Evaporative cooling: a low-budget yet energy-efficient way to improve thermal comfort in motor vehicles

The cost added by an air-conditioneer to an automobile goes beyond the retail price. During the vehicle's lifetime, the comfort charges its fare. Apart from the usual increasement on the fuel consumption, since usually the compressor is driven by the vehicle's own engine, also sacrificing the performance in certain occasions (most notably in vehicles with smaller engines, of course), a regular air-conditionner for automotive applications also implies a few more procedures to the maintenance routine, due to items such as belts and pulleys required to drive the compressor, and eventually some recharges of the refrigeration gas. A few decades ago, the most usual refrigeration gas was the CFC (Chlorine - Fluor - Carbide), also known as Freon, which is regarded as an environmentally-harmful compound due to its effect to the ozone layer, and there are still some older vehicles operating with this gas instead of the HFC-134a currently prevalent.

Brazilian Volvo N10 with an evaporative cooler. The black box behind the side ladder is a water reservoir, while the roof-mounted white one is the evaporative unit.

Basically, there are very few options for those who don't consider getting rid of some sort of cooling device in their vehicles. An option which already went quite mainstream is evaporative cooling, which got favored by many truck drivers in South America (most notably Brazil, Argentina and Uruguay) due to the ability to remain operating after the engine shutdown, since it doesn't rely on an engine-driven compressor to perform its duty. Actually, has no compressor at all... Its operation is all-electric, but it doesn't put a significantly greater charge over the electric system than a simple forced-ventilation device would do...

Evaporative coolers are cheap and easy to retrofit even into older vehicles, such as this Mercedes-Benz L-1113

Considering some special applications such as hybrid-electric vehicles, like the Toyota Prius and the Chevrolet Volt, developed under a specific goal on energy-saving, the evaporative cooling is even more worthwhile. Due to the air-conditioneer of those models getting an electric-driven compressor, allowing it to remain operating when the internal-combustion engine is off, the loads at the electric system are significantly increased. An evaporative cooler has a lower energy expense, which seems to make more sense regarding a "clean" vehicle...

Another advantage is the purchase cost of an automotive evaporative cooler (often quoted in the United States and Canada as "swamp cooler"), which is on average more than 2 times cheaper than a regular air-conditioneer. Depending on the size of the vehicle's cab, the cost advantage can get even more favorable for evaporative cooling...

Brazilian Volkswagen Transporter (Kombi) fitted with 2 evaporative units: cheaper than an air-conditioneer and doesn't overload the underpowered 1.4L EA-111 flexfuel engine...

The operative principles are also different: an average air-conditioneer works thru the compression of a gas which circulates inside a set of metallic serpentines, leading to a partial condensation of the air humidity inside the refrigerated compartment, and subsequently the condensed water gets expelled to the outside. It's worth to note the dried air is often pointed as a source of respiratory nuisance. Meanwhile, an evaporative cooler has water directly sprayed into the air. The water ends up evaporating due to the heat absortion. A set of texturized filters avoid any ocurrence of dripping, in spite of the increasement on the air humidity.

This coach, fitted with air-conditioneer, has sealed windows, avoiding the passengers to keep them open.

It's also worth to remember that a regular air-conditioneer has its efficiency decreased when there is too much external air flow entering, while an evaporative cooler actually works more effectively when there is some external air flow. This ends up leading to some advantages for this setup, due to the health benefits of a higher ratio of air renovation inside the vehicle.

Brazilian metropolitan transit articulated bus, with a 210hp engine. Too underpowered for its size and weight, but even though a regular air-conditioneer wouldn't be the most suitable solution, it's not so hard to retrofit an evaporative cooling setup. The most usual evaporative units can easily bolt into the roof hatches...

In spite of the purchase and maintenance costs advantage, evaporative cooling has only two major disadvantages: its efficiency slightly decreased when the ambient relative air humidity is already high, and it can't provide heating autonomously. But after a detailed case study, pointing out the pros and cons of the operational scenarioes, evaporative cooling can be revealed as the best option for many customers.