Solartherm was founded in September of 2012 by University of Nevada Reno students Chris Aguilar, Adam Brown, Pete DeLosa, Nate Josefowicz, and Luis Pavia. The group had worked together on a variety of project prior to the capstone course and it seem fitting that they work together on their senior project. The team has a variety of backgrounds and interests, but the common interest that they shared was a desire to do something related to renewable energy. Pete Had just returned from three months of traveling and living out of his pick up truck. He suggested that the group develop a way to use the energy from the sun to preserve food so that people on extended travels might not have to rely so heavily on the availability of ice. The end result is the Solar Cooler, which can maintain an internal temperature of 5 C in a 35 C environment. This temperature doesn’t eliminate the need to put ice in the cooler but it does extend the life of that ice by several days. For more information about the design process of the Solar Cooler please click on the links above.
Many people spend time traveling and enjoying a variety of outdoor activities, particularly during the summer months. These people are often faced with the challenge of taking food with them to these activities and keeping that food from spoiling on longer trips. The most common solution to this problem is to put food in an ice chest that can be taken along. This solution is simple and generally effective, but what about extended trips? Even the best ice chests can only keep ice for 4 or 5 days during the hottest part of the summer. Water from melted ice seeps into food packages and ruins food. Running out of ice also leaves the user’s food at risk to spoil, and getting more ice isn’t always convenient. What if you were camping a couple hours from civilization when you ran out of ice?
In order to evaluate the success of the project the team came up with some goals that they should try to obtain with the final prototype. Those goals along with the engineering specifications are listed below. The team was able to satisfactorily meet all of the goals and in some cases excede them.
- Provide enough refrigeration to extend the life of ice in an ice chest by at least 24 hours
- Provide this refrigeration through a means that is self-sustained, i.e. solar power.
- Keep the system as easy to use as an everyday ice chest
- Minimize the amount of weight the user has to lift at one time while using the system.
- Make the final product aesthetically pleasing and safe for the user.
- Maintain internal temperature no greater than 5° C
- Produce at least 50 watts of power
- Less than 20% loss of ice chest capacity
- Require the user to lift no more than 50 lbs. at a time
- Maintain exposed hot surfaces below 110° F
EVOLUTION OF THE DESIGN
The Solar Cooler was created to reduce the problem of running out of ice on longer trips. The original design used an ammonia absorption cooler similar to those used in RVs. The idea was to replace the propane burner that normally powers these coolers with a solar thermal collector. As the team began researching it was discovered that the gas absorption system was very likely to be well outside the budget of the class. Also, a solar collector large enough to provide the heat required to the boiler of the gas absorption cooler would be much to large for the system to be portable. If the system isn’t portable then it isn’t really a solution to the problem.
How it works
1 Heat is applied to ammonia and water solution in the boiler.
2 Ammonia separates from water as it reaches boiling temperature, the separated water flows into the absorber.
3 Ammonia enters the condenser, where it loses heat and turns into a liquid.
4 Ammonia enters the evaporator to be mixed with Hydrogen. Due to their difference in pressure the overall mixture pressure is dropped, this creates a cooling effect in the refrigerated space.
5 The Ammonia and Hydrogen gases flow into the absorber, where they are mixed with the separated water from step 1.
6 Ammonia mixes with water and releases hydrogen, which flows back to the evaporator.
7 The new water and Ammonia solution flows back to the boiler, where the cycle will be repeated.
The Solar Cooler was redesigned and the gas absorption system was replaced with a thermoelectric module (TEM), and the solar collector was replaced with a photovoltaic (PV) panel. The PV panels were built into a separate collapsible unit that when collapsed can be easily carried by its convenient carrying handle and when open can be used to provide shade for the ice chest itself. The TEM draws 4 amps at 10 watts and can maintain an internal temperature of 3 degrees C. This temperature is equivalent to that found inside any home refrigerator. Ice is still required for the Solar Cooler though, since it won’t be able to run at night. Ice will still melt in the Solar Cooler but very slowly.
