Thermoelectric coolers are solid state heat pumps that operate on the Peltier effect, the theory that there is a heating or cooling effect when electric current passes through two conductors. A voltage applied to the free ends of two dissimilar materials creates a temperature difference. With this temperature difference, Peltier cooling will cause heat to move from one end to the other. The array of elements is soldered between two ceramic plates, electrically in series and thermally in parallel. As current flows through a material, heat is generated.
Thermoelectric material is no different.
There is a point where the heat generated internally offsets the TECs ability to pump heat. Each TEC has a limit on how much heat that it can pump. This limit is referred to as Qmax. The current associated with Qmax is referred to as Imax. The corresponding voltage across the coolers is referred to as Vmax.