LOW←TECH MAGAZINE English

How to Brew Solar Powered Coffee

Sun, 09 Nov 2025 00:00:00 +0000

Image: The solar powered coffee maker that we build in this manual. Photo by Marie Verdeil.

There are many different methods for making coffee, some more energy-efficient than others. However, there are no coffee makers that you can power with a small solar PV panel. For example, a commercially available 12V DC drip coffee maker requires a solar panel of 300 watts to brew coffee and keep it warm.

The key to making a more energy-efficient coffee maker is insulation. Regardless of which conventional coffee maker you purchase, it will typically have little to no heat insulation, and most of the heat generated by the energy source will be wasted into the environment. Therefore, we made an insulated solar electric coffee maker ourselves.

Our coffee maker operates on the same principles as our solar-powered oven and runs on a 100W solar panel. We embedded an Italian coffee maker—a moka pot—in a mortar slab, surrounded by cork insulation and a layer of ceramic tiles.

We embedded an Italian coffee maker—a moka pot—in a mortar slab, surrounded by cork insulation and a layer of ceramic tiles.

Image: Connecting the coffee maker to a solar panel. Photo by Hugo Lopez.

Image: Cross-section of the coffee maker. 1. Tiles, 2. Cork, 3. Mortar. Illustration by Marie Verdeil.

The cooker has an electric resistance heating element integrated inside, which is directly connected to the solar panel without a battery, solar charge controller, or voltage regulator in between. Although it’s solar-powered, the coffee maker can be located inside your kitchen or next to your bed—only the solar panel needs to be outside.

Although it’s solar-powered, the coffee maker can be located inside your kitchen or next to your bed—only the solar panel needs to be outside.

The moka pot was invented in 1933 and uses pressure rather than gravity or a pump to brew coffee. It consists of a bottom chamber (a boiler that acts as a base), a funnel filter with a plate and a rubber joint, and an upper chamber where the coffee is collected. Just before the water boils, the steam increases the pressure inside the heating vessel, pushing the water through the filter and the ground coffee. The moka pot is an energy-efficient appliance, comparable to a pressure cooker for food.

How to brew solar-powered coffee

Once the solar panel receives sunlight in the morning, the coffee maker will start heating up. Consequently, if you put water and coffee in the machine in the evening, the sun will brew your coffee in the morning. Coffee is often a collective beverage and our coffee maker is most practical when several people use it. The first coffee takes roughly one and a quarter hours to make. However, once the mortar slab is warm, subsequent brews take only 20-25 minutes.

Preparing the coffee works similarly to a regular moka pot. You unscrew the top part and remove the funnel, fill the bottom part with water until the valve, place the funnel back, fill it with ground coffee, and then screw the top part back on. To serve the coffee, you unscrew the top part using the longer handle we attached to the moka pot. The handle was inspired by the Arabic raqweh.

Image: Unscrewing the top part of the moka pot. Photo by Marie Verdeil.

Image: Preparing coffee. Photos by Marie Verdeil.

Image: Serving solar-powered coffee. Photo by Marie Verdeil.

Our coffee maker is the first insulated cooking device we built, and there is room for improvement. It doesn’t produce the characteristic gurgling noise when the coffee is ready, probably because the temperature isn’t high enough.

Also, not all water evaporates. Therefore, cleaning involves placing the base under the tap, letting water run in, shaking, and then turning the coffee maker upside down to empty the base. That’s a bit impractical, because the appliance weighs 10 kg.

Because the coffee collects in the upper chamber, it is not kept warm by the mortar slab and the cork insulation. We solve this by wrapping one or more towels around the top part to keep it warm. Alternatively, you could incorporate heat insulation into the design, for example, with a tea cozy.

What you need

Image: The moka pot that we started with. Photo by Marie Verdeil.

