Experiment | Why are our cities getting hotter?

It is too hot in the summer. It is scorching hot under the sun. Everyone will complain that the sun is too strong. But do you know that the hotter summer in Shanghai cannot be blamed on the sun. The heat emitted by the big city itself can be as high as two-fifths of the heat from solar radiation.

The temperature in the city center is generally about 1°C higher than the surrounding suburbs, and can reach up to 6°C. The highest temperature in Shanghai often approaches the historical high. At 10 am on July 15, the temperature in the center of Shanghai was 34.6 degrees, while the Dongtan Wetland in the suburbs was only 29.4 degrees. The city center was 5.2 degrees hotter. If you go to the city center, the actual temperature you feel will be higher than the official temperature of the meteorological station. This is the urban heat island effect.

In order to explore the secret of the urban heat island effect, I took a temperature thermal imager from a power electronics experiment and walked along Huaihai Road all the way to Xujiahui, a benchmark for temperature in the center of Shanghai, to find the root cause of the Shanghai heat island.

The urban heat island effect mainly comes from the absorption of solar radiation by buildings and asphalt roads and the hot air discharged by air-conditioning equipment in the buildings towering within the city.

Buildings continue to grow taller and taller, and river banks are covered by buildings, blocking the flow of air and exacerbating the high temperatures inside the city.

At the same time, the dense urban population, the heat emitted by shopping malls, restaurants and vehicles, and the energy consumption of residents' lives all add fuel to the fire.

Urban "heat island disease" should first be blamed on cars. Fuel-powered cars are inefficient, with the highest engine efficiency at 40%. Only electric cars can cure this urban disease, and the efficiency of electric cars can easily reach 70%-80%.

Figure: The electric car engine front compartment is equipped with a motor driver, but it generates little heat and the temperature is very low

Figure: The large heat source of the rear engine of a diesel bus and the cool back of an electric bus

The engine and water tank in a fuel car are the main heat sources. The rated power of a bus is about 80 kilowatts, and the efficiency is 40%, which generates up to 58 kilowatts of heat. The heat is discharged into the city through the engine, water tank and exhaust gas, and the temperature is above 95 degrees. The hottest part of an electric car is the IGBT chip in the motor driver. The entire bus only needs 3 EconoDUAL™3 IGBT modules to drive a fully loaded bus. The core is 18 2 square centimeters 120 microns thick IGBT chips. When running at rated power, the total heat generation power is less than 1000 watts, and the chip temperature will not exceed 80 degrees, which is very efficient. This heat generation is equivalent to the heat generated by 3 passengers running behind.

Do you know that EconoDUAL™3 is the preferred technology and product for electric commercial vehicles? Each heat-generating chip on the right side of the above picture is 2 square centimeters.

Figure: From a bird's-eye view of the Xujiahui flyover, a large proportion of green-licensed vehicles are seen, and the asphalt surface is hot

Fortunately, almost all of Shanghai's buses are electric. Standing on the overpass, you can see that electric cars and electric logistics vehicles account for a large proportion, which contributes to reducing the urban heat island effect.

The second culprit of the urban heat island effect is air conditioning. People need coolness and machines and equipment need to be cooled. Air conditioning continuously moves indoor heat to the outdoors, causing heat waves outdoors in the city.

At night, the Huangpu River was very cool and comfortable. I took a walk to the Shanghai West Bund Dream Center to take thermal imaging photos of the air conditioners. Around the Dream Center, most of the dozen large commercial air conditioners were running, trying to move the indoor heat to the outdoors.

Where does the heat come from? Transparent buildings do not absorb much heat during the day, and the heat flows upward. The main source of heat indoors is people. Please do not underestimate the heating capacity of the human body. According to the calculation of extremely light activities, the heating power of the human body can reach more than 130 watts. In the dream center tonight, human body heat and air conditioning energy consumption are the heat sources of urban heat islands.

Picture: The heat waves in front of the commercial air conditioner, the dense air conditioners and crowds in the Dream Center

Nowadays, almost everywhere in the city there is air conditioning, as well as fire for cooking, hot water for bathing, refrigerators, washing machines and air conditioners... heat is radiated everywhere like crazy.

Transformer in residential area/air conditioner and kitchen of Fengyu Shengjian

Pan-Fried and Stove

Cities make life better. To improve the quality of life, we need to consume a lot of energy and generate a lot of heat, which makes cities suffer from "heat island disease". Where is the way out for us humans? Please see the previous article: Power semiconductors make life better .

Note: Thermal imaging temperature measurement is not calibrated according to the object's reflectivity, and the temperature value is not used as a reference.

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