Scientists calculate the energy required to store wind and solar power on the grid

Sep 09, 2013
Grid operators routinely shut down wind farms that produce surplus clean electricity. Credit: Charles Barnhart, Stanford University

Renewable energy holds the promise of reducing carbon dioxide emissions. But there are times when solar and wind farms generate more electricity than is needed by consumers. Storing that surplus energy in batteries for later use seems like an obvious solution, but a new study from Stanford University suggests that might not always be the case.

"We looked at batteries and other promising technologies for storing solar and wind on the electrical grid," said Charles Barnhart, the lead author of the study and a postdoctoral scholar at Stanford's Global Climate and Energy Project (GCEP).

"Our primary goal was to calculate their overall energetic cost – that is, the total amount of fuel and electricity required to build and operate these storage technologies. We found that when you factor in the energetic costs, grid-scale batteries make sense for storing surplus solar energy, but not for wind."

The study, which is supported by GCEP, is published in the online edition of the journal Energy and Environmental Science.

Climate change and renewable energy

Most electricity in the United States is generated at power plants that run on coal and natural gas – fossil fuels that contribute significantly to global warming by emitting large amounts of carbon dioxide. Solar and wind power are emissions-free and renewable, but depend on sunlight or wind to operate.

"For the grid to function efficiently, power supply needs to match power demand at all times, but with renewables, that's not always the case," Barnhart said. "For example, wind farms sometimes produce too much electricity at night when demand is low. That excess energy has to be stored or used elsewhere. Otherwise it will be lost. However, the U.S. grid has very limited storage capacity."

A wide variety of technologies are being developed to address the lack of grid-scale storage. The Stanford team looked at several emerging technologies, including five battery types – lead-acid, lithium-ion, sodium-sulfur, vanadium-redox and zinc-bromine.

In a previous study, Barnhart calculated the energetic cost of building and maintaining each of the five battery systems for grid-scale storage. Lead-acid batteries had the highest energetic cost, lithium-ion the lowest, he found.

"We calculated how much energy is used over the full lifecycle of the battery – from the mining of raw materials to the installation of the finished device," Barnhart said. "Batteries with high energetic cost consume more fossil fuels and therefore release more carbon dioxide over their lifetime. If a battery's energetic cost is too high, its overall contribution to global warming could negate the environmental benefits of the wind or solar farm it was supposed to support."

For this study, he and his colleagues calculated the energetic cost of grid-scale photovoltaic solar cells and wind turbines.

"Both wind turbines and photovoltaics deliver more energy than it takes to build and maintain them," said GCEP postdoctoral scholar Michael Dale, a co-author of the study. "However, our calculations showed that the overall energetic cost of wind turbines is much lower than conventional solar panels, which require lots of energy, primarily from fossil fuels, for processing silicon and fabricating other components."

To store or curtail?

Next the scientists looked at the energetic cost of curtailment – the practice of shutting down solar panels and wind turbines to reduce the production of surplus electricity on the grid.

"Curtailment of renewable resources seems wasteful," Barnhart said. "But grid operators routinely curtail wind turbines to avoid a sudden, unexpected surge of excess electricity that could overload transmission lines and cause blackouts. Curtailment rates in the U.S. will likely increase as renewable energy becomes more prevalent."

Shutting down a clean source of electricity seems counterproductive, but is storing surplus energy in batteries a practical alternative?

To find out, the researchers compared the energetic cost of curtailing solar and wind power, versus the energetic cost of grid-scale storage. Their calculations were based on a formula known as "energy return on investment" – the amount of energy produced by a technology, divided by the amount of energy it takes to build and maintain it.

Using that formula, the researchers found that the amount of energy required to create a solar farm is comparable to the energy used to build each of the five battery technologies. "Using batteries to store solar power during periods of low demand would, therefore, be energetically favorable," Dale said.

The results were quite different for wind farms. The scientists found that curtailing wind power reduces the energy return on investment by 10 percent. But storing surplus wind-generated electricity in batteries results in even greater reductions – from about 20 percent for lithium-ion batteries to more than 50 percent for lead-acid.