Moka pot. Choose the size you need. If you get a second-hand device, make sure that the rubber seal is still in good condition. Also, ensure you can easily take off the handle to replace it with a longer, straight handle.
Mortar. Any construction mortar will work. Make sure it’s not too coarse. Read more about mortar in our solar oven manual.
Metal reinforcement. We pour quite a big chunk of mortar, so we reinforce it with a metal mesh to prevent it from breaking. We use a frying sieve. Chicken wire mesh shaped in a cylinder will work as well.
Cork. For thermal insulation, we use cork sheets available at home decor shops.
Tiles. Read more about tiles and tiling in our solar oven manual.
Chimney. Because the water chamber of the coffee pot is sunk in mortar, you need a way to maintain an opening around the valve to release pressure if the coffee maker overheats. We use a metal pastry bag tip, but any metal tube will work. You can also cut a piece out of an aluminum can and roll it up.
Nichrome wire, heat-resistant electric cable. These are the components for building an electric resistance heater (see our separate manual).

Step 1: Thermal mass and electric resistance heating

Image: Step-by-step instructions (figures 1 to 9). Illustration by Marie Verdeil.

fig. 1 — Start by making a mold for the mortar base. Since the moka pot is circular, we decided to shape our cooker’s heating element into a larger cylinder. Get a wooden board. Use a plastic sheet to form a cylinder and tape it to the board with duct tape. We use an old plastic office folder and shape it around a round cork pad that we place on the wooden board.
fig. 2 — Prepare some mortar and pour a layer of about 2 cm at the bottom of the mold. Let it cure for a couple of hours.
In the meantime, prepare your electric resistance circuit based on the instructions in our manual. We made a circuit with two nichrome wires in parallel (2 x 64 cm) for a resistance of 3.2 Ohms at 18V. It draws around 5.5 Amps.
fig. 3 — Make two holes in the plastic mold to weave the heat-resistant electric cables through. Distribute the resistance wires evenly on the surface of the mortar layer. Make sure they do not touch each other.
fig. 4 — Prepare some more mortar and pour it on top of the first layer so that the electric heating circuit is completely submerged.
Add the metal mesh. Gently press it into the mortar. Ensure it doesn’t touch the nichrome wire and that there is sufficient space in the center for the pot to fit. Add a bit more mortar if necessary so that the mesh is trapped at the bottom. Wait for a couple of hours for it to set.
fig. 5 — Cut the chimney tube at an angle to fit snugly around the coffee pot valve while pointing up. Attach it to the moka pot body with hot glue or clay to ensure it stays in place while pouring mortar. It will later be submerged in mortar.
fig. 6 — Place the coffee pot on top of the mortar layer, in the center. Remove the handle; otherwise, its tip would get stuck in the next mortar layer. We’ll replace it with another handle later in the building process.
fig. 7 — Prepare more mortar and pour it into the mold, around the coffee maker. Fill the mold up to the screw rim of the coffee pot. Let the mortar cure for at least 24 hours.
fig. 8 — Once hardened, remove the plastic mold sheet.
fig. 9 — Let the base dry for an additional 48 hours. Position the base on top of a grid to allow the bottom to dry as well. The mortar should feel completely dry to the touch.

Image on the left: The mold for the mortar slab. Image on the right: The electric resistance heating next to the mold in which the mortar is cast. A metal mesh sieve acts as a reinforcement inside. Photos by Marie Verdeil.

Image on the left: The electric resistance is embedded in the mortar. Image on the right: The moka pot, with the original handle removed, in the mortar slab. The chimney is glued to the base. Photos by Marie Verdeil.

Step 2: Insulation

Image: Step-by-step instructions (figures 10 to 15). Illustration by Marie Verdeil.

