Consortium Pöyry - LEI EIA Report 27 March 2009 263 Climate change, sensitivity to temperature rise and wind data Sensitivity of the used model to the changes of meteorological conditions was estimated using the following modified input data scenarios: (1) Air temperature rise +2°C demonstrating climate change with NNPP thermal load of 3 160 MWreleased; (2) Wind data from INPP meteorological station. The air temperature rise scenario reflects predicted conditions from year 2040 onwards as the result of climate warming. The air temperature of the warm period (April- October) is predicted (greatest predicted change) to rise 2.2 oC in Lithuania by 2040– 2069 compared to the temperatures in 1961–1990 (Bukantis and Rimkus, 2005). In the INPP wind scenario the measured wind speed and direction from the INPP meteorological station was used instead of the Dukstas station data. This scenario reflects the effect of possible errors in wind data on model results. The wind measured at the INPP station is likely to be more representative of the actual lake conditions than the Dukstas station data. However, the Dukstas station data was used, because data for all required simulation periods was not available from the INPP station. Figure 7.1-64 shows the time-dependent effect of the climate change scenario compared to the present inlet and outlet 3 160 MWreleased scenario. The +2°C increase in air temperature warms the lake water about the same amount. The climate change result curve also resembles closely the curve with 5 200 MWreleased NNPP thermal load scenario at the present climate (see Figure 7.1-54). CC > 30 C 3160 MW 90 CC > 28 C > 30 C 80 > 28 C 70 60 50 Area[%] 40 30 20 10 0 01/06 16/06 01/07 16/07 31/07 15/08 30/08 14/09 Figure 7.1-64. Proportion of the lake surface area heated to over 28 oC and 30 oC by NNPP capacity of 3 160 MWreleased in year 2002 and in climate change scenario. Using the INPP wind data causes some changes to the sizes of the areas warmed over 28 and 30 degrees as shown in Figure 7.1-65. There seems to be a period of weaker winds in the beginning of June 2002, that increases the warmed up area size. Peak area size in the beginning of August is somewhat larger than in the simulation using Dukstas station data, but the number of days when the 20 % limit is exceeded is lower. However, the differences caused by using these two wind data sets are generally small. Consortium Pöyry - LEI EIA Report 27 March 2009 264 > 30 C, INPP wind 3160 MW 90 > 30 C, Dukstas wind > 28 C, INPP wind 80 > 28 C, Dukstas wind 70 60 50 Area[%] 40 30 20 10 0 01/06 16/06 01/07 16/07 31/07 15/08 30/08 14/09 Figure 7.1-65. Effect of using INPP station wind data on the proportion of the lake o surface area warming to over 28 C at NNPP thermal load of 3 160 MW released in 2002. Sensitivity to water level change To investigate the lake temperature response to water level change, a lowered water level scenario was computed. A water level reduction of 0.9 m was used since it is the minimum allowable water level in the lake according to the existing regulation. The change in the warmed up area for NNPP thermal discharge of 2230 MW was computed for year 2002. The result is shown in Figure 7.1-66. Compared to normal water level scenario, the area warmed over 28°C grows on the average by 1.3 %. Near temperature peaks the raise in warmed up area is larger. The average raise in absolute temperature in the middle of the lake in point P24 was 0.2°C. Thus the impact of water level lowering to the lake temperature is rather small. 2230 MW water level -0.9m 90 water level normal 80 70 60 50 Area[%] 40 30 20 10 0 01/06 16/06 01/07 16/07 31/07 15/08 30/08 14/09 Figure 7.1-66. Effect of lowering the water level in the lake by 0.9m to the proportion of the lake surface area over 28 oC by NNPP discharge of 2230 MW released on the year 2002. Consortium Pöyry - LEI EIA Report 27 March 2009 265 Conclusions of the thermal modelling Thermal load levels It can be concluded, that if the present criterion for lake warming (maximum 20 % of the lake surface layer warming to over 28 degrees) is used the maximum allowable thermal load to the lake during the summer months will be approximately 1 390 MWreleased. However, by reducing the thermal load during the warmest time in the summer, the maximum allowable thermal load can be essentially bigger. The exemplary modelling results show e.g. that reducing 3 160 MWreleased thermal load to half during the warmest time would keep lake temperatures below the present criterion, possibly with few days of exception. The present criterion is relatively