The Sunflower is a passive solar device I designed that uses a Thermo-Electric Generator (TEG) module. It obtains the heat for operation from the sun, to heat the hot side of the TEG and uses cool ocean water to cool the opposite side.
Step 1: Thermoelectric Power Generation
Solar Thermoelectric Power Generator
In 1821 Thomas Johann Seebeck discovered the thermoelectric effect, which is the generation of electric current from heat. He discovered when a junction of two dissimilar metals are heated through a temperature gradient, the junction produces a small but measurable electric current. Twisting the ends of two dissimilar metal wires to produce an elongated junction one can create a junction that produces a few millivolts.
Inversely when a voltage is applied to this junction, it creates a temperature difference discovered in 1834 by Jean – Charles Peltier and is known as the Peltier Effect.
Together these combined effects are called the Peltier–Seebeck effect and forms the basis of thermo-electric generators (TEG) and thermo-electric cooling (TEC).
Modern thermoelectric generators use PN junctions manufactured from semi-conductor materials instead of dissimilar metals, see figure 1. However many thermocouples still use dissimilar metals for their construction.
There are a number of electronic companies that offer surplus TEC modules, see suppliers list. Many TEC modules can be used as TEG modules. Although a TEC pressed into service as a TEG will have a lower efficiency than a TEG module. But a TEC still works well enough as a demonstration model.
The Future of Thermoelectric:
Volkswagen and BMW have manufactured a line of thermoelectric generators that are powered by the waste heat of their car’s internal combustion engine. By doing so, they can use a smaller alternator on the engine for its electrical power needs. This contributes to the car’s improved efficiency by reducing the work required of the engine.
Step 2: Components
Thermoelectric Power Generator
Heat Sink (see text)
1" thick flat Styrofoam, glue, 6-32 x 2" long nylon screws and nuts
Step 3: Construction
The topside of the aluminum plate is painted with a high temperature black paint to improve heat absorption from the sun.
The heat sink should be larger than the TEG module.
If the heat sink has mounting holes, use them as a guide to drill matching holes in the center of the aluminum plate. If the heat sink does not have mounting holes, drill a 3/16" hole in each corner of the heat sink. Use those holes as a guide to drill matching holes in the center of the aluminum plate.
Drill a 1/4" hole to pull through the power leads from the TEG module. The 1/4" hole should be located close to the mounting holes.
Next attached to the hot side of the TEG module to the aluminum plate, centered between the four holes you just drilled in the aluminum plate. Use thermal grease between the aluminum plate and TEG module.
Next secure the heat sink to the cold side of the TEG module. Use thermal grease between the heat sink and TEG module. Secure the heat sink to the plate using the four nylon 6-32 machine screws and nuts. The reason we use nylon is that plastic has a lower thermal conductivity than metal, to keep the top plate hot and the heat sink cold.
Next 1" thick styrofoam is attached to bottom side of this plate. I used a spray adhesive on the aluminum plate to secure the Styrofoam. The Styrofoam performs two functions; one as a flotation device to keep aluminum plate and TEG module floating in water, and the second as insulation to keep the water from touching and cooling down the aluminum plate.
The Styrofoam may degrade from the heat of the aluminum plate baking in the sun, but for the prototype, I’m checking function, not longevity or service.
On the opposite side of the TEG module a large deep heat sink is attached. This heat sink is partially submerged in the cold ocean water. This keeps the heat sink cool, which in turn keeps the cool side of the TEG module cold.
Float the sunflower in ocean water and allow time for the top aluminum plate to get hot in the sunlight. Within a few minutes the TEG will begin to generate electric power.
Step 4: Testing
The day I tested the device the temperature was about 80 degrees. In place of an ocean I had to use a cooking pan filled with ice water. You may question the use of ice in the testing water. Without the ice, the water volume in the pan is so small that I was worried it would have reached ambient outside temperature (80 F) before the tests could be complete.
I placed the sunflower’s heat sink into the pan of water and left it in the sun for fifteen minutes, before I began taking some power output measurements from the TEG module.
The voltage output at 15 minutes was approximately 0.8 volts, see photo. At the half hour mark I checked the current output. It was an impressive 160 mA, far more current than I had expected.
Step 5: Improving the Design
The sunflower has not been optimized. To bring this project further, I would do heat studies relating to the thickness of the aluminum material, The current sheet size may be large enough to handle two or three and possible four TEG modules at a time. A little thought can bring this project further along and make it even more impressive.
A simple way to improve efficiency of the SunFlower, can be accomplished by increasing heat
capture and decreasing heat dissipation by covering the black aluminum plate with glass, with a 2 inch dead air space. Much like in a flattened solar oven or solar water heater. This would increase the temperature and reduce heat loss from the black plate to the air and wind, improving its efficiency.