Mini solar tower powered by fresnel lens

This is an attempt to generate electricity using a very small scale solar tower, or heat tube as I sometimes call it. The source of concentrated solar energy is a large fresnel lens from a rear projection TV.

Though I wasn't able to generate electricity, I did get a small turbine to turn.

Mini solar tower (v7) with Fresnel lens.

The amount of insolation for Ottawa, Ontario, Canada for July is around 866 watts per square meter. My fresnel lens is 2'x4' or 0.74 square meters. 0.74 x 866 means my fresnel lens should be receiving 640 watts. Given the number of grooves per inch of my fresnel lens I'm sure it's quite inefficient and so the absorber will receive much less than this and I've read that solar towers are not very efficient but even 6.4 watts is 1% efficiency which seems like it'd be possible.

How the mini solar tower works

A dark metal object called the absorber is heated by concentrated sunlight (see diagram below.) The heat is transferred to the cooler air around it in the tube. When air is heated it rises. The rising hot air turns the fan which acts as a simple turbine. The fan motor, when turned by the blades of the fan, acts as a generator, producing electricity. Meanwhile, cool air enters the bottom to replace the hot air and the process continues.

How it works.

The fresnel lens I'm using is one I got from a rear projection TV and normally use for solar cooking. It's suited for this application because it's a linear type of fresnel lens; the focal point is actually a line about 2 to 3 inches long and a half inch wide. This is perfect for a tower/tube, which is an elongated object.

Solar tower testing done so far

Here are the tests I've done so far.

June 21 and 22, 2011/Versions 1, 2 and 3 - First attempts. 1 1/2 inner diameter ABS tube with heatsink as absorber. Internal heat only up to 65C (150F). No noticable airflow.

July 1, 2011/Version 4 - Extruded polystyrene insulation shell. Internal heat in excess of 165C (330F) melted the polystyrene. So more heat but no noticable airflow yet.

July 8, 2011/Version 5 - Long aluminium tube, high temperature insulation. This one is about as good as I can do and yet it only partially rotates an small paper turbine.

July 12, 2012/Version 6 - Large area mini screen solar air heater. I made the area where the sunlight was converted to heat much large so the temperatures would be lower and easier to work with. I did this by making a small screen type solar air heater and placing it inside the fresnel lens' light cone, away from the focal point. This time it turned a bigger paper turbine rapidly!

July 19, 2012/Version 7 - Large area mini screen solar air heater but with a tower this time! This is simply version 6 but with a tower added. It turned the paper turbine faster with the tower than without, but it still didn't turn the PC fan. This is likely the last attempt I'll make as I think I've pushed it as far as I can go.