Wind power is one of the fastest growing sources of power generation in the US and is providing substantial economic and health benefits along with energy security. According to the Energy Information Administration, wind generation accounted for 4.7 per cent of the total electricity production capacity of nearly 74 GW in 2015. This represents a doubling of the share of wind generation from 2.3 per cent in 2010. Given the recent increase in wind energy penetration across the country, curtailment of variable wind generation has become more widespread. While curtailment of generation is normal in a complex power system, owners of wind power plants, which entail no fuel costs, are concerned about its impact on project economics. Operator-induced generation curtailment typically occurs because of congestion in the transmission network or due to lack of transmission access. Recent research on renewable grid integration suggests that transmission expansion is one option to increase system flexibility and thereby mitigate curtailment and other integration challenges.
Against this background, in January 2017, the US Department of Energy (DoE) released a report by the National Renewable Energy Laboratory (NREL), which confirms that adding even limited electricity transmission capacity can significantly reduce the costs of expanding wind energy to supply 35 per cent of the US electricity by 2050. The report, “Reducing Wind Curtailment through Transmission Expansion in a Wind Vision Future”, affirms the findings of the DoE’s Wind Vision study released in 2015, which showed that a future in which wind provides 20 per cent of the US electricity in 2030 and 35 per cent in 2050 is achievable and will provide significant economic, energy security, and health benefits to the country. The report attempts to better assess the operational feasibility of a high wind future as portrayed in DoE’s 2015 Wind Vision study in western US. The following are the key findings of the latest DoE report.
Core wind vision scenarios
According to the report, there are several reasons for variable energy curtailment. These include insufficient transmission capacity, also referred to as grid congestion, or the inability to reduce output from other generating sources. Higher penetration and integration of renewable energy along with minimum curtailment require infrastructure changes such as increased transmission capacity or more flexible, fast-start generation sources such as gas-fired combustion turbines.
High penetration of wind energy could lead to high levels of curtailment if the generator fleet and transmission network remain unchanged. To assess whether increased penetration of wind and solar requires major changes to the infrastructure, two distinct scenarios have been analysed. The first scenario, referred to as the “Reference” scenario, includes high levels of wind (37 per cent) and solar penetration (12 per cent) without major changes in the infrastructure. The second scenario represents the view of the earlier Wind Vision study, which envisaged transmission capacity expansion with high levels of wind penetration.
The generation mix is largely similar in both scenarios with a high level of wind energy penetration (see Figure). The small differences are largely caused by differences in generation capacity as the generator fleets in the two analyses were not identical. However, one major difference between the two scenarios is the level of curtailment. As depicted in the figure, curtailment of renewable sources (in red) is much higher in the Reference scenario as compared to the Wind Vision study scenario.
Therefore, the study emphasises that without new transmission capacity, wind generation will face significant curtailment. Although the Wind Vision study scenario indicates a wind penetration of 37 per cent in western US (with around 12 per cent solar penetration) by 2050, the Reference system only achieves 28 per cent wind penetration due to the curtailment.
The study also assesses the relative impacts of transmission or generator inflexibility on curtailment. The findings suggest that retiring baseload coal in an unchanged transmission network marginally reduces curtailment in a high wind penetration scenario. The study further discovers that high wind penetration with no transmission constraints, referred to as the copper sheet scenario, has only 0.5 per cent curtailment.
Transmission build-out scenario
First transmission build-out scenario: To estimate the value of additional transmission assets in a capacity-constrained scenario, the authors have examined the case involving the addition of four new lines with an aggregate capacity of 10.5 GW. The four transmission lines are the Mountain States Transmission Intertie, the Zephyr Power Transmission Project, the TransWest Transmission Project, and the SunZia Southwest Transmission Project. According to the study, the addition of four projects has a definite impact on the Reference scenario. Notably, the annual curtailment is reduced to 7.8 per cent from 15.2 per cent due to heavy utilisation of new lines indicated by high utilisation factors. Thus, enforcing suboptimal transmission flows shows the importance of real-time balancing for reducing costs and curtailment.
Second transmission build-out scenario: The study further assesses the value of transmission capacity to reduce wind curtailment and production costs. This scenario is examined by adding 8 GW of transmission lines to the previous scenario. The results of this scenario indicate that this additional build-out reduces curtailment marginally by 1.6 per cent (from 7.8 per cent to 6.2 per cent). The reduction in curtailment is small as compared to that seen in the first transmission scenario, which is the most transmission-constrained system. The addition of the first 10 GW into a more congested system has a greater impact on curtailment reduction as compared to the next 8 GW. Therefore, after a given point, increasing transmission capacity further is useful, but yields diminishing returns.
Third transmission build-out scenario: For the third transmission build-out, the study incorporates additional 3,000 MW lines transmitting energy from Montana and Wyoming to southern California. This scenario witnessed a curtailment reduction of 1.8 per cent from 6.2 per cent to 4.4 per cent annually. Further, the new transmission build-outs decrease imports and exports in each state. The rise in imports depicts better use of wind energy in every state and not just in the states in which each line begins and terminates. This is especially obvious in the difference between the Reference and the first transmission build-out scenario. Although the second and third transmission build-out scenarios show less dramatic changes, the Reference case is the most transmission-constrained system (as mentioned above). Thus, it can be claimed that the addition of the first 10 GW of transmission capacity into a more congested system has a greater impact on wind curtailment than the subsequent additions.
In practice, additional transmission capacity can reduce total generation costs, primarily by transmitting lower- cost energy to demand-intensive regions. The other important finding of the study suggests that the proposed transmission of wind energy can help avoid generation costs and emissions, but with diminishing returns.
The best wind resources in western US are often not located near the most populated regions. Therefore, the wind energy needs to be transported hundreds of miles from where it is generated to where it is needed. This study highlights the inefficacy of the existing transmission facilities, which results in large wind energy curtailment.
To assess the value of transmission in mitigating wind curtailment and generation costs, the study uses a suite of three transmission expansion scenarios. Based on an analysis of multiple scenarios, it discovers that with no transmission constraints, wind energy curtailment can be reduced to 0.5 per cent. Moreover, the avoided wind curtailment can substantially reduce annual production costs and carbon dioxide emissions. Meanwhile, greater transmission expansion was found to lead to more benefits in terms of curtailment and cost reduction. However, the marginal benefits of additional transmission projects were found to decline beyond the initial build-out. Overall, transmission expansion is likely to play a vital role in facilitating the efficient usage of renewable resources with 37 per cent wind penetration and 12 per cent solar penetration. Going forward, further research is needed to understand how transmission infrastructure is utilised and the degree of sub-optimality present in the real transmission networks.