The current model utilizes an erythritol PCM, but its usage only spans over four to six months because of degradation. One design change for more feasible solar cooking lies in the phase change material used.
Most stoves last between 13-15 years, so the demand for a six-month maximum cooker is low. Changing the phase change material to one that degrades more slowly over time raises the demand of insulated solar electric cookers.
Sodium Nitrate and Potassium Nitrate
One possible phase-change material is a film made up of of 60% sodium nitrate (NaNO3) and 40% potassium nitrate (KNO3) [1]. This mixture is a eutectic material with a melting point of 220 degrees C and a latent heat of fusion of 110.0kJ/kg [1]. Another common eutectic material replaces sodium nitrate with sodium nitrite (NaNO2) at 53% keeping 7% sodium nitrate. This changes the melting point to 145.2 degrees C and a latent heat of fusion of 101.5kJ/kg [1].
The sodium and potassium nitrate mixture can be doped with different nanofluids to change both the latent heat of fusion and the phase change temperatures while only slightly altering the melting point [2]. Silicon dioxide (SiO2), aluminum dioxide (Al2O3), and titanium dioxide (TiO2) may be used for this purpose [2].
Silicon dioxide and aluminum oxide can change the latent heat of fusion to 126 kJ/kg and 127 kJ/kg respectively, while keeping the phase change temperature from 220 to 230 degrees C [2]. Combining the two lowers the phase change temperature between 209 and 223 degrees C [2]. Titanium dioxide changes the latent heat of fusion to 115 kJ/kg [2]. The highest possible melting temperature reaches up to 250 degrees C, but due to the variable nature of the PCM, this is not a resting temperature.
Advantages
One advantage of the Sodium Nitrate and Potassium Nitrate PCM is the high melting point between 220 to 250[2]. A higher melting point means that it will cook at higher temperatures. This means that foods can cook faster and with greater efficiency because of the Carnot efficiency equation.
Potassium Nitrate is classified as an oxidizer and a skin irritant and an eye irritant, but is not considered toxic [14], Sodium Nitrate is classified as an irritant, but is considered toxic because the LD50, or lethal dose of 50% of the population is 3g/kg [13]. However, to put this into perspective, table salt has an LD50 of 3g/kg as well and is consumed in cooking [15]. Sodium Nitrate is also used as a food preservative [13]. Sodium nitrite also has an LD50 of near 3g/kg (2.6g/kg), and also acts as a preservative [12]. Therefore, none of the ingredients are toxic.
The varying nature of the PCM with the different dopants and varying latent heat of fusions, changes to the manufacturing can be made quickly. Different dopants could be used to quickly change the latent heat of fusion and the melting points per manufacturing specs. This allows for specialized cooking procedures.
It has an extremely high thermal stability at 590 degrees C, which means that it has high durability and low degradation[17].
Disadvantages
While none of these materials are toxic to humans, sodium nitrite is classified as an environmental hazard to marine life. Therefore, if this is mass-produced and the 53% sodium nitrite is used, then it would pose a potential hazard to aquatic life.
The highest possible latent heat of fusion that can be produced at 127 kJ/kg is still lower than that of erythritol at 167 kJ/kg [1]. Therefore, the temperature fluctuation will be much higher than that of erythritol because the phase change occurs much more quickly.
D-Mannitol
D-mannitol is a sugar alcohol that is used in medicines that could work as a phase-change material [1]. The melting temperature is about 167.0 degrees C and the latent heat of fusion is between 263kJ/kg and 268kJ/kg[3].
D-Mannitol has polymorphic phase properties, which can undergo different polymorphic phase transformation. This essentially describes the polymorphic state that D-Mannitol is in [3]. The "thermal properties" at different polymorphic transformations do not remain "the same" [3]. There are four different phases of D-Mannitol: the α-Phase, the β-Phase, the γ-Phase, and the δ-Phase. The γ-Phase would not be viable because it "only appears in water solutions" [3]. The δ-Phase and the α-Phase are metastable, while the β-Phase is stable [3].
When testing the PCM, the δ-Phase and the β-Phase were present, but the α-Phase is not. The melting points of the D-Mannitol were shown at both 167 degrees C and 155 degrees C, so these are the two viable melting temperatures for D-Mannitol [3]. The latent heat of fusion remains the same because the material is the same. The temperature phase change range remains between 130 degrees C and 170 degrees C [3].
Advantages:
According to the National Institute of Science and Technology, as of January 11, 2016, the "Standard Reference Material," or the amount used for studying as a standard value, or 50 grams, is not classified as either a "Physical Hazard," or a "Health Hazard" [16]. It is still classified as an irritant, but this shouldn't pose a problem [16].
The Latent heat of fusion is a maximum at 268kJ/kg [3] in the stable β-Phase throughout polymorphic phase transformation. A high latent heat of fusion means that the temperature during the phase change does not change too quickly.