"Ideally, the energetic cost of curtailing a resource should at least equal the amount of energy it cost to store it," Dale said. "That's the case for photovoltaics, but for wind farms, the energetic cost of curtailment is much lower than it is for batteries. Therefore, it would actually be more energetically efficient to shut down a wind turbine than to store the surplus electricity it generates."

He compared it to buying a safe. "You wouldn't spend a $100 on a safe to store a $10 watch," he said. "Likewise, it's not sensible to build energetically expensive batteries for an energetically cheap resource like wind, but it does make sense for photovoltaic systems, which require lots of energy to produce."

Increasing the cycle life of a battery would be the most effective way to improve its energetic performance, Barnhart added. Conventional lithium-ion batteries last about four years, or 6,000 charge-discharge cycles. Lead-acid batteries only last about 700 cycles. To efficiently store energy on the grid, batteries must endure 10,000 to 18,000 cycles, he said.

"Storing energy consumes energy, and curtailing energy wastes it," Barnhart said. "In either case, the result is a reduction in the overall energy return on investment."

Other options

In addition to batteries, the researchers considered other technologies for storing renewable energy, such as pumped hydroelectric storage, which uses surplus electricity to pump water to a reservoir behind a dam. Later, when demand for energy is high, the stored water is released through turbines in the dam to generate electricity.

"Pumped hydro is used in 99 percent of grid storage today, " Barnhart said. "It works fantastically from an energetic perspective for both wind and solar. Its energy return on investment is 10 times better than conventional batteries. But there are geologic and environmental constraints on where pumped hydro can be deployed."

Storage is not the only way to improve grid reliability. "Energy that would otherwise be lost during times of excess could be used to pump water for irrigation or to charge a fleet of electric vehicles, for example," Dale said.

It's important for society to be energy-smart about implementing new technologies, Barnhart added. "Policymakers and investors need to consider the energetic cost as well as the financial cost of new technologies," he said. "If economics is the sole focus, then less expensive technologies that require significant amounts of energy for their manufacture, maintenance and replacement might win out – even if they ultimately increase greenhouse gas emissions and negate the long-term benefits of implementing and solar power."

"Our goal is to understand what's needed to build a scalable low-carbon energy system," said co-author Sally Benson, the director of GCEP and a professor of energy resources engineering. "Energy return on investment is one of those metrics that sheds light on potential roadblocks. Hopefully this study will provide a performance target to guide future research on grid-scale energy storage."

Adam Brandt, an assistant professor of energy resources engineering in Stanford's School of Earth Sciences, also co-authored the study.

Explore further: Japan gov't calls on citizens to stockpile toilet paper

More information: "The energetic implications of curtailing versus storing solar- and wind-generated electricity": pubs.rsc.org/en/content/articl… e41973h#!divAbstract

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holoman
2.1 / 5 (11) Sep 09, 2013
How about producing hydrocarbon fuels with the excess energy ?

http://www.prweb....8185.htm

SolidRecovery
1 / 5 (12) Sep 09, 2013
Insightful analysis. I wonder what numbers we are looking at for compressed air storage. The energy density is somewhere in between the battery and the hydro, but the initial costs would be lower than the battery.
Eikka
1.7 / 5 (6) Sep 09, 2013
"For example, wind farms sometimes produce too much electricity at night when demand is low.


Not only at nights, and not only sometimes.

Wind farms produce power at a peak-to-average ratio of 5:1 which means that if you wish to produce significant amounts of your grid's power with wind turbines, you overshoot your demand pretty much every time the wind picks up, night or day.

This comes from the simple fact that doubling the wind speed increases the power by eightfold, and low wind speeds happen more often than high wind speeds, so you get most of your energy from the rare but powerful high winds. Cutting these high winds off then loses you lots of energy.

Let's say you want to produce 50% of your energy with wind like Denmark is hoping to. It means you have to deal with power surges that are up to 2.5 times your entire grid's demand. You have to toss up to 60% of your peak production away because it won't fit the grid.

