Hidden Benefits of Electric Vehicles for Addressing Climate Change

There is an increasingly hot debate on whether the replacement of conventional vehicles (CVs) by electric vehicles (EVs) should be delayed or accelerated since EVs require higher cost and cause more pollution than CVs in the manufacturing process. Here we reveal two hidden benefits of EVs for addressing climate change to support the imperative acceleration of replacing CVs with EVs. As EVs emit much less heat than CVs within the same mileage, the replacement can mitigate urban heat island effect (UHIE) to reduce the energy consumption of air conditioners, benefitting local and global climates. To demonstrate these effects brought by the replacement of CVs by EVs, we take Beijing, China, as an example. EVs emit only 19.8% of the total heat emitted by CVs per mile. The replacement of CVs by EVs in 2012 could have mitigated the summer heat island intensity (HII) by about 0.94°C, reduced the amount of electricity consumed daily by air conditioners in buildings by 14.44 million kilowatt-hours (kWh) and reduced daily CO2 emissions by 10,686 tonnes.

HII mitigation and reduction of air-conditioning energy consumption and CO 2 emissions. The average HII was estimated at 3.0uC in the summer of 2012 in Beijing. Heat emissions, which are mainly caused by vehicles and air conditioners in buildings, contributed about half of the HII in Beijing 16 . The daily heat emitted by air conditioners was 4.32 3 10 14 J. The decreased heat emissions from the replacement are 1.69 times higher than the emissions of air conditioners in buildings, which would mitigate the summer HII by about 0.94uC (Fig. 2). Because of the reduction of HII, the energy consumed by air conditioners in buildings would decrease by 12.03%. The amount of daily energy that could be saved is 14.44 million kWh, which could reduce CO 2 emissions by 10,686 tonnes per day (Fig. 2). The results are described in Fig. 2.

Discussion
Air conditioners used in vehicles are dispersed, and the energy consumed by them is difficult to calculate. The energy saving and CO 2 emissions reduction are underestimated, but the benefits are still very remarkable.
According to the definition of specific heat capacity, when specific heat capacity is a constant, temperature variation is proportional to the heat variation. According to Ref. 17, at standard atmospheric pressure, the specific heat of dry air is 1.005 kJ/(kg 3 uC) at temperatures ranging from 0uC to 60uC. The average temperature in summer of Beijing is about 24.6uC 18 , so the specific heat capacity of air could almost be regard as a constant in our model. Thus, it is reasonable to assume that the relationship between heat emissions and HII is linear.
There are many reasons for UHIE, three of which are identified as critical factors: the difference in heat emissions, more aerosol particles and different thermal properties of the ground surfaces. It has been found that pollution aerosols have a positive impact on HII in some places 19 , while some other studies have found that aerosols have a negative impact on HII 20 . The impact of aerosol particles on HII is also highly non-linear and uncertain 21 , therefore, they are not taken into consideration in this model. As to the third factor, the replacement of CVs by EVs is a virtual replacement, which does not change the ground surfaces of Beijing, the thermal properties of the ground surfaces are regarded as unchanged in our model.

Methods
The methods used in this research are summarized in Fig. 3.
First, we analysed the decreased heat emissions caused by the replacement of CVs with EVs. Second, based on the statistics of the contribution of air conditioners in buildings to UHIE and the assumed linear relationship between heat emissions and HII, we deduced the impact of anthropogenic heat emissions on HII. Finally, according to the impact of HII changes on air-conditioning consumption in buildings, we achieved the decreased air-conditioning energy consumption by the replacement.
Heat emissions ratio of EVs to CVs. Energy consumed by CVs is all converted to heat and eventually emitted to the air. Engines of CVs convert fuel energy into thermal and mechanical energy. Then the mechanical energy is converted to heat by overcoming mechanical friction, wind and tire rolling resistance. Energy consumed by EVs is also converted to heat eventually.
In Beijing, the average fuel economy of light-duty vehicles was estimated to be 20.6 miles per gallon in 2012 12 . The heat emitted by gasoline combustion per gallon is 130 million J 22 . Therefore, the average heat emitted by CVs per mile would be: where P 1 is the heat emissions per mile by a CV, E 1 is the fuel economy, Q 1 refers to the energy contained in a gallon of gasoline. The electricity consumed by an EV per mile in China ranges from 18 kWh to 25 kWh per 100 kilometres for different models 23 , and the average is estimated at 0.346 kWh per mile. 1 kWh is equal to 3.6 million J. The heat emitted by an EV per mile would be: where P 2 is heat emissions per mile by an EV, E 2 is the electricity per mile consumed by an EV, and Q 2 is the energy contained in 1 kWh. According to equations (1) and (2), heat emitted by EVs per mile is 19.8% of that by CVs, as shown in equation (3): where r is the ratio of heat emitted by EVs to that by CVs.
where h 1 is the heat emissions from Beijing's thermal power plants when 1 kWh is produced.
Reduction of heat emissions. In Beijing in 2012, the daily heat emitted by CVs (H 2 ) was as following.
H 2~N1 |L|P 1~( 5:2|10 6 )|30|(6:31|10 6 )~9: In the summer of 2012, the average load of air conditioners in buildings was approximately 5 million kW 27 . Therefore, the daily heat emitted by air conditioners (H 3 ) in buildings was: where P 5 is the average load of air conditioners and N 2 is the number of hours per day. If CVs were replaced by EVs, the reduction of daily heat emitted by vehicles (H 4 ) would be as following.
:0|0:5| 7:29 4:32z7:29~0 : where DHII is the decreased HII resulting from the decreased heat emissions with the replacement and k 1 is the contribution of heat emissions to HII in Beijing.
Reduction of air-conditioning energy consumption. If HII were to decrease by 1uC, the energy consumed by air conditioners in buildings would decrease by 12.8% during the summer in Beijing 11 . Although the estimation in Ref. 16 is based on data from Beijing in 2005, air-conditioning energy consumption has taken an increasing proportion of total energy consumption in recent years 23 , which ensures the validity of our estimation. The reduction of HII resulting from the replacement is near 1uC. We assume the reduction of HII and air-conditioning energy saving is a linear relationship. If CVs were replaced by EVs, during the summer in Beijing, the energy consumed by air conditioners in buildings would decrease by: where k 2 is the percentage of the decreased energy consumed by air conditioners in buildings.
The amount of daily energy that could be saved is 14.44 million kWh, reaching 26.75% of the total electricity consumed by EVs, as shown in equations (12) and (13): where DP 5 is the decreased energy consumed by air conditioners in buildings with CVs replaced, and k 3 is the ratio of DP 5 to energy consumed by EVs. With the decrease in air-conditioning energy consumption, less heat would be emitted, which would also contribute to mitigating UHIE and energy saving.
Reduction of CO 2 . In 2012 in China, 740 g of CO 2 was emitted when 1 kWh of electricity was supplied to consumers 30 . Therefore, when 14.44 million kWh are saved, CO 2 emissions could be reduced 10,686 tonnes. The data in this paper are mainly from the government of Beijing and the State Grid Beijing Electric Power Company. In this paper, we have to use some data of other years because some data of 2012 are not available. Therefore, our estimation of the benefits of replacing CVs with EVs is slightly lower than its actual contribution.
According to the analysis and estimation above, the replacement of CVs by EVs can substantially alleviate UHIE in the summer in metropolitan areas, which can improve the local climate, significantly reduce air-conditioning energy consumption and greenhouse gas emissions, thus helping to address global climate change.