From observations to modeling: Investigating the heat mitigation potential of public spray mist cooling in Zurich

Residents of urban areas are disproportionately affected by heat stress due to the combination of global warming and increasing urbanisation[1]. This not only affects the quality of life, but also poses a significant health risk and has been shown to lead to increased mortality rates[2]. However, due to the complex nature of urban climate, the impact of such interventions can vary depending on the local conditions and are thus hard to predict.

Figure 1) Left: Image of the misting system. Right: Locations of the misting system (blue circle) and the nearby measuring stations. Station 110 serves as a reference. Left photo from Tabea Vogel, https://www.stadt-zuerich.ch/ted/de/index/gsz/aktuell/aktuelle-projekte/Nebelwolke-Turbinenplatz.html, accessed: 04.01.2023. Right aerial image from www.geo.admin.ch, Swiss federal authorities.

In this study a real-world implementation of a spray mist cooling system in the city of Zurich is investigated. A ring carrying 180 high pressure nozzles was installed on a public square, as shown in figure 1a. Studies on spray mist cooling are scarce and inconclusive, as it depends on various operational, environmental and experimental factors[3]. State-of-the-art measuring stations[4] have been deployed for continuous measurements of temperature, humidity and other parameters during summer 2022, as shown in figure 1b. The measurements showed a weak cooling effect that was most pronounced south of the cloud, as shown in figure 2. A mean effect of -0.7°C was measured, with the strongest cooling of up to -2.5°C. The impact of the cloud was most pronounced at 25°C. A dependency on relative humidity and wind direction was measured, with the largest effect measured at low relative humidity downwind of the misting system. Outside of the operational hours no temperature difference was observed.

Figure 2) Mean difference between the mean temperatures of the measurement sites and the reference station from 12 July to 26 September during operating hours

The field experiment supports model development as an ideal case for model validation. The effect of the misting systems on heat and moisture fluxes have been implemented into the urban LES model PALM[1]. The parameterised cooling system in PALM was then used to investigate variations in placement, weather conditions and amount of sprayed water.

[1] Keith Oleson u. a., „Interactions between urbanization, heat stress, and climate change“, Climatic Change 129 (1. April 2013), https://doi.org/10.1007/s10584-013-0936-8.

[2] Sally Howard und Geetanjali Krishna, „How Hot Weather Kills: The Rising Public Health Dangers of Extreme Heat“, BMJ 378 (14. Juli 2022): o1741, https://doi.org/10.1136/bmj.o1741.

[3] Giulia Ulpiani, „Water Mist Spray for Outdoor Cooling: A Systematic Review of Technologies, Methods and Impacts“, Applied Energy 254 (November 2019): 113647, https://doi.org/10.1016/j.apenergy.2019.113647.

[4] BARANI DESIGN Technologies, „MeteoHelix IoT Pro Datasheet“, 18. August 2022, https://www.baranidesign.com/meteohelix-pro-weather-station.

[5] Maronga et al., Geosci. Model Dev., 13, 1335–1372, https://doi.org/10.5194/gmd-13-1335-2020, 2020