rigid and according to exemplary model calculations, already small changes in the criterion would allow relatively big flexibility in the thermal loads. For example, if the criterion for lake warming would be set at 20 % of the lake surface warming over 30 degrees instead of 28 degrees, the maximum allowable thermal load to the lake would be approximately 3 160 MWreleased even during the warmest summer months. One must however note that these exemplary results are calculated for only one year and in practise the maximum allowable thermal load would vary between years depending on the weather conditions. The ecological aspects of different changes in the criterion (temperature and its definition depth area, allowed exceeding in time) are evaluated later in the chapter “ecological impacts of thermal load”. Inlet and outlet locations The current outlet is the best alternative when the area warmed up is used as criteria. However, the different outlet options do not significantly differ from each other. The present NPP outlet position allows the cooling water to spread efficiently to the main part of the lake, allowing both cooling by heat exchange to atmosphere and mixing to cooler lake water. The southern outlet position is more confined and shallow, which restricts the warm outlet water mixing with cooler lake water thus reducing the surface area where the cooling to atmosphere takes place. Dividing the outlet to two locations was no better than the present outlet option when comparing the average size of warmed up areas. However, the divided outlet option had a small advantage in the warmest day giving a somewhat smaller value for the area exceeding 28 °C, which is explained by higher than 30 °C temperatures near the southern outlet. Western inlet option had on the average 0.1°C cooler inlet water temperature compared to the present inlet, and the area warmed up was therefore somewhat smaller. Otherwise the behaviour of the scenario was similar to the present inlet option. The temperature difference is explained by a larger distance to the outlet position. In the deep inlet option simulation, the cold water storage of the deeper part of the lake was depleted in the beginning of the simulation, after which the inlet temperatures did not differ from the present inlet option. Additionally, in the deep inlet option after the thermocline of the lake is destroyed, the mixing of warmer water to deeper layer is increased raising the total heat storage in the lake. As a result, the deep inlet option produces higher surface temperatures during the warmest periods compared to the present inlet option. Consortium Pöyry - LEI EIA Report 27 March 2009 266 Climate change, sensitivity computations and model accuracy Sensitivity analysis computations showed that years 2001 and 2003 had both at least equally warm weather periods compared to year 2002. The climate change scenario with +2.0 °C temperature rise in summer months produced about 2 degree rise in water temperatures. The possibility of error in the model simulations is mostly related to wind data and surface energy balance computation. Regarding the wind data, the used model does not take into account the possibility that the lake may modify the atmosphere and, for example, generate air movement as a result of warming of the lake. This may give too low values for the lake cooling, especially for situations when the wind speed is low. Therefore the highest peak temperatures, typical for the warm, low wind days, may be somewhat overestimated. The lake surface energy balance model does not currently take into account the stability of the atmosphere, but uses neutral stability assumption. This may, in case of warm water and cooler air, give smaller values for cooling than in real situations. Also, in the model only one wind speed and direction value is used for the whole lake, whereas in reality the wind is different in different parts of the lake. The representativeness of the meteorological data for Lake Druksiai can also be questioned, as the nearest meteorological station (Dukstas) with weather data available for all the calibration and simulation periods is located 17 km away from the NNPP location. In year 2002 the overestimation in the modelling is on the average 1ºC, and in year 2003 the overestimation is on the average 2ºC. However, during the warmest periods within each year the overestimation is lower. The model calibration results for years 1989 and 1991 show a similar behaviour. It should also be noted that during warm summer periods lake water temperatures rise to high values also naturally.