The Impact of Parking PV-powered Cars in the Sun on Indoor Temperature and Air Conditioning Load

Research conducted by scientists from the University of Lisbon in Portugal has shed light on an important issue surrounding vehicles that integrate photovoltaic (PV) modules - the impact of parking these cars in the sun to recharge them and the subsequent increase in indoor temperature. This increase in temperature then demands more power for the air conditioning (AC) system to reach comfortable levels for the driver.

The researchers acknowledged this "parking dilemma" and set out to understand the thermal properties of a passenger car under typical summer conditions. Their aim was to explore the vehicle's thermal performance when parked in the sun.

To conduct their experiment, the scientists used an Alfa Romeo 156, a mid-sized car with a light grey exterior and black leather interior. They installed a custom-made solar irradiance measurement instrument on the car's roof, and an infrared thermometer was placed on the gear shift console to measure indoor temperature.

The researchers collected data on outdoor irradiance and indoor and outdoor temperatures under different shade conditions at various times of the day. They parked the car in the shade from 7:00 am until around noon, and then exposed it to direct sunlight from midday until 8:00 pm.

Experiment II involved heating the car cabin using an electric heater coupled with a fan while the car was parked in the shade. The experiment lasted for approximately an hour, with the heater set to an average power of 500 W.

The initial cabin and outside temperatures were 21.5°C. The researchers observed a linear growth in cabin temperature over time, with a total of 390 Wh required to increase the temperature by 3°C. A slope of approximately 4°C/hour was obtained after fitting the data points.

Using the results from both experiments, the researchers developed a simplified thermal model of the vehicle, considering solar gain and heat gain. They used a set of equations to analyze the impact of parking time on PV generation and the extra air conditioning load.

The study revealed that the effect on the air conditioning load is substantial for vehicles with a low PV installed capacity. The critical parking time, which represents the minimum amount of time needed for PV generation to exceed the additional air conditioning load, was found to be approximately 2 hours for a 0.5 kW PV system.  However, for systems with more than 0.8 kW installed capacity, parking in the sun consistently had a positive impact on the vehicle's driving range.

The researchers emphasize the need for further exploration of this concept, particularly with actual vehicles equipped with vehicle-integrated PV (VIPV) systems.  They also suggest examining the concept of solar-powered buses, presenting an opportunity to extend the benefits of PV integration to public transportation.

In conclusion, this research highlights the importance of considering the impact of parking PV-powered cars in the sun on indoor temperature and the resulting air conditioning load. Understanding these dynamics can inform the design and implementation of PV systems in vehicles, ultimately enhancing their performance and driving range.