## Photovoltaics for cars: the best solution from the natural source to the wheels

There are **important differences** between the use of solar energy and all other modes of powering the cars. Solar photovoltaics is **the only case** in which the primary energy can be used on the car, coming **directly from nature**, in a sort of “short supply chain”. In all other cases, an “energy vector” is used: the primary energy is “incorporated” into a “vector” (fossil fuel, bio-fuel, electricity, hydrogen …), transported, distributed, sold and taxed.

It is a relevant difference: in fact, energy transmission always involves energy consumption and, in many cases, CO2 emissions, energy costs and emissions that would instead be avoided by **using direct primary energy on the vehicle**, through photovoltaics.

### What contribution can photovoltaics make on a car?

Let’s now try to **evaluate the savings** that can be achieved **by integrating the photovoltaic system into a vehicle.** The skepticism that has accompanied the use of solar energy in the automotive field for a long time is due to the fact that the power of a solar panel that can be housed in a car with a normal size, of 300 W, is much lower than the power of a medium-sized car, of the order of 50-60 kW. **But this observation is as elementary as it is misleading:** it would be correct to think so if both systems always operate at maximum power. In the case of a car, this happens almost only when you run at Le Mans “24 hours”.

In reality, most of the motorists (about 50%) use the car mainly in the city, for no more than an hour a day and almost always with only the driver on board. In these conditions, the “average” power required in an urban environment is of the order of 8 kW, considering the partial recovery of the power needed to brake. If the car is used one hour a day, **the daily energy required for traction is therefore equal to 8 kWh.**

Now let’s go back to the solar panel: not even this, of course, always works at maximum power, at least because at night the sun goes away. However, if we consider a panel exposed in a sunny place, the energy obtainable is roughly equal to that obtainable in about ten hours per day operating at a power of 2/3 of the maximum power: in the case of a panel with a maximum power of 300 W , we can estimate the daily energy obtainable on an average day by multiplying the average power (200 W) for 10 hours, obtaining a daily energy of 2 kWh. **The photovoltaic panel can therefore provide about 25% of the approximately 8 kWh per day** required for traction, in a typical urban use, a value well over 0.5% that an apparently common sense but superficial analysis, based only on the maximum power , he would have attributed to him.

The Flexible photovoltaic used in the LIFE-SAVE project is developed by Solbian.

**A more precise evalutation of the potential contubution achievable by PV panels**located on a car (in almost horizontal position) is given bt the following graphs. The values of mean and maximum daily net energy are computed for different cities at various latitudes by

**using the database PVWatts, developed by the NREL (National Renewable Energy Laboratory)**

**of US**.

**TMY (Typical Metereological Year)**, that takes into accout the mean real expected weather.

**the maximum daily contribution can be over 3000 Wh/day**in some locations, while

**mean contribution**(considering also winter time)

**range from 1.000 to 2.000 Wh/day**.

**significant increase of energy harvesting**. Average yearly daily contributions range from 2000 and 3000 Wh for most world locations, with maximum values exceeding 5000 Wh/day in some locations. These results have been obtained considering 2 m^2 of panels in horizontal position and 2 m^2 on vertical position, on car lateral sides.