One drawback with the study is that no consideration been taken to that fact that the Swedish electricity system today have a large part of non-interruptible power generation, i. e. electricity generation that not can be shut down when the sun shines. This concerns for example a part of the hydroelectric power generation, nuclear power and some industrial power generation. [3] has discussed this and calculated the non-adjustable power generation in Sweden to be 6,9 GW (July) — 12,7 GW (winter). Nuclear power stands for the largest part of this, varying from 4,9 GW (summer) to 9,4 GW (winter). No detailed calculation has been performed on how this will change the possibility for large scale solar electricity generation in Sweden, but a rough estimation is that this almost half the possible PV area that can be installed in Sweden without significant overproduction. It is obvious that a large share of non-adjustable nuclear power in the electricity mix has a negative impact on the possibility to large scale introduction of solar electricity in Sweden — at least when assuming the usage pattern as today and when the possibility to store electricity not is taken into account. This has to do with the very uneven access to solar irradiation over the year in Sweden, in combination that the electricity consumption is substantially lower during the summer at the same time as only half of the nuclear power plants in Sweden are shut down for yearly maintenance.

Since solar electricity is seen as one important, and probably necessary, large scale electricity source in the future it is important that we study future incorporation of solar electricity in the electricity grid thoroughly. For this, we should not be satisfied with how the usage profile looks like today, but should study how the usage profile could be changed to facilitate large scale solar electricity. It is also important to remember that other renewable energy sources like wind and wave power also are non-adjustable and all these energy sources have to be studied together. Although out of the main scope of this paper, some suggestions can be done how to increase the share of solar electricity in the future electrical system:

• Introducing movable (N-S tracking) PV technology will increase and even out the daily electricity production profile, and thereby increase the daily production period. It is important to remember that when PV becomes a large energy source in the grid, it is favourable to rather place stationary modules to south-east and south-west than to the south. In Sweden, south vertical PV modules give a higher winter output, although the annual performance is approx. 25% lower than a tilted module.

• The different electrical grids in different countries have to be coordinated to even out the electricity consumption, which will be favourable for solar electricity. The consumption profile in Sweden with much higher electricity usage during winter is for example very different from the consumption profile in more south countries in Europe and coordination of the different electrical grids should be favourable. ABB has developed the HVDC (High Voltage Direct Current) technology, which is used to transmit electric power over long distances with low electrical losses [4], and this makes remote production of solar electricity and connection of different grids

interesting. Countries like Sweden should not hesitate to invest in solar production technology in other countries with more even electricity production profile, like south of Europe or North Africa.

• Storage technology has to be further developed so it is possible to store energy from day to night or even during longer periods to extend the use of solar electricity. It is still questionable if small scale storage, like batteries for single houses, is a good choice for large scale introduction. Studies within the solar electricity programme SOLEL in Sweden has shown that the energy pay-back period for stand alone systems can be as long as 15-20 years, mainly because the batteries in the system [5]. A more possible solution in the energy changeover that will be necessary for facing out fossil fuel in the future, it is better with large pump hydro power plants, common for several countries. To be able to use large scale solar electricity in Sweden, with the very uneven access of solar radiation due to the high latitudes, cooperation with other countries, both concerning energy production and energy storage, is necessary.

• The electricity usage pattern may change in the future to get a profile that better matches available renewable energy sources. Ten-fifteen years ago, there was a potential lack of electricity during winter daytime in Sweden, which cause a tax system with higher electricity price during these periods. This had the consequence that families, and also industries, tried to situate electricity consuming activities like washing, drying and hot water preparation to evenings and nights as far as possible. Similar tax systems may be necessary in the future to enable larger share of non adjustable renewable electricity production.

Finally, the calculation of A0 that is done in the paper should not be seen as a fixed number, but more an indication of the magnitude of possible PV area that can be installed in Sweden. In the paper both PV technology and electricity consumption are assumed fixed, but this will change in the future. Higher PV performance would reduce A0, but as the share of electricity in the energy mix is growing, even if the energy demand is fixed or slightly decreasing, this would result in larger A0. Further studies, like how PV production match wind energy production, is also planned.

Acknowledgements

We want to thank Kent Boijesson at SERC, Hogskolan Dalarna for helping to organise older weather data and NordPool for using their data for Swedish electricity consumption. The study has been performed with financial help from Elforsk programme Solel.

References

[1] www. NordPool. com (July 2008)

[2] Estevez, Nicoals Sebastian, Photovoltaic Power in the Swedish Grid: to deal with Solar Electricity Overproduction in the Future. Master thesis work, Hogskolan Dalarna, Borlange, Sweden (2007).

[3] Olsson, Catrine och Edin, Niklas, Systemanalys av solkraft vid samkorning med det svenska kraftnatet med lagring av solenergi i vattenmagasin. ("System analysis of solar power in the Swedish grid with energy storage is hydropower ponds”). Thesis, Lund Technical University, Institutionen for Varme — och Kraftteknik, Sweden (1995).

[4] www. abb. com/hvdc (July 2008)

[5] Spante, Lennart, et. al.: Solel 97-99; Ett branschgemensamt FoU-program, slutrapport, Report 00:9, Elforsk (Downloadable with english summary from www. elforsk. se) (2000)