Thermoelectric cooler (TEC) or Peltier create a differential temperature on each side of it. One side gets hot and the other side gets cool when it is powered up. Application of differential temperature on it can generate electricity.
When DC power source applies to a TEC, heat will be transferred from one side of the module to the other. It creates a cold and hot side. They are also used in industrial areas, computer CPU coolers, portable refrigerators, medical instruments cooling device etc.