The basic function of a TEM is shown in the figure below.
Thermoelectric Energy Flow
The figure shows the basic energy flow for a thermoelectric heat pump. Power is supplied to the the TEM creating a temperature gradient between the two sides. Heat is then absorbed by the cold side and rejected by the hot side.
For the final prototype Solartherm uses two 50W PV panels that can be easily collapsed into a custom designed carrying case for transportation and quickly set up when ready to use.
The Cooler portion of the ice chest is simple to construct. The cooler is purchased separately and a hole is cut in the lid of the cooler. The hole needs to be 5” x 5” for the TEM to be inserted into and sealed to the cooler. Once the TEM is in place, the cover is mounted to the TEM using the holes provided in heat sink mounting bracket. The fan and TEM are wired with both hot wires (both Red) connected to Hot port on plug. The process is repeated with the common wires.
The main construction of the Brief Case frame will be made of 16 GA galvanized sheet metal. The parts are designed in SolidWorks and a flat pattern is produced and imported into a punch CNC machine. After the sheet metal has been punched by the CNC machine each part will need to be broke according to the angles given on the engineering print. Each part is then welded as shown on the assembly print to make the rigid frame. Piano hinge, the length of the frame, is riveted to the flat stock on the bottom of the frame. Once in place one of the solar panels is placed in the frame with the PV side up. The one side of the solar panel next to the piano hinge is then drilled and riveted to the hinge. Sliding track is then attached to both sides of the one solar panel. The Tracks are attached (via screws) as far away from the piano hinge as possible to minimize the overlap of the top solar panel. The two pistons are then attached to the pre-punched hole in the side of the case and to the drilled hole in the top of the solar panel as shown in the engineering drawing. Once the tracks and pistons are mounted the top solar panel can be attached to the sliding portion of the track. Last of all the handel is bolted to the top of the case and the pins are inserted to lock the panels in the stowed position.
Wiring the solar panels to the controls is the final step. A charge controller and Pules Width Modulator (PWM) are used to regulate the voltage and to properly charge the battery. The battery is used to supply a constant steady voltage and current to the TEM. The controller is mounted on the bottom of the case. The panels are wired in parallel to keep the voltage the same but increase the amperage. The Solar panels will be wired to the PWM. The Charge controller is wired to the output of the PWM and then wires are run from the Charge Controller to both the battery and the TEM. A 12 foot extensions cord is used to bridge the distance the cooler is from the Brief Case.
TESTING & RESULTS
The first test was conducted on the ice chest prior to inserting the TEM. A 4.54 kg block of ice was put in the ice chest, and it was placed outside for several day. Over the test the daytime high temperatures were between 50 and 60 degrees F. After 139 hours only 0.276 kg of ice remained. During the trial the ice melted at about 0.03 kg/hr.
Based on calculations done by the team, 3 degrees celsius is enough to extend the life of 1 kg of ice by about 4 days. The target ice life extension was 1 day, so we can say that the Solar Cooler project was a success. The user of this product would not need to put any ice in it at all during the day. It is however, still recommended that ice be used in conjunction with the TEM in order to ensure that food is preserved over night while the sun is not powering the Solar Cooler.
The Solar Cooler is able to stay well below 5C. The fact that it works indicates that at least 50 W is being delivered to the TEM. (This is how much is needed to run the TEM.) The volume to the TEM inside of the ice chest reduces the total capacity by less than 5%. The solar panel array weighs in at 43 lbs. No surface on the outside, including the PV panels is too hot to place a hand on. All specifications set by the design team have been achieved. Therefore, the Solar Cooler is considered to be a complete success.
Park, C. Intermediate Heat Transfer Lab: Thermoelectric Heat Pump. University of Nevada, Reno: 2012.
Gas-Refrigerators (website). Retrieved from