In the summer of 2023, Seville, Spain, recorded 47.4°C — the hottest temperature ever measured in a European city. Seville regularly bakes above 40°C for weeks at a time, and climate projections suggest the heat will only intensify. Conventional air conditioning is part of the problem: it requires enormous amounts of electricity and pumps waste heat back into the streets, making the city hotter.
So the city tried something different. Something very, very old.
**3,000 Years of Persian Wisdom**
The CartujaQanat system, installed in the Isla de La Cartuja neighbourhood, is based on the *qanat* — an underground water management system developed in ancient Persia more than 3,000 years ago. Qanats were horizontal tunnels dug into hillsides to channel groundwater to the surface using only gravity. They transformed arid landscapes, made desert agriculture possible, and spread across the world from Morocco to China. Some ancient qanat systems are still operational today.
The principle Seville borrowed is simple: underground temperatures stay cool even when the surface scorches. Water stored underground at night — when ambient temperatures drop — retains that coolness during the day.
**How CartujaQanat Works**
The €5 million project, partially funded by the European Union, became operational in 2022. Its mechanics are elegant:
At night, approximately 140 cubic metres of water is cooled underground, taking advantage of the naturally lower temperatures. To accelerate the process, solar-powered pumps push water to the roof, where it is sprayed in a thin sheet across a 380-square-metre surface — a method called a *falling film* — before draining back underground, chilled by evaporation.
During the day, as outdoor temperatures climb, the cooled water circulates through ceiling systems in nearby buildings, absorbing heat from inside. Separately, water is channelled through subterranean pipes to cool air, which is then released via underground ducts into public spaces, parks, and streets. Mist nozzles spray fine water droplets outside to further lower temperatures through evaporation.
The system can cool up to 36,000 square metres of air per hour.
**The Numbers That Matter**
Researchers from the University of Seville measured the impact during the summer of 2025. On the hottest days, indoor areas served by CartujaQanat were as much as **12°C cooler** than the outdoor temperature. In a city regularly hitting 43°C, that's the difference between a dangerous heat environment and a liveable one.
But the most remarkable figure is the energy balance. CartujaQanat's rooftop solar panels generated **55,000 kWh** in summer 2025. The entire system consumed just **42,000 kWh**. The ancient cooling system produced 31% more energy than it used.
That is not a rounding error. A climate adaptation project is actively contributing power to the grid.
**Why It Matters Beyond Seville**
Conventional air conditioning is locked in a vicious cycle: as the planet warms, more AC is installed; more AC increases electricity demand; increased demand from fossil fuels accelerates warming; which requires more AC. The International Energy Agency projects that air conditioner energy consumption will triple by 2050, becoming one of the largest drivers of future electricity demand.
CartujaQanat breaks that cycle. It is not air conditioning. It does not pump heat back into the street. It stores coolness that nature provides at night, then distributes it during the day, using ancient hydraulic physics and minimal active energy.
Seville is also pursuing a companion project, LIFE Watercool, which is integrating water-based cooling infrastructure into bus stops, schools, and public squares across the city. Together, these initiatives represent an emerging model: cities learning to work with thermal physics rather than against them.
An engineering concept that helped build Persian civilisations, moved food across medieval empires, and watered Central Asian farmlands for millennia is now helping a 21st-century European city survive its own summers.
The ground beneath the heat has been the answer all along. 🌊
*Sources: University of Seville research data (Summer 2025) · CartujaQanat project documentation · LIFE Watercool · Al Jazeera Features (2024) · Positive News (Week 10, 2026) · World Halffull*
