A challenge for system operators is maintaining reliability with increasing penetration of intermittent renewables. For example, at the end of the day when solar production is declining and residential demand is increasing, significant ramping of non-solar resources is required. Similar challenges can also arise as a consequence of deviations in output from wind or solar facilities relative to weather forecasts over time periods ranging from minutes to hours.
The good news, which should give confidence to operators in the US and elsewhere, is that high penetration levels of renewable generation are not only technically feasible but are already being managed without compromising reliability. This also suggests that an increasing share of renewable energy can be integrated into the electricity system going forward.
Integrating increasing renewable generation
In a new report from the Brattle Group for the Advanced Energy Economy Institute (AEEI), two cases are reviewed – the Electric Reliability Council of Texas (ERCOT) and Xcel Energy Colorado (or Public Service Company of Colorado, PSCo), both of which have managed to successfully integrate increasing amounts of variable renewable energy resources at costs that have generally been small to modest. For example, in 2014, 10% of total electricity demand in ERCOT was met by wind power, with wind's contribution reaching nearly 40% at certain times. ERCOT estimated the cost of integrating its first 10,000 MW of wind, approximately the capacity currently deployed, to be about US$0.50/MWh of wind generation. Xcel Energy Colorado gets nearly 20% of its electricity from wind.
These organizations have used well-established and widely available methods and technologies such as:
• Changes in ancillary services, which manage short-term mismatches between electric supply and demand, with fast-ramping gas-fired generation, demand response, storage and other technologies
• Improved forecasting of production from wind
• Increased flexibility of fossil power plants on the system
• Evolving capabilities of renewable generation itself to contribute to reliability
• Expansion of transmission infrastructure.
In addition, newer approaches under development have included utilizing large-scale storage, dynamically managing the capacity of transmission lines, and allowing demand response to play a bigger role in managing system variability (and emergency situations).
Renewable penetration above 50%
The success to date of these shows that integrating variable renewable energy at penetration levels of 10-20% on average and at times above 50% – i.e. high relative to the current levels in most of the United States – is possible, according to the report. Integration challenges in other parts of the United States, and elsewhere, will differ due to both the mix of renewable resources and the make-up of the existing electric system. For example in Germany renewable generation reached over 28% in 2014 without reliability concerns, although debate is ongoing about the reliability and resource adequacy impacts of the rapid renewable deployment there.
With approaches similar to these, utilities should be able to integrate increasing shares of variable renewable generation using well-established tools and technologies. Further, ongoing technological progress and learning will likely allow the integration of even larger amounts of renewable generation in the future.
Preparing T&D for the future
The report notes that a future electric system characterized by a rising share of renewable energy will likely require concurrent changes to the existing transmission and distribution
(T&D) infrastructure. These are not covered in the report, but utilities, grid operators and regulators are nonetheless advised to plan for needed future investments in T&D, given the lead times and complexities involved.