For an application in Enclosure Cooling using TEC (Thermoelectric Cooler), both heat sink and cold sink can be defined separately, and as the fan is used to increase heat transfer over the sink, it becomes an active sink.
In the figure below there are two active sinks (heat sink/cold sink + fan ) and TEC, as both consume power.
Its a “through mount” air conditioner. It can be through the enclosure (“through mount”), external to the enclosure (“flush mount”) or inside the enclosure (“internal” or “recessed” mount). Typically, through-mounted systems are the least expensive and offer greater cooling capacity, although the true driving factor when choosing ought to be the design of the enclosure and location of other equipment.
From inside of box to outside, the basic parts used are:
Cold Sink Fan – Cold Sink – Cold Sink Extender (CSE) – Thermoelectric Cooler (TEC) – Heat Sink – Heat Sink Fan
Cold Sink Extender (CSE) or Heat Sink Extender ( HSE) , either can be used depending upon the application and footprints of heat or cold sinks. Sometimes, it not beneficial to use either of the two. One of the main reason of using extenders is to fill in the insulation thickness wall area. Generally CSE generates less entropy than HSE, so is preferred sometimes.