This construction manual is under continuous revision with new learning and improvements. It was largely compiled during my 1-year trip to Africa/India/Nepal, which I document in this blog. Please see our ISECooker Research Page. In updating this page, I have removed dated or superfluous material, which may still be of value to the reader. You can find this removed material in Construction Manual Archives.
Insulated Solar Electric Cooker (ISECooker) is an insulated electrically-heated cooking chamber, allowing the user to cook over a long period with low power from a solar panel, from the grid, or from a combination that includes any electrical sources. Energy can be stored thermally or in a battery. Low-power cooking saves money by reducing electrical bills (if grid connected) or reducing the number of solar panels purchased. The simplicity of ISECooking technology allows it to be manufactured in locations where it is used. This manual describes basic concepts behind ISECooking rather than a step-by-step process. Construction details will differ in each location depending on availability of parts, resources, construction equipment and expertise.
Topics to cover:
1. What is 100 W?
2. Different designs for storing and delivering energy.
3. Thermal Conductivity
4. Different heating technologies
5. Drawing power from the solar panel
6. Insulation and construction materials
7. Safety and corrosion
8. Cooking
9. Developing a business collaboration
10. Instrumentation: Things you may want to buy
1) What is 100 W?
- 100 W solar panels are presently manufactured in China for about $10, and retail for about $50 in the USA. Prices in Africa and Asia vary depending on import taxes. The continued decrease in solar panel cost makes higher powered designs more compelling.
- 100 W will bring 1 kg (1 liter) of water to a boil in an hour. A 200 W solar panel will bring 1 kg of water to a boil in half the time. A 1000 W microwave oven takes about a tenth as long: 6 minutes. Hence, a full day of sunlight with a 100 W solar panel would boil about 5 kg of beans, rice, stew, potatoes,… enough for a reasonably sized family.
- 100 W is not enough power to cook anything without insulation because the heat would escape, preventing the food from getting hot enough to cook. Hence it is crucially important that the chamber be insulated. The most important part to insulate is over the top of the pot – this is also the hardest part to insulate.
- 100 W for 5 hours is 500 Wh or 1/2 a kWh. Most car batteries are about 1 kWh.
- 1/2 kWh would also raise the temperature of 10 kg of aluminum from room temperature to 250 C, providing an additional way to store a day’s worth of solar energy, for high-power cooking.
2) Different Designs for Storing and Delivering Energy
The above pictures show different designs, from left:
1) Heater connected directly to cook pot, providing the best thermal connection between the heater and the food. It is simple to construct, but is hard to clean. People do not want to use a pot that has wires connected to it.
2) Removable Pot with a “Heated Nest” is easier to use.
3) Thermal Storage can be added directly to the cook pot by making the base very thick (see below).
4) Even with thermal storage, food can slow cook in a separate container (green), especially if there is an insulating layer between the (green) cook pot and the thermal storage.
Direct Connect: A solar panel is directly connected to the heating element (section 4). There is no energy storage except in the food itself. It is the least expensive, easiest to build, and least likely to need maintenance.
Removeable Cook pot: In all ISECooker designs, the electric heater can be connected directly to the cook pot, which is simple to build, but hard to use and clean because there will be electric wires coming from the pot. However, a removeable cook pot can be made by two methods:
- Detachable electrodes: The heater is cemented to the bottom of the cook pot and the electrical connection is made via electrodes that make contact when the pot is placed in the insulated receptacle. One sees this in popular teapots everywhere. We built a detachable electrode into our prototype on Saturday, Dec. 24, as recorded on my blog.
- Heated Nest (much easier to build): An ordinary cook pot can be heated by placing it into a stationary thermally conducting receptacle that is heated by a permanent electric heater. We made a very nice heated nest in Zambia with Clement end of February from a thin-walled (spun) aluminium pot. We cut the sides so the aluminium cook pot would fit inside it. At 100 W, there was only a 5 C difference in temperature between the nest and the pot, indicating good thermal connection between the heated nest and the cookpot. Pictures below right are from an ISECooker made with Salma in early December, 2022. What’s nice about it is the thick aluminium foil is on both sides of the heaters, effectively taking heat from both sides of the heaters to the cook pot. The heater is a visible ring of diodes (at left), as reported in our “Hot Diodes” Paper. We have since abandoned diode heaters in favor of resistive heaters, described below.
The advantage of the heated nest is that the pot is lighter and simpler, and many different pots can be heated with the same heated nest. Having the heater connected directly to the cook pot with detachable electrodes provides better thermal connection between the heater and the food, but is more difficult to build and maintain.
