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Solar Thermoelectric Generator

This is a conceptual design for a thermoelectric generator that can be made of ordinary materials. The design is intended for parts of the world where silicon technology isn't available or isn't affordable, where a solution needing little or no mechanical skill is desirable, and where a small amount of power is needed to charge batteries for a radio or low-power light. 
 
 
Introduction
Refer to the Applications drawing below.  In this design, the thermoelectric generator's upper heating surface is made of an insulating material such as fired clay, stone, or porcelain.  During the day (a), sunlight heats the surface and provides energy to charge a small battery pack.  At night (b), a charcoal cooker set on top of the heating surface provides energy to operate a small appliance such as a radio.  In this case, the generator is capturing energy from the cooker that would normally be dissipated into the floor and wasted.
 
 
Construction Overview
Refer to the Exploded View drawing below.  The generator consists of an array of thermocouples made from two different metals such as copper and strips cut from tin cans or discarded copper and aluminum wire (a,b).  It is best to join the metals by soldering or welding, but if that isn't possible the joints can be made by crimping the metal strips or twisting wire pairs together. The top surface of the generator (c) is painted flat black to absorb solar energy and heat the top thermocouple junctions (a). The lower thermocouple junctions rest on a cooling sheet (d), and there is an airspace (e) between the heating surface and the cooling surface. Thus, the top thermocouple junctions are hot and the lower junctions are cool, and this differential generates a thermoelectric current (f). To provide additional heat-sinking capacity, the cooling surface rests on metal pipes (g) driven deep into the earth.  If water can be made to circulate through the pipes using gravity feed (i.e. diverting water from a stream) the cooling effect will be much better, but this is not a requirement.  The generator’s output supplies a charge controller (h), which may be just a simple diode and low-cost regulator (7812, LM317, etc.) or a commercial unit. The battery under charge (i) is supplied by the charge controller.
 
The main drawback of this concept is that thermocouples made from common wire alloys probably generate very little power, but this is offset by the low cost and simplicity of this design (provided raw materials like discarded wire, scrap metal and pipe are readily available). Experimentation will show whether or not this is a practical solution.

 
 
Construction Detail
Refer to the Schematic drawing below.  This drawing shows the hot and cold surfaces (a,b) and the arrangement of the metal thermocouples (c,d). The hot surface (a) is made of any durable insulating material painted flat black.  If used both for solar and cooking heat application, the upper surface can be made of stone or porcelain so that either a metal solar heat collector or a metal charcoal cooker can be placed on top of it as a heat source.  The exact configuration isn't important, but what is needed is maximum heat transfer and no electrical conductivity.  The cold surface (b) is a slab of fired clay, porcelain, slate, etc. that is a good thermal conductor, but an electrical insulator. It should be shaded and painted a light color to minimize energy absorption. Alternating strips of copper (c) and scrap tin (d) are soldered or brazed together and bent into the undulating shape in the illustration. Each connecting joint is a thermocouple junction (e,f). The upper thermocouples (e) contact the hot surface, and the lower thermocouples (f) rest on the cold sheet. Opposite ends of the metal strips provide a voltage potential (g). If using copper and aluminum wire, the wires can be wrapped on a supporting flat panel, and the ends tightly twisted at each thermocouple junction.
The thermocouples can be glued to the upper surface using a high temperature epoxy such as JB Weld or can simply be threaded through slots cut in the material. The lower thermocouples make pressure contact with the cooling plate so that the unit can be taken apart for maintenance. The schematic illustration shows just one portion of the thermoelectric generator. It will be necessary to connect a large number of such cells in series to obtain a useful voltage and in parallel to obtain a useful current level. The thermoelectric generator will need to generate more power than is required to charge the battery. For example, design the generator to output 14 volts at 250 mA and regulate that down to 12 volts at 200mA (about 6W). I can tell you that it will probably take a lot of thermocouples to generate that kind of power, and you would need to cram them into about one square meter, if possible.  You might choose to make modular units that can be wired up in a "farm" for a more scaleable solution.
 
