As a general rule, I like solar energy and wish it were actually energy efficient (i.e., I wish it didn't take more energy to create the things than they will produce in their lifetime before they break down).
Anyway! A company called Infinia is building a new type of solar energy . . . . capturing device (as opposed to panel). It's a solar electric dish that focuses solar radiation on a motor suspended in front of it (where the receiver would be on a normal satellite dish). The motor is a "free-piston" Stirling engine that uses changes in air pressure to drive a motor, generating electricity.
The company claims that the system is (a) cheaper than traditional photovoltaic (PV) solar energy systems - which, in theory, means that it takes less resources (energy as well as materials) to produce, which means that it may be (1) economical and, (2) therefore, energy efficient (the theory being that being non-economical is a symptom of being non energy efficient - if you don't get it, ask me or Alex), (b) manufactured without producing any harmful pollutants (which means it doesn't do one part of the environment a favor at the expense of another), (c) is recyclable ('nuff said), (d) 24% efficient, which is about twice the going rate, (e) aesthetically pleasing (you can decide for yourself), (f) capable of producing 9 megawatt-hours per year, and (g) available in 2008. For more details, see the website.
To point (f), this figure is probably assuming optimal conditions, do, for the sake of the following, let's say that a typical residential install would probably produce 66% of optimal, i.e., 6 megawatts. Why 66%? Well, I was going to figure out the average amount of energy produced each month (using the earth rotation, etc.), how many clear, partly cloudy, and cloudy days there are per year on average for my neck of the woods, etc., but I got tired of it because there are a lot of variables, so I just went for 66% because it seems not-too-optimistic and not-too-pessimistic.
Anyway! The average price of residential electricity in the US is $.08/kW-h (as of 2003 according to the DOE). Thus, if the system produced 6 MW-h per year, it would be producing $480/year worth of electricity (I'm glossing over the issues of whether a single residence would be able to consume all of the energy being produced at any time and whether or not it would be able to feed back into the utility grid using a net metering system). Now, lets say that the system lasts for 20 years. Why 10? Well, ...... no, don't worry, I'm not going to do that again. GE says that its residential solar panel system will last for "many" years, though the batteries will probably need to be replaced every 5-10 years. Thus, I guess that the system could last for, say, 30 years. Again, taking the idea that 66% is neither overly-optimistic, nor overly-pessimistic, I settle on 20.
So! Let's say that the system works as outlined above - oh, and let's throw in that it maintains its efficiency throughout its life cycle and, for the sake of argument, let's just hold the price of electricity static. Let's also say that the interest rates (savings rates, not mortgage rates) in this country average 4% over the foreseeable future. (Let's not even get into how I came up with that). I don't have any information on how much the system will cost, but, it seems that the system would pay for itself in whatever year corresponds to its cost below:
(Note: this assumes that interest compounds only annually, not monthly as it usually does. Thus, the monetary numbers are a little low).
Now that I have rambled to my heart's content, I'll stop :-)