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Energy Storage: The Final Barrier to Wind and Solar’s Success

Perhaps the biggest shortcoming of solar and wind power is their intermittency. In locations like Hawaii, where I live, wind and solar power are already competitive on price. My fossil-fuel supplied electricity typically costs above 40 cents a kilowatt-hour, and wind and solar power can compete with that. But since they can’t supply power that is available on demand (firm power) they must be backed up by power sources that can provide power when the sun isn’t shining and the wind isn’t blowing.

This scenario could change dramatically if cost-effective energy storage solutions were developed. I consider this to be the most important unresolved problem in the energy business. A company that develops a way to efficiently and economically store intermittent energy for on-demand use will be a game-changer.

The ideal power storage solution would be able to store energy densely, at a reasonable capital cost, and would be able to return that power at high efficiency. For instance, if we put 1 unit of power into the storage system and we actually got 1 unit back out when we needed it, the system would be 100% efficient. Real-life efficiencies will be less than 100%, but the higher the efficiency, the more desirable the storage option.

A new report on energy storage from Navigant Research predicts that the market for energy storage for the electric grid could surpass $30 billion annually by 2022. Some of the potential options include batteries, pumped hydropower, compressed air energy storage (CAES), flywheels, hydrogen, and fuel cells. And of course nature also has a built-in storage mechanism for solar power (albeit an inefficient one) called biomass.

The following figure highlights the biggest problem with most energy storage options — the energy density is simply too low. Energy density measures the amount of energy stored per unit of volume or weight. What this means is that for a given volume or a given weight, the storage options can store only a tiny fraction of energy relative to liquid fuels.

Related article: Verizon Prepares for $100m Renewable Energy Investment

Energy Density

The most energy dense options possible would be those at the top and to the right of the graphic. Were Uranium-235 included, it would have appeared at the top right corner of the graphic. The least energy-dense options would be those at the bottom left of the graphic, which is where we find batteries, flywheels, and compressed air. At this scale, the energy density of these storage options appears to be near zero. Relatively speaking, gasoline contains more than 50 times the energy of the same volume of a nickel-metal hydride battery.

Pumped hydropower storage (PHS) is a storage option that is already commercially used in some conventional power plants. The concept is that off-peak power is used to pump water up to a reservoir at a higher elevation, and then returned through turbines to produce electricity. A March 2012 report by the Electric Power Research Institute (EPRI) indicated that PHS accounts for 99 percent of utility-scale storage capacity worldwide. PHS has a reported round-trip efficiency of about 75 percent, considerably higher than that of many other storage options.

There are around 50 PHS systems of at least 1 gigawatt (GW) installed around the world, with another 15 or so facilities of this size under construction. Operating facilities exist in the US, China, Japan, South Africa, Russia, Australia, South Korea and in a number of European countries. The largest facility in the world is a 3 GW system in Bath County, Virginia.

The primary advantage of PHS is that very large amounts of power can be stored for long periods of time, but accessed quickly. The major disadvantages are that initial capital costs are high and the technology is limited by geography to locations that can host a large reservoir at a significantly higher elevation than the power station.

Compressed air energy storage (CAES) is the second largest category of utility-scale energy storage. In a CAES system, off-peak power is used to compress air into a storage reservoir, which is later released through a turbine to produce electricity as needed. This reservoir is typically an underground cavern, but some work is being done to develop these systems under water, in enclosed bags that expand against the outside water pressure.

The first utility-scale CAES facility was built in Germany in 1978, utilizing a salt dome as the reservoir. The first system in the US was built in 1991 in Alabama. A salt cavern is also used in this system, which can compress air up to 1100 pounds per square inch (psi). Other projects are under development, with the US Department of Energy providing funding in some cases.

Related article: Giant Concrete Spheres: New Form of Energy Storage for Offshore Wind Farms

As with PHS, CAES is limited by geography. Further, the cycle efficiency of the systems currently operating is reportedly 40 percent or less — much lower than with PHS.

Hydrogen is one of the more energy-dense storage options by weight. One kilogram (kg) of hydrogen compressed to a pressure of 150 bar (2,175 psi) actually stores a lot more energy than one kg of gasoline (the horizontal axis is energy density by weight). As shown in the graphic, compressed hydrogen contains more than three times the energy of gasoline per kg (142 megajoules for hydrogen versus 46 megajoules for gasoline).

However, hydrogen falls short when it comes to volumetric storage (the vertical axis). One liter of gasoline contains over 20 times the energy of one liter of 150 bar hydrogen. Thus, one of the limitations of hydrogen as a fuel is that the range of a vehicle running on hydrogen will fall far short of that vehicle utilizing a similar-sized gasoline tank.

Nevertheless, the German utility E.ON (OTC: EONGY, Frankfurt: EOAA) is investing in a hydrogen-based storage system. In 2012 E.ON contracted with Canada’s Hydrogenics (NASDAQ:HYGS) to install a power-to-gas system in Falkenhagen, Germany. The idea in this case is that off-peak power is used to make hydrogen from water by electrolysis, and then the hydrogen is injected into a natural gas pipeline. The hydrogen-natural gas mixture can then be used as needed for power production, or for heating.

