 - - 4 708.69 Scenario 2b, 2020 - 1 768.31 1 870.30 1 067.77 - - 4 706.38 Scenario 3a, 2013 397.07 1 788.38 1 887.69 1 082.07 271.41 233.57 5 660.19 Scenario 3b, 2020 397.07 1 787.60 1 887.11 1 081.72 271.19 233.21 5 657.90
5.2.4.3 Energy losses due to irrigation demands
The energy losses due to irrigation demands at each plant have been estimated. The results are shown in Table 5.2.8.
Table 5.2.8: Energy losses in GWh/year due to irrigation demands
Case NN5 NN3 NN2 NN1 NL2 NL1 Total
Present situation 2007 - - - 2.70 - - 2.70
Scenario 1a, 2013 - - 8.97 3.15 - - 12.12
Scenario 1b, 2020 - - 9.76 3.47 - - 13.23
Scenario 2a, 2013 - 11.29 9.09 3.15 - - 23.53
Scenario 2b, 2020 - 12.22 9.90 3.47 - - 25.59
Scenario 3a, 2013 0.47 11.37 9.14 3.19 2.09 2.14 28.40
Scenario 3b, 2020 0.47 12.30 9.95 3.51 2.30 2.36 30.89
5.2.5 Total benefit for NN1 and NN2 hydropower plants from the new reservoirs
When the benefits of the new reservoirs for the energy production in NN1 and NN2 are estimated, the energy productions in 2013 and 2020 must be compared to the energy production in NN1 in the same year but without the new plants.
Table 5.2.9 shows the energy-generation benefits of the new reservoirs, having taken the evaporation into consideration.
Table 5.2.9: Increased energy production (GWh/year) in NN1 and NN2 due to the additional reservoirs
Case NN1 NN2
Scenario 1a/b, 2013 and 2020 75.4 -
Scenario 2a/b, 2013 and 2020 81.2 35.5
Scenario 3a/b, 2013 and 2020 95.1 52.7
According to Table 5.2.5 above, the total energy losses at NN1 during the filling periods of the new reservoirs is 412 GWh. When the new plants are in operation, the energy production in NN1 will increase by approximately 95 GWh/year.
When the reservoirs of NN3 and NN5 are filled, the energy loss in NN2 will be 143 GWh (see Table 5.2.6 above). When NN3 and NN5 are in operation, the energy production in NN2 will increase by approximately 52 GWh/year.
5.2.6 Installed Capacities in the Scenarios
The installed capacities in Table 5.2.10 are in accordance with the feasibility studies given to the consultant by DoE in October, 2007.
Table 5.2.10: Installed capacities, exclusive NB1 and NB2
Reservoir Installed Capacity (MW)
Nam Ngum 5 100
Nam Ngum 3 440
Nam Ngum 2 615
Nam Ngum 1 155
Nam Lik 1 61
Nam Lik 2 100
The total installed capacities in the scenarios have been put together in Table 5.2.11 below.
Table 5.2.11: Installed capacities in the scenarios, exclusive NB1 and NB2
Case Installed
Capacity (MW) Scenario 1a/b, 2013 and 2020 770 Scenario 2a/b, 2013 and 2020 1 210 Scenario 3a/b, 2013 and 2020 1 471