Sunday, April 22, 2007

Smelling Salts

That last entry was a little gloomy so let's revive ourselves a little and shake off the stupor with a few thoughts on energy storage. So far, we've given a little consideration to flywheels, biofuels, gravitational potential energy and load shifting (demand side management). Let's consider something that is just a little bit like a battery. Batteries are chemical storage of energy. Sometimes they are just constructed as chemical reactions waiting to happen (as when you stick two different kinds of metal into a lemon) and sometimes they are made so the chemical reaction can go forward or backwards depending on whether the battery is charging or discharging (like the lead-acid battery in your car). Batteries are an important kind of energy storage and they are already used in many off grid real energy systems. Their technology is also improving pretty rapidly as hybrid electric vehicles start to prove themselves and push demand.

Here I want to talk about chemical storage of solar energy because of a new discovery. Ammonia is used in making fertilizer and the process for making it is called the Haber-Bosch process and it works by stripping hydrogen from natural gas and combining it with nitrogen from the atmosphere at high pressure and temperature using and iron catalyst. This is an incredibly big deal because this process allows us to grow grains on the same piece of land season after season, in your eye Mr. Malthus! This is also why ethanol fermented from grains gives so little back as a biofuel. The natural gas feedstock and the heat from burning natural gas to get the needed high pressure and temperature mean that a lot of fossil fuel is used to make this kind of ethanol. Rooted plants don't turn solar energy into stored energy with a high efficiency in any case though they do great things for our food chain and air supply.

Ammonia is also a fuel. It oxidises to nitrogen and water and it can be used in turbines or fuel cells. In Australia there has been some work on using ammonia as a substitute for water in the basic hydrogen fuel picture. Under high pressure, ammonia is heated by concentrated sunlight over a catalyst and dissociated, the reverse Haber-Bosch process, and then recombined to produce heat again at a later time. But this requires the high pressure of the Haber-Bosch process. Oxidizing ammonia can occur at close to atmospheric pressure which means less equipment. What has caught my attention is the idea of synthesising ammonia using solar power by making aluminum nitride first. This can occur at low pressure. The next step is to mix the aluminum nitride with steam to produce ammonia and recover the aluminum oxide that was reduced using solar power to form the aluminum nitride in the first place.

Reduced? Oh No! That involved carbon! Well, the folks who thought this up were interested in fertilizer and they found a way to avoid burning fossil fuels needed to get the Haber-Bosch process to go, but they still need the feedstock. That is a big step by itself, but if we want a fuel, we'd better not be sending carbon into the atmosphere just to make it. Not to worry. When methane is used as the carbon source you get carbon monoxide and hydrogen out in addition to the aluminum nitride. These can be recombined using a catalyst to get methane back so the whole thing can be closed cycle. A biological process might be even better, cycling the carbon monoxide through ethanol and then cracking. One then gets a hydrogen stream for free.

Does this compete with the efficiency of photovoltaics? It might not matter. By the time we are ready to say energy storage is the crucial next step, real energy will be quite a large part of our energy use and so long as we're there, we can be a bit extravagant if we're not encroaching on arable land. My guess is that it could compete because the main step, the endothermic production of aluminum nitride, should stick: that is, one is not going to be fighting the reverse reaction since the carbon monoxide carries the oxygen away. This is the step at which energy is stored and so long as the reaction goes quickly enough this will be the main draw on the solar power rather than the radiative and convective losses. The ammonia formation is exothermic and can probably be used to help pre-heat the main reaction. Burning the ammonia in a turbine may not be as efficient as using it in a fuel cell but it may also fit the on-demand profile for this kind of energy storage a little better.

Remember, we're thinking about energy storage ahead of time because this is where real energy is taking us. We have a whole list of posibilities and any one or two might do the job. I thought I'd poke at this one because I thought the idea that we can make ammonia from the Sun is pretty neat. Clover can do it too so be sure to plant some in your yard so you won't need all of that lawn fertilizer every year. You only need about a pound of seed which costs about four dollars and you'll be good for the next twenty years or more.


Anonymous said...

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J said...

Is it possible to reverse the process, thereby creating organic Nitrogen, and Hydrogen gas?
Using solar energy, we could take nitrogen pollution from farms, and cattle feed-lot operations, etc...
and synthesize the whole Nitrogen cycle, and decouple it from dependency on natural gas.

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