The Insulated Solar Electric Cooker (ISEC) is a double walled Aluminum pot with a resistive heater directly connected to a solar panel whose goal is to create and disseminate cheap solar cookers in rural areas that primarily rely on biomass for cooking. Phase Change Materials (PCMs) like Erythritol, a sugar substitute, take a tremendous amount of energy to melt, and when they solidify they release the energy. Through the use of PCMs, the ISECs will produce enough heat to cook food even after the sun sets. However, PCMs like Erythritol degrade over repeated heat exposure, so the purpose of this experiment is to evaluate its viability for the ISEC

Gamoji, Sudheendra

English

DigitalCommons@CalPoly

 Gamoji  1  Thermal Degradation of Erythritol  Sudheendra Gamoji  California Polytechnic State University, San Luis Obispo  PHYS 461, 462  Dr. Peter Schwartz  Nov 23, 2022  Gamoji  2  Outline  ISEC Project ……………………………………………………………………………………….3  Phase Change Materials and Erythritol ……………………………………………………………4  Initial Design ………………………………………………………………………………………5  Data ………………………………………………………………………………………………..6  Analysis …………………………………………………………………………………………..10  Data from the Differential Scanning Calorimeter (DSC) ………………………………………..11  Conclusion ……………………………………………………………………………………….. 12  References ………………………………………………………………………………………..13  Gamoji  3  ISEC Project  The Insulated Solar Electric Cooker (ISEC - see Figure 1) is a double walled Aluminum  pot with a resistive heater directly connected to a solar panel. The goal is to harness the power of  the sun to reliably cook food and to ultimately disseminate in areas in the world that still  primarily rely on biomass––coal, wood, etc––cooking, such as rural India, sub-Saharan Africa,  etc. It can be built for under $100, but requires insulation to better contain heat. The project was  started to alleviate the environmental and health burdens arising from biomass cooking in rural  areas, where it is estimated that around 3 billion people in the world use biomass for daily  cooking, and 4 million die from health complications due to it annually.  1  Figure 1: Diagram of ISEC with Phase Change Material  The device,  hooked to a 100 W solar panel,  can boil 1 L of water per 1 hr of sunlight, and  therefore can use the boil-and-simmer method, it can cook about 5 kg of food in a day.  1  In order  1  Osei, M., Staveland, O., McGowan, S., Unger, J. B., Christler, N. R., Weeman, M., Strutz, M. E., Walker, M., Maun, M. B.,  Dunning, N. C., Bekheit, M. M., Abraham, J. C. P., Cox, L., Gius, G., Hansel, O., Amoafo, E. O., Hugo, N., & Schwartz, P.  (2021, April 26). Phase change thermal storage: Cooking with more power and versatility. ScienceDirect.  Gamoji  4  for the ISEC to completely replace biomass cooking, it needs to produce power at night as well,  which is achieved with Phase Change Materials  Phase Change Materials and Erythritol  Phase Change Materials (PCMs) are substances that can absorb or release energy when  transition phases (melting absorbs energy, solidifying releases energy). The operating principle  revolves around a PCM’s latent heat of fusion. As it approaches its melting point, the material  will undergo an endothermic process, and the absorbed energy will remain in the liquid PCM  until it begins cooling. The cooling process is exothermic, so when the PCM is changing phases  from liquid to solid, it will release all the energy building in its system. PCMs are characterized  by their high heats of fusion, so they can provide more heat than materials with lower heats of  fusion. Since the ISEC solely relies on solar power to cook, it is only viable during daytime.  Since most communities across the world have at least one meal towards sunset/nighttime, PCMs  step in and allow the ISEC to harness the power of the sun even after it sets. Latent heat storage  allows the ISEC to produce greater power and cook for longer periods.  Erythritol (ET), the focus of this paper, is a sugar alcohol with a Latent Heat of 315 J/g,  and its melting temperature is around 121℃. Organic PCMs are great sources for thermal  storage, but repeated use begins to degrade the material. As ET degrades, its melting temperature  will drop, and there is a decrease in the heat of fusion and it will turn from clear/off white to dark  brown/black, as shown in Figure 2. Once the melting point decreases below 100 ℃, the PCM  has limited value to cook food.  Latent Heat  Melting  Temperature  Heat Capacity  Thermal  Conductivity  315 J/g  121 ℃  2.87 J/g*K  0.33 W/m*K  Table 1: Erythritol’s properties  Gamoji  5  Figure 2: (left) fresh sample of Erythritol, melted for the first time  (right) degraded sample of Erythritol after months of cycling  Initial Design  This project was largely conducted during the COVID-19 pandemic, where access to Cal  Poly’s lab and its materials was restricted or prohibited due to the nationwide lockdown. Most of  the materials and instruments used for the project were purchased from Amazon, and  experiments were conducted in a garage.  