Car solar heat

With Summer, thoughts turn to how to keep the cabin cool: Potential of solar reflective cover on regulating the car cabin conditions and fuel consumption

The paper begins:

Most vehicles have a large glazed surface area with respect to its total cabin area. For a general mid-size sedan cabin, its total glazing areas is ∼4.21 m2 (29%, 27% and 43% for the windshield, rear window, and total right and left side windows, respectively) . . . Unfortunately, when vehicles are parked in an open parking lot under direct sunlight, the inclination of the transparent surfaces allow a large percentage of incident solar radiation to pass through it. Rugh and Farrington [47] claimed that 50–75% of the thermal energy entering the car cabin is due to transmitted and absorbed solar energy from glazing. Windshield alone accounts for more than 40% of heat transmitted into the cabin​

Reducing the paper from the most to least effective techniques:


So this was my morning project wearing sun glasses and 50% sunscreen:

• double aluminum layer over foam - exterior reflects the sun light and protects the interior from and radiant heat with foam and a second aluminum layer.
• clothes line cord - along the top, folded, and aluminum taped, the ends are anchored in the inside sun screen posts. One problem is closing the window will crease the rubber seal. So a future version will use a thin, flexible material that will not crease the rubber seal.
• bottom fits under the windshield wipers so the wind won't blow it away.
• should work in winter too - keeping the interior heat inside instead of radiating out to space.

Bob Wilson

 

Test configuration:

This configuration maximizes charing the car by solar energy from the combined solar panels and solar panel storage battery. It minimizes the risk of using expensive Grid power.

Using the Tesla Charging, circuit metrics:

• Data collected by SPAN breaker box from circuit, "Tesla charger", while 2019 Model 3 is plugged in. This includes the overhead and battery charging power.
• 9:30 - 13: 18 - Charging the Tesla, circuit breaker panel to battery
  • Tesla car charging set to 20 A at 240 VAC or 4.8 kW rate into battery
    • 4.8 kW is maximum solar array output, 20 A.
    • 5.0 kW is the maximum solar array battery output, 22 A.
  • Tesla Universal charger reports 228 minutes, 17.9 kWh during Tesla charging, no cabin cooling
  • SPAN reports 17.8 kWh, 5.6 kW rate to Tesla Universal charger
  • ~800 W lost between circuit and Tesla battery, ~86% efficiency charging car
• 14:00 - 16:00 - Cabin cooling of car (2019 Tesla Model 3 Std Rng Plus) at 72 F
  • No Tesla car, battery charging
  • Windows closed in afternoon sunlight
  • Tesla Universal charger reports 120 minutes, 2.2 kWh during closed windows, cabin cooling
    • Afternoon shade stopped heating car interior
  • SPAN reports 2.2 kWh of cooling car
  • No way to detect power lost between circuit and Tesla cooling

It is well known that there are EV car charging losses converting L2 AC power to battery charging voltages so the ~85% efficiency is well within expectations. The 20 A charging rate was selected so between the solar panels and solar panel storage battery, no expensive grid power would be needed. The car is ONLY charged from solar panel generated power.

Continuous cooling of the closed car shows an initial, high power peak ~2 kW followed by ~1.2 kW sustained cooling load. Apparently the car cabin cooling runs at ~2 kW or 1.2 kW. Regardless this will let me measure the effectiveness of different sun screen systems to maximize cooling while minimizing power required.

Bob Wilson