Integrating Renewable Energy Part 1: Physical Challenges

Written by Kasparas Spokas

Meeting climate change mitigation targets will require rapidly reducing greenhouse gas emissions from electricity generation, which is responsible for a quarter of all U.S. greenhouse gas emissions. The prospect of electrifying other sectors, such as transportation, further underscores the necessity to reduce electricity emissions to meet climate goals. To address this, much attention and political capital have been spent on developing renewable energy technologies, such as wind or solar power. This is partly because recent reductions of the capital costs of these technologies and government incentives have made this strategy cost-effective. Another reason is simply that renewable energy technologies are popular. Today, news articles about falling renewable energy costs and increasing renewable mandates are not uncommon.

While capital cost reductions and popularity are key to driving widespread deployment of renewables, there remain significant challenges for integrating renewables into our electricity system. This two-part series introduces key concepts of electricity systems and identifies the challenges and opportunities of integrating renewables.

Figure 1. Schematic of the physical elements of electricity systems. Source:

What are electricity systems? Physically, they are composed of four main interacting elements: electricity generation, transmission grids, distribution grids, and end users (Figure 1). In addition to the physical elements, regulatory and governance structures guide the operation and evolution of electricity systems (these are the focus of part two in this series). These include the U.S. Federal Regulatory Commission (FERC), the North American Electric Reliability Council (NERC), and numerous state-level policies and laws. The interplay between the physical and regulatory elements has guided electricity systems to where they are today.

In North America, the electricity system is segmented into three interconnected regions (Figure 2). These regions are linked by only a few low-capacity transmission wires and often operate independently. These regions are then further segmented into areas where independent organizations operate wholesale electricity markets and areas where federally-regulated vertically-integrated utilities manage all the physical elements (Figure 2). Roughly two-thirds of the U.S. electricity demand is now located in wholesale electricity markets. Lastly, some of these broad areas are further subdivided into smaller balancing authorities that are responsible for supplying electricity to meet demand under regulations set by FERC and NERC.

Figure 2. Left: North American Electric Reliability Corporation Interconnections. Right: Wholesale market areas (colored area) and vertically-integrated utilities areas (tanned area). Source: &

Electricity systems’ main objective is to orchestrate electricity generation, transmission and distribution to maintain instantaneous balance of supply and continuously changing demand. To maintain this balance, the coordination of electricity system operations is vital. Electricity systems need to provide electricity where and when it is needed.

Historically, electricity systems have been built to suit conventional electricity generation technologies, such as coal, oil, natural gas, nuclear, and hydropower. These technologies rely on fuel that can be transported to power plants, allowing them to be sited in locations where electricity demand is present. The one exception is hydropower, which requires that plants are sited along rivers. In addition, the timing of electricity generation at these power plants can be controlled. The ability to control where and when electricity is generated simplifies the process by which an electricity system is orchestrated.

Enter solar and wind power. These technologies lack the two features of conventional electricity generation technologies, the ability to control where and when to generate electricity, and make the objective of instantaneously balancing supply and demand even more challenging. For starters, solar and wind technologies are dependent on natural resources, which can limit where they are situated. The areas that are best for sun and wind do not always coincide with where electricity demand is highest. As an example, the most productive region for on-shore wind stretches along a “wind-belt” through the middle of U.S. (Figure 3). For solar, the sparsely populated southwest region presents the most attractive sunny skies (Figure 3). As of now, long-distance transmission infrastructure to transport electricity from renewable resource-rich regions to high electricity demand regions is limited.

Figure 3. Maps of wind speed (left) and solar energy potential (right) in the U.S. Source:

In addition, the timing of electricity generation from wind and solar cannot be controlled: solar panels only produce electricity when the sun is shining and wind turbines only function when the wind is blowing. Therefore, the scaling up of renewables alone would result in instances where supply of renewables does not equal customer demand (Figure 4). When renewable energy production suddenly drops (due to cloud cover or a lull in wind), the electricity system is required to coordinate other generators to quickly make up the difference. In the inverse situation where renewable energy generation suddenly increases, electricity generators often curtail the electricity to avoid dealing with the variability. The challenge of forecasting how much sun and wind there will be in the future adds more uncertainty to the enterprise.

Figure 4. Electricity demand and wind generation in Texas. The wind generation is scaled up to 100% of demand to emphasize possible supply-demand mismatches. Source:

A well-known challenge in solar-rich regions is the “duck-curve” (Figure 5). The typical duck-curve (named after the fact that the curve resembles a duck) depicts the electricity demand after subtracting the amount of solar generation at each hour of the day. In other words, the graph depicts the electricity demand that needs to be met with power plants other than solar, called “net-load.” During the day, the sun shines and solar panels generate electricity, resulting in low net-loads. However, as the sun sets and people turn on electric appliances after returning home from work, the net load increases quickly. Electricity systems often respond by calling upon natural gas power plants to quickly ramp up their generation. Unfortunately, natural gas power plants that can quickly increase their output are less efficient and have higher emission rates than slower natural gas power plants.


Figure 5. The original duck-curve presented by the California Independent System Operator. Source:

These challenges result in economic costs. A study about California concluded that increasing renewable deployment could result in only modest emission reductions at very high abatement costs ($300-400/ton of CO2). This is because the added variability and uncertainty of more renewables will require higher-emitting and quickly-ramping natural gas power plants to balance sudden electricity demand and supply imbalances. In addition, more renewable power will be curtailed in order to maintain stability (Figure 6), reducing the return on investment and increasing costs.

Figure 6. Renewable curtailment (MWh) and cumulative solar photovoltaic (PV) and wind power capacity in California from 2014 to 2018. Source: CAISO

Although solar and wind power do pose these physical challenges, technological advances and electricity system design enhancements can facilitate their integration. Several key strategies for integrating renewables will be: the development of economic energy storage that can store energy for later use, demand response technologies that can help consumers reduce electricity demand during periods of high net-load, and expansion of long-distance electricity transmission to transport electricity from natural resource (sun and wind) rich areas to electricity demand areas (cities). Which solutions succeed will depend on the interplay of future innovation, state and federal incentives, and electricity market design and regulation improvements. As an example, regulations that facilitate long-distance electricity transmission could significantly reduce technical challenges of integrating renewables using current-day technologies. To ensure efficient integration of renewable energy, regulatory and energy market reform will likely be necessary. For more about this topic, check out part two of our series here!

Kasparas Spokas is a Ph.D. candidate in the Civil & Environmental Engineering Department and a policy-fellow in the Woodrow Wilson School of Public & International Affairs at Princeton University. Broadly, he is interested in the challenge of developing low-emissions energy systems from a techno-economic perspective. Follow him on Twitter @KSpokas.

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