One very good way to make a heated nest is to buy two identical aluminium pots, as we did in Ethiopia and Nepal; below from left: cut vertical slits in the one we designate as the nest, smooth the cut edges, cut an annulus from a thin sheet of stainless steel (or make one out of think concrete), inserting the nest into the cooking surface, and the exposed electrodes with the nest and surface upside down.
Whether using a stainless steel or concrete cooking surface, it is important to make a hermetic seal between the inside of the nest (cooking food) and the outside (insulation). More Archived.
Thermal Storage Thermal energy can inexpensively be stored by heating metal (Solid Thermal Storage, STS) or melting a phase change material (PCM), but requires good insulation in order to prevent thermal loss. We have experimented with several different kinds of thermal storage. We no longer pursue phase change thermal storage because of complexity and safety concerns. You can read about our efforts in this publication, and in archived information.
Heavy Aluminium Pot. Aluminium is a good thermal conductor. Preheating a heavy pot itself provides very high heat fluxes directly to the food. We demonstrated this in Togo on Nov. 28 (see blog) when we heated a 5 kg aluminium pot to 250 C and boiled 1 kg of water in less than 3 seconds, corresponding to nearly 100 kW of power. We subsequently heated the pot again and stir-fried 1 kg of vegetables about 2 minutes. Both events are documented in this video. Making an aluminium pot with a 5-kg, two-inch thick base is a major task in the USA, but in unindustrialized countries, every town I visited has aluminium foundries that cast pots from recycled aluminium in sand molds. Thus, I’ve visited foundries in Uganda (Sept. 14), Togo (Nov. 21 and again Dec. 1), Ghana (Dec. 16, and Dec 19), and Malawi: Lilongwe (Jan. 21) and Salima (Jan 27).
All kinds of aluminum are recycled at these foundries (above, see July 21), and there is no quality control. Recent publications indicate cookware from recycled aluminum leech toxins (lead, cadmium, arsenic) into food, and that the leeching increases with abrasive cleaning of the pot. Thus, we no longer cook directly in cookware made from recycled aluminum. However, recycled aluminum STS can conduct heat to a cookpot made from food-grade aluminum, iron, copper, or steel. Iron may be an ideal metal because it will leech iron, an important nutrient into food. At right is a failed aluminum pour (the inner mold wasn’t properly centered with the outer mold), which nicely shows how a 1-kg spun aluminum pot snugly fits within the 8 kg STS made from recycled aluminum. Despite having the STS and cookpot in close physical contact, the thermal flow from the STS through the discontinuity reduces thermal power from about 100 kW to only about 1 kW. However, 1 kW is still sufficient to adequately cook food.
Grid Backup: The solar panels can be connected in parallel with any external DC voltage source of lower voltage than V_mppt of the solar panel. If the voltage source has a higher voltage than V_mppt, it will prevent the solar panel from providing power, and can even drive current backwards through the solar panels – don’t do that. One can buy a grid-connected DC power supply ($10-$20) of the same or lower voltage as V_mppt of the solar panel. Alternatively, the DC could come from a charge controller connected to a battery. Alternatively, an AC electric heater could also be used that can be plugged directly into line voltage, which would require two different heaters for a solar / grid hybrid power supply.
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. More Archived.
3) Thermal Conductivity
In order to cook with low power, it is essential to insulate the cook pot. Insulating a cooker is a foreign idea to every culture that I know… except India, where one can buy a “thermal rice cooker” that is an insulated pot into which you pour hot water and rice and allow to cook. Most of us are accustomed to cooking with powers greater than a kW. In particular, it is necessary to insulate above the cook pot with an insulated top, as heat rises. It is also important to not let the heaters get too hot for two reasons:
- To reduce damage to the surrounding materials.
- To reduce heat lost to the outside, because the flow of lost heat is proportional to the temperature difference.
So, while we want to heat the food, it is helpful to instead think about cooling the heaters. Thus, it is very important that there be good thermal conductivity between the heater and the food, and have good insulation between the cooking unit (especially the heater) and the outside. If the heaters are not thermally connected to the food, the heaters will rise to a high temperature and lose heat to the outside, through the insulation.
