Thermal Isolation

BSST has patented an improvement in thermoelectric performance that we refer to as “thermal isolation in the direction of flow”, or simply “thermal isolation”.  The result of this concept is that the temperature difference from one side of the thermoelectric device to the other is reduced, thereby improving performance.  How the performance improves can be explained by the following diagrams and equations.  The equation below is for the optimum coefficient of performance in cooling, denoted by øCOPT. In the diagrams the heated and cooled heat exchange fluids move horizontally across the surfaces of the thermoelectric devices.  In the conventional device, the substrate on which the thermoelectric elements and their circuitry are mounted is a good thermal conductor and the surfaces reach uniform temperatures separated by ΔT.  If the hot and cold substrates on which the thermoelectric pellets and circuitry are mounted are poor thermal conductors in the direction of flow, they can prevent those surfaces from attaining uniform temperatures and the differential temperature across all portions of the device, ΔT, can be reduced to half under many conditions. Therefore wherever ΔT appears in the formula, it is actually reduced to ΔT/2. As you can see, the coefficient of performance doubles because of the first occurrence of ΔT and is improved a bit more by the 2nd occurrence.

The following photograph shows a construction for liquids in which the hot and cold liquids are routed through alternate heat exchanger cells. Between each pair of cells is a thermoelectric pellet with the pellets alternating N and P. The electric current passes directly down the stack of cells from one end to the other.

An infra-red picture of a similar stack is shown below.  In this image, the hot and cold liquids are introduced at opposite ends (counter flow).  The colors in the picture show the cold circuit fluid (magenta ≈ 20°C) becoming cooler (blue ≈ 10°C).  The other circuit becomes warmer (from orange ≈ 30°C to yellow ≈ 40°C) from right to left.  Due to thermal isolation in the direction of flow, the difference in temperature across each pellet is nearly constant regardless of its position along the stack.