We insulate the mortar slab with a 2 cm thick cork layer on all sides (including the top and bottom). We use rectangular and circular cork sheets, 4 mm thick, which means we use five layers.

fig. 10 — Layer 5 circular cork pads for the bottom part. Use wood glue to secure them together.
fig. 11 — Use another five cork pads for the top. Cut out a circle in the middle, about 2 cm wider than the coffee pot, so that there is room to add a plaster protection to the cork. Glue them together.
fig. 12 — Start cutting and layering cork sheets around the cylinder, using painter’s tape to keep them in place. In each layer, make two holes to weave the electric wires through.
fig. 13-15 — Before adding the last sheet, decide where you want to route the cables from the electric heating resistance. You can cut away a path into the last but one layer of cork to allow the wires to pass. We let them out at the bottom. Add the last sheet of cork (fig 15.). Use some tape to secure it.
At this stage, you can run a test. We used water only, in order not to stain the cork with coffee. Test how long it takes for the water to reach the top chamber.

Image on the left: Cork sheets on top of the finished mortar slab. Image on the right: The device is covered in cork and ready to test with a wattmeter. Photos by Marie Verdeil.

Step 3: Tiling

Image: Step-by-step instructions (figures 16 to 21). Illustration by Marie Verdeil.

We chose to cover our coffee maker in glazed tiles. They are waterproof, easy to clean, and commonly used in kitchen furniture, walls and counter tops. Tiles are easy to adapt to various shapes and are aesthetically pleasing. Furthermore, they are easy to obtain and tiling doesn’t require expensive tools.

fig. 16 — First, prepare the surface for tiling. Adhesive mortar doesn’t stick well on cork, so start by covering the cork with plaster. We used plaster bands, similar to those used for medical casts. The bands also help to keep the cork sheets together and fill the air gaps. At this stage, protect the coffee maker with painter’s tape to prevent it from getting plastered.
Leave the plaster to dry for 24 hours.
fig. 17 — Once the plaster has dried, you can tile the surface. We found vintage glazed tiles on the street that we cut into strips and sanded off their sharp edges. For the top part, we use tile fragments from tiles that shattered during the cutting process. Mix approximately a cup of adhesive mortar for walls (which is stronger than adhesive mortar for floors). Using a spatula, spread some mortar at the back of each tile before pressing it against the cylinder. Leave a gap of a few millimeters between each tile for grouting. Repeat this process for the top, and ensure the chimney entrance remains clear. Leave everything to dry as indicated on the mortar packaging.
fig. 18 — Now it’s time to grout the tiles. The aim of grouting is to seal the joints between the tiles to avoid moisture and dirt getting in. Mix the grout with water following the instructions on the package. With a scraper, or an old plastic card, press the paste in between the tiles. After about 20 minutes, clean up the excess grout with a damp sponge. Smooth out each grout line with a wet finger or sponge. Leave everything until it’s dry.

Image: Tiling the plastered surface. Photos by Marie Verdeil.

Step 4: Finishing touches

fig. 19 — Add a handle. Remove the original handle of the moka pot, as it has a vertical design that would get stuck in the mortar. Replace it with a horizontal handle, which gives leverage to screw and unscrew the top chamber and helps to serve the coffee. We made a handle out of wood and attached it with a bolt and nut to the coffee pot using the metal attachment welded on the cylinder. A length of about 20 cm is ideal. Use strong wood because the handle needs to withstand significant stress.
fig. 20 — Wiring. Extend the short heat-resistant cable with a longer 1.5 mm² regular electric cable. Use shrink tape to prevent water or coffee from coming in.
Base. Add a 20 mm thick wood base and cork layer to the underside of the structure. It creates room to lift the coffee maker more easily, and protects the tiles from damaging the countertop surface.
fig. 21 — Done! Brew your first solar-powered coffee.

Image: The coffee maker is finished. Photo by Marie Verdeil.

Other ideas

You could also fit the coffee pot in mortar in an existing box, like an old wooden crate or a scavenged metal drum. Instead of tiles, you could use other kinds of coating with a cement, lime or plaster base. You could also decide to integrate the pot in the kitchen furniture counter.

Credits

Concept: Kris De Decker, Marie Verdeil.
Design: Marie Verdeil, with input from Anna Mareschal de Charentenay.
Construction & documentation: Marie Verdeil, with assistance from Hugo Lopez.
Thanks to AkashaHub Barcelona for the workspace. Living Energy Farm & Cal Poly for their pioneering work on insulated solar electric cookers.

