Tuesday, April 29, 2008

Underground compressed air storage, geothermal power and radiation from radon

During the recent Oil Drum debate on Nuclear EROEI, "Cyril R." a frequent commenter on energy related blogs made a case for "Compressed Air Energy Storage" (CAES).

In a response to a comment concerning the low capacity factor for wind, "Cyril R." stated
"the capacity factor you referenced is very low, good locations in the US get 30-40%, which is close to the average capacity factor in the US. Moreover, consider the correlation with the load to be more indicative than capacity factor. Not good for wind, but with CAES this can be cost-effectively dealt with; the CAES equipment is similar to NG turbines, i.e. they have low materials input so this won't fundamentally increase the materials input for wind."

Cyril R proposed compressed air storage in salt domes and saline aquifers which he argued were adjacent to areas of high-quality wind resources.

My initial response was to observe:

"The last time I checked, the expansion of compressed gasses has a cooling effect. If the gases contain humidity the cooling can produce condensation and even freezing. The Grand Solar Scheme recognized the problem and proposed to burn natural gas in the released air stream to reheat it. There are two problems with this approach. First as we all know natural gas is not a sustainable resource, so it is not a sustainable solution. Secondly, burning natural gas produces CO2, and thus a CAES solution would contribute to global warming."

Cyril R. responded:

". . . using biomass derived fuel for heating in combination with hydrogen. In this instance, the use of hydrogen would be interesting because of the higher thermodynamic efficiency. In adittion, there is the AACAES approach which is hardly rocket science. Just add thermal oil storage (proven industrial technology) to store the heat created during the compression stages and use it later to deal with the cooling effect of expansion."

"Cyril R.'s" burning biomass suggestion is problematic from a number of views. The collection and transportation of large amounts of biomass would be energy intensive, the use of a technology involving the extraction of large amounts of heat from compressed air into mineral oil, and then the discharge of that heat into expanding air would be an added expense to a CAES system.

In addition to "Cyril R.'s" CAES scheme, the January Scientific American, published an article on a "A Grand Solar Plan" that proposed a CAES scheme involving caves.

Following the debate an interesting problem with the CAES system occurred to me. Any CAES project involving the release of compressed air from underground storage in salt domes and saline aquifers would probably transport radon to the surface.

Let us examine the problem. Radon is a colorless, chemically inert, radioactive gas produced by the radioactive decay of thorium and uranium in the earths crust, Because it is a gas radon can be drawn into the lungs. There it produces a radioactive multiple whammy. Radon 222, if it decays in the lungs, produces a long and deadly decay chain. (222Rn (3.82 days) → 218Po (3.1 min) → 218At (1.5 s) → 218Rn (35 ms) → 214Pb (26.8 min) → 214Bi (19.7 min) → 214Po (164 ┬Ás) → 210Pb (22.3 yr) → 210Bi (5.01 days) → 210Po (138 days) → 206Pb (stable). Each isotope in the chain releases more radiation into the lungs triggering more and more carsenogenic lung tissue damage. Radon is considered to be next to smoking the second leading cause of Lung cancer for Americans. Radon exposure greatly increases the lung cancer danger for smokers.

While salt domes generally contain virtually no radon, surrounding rock does. Critics of nuclear power have long argued that radioisotopes from "nuclear waste" placed in salt domes can be transported out of them through a variety of mechanisms. Similar mechanisms could transported radon from surrounding rocks into salt dome cavities used to store compressed air. The method of creation of salt dome cavities together with the effect of compressed air on the surrounding salt and rock might tend to open up channels for radon transport from radioactive rocks into the compressed air cavity. These would include the use of water to form the original cavity, the heat and humidity of the compressed air, together with the the effects of highly pressured air on the flaws and imperfections in the salt structure surrounding the cavity. All these forces could tend to open up transport channels between surrounding radioactive rocks, and the salt dome cavity. The fluxuating air pressure, caused by compression and decompression could pump the radon from the surrounding rock into the cavity. The release of compressed air from the cavity would force radon to the surface.

The use of saline aquifers for compressed air storage is even more problematic. A recent Geotimes report, "Rooting Out Radioactive Groundwater" focus on the problem of radon in all aquifers. It states:

"Groundwater from deep aquifers is typically oxygen-depleted and has a very slow flow rate, and marginal water typically has high salinity. These alternative water resources may therefore also have high radium concentrations."

Thus it would appear that significant atmospheric radon release associated with CAES energy storage in saline aquifers is very likely, and in the case of salt dome storage is quite possible.

The presence of radon in geothermal hot water and steam used in the generation of geothermal power has also been ignored, although high concentrations of radon could be expected from molten and hot rocks. Geothermal power techniques involved in the insertion of large amounts of water into hot sub surface rocks, would almost certainly lead to the transport of large amounts of radon to the surface with hot water and steam


Robert Hargraves said...

I'm not sure the compressed air energy storage system is a lossy as you fear. The air remains in storage for only a few hours, and if it does not lose too much of its heat from the compression the extra natural gas heating may not be so severe. I don't know the numbers, though.

Charles Barton said...

Robert, Underground compressed air storage, is in effect a ground source heat pump. Some of the heat is dissipated into the surrounding rock. The air will return to the surface with less heat than it had when it entered. This problem is acknowledged by the sources I have read, and they all suggest burning natural gas in the air stream to prevent ice formation.

Anonymous said...

I have not run numbers but I imagine dilution would limit the exposure of people other than those working at or living immediately down-wind to negligible doses.

But nuclear opponents definetly should not be let of the hook that easily if they're going to raise hell when a nuclear plant discharges enough tritium into the atmosphere to give nearby inhabitants a dose equivalent to eating a banana.

Nor do they deserve to be let off the hook for all the nasty stuff that goes into PV-cells. Cadmium has a half-life of forever as I like to remind them. I get an incredulous grimace when I suggest they extract it from solar cells upon end-of-life and put it into a geological repository that has to be leak proof until the Earth is no longer inhabitable by life.

Charles Barton said...

There scattering of radioactive radon daughter products would not be exactly good for human health. We do not know how much radon would be involved, and how they would be dispersed because the Greens are not concerned about any radiation source other than nuclear power plants. In addition Radium could be transported to the surface along with water. Hence Geothermal power pants are potential sites of radium contamination, and this I regard as likely. CAES could also bring radium to the surface in ice which is evaporated as the air is heated. Thus power generating turbines are potential locations for radium contamination, while the air stream from spent CAES carries radon and its daughter products far and wide.

Unknown said...

I think the compressed air will indeed hold a considerably huge amount of energy in the underground storage and thus we can say that the amount of heat formed will be significant too. Burning the natural gas in the air is a good idea but the subsequent release of carbon dioxide is questionable too. I think more research can be done with statistics to carefully tell what is the best solution out not only for the surrounding air but for the people working and living in the nearby vicinities.


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