A multistage thermoelectric device should be used only where a single stage device does not fill the need. Figure 4, depicts DT, vs. C.O.P.max, vs. Number of stages. C.O.P. is defined as the amount of heat absorbed (in thermal watts of heat pumped) at the cold side of the device, divided by the input power (in electrical watts). This figure should help identify when to consider cascades since it portrays the effective DT range of each cascade. A two stage cascade should be thought of, somewhere between a DT of 40°C (T_C = -5°C), where the C.O.P. bars of the 1 and 2 stage devices begin to diverge, and a DT of 65°C (T_C = -30°C), where a single stage device reaches its maximum DT, and also, heat pumping "shutoff", Q_C = 0. Similar decisions must be made as to the number of stages to be considered at larger DT's. The two important factors again are DT and C.O.P.

There is another very significant factor that must always be considered and that is the cost. Usually, as the number of stages increase, so does the cost. Certain applications require a trade-off between C.O.P. and cost.

As with any other T.E. system, to begin the selection process requires the definition of at least three parameters:

• T_C Cold Side Temperature
• T_H Hot Side Temperature
• Q_C The amount of heat to be removed (absorbed by the cooled surface of the T.E.) (in watts)

Once DT (T_H - T_C) and the heat load have been defined, utilization of Figure 4 will yield the number of stages that may be required. Knowing C.O.P. and Q_C, input power can also be estimated. The values listed in Figure 4 are theoretical maximums. Any device that is actually manufactured will rarely achieve these maximums, but should closely approach this value.

Figure 4: DT vs. C.O.P. Max as a function of # of stages

Melcor offers a line of 'Standard Cascades' though there are no 'Standard' applications. Each need for a cascade is unique, so too should be the device selected to fill the need. Melcor has developed a computer aided design system to help select a device. The three parameters listed are used as inputs to the programs. Other variables such as physical size, and operating voltage or current can, within limits, be used to make the final selection. Over 40,000 different cascades can be assembled utilizing available ceramic patterns. This allows near custom design, at near 'standard' prices. When the three parameters have been defined, please contact Melcor for assistance in cascade selection.