Which of the following devices can provide "free cooling" when outdoor conditions permit?

The air-side economizer is designed to utilize outdoor air for cooling when conditions are favorable, thus providing "free cooling." This system operates by bringing in cooler outside air to help reduce or replace the need for mechanical cooling, which can significantly save energy costs. When the temperature and humidity of the outdoor air are low enough, the economizer will allow for the circulation of this air into the building, helping to maintain a comfortable indoor environment without relying on traditional cooling methods.

In contrast, the other devices listed do not directly enable this kind of free cooling. Heat exchangers are primarily used to transfer heat from one fluid to another and do not specifically dictate the use of outdoor air for cooling. Variable speed drives control the speed of motors and fans but do not directly relate to outdoor air usage for cooling purposes. Fan coil units are responsible for circulating air in a space but rely on a chilled water or refrigerant system for cooling, rather than directly using outside air for natural cooling benefits.