100% Variable Renewable Energy Power Systems: Survey of Possibilties

Abstract

Due to favorable public policy and falling technology costs more and more jurisdictions around the world are adopting higher shares of variable renewable energy into their power systems. Although some countries and jurisdictions, as part of larger interconnected systems, have been able to operate for extended periods of time with 100% of their demand covered with VRE, operating a self-sufficient power system annually with only VRE sources is not yet possible (with the exception of a few micro and mini grid systems, with state of the art technologies and practices). This study through a high-level qualitative assessment, identifies limitations of current and future power systems to host very high shares of VRE. To develop insights on power systems with high shares of VRE current academic literature was reviewed, seventeen experts in the field of renewable energy integration were interviewed and case studies of power systems already operating with high shares of variable renewable energy were analyzed. Power System Stability – a central challenge to achieving high shares of VRE: As a result of its unique characteristics (variability, uncertainty, modularity, asynchronous), the integration of VRE into today’s power systems poses unique challenges. Particularly at high shares maintaining power system stability becomes a critical challenge. Unlike synchronous generators, VRE generators do not inherently provide inertial response or governor response to frequency deviations. A high VRE share grid might not have sufficient reactive power sources to provide voltage stability. Without fault ride through (FRT) capability, VRE generator (such as wind turbine) disconnections in the event of system disturbances negatively affect transient system stability. A 100% VRE grid will require power systems dynamics to remain stable in an inertialess grid. Grid codes would need to be defined to ensure VRE generators are built in with FRT and voltage control capabilities. Industry and market structures do not yet have experience with this since power systems have historically relied on synchronous machines to provide stability. Even island power systems that have operated with 100% instantaneous share of VRE have always had conventional generators online as back up. Solutions require a paradigm shift: Experts overwhelmingly reiterated that a large power system operating with a 100% share of VRE would require transformation in technology, operational practices and market design. In terms of technological transformation, solutions that add to system flexibility such as energy storage and demand response, will need to evolve to accommodate the variability and uncertainty associated with VREs at time frames ranging from seconds to days. VRE 5 generators will need to provide grid reliability services through advanced inverters. Grids will need to incorporate information and communications technology into every aspect of electricity generation, delivery and consumption. Institutional transformations would involve changes in power system planning methods from deterministic approaches to probabilistic approaches. Apart from increased coordination between different balancing areas, communication between transmission and distributions systems would also need to evolve. Power markets would need to adequately compensate for services that enhance system flexibility, provide greater grid stability and support VRE integration. Lack of modeling exercises: Due to the lack of practical examples of large systems with very high penetrations of variable generation, researchers have focused on models to simulate behavior of such systems. However, there are only a limited number of comprehensive studies modeling the behavior of power systems with close to 100% VRE penetration. Some studies use unrealistic forecasts of energy demand; do not take transmission or ancillary service requirements into account. Most studies do not provide whole system simulation or provide simulations at the hourly instead of sub-hourly time scales. This fails to acknowledge reliability challenges a system might face at sub hourly time scales and during transient events. Majority of experts believe there is no technical limit to VRE penetration level: Experts cited challenges such as grid strength, frequency stability, and lack of controllability for mid- to long-term operations for achieving a high VRE share grid. However, many of the experts consider these challenges to be solvable. Technical limits are defined by how the system is designed, inputs the system can handle and how the system is operated. 12 out of 17 experts identified no technical limit and asserted the ability of the system to evolve and accommodate new technology and new inputs. Alternatively, experts who did see a technical limit, assumed a static grid and weighed in on the existing system’s limitations. Economic limits exist even if technical limits do not: All of the 17 experts interviewed stressed the importance of economic viability of a 100% VRE grid. Even if there is no technical limit to grid integration of VREs there might be an economic limit. Grid integration studies have found VRE penetration levels well below 100% to be economically desirable for large power systems such as the US, pan-European electricity system. However, the economic limit is not fixed and technological breakthroughs, strategic investments, or evolving social preferences can push the economic limit 6 The relevance of a 100% VRE grid: There is a lack of robust modeling studies that examine the technologically and economically optimal pathways to a decarbonized power system; therefore it is still difficult to concretely assess whether or not a 100% VRE grid is one of those pathways. However, most large power systems (peak electricity demand above 1 GW) are still far away from a decarbonized grid with most VRE penetration levels standing at below 20%. Research and conversations around a 100% VRE grid can be significant in stimulating innovation and breaking the institutional inertia that govern our power systems. Research on 100% VRE grids can also be an effective advocacy platform for pushing greater political commitment towards cleaner sources of energy.