Perry's Note

Interesting , you folks have put a lot of thought into this.
 
I heard something about how much solar power is available per square meter and did some quick web research which may aid in figuring out power(from an end user, how many of these to I need to heat my house, point of view). Some snippets from wikipedia:
 
Solar radiation reaches the Earth's upper atmosphere at a rate of 1366 watts per square meter (W/m2).While traveling through the atmosphere 6% of the incoming solar radiation (insolation) is reflected and 16% is absorbed resulting in a peak irradiance at the equator of 1,020 W/mē.[2] Average atmospheric conditions (clouds, dust, pollutants) further reduce insolation by 20% through reflection and 3% through absorption. Atmospheric conditions not only reduce the quantity of insolation reaching the Earth's surface but also affect the quality of insolation by diffusing incoming light and altering its spectrum.
In North America the average insolation at ground level over an entire year (including nights and periods of cloudy weather) lies between 125 and 375 W/mē (3 to 9 kWh/mē/day). This represents the available power, and not the delivered power.

So, using an online "watts to BTU/hour" calcuator, this would translate to a range of 427 to 1280 btu/hour available power.
 
In terms of absolute available energy when the sun is shinging(condensed from http://zebu.uoregon.edu/1998/ph162/l4.html):
Under optimum conditions, one can achieve fluxes as high as 1000 Watts per sq. meter
Summer 40 degree latitude = 600 Watts per sq. meter
Winter 40 degree latitude = 300 Watts per sq. meter
Now I guess we have to figure out the effeciency of the design and do the math.
 
Are you thinking of using solar to power the fan as well?
 
Many more questions, not enought time....