Request Heat emitted from a car during an sunny day.

Hi

After stopping my bike next to a car this morning, I felt a very hot wave of air in an already very hot day. I wondered how much heat a car can transfer to its direct environment.

I tried some calculations but I am not sure about it. My goal is to find the right order of magnitude, si I simplified the model with 3 main sources :

1. heat from the engine combustion
2. heat transmitted by metal
3. heat from cooling inside the car

If anyone can complete / correct, i would be thrilled.

common hypothesis : 35°C outside, 23°C inside, 1h drive at 25 km/h in a town. no wind. Middle car (VW Golf) in Europe.

1 - Engine combustion

1L of gasoline is 8.9 kWh, engine efficiency is 20%, engine burns 8 L / 100km. So 1h drive is 25 km = 2L gasoline = 17,8 kWh * 80% = 14.24 kWh.

It feels kind of low because exhaust gaz are at 500+°C and engines are running at 90°C but I could’nt find a better way to estimate this.

2 - Metal radiation to the environment (part of it comes from engine combustion but maybe epsilon ?)

I simplify the car as a 10m² aluminium object, so with the formula Q (heat transmitted in J) = k (heat transfer coefficient in W/mK) * A (surface in m²) * t (time in s) * delta T (temperature between air and car).

Also, I found that average temperature of car hood in sunny days is around 60°C. k is 237 W/mK for aluminium. The calculation gives 237 * 10 * 3600 * (60-35) = 213 300 kJ = 59.25 kWh.

3 - heat from AC

first I calculate how much heat is received inside

\- direct sunlight through glass = 900 W/m² (solar power) * 3,5m² of glass * 0.65 (resistance from the glass) = 2050 W

\- conduction from metal : same kind of calculation as (2) but with the car complex as resistant to the sun and around 8m², with a lambda of 4, so power is 4 * 8m² * (60°C - 23°C) = 1184 W

\- same for glass with a lambda of 5.5 and 3.5 m² = 5.5 * 3.5 * (35 – 23) = 230 W

\- ventilation of the car and 1 human body inside = 250 W

Total = 2050 + 1184 + 230 + 250 = 3714W, during 1 hour makes 3.7 kWh.

Total heat from 1+2+3 = 77 kWh.

 

What do you think ?

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