So how are we going to charge the car with mobile solar panels?
Charging the car with solar panels can bring us to places a normal petrol car is not able to go. In this article, we are going to explain how we calculated the numbers and which solutions we are going to explore in order to charge the car during our trip through Africa.
Energy consumption of the car
Starting with the car. The regular electric car we have selected for our project contains a 77kWh battery. This results in a WLPT range of 537km. But WLPT does not calculate any off-road driving with a fully-loaded car and rooftop tent. Therefore, a better estimation needs to be made.
In the graphic below, the amount of Wh/km can be found for the car. Although we leave the Netherlands in cold weather conditions, most of the kilometers will be driven in summer conditions, so we can ignore the left part of the graphic. On the roads in Africa, we estimated a speed of a maximum 80 km/hr. As air resistance is the main range killer, this influences energy consumption in a positive way. During our trip to Norway in 2020, where most of the roads have a speed limit of 80km/hr, the range of our Tesla Model 3 increased by over 30%.
But there are multiple factors that will influence our range in a negative way. First of all, we will drive with a rooftop tent which results in way more drag. In the past weeks, we drove over 2.000km with the rooftop tent in our car. Driving 85km/hr on the highway, not close behind a truck, resulting in a consumption of 150 to 170 Wh/km, not too bad! In addition, it became clear that wind can be our friend and our enemy. On a very windy day, the energy consumption driving 85 km/hr increased to 220 Wh/km. On the positive side, on our way back home we could drive 100 km/hr with the same energy consumption of driving 85 km/hr with zero wind. It might be interesting to not travel during headwinds! We are investigating a way to measure true wind while driving, so we can gain more knowledge about the influence of wind. If you have an idea how to do this, please let us know!
Four other important factors that will probably result in higher power consumption are the use of air conditioning, all/off-road tires, increased ground clearance, and the weight of the car fully loaded with all our gear and equipment.
With all factors in mind influencing the power consumption of the car, we decided to take 200Watt/hour per kilometer as a baseline. Compare this to the average energy consumption of 140Watt/hour of our Tesla Model 3 in the summertime, it is quite high, but at this stage of the project, we have to be conservative.
Amount of energy needed per day
During our holiday in Africa a few years back, we drove 150km per day on average. Driving this distance daily will result in traveling 9 months straight with the 40.000km ahead of us. In Europe we want to make use of fast charging, therefore we will drive a lot more than 150km a day. But with all kinds of possible setbacks in mind, like bad weather, we planned to be on the road for 12 months. 150 kilometers per day with an energy consumption of 200Wh/km results in daily energy consumption of 30kWh. As we need electricity for all kinds of other equipment like a fridge and induction cooking, we added 2,5 kWh, resulting in total daily energy consumption of 32,5kWh.
Charging the car
There are two ways to charge the car: with AC and DC. Charging the car with AC is like charging the car at home; the most used solution. It can be done with standard equipment already on the market for so-called off-grid power systems. AC charging can be done in 1 or 3 phases. Our car can be charged with a single-phase AC system up to 7,3kW. The limit for a three-phase AC charging system is 11kW, but a three-phase system requires way more equipment and is, therefore, no option for our project.
Electric-powered cars can also be charged with DC. This is done when making use of fast charging where you can charge the car with over 300kW if the car is supporting this. Our car can be charged with up to 125kW DC, way more than the 7,3kW of AC charging. In addition, charging the car with DC means fewer conversions as solar panels also produce DC. When charging the car with AC, the DC power from the solar panels will be inverted to AC to connect it to the grid and thus the car. The car will convert the AC power back to DC again to charge the batteries which are DC, see images below for your reference.
But before going into more detail about charging itself, there is another important topic: we need to find a charging solution that will survive our trip. The used additional battery cannot be the link between the solar panels and the charger. Pushing the 32,5kWh through the battery daily, with power rates of up to 7,3kW (AC) is undesirable. Luckily, there are already solutions for this. At home, we have a charging station that is able to charge the car with the amount of energy the solar panels produce, leveling out the power going to or coming from the grid. We will take additional batteries with us, around 10kWh, for the fridge and cooking. This battery can stabilize the power coming from the solar panels if needed.
In the figure below, coming from the Home Wizard app, you can find the power consumption of our home on a particularly sunny day. Purple means taking energy from the grid, and green means delivering energy. At 09:45 we started charging the car. A few hours later (the car was already quite full) the battery reached the required state of charge and delivering energy to the grid started again. This solution can also be used for the off-grid AC solution for our project.
Back to the AC/DC charging again. The car and a car charger communicate with each other. For AC, this is rather simple: the car will say to the charger when it is ready for charging and will indicate when the car is charged at the required state of charge. The power given by the charger can vary, as seen in the above picture. The car accepts the power that is given by the charger as long as the power itself is stable looking at frequency and voltage.
Compared to AC, communication between the DC charger and the car is more complex. Communication between the car and the charger happens constantly. The charger tells the car what power it can deliver and the car responds to what it can receive. If the above messages are in line, a so-called “hand-shake” is set up, and charging starts, or continues. This communication protocol is not that difficult, but at this stage in the project, it is unknown how the DC charger responds to sudden dips or peaks in the solar power as solar panels cannot give this handshake and shadow will influence the solar power.
We have found a company, Venema E-Mobility charge Systems, that can deliver a DC charger where solar panels can be plugged in directly. Beginning of June, together with a supplier of solar panels, we are going to test if the set-up will work. Unfortunately, this solar panel supplier is not able to provide us with solar panels for the trip. With this DC charger, we are limited to 30kW of charging power.
With all the energy losses in mind and the number of hours we want to charge, we estimated that we need at least 10kWp of solar panels, resulting in charging of around 5 hours a day. We could also charge one day and drive the other.
So what about the solar panels?
As we will take a regular electric car with us, we do not have a lot of space in or on the car. Therefore, we can not bring the standard glass solar panels, which can be found on most of the roofs, as they are thick and heavy. Instead, we will bring flexible lightweight solar panels. The weight af a glass solar panel is roughly 11kg per m2, a flexible variant is around 3,5kg per m2. Normally, these kinds of panels are installed on roofs that are not able to support the weight of glass panels. The downside is that they are up to 25% less efficient, not considering the high-end variants due to financial reasons, but they are less than 1/10 of the thickness! The panel we have in mind produces up to 240Wp. With the needed 10kWp this results in at least 42 panels or around 60m2 of solar panels with an overall weight of roughly 200kg.
A flexible panel is ±4mm thick with a junction box of ±25mm. The floor space in the back of the car is 1.700 x 1.020mm. If we take a panel of 1.500 x 1.000mm, as can be supplied, and we stack it stepped, see the below picture, it will result in a stack of 250mm thick.
We need to find a solution to store the panel safely in the car together with all the required equipment. We are thinking of a drawer system of some kind. If you have an idea, please let us know!
Before this project became as mature as it is now, these calculations helped us to see that what we are going to do is possible with realistic facts and figures. We hope you like this article and understand a little bit more about what we are working on and how we are going to do it. We are not there yet, as we need to find more partners. If you are interested or know a company that would be interested in becoming a partner, please let us know! In addition, you could help us by liking or sharing this article. If you have any comments or questions on this article or our project, please let us know! We definitely could use your help to make our project even better!