This piece was originally published in the January/February 2020 issue of electroindustry.
How to Modernize the Grid to Accommodate the Integration of Renewable Energy and Enable the Decarbonization of Society
Audrey Wang Gosselin, Engineer, ABB Inc., and Gary Rackliffe, Vice President, Smart Grids and Grid Modernization, ABB
Mrs. Wang Gosselin is an engineer in the LEAD Early Career Program. She is bringing business development opportunities to ABB with the Smart Grids and Grid Modernization team. Mr. Rackliffe is Chair of the NEMA Utility Products and Systems Division Leadership Committee.
Individuals, governments, and businesses are pushing to decarbonize society, and a core component of reducing carbon emissions is shifting to renewable energy for the generation of electricity. A transition to 100 percent clean, carbon-free electricity by 2050 is necessary in order to reduce greenhouse gas emissions by 80 percent.1 Renewable generation has grown rapidly, doubling in the last 10 years,2 and the global power generation mix is transitioning to renewable energy sources.
Though the effort to decarbonize via renewables is essential, the power grid’s limited ability to accommodate renewable generation presents a challenge to the industry. In fact, increasing renewable generation capacity is not resulting in the expected growth in electricity generated from renewables.
This trend is the result of a saturation effect—as more increments of renewable capacity are added to the grid, the energy generation contribution of these capacity increments becomes smaller and smaller. Constraints that contribute to this saturation effect include transmission limitations based on new generation locations relative to load, increased transmission distances, and transmission congestion. Other limiting factors include lack of energy storage, coordination with other generation resources, and the challenge of balancing power supply and demand.
Transmission Distance and Congestion
Unlike fossil fuel plants, the most productive renewable generation sites are often located far from large loads. For example, North Dakota has excellent wind energy potential, but the state is far from any large load centers. This challenge will become increasingly relevant with the growth of renewable wind generation in the central U.S. region and offshore.
In addition, increasing the amount of remote renewable generation without sufficient transmission resources has led to transmission lines becoming congested. Consequently, renewables may be curtailed to keep lines within their thermal and dynamic stability limits.
Transmission infrastructure must therefore be enhanced to accommodate growing transmission needs over great distances. Specifically, long-haul transmission lines, such as HVDC (high-voltage, direct current) lines, could be installed to bring large amounts of remotely generated renewable energy to market. The National Renewable Energy Laboratory Interconnection Seam Study is showing that added transmission capacity assists in the growth of renewable generation.3
Balancing Supply and Demand
Balancing the supply and demand of power is becoming increasingly challenging due to the growing diversity of the power generation mix.
An iconic visual for illustrating this balancing act is the California duck curve, shown in Figure 1. The California duck curve was first published in 2013 by the California Independent System Operator, but it can also be observed in other grid regions that have seen a growing amount of renewable solar generation and an evening system peak.
The duck curve shows the net load on the grid on a typical day in springtime. Net load is the load served by the electric system, minus the load served by variable renewable generation such as solar and wind power.4 The duck’s belly is the midday dip in net load caused by the growth of solar generation. Whereas the duck’s neck shows the ramping generation requirements that thermal peaking plants and transmission imports must fulfill to meet peak power demand in the evening.
Figure 1. The California duck curve4
The duck curve reveals a timing imbalance between supply and demand. During the day, there is a risk of oversupply of generation in the early afternoon when solar power is available; oversupply can directly impact market prices, sometimes dipping into negative pricing. In the evening, base load generation, fossil fuel peaking plants, and energy imports are needed to meet the steep change in net load and the system peak demand.
The consequence of this phenomenon is that unless transmission is available for export demand, renewable generation may be curtailed to avoid negative pricing during the day. Base load generation is needed to reduce the strain on peaking plants that must rapidly ramp up to meet evening demand.
Strategies to Facilitate Higher Levels of Renewable Generation Deployment
Considering the constraints on the grid, the challenge for the power industry then becomes about mitigating the renewable saturation effects that could lead to curtailment.
INVESTING IN TRANSMISSION TO SUPPORT RENEWABLE GENERATION
Although distributed energy resources (DERs) are essential for our renewable future, the hype of DERs should not overshadow the importance of the role of transmission and centralized grid resources in the modern grid.
