Design optimization of mist cooling for Urban Heat Island mitigation: Experimental study on the role of injection density

Climate change, heat waves and weather extremes unveil the need to counteract excess heat and its dramatic consequences on energy, economy, outdoor liveability and, above all, health. In the urban context, further concern arises from the concerted action of cities' materials, fabric, layout, density and activities, which are responsible of heat and pollutants entrapment, of wind force breaking and sweltering microclimates. Ready-to-use, high-impact, smart, cost and energy-effective countermeasures are the only ones having chances to be widely implemented in the short haul. Against this backdrop, this work presents the results obtained from an experimental campaign conducted on a single mitigation technology, meant to reach high local temperature reductions and empowered with climate-adaptive features to be applicable close to any vulnerable target (e.g. schools, hospitals, hospices..): a web of smartly controlled mist sprayers. A prototype was designed and its impacts on the local microclimate were thoroughly characterized. Notably, the nozzle density was investigated to delineate the tradeoffs between evaporative cooling global magnitude and spatial dilution: in fact, by rarefying water emission, a larger air volume can partake to the cooling as it gets harder to reach saturation; conversely the point spatial temperature drop might weaken and become negligible, jeopardizing the whole mitigation strategy. This paper discloses such a controversial point and provides guidelines for the correct design of mist cooling systems for Urban Heat Island counteraction.