How to Build a Solar Powered Electric Oven

Mon, 13 Oct 2025 00:00:00 +0000

Image: The insulated solar electric cooker that we build in this manual. Photo by Marie Verdeil.

ARTICLE

STEP BY STEP BUILDING PROCESS

Cooking’s high power use

Electric cooking devices are challenging to operate on an off-grid solar PV system. For example, an electric oven requires between 1,000 and 5,000 watts of power, while electric stove burners have an average power consumption of 1,000 to 3,000 watts per burner. If you want to use an oven and one electric stove burner simultaneously, you need a solar array of at least 32 square meters in optimal weather conditions - just to cook. ¹

You can overcome this problem by storing solar power in lead-acid or lithium-ion batteries. If these batteries are powerful enough, they can temporarily provide you with a higher power supply than your solar array can deliver. Batteries are also necessary if you want to cook after sunset or in bad weather, which is likely the case. Unfortunately, batteries account for 70-90% of the costs and the energy invested in a solar PV system. ²

Cooking thus makes it difficult to completely disconnect a household from the power grid and switch to autonomous, smoke-free, small-scale power production. ³ That is especially so when you have a small budget and limited space for solar PV panels. For example, when I attempted to go off the grid in my apartment in Barcelona by using solar panels on the balcony and window sills, it was mainly the electric cookstove that thwarted my efforts. ⁴

Image: The insulated solar electric cooker that we build in this manual. Photo by Marie Verdeil.

How to adapt an electric cooking device to solar power?

Many of the devices we take for granted nowadays were designed for an era of abundant electricity generated by fossil fuels. However, the electric oven we build in this manual demonstrates that modern appliances can be redesigned for an era of intermittent, less concentrated power sources, such as wind and solar energy. Our oven is powered by a 100-watt solar panel, small enough (50x90cm) to fit on a balcony. Furthermore, it can cook after sunset, without the use of batteries.

The key to significantly reducing the power consumption of a cooking device is thermal insulation. Our electric solar oven has 5 cm of insulation on all six sides. Its power use is further reduced by a lower cooking temperature of about 120°C (248°F). You can cook all food safely at much lower temperatures than those typical in modern cooking devices - it just takes longer.

Our oven is powered by a 100-watt solar panel, small enough (50x90cm) to fit on a balcony.

The key to cooking after sunset without batteries is thermal mass. Rather than storing electricity from solar panels in a battery for operating the cooker at night, the heat supplied by the solar panel during the day is stored in the appliance itself. Because the oven retains a high temperature at sunrise, it’s quickly ready for cooking again in the morning. Connected to a solar panel, it’s almost always preheated and ready to use.

Thermal mass also allows the cooker to continue operating after periods of clouds and rain. Likewise, opening the oven door hardly affects the temperature inside. The heat is stored in the mortar and tiles, and the air temperature quickly returns to normal when the door is closed again.

Our oven is heated by a self-made electric resistance that connects directly to the solar panel, without any intervening battery, solar charge controller, or voltage regulator. To maximize energy efficiency, the oven chamber is dimensioned around a metal oven tray and features a side door. Its shape and weight resemble those of a regular oven.

Image: A drawing of the solar electric tiled cooker. Illustration by Marie Verdeil.

The advantages of solar electric cooking

The device we build in this manual is known as an “insulated solar electric cooker” or “ISEC”. The ISEC is a more recent and more sophisticated version of the “solar box cooker”, which is an insulated wooden box with one or more transparant glass plates on top. When a solar box cooker is put in the sun, its interior reaches temperatures that are high enough to boil water and cook food. ⁵

While an insulated solar electric cooker also consists of a well-insulated box, it does not have a glass plate on top. It’s powered by a solar PV panel instead, which is connected to an electric heating element inside the cooker. One could also describe the ISEC as a fireless cooker with an electric heater inside. ⁶

Image: Two conventional, non-electric solar box cookers. Solar energy enters the glass plate and heats up the interior. Built by Audrey Belliot (Slowlab) and Marie Verdeil. Photo by Marie Verdeil.