The polymorphic phase properties of D-Mannitol allow for varied temperature changes. The phase change range is 40 degrees C between 130 and 170 degrees C [3]. There are also two spikes at different temperatures for each phase at 167 degrees C and 155 degrees C where it maintains a single phase. This allows for both concrete temperature values, and an effective latent heat of fusion at 268kJ/kg. The most stable phase, the β-Phase retains this for longest meaning that there is a high phase change range, but definite locations of both metastability and stability [3]. This is the best of both worlds meaning that there is a high phase change range, so the heat of the stove lasts longer, and a point where it holds a constant temperature for consistency.
Disadvantages:
Although the polymorphic nature of D-Mannitol allows for a greater phase change range, the material undergoes the same type of phase change [3]. This means that when manufacturing the ISEC, there is very little variety in change, so it is hard for manufacturers to control different latent heats of fusion for different devices, unlike Paraffin or the Sodium Nitrate/Sodium Nitrite/Potassium Nitrate film.
The phase change range of temperatures is higher than that of erythritol at 118 degrees C, the temperatures of the phase change range are moderate when compared to the film of Nitrates. Therefore, it would not be as useful for high-temperature cooking situations. 170 degrees C, or 338 degrees F is still lower than the standard 350 degrees F [3].
The nature of the α-Phase of D-Mannitol has not been thoroughly tested, so the α-Phase could provide a potential hazard [16]. Even the stable β-Phase has little information on the Safety Datasheet, so it should be more thoroughly tested and researched before mass-produced to the public [16].
It does have a thermal stability higher than that of erythritol, but it is not too much higher at 190 degrees C. Because the phase change range is between 130 degrees C and 170 degrees C, it is not very durable and potential degradation could occur early if not properly maintained [17].
With a latent heat of fusion of 370 kJ/kg, oxalic acid dihydrate has the ability to store a great amount of thermal energy with respect to its mass. Additionally, its modest melting point of approximately 102°C [8] means that, as the photovoltaic cell heats the material, it will undergo its phase change at a lower temperature, ensuring the storage of thermal energy in said change. This material allows for greater recyclability overall, however the temperature at which the latent heat is subsequently released is significantly lower than other options, making it potentially less viable for cooking certain types of foods.
The significant drawback of this phase change material are its toxicity [8]. Being a corrosive, skin irritating, health hazard, one must think carefully whether this material is feasible in the culinary setting. The fumes are also combustible.
Advantages:
Oxalic acid dihydrate has an expecially high latent heat of fusion, allowing the PCM to store significant amounts of thermal energy in the liquid state. Given the maximum temperature of the heating element, reaching the phase change temperature should never be an issue.
Disadvantages:
The melting point of this PCM is significantly lower than some of the others listed, meaning the latent heat of fusion will be released at a lower temperature. As a result, the temperature of the material will stall at only 102°C, making it less effective for cooking certain types of food.
Galactitol is a sugar alcohol that is reduced from galactose and is very safe if accidentally ingested. It has a melting point of approximately 190°C which is near the maximum temperature of the PV cell, meaning it may be too high. However the higher the melting point is desirable in the aspect of absorbing more energy through the day. The latent heat of fusion of this PCM is around 357 kJ/kg*K, making it a great material for storing thermal energy.
Advantages:
The high melting point and latent heat of fusion of Galactitol make it a very viable material for cooking food in the ISEC. Galactitol is not very hazardous as well, making it less dangerous in the case of any accidental leakages of the ISEC into the cooking compartment.
Disadvantages:
The high melting point may be too high for the given photovoltaic cell to reach. Upsizing may be necessary to take full advantage of this material. Galactitol is also not very durable, losing some of its thermal property stability after as little as 50 cycles [17].
Paraffin is an organic PCM with a relatively high heat of fusion which ranges from 200-270 kJ/kg depending on the type - varies with the amount of carbons the type of Paraffin. It is non-corrosive and is very stable after repeated thermal cycling which is optimal to avoid degradation. [11]
Paraffin is also safe, reliable and inexpensive. We want the PCM to be affordable and easy to use which Paraffin meets these requirements. However, it is considered a low-grade PCM which means it has a relatively low melting temperature which would not absorb as much energy as we hope to.
The most common application for Paraffin waxes as a PCM is in electronics thermal management. [18]
Advantages:
Paraffin has a high heat of fusion, and a high latent heat, ranging from 200 to 280 kJ/kg. It is also relatively inexpensive and stable over repeated thermal cycling. It is also not harmful if accidentally consumed, which is useful with cooking and having the material in close contact with food. [18]
Disadvantages:
Paraffin has a very low thermal conductivity [18]. Low thermal conductivity means that the PCM creates an uneven melt front and a large temperature gradient, which can make it more difficult to cook with [4].