Page 8 of Thermoelectric Handbook by Melcor

® thermoelectric multistage cascade devices a multistage thermoelectric device should be used only where a single stage device does not fill the need figure 4 depicts t 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 t range of each cascade a two-stage cascade should be thought of somewhere between a t of 40°c tc -5°c where the c.o.p bars of the one and two-stage devices begin to diverge and a t of 65°c tc -30°c where a single stage device reaches its maximum t and also heat pumping shutoff qc 0 similar decisions must be made as to the number of stages to be considered at larger ts the two important factors again are t and c.o.p figure 4 t 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 more than 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 typical device performance when performance vs input power is plotted for any thermoelectric device the resultant curve will appear as in the figure below an inverted parabola performance can be t th tc heat pumped at the cold side qc or as in most cases a combination of these two parameters input power can be current i voltage v or the product of iv when we refer to the tmax or qcmax we are referring to that point where the curve peaks the same is true when referring to either imax or vmax since operating at or very near the peak is relatively inefficient most devices are operated somewhere between 40 and 80 of input power max as stated devices are normally operated on the near-linear upward sloping portion of the curve when automatic or closed loop temperature control is being used current or voltage limits should be set below the max intercepts 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 · tc cold side temperature · th hot side temperature · qc the amount of heat to be removed absorbed by the cooled surface of the t.e in watts once t th tc 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 qc 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 6 tel 609-393-4178 · fax 609-393-9461 · web www.melcor.com · email tecooler@melcor.com for a list of global sales offices visit www.melcor.com

[close]