A mini split ductless system is a great way to add air conditioning to a set numbers of rooms without having to have ductwork done as you would with a central air system or install and remove multiple
window units each year. The split systems have an exterior condenser and an indoor evaporator unit that houses the cooling coil, a fan, controls to which you can add indoor blowers. Some models are designed to have more than one evaporators connected to the condenser unit. Each evaporator blower is mounted high on a wall indoor of where you want to get cooled. This set up allows you to save energy by only turning on the unit to cool the area that needs to be cooled. A well recognized brand that carries mini split unit is Mitsubishi.
Aside from being able to control cost due to controlling only which area needs the cooling. The mini split systems are generally a lot quieter especially on low compared to your average window unit. The mini split is more expensive then window or portable air conditioners but if you are only cooling off a few rooms it is definitely less expensive to set up and maintain than a central air system.