Two-Stage Evaporative Coolers

Conventional evaporative coolers, also known as swamp coolers, have followed two separate designs: direct and indirect coolers. The former passes warm air directly through moist pads, which invoke evaporation of cooler air that is then blown into the home. The latter is a closed-circuit system that sends the evaporatively-cooled air through a heat exchanger, where its "coolness" is passed onto the warm indoor air on the other side.

There's a nagging problem for both types of evaporative coolers, and that problem is humidity. While swamp coolers are energy efficient, using very little power relative to conventional air conditioners, the cool air they release is also very humid, often to the point of discomfort for residents. Therefore, swamp coolers have been limited to hot, dry climates with very little humidity. The air released by these coolers can reach humidity levels of 80 percent.

In response, innovators have devised the two-stage evaporative cooler, combining both direct and indirect cooling methods. In such a system, the warm air is first pre-cooled indirectly by passing through a heat exchanger that is then cooled by evaporation on the outside (no humidity added to the air). The pre-cooled air is passed directly through a moistened pad, picking up humidity as it cools. Obviously, some humidity is still picked up and passed into the living space, but is considerably less, thanks to that pre-cooling step. The result is end-humidity levels around 50 percent, a much more comfortable level.

Two-stage evaporative coolers are significantly more expensive than single-stage systems, but are often not too much more than conventional, whole-house systems. Energy savings over time will typically make up any difference. The decrease in humidity inherent in a two-stage cooler greatly expands the potential reach of evaporative cooling. But for now, its expense and relative obscurity keep its influence small.

Another benefit of two-stage and evaporative cooling in general is solar power. Because the coolers use such a small amount of energy in the first place, it's easy for a small set of solar panels to handle their electrical demands. So two-stage swamp coolers are now on the market. They come equipped with photovoltaic panels to run the blower and water pump. Given that heating and air conditioning is the number one consumer of electricity in the average home, energy-neutral cooling would be a big benefit to homeowners and our overstressed electric grid.

Indirect Evaporative Cooling / Direct Evaporative Solar Cooling / Two-Stage Evaporative Coolers