Eikka
1.7 / 5 (6) Sep 09, 2013
In fact, if you calculate the distribution of hours for different wind speeds using the Rayleigh distribution and multiply it by the turbine power at various wind speeds, you can simulate scenarios for different amounts of curtailment.

For example, restricting the turbine's power to 20% of its maximum output wastes half of the energy the turbine could make, but also makes it much steadier with a peak-to-average ratio less than 2:1 or more precisely a Cp of 0.58 which can be thought of as a sort of "availability" factor. The downside is, your wind electricity is now twice as expensive to make.

You could do better, and restric the windmill to 7% power, and produce a Cp of 0.75 which means the turbine would turn at a constant power nearly all the time, but you'd sacrifice 80% of the energy and that would make your wind power five times as expensive.

So it's possible to tame the winds - at a cost.
Eikka
1.7 / 5 (6) Sep 09, 2013
How about producing hydrocarbon fuels with the excess energy ?


The natural gas network in Germany for example already holds 220 TWh worth of energy. The electricity demand in Germany is about 550 TWh a year, and the conversion factor between the two is about 1/3 to 1/2, so there's actually about two months worth of storage capacity available that already exists and doesn't need to be built.

The only problem is the round-trip efficiency, because you lose 40% of your energy in conversion to methane and hydrogen. Then again, if you have to toss away half of your wind power output because it's too much for your grid, you might as well turn it into synthetic natural gas and stuff it in the gasworks.

After all, you don't have to turn it back into electricity. You can use it for chemical processes and heat just as well, where it directly offsets fossil fuel gas. You can even use it to power vehicles.

shavera
3.3 / 5 (3) Sep 09, 2013
Why not to research the cold fusion which doesn't require an accumulation and forget all these renewable energy nonsenses?


We already did. We found it wasn't possible. We moved on. Sorry. The universe just doesn't work that way.
Sanescience
1.6 / 5 (7) Sep 09, 2013
Storage is also important because if you can't provide base load power reliably but don't have the demand for full time conventional power plants, you end up with Germany's dilemma of subsidizing those sources for the numerous times they are needed plus building a new collection of 25 mega polluting coal and gas power plants to take the place of nuclear plants they are shutting down.

It also damages your industry base as power interruptions and the expense of new infrastructure is making costs uncompetitive with competitor nations.

Germany needed storage capacity "yesterday"!

http://www.forbes...-stupid/
Aaron1980
1 / 5 (9) Sep 09, 2013
use the electricity to make aluminum. aluminum is eternally recyclable and can replace many other materials that use more energy to produce and are less recyclable or not recyclable including plastics. The reason it is not used as much as it can be is because it is now too costly to make because it uses electricity to make aluminum. This extra free electricity can make aluminum cheaper and drive the price of aluminum down to be dirt cheap so everyone will use it instead of more damaging materials such as cheaper steel and plastics.
rwinners
not rated yet Sep 09, 2013
How about producing hydrocarbon fuels with the excess energy ?

http://www.prweb....8185.htm


This article is lame. Energy can be stored in many ways beyond conventional batteries. There is pumped storage. There are mechanical devices that can store energy as motion or as pressure. And there are proven ways to balance the output of conventional power plants to match the need that is being partiallly satisfied by renewables.
jimjohn32
1 / 5 (3) Sep 09, 2013
With electric cars, you just need need a smart grid and you will all the storage you need in millions of electric car batteries. Just lower the price when there is excess electricity and watch the free market work.
RealScience
not rated yet Sep 09, 2013
Aaron - that's a good way to approach the problem (similar to Iceland, Quebec, and the area around the Columbia river bringing in aluminum refining to soak up extra hydroelectricity production).
But with intermittent sources it isn't quite as simple. Electricity is ~1/3 of the cost of aluminum, so near-free electricity would be great. Ore cost would stay roughly the same with intermittent production. However capital costs (financing and depreciation on the plant) would rise almost proportionately with the intermittency. And maintenance costs per ton would also rise with frequent shutdown/startup of the process. One would still need a fairly consistent excess of power most of the time or the higher capital and maintenance costs would more than offset the near-free electricity.
Perhaps a less-efficient aluminum production process better suited to intermittent operation might have capital costs enough lower to make it work, but that could take many millions of dollars of R&D.
rwinners
1.3 / 5 (3) Sep 09, 2013
How about producing hydrocarbon fuels with the excess energy ?

http://www.prweb....8185.htm


How about hydrogen...
antialias_physorg
5 / 5 (1) Sep 10, 2013
OK, as far as the article goes.

