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Smoothing Out Solar Production

An oversupply of electricity passed onto the utility grid can damage mechanical components, stress generators and adversely affect market energy prices. Managing the potential for oversupply situations is a significant challenge for utilities adding renewable power — especially solar — to their generation mix.

In 2013, the California Independent System Operator (CAISO) introduced its “duck chart.” The graph, which charts the net load curves for typical days, build a shape resembling a duck’s tail, belly and neck during certain times of the year. The duck chart illustrates the potential for overgeneration of solar photovoltaic (PV) electricity. As California seeks to meet clean energy goals, the visual emphasizes the challenges of intermittent solar power, including overgeneration and insufficient energy storage.

When the Sun Shines

Let’s consider the energy requirements for a single, springtime day in California. The duck curve shows the difference between forecasted electricity load and expanded energy production. With solar PV production in the mix, abundant afternoon sunshine could create an overgeneration situation, followed by a ramp up of conventional energy production to supplement demand when the sun sets.

As the duck curve materializes or the curve grows, grid restrictions may increase in CAISO and other balancing authorities across the U.S. This curtailment of production may mean lower capacity and reduced profitability for solar PV plants.

Creating a Storage Solution

Energy storage is one potential solution to this challenge. Instead of passing generated solar PV energy directly onto the grid, the energy can be stored and shifted for use during the peak hours identified by the duck curve. Economical energy storage solutions still being developed could make this approach a future requirement for balancing authorities.

Looking forward, plant owners and developers must reconsider plant designs to optimize how storage systems are charged, minimizing stranded solar production and capital costs. To evaluate this approach, let’s assume the energy storage to be used is a battery.

Many recent installations of utility scale PV plants use solar tracking systems to maximize the energy output per dollar of capital cost. These tracking systems, however, may not be best suited to charge a battery. The daily energy output of a tracking system during the summer can far outpace the daily energy output on a winter’s day in the same location. If a battery is sized for one season, it will be either overwhelmed in the summer or underutilized in the winter. Either scenario causes a stranded asset.

But what if a solar PV plant could be designed so daily energy production is the same in the summer and the winter?

Installing a high-angle, fixed-tilt solar PV system could smooth electricity generation year-round by capturing winter irradiance while depressing summer output. That would lead to a flatter daily output. Such a PV system design prevents both over- and underutilizing the battery across the seasons. Because a fixed-tilt system can be implemented with a lower capital cost, this design philosophy may even yield a lower levelized cost of energy (LCOE) to charge a battery compared to a tracking system.

Anticipated grid restrictions, coupled with ambitious renewable energy production goals, require solutions to address the duck curve/overgeneration problem. When using solar to store energy, producers should investigate design modifications for steady PV production with minimal impact on the levelized cost. Solar arrays sized for battery storage — not the grid — offer longer-term production and income stability for power producers.

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Dan Clark
Written by Dan Clark

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