A downside of renewable energy particularly, wind and solar technologies, is the volatile supply of power. Not only may a scarcity of electricity result in a power blackout, but an oversupply can also lead to grid instabilities as they alter the frequency within the network. For example wind energy in East Germany during strong wind conditions can provide up to 12 GW, which is more than all German coal and gas fired power plants considered together. This is not critical as long as there is enough electricity demand, but may lead to grid instabilities in cases of insufficient demand as there is not enough electricity storage capacity available. To get rid of excess electricity, transmission system operators (TSOs) often have to pay an extra fee to the electricity market (EEX -- European Energy Exchange, Leipzig). Otherwise wind park operators have to be convinced to stop the wind turbines immediately in order to prevent grid instabilities and blackouts. Conversely wind turbines must be stopped due to safety reasons if the wind speed exceeds 30 m/sec. This scenario may cause, within one hour, power gaps equal to the performance of two nuclear power plants. In such cases conventional reserve power plants are required to step in instantly.Agreed, as to solar, wind and the limitations on hydro storage.
In addition, the location of e.g. windfarms (onshore and offshore) is often far away from the centres of demand. Electricity has to be transported from sparsely populated regions to large electricity consumers in metropolitan areas. Therefore, new energy infrastructure (new high voltage transmission lines, transformers and energy storage capacities such as pumped-storage hydropower plants or thermal storage facilities) are needed.
Renewables still account for only a tiny share of energy consumption, because of the expense--and that won't change any time soon. Heck, after promising a major commitment, even Google's abandoning renewable energy production.
(via The Telegraph (U.K.))