What is the informal term for an evaporative cooler?

The informal term for an evaporative cooler is indeed "desert cooler." This designation is widely recognized because evaporative coolers are particularly effective in dry, hot climates, such as deserts. They work by using the natural process of evaporation to cool air, making them a popular choice in areas where humidity levels are low.

Evaporative coolers draw warm air through water-saturated pads. As the air passes through these pads, the water evaporates, cooling the air before it is distributed into the living space. This process is efficient and environmentally friendly, as it uses significantly less energy compared to traditional refrigerant-based air conditioning systems.

The other options, while related to cooling systems, do not accurately describe an evaporative cooler. An air conditioner specifically refers to a system that uses refrigerants to cool indoor air, a refrigerant chiller also uses refrigeration cycles, and heat pumps are devices that can both heat and cool spaces but operate on different principles than evaporative coolers.