Image: An insulated solar electric cooker is powered by a solar PV panel, which is connected to an electric heating element inside the cooker. Photo by Marie Verdeil.

The conventional solar box cooker is a very simple device that works without electricity and is cheap and easy to build. ⁷ By comparison, the ISEC is a bit more complex to build and requires a high-tech solar panel. However, solar electric cooking has several important advantages that can make the extra effort worthwhile:

An electric solar oven can be located inside your kitchen. Conventional solar box cookers only work when they are outside in the sun. That is great for events and picnics, or if you have a garden. However, for many people, it would be more practical to have their cooking appliance in their kitchen. The ISEC makes this possible because only the solar panel needs to be outside. In winter, having the cooker inside will also increase its energy efficiency. It will lose less heat to the environment due to cold and wind.
An electric solar oven can be insulated on all sides. Solar box cookers cannot be insulated on the top side; otherwise, solar radiation cannot enter the appliance. ⁸ In contrast, an ISEC can be insulated on all sides, making it more energy efficient than a non-electric solar box cooker. You can further increase the oven’s insulation by draping one or more wool blankets or carpets over it, which is not possible with a non-electric solar box cooker.
An electric solar oven works well in cloudy weather. Conventional solar cookers require full sun to function effectively. That is especially true for parabolic cookers, which concentrate solar rays at a focal point; however, solar box cookers also exhibit low performance during cloudy weather. In contrast, the electric solar cooker can get around that problem by using more or larger solar PV panels. During sunny days, you can use the excess solar PV capacity for other purposes. ²
An electric solar oven needs no attention. Solar box cookers need to be turned towards the sun at least every 15-30 minutes. Parabolic solar cookers require even more frequent movement. In contrast, an ISEC requires no attention. Of course, you could turn the solar panels towards the sun every 15 minutes, which will speed up the heating of the ISEC’s interior. However, solar panels are less sensitive to solar orientation than solar box cookers. Once you have placed food in the insulated solar electric cooker, you can leave it alone.
An electric solar oven allows you to cook after sunset. By embedding the electric heating element in a material with a high thermal mass, an ISEC can remain at high temperature for many hours after sunset. While several methods exist to add heat storage to a conventional solar cooker, they are complex and don’t work very well. For example, adding thermal mass to a conventional solar box cooker would make it too heavy to move around and follow the sun. An ISEC with thermal energy storage is also heavy, but it can remain stationary.

Our choice of building materials

Low-tech Magazine did not invent the ISEC. Our experiments with insulated solar electric cookers, which began in the summer of 2024, are inspired by the work done at Cal Poly University and Living Energy Farm ⁹¹⁰, which we described in an earlier article on direct solar power. ² We borrowed ideas and knowledge from the manuals made by these pioneers, but we also saw some room for improvements, mostly in the choice of building materials. We also applied the concept of insulated solar cooking to a DIY coffeemaker.

Rather than glass wool, sheet metal, and plastic buckets, we have chosen to build our cooking device with tiles, cork, plaster, wood, and mortar.

Rather than glass wool, sheet metal, and plastic buckets, we have chosen to build our cooking device with tiles, cork, plaster, wood, and mortar. These materials are easier to obtain and to work with, and they are more aesthetically pleasing. We aimed to design an appliance that people would actually want to see in their kitchen, and which can be built and repaired with only a few standard tools. It remains to be seen how durable our material choices will be in the long term. Still, for now, the device has been operated extensively for several months without any significant signs of damage.

Image: Some of the materials we have used. Photo by Marie Verdeil.

Our solar electric cooker consists of several key components: the structure (a wooden box), the electric heating element (nichrome wire), the insulation (cork), the thermal mass (mortar and tiles), and a solar PV panel.

