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By Robert Rapier on Jul 16, 2007 with no responses

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.