5.1 Circuit
One of the most important parts of this preliminary project are the considerations about the electric components of the vehicle and the circuit. Unlike some of the preceding sections, this one is not based on earlier theses; in fact, the introduction of the electric propulsion system is the key difference between this work and previous works.
Another factor that adds to the relevance of this section is its importance to the safety of the user, which has always to be a major concern when dealing with electric systems. Although the currents and voltages are not high in this project they are enough to provoke injuries. This is why it is so important to carefully design and implement the circuit (avoiding malfunctions such as short circuits), to properly isolate the wiring and electric components (as well as firmly hold them in place) and to include components in the system that can allow to shut it down in case of necessity. Furthermore, usually it would be necessary to isolate and protect the constituents of the electric system from being subjected to sand, dust and water (the exposure to these is very likely due to the kind of utilization expected from this type of vehicle). However, the components are taken from an e-bike specially intended to be used on the beach, and therefore the components are already capable of dealing with those problems, since they were designed specifically for it.
The constituents that are going to be part of the circuit are already mostly defined, since they were chosen in the concept selection stage:
36V, 500W electric motor (geared hub);
Lithium ion battery, with 48V and 10AH (480Wh);
Controller;
Throttle (manual/twist).
Besides these, are going to be included the following elements in order to contribute to avoid accidents in the electric part of the vehicle:
Main circuit breaker (figure 46): The function of this device is to allow a quick disconnection of the batteries from the rest of the components, immediately interrupting the circuit. It is a safety component which is supposed to be used in case of emergency (e.g. the detection of sparks, flames, etc.).
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Figure 46 – Main Circuit Breaker [34]
Safety fuse: It interrupts the circuit when an unexpected spike of current occurs due to a short circuit or accident. Normally they are placed in-line with the battery. While the circuit breaker may be manually activated in case the occupant detects an anomaly, the safety fuse does it automatically (adding a second layer of protection).
Besides these, the only other elements necessary are the wires to do the connections. Their isolation must be without defects and during use they should be held closely to the chassis (using supports) to avoid being pulled.
The electric circuit implemented (figure 47) is very simple due to the characteristics of the problem (only a few components are necessary and in a simple configuration). The control is done using a bought controller (used specifically with the motor and battery of the vehicle) and for that reason it is presented in the circuit as a black box (it is unnecessary to present the arrangement of its inner components).
Figure 47 – Layout of the electric circuit to implement
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In figure 47 is presented the circuit that will be used in the vehicle. It is a simple layout that is typical in electric vehicles. S1 represents the throttle, S2 the fuse and S3 the circuit breaker.
5.2 Location and placement of the components
Motor (1): It comes integrated in the wheel of the e-bike (placed in the front), and cannot be removed from it. To make the transition to a fully wind powered vehicle the wheel has to be replaced.
Battery (2): This component is placed in a compartment (the battery holder), which is itself situated beneath the horizontal bar of the tripod, and connected making use of 4 bolts and nuts which go through the lateral supports specifically design for that purpose. Batteries are susceptible to damage caused by vibrations and impacts, which may cause their irreparable failure (a very serious problem when considering their high cost) and even pose a danger to the occupant. For that reason, in the inner walls of the battery holder are used silicon foam pads specifically design for this purpose. Besides absorbing energy of the shocks, this material is very resistant to high temperatures and flames, a key property in this type of application [63].
Throttle (3): Positioned beneath the seat (like the brake lever, but on the opposite side). It was considered the use of a handlebar to steer the vehicle, and in this case it would be placed in there. Once it was decided that would not be the best option, this location was chosen for the throttle because it is easy to reach and does not present safety concerns.
Controller (4): Placed behind the seat, a location that offers some protection. This component does not need to be touched during use, so there is no inconvenient in this location. It is held in place by being put in a support integrated in the seat.
Main circuit breaker (5): Given its function and importance, it must be in an accessible location.
For that reason, it is also placed beneath the seat, but in a posterior position relatively to the throttle since, unlike this element, it is only used sporadically.
