From Stanford University
Stanford scientists calculate the energy required to store wind and solar power on the grid
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 energy 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 wind 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.
This article was written by Mark Shwartz of the Precourt Institute for Energy at Stanford University.
I calculated many times the cost of storing energy and with every new technologies and promises I saw, it was always 2×3 times minimum the cost per kw/h of the source. So let say you spend 25cents/kwh for wind, you will pay 50-75 cents/kwh for your storage cost. I would rather pay for nuclear at a fraction of the cost.
“contribute significantly to global warming by emitting large amounts of carbon dioxide. Solar and wind power are emissions-free and renewable”
Fail.
Perhaps other methods other than battery storage, I heard of this (Storing electricity with compressed air)http://robertkyriakides.wordpress.com/2008/04/02/storing-electricty-with-compressed-air/ perhaps an idea?
By all means let’s put some large holes in the ground to dig up lead, zinc and other heavy metals to store electrons. Idiots.
“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.
But if you are relying on that excess energy to power your pumps are charge your vehicles and there is no excess for two or three days then your fields don’t get irrigated and your vehicles stop running.
Now you could say that in those cases then use non-excess energy to run the pumps and charge the vehicles. But then you are making them normal loads on the system (as they actually already are) and they no longer exist as a source for the excess energy.
The only solutions for excess energy are storage or waste. Using it for something that you would do anyways is a zero sum game and accomplishes nothing.
Main reason against: COPPER THEFT BAROMETER http://www.sacci.org.za/index.php?option=com_content&view=article&id=609:copper-theft-barometer-press-release&catid=9:press-office&Itemid=40 will quickly acquire a BATTERY THEFT BAROMETER
Is it true that in California, hydro-electric power is NOT considered renewable energy?
[REPLY: Sadly, yes … because if they did, we’d already be meeting the renewable target. It’s criminal but true. -w.]
Any use of the word “renewable” when refering to energy is an automatic fail for any scientific paper. That’s not my rule, it’s Thermodynamics.
By using the criteria that storage had to be competitive with generation does that mean the least cost efficient power to produce is the most is the most cost effective to store? Also, as a chemist who knows a little bit about electrochemistry, it would be interesting to look at the scale of the battery backup being discussed along with their durability over the long hall. The Li batteries I know of can only take 500 cycles before they are substantially degraded. The pumped hydro seems to make the most sense to me and from a solar point of view, many areas that are deserts are that way because they are on the leeward side of a mountain range so topography should work in favor of this type of storage in those circumstances.
Many years ago I read that raising a weight was an almost perfect way to store energy, a bit like a grandfather clock. Might sound daft but surely some sort of clockwork mechanism might work without having to use costly batteries at all, these things are very tall.
Use the “surplus” electricity to convert water to browns gas. Store the browns gas. When extra electricity is required, burn the browns gas in direct gas turbine generators.
Simple
‘But there are times when solar and wind farms generate more electricity than is needed by consumers. ‘
The real problem is the time they cannot , which are far more often . Indeed none of the renewable fan club can answer how much money or R&D would it take to overcome the reality caused intermittent nature of supply from this source .
Worse the major demand comes at the worst time for either of these sources, winter , low sun , not much wind , but lots of need.
Frankly these cannot meet demand without massive cuts in that demand , which is the real green agenda.
@DGP “Any use of the word “renewable” when refering to energy is an automatic fail for any scientific paper.”
That brings up an interesting question I have never seen addressed. Both wind and solar obviously are removing energy from the normal cycle. What effect does this have.
Wind turbines most be causing a reduction in the energy of the wind and thus its velocity. True in small numbers turbines effect can be ignored, although even the builders of wind farms have to place the turbines so they don’t block each other. But what is the long term effect of large numbers of turbines changing wind patterns?
