In response to the very real challenge of climate change, the share of electricity generation from renewable sources is expected to increase substantially over the next few decades. This increase is expected to come largely from PV solar and wind energy. The state of California has recently approved legislation which requires that one-third of California's electricity come from renewable sources by 2020. Much of the increase is expected to come from wind and solar. The nameplate capacity levels of solar PV in California in 2020 is expected to be almost 6,000 MW which is over 100 times the 2007 levels (Hawkins, 2008). This is more than the capacity of the current fleet of nuclear plants in California. Wind energy capacity in 2020 is expected to be almost 13,000 MW which is five times the 2007 level.

Very little is known about the likely operational effects of this energy transformation. For while both solar and wind are renewable, they have the disadvantage of not being fully dispatchable. As a result, system operators manage these energy resources by forecasting production levels. The accuracy of the forecasts is critical since the stability of the system requires that the amount of power generation in a control area match exactly, on a near-instantaneous basis, the system load, net of losses and interchange with other control areas. Unfortunately, very little is known about the accuracy of the wind and PV solar forecasts in California.

This paper assesses the operational impacts of integrating PV solar and wind energy into the power grid by examining the operational challenges of PV solar and wind energy in Germany, a country whose power grid has a high degree of operational transparency. Germany is also a recognized world leader in embracing renewable energy. In the TenneT transmission control area in central Germany (http://www.tennettso.de/pages/tennettso_en/index.htm ) wind energy and PV solar energy accounted for approximately 17 and 11 percent, respectively, of vertical load during the day light hours over the period 1 March 2010 through 30 April 2011. While these penetration levels are seemingly modest relative to the increasingly cited goal of 100 percent renewable energy, it may be of relevance that there are instances in which both PV solar and wind energy in the TenneT system actually exceed its vertical load. Wind and solar penetration levels are also significant in the 50Hertz ( http://www.50hertz-transmission.net/en/index.htm?languagevariantid=ENG&lang=en&switchLanguage=yes) and Amprion ( http://www.amprion.net/en) systems.

The paper proceeds by first assessing the accuracy of the PV solar and wind forecasts relative to the accuracy of forecasting load. The analysis then considers the operational challenges associated with the integration of these renewable technologies.