Solar Mythbusters #2: Intermittency is a barrier to solar development?

Myth: The intermittent nature of solar photovoltaic (PV) generation prevents it from being a dependable energy source for utilities. 

This is the second in our Solar Mythbusters series see #1 on Clouds here.

By definition, solar energy (PV without storage) is intermittent because it’s not available at night and generation levels can be impacted by clouds and temperature changes during the day. The term has – at times – become a black eye for solar industry proponents because it’s commonly painted by utilities as an enormous integration challenge.

Framing intermittency as a barrier to solar development over-blows the issue and incorrectly suggests that solar is unpredictable and operationally cost prohibitive to integrate on a grid system. Despite utilities occasionally admitting that solar does not present a major challenge, the recent war on solar has fueled a more negative opinion. Before digging deeper into SACE’s rebuttal to this myth, it’s worth noting that utility concerns regarding intermittency challenges are typically related to very significant levels of solar penetration (typically above 15% at peak power, though numerous studies demonstrate non-issues well above 30%). Yet, solar energy currently accounts for less than one percent of annual U.S. electricity generation.

Solar is intermittent, but it’s also predictable

There’s an important distinction between intermittency and predictability. Although solar (without storage) is non-dispatchable – meaning it doesn’t provide guaranteed power that can be ramped up or shut down quickly – it is relatively predictable for both short and long-term planning.

We know where and when the sun rises and sets every day, and we can project annual load profiles based on solar system configuration and historical solar irradiance data for any given location. Satellite-derived data can also be used for estimating not only historical solar irradiance, but even current (real-time) and forecasted values as well. For example, Clean Power Research’s (CPR) web-based resource – SolarAnywhere – provides such data, with forecasts up to seven days. This level of sophistication allows utilities to optimize system fleets and locations based on historical solar data, while also providing forecast ability for planning and scheduling regulation reserves. What’s more, it allows utilities to account for “invisible” generation occurring behind customer meters.

Source: Clean Power Research

While a cloud could disrupt the output of a single system, that one interruption is diluted when looking at the aggregate output of numerous systems spread across a larger area (e.g., utility territory). As shown by the CPR data in the figure to the right, 62.7 megawatts (MW) of solar produced by over eight thousand systems has a more manageable energy pattern compared to a single solar system with an equal capacity (i.e., 62.7 MW). It’s important for utilities to account for the smoothing benefits of geographically dispersed PV systems, since this results in both lower variability and less prediction error. Being able to accurately model the generation of thousands of solar systems (past, present, or forecasted) also demonstrates the benefits of geographic diversity in solar load prediction and planning.

Fuel savings outweigh costs associated with operational demands of intermittent renewables

To accommodate the intermittent fluctuations of high penetration levels of solar, grid operators would likely have to cycle power plants. A recent study by the National Renewable Energy Laboratory (NREL) found that increasing the amount of renewable energy on the grid increases maintenance costs for coal- and natural gas-fired power plants, but the savings generated by not buying fossil fuels far outweigh those costs.  The study modeled increasing the amount of renewable energy – specifically wind and solar – to 33 percent of the total electricity running through the electric grid in 13 Western states in 2020. In turn, fossil plants had to be ramped up and down, or cycled, to accommodate for changes in power production from the wind and solar farms.

The study found that the increased maintenance costs associated with cycling would total between $35 million and $175 million a year across those states, but that the savings from reduced fossil fuel needs would by about $7 billion a year, far outweighing any cycling costs. In addition, the study found that emissions of carbon dioxide dropped by billions of pounds and that sulfur dioxide and nitrogen oxide dropped by millions of pounds due to increased use of renewable energy, while increases in those emissions due to cycling were comparatively negligible.

The results of this study demonstrate that the operational benefits of including high-levels of intermittent renewable resources – such as solar – greatly exceed the costs inflicted on fossil plants that need to ramp up and down to fill energy production generation gaps.

Technology support

While solar can achieve relatively high penetration levels on most grid systems with little trouble, there are current technical solutions in use today to address voltage concerns raised by unique circumstances, such as when there’s a high concentration of solar beyond a single transformer, or large-scale solar on a feeder. PG&E, a national leader in solar capacity, states that for their system overall, “distributed generation is low at about 5% of system peak,” and “system impacts are manageable.” In addition, on a national level the utility grid infrastructure is seriously lagging, with 70% of transmission lines and transformers over 25 years old, and in need of upgrades with or without distributed generation.

That said, there are more substantial advances in technology that are and will make the grid more resilient to higher levels of solar. For example, smart inverters are currently being retrofitted on solar systems across Germany, and here in the U.S., the Western Electric Industry Leaders (WEIL) group is proposing that standards and regulations be updated to require them in new solar installations. Basic smart inverters – which only add about $150 to the general inverter cost – can help reduce voltage fluctuations. “Smarter” inverters could be capable of communicating with the incumbent utility. Similarly, smart meters, batteries, and other devices are advancing rapidly and building momentum as real players in the electricity system that will improve the transparency and planning options for system operators, and ultimately enable increasing levels of intermittent resources such as solar.

Can’t do solar because it’s intermittent? BUSTED! Intermittent yes, barrier to solar development – no way!

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