Thermogen

Have heat, need electricity?

DSC_0834

The thermoelectric effect has been used since the 1930’s to generate electric power, but it really hasn’t been used widely in consumer products to generate electricity.  While inefficient, this are suitable for some applications with relatively low electrical power requirements, combined with extreme environmental conditions.

Historical Background

In the 1940’s, the Soviet government produced kerosene-thermocouple generators to power radio amplifiers.  Fuel up, Plug in, Tune in!

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http://blog.modernmechanix.com/2005/11/18/kerosene-radio/

The Soviets also have used radioisotope thermoelectric generators as power supplies in remote locations, such as lighthouses and navigation beacons in far Siberia.  The US used them for remotely located radar stations in Alaska and probably northern Canada.  If you see one of these, don’t bust it open.  Inside there’s a slug of strontium-90 or plutonium, and you’ll likely die of radiation poisoning from exposure to the source inside.

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-Wikipedia

Thermoelectric generators have also been a mainstay of the space program, powering deep space probes where the solar flux won’t yield much energy.

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-Wikipedia

The black tubes are the RTG’s

Principle of Operation

 

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-CUI

the principle of operation is incredibly simple.  With power supplied as shown above, one side gets cold and the other gets hot.  Run in reverse, heating one side and cooling the other side, a voltage is generated.  N and P typed doped bismuth telluride (Bi2Te3) is the most efficient semiconductor for this application, but other materials have been used in the past.

 

Circuitry

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DSC_0831

From the first version, I added a 3-Farad ultracapacitor and Pololu boost regulator, which acts as a DC-DC converter, stepping up voltage from 1.5V to 2.8V, the minimum forward voltage for the LEDs.  The capacitor smoothes out the voltage supplied to the boost regulator, and provides about a minute of light with no input from the thermoelectric elements.

Construction

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Thermodynamics

 

As assembled, the stack is as thermally efficient as possible, with a copper base plate, and thermally conductive matting between the copper substrate and the thermoelectric element.  Unfortunately, a double-sided adhesive film was only available in a product with a relatively low thermal conductivity, but the impedance is not terrible considering its thickness.

The heat sinks used here are relatively dense for the application, being intended for use in forced-convection applications.  They do dissipate enough heat to provide significant energy flow under natural convection in this arrangement.

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Most of the energy is dissipated as heat, but a small amount is converted to electricity.

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Based on this chart from the manufacturer, CUI, the theoretical efficiency of the elements used is 1.25%, given the temperature difference.  That’s not very good compared with even low-efficiency PV panels, but PV’s don’t work at night, requiring battery or capacitor storage.  As the thermoelectric elements work directly from heat, this is a fine solution for the application.

Doing some calculus yields the theoretical voltage of the assembly, based on the electrochemical properties of doped bismuth telluride.

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Experimental tests yielded a lower output than predicted, but that is expected due to system inefficiencies.

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Cost Analysis

 

 

Thermoelectric

Gasoline Generator

PbA Battery

Marine Deep Cycle

Kerosene Lantern

Lighting system

LED lights

Fluorescent Lights

Fluorescent Lights, Inverter

Kerosene Lantern

Associated Hardware

LED lights

 

 inverter

mantles

Recurring Needs

 

gasoline, oil

battery replacement

kerosene, mantles

Expected Life

20 years

10 years

3 years

(900 cycles)

20 years+

electrical Power Output (W)

0.3

1000

750

0

Initial Cost

200

500

165

65

recurring costs

0

$2/day for fuel

$0.25/ day for recharging

$1/day for fuel

(US white gas price)

First year cost

200

1300

240

400

Second+ year cost

0

600

75

300

5 Year Cost

200

3700

540

1600

toxic impact

 

gasoline, oil

Lead, Sulfuric Acid

Mantles, Hydrocarbon Fuel

hazards

 

fire

 Sulfuric Acid

fire

 

The total cost of the thermoelectric system is about $200 US, substantially less than the other options for portable electrical power/lighting used in food carts.  The cost breakdown above does not include lighting for the gasoline generator and lead-acid battery installations, which would total about $100-200 more.  The amount of electrical power generated is orders of magnitude greater for the gasoline generator and battery arrangements, making those suitable for more power-intensive situations like an ice cream truck.  But if all you have is a griddle, and all you need is some light, thermoelectric is the way to go.

Analysis

 

This is a promising technology with many potential applications.  While not in wide use, it could be used to good effect to provide lighting, displacing other energy sources which are highly polluting and have relatively high costs.  The barrier to entry into the market is not insurmountable with a kit solution, and in volume the cost of devices would drop.

An issue is the fragility of the commercial elements I used – they can be overheated, which was a problem I ran in to, but there are higher temperature rated elements available.

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