Some discussions on the self-cooling system for submersible sewage pumps

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I would like to ask everyone here about the self-cooling system of submersible pumps. As far as I know, submersible sewage pumps are generally submerged underwater, with the motor exposed above the water surface by no more than one-third or one-half; however, this results in insufficient sewage removal. To overcome the cooling issues of motors, self-cooling systems have been developed, including external circulation self-cooling systems and internal circulation self-cooling systems. The external circulation self-cooling system utilizes the fluid in the high-pressure area of the pump to circulate around the motor stator casing in order to cool the motor; it is an open system ； The internal circulation is closed-loop; a circulating cooling impeller that operates in sync with the motor shaft is used to drive the coolant around the motor, after which it passes through a heat exchanger to dissipate heat, thereby cooling the motor. The first type has the drawback of prone clogging of the cooling circulation channels, while the second type seems to be rarely seen among manufacturers on the internet; most high-power submersible sewage pumps still use external circulation self-cooling systems. So the question is: those who are familiar with this area could they analyze and explain the advantages and disadvantages of these two cooling methods in terms of price, lifespan, efficiency, etc.? Why is internal circulation so rarely seen in the market? I asked some manufacturers, and they actually said that the internal circulation system needs to be imported?