253 Thermoelectric Genera

The turbine recuperator technique involves some very high technology mechanics to make the system work. It prompts the questions: Is there any other way of achieving the same objective? Is a solid state solution possible?

Thermoelectrical devices were invented in 1821 and are perhaps best known today for the small fridges we have on our cars and boats to cool food and drinks. An array of bismuth telluride chips 40 mm square can produce 60 watts of cooling with a temperature differential of 20°C. If we go back 60 years to the 1930s there were thermopiles which one placed into a fire and the pile provided the current for a vacuum tube radio. It is only very recently that here in the UK a group of engineers started to ask the question 'Why are thermopiles so inefficient?' What happens to the 96% of the energy consumed that does not appear at the output terminals? Why is the output voltage so small - typically microvolts per °C at top temperature?

At Southampton University Dr Harold Aspden soon identified the answer to the efficiency question. The energy was being consumed by circulating currents within the device. It was then realized that if a dielectric was placed between the thermopile layers, and the pile was oscillated mechanically, that an AC voltage could be obtained up to 50 times the amplitude of the original DC voltage, Fig. 2.23. This oscillation has been tested with frequencies from DC to RF and the process holds good across the spectrum. Dr Aspden has concentrated his efforts on producing thermopile arrays for use on the roof of a building, with temperature differentials of20-40°C.

However, if we return to our waste heat recovery problem we are dealing with top temperatures of 600°C plus and consequently alternative materials will be required and the number of stages in series to produce a given voltage will be reduced. But, with a top temperature of 30°C existing, devices can convert 20 W of power with an efficiency of 25%. It should be emphasized that this work is at an early stage of development at this time.

The thermopile elements suitable are iron and constantin 40% nickel/60% copper (Type J thermocouple material); at 600°C, with mechanical excitation, a voltage of 300-500 mV per stage can be achieved, hence 500 cells in series would produce 216 V DC. The circulating current in each cell is proportional to the temperature difference but the output AC voltage may be controlled by adjusting the amplitude of the mechanical excitation. The most interesting point is that to give 10 kW a suitable unit could be very compact - our calculations suggest about 100 mm cube. We believe the mechanical excitation is best supplied by ultrasonic piezoelectric transducers driven by a HiFi amplifier. The power required is around 200 watts. One interesting point is that the unit offers reversible power flow. How? It can be converted from refrigerator to heater and act as a braking device.

CONVENTIONAL THERMOPILE

i-temperature gradient -

CONVENTIONAL THERMOPILE

i-temperature gradient -

junctions series connected with d.c. output junctions series connected with d.c. output aspden thermogenerator aspden thermogenerator

THERMAL
Fig. 2.23 Aspden thermogenerator and its control system (below).
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