Reducing CO2 emissions by curtailing renewables: Examples from optimal power system operation

To lower CO2 emissions, policy makers want to integrate as much variable renewable energy (VRE) as possible into power systems. This has been translated into targets for VRE as a share of total electricity generation and policies that aim to maximize the use of electricity available from VRE sources. However, in this paper we demonstrate that it is a misconception that maximizing VRE production always lowers CO2 emissions. In fact there are many constraints in power system operation that can lead to situations when curtailing VRE reduces both costs and CO2 emissions. In this paper we identify these situations and constraints, and illustrate them with several examples. The examples show how different constraints from optimal power system operation, using economic dispatch (ED) and unit commitment (UC), can combine to create the seemingly paradoxical result that curtailing VRE reduces both costs and CO2 emissions. Broadly defined these situations can occur 1) due to network constraints which create the need for inefficient redispatch actions if VRE is not curtailed, 2) due to increased need for ramp capability and cycling from other units, and 3) due to reserve/security requirements which can be satisfied more efficiently by allowing VRE curtailment. To achieve the most economical and efficient operation of power systems, instead of VRE curtailment being seen as a measure of last resort to preserve system security, VRE should always be optimally dispatched through markets based on its true cost, thus maximizing the value of VRE to the system rather than its output.