Environment

Scientists challenge economics of storing renewable energy

Scientists challenge economics...
New research examines the economics of storing energy from renewable sources (Photo: Shutterstock/luchschen)
New research examines the economics of storing energy from renewable sources (Photo: Shutterstock/luchschen)
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A representation of the break even point for curtailing or storing energy, with the x-axis representing the percentage of the energy production being curtailed or stored and the different colored lines representing different storage options
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A representation of the break even point for curtailing or storing energy, with the x-axis representing the percentage of the energy production being curtailed or stored and the different colored lines representing different storage options
Representation of the battery cycle life required to make storage an energetically viable option for wind power, with horizontal lines representing current technology
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Representation of the battery cycle life required to make storage an energetically viable option for wind power, with horizontal lines representing current technology
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New research examines the economics of storing energy from renewable sources (Photo: Shutterstock/luchschen)
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New research examines the economics of storing energy from renewable sources (Photo: Shutterstock/luchschen)

True or false: solar and wind power are freely available and clean, and thus should always be stored when they generate more energy than the grid can use? It's easy to assume that renewable energy should never be turned off, but scientists at Stanford have done the math to find the break-even point where storing energy is better than "wasting," or curtailing, that energy, and their findings aren't necessarily as you'd think.

Though curtailing energy production results in an immediate energy loss, avoiding that loss through energy storage also requires an investment of energy, either through manufacturing batteries or building infrastructure. However, not all storage is the same, nor are the energy demands of creating wind and solar farms equivalent. By expressing all the energy in equivalent terms, the team could compare the return of energy garnered from solar and wind with the energy stored in batteries, per unit of energy required to build either (expressed as EROI for return of energy, and ESOI for the storage of energy).

Because solar panels are more energetically expensive to produce than wind turbines, the EROI values differ by a factor of ten. When looking at various types of batteries, even more efficient flow batteries, all had much lower ESOI values than the geologic energy storage methods studied, which were compressed air energy storage and pumped hydroelectric storage.

Factoring in these differences, the study's results show it’s currently always a better option to store solar energy because of the high energetic cost to recoup. However, the only storage options that are always better than curtailment of wind are geologic methods, with battery storage becoming better than curtailment depending on the fraction of energy being used in the grid instead of being stored or curtailed. In the graph below, you can read the ϕ in the bottom axis as representing that fraction. At the far right, if 100 percent of the energy is being curtailed or stored (i.e., none is going to the grid), then storing it is just barely a better option with any battery type. But at other rates it depends on the battery type.

A representation of the break even point for curtailing or storing energy, with the x-axis representing the percentage of the energy production being curtailed or stored and the different colored lines representing different storage options
A representation of the break even point for curtailing or storing energy, with the x-axis representing the percentage of the energy production being curtailed or stored and the different colored lines representing different storage options

It seems counter-intuitive that wind energy should be so cheap yet benefit in most cases from curtailment. But Michael Dale, one of the co-authors of the study, compared it to storing valuables in a safe. "You wouldn't spend $100 on a safe to store a $10 watch," he writes. In some situations it may even be preferable, in terms of energy expenditure, to build a new wind turbine rather than build storage for existing turbines.

The authors also make it plain that they’re condensing the question down to comparing one variable: the energetic trade-offs involved. But relying on economics alone avoids these considerations, and can even turn so-called green energy into the opposite. The authors calculated how much the life cycle of batteries would need to improve before becoming a viable option for wind – by a factor of two to 20, depending on the type of battery. But more importantly they also encourage the development of technologies that can use the otherwise curtailed energy in applications that aren’t harmed by being intermittent, such as systems to pump or purify water.

Sources: Stanford University, Royal Society of Chemistry

26 comments
Slowburn
The electricity that come off windmills is lousy with spikes and dips because of the way turbulence effects the power generated by the blades. if the windmills were pumping water or compressing air these variances in energy capture would be of very little importance because the electrical generation would come from stored energy. Had the Wind Energy enthusiasts gone this way from the start they could have become a reliable peak load producer and the price the charged for the electricity would have been nicely profitable for the producer, and reasonable for the utilities at the same time because of the expense of other peak load generating systems. I like pneumatic because those towers would make a dandy air tank.
JoeT
Unless and until we find a low cost, long life battery, I imagine that it would be less expensive, and more effective, to employ a smart-grid and smart-home/building approach that will throttle demand quickly, down or up, as a means of "storing" or "releasing" electricity. For example, if the wind were blowing especially hard for a short amount of time, the utility provider could signal (via 3G or FM radio subcarrier or the 'net or whatever) the customer's home or building controller to temporarily use more, or less, electricity. The air conditioner could come down or up 0.5 degrees F, the electric water heater could climb 5 degrees, etc. A network of large buildings with hydronic heating and cooling systems could engage thermal ice storage systems. It would be turned on when power was in excess and the ice would be consumed when power was in short supply. 300 gallons of water, converted to ice, stores about 90 KWH of electricity. Of course, the quintessential storage application would be thousands of parked electric cars, all tied to their chargers, waiting for cheap electricity to begin, and then getting an unexpected price break in the middle of the day whenever the utility has extra power on its hands. This is to say that electric cars enable the use of much more wind and solar power.
The Skud
I agree with Slowburn - raising water for hydro power or some sort of compression system for air, with turbine use later would have been a better option. Windmills - and solar panels - were sold to the public as cheaper (and 'greener') than coal-fired power, but will always suffer from gaps in the wind or sunshine supply, making them intermittent at best without storage.
corgidaddy
The Gizmag article titled "Graphene-based supercapacitor a step closer to commercial reality" represents a solution for storage of excess power from wind power sources. http://www.gizmag.com/graphene-based-supercapacitor/28579/ No doubt the scientists who conducted this research were not informed of this development as this newest technological breakthrough is too new for their consideration.
Sascha Humphrey
Of course energy storage doesn't have to involve batteries or their like, you can store energy in form of hydrogen for instance! http://phys.org/news/2013-07-oftwo-worlds-solar-hydrogen-production.html
Димитър Мирчев
Assuming we add grid energy storage only for the purpose of storing energy this may be true. But utilities and end consumers add batteries for various of reasons and storing energy is not even the main one.
Riaanh
@JoeT, I really like your idea. The electric water heater alone would probably make it worthwile. Heating the water of thousands of households when you have the energy would definately lead to a much reduced peak power demand.
Siegfried Gust
Agree that using the tower of a wind mill for energy storage seems to be an obvious next step in their development. But pumped fluid storage seems like it would be inherently for efficient than compressed gas systems due to losses associated with heat created during compression. I could even imagine a system that worked by raising and lowering a weight inside the tower that might have an efficiency advantage due to the direct mechanical linkage made possible that way.
Slowburn
@ Sascha Humphrey Did you read the article only 5% of the solar energy was converted to hydrogen. Using platinum as the catalyst it take 1.8 watt hours of electricity to produce 1 watt hours worth of hydrogen. Then you have to compress it for storage.
Bob Stuart
There is also a big environmental cost to not storing wind power that should be factored in. As more renewables enter the system, the need for storage and baseload increase. It makes sense to use wind to pump water directly, and generate hydropower on demand. Operating a short inclined railway for tankers would be a good match for kites, and there are locations where reservoir facilities are underused.