Evaporative cooling: cuts temps by 15°C, saves 90% energy

Evaporative cooling: 15 degrees cooler only if humidity is low

In June, with the thermometer soaring in Seville, twelve units installed on the roof of San Agustín school kept classrooms between 22 and 25 ºC, housing 50 teachers and nearly 700 students. No compressor, no fluorinated refrigerant: just water evaporating through honeycomb-shaped cellulose panels. The narrative accompanying this technology promises to retire air conditioning: up to 15 degrees lower temperatures and 90% less electricity consumption. The fine print lies in the map and the hygrometer.

What is evaporative cooling and how does it cool without a compressor?

The principle is basic physics. Water absorbs heat as it changes from liquid to gas, stealing that heat from the surrounding air. A fan draws in outside air, forces it through a soaked panel, and the air exits cooler; ducts then distribute it throughout the building. Australian manufacturer Seeley International sells its Breezair units with this promise: a drop of 10 to 15 degrees during peak heat hours and up to 90% energy savings in hot, dry climates. Its distributors in Spain, BioAire and AustralAir, offer a 2-year warranty on components, 10 years against corrosion, and 25 years for the external structure.

Compared to traditional air conditioning, the ecological argument holds: there are no fluorinated gases, and the air is completely renewed—in an industrial warehouse, the system moves that volume about 25 times per hour. Water is the only consumable. And that is where the bill hidden from the headline begins.

Why doesn't it work on the coast?

Because humid air can no longer absorb more water, and without evaporation, there is no cooling. That is the Achilles' heel: in coastal areas or on muggy days, the unit falls short and barely ventilates. The psychrometric calculation circulating among technical profiles makes it clear: in a dry city like Madrid, at 40 ºC and 50% relative humidity, adding water to the air raises humidity to 80%, and the temperature drops to 32 ºC, not 25. With heavier air, the margin narrows even further.

Conventional air conditioning plays in another league in that terrain: it cools and, incidentally, dehumidifies, so the perceived temperature falls more than what the thermometer shows. Evaporative cooling does the opposite: it adds moisture. In the arid heat of Córdoba, starting at 45 ºC with the promised 15-degree drop, the math leaves rooms at around 30 ºC. And when you turn off the fan, the heat returns to an environment more humid than before.

The botijo mechanism: nothing new under the sun

Traditional architecture has used it for centuries. Wind towers, courtyards with water, lattices, and jets cooled homes long before electricity existed: adiabatic cooling and construction in southern Spain and North Africa share the same principle. The domestic example is known to anyone who grew up in a village: the botijo (a porous clay water jug). Porous clay, water filtering and evaporating on the outside, and the water inside staying cooler.

Domestic experience points in the same direction. In Jaén, there was a frescaria (cooling room) for years that provided some relief; in other homes, simply mopping the floor in the afternoon was enough to reduce the stifling heat. And industrial warehouses from the seventies and eighties already had evaporative coolers on their roofs. As a disruptive invention, it is hardly new.

Is there a risk of Legionella in an evaporative cooler?

The industry maintains that there is not: the units neither store nor atomize water, and industrial models are sold with a certificate of non-propagation of the bacteria and a self-cleaning system that empties the tank daily. Technical doubt persists regarding whether high-estimulante ilegal air passing through a wet panel drags droplets. It is a question, not an accusation: there is no record of outbreaks linked to these devices.

The point where all parties agree is maintenance. Cellulose panels must be replaced periodically, like filters, and remain wet for months. Mold, limescale, and bacteria thrive there if the equipment is neglected. Professional inspection when the device smells bad or has been idle for a long time ceases to be a luxury.

Where they are already used: from Hipercor to bar terraces

More than one might think. Supermarket chains with high ceilings and warehouses with sandwich panels without false ceilings are the natural habitat for these units: there are installations in hypermarkets and warehouses on the outskirts of Madrid, in medium-sized spaces, and logistics centers. The higher the ceiling, the more cost-effective it is to move that volume of air.

In hospitality, the issue is different. For a terrace, installation falls under the activity license, and some venues have already been forced to remove undeclared elements from their files. How many operate without appearing on any paperwork is an unanswered question.

How much does it really save and at what price?

Evaporative units can be 80% cheaper than conventional air conditioning, according to the manufacturer itself. Compared to a split system, electricity costs plummet because there is no compressor: just a fan and a water pump. But water is consumed, paid for, and requires installations designed for it. And maintenance—panels, cleaning, inspection—reappears every season. The big savings concentrate in hot, dry climates, exactly where the air conditioning bill hurts the most.

With that arithmetic, the headline of 15 degrees only holds up in inland arid zones and during peak hours. In Madrid, on a day of 40 ºC with air half-loaded with humidity, the best calculation leaves the house at 32. The invention called to retire air conditioning demands, first of all, a climate that not all of Spain has.