**7. Other renewable energy sources**

The contributions that renewable energy sources, other than wind, make to explaining wind overcapacity are also discussed. Similarly to results obtained for nuclear, hydropower is not statistically significant. It is a well-developed renewable energy source, with widespread use in Europe for electricity generation (Balat, 2006), which is relatively stable and mature. Its own characteristics imply its use in base load generation. However, recent developments in this source have involved engaging hydropower with wind power. Indeed, wind power combined with pumped hydro power stations can be useful in meeting electricity demand in peak-load periods. In off-peak, wind overproduction can be used to pump water to an elevated reservoir to later be re-used back in the lower reservoir (Dursun and Alboyaci, 2010). This new technology adds storage capacity, not in terms of energy storage but rather in terms of energy generation storage, thereby giving wind energy a new role as a backup to hydropower. In the future, this could help to mitigate overcapacity effects.

Solar energy merits our attention since it is an increasingly common renewable energy source in Europe. The results show that solar energy does not prove to be statistically signif‐ icant in explaining wind overcapacity. Nevertheless, a popular and advantageous solution, stated by Nema et al. (2009), is the mix of wind and solar considering that their integration makes them less exposed to intermittency. Supporting such systems, more especially in re‐ mote areas, may be a solution to reducing overcapacity. It can be followed by European partners, namely in regions where land space and natural resources allow this investment. A case in point is the recent investment in southern Spain in solar thermal plants with a ca‐ pacity of 300MW at their completion in 2013. Solar thermal power plants use several availa‐ ble technologies such as power towers, parabolic troughs with heat storage, sterling dishes and concentrated/non-concentrated solar power. Consequently, by combining this mix of technologies, the power plants can operate without sunlight at total capacity for 7.5 hours.

When there is more wind installed capacity, the number of intermittent wind power plants also logically increases. Our results support this assumption since the growth rate of wind capacity positively influences overcapacity. Regarding the impact of industrial, municipal and renewable waste power, it seems to overlap wind energy due to positive signs in our results. In fact, with more waste processing for energy generation, there appears to be a sub‐ stitution effect for wind energy. This result is in line with the growing use of waste for elec‐ tricity generation in recent years, ranging from 2% in 1998 to 5% in 2009.
