I was browsing the site as I am often wont to do, and spent quite some time revisiting old 'ibles covering various solar projects, almost all of which are heat related. I understand from general consensus that PV (photo voltaic) solar systems are very inefficient and one of the worst ways to harvest solar energy (when considering the cost of construction and the energy transfer capacity, etc.) Almost everyone who knows what they are doing suggest that the best (and easiest) way to harvest solar energy is to collect the heat - be it to heat air or water, cook food, or other such purposes.

Now, I have recently been fascinated by a discovery relatively new to me, peltier units. These devices, for those who don't know, can work in one of three ways. They can harvest and/or produce either heat, cold, or electricity.

Basically, if an electric current is run through a unit, it produces heat on one side and an equal amount of 'cold' on the other. Yes I know you can't produce cold - but for the layman, that is what we call it.

Or, if one side is heated while the other side is cooled, the unit produces an amount of electricity from the difference in temperatures. One example I have seen and plan to make use of is in vehicles - if you place one or more of these on your exhaust manifold, or any other location where the engine produces waste heat, and airflow can reach it to cool the other side, you generate electricity from the waste heat energy that would otherwise just be . . . well, wasted.

But it occurred to me today, if you had a unit attached in the middle of a solar collector, focusing sunlight on one side of the peltier unit, and isolate the other side of the unit in some cooling medium (perhaps even just air with a heat sink of some sort - more likely a cooling liquid though), how effective would this be at harvesting solar energy (heat) into electricity?

Actually, I have been using the more common name of peltier, but I believe that for this application a seebeck unit (which is nearly the same, but different) would be required. As I understand it, Peltier units are used more as coolers and are almost always constructed with Bismuth Telluride (Bi2Te3) and used around room temperature and below; while Seebeck units are good for power generators are often constructed of PbTe or, SiGe as well as Bi2Te3 and are used at much higher temperatures.

From http://en.wikipedia.org/wiki/Thermoelectric_effect :

The effect is that a voltage, the thermoelectric EMF, is created in the presence of a temperature difference between two different metals or semiconductors. This causes a continuous current to flow in the conductors if they form a complete loop. The voltage created is of the order of several microvolts per degree difference.

((more info can be found at the referenced wiki page on formulae etc.))

Just curious on if this would ever be a decent way to harvest solar energy into electricity. I have also thought up using a thermal siphon solar heating system in which the liquid flows past one or more water wheels which power a small (and obviously slow) generator - but at that point I believe it would be so inefficient and limited in use as to just have absolutely no practical application. The peltier unit (used as a seebeck unit) would probably do better, but I am not certain, which is why I post the question.