Material taken from the ISEC Construction Manual that may be dated or superfluous, but may also have some value.
From removeable cookpot:
- under pictures from Nepal:
An additional benefit of this design is that the nest makes a good barrier between the insulation and the food, so you don’t need to mold the cooking surface around the nest. Rather, you can make a flat annulus (ring) to fit around the rim of the nest. If there is concern that the slits in the nest will prevent a hermetic seal between the food and insulation, aluminium foil can be glued over the slits with high temperature RTV glue (gasket maker).
From Thermal Storage:
– Phase Change Thermal Storage (PCTS). More heat can be stored by melting a PCM than heating a solid. Additionally, with PCTS, most of the heat is returned at the melting point, so the cooking can be more controlled. The downside of PCTS is that you have a pot or container with a hot liquid in it. We’ve mostly worked with two PCMs: Nitrate Salt mixtures (melting point 220 C – 300 C) and erythritol (melting point 118 C). Sunbuckets has promoted their nitrate salt PCTS in moveable aluminium housing. However, I’ve heard informally, that the housings can leak, and nitrates accelerate burning, so that’s a problem. Matthew Alonso wrote his PhD thesis designing and evaluating Sunbuckets, comparing the nitrate salts to a solid piece of aluminium. In the conclusion, if you read between the lines (which he personally pointed out to me) is that solid aluminium is better than nitrate salts. We published our experiments with a stationary pot housing erythritol PCM. It worked great, and I used it for 3 months in my house nearly every day. However after 3 months, the erythritol degraded, with a reduced melting point and reduced latent heat.
We will pursue STS over PCTS for domestic use because of the safety and simplicity. However, for industrial uses such as large restaurants and public facilities, nitrate salt PCTS would prove more effective and less expensive. The larger scale would facilitate longer thermal retention times. Nitrate salts are used to store thermal energy from concentrated solar fields for night-time electricity generation, providing significant precedent and guidance.
Detachable Aluminum Mass. Having a heavy aluminium pot prevents one from quickly heating food in the morning (for instance), and also makes it difficult to heat something slowly. One solution is to have two pots: one with thermal mass, and one regular pot. Another solution is to have a thermal mass that can be moved into and out of thermal contact with the heater and the cook pot. One device we built worked very well and is well documented in this final report. The downside of this design is a much lower power flux caused by the heat needing to cross two metal-metal interfaces. This design was able to achieve power fluxes of 600 – 700 W, less than 1/100 that of the “heavy aluminium pot” thermal storage design (above). Thermal conductivity across the interface can be improved by placing a drop of oil in the interface. However, this would require a substance that would remain liquid and also not burn over a temperature range of 25 C – 300 C. If you know of a nonpoisonous substance that can do that, please inform us.
Battery storage: Batteries and the necessary charge controllers add considerably to the cost. This cost, like that of solar panels continues to decrease. In another 10 years, the battery storage systems may be very common. If a solar panel is connected to an ISECooker and a PWM charge controller charging a battery, the ISECooker will use the electricity available in between the power pulses of the charge controller, using otherwise wasted electricity. However, a full home solar electricity system could also operate where the ISECooker draws power through the charge controller from the battery, which is charged by solar panels.
Battery Back up: solar panel to deliver more power to the ISEC.
As an example, a solar panel with an MPPT of 18 V and 6 A with an ISECooker resistive heater of 3 Ohms, delivers 108 W under full sunlight. Under half sunlight, the panel COULD generate 54 W at V_mppt. However, with a current of only 3A, the voltage across the 3 Ohm heater would drop to only 9 V, delivering 27 W of power from the solar panel. If a back up power supply connected in parallel engages at (for instance) 16 V, the current to the ISECooker would be 5.3 A, providing 85 W, with the solar panel providing 3A*16V, or 48 W.
A small 12.7 V battery can be charged with a PWM charge controller in parallel with the ISECooker, so that the ISECooker uses all the power that the PWM doesn’t use to charge the battery. If the battery (through the charge controller) is connected in parallel to the ISECooker (with a diode so the panel is not directly connected to the battery), the voltage on the ISECooker would be 12 V (12.7 V – 0.7 V), delivering 4 A of current for a total of 48 W. But the solar panel would be providing 3 A, and thus 36 W of the power (12 W more than with no battery backup). The improvement in efficiency from the solar panel would be greater if using higher backup voltage (as long as the back up voltage is lower than the solar panel V_mppt).
6) Insulation and construction materials
Supporting the cook pot from below, and sealing the top surface with a thin, insulating surface. At Cal Poly, we have had success using fiberglass fabric for the top cooking surface (and the bottom surface of the insulating top). We have suspended the heater/cook pot inside of a cylinder of thin welded-wire mesh, which provides excellent insulation, but is likely not strong enough to hold a pot through robust stirring as with making nsima. We’ve also supported the pot with fire brick, which is thinly-sliced (improving insulation) and packed between fiberglass wool under the pot to provide stability. These are excellent methods except that fiberglass wool and fiberglass fabric is hard to clean and hard to find in the tropics. In Togo, we were able to buy Teflon fabric, which is good to temperatures of about 200 C, but ultimately degrades. Also, the fabrication of Teflon has considerable environmental impact.