Weekly Progress

Week One

Our group was set up in a random meeting of interests. We decided to work on novel phone chargers. We started brainstorming and we scratched through a few ideas before the class time was over. Later that evening, on a bus ride back home one of the members hatched the idea of a solar panel powered, refrigerated bag - and thus was the genesis of our project.

The Group In its First Meeting

Week Two

We have finally devised an efficient way to make the refrigerator. Now, it is time to advance from the theoretical side of the project to the practical parts. We still have some unresolved issues with the cooling system. One thing we are not sure about is the total power required for the entire system to function smoothly.

Week Three

At this moment, we are looking for the potential battery for the project. We looked out for three options like lithium acid, lithium polymer, and AAA batteries. We researched and found out some facts that lithium acid is not safe and are hazardous to use in specifically in a backpack. Additionally, AAA batteries are exorbitant in price and the system discharges at a rapid rate because, in the backpack’s system, the mechanism requires a high current draw. To sustain the system, the battery requirement can be fulfilled by the option which is financially viable, environmentally sustainable and long lasting. So the best option for the project is lithium polymer because the battery meets all the need for the system and has highest energy density amongst it’s competitors.

Week Four

One of the major roadblocks of the project was to create the cooling unit. The backpack must be equipped with a sustainable system which would lower the temperature within the refrigerator chamber to keep the products chilled. The team started brainstorming on the ideas for refrigeration and the first thing that came up mind was a compressor. Although one of the teammates came up the idea that fitting a compressor unit within a 10x6x6 inch compartment is unfeasible. Additionally, the compressor system consists of high-pressure air intakes which are not safe. Furthermore, the components of the compressor system are hazardous for our environment.

After thorough research, the only viable system that fitted the backpack is a thermoelectric cooling unit. Thermoelectric coolers are available in small sizes. They are environmentally friendly and it is considerably cheaper compared to a compressor cooler. The only drawback is that it consumes more energy than a traditional compressor. The reason being thermoelectric coolers operate constantly while compressor coolers cycle on and off as necessary to maintain interior temperatures. Not only does the continual power draw mean higher energy bills, the constant use means parts are likely to break down and need replacement sooner.

To tackle this issue, the team decided to add a digital thermometer to the cooling chamber. As a result, the user will know when the temperature inside is low enough, following which the user will turn the system off. The interior of the chamber will be completely insulated, which will maintain the temperature inside even after the system is turned off.

Thermoelectric Cooling Unit

Peltier Unit

Working of the Thermoelectric cooling unit

Week Five

The backpack requires specific insulation for optimum performance of the cooling system. Since the backpack original fabric is unable to satisfy the needs, the team looked out for specific options which can be apt for providing insulation. In order to match this need. The team started looking for potential options which are cost efficient and not thick. The thickness of the fabric was an important thing to consider as it will increase the total weight of the bag. The team came up with several materials like aluminum sheet, aluminum mylar sheet and quilted fabric. We tried specific combinations of the materials and the best worked out for us was aluminum mylar sheet and quilted fabric. Since Aluminum insulates around 50% of the heat whereas aluminum mylar insulates nearly 100’’5. Henceforth, by using aluminum mylar and quilted fabric, we were getting optimum efficiency. Therefore, we chose them to insulate our backpack.

Aluminized Mylar Sheet

Quilted Fabric

Week Six

By midway, the team decided to work on their cooling unit. The cooling unit comprises a Peltier cooling unit, one 2000 RPM & one 3000 RPM fans for throwing hot and cold air, heat sync and a power source.  The main idea of the team was to construct a unit which comprises a Peltier unit sandwiched by two heat syncs. The heat sinks is a conductive material to draw in heat and cool respectively. The hot side has large heat sync and the cool side has smaller heat sync. The hot side also has a 12V 0.2 A fan attached to it which acts as an exhaust to cool down the other side. The exposed gaps between the adjacent heat sinks after placing the peltier unit in the middle is covered by Styrofoam in order to prevent unwanted conduction. The team tried several configurations for the system by shifting the positions of fans with different RPM.  In one configuration, the higher RPM fan was placed on the cool side of the Peltier unit. This configuration was not effective because the heat was not blown out in a proper manner which was increasing the temperature of the system. This also increased the discomfort for the user as the back of the user may be heated up. The team figured out the solution for the problem and decided to keep higher rpm fan on the hot side of the Peltier for the proper exhaust system. This configuration worked in the experiments and provided the optimum user experience. By this point, the team was almost done with the cooling system. 

Week Seven

The team tested the cooling unit which comprised of Peltier cooling unit. After several experimental running and upgrading the system, the team figured out the main issue related to the system. The problem was related to the RPM of the fans. After switching the positions of the fans, the system was not up to the expectations. The system was cooling the chamber in 3 times of the desired cooling time (The desired temperature 12-16 °C).The team faced the problem with the RPMs of both the fans and finally decided to use 10,000 RPM fans for both the cooling and exhaust unit.  Furthermore, the efficiency of the product could be increased by 30% by changing some components like heat sync and power cables. The team decided to switch heat sink which increased the efficiency. Another reason for switching heat sync was that the previous sync was not as effective at conducting as the team had expected.

Week Eight

By this time, the team decided to design cooling compartment. The main aim for the team is to construct a light weight compartment which is properly insulated and can be fitted inside the backpack. For making compartment, the team looked through several options like aluminum, Styrofoam or Cardboard. Aluminum could hurt the user if the bag is torn and Styrofoam could be easily broken. The team finally decided to make the frame of the compartment using cardboard with high tensile strength. Although using cardboard has one drawback: Insulation. The team started looking solutions for the insulation. From their research done in Week Five, the team decided to insulate the compartment with a quilted fabric layer sandwiched between two Mylar sheets. The team used this composition to minimize the chance of leakage and lack of insulation. 

Insulation 

Cooling Compartment

Week Nine

By this time, the team started building the main circuitry of the system.  Each component including solar panels, cooling unit and power source was connected and team experimented with the parts. After few testing’s, the team researched to increase efficiency and the way they choose was using a voltage booster circuit. 3-32V to 3-35V DC Voltage Booster circuit was used with battery output to increase the efficiency of the Peltier unit - to cool the system quicker. The outputs of the battery were soldered to the booster circuit.

Testing of the Product

Week Ten

By this time, all the individual components of the project was working as expected. This was now the time to bring all the components together in the form of a utility backpack. The solar panels were fitted to the back of the bag and the charging circuit was set.The cooling chamber was placed inside the backpack and the product was now ready for presentation.