Below, please find a table of thermal conductivities of some common substances that we use.
| Substance | Thermal Conductivity (W/mK) | Max temp (C) | Specific Heat (J/gK) | Vol Specific Heat: kJ/L*K | Density, kg/L, or relative to water |
| Copper | 385 | 1085 | 0.385 | 3.4496 | 8.96 |
| Aluminum | 250 | 660 | 0.897 | 2.422 | 2.7 |
| Iron | 80 | 1538 | 0.412 | 3.537 | 7.86 |
| Steel | 45 | 1425 | 0.466 | 3.756 | 8.05 |
| Stainless Steel | 15 | 1450 | 0.5 | 3.75 | 7.5 |
| earth, stone | 2 to 3 | 2 to 3 | |||
| concrete | 2 | 2.4 | |||
| Sand | 0.3 | 1.6 +/- 0.1 | |||
| Fiber glass | 0.04 | 1000 | |||
| Wool | 0.04 | 570 | |||
| Polyethylene fiber | 0.04 | 260 | |||
| Styrofoam | 0.035 | ~ 100 | |||
| Air** | 0.026 (at room temperature) |
Chicken Feather 0.03 ?? ~300
**The low thermal conductivity of air does not mean that leaving a pot in open air will insulate it. The convective flow of bulk air around a hot pot will quickly cool the pot. The purpose of fibers in insulation such as fiberglass and wool is to stop the bulk flow of air, so that the heat must conduct through the air rather than convect. It is actually the trapped air, not the fibers that insulate. Thus packing more fibers of insulation will not improve the insulation. The insulation will increase with increased distance between the hot and cold surfaces. So, increasing the thickness of the insulated chamber will improve insulation.
Unfortunately, the thermal conductivity of air (and thus, any kind of fiber insulation) increases with increased temperature because of increased molecular speed with increased temperature. The graph from the Engineering Toolbox (right) indicates the thermal conductivity of air will double with a change in temperature from 20 C to 550 C, increasing rate of heat loss at high temperatures.
Keep in mind:
- Heat will travel away from the cooker through the insulation and also through the supporting receptacle. Thus, the structure that supports the cook pot, connecting it to the outer casing must be reasonably thin and reasonably nonconducting. For instance, the annulus (circular ring, the counter top) between the top edge of the cookpot and the top edge of the outer shell could be made from concrete or thin steel, but not aluminum, which is highly thermally conductive.
- gaps between metal surfaces, anything that isn’t a solid piece of metal will greatly reduce thermal conduction.
4) Heating Technologies
We’ve used three different kinds of heaters. Resistive heaters, Diodes, and Positive Thermal Coefficient (PTC) Thermistors.
Resistive Heaters: mostly made of Nickel Chromium (NiCr) resistive wire. The advantage is they are inexpensive and are easy to find in most countries. We have made heaters many ways, which is well documented in the Construction Archives. The problem is that the junction between the red-hot NiCr wire and the thick power cable corrodes. There are three solutions that have time-proven success.
- Purchased electric heating elements: used as intended without cutting open.
- Any kind of high power resistive heater, usually ceramic heater. The ceramic heater can be sealed in concrete or cemented to the cookpot with high-temperature adhesive.
- Low resistance, low temperature electrodes can be added to a NiCr heating element by folding a section of each end over on itself twice and twisting them. See below
Ethiopia supports a cottage industry that has perfectly solved and tested this challenge… constructing stoves, especially mitads for making the traditional Ethiopian bread, injera. While you can buy injera stoves made in large factories, they are also made in small shops by winding an electric coil through a path carved into a ceramic disk.
The electrode connection is made by folding the coiled wire over on itself twice and tightly twisting the four wire sections with pliers. It makes a perfect connection because it’s the same wire, and the electrode’s resistivity is less than that of the coil because it consists of four wire sections. The pictures below show the process in this video.
The grooves that hold the heater wire are etched into rather soft, machinable ceramic disks with a large compass-like tool. However, in Nepal (Bhaktapur), the indentation for the coiled NiCr wire is pressed directly into soft clay before firing, resulting in indentations that better hold the heater wire. At right are two 200 W heaters: at left is 8 Ohms for the ~ 36 V output of two (100 W) solar panels in series; at right is 250 Ohms for (~ 220 V) line voltage. Both heating elements have low resistance folded electrodes that should be a few cm long to maintain the junction between the hot NiCr wire and the copper power cable.
While trying to make the low-resistance electrodes for the thinner wire, the wire is prone to breaking during the twisting. I was finally able to make an electrode for the thin wire by making an electrode with slightly thicker wire winding through one layer of thin wire. In the pictures below, the thinner wire is in the lower right. From left, the first folding; the second folding with the thin wire woven in; twisting the electrode tight, finished electrode with the thicker wire cut away.
I cut an electric heater open and was surprised to see that the ~ 2mm diameter steel electrodes are not spot welded to the NiCr wire as I thought they were. They are just wrapped around inside a machined thread. Also the steel electrode extends into the heating element… all the way to the coiled cooking surface. We have not yet attempted this construction method.