The primary design involved using 4 glass beakers with nichrome wire wrapped around  the length of it. Each beaker was filled with 200g of ET with thermal switches and fuses ranging  from 120℃ to 180℃ in 20℃ increments. A 12 V DC Power Supply was connected to each  beaker, with the resistance of the nichrome wire measured around 3𝛀. The goal of the  experiment was to cycle the same amount of ET at different temperatures and observe its rate of  degradation as it changes phase repeatedly.  Gamoji  6  The second design involved using 4 stainless steel beakers (to spread the heat more  evenly) and ceramic resistors (three 1𝛀 resistors connected in series per beaker) with Four 12 V  Power Supplies. The third design involved 4 stainless steel beakers and pieces of resistive  heating coils, connected in parallel to a 240 W DC Power Supply set to 12.6 V and it was  surrounded by thick layers of rockwool for insulation.  Data  Figure 3: Data recorded over 160 hrs shows phase change around 112℃  Figure 4: Same beaker from Fig 3, 1 month later, shows phase change around/lower than 110℃  Gamoji  7  Data collection proved quite difficult due a variety of complications. The goal was to  keep the beakers heated between 3 days to one week with continuous heat and then observe how  many times the ET achieved phase change, which was partially achieved in Figures 3 and 4.  Figure 3 shows data collected from March 1 to March 8, 2022 from a beaker whose thermal  switch was set at 160℃. Throughout the graph, around 112 ℃, the graph hits a flat section  where the blue line is bolder, indicating that the beaker stayed at that temperature for longer than  others in the same range, which is a sign that the Erythritol is undergoing phase change. One  month later, in Figure 4, data collected from April 1 to April 6, 2022 from the same beaker  shows a slight drop in the melting temperature, as the phase change line is now at the bottom end  of the temperature range, signifying that the since the melting point could have lowered or that  the sensor did not register phase change and got shifted, the temperature switch range must also  be lowered to more accurately capture the phase change temperature.  Due to the lockdown, the first 7 months of this experiment was conducted remotely. The  first design, featuring glass beakers and nichrome wire, could not adequately spread heat evenly  so some parts of the ET melted quickly while others remained solid, and since the temperature  sensor was attached to the side of the beaker the thermal switch triggered too early. Since glass is  a poor conductor of heat, when the nichrome wire’s temperature rapidly rose (peak temperature  was around 250℃ to 300℃), the glass began to crack due to prolonged exposure. After four out  of eight beakers cracked and spilled hot ET onto the labstation, the second design was adopted.  The second design, featuring steel beakers and ceramic resistors, provided more  promising results, but also exposed more problems. The power supplies were not intended to be  used continuously for days on end, so they began to malfunction due to a rise in the power  supply’s internal temperature. Eventually, they stopped conducting any current at all because the  Gamoji  8  circuits in the block had completely burnt and the plastic container housing the circuit had  partially melted. In addition, the ceramic resistors were also cheap and of poor quality, so they  too stopped conducting current after a few weeks.  Figure 5: A skewed data set where the thermocouple was sitting too close to the nichrome wire  Figure 6: Another example of skewed data  The third design, featuring a 240 W power supply and resistive heating coils, was by far  the most effective design, although it still posed a lot of problems regarding data analysis. The  heating coil was difficult to cut and shape, so any adjustments were difficult to make. The  Gamoji  9  thermocouple, now submerged into the beaker of ET, would read skewed data because of the  way the coil wrapped partially around the beaker (one side was hotter than the other), as shown  in Figure 5, where the upper temperature (which should be 160℃) climbs near 170℃ and the  lower temperature (which should fall to around 110℃) rests around 157℃. Figure 6 shows  another example, where the maximum rests at 130℃ and the minimum (the graph flattens,  signifying phase change) rests at 100℃, but both bounds should be about 20℃ higher.  