As an application of Thermal Energy Storage, the freezer in the ice cream bus will be engineered to store energy overnight without the use of a battery, significantly cutting down costs. Instead, a sizable amount of ice can be used to maintain a below-freezing temperature in the freezer to keep the ice cream from melting. The concept used is the constant temperature that is maintained when ice undergoes a phase change into water. If the phase change point can be brought to the ideal temperature of ice cream, the same temperature will be maintained within the freezer until the ice completely turns into liquid. Such a method implies the ice cream will maintain its light texture even without solar power at night. The phase change itself can take up to more than ten hours which is ample time to maintain an ideal temperature through the night when atmospheric temperatures are relatively low as well. So long as insulation is optimized to prevent a significant amount of heat from entering, ice will serve as a better alternative to batteries in keeping a freezer at ice cream temperature through the night.
Design for the Ice cream Bus
We envision a simple circuit where a solar panel will provide voltage (~72V) that will be converted to power a freezer located inside a bus. While simple in idea, there are multiple factors at play. The first step after harnessing solar power energy involves regulating the voltage and current received, a process that will be performed by a charge controller. The charge controller will act as a DC to DC converter (accepting up to 100V) and regulate the flow of charge to safe amount of 9 Amps for a standard 12V deep cycle battery. There will be another connection from the charge controller to an inverter which will cause 12V of DC to be converted into 120V of AC. A switch and fuse would be placed between that connection to power the device on or off and to prevent damaging the inverter with too large current flow respectively. The final step is connecting the inverter to the AC motor in the compressor of a freezer. The process should result in a simple, but cost-effective solar-powered freezer.
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| A schematic of how a solar panel can supply power to a freezer. |
Since one of the goals of this project is to reduce cost, we decided to spend more money on the charge regulation aspect of the schematic while taking a cheaper route on the inverter. A better quality charge controller ensures more safety between the output of the panel and input into the battery. As seen below, the charge controller is easily able to connect to a 72-Volt 435-Watt Solar Panel and is ready to send 9.1 Amps to a load (represented by the light bulb).
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| The EPEVER Solar Charger 40A is able to handle up to 100-Volts of input. |
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| Mike and Wilson connect the Solar Charger to the SunPower E20 Panel and 12-Volt car battery |
With the arrival of the inverter, the next step of the project was to set up the electronic system itself. A 12-V battery was connected to the battery outlet in the charge controller while the relay (the black cube in the center of the panel below) was connected to the load outlet in the charge controller before it went through the inverter. The purpose of installing a relay was to have the system shut off automatically, as opposed to having someone turn the system off at night to prevent the solar panels from excessively draining the batteries and ruining the charge controller. The system was a success because as soon as night fell, the charge controller was disconnected as seen below.
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| This panel contains a mounted charge controller, inverter, and relay all used to operate the freezer in the ice cream bus during the day. |
With the electrical system functioning, we moved on to installing the rack itself. Dr. Heston and Wilson were working on extending the rack that came with the bus in order to provide the panel with enough support.
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| Wilson and Dr. Heston install the rack for the solar panel. |
Here the panel is held up at one of the four angles it was designed to lock in place.
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| A front view of the solar panel installed on the bus. |
As a more thorough explanation of the racking system, the two bars hold the rack into place with pins at each end locking each bar from moving or sliding down. When the panel needs to be adjusted, we simply take out the pins, slide the bars to the next level and lock the bars into place with pins in another position.
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| Wilson demonstrates the racking system for the bus. |
In this final image, Adeel and Wilson showcase their work.
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| Adeel and Wilson pose by their finished project with the ice cream bus. |