We no longer develop diode heaters or PTC (Positive Thermal Coefficient) heaters. However, we explain these technologies in the Construction Archives, and explain why we no longer work with them, although they have attributes that may make them the appropriate heating element under some circumstances.
At the end of my visit in Kathmandu (see blog), I experimented with making closed NiCr ceramic heaters.
I received “leather hard” clay heaters from Hari in Bhaktapur. I inserted the NiCr coils of the appropriate resistance with thick-wound leads at each end, and let them dry. I covered some of the channels, allowing a hole in the side to vent the air space.
I sent them back to Hari, and he fired them. See below, before and after firing.
See below, center that I failed to adequately join the surface. Likely the covering clay was wetter than the underlying heater core, so when it dried, it shrank more than the underlying heater core, and detached.
5) Drawing Power from the Solar Panel
The amount of power drawn from a solar panel depends on the panel, the amount of sun, and the voltage at which the load (cooker) is drawing power. Below are data I took from a solar panel on my roof in California. With the knowledge that Power = Current * Voltage; P = IV, one sees several important characteristics:
- Lower sunlight means less current (for fewer light photons exciting fewer electrons). But the maximum voltage remains about the same for all solar intensities. **
- The panels produce no power at their maximum voltage, because the current is zero.
- The maximum power is produced at about V_mppt = ~17.5 V (for these panels), for all solar intensities. Thus, we always want to be drawing current from the solar panels at V_mppt for all light intensities. Because the power curves are pretty flat on the top, one only needs to be close to the V_mppt to draw near maximum power. Also, it is better to err on the side of lower voltage than higher, because the power drops of much faster for voltages higher than V_mppt than for lower voltages.
** It’s actually more complicated: Lower solar intensities also slightly reduce V_mppt, and the maximum voltage. However, increased panel temperature decreases voltage. Because the panel is usually hotter when the sun is strong, these two effects largely cancel; meaning that yes, V_mppt changes very little with changes in solar intensities.
A resistance heater will draw the most power if the resistance (R = V/I; from V = IR) is chosen to draw power at V_mppt. However, because V = IR, the voltage on the heater will drop with decreasing current (decreasing solar intensity). The “working point” is the current and voltage that the load and solar panel are experiencing. The working point is where the solar panel output curve intersects the (straight line, V = IR) resistance curve of the load. Below, left, see three working points where the red resistance line crosses the solar panel output curve for three different solar intensities. The maximum power point for 13:00, full sunlight is about 5.7 A, 17 V, corresponding to resistance of 3.0 Ohms, and power output of 97 W. Thus, the red line represents a 3 Ohm resistor. At 5:00 PM, not only does the current drop to about 2.2 A, but the voltage correspondingly falls to about 7 V, corresponding to a power of 16 W, although the power output would be close to 40 W if the working point was further to the right, at the same 17 V, requiring a resistance of 8 Ohms.
Choosing the R_mppt for one intensity, compromises power at the other solar intensities. The resulting powers are listed for the different resistances and time of day, in the table below.
| R(Ohm) | 9:30 | 13:00 | 17:00 |
| 3 | 33 | 97 | 14 |
| 5.6 | 59 | 65 | 33 |
| 8 | 40 | 44 | 47 |
The panel can be made to optimally deliver power at all solar intensities with the following strategies.
- Making the heater from several resistors or a variable resistor that can be changed in order to roughly maintain the solar panel output voltage near V_mppt. The resistors could be changed manually or by a logic circuit.
- Using a chain of diodes to heat the pot, as described in #4 above and extensively in our Hot Diodes! Paper.
- Using a voltage converter to draw power at mppt and convert the voltage to meet the resistive load.
- Providing a back up power supply for when sunlight is insufficient, maintaining V_mppt on the solar panel.
- Simple Power Optimization circuit we designed.
Changing Heater Resistance:
The current drops with lower sunlight, but V_mppt remains the same, increasing the resistance for maximum power. For example, above left, the best working point for 5:00 PM would be (18 V, 2.2 A), requiring a resistance of about 8 Ohms. Thus in order to provide optimal power between full sunlight, the resistance should be able to change from 3 Ohms to 8 Ohms. This could be done with multiple resistors, but would require the user to change the connections, although switching could be done with an electronic circuit.
Chain of Diodes:
Unlike a resistor, the voltage on a diode depends very little on the current through the diode. Thus, a chain of the correct number of diodes in series would maintain near constant voltage across the solar panel. We no longer use diodes because they are destroyed by the high temperatures needed to heat a thermal storage device. However, for heater temperatures below 250 C, diodes work well and better optimize power without an MPPT charge controller. At the same time MPPT charge controllers have become less expensive and other circuitry allows for power optimization with a standard resistive heater such as the power optimizing circuit described below. More details in Construction Archives, and in our Hot Diodes! Paper.
