Unfortunately, the electricity demand in Switzerland is higher in winter, when the production of hydro-power is lower (see for example the winter/summer production in year 2011 in Figure 23). Higher de-mands in winter are expected to prevail in the future because in most of the scenarios heat pumps replace partially fossil heating. Figure 23 shows the seasonal production mix of summer and winter for some of the scenarios in year 2050; note that the Cleantech study reports only a monthly chart, and the PSI-elc study reports daily winter/summer profiles; for both studies, the winter and summer shares were synthesized.
Clearly, PV production is lower in winter, whereas wind production is slightly higher in all scenarios (Figure 23). Total hydropower production is currently usually lower in winter in all scenarios, whereas
Figure 23: Winter/summer production mix. Cleantech: Synthesized winter/summer share based on monthly chart. PSI-elc: Synthesized winter/summer share based on daily profile for one selected scenario (POM+GAS)
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the stored-hydropower production can be slightly higher in certain years. Most scenarios of the stud-ies follow this pattern. An exception are the scenarios NEP+E and NeueKernkraftwerke of the SCS study, where the hydropower plants in total produce more in winter. This high production in winter from stored-hydropower can also be seen in the daily patterns of Table 13 in the rows of the studies SCS, Greenpeace (which uses the SCS model), and also partially of the BFE study, which reports the daily pattern of the NEP+C+E scenario. Similarly, the Cleantech study assumes that stored-hydropower produces mainly for winter demand. In our view, it must be carefully evaluated in future studies whether high hydropower production in winter is technically as well as economically feasible under different stochastic conditions. Economic feasibility should be evaluated from the system plan-ner’s as well as from the investor’s perspective.
Greenpeace does not provide an easily extractable seasonal or daily generation mix, but we can comment with help of the daily winter/summer mix in Table 13 as follows. In summer, there is no sub-stantial biomass production, more run-of-river production, less seasonal-storage plant production, less production and pumping from pumped-storage plants, no export, but power-to-gas. In winter, there is less run-of-river, more stored-hydropower production, and no substantial production and pumping of pumped-storage plants [44] (Fig. 3.7, Fig 3.8). In winter, PV production is relatively high because the study assumes also installation in alpine areas. In this study, the highest imports are during October to December [44] (p. 30).
8.2 Hourly variation and storage
The dispatch problem may be defined as the problem to match in every minute demand and supply over a yearly time horizon under stochastic solar and wind production, and under uncertain water inflows for hydropower as well as under constrained storage options. The dispatch problem will be-come more complex in the future because most of the envisaged scenarios assume an increasing variation in production by more PV and wind power. On the demand-side, some of the load can be flexibilized to counterbalance those variations: For example, the SCS, VSE and the ETH/ESC study try to quantify such demand side management. Generally, if the inflexible part of demand is still lower than production, then electricity must be stored, exported or discarded. On the other hand, if stochas-tic production is too low, then it must be augmented by flexible production or by imports.
As of today the short-term variations are mainly balanced using hydro-power plants (“ancillary ser-vices”). The VSE study, which is focused on the power sector, evaluates also the costs of more ancil-lary services in the future, that is, the ramping costs as well as the opportunity costs. The service costs are estimated to be 40% higher in year 2050 than today in VSE’s Scenario 2, which has a rela-tively large amount of wind and solar production in year 2050, and they are estimated to be 60%
higher in Scenario 3, which has very large shares of solar and wind [46] (p. 93).
The optimal short-term dispatch of hydropower is a non-trivial optimization problem because the hour-ly dispatch influences the yearhour-ly production, and the water-inflow is varying from year to year. In addi-tion, interconnected reservoirs and bounds on the feasible water levels in rivers reduce the operation-al flexibility. The sum of the peak capacities of operation-all hydropower plants in Switzerland (14 GW in 2012) overestimates the available production capacities; for example, the average load factor of all hydro-power in 2012 is approximately only 30% [5]. With more inflexible generation by solar and wind, pumped-storage plants may be used to store excess production and increase load factors. In the studies, it is generally assumed that the pumps in pump-storage plants will have 5 GW peak capacity, which usually assumes that the project “Lagobianco” of the company Repower will be built (this pro-ject was shelved recently in 2013). The common generation pattern as of today with pumping during night is shown in Table 13.
In the BFE study, the hourly dispatch is investigated in the scenarios WWB+C and NEP+C+E; the C variant means that central CCGT plants are allowed and annual net-imports are zero. In scenario WWB+C in winter in year 2050, the relatively high capacity of the CCGT plants in combination with the hydro-storage plants are able to balance the low winter-production from run-of-river and from PV [33] (Fig. II.3.21, p. 815). In this scenario, throughout the year, pumped-storage hydropower is not heavily required. In scenario NEP+C+E, which has more renewables and less CCGT plants, pumping is more needed as follows. In summer, the pumped-storage hydropower is engaged every day, whereas the hydro-storage (without pumps) plays a minor role [33] (Fig. II.3.24, p. 818). After year 2045 additional storage capacity is required in summer [33] (p. 828) or exports/curtailing must happen on a regular basis. In winter, the hydro-storage must be used extensively after the full nuclear phase-out in year 2034, which requires special management to safely cover production during different
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ter profiles [33] (p. 826, 828). Pumping may occur at mid-day in summer when there is excess PV and wind generation, which is the reverse of today’s operation (Table 13).
Table 13: Examples of daily summer/winter generation pattern: Today and year 2050 Study
(Scenarios) Summer Winter Legend
Today’s pro-duction (2012) [5]
(20.6.2012) (19.12.12) Today’s
trad-ing (2012) [5]
(20.6.2012) (19.12.12)
Greenpeace (2050)
(legend is in opposite order of chart colors; demand profile is identical to SCS study)
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SCS, NEP+E