Solar energy can also be used to generate cool air. There are two kinds of solar cooling systems: desiccant systems and absorption chiller systems. In a desiccant system, air passes over a common desiccant or “drying material” such as silica gel to draw moisture from the air and make the air more comfortable. The desiccant is regenerated by using solar heat to dry it out. Absorption chiller systems, the most common solar cooling systems, use solar water heating collectors and a thermal-chemical absorption process to produce air-conditioning without using electricity. The process is nearly identical to that of a refrigerator, only no compressor is used. Instead, the absorption cycle is driven by a heated fluid from the solar collector.