Thursday, December 13, 2007

Cheaper than Coal? More discussion of a phosphorescent solar cell

The following is a continuing discussion of the development of a solar cell based upon phosphorescence begun in my first post on the subject located here.

The Auger Effect, Resonance, and Ionization

In my previous post I discussed employing an effect similar to that used in a laser in order to create an amplified electrical current.

One of the problems with my discussion of this topic is confusion on my part concerning the process of ‘ionization’ and so therefore I wanted to say a few words about this subject so that no one else will be confused by my confusion on the subject.

In the process of ionization an electron is stripped or ‘kicked out’ of an atom. An electron is not ‘created’ it is ‘kicked out’ and when an atom is missing an electron it is referred to as an ion. A phosphorescent medium can be thought of as a type of ‘semiconductor’ and so therefore ionization is possible.

By the way, you cannot ‘create an electron’. Rather what you can do is create an electron-positron pair, provided that the absorbed radiation is in the form of gamma rays, and so therefore such a device would only be useful in outer space and not down here. Furthermore and electron and a positron annihilate each other releasing a gamma ray, and exposure to a gamma ray is much worse than becoming sun burned, and given that it is unlikely that people will be converted into the Incredible Hulk by exposure to such gamma radiation, and given that such a device would probably just kill everyone who was exposed to it, this an idea not worth considering down here on earth, not that there is much to worry about since the earth’s magnetic field protects us from gamma rays and therefore such a device would not work here in any case.

One of the reasons why silicon is currently used for solar cells and phosphorescent material is not being used is that with phosphorescent material you get that nasty problem of the emitted photon, and we want useful electrons. However at the same time the metastable state that we find in phosphorescent material, which gives us that useless photon, is also the source of a potential battery, so that with a phosphorescent solar cell the collector is also the battery, whereas with silicon while you don’t have the problem of emitting a photon, you require a separate battery or you must use the power in real time. Now given Einstein’s famous equation E=MC(2), we know that it is possible to store a great deal of energy within a very small quantity of matter and so therefore the promise offered by the more troublesome and stubborn phosphorescent material seems well worth the effort to exploit, since it brings with it the promise of a large amount of energy and promises to be much more powerful than a silicon solar cell for that reason.

Now if there is some other method of extracting electrical energy from phosphorescent material, other than exploring ionization, at the moment I cannot think of what that might be. Perhaps I might come across something else in my research but at the moment ionization and exploiting the Auger effect would seem to be about it. You see, we have a problem with phosphorescent material in that the energy it releases is in the form of a photon, and we don’t want that. In the process of ionization an electron is kicked out of an atom, creating an ion, and we can think of that electron leaving behind a kind of ‘hole’ which can then be filled by an electron, and once again, when the hole is filled the electron would release energy in the form of radiation (a photon once again) so we would hope that once the electron is replaced it has already lost enough energy (we used it) that there isn’t much left to radiate.

Anyone who is interested in an introduction to ionization might consider consulting the link to the wikipedia page on the subject. For our purposes we would be more interested in the more modern treatment of ionization (based upon quantum physics) rather than the older classical treatment.

Photosynthesis and the Electron Cascade

Plants employ solar collectors (their leaves) and they also employ a small battery in which they store electrical energy at the center of each leaf. You see, a plant, like us, really has not use for a photon, and so therefore a plant stores electrical energy in electrons.

Like us, plants also have a problem in that only the surface of the leaf is useful as a solar collector, and so therefore plants employ this very interesting effect of creating an electro-chemical electron cascade effect, sending the energy converted from sunlight cascading downward towards the center of the leaf where it is stored as useful electrical energy in a form of chemical battery.

Now in order to exploit Einstein’s equation, we must mimic the plant, because we have the same problem confronting a plant, in that just as the surface layer of the leaf of the plant is the only useful portion of the solar collector so to only the surface layer of the phosphorescent medium is a useful solar collector. Therefore we need to create a type of electron cascade effect, much as we see the plant doing in its leaf, since this is the sensible solution to that problem, and given that it is sensible that explains why we see plants employing that solution, and so must we.

For this reason I am now picturing a modification of the design of the solar collector. What we need is a metal disk upon which the phosphorescent medium is applied as a coating.

One way of thinking about the phosphorescent solar cell is that the cell is like a big capacitor. In a capacitor a charge accumulates upon two metal plates because of the difference of voltage between the two plates and the charge must build up because of the presence of an insulator or dielectric between the two plates (sometimes this is nothing more than air). Another way of thinking about the phosphorescent solar cell is to compare the metal plate and the phosphorescent medium to an anode and a cathode.

The purpose of introducing a metal plate into the design is to allow us to use techniques based upon electronics to imitate what the plant accomplishes using chemistry (an electron cascade effect, which frees up the surface to function as a solar collector by moving the charge carriers down to the center of the medium, in order that the solar cell can function properly as a battery).

Stochastic Cooling

One of the problems our solar cell will encounter is that phosphorescence is a temperature dependant phenomenon, and for this reason, because we are not operating our device at super cool temperatures, the battery will have a tendency to leak, releasing photons (which is why a phosphorescent toy glows in the dark).

In my previous post I discussed an intuitive hypothetical means of preventing such leakage by employing resonance and damping to create a type of electromagnetic trap. It turns out that this idea relates to a form of electromagnetic cooling known as ‘Stochastic Cooling’ which employs electromagnetic traps and radio frequencies. to create a damping effect that results in cooling. So it would seem to be possible to operate such a solar cell at ambient temperatures while at the same time the device itself remains cool. This would reduce the efficiency of the device since power would have to be directed towards cooling, which would a percentage of the energy not available to the end user (the sun provides about 1000 watts per square meter at the equator, and that figure would be less at higher or lower latitudes). We also probably do not need ‘super cooling’, and what is required for our purposes is that the device by ‘just good enough’, and even if it leaked a little that also wouldn’t be problem, just so long as it did not leak a lot, and was therefore a useful device. We should also keep in mind Einstein’s equation (E=M(2)) and this suggests that if we design a device whereby we can keep ahead, it all adds up.

Other Angles

There are a few other approaches to getting useful energy out of phosphorescent material that I did not mention above. Both involve doping. Think of a plate of food. You sprinkle salt and pepper on the food and in a way we could say that you were doping the food with salt and pepper.

It has already been demonstrated that doping a phosphorescent medium with rare earth minerals results in a large increase in the battery life (the substance will still be found glowing after hours have passed). Unfortunately the term 'rare earth mineral' sounds expensive to me, and perhaps it might be possible to achieve similar results with further experimentation with not so rare earth minerals.

Researchers working with DNA have found it necessary for their purposes to invent a technique referred to as 'phosphorescent quenching by means of electron transfer.' The way this works is that the medium is once again doped, this time with donors (the phosphorescent substance which will be transferring an electron) and receptors (a substance which will receive a donated electron). This quenches the phosphorescence preventing the release of a photon, and it is possible that a similar idea could be employed in a solar cell which would simplify the design. We could imagine a design somewhat similar to a conventional silicon solar cell where a charge is separated between 'holes' and charged particles, however such a device, while it could conceivably work much like a silicon solar cell, would lose its function as a battery since it is the 'metastable state' of the phosphorescent medium that provides the battery potential. However, given that silicon solar cells are only 12 percent efficient, it might be interesting to find out if a phosphorescent solar cell which functioned like a silicon cell would prove to be more efficient than a silicon cell.