Buck Converter:
We can use a buck convertor to take current from the solar panel at V_mppt and deliver increased current and reduced voltage to the load at constant power. By drawing less current from the solar panel, the solar panel output voltage increases to V_mppt. More details including the circuit we made to control the buck convertor in Construction Archives.
Battery Backup or grid connection will improve ISEC in two ways:
- It will provide back up power allowing cooking when sunlight is not adequate.
- The voltage of the solar panel and load are at least as high as the backup voltage, allowing the 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.
BACK UP VOLTAGE SHOULD NEVER BE GREATER THAN V_mppt. If the backup voltage is greater than V_mppt, the backup power supply will stop the current from the solar panel and possibly drive current backwards through the solar panel itself, heat the solar panels, possibly destroy the solar panels, and waste power from the backup power supply.
Power Optimizer Circuit:
Simple Power Optimization circuit we designed. We designed a circuit that when the voltage drops below a set point (for instance, 16 V – 17 V for the solar panels shown above, disconnects the load while charging a capacitor. When the voltage exceeds the set point, the load connects again, drawing current from the circuit and the charged capacitor. This circuit is being revied for publication. One version of the circuit is shown below.
6) Insulation and Construction Materials
The heated cook pot needs to be suspended in insulation in a way that is secure and doesn’t conduct heat to the outer housing. Additionally, the insulating chamber should be sealed to prevent insulation from getting in the food, and to prevent food, water and condensate from getting in the insulation. If the cookpot must be firmly suspended in the insulation requiring rigid connection to the outer shell either from below, or through the “countertop” on the rim, or both.
Holding the cook pot in/on a strong surface that is supported on the rim of the outer housing. This surface must also be reasonably thin and insulating to limit thermal loss. This surface could be made from thin stainless steel, high temperature composites such as Bakelite (which is expensive), or a thin slab of concrete, plaster, or ceramic.
Concrete slabs must incorporate steel mesh, such as rebar in large concrete slabs.
Care must be taken to make the slab uniform thickness and keep the mesh near the center of the concrete.
To make thin, strong concrete surfaces, please observe several things:
1) Contain a steel mesh of about the same size as the thickness of the concrete, as thin as 5 mm.
2) Press the concrete between smooth, plastic surfaces so that it doesn’t stick after it cures.
3) Vibrate the concrete immediately after pouring. This will liquify it, allowing the air bubbles to come to the surface, below. You can use a massage vibrator, or you can even pick up the entire assembly and drop it many times.
We used plaster of Paris (Dec 16, 17, 19, 24, Ghana), and wall plaster (Jan 15, Malawi), below.
Clement and I built a nice concrete ISECooker in Lusaka Zambia, and even made the bottom surface of the insulated lid from concrete. Using two wash basins (one deeper than the other) the ISECooker closes like a clam shell. We used polyethylene (melting temperature ~ 260 C) insulation from pillows (cost ~ $3.00 per ISECoooker), and we put a thermal switch (T = 180C) and thermal fuse (T = 220 C) on the underside of the concrete receptacle. The maximum working temperature of the aluminum pot is determined by the distance between the aluminum pot and the thermal switch. See section 7.
The section of electric heating element is glued into the nest with RTV glue (this is a bad idea, as the heater itself easily attains temperatures exceeding that of the RTV glue; thin handles made of steel cable can be casted into aluminum pots, providing handles that:
1) don’t burn your hands because they are thin and steel (a poorer conductor than aluminum).
2) are flexible, so they lie flat under the top insulator and spring up when the top is removed.
Clement demonstrated the thermal storage capability with this short video.
We’ve usually made the outer housing from plastic wash basins, although we got a large aluminium pot at a good price in Nepal. The plastic wash basins are seen as ugly garbage cans, which is probably a deal-breaker for the consumer market. We also built the entire receptacle surface and outer housing out of clay (right), producing a beautiful product. 
Clay may however be more expensive and heavy and more fragile than concrete… not a problem for a stationary cooker… like in the USA, we don’t move our ranges around. However, so far people have prioritized ability to move the cooker. It may be sufficient to make just the top (annulus) cooking surface from clay (ceramics), but we haven’t tried that yet.
Thermal degradation of concrete: It’s widely accepted that concrete is not high-temperature stable – that it disintegrates at temperatures above 70 C. However, we’ve not experienced this from either our concrete nor plaster receptacles in the Malawi vocational schools, where temperatures routinely exceed 300 C. Likely, thermal disintegration is caused by differential expansion because of uneven heating. We may be protected from thermal degradation by doing two things:
1) The steel mesh stabilizes against cracking. Indeed Andrew at ASEI reports that without internal steel mesh, the receptacle surface disintegrates with a singe heating.