In figure 48 are identified and illustrated the locations of the mentioned elements.
Figure 48 – Location of the different components of the electric system
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5.3 Expected consumptions and battery duration
Knowing all the components of the system and their characteristics, is possible to do an estimate of the consumptions in different wind conditions, and therefore the time of use available. The two extreme circumstances are when the vehicle is being operated only resorting to the wind power (electric system removed) and when the wind is negligible and therefore the kart only has the motor as a power source.
The battery has 480Wh of energy storage capacity and it is going to be assumed in every case the use of the vehicle is started with full charge. It is important to notice this is another example of a situation where accurate values could only be determined by doing tests with the vehicle (instead, an approximation is utilized).
In section 2.4.2, in order to choose the necessary capacity of the batteries, was made the following approach to the problem: it was considered the average utilization of the vehicle was 4 hours and that in normal wind conditions the energy drained from the battery in this period corresponds to the use of the motor at full power during one hour. This 4 hour period is therefore from that point onwards set as the average expectable duration of the battery in regular wind conditions.
On the other hand, and considering the utilization of this type of vehicle requires in most cases a moderate/high speed, in a no wind situation (or when it is very weak) the expected duration of the battery will be close to one hour (the motor will be working close to full power to compensate for the lack of thrust from the sail). This is a very short duration that in most cases would not satisfy the needs of the user: it is important to remember, however, the electric power system is supposed to be only an assistance to the wind propulsion since the vehicle, namely the battery, was not design to operate in these conditions (a greater battery capacity would be necessary, and this would translate in an important cost increase).
The batteries should be fully charged before being used and fully discharged after, mainly to maximize the time of use in each utilization (lithium ion batteries are not as susceptible to being damaged from
“memory effect” as other types).
These numbers can be utilized in the future as reference points when testing the vehicle. It is also expected the values to some extent vary with the type of terrain. Either way, since the motor is controlled with a twist throttle that allows to regulate the power gradually, it’s possible for the user to roughly predict the duration of the batteries.
Finally, this discussion is only valid for a new battery since, as mentioned before, even lithium ion batteries, correctly stored and only subjected to complete charge/discharge cycles tend to loose capacity over time with each utilization, although it only becomes relevant after several hundred repetitions. The expected life of the equipment in analysis is around 1000 cycles (from this moment is considered the battery has lost too much of its capacity and should be replaced).
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5.4 Cost analysis
In this section it is going to be made the discussion of the cost of the implementation of the electric power solution in the land yacht, including both the direct and the indirect shares. The cost of parts not related with the existence of an electric motor (structure, sail, etc.) will not be considered. Once again, this option was taken to focus attention primarily on the exploration of the specificities of the hybrid vehicle, and also because in previous works a cost analysis of the standard components was already done. There are going to be considered two situations: prototype stage and production stage (although only in the first situation can be found exact values).
5.4.1 Prototype
In this phase, the components of the electric system are all taken directly from the e-bike Shuangye A7AM20 beach model. Therefore, to build the prototype it would be necessary to purchase it, at a cost of 630€ [37].
There is still another factor to consider: since the land yacht projected in this work has 3 wheels and a fourth one is required to replace the motored front wheel when the user opts to use the vehicle without the electric system (when the wind speed is high enough), two extra non-motored wheels are needed.
This represents an estimated extra cost of 100€. The other elements of the electric system (main circuit breaker, safety fuse, wiring) have a negligible cost when compared to the former shares.
These portions englobe every cost with components that have to be bought except the sail and fasteners. All other cost are related with the purchase of the necessary aluminum and steel tubes and the production operations required to obtain the chassis.
5.4.2 Production
In a situation of production of the vehicle, the unitary costs with the electric components would obviously be smaller, since they are now bought in bulk and without the need to buy them together with extra unused parts. It is, however, difficult to quantify this difference given that it would greatly depend on the quantity of vehicles produced and the specific research done to find suitable suppliers. This research would also determine whether the different elements would still be bought as a set or whether it would now be advantageous to acquire them separately.