From (3), (13) and (14), it is shown that when Th -Tc>△Tmax, the cooling capacity qc<0. At the same time, TEC cannot cool, but start heating, which is not allowed in practical work. So the working current should be controlled in range of I0IIsat, which can ensure the cooling capacity qc≥0. Although based on the analysis of the cooling capacity qc, the general range of working current can be determined, but in practical work, the refrigeration coefficient is still an important indicator. From (9), when the working current of TEC satisfies the following relations, the TEC can reach the maximum refrigeration coefficient βmax:

Where, Iopt is the working current in the condition of meeting the optimal refrigeration coefficient; Z is the figure of merit, ;.The relationship among current I, cooling capacity qc and β is shown in the figure 3. When Ish<I<0 and W<0, TEC is in the power generation mode. When I<Ish、0<I<I0 and I>Isat, the cooling capacity of TEC qc<0 and it will heat the cold load, which should be avoided as far as possible in practical work. When 0<I<Isat, the amount of heating load is small because there exists the Peltier effect reducing the impact of heat resistance in the cold end. Only when the current meetsI0<I<Isat, TEC can work normally in the cooling mode; when I=I0 or I=Isat, it cannot cool. The cooling capacity qc is a quadratic function about current I, so qc is symmetric in the two interval, I0<I<Isat and Imax<I<Isat. However, when considering that the refrigeration coefficient β increases with the current increasing in the range of I0<I<Iopt and decreases with the current increasing in the range of Iopt<I<Isat, so he refrigeration coefficient should be considered at the same time with improving cooling capacity and the working current in Imax<I<Isat is not selected in general. Obviously, TEC has two extreme ways of working, the maximum refrigeration coefficient operation mode and the maximum cooling capacity operation mode. The former is the operation mode of variable voltage and variable operating current; the latter is the operation mode of constant working voltage and working current. But in the actual work, the hot and cold end temperature of thermoelectric cooler is changing, and it generally works between the maximum refrigeration coefficient and the maximum refrigeration capacity, so the current needs to be regulatedaccording to the actual situation.