Of course energy is lost when water is broken down into hydrogen and oxygen. The efficiency of electrolysis of water into hydrogen can be as high as 85 percent. Hydrogenics reports that the efficiency of its hydrogen fuel cells “is greater than 55 percent” in converting hydrogen into electricity. So we could expect the cycle of converting off-peak power into hydrogen and then back to electricity during peak demand would be (0.85) * (0.55) = 47 percent efficient. In other words, a little more than half of the power sent to storage is wasted.

This also implies that the value of peak power would need to be more than twice the value of off-peak power to make such storage profitable. For example, if off-peak power is worth a nickel, and peak power is worth a dime, then a nickel’s worth of power sent to storage is only worth 4.7 cents (47% of a dime) at peak demand. On the other hand, if peak power is worth 15 cents in this scenario, a nickel’s worth of off-peak power can be turned into 7 cents of peak power (47% of 15 cents).

Batteries comprise an enormous global market, and are often used in personal solar systems to provide power at night. But batteries are seldom used to back up power plants because they have low energy density, are expensive, and have a limited lifespan. However, a great deal of research is being devoted to the development of advanced batteries, which are projected to reach gigawatt levels of utility-scale storage over the next 10 years.

Governments are investing heavily in the development of utility-scale storage, and a number of utility-scale storage possibilities are still in development. This is an area that promises to grow rapidly in the coming years given the number of countries implementing aggressive renewable electricity standards.

By. Robert Rapier




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Leave a comment
  • Joe Elchert on May 02 2013 said:
    Hello, i'd like to comment on your assessment of Hydrogenics' product.
    "Of course energy is lost when water is broken down into hydrogen and oxygen. The efficiency of electrolysis of water into hydrogen can be as high as 85 percent. Hydrogenics reports that the efficiency of its hydrogen fuel cells “is greater than 55 percent” in converting hydrogen into electricity. So we could expect the cycle of converting off-peak power into hydrogen and then back to electricity during peak demand would be (0.85) * (0.55) = 47 percent efficient. In other words, a little more than half of the power sent to storage is wasted."

    ok so, you say 85 percent from electrolyser to up to 55 percent for fuel cell. Please elaborate on direct injection into the natural gas pipeline. With energy storage of renewable, any way to store large amounts of energy for large amounts of time to miles and miles away is going to garner attention. This power to gas creates a "banking" of wind/solar power. Not sure why it is necessary to even combine the fuel cell portion to the equation. For this would be a specific application for a remote place where hydrogen isn't supplied by other means or simply a station.

    The dollars and sense of this have been figured out and there is value here for both the utility, their customers, and Hydrogenics. Looking forward to a new article with the facts. You took one application diluting the numbers for power to gas. Where as everyone who understands this tech knows that there are multiple applications and varying roi/margins for each of these. Do a study on what municipalities/power companies do with excess renewable energy and how much they sell there "dump" energy for to their neighbors. To save you some time ITS cheap!
  • SA Kiteman on May 02 2013 said:
    There may be a more efficient way to store vast amounts of electricity cheaply and efficiently. And it may work very well for an island like Hawaii. The mechanism is essentially a launch loop laid on its side. It amounts to an enormous, incredibly high speed fly wheel but it not limited in velocity by the strength of the material. Wikipedia has a fairly good starter article on launch loops.
  • Bob Wallace on May 03 2013 said:
    "if off-peak power is worth a nickel, and peak power is worth a dime, then a nickel’s worth of power sent to storage is only worth 4.7 cents (47% of a dime) at peak demand. On the other hand, if peak power is worth 15 cents in this scenario, a nickel’s worth of off-peak power can be turned into 7 cents of peak power (47% of 15 cents)."

    EOS is installing their zinc-air batteries in New York. They claim 75% efficiency, $160/kWh and 6,000 cycles.

    To get one kWh from off-peak to peak you'd have to start with 1.33 kWhs or 6.7 cents worth of five cent electricity.

    $160/kWh and 6,000 cycles would mean 2.7 cents per kWh cycle. We're now at 9.4c/kWh.

    Toss in some BOS costs and profits and it looks like 5 cent offpeak could be sold profitably into 15 cent or less peak hours.

    EOS thinks they'll have cycle life up to 10k soon. That would shave off a penny.
  • Guest on May 03 2013 said:
    A new GALLUP poll says over 70% of Americans want more WIND and SOLAR energy, so your article is in line with what Americans want.
  • SA Kiteman on May 05 2013 said:
    hope they hurry up and figure out that these ources are so diffuse that their portion will likely destroy habitat for our already stressed wildlife. Wind, solar, and bio-fuel are frequently bad for the ecology.
  • Ashley Lionell on May 02 2014 said:
    A new form of innovative energy storage is making its way to the markets. My firm, Mark Power is developing it. In my view, it is the ideal energy storage that people would want: ultra low cost, high round trip efficiency, zero discharge and capacity degradation, high ramp rates, made entirely out of sustainable green materials and practically unlimited lifetime. Brace yourselves for the novel solution's disclosure this May 2014.

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