Since most of the recorded data was unusable, towards the end of the project the  apparatus was modified for an alternate approach to data collection: measure the current melting  points of each beaker, after several months of temperature cycling (August 2021 to May 2022)  and compare them to the initial melting point. However, the data collected showed extremely  erratic spikes that couldn’t be explained by physical phenomena, as shown by Figures 7 and 8.  Figure 7: ET in three beakers was heated to 170℃ on a hot plate and left to cool  Gamoji  10  Figure 8: ET in a beaker heated to 150℃ and left to cool  Figure 7 shows three beakers (differentiated by red, blue, and green) whose temperature  shows drastic rises and drops within seconds of each other, which leads to the conclusion that the  data collection was flawed in some way.  Possibly, the thermocouples were faulty and not viable  for further use. Figure 8 shows a gradual temperature decrease with no phase change plateau,  followed by a sharp rise in temperature and then a gradual decrease with no phase change  plateau. Since the thermocouple was submerged into the melted ET, it likely shifted positions as  the ET solidified, which could explain the sharp rise, but the cause remains unidentified.  Analysis  Over the course of 9 months, four steel beakers, each filled with 200g of previously  unused Erythritol, was cycled at 120℃, 140℃, 160℃, and 180℃. Throughout the course of the  experiment, the data collection was inconsistent and difficult to analyze. Through the limited  Gamoji  11  data that was usable, from Figures 3 and 4, the phase change plateau clearly shows a decrease in  the melting temperature of ET, which indicates that its capacity for thermal storage dropped.  Data from the Differential Scanning Calorimeter (DSC)  Differential Scanning Calorimetry is a technique that measures the heat a sample takes  in/gives off when it is heated, allowing the determination of things like recrystallization energy,  latent heat, and specific heat. DSC is particularly useful in observing phase transitions; as a  material begins transitioning to a different phase, more or less energy needs to go into the system  to keep it at the same temperature to complete the transition. The amount of heat flow depends  on whether the process is exothermic or endothermic.  Figure 9: DSC results of Erythritol samples at varying degrees of degradation  2  Figure 9 shows the phase transitions of Erythritol samples of varying degrees of  degradation as they are taken from a temperature range of 25℃ to 180℃. The horizontal axis  2  Data obtained from Daniel Nagy, a researcher on the ISEC team  Gamoji  12  displays data in ℃, while the vertical axis shows data in Power/mass [W/g]. This metric  describes the amount of energy spent in a given amount of time per gram of Erythritol.  Erythritol’s cooling process is exothermic, as is visible in graphs blue, purple, and black. The  melting point for the blue and purple samples seems to rest around 110℃, while the melting  point for the black sample shows the melting point around 125℃.  The blue and purple samples  (much more degraded than the black sample) also release less energy  ( the intetraged area undr  the curve) than the black sample in their solidifying process. Erythritol’s degradation, therefore,  reduces its energy output.  Conclusion  Though temperature regulation and accurate data collection became significantly  difficult, some data provided promising results. The data from Figures 3 and 4 suggest that from  August 2021 to May 2022, ET’s melting point lowered from 121℃ to 108℃ (estimated). The  two figures show consistent flat sections around roughly the same temperature, indicating that  the sample went through phase transition or reached its melting point. Physically, it transformed  from a clear to black solution, signifying that the sample physically degraded. Based on the  observed rough rate of degradation, Erythritol is not a viable option for phase change in the  ISEC.  Gamoji  13  References  Osei, M., Staveland, O., McGowan, S., Unger, J. B., Christler, N. R., Weeman, M., Strutz, M. E.,  Walker, M., Maun, M. B., Dunning, N. C., Bekheit, M. M., Abraham, J. C. P., Cox, L.,  Gius, G., Hansel, O., Amoafo, E. O., Hugo, N., & Schwartz, P. (2021, April 26).  Phase  change thermal storage: Cooking with more power and versatility . ScienceDirect.  Watkins, T., Arroyo, P., Perry, R., Wang, R., Arriaga, O., Fleming, M., O’Day, C., Stone, I.,  Sekerak, J., Mast, D., Hayes, N., Keller, P., & Schwartz, P. (2017). Insulated Solar  Electric Cooking – Tomorrow’s healthy affordable stoves?  Development Engineering ,  2 ,  47–52. https://doi.org/10.1016/j.deveng.2017.01.001 

Thermal Degradation of Erythritol

https://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1240&context=physsp

Thermal Degradation of Erythritol

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