$1-1)$ Why is the reversed Carnot cycle executed within the saturation vapor dome not a realistic model for refrigeration cycles? Please also describe what really happens in an actual refrigeration cycle to make it realistic. $($You may need to use a keyword like 'saturated vapor', 'superheated vapor', 'two$-$phase compression'$)$

$1-2)$ Suppose that a heat engine uses $100$ J of heat energy and operates with $40\%$ of its thermal efficiency. The work generated from the heat engine $($you can calculate it easily!$)$ is then used as an input work for heat pump to give heat to a room. Please suggest the minimum COP of heat pump that makes it worth. Otherwise, we may just use the original heat energy to heat the room.

$1-3)$ Please write down the definition of Joule$-$Thomson coefficient and explain what it means.

$1-4)$ Please explain at least single reason why we need to use absorption refrigeration system with ammonia $+$ water, rather than using ammonia only for simple ideal vapor refrigeration cycle.