Solar Powered Car Interior Ventilator

[Astra]

I have come across some window mount unit solar cell that power little exhaust fan that claims to coll down the car cabin that parked under hot summer sun.

However since the fan is not powerful enough to drive the heat out of the car effectively, is it enhance it with:
1. larger solar panel (higher output)
2. high flow rate axial fan (to minimize required amperage to power)
3. air temperature sensitive thermostat (to provide continuity when temperature reached a 'set point' say.. 37C)
4. rechargeable battery unit to prevent fan speed fluctuation due to inconsistent solar panel output

anyway, this is how roughly my idea works. Might be some installation issue with the exhaust fan, but let us just assume that it can be installed and it can drive the hot air from cabin out.

The ideal condition of this setup is try to reach equilibrium temperature with outside ambient temp. However i was told that this is not possible due to increased entropy level in car interior?

 

[Topher925]

I don't really see why entropy has much to do with anything since its production would be so small it can be neglected. The reason why it can't reach ambient temp is because your cooling it with ambient air. You may be able to get close to ambient but you will never be able to instantaneously remove heat from a surface the same rate as it is absorbed, or not in this case anyway. Let's look at Newtons law of cooling which applies for only convection that is removing the heat not adding it (radiation);

q = hA(Tc-Ta)

where q is the rate of heat transfer, h is the transfer coefficient, A is the surface area, Tc is the car temp, and Ta is the ambient air temp. If you take the limit of Tc/Ta as Tc -> Ta it will converge to 1 but never quite actually reach it in reality. In order to do this it would take an infinite amount of time and a hell of a lot of airflow. While mathematically it is sort of possible, practically it isn't.

 

[Astra]

Topher925,

Thanks for clearing up the air, so basically i can say that the rate of heat transferred out equal to:

$$\dot{}q$$=$$\dot{}m$$C_p$$\delta$$T

where $$\delta$$T is the difference between outside ambient air temp vs heated cabin air temp.

Cp just assume 1.005 since it does not change much.

$$\dot{}m$$(mass flow rate) obtained from (volume flow rate of exhaust fan) x (air density)

$$\dot{}q$$=$$\dot{}m$$C_p$$\delta$$T = hA(Tc-Ta)

i can assume we can theoretically find the required exhaust fan air flow rate to cool down the car interior temp to its outside ambient temp?

 

[wanteh]

astra :
where can i get about information theoretically to find the required exhaust fan air flow rate? whould you reply to my email? any 1?
wan_teh36@yahoo.com