But there is one niggle: As soon as we have a sizeable chunk of the energy supplied by alternative sources the calculation starts to change (as now you're not drawing the power to make the batteries/storage systems from fossil fuels anymore). So over time it becomes more advantageous to supply even wind with battery backup.

so there's actually about two months worth of storage capacity available that already exists and doesn't need to be built.

The problem is energy dependence - as germany produces only a few percent of the gas it actually needs. The rest comes (in large part) from russia via the ukraine. Even given that russia is a 'solid' business partner the ukraine has squabbles with them over payments (russia changing pricing for the ukraine rather willy-nilly every now and then - and the ukraine not willing to comply). Which has led in the past years to the ukraine just shutting down the pipes in retaliation for a time.
Eikka
1 / 5 (4) Sep 10, 2013
The problem is energy dependence - as germany produces only a few percent of the gas it actually needs


Which is more the reason to make more gas yourself, from biomass, waste, and excess renewable energy. Fraunhofer Institute already has a process for synthesizing methane that works at 60% efficiency.

Windmills and solar energy cannot exist without the natural gas anyhow, because it is used for load matching these renewables into the grid, and as such will always be producing a significant portion of the energy on the grid. It's just fitting that you can kill two birds with one stone: reduce renewable energy intermittency by curtailing maximum output power, and store the excess into methane to be used later for load balancing, heating, chemical feedstock and vehicle fuel.
Eikka
1 / 5 (3) Sep 10, 2013
With electric cars, you just need need a smart grid and you will all the storage you need in millions of electric car batteries.


Electric car batteries are actually insufficient for grid energy storage.

It's not a matter of how large the batteries are, but how much people drive with them. Suppose you do 15,000 miles a year, so on your average day you drive 41 miles, and at 350 Wh/mi your average daily consumption is 14 kWh. Multiply that by a million, and you get 14 GWh.

That is, you can't put in more than 14 GWh per day into the million electric cars, because their batteries fill up eventually. One windy afternoon and that's it. Not to mention the fact that they aren't all plugged in all the time.

What you'd want for a country the size of US is at least 200-300 Terawatt-hours of storage capacity. What you get with electric cars, even if you had 200 million of them, is about 2-3 TWh.

antialias_physorg
not rated yet Sep 10, 2013
Which is more the reason to make more gas yourself, from biomass, waste, and excess renewable energy.

Exactly. Though the amount one can produce via biomass is limited in comparison. Germany doesn't have space to spare. Everything is pretty much in use (consider that germany has 80mn people living in an area about 30% larger than Texas).
Popular opposition has also stopped fracking in germany (thankfully).

In any case we'll need to look at a combination of energy storage methods. I don't think there is a one-size-fits-all solution. And then there's always the pollution issue with combustion of hydrocarbons.

Windmills and solar energy cannot exist without the natural gas anyhow, because it is used for load matching

What exactly prevents battery (or other storage methods)-matching from replacing gas matching?

Humpty
1 / 5 (7) Sep 10, 2013
Yeah fuck the batteries right off and use hydroeloectric - to pump the water back up hill to fill the dams again...

There are tons of them in existance - and they already are HUGE energy storage mediums - ideal for peak surges etc....

At best, raise the current output and reverse pumping capacity by adding a whole heap more turbines.
Eikka
1 / 5 (3) Sep 10, 2013
Exactly. Though the amount one can produce via biomass is limited in comparison.


It is one input of carbon to the system. Methane can be made directly out of CO2 and water using electricity at a reasonable conversion efficiency, which is what the Fraunhofer institute project is about.

And then there's always the pollution issue with combustion of hydrocarbons.


