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Today is wisdomwednesday. A bi-weekly item in which we put a spotlight on one of our technological innovations. In this first wisdomwednesday of 2019, our electrical engineer Hidde will explain something about the battery in our solar car. How does a battery cel work, how much energy can it store and how is it possible that the solar car can still drive around when no sun is to be found?

Driving without sunlight
The solar cars’ battery is used to store solar energy. This energy is generated through the solar panel when enough sunlight hits the panel. A large portion of this income goes directly into powering the engine. This allows us to use the energy generated by the panel as efficiently as possible. The ‘extra’ energy which is generated is stored in the battery and can be used to power the motor when there is little or no sunlight available. Even when the solar car isn’t driving the panel charges the battery!

How does it work?
The battery exists of multiple rechargeable battery cells. A battery cell has the same properties as a regular battery, only the shape and size are different. Just like in a normal battery, chemical energy is converted into electrical energy when discharged and vice versa when charging. The way that the chemical energy is stored depends on the materials that are used in the battery. The battery used in RED Shift for example is made from litium-ion battery cells. In this type of battery cell the lithium ions are the items taking care of the charging and discharging of the battery.

Enough buzz
As Solar Team Twente, we are looking for battery cells which have a high energy density. This because the regulation of the Bridgestone World Solar Challenge put a limit on the total weight of battery cells allowed in the car. But at the same time the battery must deliver sufficient power to the engine. To achieve these amounts of power the battery cells are connected in series. Where for each cell the negative terminal is connected to the next battery cells’ positive terminal. The power of the two cells now allow for more overall power of the battery. By connecting enough battery cells in series we can achieve the right amount of power which is necessary to overtake our competition during the race!

Our electrical engineer Hidde has selected two of the most frequently asked questions on the topic of the battery which he will answer for you!

How is the energy of the solar panel stored in the battery?
The power from the panel is the same as the power from the battery. However, the power from the panel varies. This depends on the amount of sun which shines on the panel. Part of the power is used to power the engine directly and to keep on driving the car. The remaining power is stored in the battery. When this is full the connection between the panel and the battery is cut-off. This is done from a safety perspective, the battery can overload when too much power goes into it. The power in and coming out of the battery depends on the revenue from the solar panel, which we sadly cannot control. By reviewing the revenue from the panel, the speed of the car and deducing the usage of the solar car from this, we can estimate the amount of power in the battery. These estimations and calculations are very important for our strategy expert to determine the optimal racing strategy!

Why is it important for the strategy to use a battery?
The battery is of major importance to our strategist. It assures that the speed of the car is not only depending on the revenue from the panel (and thus enough sunlight) but is backed by a battery which can be used to power the car. Without battery all revenue from the panel would go straight to the engine. This is however not always possible, e.g. when dealing with a speedlimit. When using a battery it is possible to store some of this energy for later or if there is not enough sunlight available to maintain a constant speed. A very good example of this is when dealing with clouds, if the strategist knows that beyond the clouds there is clear sky, this could conclude that they will use the battery to drive through the clouded area faster to get to the clear skies where more revenue can be made from the solar panel. The battery could be seen as a safety buffer for strategy!

Wisdomwednesday recap:

  • Solar energy is converted into electrical energy, which is stored in a battery
  • Battery cells are about the same as rechargeable batteries, just different in shape and size
  • The regulations of the Bridgestone World Solar Challenge challenge us to think about increasing the efficiency of the battery by limiting the total weight of the battery cells we are allowed to put in the car