Implementation
Refer to the Implementation drawing below.  From hard, dry, seasoned wood, make a frame (a) by gluing together two narrow strips of wood and a flat backing.  The two thin strips should be slightly shorter than the backing to allow a narrow shoulder at each end of the backing.  This shoulder will slot into the frame (d).  Cut a series of slots in the narrow strips (b) for the length of the strips.  Thread alternating metal strips through the slots (c) for the length of the strips as shown.  Make as many of these assemblies as desired, and slot them into a frame (d).  Interconnect groups of these assemblies in series for more voltage and in parallel for more current, as desired.  To use the framed assemblies, simply place the frame on a cold surface such as a slab of marble or fired clay and then set a second hot surface on top of the frame.  A temperature difference between the hot and cold sides will generate electricity.  For example, you can put a metal cooker on the top marble slab and set the whole unit on a stone floor.  The top slab will get hot and the stone floor will draw heat away from the bottom slab.  Alternatively, you can cool the bottom slab with a water pipe and set a piece of black metal on top so that the metal gets very hot in the sun.  This will generate electricity from sunlight.  Make sure that the wood and glue you use can withstand high temperatures.  It is very important to use an insulating material like stone, ceramic, or fired clay for the top and bottom slabs.  Do not set metal objects directly on the generating assemblies, because this will short them out.
 
 
 
 
 
 
 
Cautions
  • Do not hook the generator directly to a battery to charge it. Be sure to use a charge controller or, at a minimum, a diode and current-limiting resistor. If you hook up a battery directly, and the resistance of the generator is too low, then it will get hot and will probably ruin your batteries or even make them explode. Don't ever try to recharge non-rechargeable batteries.
  • Since the generator stand provides a good ground, there should be a lightning arrestor or other tall object nearby to deflect lightening strikes.
  • The charging battery should be kept inside a ventilated, solid chamber, both to keep the battery cool, and to contain any accidental explosion of the battery.  The battery could even be below-ground as long as it's protected from flooding water.  Ventilation is crucial.
  • If the thermocouple joints are soldered, make sure the solder can withstand the heat generated in the unit. If the thermocouples are twisted together, the joints may need ongoing maintenance due to corrosion/oxidation.
Disclaimer
I am definitely not an expert on thermoelectric systems and I have no idea how to actually build one. Be sure to do a lot of research and build models before attempting a full-scale system. As it stands, this system is completely imaginary. I haven’t done any experiments or built any models. It may turn out that the device is completely impractical.
 
 
Guidelines for Students
Suppose you want to take this as a design project.  Here are a few guidelines I'd offer:
  • There is no electronics store like Radio Shack next door, but you can scavenge materials like wire, sheet metal, pipes, glass, bricks, stones, fired clay, and so forth.
  • In the environment where this device is used, technology is expensive, and labor is cheap, so a simple, but labor-intensive solution that can make use of a variety of available materials is best.  Assume that simple tools and skills are available, but the less dependence on technology (i.e. soldering, welding) the better.
  • Prefer a solution that is flexible, dead simple to implement, and robust over something elegant, efficient, or complicated.  Hint:  If your solution involves a parabolic reflector, you are probably on the wrong track.
  • Initially, focus more on generating a flexible and simple solution, not a solution that generates a lot of power.  A few volts at a couple of hundred milliamps is sufficient to charge batteries or run a radio, perhaps something roughly one square meter producing in the 10 to 20 watt range with a minimum target of 5 watts.
  • Since the charge controller uses electronic components, assume that some kind of commercial charge controller is available and just focus on generating the raw electrical supply, i.e. separate the thermoelectric generator problem from the battery charging/regulation problem on the first pass.  Assume the user can purchase a battery pack with included charge regulator.
  • For prototyping, try using a charger that accepts DC input from a wall wart.  I have a Polaroid charger at home that accepts 12VDC at 650mA (8W), charges 4 AA cells, and costs roughly $20USD.
  • To simplify things, assume the builder has a voltmeter for testing and setup.  Eventually, it might be possible to publish a table with the approximate output of different combinations of metal at various dimensions.
  • Assume the user will need to maintain the device.  Make it easy to take apart and make it modular so it's easy to replace/repair failing components.
  • Keep safety factors in mind.  Don't design a solution that is likely to produce noxious fumes or that might blow up and kill somebody.
References
I found a link to some antique thermopiles here that will give you some idea what you are up against: http://www.dself.dsl.pipex.com/MUSEUM/POWER/thermoelectric/thermoelectric.htm 
 
You can, of course, start with the Wikipedia article to begin researching this topic and get some ideas:
 
A practical expectation for the output of a one square meter silicon solar panel is roughly 65 watts (about 6.5% effective efficiency), so aiming for something in the 5 to 20 watt range (0.5% to 2% efficiency) for a thermoelectric generator seems reasonable.  I think 10 watts is probably a good target with 5 as the minimum and 20 the likely upper limit.  Here's one of many calculations I found.  This one seems reasonable to me:
 
Contact
If you build this or try any experiments, I’d love to hear about it. My name is Craig Hyatt and my email is craighyatt@live.com.
 
September 24, 2009