Methane gas and hydrogen are pretty clean burning. There's no particulate matter coming out of a CNG engine, and the CO2 is recycled back. In the future, fuel cells solve the remaining issues such as incomplete combustion putting out CO and NOx.

What exactly prevents battery (or other storage methods)-matching from replacing gas matching?


Cost, space, and practicality issues, environmental concerns in sourcing the materials or building e.g. dams, unkown future availability of the essential raw materials, and the fact that the gas network and the powerplants already exist.

Neinsense99
1 / 5 (5) Sep 10, 2013
Yeah fuck the batteries right off and use hydroeloectric - to pump the water back up hill to fill the dams again...

There are tons of them in existance - and they already are HUGE energy storage mediums - ideal for peak surges etc....

At best, raise the current output and reverse pumping capacity by adding a whole heap more turbines.

You probably mean to fill the reservoirs that are upstream of the dams, not the dams themselves, which are just the barrier.
antialias_physorg
not rated yet Sep 10, 2013
Methane gas and hydrogen are pretty clean burning.

Sure. That's why they're infinitely preferrable to coal. But if we can go even cleaner with little effort (hydrogen or straight batteries) - why not do that?

Brazil uses a lot of ethanol vehicles. And there's still a very distinct smell to using that fuel.

Cost, space, and practicality issues

We're experimenting with old mines for hydro storage. Salt batteries are also very cheap. And while they don't charge/discharge quickly that's not a big issue for grid applications. Then there's the idea of building oceanfloor hydro storage (which would combine extremely well with off shore wind). There's a lot of good ideas about (almost too many).

I'm guessing gas will be a mid term tideover - but probably not the solution (or, just a part of the solution for emergency uses).

Burning stuff just seems so 'last milennium'.
Sanescience
1 / 5 (6) Sep 12, 2013
Which is more the reason to make more gas yourself, from biomass, waste, and excess renewable energy.

Exactly. Though the amount one can produce via biomass is limited in comparison. Germany doesn't have space to spare. Everything is pretty much in use (consider that germany has 80mn people living in an area about 30% larger than Texas).


That can't be right, there is no way Germany is bigger than Texas! I would say 30% *smaller* (area) is an over estimate. More like 1/2 the size of Texas. Which actually favors your statement.

If you believe Lockheed Martin (Skunk Works) a fusion system the size of a shipping container making 100MW has been "solved" and they will have a prototype in 4 years and a production line in 4 years after that. What will become of all these titanic expensive investments of green energy? They will either push back to stall fusion deployment or they will have to eventually take multibillion/trillion dollar write offs.
kochevnik
1.1 / 5 (7) Sep 12, 2013
@Eikka Windmills and solar energy cannot exist without the natural gas anyhow, because it is used for load matching these renewables into the grid
An abjectly ludicrous and desperate statement, much like your president about Syria. Even more absurd because your war is all about cutting Russia off from selling natural gas to Europe and feeding gas from Saudi Arabia in it's place
wwqq
5 / 5 (1) Sep 17, 2013
use the electricity to make aluminum.


Aluminium smelting is a large capital expense. If you operate it 10% of the time you are going broke. Aluminium smelters operate as close to 24/7, 365 as humanly possible. YOU PAY THEM if you can't provide electricity, they are a bitch to shut down and restart; they do not load follow.
Eikka
1 / 5 (4) Sep 20, 2013
But if we can go even cleaner with little effort (hydrogen or straight batteries) - why not do that?


There's no difference in pollution between methane and hydrogen as fuels as long as the CO2 is recycled back. Batteries on the other hand have poor ESOI and they cause other environmental damage by requiring us to dig up materials.

Methane is denser than hydrogen, so it's much more convenient to store and transport, and doesn't have the habit of diffusing through metals and being extremely easy to explode.

An abjectly ludicrous and desperate statement


Pray tell, without nuclear power, coal, oil, peat and other fossil fuels you can stockpile and use as needed, how do you meet the difference?

Remember that massive battery storage doesn't yet exist - if ever - and hydro power is already built to the maximum, and with 5:1 ratio of max:avg in renewable output, you're looking at producing the vast majority of your total energy by this "other way" that isn't gas.