For example, Figure 2 shows the disparity between wind capacity and transmission capacity in West Texas. From 2007 to 2011, the Electric Reliability Council of Texas (ERCOT) significantly curtailed wind generation to keep transmission lines within their stability limits. Despite ample wind power production during these years, wind generation could not be delivered to customers due to transmission capacity limitations.
Figure 2. Total wind generation and transmission capacity in West Texas5
ABB conducted a series of studies on transmission line capacity, reactive power compensation to allow optimal power flow, and economic impacts. These studies allowed ERCOT investments to enhance the Texas transmission system and connect wind energy generated at remote wind farm locations with the more populated areas in Texas.
INVESTING IN ENERGY STORAGE SYSTEMS TO CAPTURE AND STORE RENEWABLE ENERGY
Utilities are responsible for meeting system demand and maintaining stable voltage and frequency on the grid, but these requirements are complicated by the variability of renewables. Also, renewable generation is non-dispatchable and does not have 100 percent availability. Energy storage can be used to capture oversupply of renewable generation to shift this energy to help meet peak demand. Storage can also provide capacity firming services to complement renewables to maintain voltage and frequency stability on the grid.
The following are two primary uses of storage to mitigate the integration of renewables:
- Excess generation can be captured and stored for use during peak demand. This approach is currently being used to address the California duck curve, which is used to show power production over a day and the imbalance between peak demand and renewable energy production. Energy storage systems are charged midday during over- generation hours and discharged in the evening during peak demand.
- Storage systems can respond to sudden changes in frequency to maintain generation at a committed level. This approach, known as “capacity firming,” was implemented by ABB in Hawaii to aid in firming-up solar generation on Kauai. The battery energy storage system that was installed, shown in Figure 3, can quickly respond to changes in generation levels, optimizing the efficiency of the solar photovoltaic (PV) plant and allowing for a smooth power output onto the grid.
Figure 3. Battery energy storage systems installed by ABB in Hawaii5
INVESTING IN DEMAND MANAGEMENT TO ‘DISPATCH’ LOAD TO MATCH RENEWABLE GENERATION
The future grid will include shaping demand to match the characteristics of renewable generation. Today, the grid is managed by forecasting load and dispatching generation to meet peak demand. By reversing this idea—managing the load to respond to the available renewable generation—utilization of renewable generation assets will increase.
One approach for implementing this concept of demand-side management is to incentivize power consumption at certain times of the day with time-of- use rates or real-time pricing.
Considering the duck curve again, moving demand from the evening to the afternoon when solar PV generation peaks will decrease system generation capacity requirements and increase solar PV generation asset utilization. Both effects improve the economics of the grid.
Looking to the future, demand-side management will prove to be critical with the growth of the electric vehicle market. EV charging could dramatically increase peak load, potentially exacerbating the effects of the duck curve, unless the effects of this added demand are preemptively mitigated with managed charging strategies.
- Renewable generation must be immediately consumed, stored, or transmitted to other regions needing generation. The energy timing and geography constraints must be addressed, or renewable generation will be curtailed, resulting in diminished benefits from renewable
- Utilities must modernize their grids now to address the saturation effects that can lead to curtailment of renewable
- A modern grid will need transmission capacity along with storage to accommodate higher levels of renewable
- Finally, a modern grid will also need active shaping of customer load to align demand with renewable generation characteristics. ei
- ScottMadden Energy Industry Update, “How Soon is Now?” ScottMadden , 2019, bit.ly/EnergyIndustryUpdate
- Cara Marcey, U.S. Energy Information Administration, “U.S. Renewable Energy has Doubled Since 2008,” March 2019, bit.ly/GenerationDoubled
- Joshua Novacheck, “Interconnection Seam Study,” National Renewable Energy Laboratory, https://www.nrel.gov/analysis/seams.html
- ScottMadden Energy Industry Update, “Everything Counts in Large Amounts,” ScottMadden , 2019, http://bit.ly/LargeAmounts
- ABB, “Meeting the Challenge of Modernizing the Grid to Stabilize the Integration of Renewable Energy,” ABB Power Grids North America, Raleigh, 2019, bit.ly/ABBReport