A recent heat wave in eastern Australia reminded us that we are in the Australian summer. In another record hot year globally, as heat waves become more frequent and intense, our cities make us hotter. This is a city’s heat island, where urban temperatures can be significantly higher than the surrounding rural areas.
So the question is what can we do to keep our cities cool.
Why is the city hotter?
Temperature differences are caused by a number of factors, including dense building materials that absorb more sunlight energy, fewer trees providing shadows, and less soil cooled by evaporation. Buildings can also behave like husky hair, reducing wind speed and preventing heat from radiating into the night sky. Most importantly, waste heat from car engines, air conditioners, and other energy uses increases overall air temperature. Why is it so important? Even a slight increase in temperature will drive up overall energy demand. Under the most frequent use of the power grid, the energy bill of about 40 hours per year is only 40 hours.
The most extreme high-temperature events could distort train lines, cause rolling blackouts, and lead to billions of dollars in lost productivity. This is not only bad for our wallet. Heat stress can damage organs or exacerbate existing diseases. Extreme heat events have killed more Australians since 1900 than forest fires, hurricanes, earthquakes, floods and severe storms combined.
So, what can we do?
There are many measures individuals can take to reduce the impact of heat on the home, such as installing light-colored roofing materials, insulation or air conditioning. But the situation becomes even more complicated when considering the entire city and how these small actions interact with each other and with the climate.
Air conditioner
Air conditioning can save lives during heat waves. But only by dissipating heat and the extra energy that runs the system can our homes become cool. In addition to increasing outdoor air temperatures in the short term, burning fossil fuels can also exacerbate global warming. No air conditioning may solve this problem.
Trees and park
Trees not only shade, but also cool the air, because the water evaporated from the leaves consumes energy, reducing the peak temperature by 1-5 ° C. Most urban planners agree on the broad benefits of urban vegetation, and some large city councils have developed urban greening strategies.
However, urban trees can be a problem for some councils. They use water, are expensive to maintain, can damage utilities and property, and can worsen air quality rather than improve it. Larger cities usually consist of dozens of councils. Getting them to agree is a major challenge.
White roof
We know that black surfaces get hot in the sun, but the demand for dark roofs still far exceeds the demand for light colors. A more reflective roof can reduce home energy costs and the overall temperature of the city.
White roofs are most effective in warm climates, because in cold climates a balance must be struck between cost savings in summer and increased heating costs in winter.
Green roof and wall
Green roofs and walls are building structures with integrated vegetation. They provide cooling by blocking buildings and evaporating from the leaves. Generally, compared to white roofs, they have a lower cooling effect, are more expensive to install and maintain, and require more water and energy.
But they do look good, they can improve biodiversity and make people happier.
Pavement watering
Before extreme heat waves, it is possible to reduce the temperature by wetting buildings and pavements. This is a Japanese tradition and is now considered in major cities such as Paris.
However, temperature and humidity are important factors that cause thermal stress. Therefore, pavement should be watered only if the additional humidity does not increase the thermal stress.
Large roof solar
Solar panels convert solar energy into electricity, so less energy is needed for the entire network. If enough roofs are covered with solar panels, will this reduce the air temperature?
Maybe a little bit. Other benefits include reduced energy required for cooling (because the roof is blocked by panels), and stable, lower-cost decentralized renewable energy systems.
Building density
Buildings with a lot of thermal mass (think solid, double-storey brick houses) may be an effective way to keep the indoor temperature more stable. Absorbs heat during the day and releases heat at night. The same idea can be applied to the whole city.
Urban cool islands may form in high-density cities such as Hong Kong, as high-rise buildings can provide additional heat capacity and shading. For similar reasons, the compact layout of streets in traditional Arab and Mediterranean cities makes these streets cooler.
Shade structure
Installing light-shielding structures on streets, sidewalks, and roofs can reduce the surface temperature of materials and reduce the absorption and radiation of heat back to the streets. Shading structures need to be designed so that they do not restrict airflow and trap heat and air pollution in the street.
Which is the best?
To find the most effective method, we need to be able to model physical models of different types of cities, different climates, and different strategies. We can then assess the impact on the economy and health and develop appropriate plans to maximize our gains.
Here, we focus on heating issues in cities, but there are other important issues such as air quality or flooding. In cold cities, urban heat islands can actually be a good thing. Every city is different; each plan requires a tailor-made comprehensive plan across the metropolitan area, which is then implemented locally by parliament, businesses and families.
https://theconversation.com/when-the-heat-is-on-we-need-city-wide-plans-to-keep-cool-70738