Effects of wind intermittency on reduction of CO2 emissions: The case of the Spanish power system

Renewable energy sources are believed to reduce drastically greenhouse gas emissions that would otherwise be generated from fossil fuels used to generate electricity. This implies that a unit of renewable energy will replace a unit of fossil-fuel, with its CO2 emissions, on an equivalent basis with no other effects on the grid. But, the fuel economy and emissions in the existing power systems are not proportional with the electricity production of intermittent sources due to cycling of the fossil-fuel plants that make up the balance of the grid (i.e. changing the power output makes thermal units to operate less efficiently). This study focuses in the interactions between wind generation and thermal plants cycling, by establishing the levels of extra fuel use caused by decreased efficiencies of fossil back-up for wind electricity in Spain. We analyze the production of all thermal plants in 2011, studying scenarios where wind penetration causes major deviations in programming and we define a procedure for quantifying CO2 reductions using emission factors and efficiency curves from existing installations; the objective is to discuss the real contribution of renewable energy to the environmental targets: the results show that CO2 reductions are still relevant at high wind penetration, whilst we also suggest alternatives to improve reliability of the power system. (C) 2013 Elsevier Ltd. All rights reserved.