3. Feasibility: Prototyping, Analysis, Simulation

Developing a Feasible Design for the Overall System

In order to start developing a functional prototype, the team needed to decide on a system set-up that would be used throughout the duration of the project. The figure below showcases the decided set-up that will be used for analysis, prototyping, simulating, and testing. Please note the system is not drawn to scale.

System features are described and justified as follows:

• Electricity sourced from an outdoor home receptacle: Used to power the pump. It is assumed that the receptacle will provide 120V to the system for the pump to use.
• Central fuel tank: located close to the home and the pump. Occasional refilling of the fuel tank will need to be completed manually by the user to the main system.
• Distance between the first torch and the pump: The team decided using 15, 30, 60, and 90 ft of tubing would be appropriate for simulating various set ups. Generally, the distance between the pump and the first torch will be variable for each customized home set-up. In fuel analysis, these dimensions would allow for easy scaling. It was also noted in this discussion that the app should include in its setup menu a selection of various pump to first torch lengths, so that the microcontroller can communicate the appropriate flow rate of the fuel accordingly.
• Distances between torches: citronella oil is effective in approximately a three foot range. The ensure the spheres of coverage overlap, or team will be using a 5ft standard spacing between consecutive torches.

In order to proceed with system and subsystem development, the system was broken down into mechanical and electrical subsystems. Some focused analysis on fluid dynamics and power was conducted and described next.

Mechanical Considerations

Pump Flow Rate Feasibility

To determine if the chosen fluid pump will meet the requirement of moving fluid through the torch network and into the torch fuel tanks this calculation will calculate the maximum pump head required including frictional losses in the tubing system. Since the tubing is flexible we will make the assumption that the tubing is 40 feet long in a straight line and has a diameter of one eighth inch, and ignore frictional losses in curves .After travelling through the tubing system to the torch, the fluid must be transported 6 feet vertically up the torch base at a minimum rate of approximately 0.013 gallons per minute to sustain the flame.

As a result the pump in the base of the torch must have a total head value of at least 6.004 feet (1.83 meters) to allow the last torch in the series to function properly. Our best candidate at this time has a head value of approximately 2 feet per the pump's specifications, this means the pump we had originally intended to use will not be sufficiently powerful to run the torch system under these conditions. Please note this model is very rough and the conditions are heavily subject to change in future analysis.

Electrical Considerations

Sensor Feasibility

Concept 5 and 6 from the concept selection in the System Level design showcased using an ultrasonic sensor or a capacitive touch sensor would best meet our requirements for the project. Using the products benchmarked for the sensor selection, it was apparent that off-the-self ultrasonic sensor within our price range would not meet the range needed for accurate measurements and would be difficult to mount. As a result the team is looking into potentially redesigning the head of the torch using machinery on campus. This will need to be further discussed and decided upon in the next phase. In the meantime, Seeed Technology's Capacitive Water Level Sensor seems like a promising alternative that would be easy to integrate into our system. After some investigating, the team discovered it would be possible to redesign the product to fit our needs. Overall, it seems it would be better suited for our project with the current torch materials we have. More information on redesigning process of this sensor is described in the next section.

Power Feasibility

Since the System Level Design phase, the team has had to revisit some concepts selected surrounding the valves in order to proceed with power subsystem development for the individual torches. Upon review of the team's system level design, the client updated the team with his exploration in using a mechanical float to control the refilling of the individual torches with fuel. The team has taken the concept into consideration and focused on implementing it into their own system, thereby reducing reduce the amount of power required from the solar panels for each torch. At this time, the team has decided to move forward with prototyping a system without an electromechanical valve and understand the risks associated with doing so include potentially redesigning the power subsystem if a valve would need to be added down the line.

With the new design in mind, using the power information concluded from the Voltage Power and Current Compatibility table in Section 2: Prototyping, Engineering Analysis, and Simulation, a small power sub-system was developed as seem in the block diagram below.

The red lines represent the positive/Vcc leads and the black lines represent the negative/ground leads.

Five major components make up the solar powered subsystem: solar panel, charger, LiPo rechargeable battery, voltage regulator, and the load(s). The loads taken from the Voltage Power and Current Compatibility table in Section 2: Prototyping, Engineering Analysis, and Simulation were used to understand exactly how much power must be generated from the solar panels in order to operate which was approximately 220 mWh or 62 mAh. Looking at the specifications for the microcontroller, level sensor, and transceiver, it was noted that the operating voltage for the subsystem needs to be around 3.3V. After lots of searching only one rechargeable coin cell battery was found to fit the requirements. However, the battery was not ordered due to the fact it is an obsolete part and might jeopardize our system design later on. Therefore, a 3.7V LiPo rechargeable battery was implemented into the system design and ordered along with a voltage regulator. LiPo batteries are easy to come by and made into many shapes and sizes making it an ideal choice for the design. It should be known that the chosen voltage regulator also requires two different sized capacitors to operate, one 100nF and one 10uF capacitor. Next, the charger was chosen. Adafruit's Universal USB / DC / Solar Lithium Ion/Polymer charger was integrated into the design, since it is small and has a variety of ports and pinouts making prototyping easy.

The final component selected were the solar panel cells. The first couple parameters considered was the size and voltage a the maximum power point. This value should be relatively high (between 5-12V), so our team can ensure it will meet the demand of our circuit. The maximum power generated from this cell is 132.3mW in full day sun. According to the graph from the data sheet below, overcast skies can cause the power generation to decrease dramatically as it follows a logarithmic scale.

Assuming that part sun reduced the power generated logarithmically to 13.23mW and only 8 hours of daylight is received per day, a single IXOLARTM High Efficiency SolarMD cell (SM500K12L) could only produce approximately 106mW per day. This calculation was evaluated simply by multiplying the power generated in part sun by the hours of available sunlight. Based on the 0.22 Wh need from the loads, approximately three of the solar panels selected will be needed to power the torch.

The table below tabulates the components, quantities, output voltage/current, and the datasheet for each of the components selected for prototyping to further demonstrate feasibility.

Relevant Files