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Heat air, water and generate electricity from solar tubes.

by taylaron
Tags: electricity, generate, heat, solar, tubes, water
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Oct20-13, 09:41 PM
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Greetings PFers
I’m thinking about building a Tiny House similar to the one below, except outfitted with solar power.

A Tiny house is a small (~120 sqft) home on a trailer bed. I’m interested in designing one that is equipped with a solar water and air heater as well as power generator. Because solar thermal is not a complete solution to heating air, water or generating power, it should be able to be switchedbased on demand. The solar water heater would simply supplement the on-demand propane heaters.

I’m thinking about using an array of Solar Tubes (Glass evacuated thermal collectors) on the roof

At the end of each tube is a brass cylinder where the heat is transferred. The question is what method is most efficient and practical for transferring heat to electricity to charge a battery in this application.
I’m not using Photovoltaics because they’re so expensive right now and it’s not worth the investment with more efficient panels coming out in the near future.

Conversion options that I see:
1. 12volt Peltier device (10-15% efficiency. Requires temp. differential). Direct conversion.
2. Sterling Engine connected to a 12 volt PM DC generator
a. 25% efficient Stirling * 75% DC generator = 19% overall efficiency

With these rough calculations, it looks like ~80-90% of the energy collected by the solar thermal tubes will be lost regardless of generation method. In the interest of simplicity, I would go with the Peltier device option to generate electricity.

To produce hot air or water, the oil which would circulate through the system would pass through a refrigerator radiator which has two tubes, one originally for coolant and one for cooling water going to the spigot. The hot oil and soon-to-be hot water would pass through the two tubes in the radiator and the water would go into an insulated container for later use while the oil is recycled.

Hot air would be produced by stopping the flow of water through the radiator and passing air through the radiator fins.

I also envision each solar tube having it's own parabolic dish along it's length if it would fit in the roof. The top of the solar collector would be covered in corrugated plastic to prevent glare off the roof.

If I've missed an alternative or have any corrections, I would appreciate your input.

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Oct23-13, 02:50 AM
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Hi taylaron! You've been absent!

As for your project, I like the Winston Compound Parabolic Collector:

"The students were led by Professor Roland Winston, and their system uses what are known as External Compound Parabolic Concentrators (XCPCs). Each device concentrates sunlight onto a specially-designed collector tube, which converts that sunlight's energy into heat. The combined solar heat from an array of these XCPCs can be fed into a building's heating system, used to generate electricity, or to run an absorption chiller-style air conditioner - these generate cold air in a process that utilizes heat, as opposed to electricity created from heat."
Oct23-13, 02:14 PM
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Hey Bobbywhy, nice to hear from you.

Winston's compound parabolic collector sounds very similar to my own, except his team designed it to cool air. I'm interested in being able to switch from heat to cool if practical. Convection driven air conditioners instead of pump driven air conditions are less efficient, but the fact they use heat as a source of energy cannot be ignored.

I'm curious what system they used to absorb the solar radiation and what they used as reflective material. 3M's solar reflector film is very* Expensive.
After staring at the picture, it looks like they simply made their own solar tubes. Instead of being cylindrical, they used a triangular form factor. Wither or not they're in a vacuum or simply surrounded by Xenon or something, i don't know. I cannot find any more information on their construction unfortunately.

Bobbywhy, what method do you think I should use to generate electricity?


Oct23-13, 09:21 PM
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P: 1,909
Heat air, water and generate electricity from solar tubes.

My preference for generating electricity is one with no moving parts: Thermionic Electrical Generators using the "Seebeck effect".

Here are a few sources of units for sale, with outputs as high as 76 watts!
Small Thermoelectric Generators


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