Grid Storage Cost-Effective for Solar, Not So Much for Wind

Renewables like solar and wind aren't on call 24/7. If we expect to count on power from those two intermittent resources 24/7, we need to find a way to store that power, so that we can flick a switch on on a windless night and have the lights go on.

This isn't news: we've covered the storage issue frequently here at ReWire. But a study published recently is news: it suggests that on a cost-effectiveness basis alone, storage may make a lot more sense for solar energy than it does for wind.

In theory, electrical power storage should work the same whether the power it's storing comes from solar, wind, coal, or scuffing your shoes on a carpet. But in reality, energy storage systems take energy to create. That's true whether we're talking about synthesizing chemicals to make batteries, building a reservoir and turbines for pumped storage, or enclosing a cavern for compressed air storage. The energy it takes to build a storage system is referred to as that system's "embedded energy," and that embedded energy has to be accounted for in planning the grid. Otherwise, we could easily use more energy just building the batteries than we conserve by saving up solar power for nighttime use.

In the study, published in the journal Energy & Environmental Science, researchers from Stanford University examined the embedded energy in several types of chemical batteries, as well as pumped storage. They compared that embedded energy to that involved in building solar panels and wind turbines.

The results with regard to wind were rather surprising. It's common practice to shut down wind turbines when they're producing more power than the grid needs. The practice is called "curtailment." When a wind turbine is curtailed and not producing energy, it's not paying back the energy we spent to build it. This ratio between embedded energy and the energy produced is called Energy Return on Energy Invested (EROEI).

You might think it would make sense to build storage batteries and attach them to the grid so that we could charge them with that excess wind power rather than curtailing turbines. But the Stanford team discovered that it would actually cost between two and five times as much embedded energy to store that excess wind power in batteries than we lose by curtailing the turbines.

Solar and storage turn out to be a better match for a counterintuitive reason: solar costs more embedded energy, so we have more to lose by not storing the power it produces. On average, utility-scale solar and battery storage have pretty similar levels of embedded energy. It thus makes more sense to use those batteries to store solar panels' excess energy, because we'd be wasting more embedded energy by not using the solar.

According to the study's lead author Charles Barnhart, a postdoctoral scholar at Stanford's Global Climate and Energy Project, the embedded energy contained in grid storage in batteries could be reduced dramatically by increasing the number of discharge cycles the batteries can deliver. Typical lead-acid batteries can be charged and discharged about 7700 times, and lithium ion batteries about 6,000 times. To store energy in an energy-effiicient way, says Banhart, we'll need to produce batteries capable of as many as 18,000 cycles. That would mean batteries that could charge and discharge once daily foor almost 50 years.