AeroWorker

Electric Vehicles and the Urban Heat Island Effect

Aug 31, 2018

How much heat from ICE vehicles would it take to vaporize the water in a Olympic swimming pool, and how much heat would electric vehicles emit proportionally.

I was reading an article on cement and how the world is running low on the right kind of sand to make the best cement which was interesting. For example the kind of sand I use here on my property to make cement isn’t as strong as the water eroded kind of sand because desert sand is wind eroded and fractures, not rounded and polished like water eroded sand.

The article went on to talk about the Urban Heat Island effect which is caused by all the cement and asphalt surfaces in our cities which store and release heat. Just making surfaces with a lighter albedo would help a lot with mitigating this problem. Then they just happened to mention in passing that the heat emitted from cars and trucks contributed to the Urban Heat Island effect. That is all they said.

This passing comment got me to wondering; just how much heat do internal combustion engines in cars and truck contribute to our urban environments. Inquiring minds wanted to know. Possibly this is a fact you could google or research but that isn’t challenging enough, I wanted to figure it out if I could.

First of all you will need a series of assumptions you can assign reasonable numbers to that will at least get you to a ballpark figure. I chose our city here in Southern New Mexico, Las Cruces which has a population of around 70,000 people with about 100,000 people residing in the county of Dona Ana.

For a working number of vehicles I chose 50,000 vehicles spending one hour a day in the city burning one gallon of gasoline each. It doesn’t matter if there are less vehicles spending more time driving, or more vehicles spending less time, this was going to be my average.

Just for grins I had them average 22 MPG traveling on average 22 miles. This would be helpful later when I decided to calculate my values for electric vehicles. One gallon of E-10 gas contains 111,836 BTU of heat energy. ICE cars and trucks averaging 20% thermodynamic efficiency have to reject the 80% left over heat into the environment to keep the vehicle from overheating.

That amounts to 89,469 BTU per vehicle. Fifty thousand vehicles would collectively contribute 44,734.5 Therms of heat per day. That is the same as saying 4.47345 billion BTU to you and me. That is a really large number and while it is a nice result it doesn’t do us much good, we need a visual.

How much heat energy would you need to boil all the water away in an Olympic size swimming pool I wondered? It turns out there is 2,500 tons of water in such a swimming pool which translates into five million pounds (5,000,000 pounds). Now we are getting somewhere.

Let’s set a starting temperature for the pool water at 80 F, and a vaporization point of 202 F because we are at elevation here in southern New Mexico. First we have to take the heat and get the water heated to 202 F which means raising the temperature 122 degrees. So 122 x 5 million = 610 million BTU. That leaves me 4.095 billion BTU left over.

The latent heat of vaporization is 970 BTU per pound of water. Now 970 x 5 million = 4.85 billion BTU. That is not enough heat but it is close. You have 4.095/4.85 = .84432 or 84.432% of the water in an Olympic pool could be vaporized by the heat coming from 50 K cars burning one gallon of gas each. This assumes perfect heat transfer to the water which doesn’t exist but that is a separate matter. This is after all just a thought experiment.

So far so good, but then a EV World blogger like me just has to know just how much heat would a 50 K fleet of electric cars emit in the same scenario? Let’s assume our EV’s are 95% efficient at using their energy. They are not going to use the same gallon equivalent because they are more efficient.

The fraction would be 108/22 = .203 or 20.3% x 32,777 watts = 6,653 watts or 6.653 kWh. Since the EV’s are 95% efficient that means they only reject 0.05 or 5% of this energy as heat. That is 332 watts or .332 kWh. There are 3,412 BTU is a kWh so .332 x 3,412 = 1,135 BTU per vehicle. Multiplied times 50 K vehicles, 1,135 BTU equals 56,750,090 or 56.75009 million BTU.

Four point eight five billion (4.85 billion/.0567 billion = 0.0117 or 1.17%. There you have it replacing our internal combustion fleet with electric vehicles would reduce the heat contribution to our urban heat island effect by a factor of one hundred to one or to 1.17% of the value. So now I have one more nightmare scenario to contemplate when I am driving my EV around town. Not only to have this visual picture of all the piston engines around me pumping out air pollution which my car is not but now I have this mental picture of a large boiling swimming pool to top it off.

My calculations in this exercise could have been more accurate if I just knew what the sales of gasoline and diesel fuel were in my city. Then I would not have to invent a hypothetical. I could just take the monthly figure and convert it into an average for one day. It would of course vary though out the whole year but this is where an average could come to the rescue.

I invite anyone who has access to such a figure to do the calculations. I personally would love to hear your figures on this problem. The other thing I do not know is just what percentage of heat in this effect comes from vehicles as opposed to is just being released from surfaces. These things are all probably known and out there. All I wanted to do here is boil some water for tea, you know for a BIG GULP cup.

Addendum: I got a comment on Ecomodder to the effect that ultimately all the energy in both types of vehicles is returned to the environment. I agree but for the purposes of my story I thought it was more interesting to elucidate the particulars of both vehicles in the way I did to showcase their relative efficiencies or inefficiencies as the case may be.

Just comparing total energy in then out still makes my case. Originally I set out to vaporize all the water in an Olympic size swimming pool and using all the BTU from a gallon of gasoline for each of the 50K cars does that rather handily. The heat energy returned from my hypothetical EV would be characterized as 22/108 or approximately 20% of the ICE vehicle. Either approach is entertaining or appalling depending upon your outlook regardless.

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