2) The receptacle surface is thin, reducing the temperature difference across the thickness.
Lastly, if thermal disintegration appears, it may be worth investigating improvements by using Firecrete Super Refractory Castable Cement, reputedly high-temperature cement.
Stainless Steel:
In Nepal, we bought a 4′ x 8′ sheet of .5 mm stainless steel (SS) for $36: enough for eight cookers (see blog, April 28) – $.50 per ISECooker. Stainless is the most popular cooking surface and, with a thermal conductivity of 15 W/mK, a 0.5 mm SS surface will lose less heat than a surface made from 7 mm-thick concrete. Because we had a full heated nest (pictures below), there was no need to mold the receptacle surface around a pot. The top receptacle annulus surface was cut from the stainless steel sheet in a few minutes. Below, notice from left (cook pot in heated nest), upside down pot in SS rim with protruding heater electrodes, directing the cutting of the SS sheet, assembling the prototype, and workings grinding down the cut edges. Although the thin stainless is not as rigid as a 7 mm concrete slab, it is strong enough to hold the pot in place when the perimeter is screwed down to the aluminium pot rim. Ultimately, we inserted a piece of ceramic between the nest and bottom of the outer shell, to prevent the nest/STS/pot from sagging under its weight.
Final Design manufactured in Kathmandu
For details, see the blog, Friday, July 28, 2023
NYSE has finished manufacturing the ISECookers, in fine form. Above, from left: The closed cooker, the cooker opened exposing the heated nest, the ceramic heater in the heated nest, the 7 liter cook pot in the heated nest, the 8 kg STS (solid thermal storage) in the heated nest, the 4 liter cookpot in the STS in the heated nest, the 7 liter cook pot in the STS in the heated nest. Because all the pots are made in the same shape, the pots are interchangeable. However, the insulating lid cannot be closed when the large, 7 liter pot is heated with the thermal storage.
September of 2023, I received the data below from Ashutosh at Kathmandu University.
We are able to estimate the thermal power loss from the temperature rate of change of the 9 kg of aluminium in the ISECooker. We see that we need 30 W to keep the ISECooker at the boiling point of water, and with ~ 120 W of input power, this cooker will not achieve a temperature much higher than 300 C.
We experimented with soft “countertops” including fiberglass fabric and Teflon. For a number of reasons these may not be adequate, but more information is found In the Construction Archives.
7) Safety and Corrosion
Don’t start a fire! We can prevent a fire from starting by keeping all temperatures below the combustion temperatures of the surrounding materials. Thus, there are two solutions:
- Insulate the cook pot / heater with materials that don’t burn, such as fiberglass and rock wool.
- Control the temperature of the cook pot.
The temperature of the cook pot full of water or wet food will never exceed 100 C because extra heat will be used to boil the water, but the temperature of the heater could be considerably hotter, especially if the heater is not well thermally connected to the food. Additionally, at some point, a user will leave an ISECooker on without food in it (or all the water will boil away), so we can not rely on there to always be food in the cooker. Apart from this, extra care should be taken to insulate around the heater as well as thermally connect the heater to the cook pot – for instance by encasing the heater with the cook pot within an aluminium sheet or an aluminium pot, we call a “heated nest”.
Fiberglass and rockwool are ideal high-temperature insulations, but are difficult to find in Africa. Perlite is a locally available insulator, and wool is very good with its high combustion temperature. Organic substances such as cotton, wood chips, paper, rice hulls, and straw are poor choices: they can burn, absorb water, and rot. However, these substances could be used as part of the outer insulation shell, far away from the hot cooker, but are not recommended. Polyethylene fibers (like inside pillows) works great, but melts at 270 C. Chicken feathers should work great, but no one has tried them. The start to decompose at 200 C, but should remain reasonably intact through 300 C.
