Echo, our newest boat, is designed to fly even more efficiently and reliably! Did you know that Echo is the fifth letter of the NATO phonetic alphabet? Echo is the result of 5 years of Solar Boat Twente, and at the same time an evolution (echo) on Twentsch Pegasus, the boat of last year’s team.
Together with the solar panels, the hull remained the same. It is made out of carbon fiber in a sandwich structure, where the core consists of foam. This structure and the shape make for a lightweight, strong and hydrodynamic boat. It is 6m long, 1.6m wide and weighs below 120kg!
The solar panels are one of the most notable parts of our boat, and generate the incoming energy. This year we are using monocrystalline solar panels that have a lightweight finish, and a total area of 6 square meters. With an efficiency of 24.8%, up to a quarter of the incoming solar energy is converted to electrical energy. This is done with the aid of Maximum Power Point Trackers whose job it is to extract as much power from the solar panels as possible, at any time.
The back strut performs multiple tasks, all in one part.
Firstly, it houses the motor, which drives the propeller. Less than 1.5kW of the motor is required to make the boat fly at 25km/h! Unfortunately some heat is produced, and therefore the motor must be cooled. Since it is always in the water, this is done without extra weight or power consumption.
Secondly, the propeller is mounted on the back strut. The two bladed pull-propeller is made out of aluminum and is a lot lighter and more efficient than the previous one, generating more forward thrust with the same power input. Additionally, there is a second propeller with three blades. This one can be used at higher speeds, making for a top speed of around 45km/h.
Lastly the rear hydrofoil is mounted below the back strut. The internal system to change the angle of the hydrofoil has been made smoother and stronger, making the rear hydrofoil more responsive than before.
The front hydrofoils got a completely new design. In order to make them more robust and the actuation simpler, a design with flaps has been adopted. This means that, unlike the other boats, the main part of the wing has a fixed angle and only a small part of the wing has an adjustable angle (just like in an airplane). This new design is not only stronger, but also a lot more hydrodynamic: for the same lift, the drag is reduced with 30%! The hinge is made of a single layer of carbon with an elastic epoxy, making for a light system that does not affect the drag.
The electronics are responsible for all the incoming energy of the solar panels, the temporary storage in the battery and the correct conversions and release of power to the other devices in the boat.
This year, the focus was mainly on the battery. Because this one is cooled with a phase changing material, it could get a lot more compact this year. The electronics therefore, together with some other improvements in this department, allowed for a better weight distribution. Additional sensors have been integrated such that for instance measures like battery temperature and incoming power of each solar panel can be measured better. In the battery, a custom busbar reduces the resistance for a more efficient power output. Additionally, the battery has been characterized extensively. These test results, together with an improvement in the software of the battery management system, allow for a better rea-time estimation of the remaining capacity of the battery. This is important for a good race strategy.
Lastly, some new safety systems have been installed to reduce the risk of failing.
Since no device has a 100% efficiency, heat will always be produced as a side product. The three main devices that need to be cooled are the battery, the motor controller and the motor itself.
We chose to use a phase changing material (PCM) to cool the battery this year. Because we are using a non-conductive PCM, the battery can be submerged in it for a better heat transfer. The PCM takes up a lot of heat during the phase change from solid to liquid. This way, the battery is kept at constant temperature. Besides, the use of PCM does not require any additional power consumption, only adds a little bit of weight (10% of the battery mass), and makes for a more compact battery than in the case of air cooling.
The motor controller is being cooled with a di-electric immersion fluid. This means it is submerged in a non-conductive liquid that is pumped through a closed loop system. Also here, this makes for a more efficient heat transfer, allowing the cooling system to be smaller and consume less power. The liquid is in turn air-cooled by a radiator at the back of the boat.
As described before, the motor is passively cooled because it is mounted in the nacelle in the water. The aluminum strut is a good heat conductor and so the heat generated by the motor is directly dissipated by the water.
If the battery is the heart of the boat, the software is its brains! There are three main aspects to it, namely the internal functionality, data collection and the control system. The internal functionality is all about providing the backbone for the boat to operate successfully. This includes setting up all the internal communication and ensuring reliable behavior for any safety critical features, like turning the motor off or pressing the emergency button.
The second task of the software is collecting and storing the data from every sensor and every device. This data must be stored and sent to shore reliably, which is vital for real-time analysis of the boat, and optimizing and tuning the (control) systems in the boat. This telemetry system has been greatly improved, including a custom data visualization webtool.
To be able to hydrofoil stably above the water we needed to develop a whole new control system. In essence this is just a function that takes the boat’s sensor data as input and outputs the necessary position for the hydrofoils in order to maintain stable flight. However, designing this control system involves making a complete model of all the physics of the boat, and a lot of simulation before it can be safely applied on the boat. This control system is then run on a dedicated microcontroller to ensure reliable and fast execution.