ABSTRACT
While some hydro installations only make use of the run of a stream of water, power plants in several of the most mountainous regions can benefit from natural or manmade water reservoirs, allowing more-orless complete timewise decoupling between filling the reservoirs by precipitation and letting the water down through the turbines to satisfy an energy demand. Figure 8.1 shows such a reservoir. Figure 8.2 shows the yearly variations of the combined water levels of all the Norwegian reservoirs used for hydropower generation. In addition to the results for a typical year, the figure also provides curves for the highest and lowest water-level years in the recent past. They primarily reflect the variations in annual precipitation, but are also influenced by variables such as temperature and timing of snowmelt that affect the ratio between runoff and evaporation. The high-latitude location of Norway makes the runoff available for hydropower exploitation highly dependent on when the snow and ice accumulated during winter melt, usually sometime in May and June (cf. Figure 3.3). From August, the hydropower utilization exceeds the filling of the reservoirs, and the water level drops more or less regularly until the following May. The water-level difference between high and low for 2005 is equivalent to a potential power production change of some 50 TWh, leaving much room for using the hydropower to smoothe intermittent energy production attached to the same grid system. For example, in the scenario depicted in Figure 5.21, Norwegian hydro covers the entire intermittency deficit of a 100% Danish wind-based electricity system by 5 GW of transmission capacity between Norway and Denmark, and with an influence on hydro reservoir filling amounting to less than or about 2% (Sørensen 1981). Figure 8.3 further illustrates the issue by giving the weekly energy input into the Norwegian
reservoirs during both a typical year (1985) and an atypical year (1990), rather than the accumulated energy level of Figure 8.2 (with water level being expressed in stored energy units).
