Designing Lightyear One: the solar roof

Simone Regondi — PV Engineer

At Lightyear, we all appreciate the unique value of Lightyear One’s solar roof; it is a direct clean energy source and a core technology we are developing. As a photovoltaics engineer I am working on integrating the solar cells, both electrically and mechanically. Currently, we are mainly focusing on process optimization, improving technology maturity level and bringing parts to manufacturing quality.

The 3 main elements of Lightyear One’s solar roof

Lightyear One’s solar roof is composed of 3 main elements. The top layer, made of glass. It protects the solar cells while letting light in, and gives a smooth surface to the roof. The second layer is the photovoltaic (PV) module containing the solar cells. This module is laminated together with the third element: the supporting structure. The solar roof has a unique curvature in order to maximize the aesthetics and aerodynamic performance of Lightyear One. To create the right strength and optimize the weight of the roof, the supporting structure and the method used in the glue process are being developed in-house.

This last part – keeping the PV module lightweight while making it sturdy – is also one of the most challenging aspects. We are working with different partners such as TNO to ensure that the product is safe and reliable. In order to test the mechanical properties, we performed a ball drop test and different static load cases. We also conduct thermal tests to address the suitability of the bill of materials (a list of raw materials), as well as take measurements for electrical performance.

Aesthetics of the solar cells are very important for Lightyear One. On the left you see a design prototype of the solar roof. On the right, you see a functional prototype where we left out the steps that enhance the aesthetics. The difference is clearly visible.

Good looking and efficient

We designed the roof to be slightly curvy for aerodynamic and aesthetic reasons, while maintaining focus on surface optimization. What is great about the rear-contact crystalline silicon cells we use, is that we can increase the power conversion efficiency. This also eliminates the metal busbars on top of the cells, improving the aesthetics of the roof. We are still innovating in applying conductive back-sheet technology to vehicle-integrated photovoltaics and this brings huge advantages. We can develop a system for a completely automated photovoltaic module fabrication, and we have extreme flexibility in deciding cell size, location and orientation.

The solar area

In the Netherlands the solar area of Lightyear One can deliver a peak power of around 1250 Wp and can provide about 700 kWh per year. This translates, for example, into a ski trip from Amsterdam to Innsbruck (1802 km back & forth) in which you only have to charge the car twice. Predicting the energy yield is always a challenge for vehicle-integrated photovoltaics because of the changing environment. This includes weather conditions, time of year and location. The model used for the calculator on our website includes a lot of different sources for this combined with a loss factor due to shadow.

Currently, we are working on validating this model. We have developed devices to collect solar irradiance data and put them on different test cars so we can calculate the energy yield as well as the losses due to shadow on the car. Based on all the collected input combined with our calculations and simulations, Lightyear One is expected to generate around 50-70 km extra range per day, during a Dutch summer.

Due to all this energy however, the solar cells can get warmer in certain conditions. Normally when you have a PV structure on your roof you receive 1x the sun’s energy. When your car is parked next to a reflective building however, the car can be exposed up to 1.5x the energy. Together with the Energy Research Centre (ECN) at TNO, we conducted tests to determine the reliability of the solar panels at elevated temperatures. The test was based on the car receiving twice the normal amount of energy, and the structure successfully passed.

After all this, you might still wonder how far you will get with 1 hour of sunshine on an empty battery. Imagine yourself driving through the blooming Dutch countryside at around noon, with sunny weather and barely any traffic. You will have 15 km to get to any power outlet in order to start charging at the same speed (km/hr) a normal EV would do at a special charging station.

 

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