Storing Renewable Energy

First, thanks to all who provided input for the renewable diesel essay. The comments were useful, and will help me to strengthen the chapter. Second, I had said that today I would comment on Benjamin Cole’s Seamless Transition to a Post-Fossil Economy. Frankly, I think other readers adequately addressed this, and even Benjamin realizes that a seamless transition is unlikely. So I will leave that one as is.

One of my major interests is storage systems for renewable energy options that would be characterized as intermittent. Solar and wind would fall into this category, and their intermittency really limits their ultimate potential. If wind turbines must be backed up by coal-fired power plants, it lessens the benefit. Therefore, the development of storage systems for intermittent sources of renewable energy is critical.

Previously, I wrote about Compressed Air Energy Storage (CAES). This is a system in which excess energy is used to pump compressed air into a storage cavern, which can then be bled off through a turbine when the wind is not blowing. There are clearly limitations to such a system. One must have access to both a good source of wind power, and a large, airtight cavern. This will limit CAES to specific niches.

However, this weekend I read about a system which, if successfully commercialize, would be more universally applicable. These ideas are to solar what CAES is to wind:

Groups Store Renewable Energy to Use on Rainy Days

First they frame the problem:

Scientists and engineers are struggling to find ways around a major obstacle to the growth of renewable energy: the fact that inexhaustible sources of energy, such as the sun and the wind, are undependable.

Solar power doesn’t work at night or on cloudy days. Wind is notoriously fickle, often dying down in the late afternoon just as electricity demand is peaking.

Then they cover the research under way:

The Department of Energy is researching ways to store energy at solar power plants that use thousands of mirrors to concentrate the sun’s rays on pipes filled with oil. The oil, heated to 750 degrees Fahrenheit, turns water into steam, which drives an electric power generator.

In one design from the Sandia National Laboratory in Albuquerque, N.M., excess heat is channeled into tanks of molten salt - a mixture of sodium, potassium and nitrogen that melts at 430 degrees Fahrenheit - where it can be stored for up to a week. The stored heat then can be transferred to a “heat exchanger” to boil water to make steam to run a generator at night or whenever necessary. Several power plants under construction in Spain plan to use this concept.

Another approach being tested at the University of Stuttgart in Germany would run pipes of fluid heated by the sun through a solid block of concrete. The concrete holds the heat for later use. To recover it, cold fluid is passed through the pipes, picking up heat on the way.

Molten salt is already used in some applications in the chemical industry to dampen temperature fluctuations in reactors. Even loss of power to a chemical plant would result in the reactor maintaining something close to reaction temperature, allowing a much quicker restart when power is restored. So, this is not pie-in-the-sky. These technologies are in use. I am glad to see that someone is attempting to use them to store solar energy, because I firmly believe it is the future.

In fact, I did a calculation this weekend for the book chapter I am writing that I believe demonstrates that there is no way we will be able to grow our way out of our petroleum dependence. The efficiency of photosynthesis is just too low to make that possible on the land we have available. So, in the long run it has to be solar, wind, and probably nuclear power.

Compressed Air Energy Storage

I have always been a big fan of wind power. But one of the knocks on wind is that it is intermittent. Since electrical demand probably won’t match up very well with wind fluctuations, installed wind capacity does not displace conventional power generation in a 1 to 1 ratio. For example, I have seen it claimed that 2,000 megawatts of installed wind energy still requires 1,800 megawatts of standby power for when the wind isn’t blowing. (1)

Clearly a storage system is needed. During times of high wind flow and low demand, the excess energy could be stored in something akin to a giant battery. When the wind isn’t blowing, users would pull from the “battery”. I have given a lot of thought over the past couple of years as to just what form such a storage system would take. I could envision several different options. One, air could be compressed into a storage system and then discharged through a turbine as needed. Two, water could be pumped uphill, and then be allowed to flow back through a turbine as needed. Three, water could be split to hydrogen and oxygen. I don’t like this option as much, because most electrolysis is inefficient and hydrogen storage is problematic.

(Incidentally, a variation of this third option was recently brought to my attention, in which excess wind power is used to make methanol, ethanol, or even ammonium nitrate fertilizer. For an excellent comprehensive overview of this option, combined with an impressive overview of wind energy potential in the Great Lakes area, see David Bradley’s report A Great Potential: The Great Lakes as a Regional Renewable Energy Source.)

Imagine my surprise this weekend to learn that while I have been daydreaming about a wind energy storage system, someone is in the process of doing it. Furthermore, others have previously blogged on it. I felt a bit like Rip van Winkle after waking up from his long nap. How could I have missed such an important development? The storage system is called compressed air energy storage (CAES). A January article from MSNBC explains the concept in Store wind power for later use? Cities bet on it:

A group of Iowa cities intends to not only harness the wind, but also capture it, store it underground and use it to help make electricity when demand peaks.

Members of the Iowa Association of Municipal Utilities have invested in a proposed power plant that would use wind turbines to drive compressed air into underground aquifers. The air would be released to generate electricity when needed.

The plant will use power from its own wind turbines, supplemented by cheaper electricity bought at off-peak times, to force air into rock formations at least 2,000 feet underground.

Current plans call for pressurized storage of tens of billions of cubic feet of air in rock formations deep underground. (2)

If you think I was surprised by that, imagine my surprise upon reading this from the same article:

Only two other underground compressed air plants are in operation. A plant in Huntorf, Germany, was built more than 23 years ago and a plant in McIntosh, Ala., is 11 years old. Both store compressed air in underground salt caverns.

Iowa’s project is unique in that it would use wind power to store the air and combine it with massive underground storage capacity.

The Germany and Alabama plants store hundreds of thousands of cubic feet of air in a thermos-bottle shaped container installed in the salt mines. The Iowa project would use naturally occurring pockets embedded in sand or sandstone formations sealed by shale or other rock.

So, a plant in Alabama has been using compressed air storage successfully for 11 years, and I didn’t know about it until this weekend. The only difference is that they aren’t using wind to do it. The Iowa plant will be the first to do that, but others will probably follow.

To be sure, such a storage system is not universally applicable. You need some kind of large, airtight, underground cavern. There are a lot of these in the United States, but they need to be located near a source of wind. Although, now that I think about it, I see no reason such a system couldn’t also be paired with solar or tidal generation systems, storing their excess energy using the same concept.

The plant is scheduled to come online in 2010. I wish them great success, and look forward to hearing reports after they start up.

References

1. “Airtricity’s rise and rise leaves criticism blowing in the wind”, Irish Examiner, April 30, 2005.

2. “Store wind power for later use? Cities bet on it”, MSNBC.com, January 4, 2006.