Several temperature switches and thermostats are available (see section 10) but be careful because DC current is hard on AC switches. There is an inexpensive DC thermal switch available for different temperatures, the highest temperature switch turns off at 180 C and turns back on at 140 C. In case the thermal switch fails in the “on” position, a thermal fuse of higher temperature can be added in series. If you want the cookpot or thermal storage to get hotter than 180 C, the switch can be moved further away from the cook pot and heater. For instance, the temperature switch can be placed on the opposite side of the supporting receptacle (see right), or even further away from the cook pot. For instance, if you place a 180 C switch half way between the cook pot and outside shell it will turn off when the temperature at this point is 180 C, which should be about the average between the temperature of the cook pot and the ambient (T_a): We estimate the temperature of the cook pot then to be about:
T_cp ~ 2*T_s – T_a
So, if you use a 180C thermal switch in an area with an ambient temperature of 30 C, the cook pot will get to 330C before the thermal switch turns off. The further the switch is from the cook pot, the hotter the cook pot will get before the switch turns off. If the thermal switch is not on the cookpot, there will be a time delay between the cookpot temperature and that of the thermal switch. So, it is possible for the cookpot to get too hot before the thermal switch turns it off. As we discuss above in “corrosion”, DC is hard on switch electrodes. You should be careful not to use AC switches and thermostats on with DC power supplies because the electrodes may burn out. As an extra safety precaution, you can include a thermal fuse next to and in series with the switch. The fuse should blow at a higher temperature than T_s, so that it only blows if the thermal switch breaks in the “on” position.
Be careful to confirm that any switch/fuse you use can handle the voltage and current of the circuit.
Toxicity of Recycled Aluminium:
As discussed in section 2, when recycling aluminium from (for example) automobile engines, impurities may be introduced, which can leach out into food, especially if the food is acidic (such as tomatoes). A 2017 study showed that food cooked in pots made in unindustrialized countries from recycled aluminium contained alarming amounts of aluminium, lead, cadmium, and arsenic. A 2022 study indicates that leaching of heavy metals is greatly accelerated by both abrasive cleaning as well as higher temperatures. An NPR story indicates 5 million deaths annually from lead- induced CVD (Cardiovascular Disease), and introduces Pure Earth, an organization dedicated to reduction of lead and mercury pollution.
Teflon makes poisonous vapors at high temperatures (> 300 C)
Many collaborators have used Teflon-coated fiberglass membrane to separate the metal heating cooking assembly from the insulation. At temperatures greater than 300 C (572 F), toxic vapors are released that cause “polymer fume fever” which can be fatal.
More Difficulties with DC electricity:
- Stopping DC current is harder on the electrodes in a DC switch.
- DC voltage is more likely to corrode wire junctions.
If you were pushing a car to speed it up, the momentum would grow with the moderate force you provide. If you needed to stop the car very quickly, you would need a very great force. For instance, if you ran the car into a brick wall, the brick wall would stop the car quickly, but with a destructively large force. When you close a switch, completing a circuit, the voltage pushes the current to rise to I = V/R. This current running through the wires has “electrical momentum”. When you open the switch again, the electrical momentum ends in a short time, which you see as a high-voltage electrical arc, that degrades the electrode surfaces in the switch after many switches. This is less of a problem for AC because AC voltage and current pass through zero 200 times per second, so the pulse and discharge ends by itself in 1-4 milliseconds.
Metals at a junction will corrode if there is a voltage difference between the metals. This can happen if there is an applied voltage or if the metals are different, such as copper being connected to a NiCr wire. It seems that the negative terminal corrodes where the stranded copper wire connects to the copper ferrule (which connects to the NiCr wire). This is less of a problem with AC circuits because the polarity of the junction constantly changes 50 or 60 times a second, so the applied voltage has no effect. The solution is to keep the junction with the copper wire (especially the stranded copper wire) cool. This can be done by making low resistance, low temperature leads to the NiCr heating element by folding the NiCr wire over on itself several times at each end, and twisting the wires together. See pictures in section 4, heaters.. This creates a low resistance electrode, reducing the temperature of the junction. Additionally, single stranded copper wire should be connected to the heater electrode. The single stranded copper wire can be connected to finely stranded wire outside of the insulation at room temperature. It is not recommended to solder NiCr to copper for two reasons:
- Solder doesn’t stick to NiCr well.
- The solder will certainly melt during operation.
Melting solder by itself is allowable in other situations. For instance, if heating with a diode chain, a small amount of solder can be applied to the twisted wire junctions. The solder will melt during operation but not run if only a small amount is used. Care must be taken to not allow the molten solder to short the diodes, and the entire junction should be buried in high-temperature glue or concrete. I discourage the use of solder.
The corrosion problem is commonly solved in industry by spotwelding the NiCr wire to a steel wire that is long enough to be cool at the other end, to which the copper wire can be connected without corroding. ASEI has been able to spot-weld the NiCr wire from electric stove heating elements to small finishing nails and then stabilize the junction in concrete; see blog for September 25.