Solar panels absorb energy that would have been absorbed by the earth and buildings which would have then been re-radiated back to the sky. But if absorbed and converted to electricity it isn’t re-radiated, or at least isn’t immediately re-radiated. Wouldn’t a large number of solar farms cause an imbalance in the downwelling and upwelling solar radiation? Aren’t greenhouse gasses supposed to be bad because they cause just such an imbalance? What might be the effect on global temperatures if there are lots of solar farms in operation?
Friends:
The article says:
OK. Storeage works for solar but not for wind.
The article also reports
In other words, the energy required to build a solar farm is approximately doubled by use of the putative battery storeage.
And the article says
Barnhart said.
Have these people never heard of money?!
Nobody cares about the “energy return on investment”.
Everybody cares if the monetary return on investment is decreased because their electricity bills go up.
Richard
“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,”
But those electric vehicles have batteries in them, so what’s the difference? Surely all you’ve done is change the location of the battery?
I don’t accept the intial premise of this paper that ‘there are times when solar and wind farms generate more electricity than is needed by consumers.” Also, I see no mention of the power lost in the conversion from AC to DC and back again (inverters).
Futhermore, if in fact solar and wind actually produced more than was needed, it would just be a simple matter of turning down the back up fossil or nuclear plant that are always required.
“However, the U.S. grid has very limited storage capacity.”
Where does the grid have any storage capacity?
Leaves are the only renewable solar panel that works, they capture solar energy and remove CO2 from the atmosphere, and they are biodegradable when their useful lifespan is over. Wood and other cellulose materials are the best batteries for solar energy, animals are also stored solar energy. Both are renewable and biodegradable, and are much cheaper, more efficient and require no toxic mining operations. As far as it goes all “fossil” fuels are stored solar energy, great for long term storage.
Fail-as soon as you read “fossil fuels that contribute to global warming”
I dont know why more people dont push the MIT/JET energy NANOR or E-Cat or LENR technology….
The 2 law works perfectly well if you draw the box around the whole universe…
why not use gravity storage.
One of the largest pumped lake storage systems was built in Scotland , not for wind or solar but for nuclear. Since to even pretend to be economically viable (without the hidden subsidies) NP needs to run pretty much flat out 24/7 for it full plant lifetime.
In order to us the off peak you either need something like gravity storage or 1970’s bonanza of cheap off-peak tariffs and thermal storage in the form hot bricks in everyone’s house aka storage heaters.
Those usually moaning about the inconsistency of solar etc ignore the fact that NP has a similar but opposite problem, that it HAS to run even when you don’t need it.
Apparently the turn around efficiency of pumped water storage is surprisingly good when done on a large scale.
All our forms of energy are solar based, where else could we get energy from? Wind, hydro, nuclear and hydrocarbons are all dependent on the sun at some stage. The only solar idea that doesn’t make any sense is photovoltaic, and that is the hole we dump the most money in.
These people are so far behind the curve even crampons wont help them.
The global warming ’caused’ by mankind’s fossil fuel use is approaching zero. The rest of their guff is, well … guff.
http://www.edie.net/library/view_article.asp?id=6281&title=Coire+Glas+pumped+storage+one+step+nearer+
http://en.wikipedia.org/wiki/Cruachan_Dam
“The station is capable of generating 440 MW of electricity. It can go from standby to full production in two minutes, thus it is used to deal with periods of peak demand on the grid. “
The best scalable pumped hydro I have read about involves using large pipes placed offshore. Water is pumped out into the ocean, and then allowed back in. Let’s also consider linking conventional hydro to other renewable in order to load balance (since hydro is almost an instant on). The full (non-conventional dam) back up need not be weeks, but probably ~2 days to balance a month or so of load. When the reserve drops to ~25% extra generation can be turned on to both power the grid and return the reserves to ~80-90%. These would not need to be quick peaking plants, but could take at least a day to turn on. Also coupling to some energy intensive controllably intermittent uses (desalination, smelting) could be a plus as well as long as those have a spin up/down time on the order of hours not days or weeks.
This might even allow fossils to move to an arena of almost exclusively base-load type operation at which they are the most efficient.
However, it would be a lot of moving parts and may not work.