8) Cooking
What EVERYONE does WRONG: Your ISECooker will not function unless it is insulated. The ISECooker’s insulated top must be on ALL THE TIME. Please check yourself: feel all surfaces. If you feel warmth, this is where the ISECooker is losing heat and this is a problem. You can open the top of your ISECooker only for a moment to put food in, take food out, or stir… just for a very short time. People like to leave the top of the ISECooker open to watch the cooking. If you leave the insulating top off the ISECooker, it will lose much heat and will cool. Most of the pictures on this website show the ISECookers open, to show the inside. HOWEVER, ISECookers need to be closed when heating up and when cooking. If it feels hot, then heat is being lost
Cooking without thermal storage is good for slow cooking, boil and simmer, and sautéing: You fill the ISECooker in the morning and the food is done in the evening. If there’s no sun, you have to use a different cooking technology. With some adaptation, you can do many different things. If there is water in the ISEC, the temperature will not exceed 100 C. So, if I want to brown my onions or mushrooms, then I sauté them first. When they are caramelized, I add everything else. At the low power of 100 W, cooking chicken and vegetables produces a lot of “soup”. When I come home at the end of the day, I push the food to one side with a spatula and add dry rice. The will cook in the hot ISECooker even if the sun is down. This way, I have stew and rice. And the rice is the best rice you’ve ever tasted because it is cooked in broth. Also, for reasons not clear to me, cooking dried beans in the direct connect ISECooker produces the best beans I’ve tasted, and at 100 W, unless they dry up, you’ll never burn them.
9) Developing a Business Collaboration
We started building ISECookers in 2015, but we still don’t have large scale, or even medium scale production. There are minor challenges in both the technology development, the industrial production and the dissemination (adoption). We must do everything we can to build a technology that is effective, inexpensive, convenient, beautiful, and understood. It is NOT important that one person or shop does all the work by themselves. I have heard many collaborators say “I can do it all myself!” I have seen that wide collaboration is necessary to make the best product. We are not in competition with each other, we are in competition with large multinational corporations outside of Africa. We need to work together.
Business Development: There must be a factory, even a small factory as well as a team with deep academic understanding of electricity and thermal flow. If you have had electricity/electronics training, this is NOT the same as having a PhD in electrical engineering or physics. It is important that there be a university or engineering partner – at least until we have a clear step-by-step path to building a working ISECooker in your area. Likewise, an academic team is not set up to produce ISECookers either in motivation or infrastructure. Having an industrial partner is necessary.
Production: we should build a collaboration of every enterprise that is the best at what they do. For instance if you need specialty pots, then find a collaborator who casts or spins pots, rather than developing that technology yourself. Another example is if you want clay pots to be part of the ISECooker, you should partner with a clay studio.
10) Instrumentation: Things you may want to buy
Power Meters:
There are some specialty electronics that are helpful in building and using ISECookers. While digital multimeters are necessary and can be used to measure power (P = I*V), an inexpensive power meter is available: $10 power meter (PZEM-031), that continuously provides voltage, current, power, and integrated energy over time. They are placed between the solar panels (or other power source) and the ISECooker. These are helpful in experimentally developing your ISECooker prototypes. However, for only $10, it may be a good addition to all the ISECookers you sell. The integrated energy (lower right) can be used to calculate the total CO2 over time displaced by the solar electric cooking.
Thermal Switches:
A thermal switch or thermostat and/or thermal fuse prevents the ISECooker from gaining excessively hot temperatures. The cheapest functional thermal switch we’ve found (pictured at right) is for DC motors and has an “off temperature” as high as 180C. The “on temperature” is 40C lower. So, the one that turns off at (T_s) 180C turns back on again at 140C. Lower temperature switches are available.
The use of thermal switches (above left) and thermal fuses (above middle) is discussed in section 7. Above right, the thermal switch and fuse are connected in series, and glued to a concrete heated nest with high temperature epoxy.
Measuring Temperature:
I like to use thermocouples because the temperature sensor is just a junction between two wires of different metal composition. Thus, the probe itself is tiny and the wires run out to a reader that shows and/or records the temperature. There are a number of hand-held thermocouple thermometers for as low as $20 on the market and data loggers for more than $100. However, there are also several inexpensive ones I’ve seen (such as the TM-902C for about $2 through Alibaba), but never used. It would be a good idea to try an inexpensive one to see if they can be used to display temperature on every device.
Some thermocouple readers take more than one thermocouple probe, allowing you to read/record temperatures in more than one place. When using these, be careful to see if there is “cross talk” between the probes. That is, if both probes are connected to the same piece of metal, does it affect the reading of either probe? If there is “cross talk”, then you must make sure that the thermocouple probes are electrically isolated from each other. One way to do this is
to cover each probe (or all but one probe) with a thin coating of high-temperature glue. For $1,000, Omega makes a high-quality data logger that doesn’t have cross talk problems (right).