Ceramic & terracotta tiles

The use of tiles is the distinctive feature of our design. Most ISECs built to date use aluminum for the inside oven compartment and metal or plastic (for example, a bucket) for the outside. However, making a water-tight aluminium box is not easy and requires specialist tools. Plastic works well as an outer shell, but it looks rather bad and may become brittle over time.

The use of tiles is the distinctive feature of our design.

In our oven, thick terracotta tiles form the cooking chamber, providing a waterproof, fireproof, and easy-to-clean interior surface. The tiles prevent water from entering the insulation layer or the electrical system, and they ensure that the insulation does not become damaged by heat. ¹¹ We also put tiles on the outside of the cooker, where they please the eye, protect the device against water damage from the outside, and make it easy to clean.

Image: The oven chamber, made from thick terracotta roof tiles. Photo by Marie Verdeil.

Tiles are easy and cheap to obtain: we collected all of them on the streets of Barcelona. Tiling can be accomplished with minimal skills and tools. You need a tile cutter if you want to cut tiles shorter, but you can avoid this by choosing the right size of tiles or by applying a “trencadís” technique, made famous by architect Antoni Gaudí. This technique involves dropping tiles so that they break and rearranging them altogether as a mosaic.

Using a tile cutter requires some practice, and you will likely break a few tiles as you learn. Make a groove in the tile by passing the sharp tungsten wheel several times across it. Then apply some pressure on both sides with your hands: the tile should break along the line. Some tiles are more challenging to cut than others. To fix tiles to wood, use a cement-based adhesive mortar. To attach tiles to cork, we applied a layer of plaster bands in some cases, as mortar doesn’t adhere well to cork. Use grout to seal the joints between tiles.

Cork or wool for insulation

Insulation is key to the workings of all solar cookers, including the ISEC. It’s the insulation that allows cooking with a very small solar panel. The heat accumulates over time because the insulation slows down the release of heat from the cooker to the outside environment. For insulation, we use a 5 cm layer of expanded cork on all six sides of our device. Expanded cork is a natural insulation material that provides excellent insulation. It’s made from cork waste, bound using steam. Standard cork sheets also work well.

For insulation, we use a 5 cm layer of expanded cork on all six sides of our device.

Cork is expensive, and it’s not a material you easily find on the streets. You can obtain a cheaper but more labour-intensive insulation material by cutting up discarded and second-hand wool clothes and blankets. Apart from cork and wool, there exist many other insulating materials. However, many are toxic or unpleasant to work with, and unlike cork and wool, they are often flammable. Cotton and cellulose are cheap and sustainable (waste) materials, but they do not insulate as well as cork or wool.

Image: The expanded cork insulation. Photo by Marie Verdeil.

Mortar for heat storage

We made the base of our cooking device out of mortar, a material that retains a lot of heat. Thermal mass is the key to cooking after sunset. The solar panel stores heat in the oven rather than electricity in a battery. The mortar serves a dual function: it also safely encapsulates the electric heating element (see below). Every ISEC with a self-built heating element will have some thermal mass; however, we have made a thicker slab to enhance heat storage. The tiles of the oven chamber provide extra thermal mass.

The solar panel stores heat in the oven rather than electricity in a battery.

Mortar is composed of cement, sand, and water. Sold as a powder in bags, it must be mixed with water before application. You can also buy a bag of cement and mix it with sand and water to obtain mortar. Follow the instructions on the packaging for the powder-to-water ratio. Once cured, which takes several days, mortar becomes (and remains) hard. There’s no need to use refractory cement, which is made to withstand high temperatures in fireplaces and pizza ovens, because the temperature in our oven is not that high. Sand is an alternative material with a high thermal mass.

Heating element and electric system

Our oven is heated by an electric resistance element, which is connected directly to the solar panel. We initially used commercial heating elements in our prototypes, which yielded disappointing results. Therefore, we decided to build our own, based on the manual provided by the Living Energy Farm. ¹² Building your own heating element involves extra work, but it’s worth the effort.

Many commercial heating elements have built-in thermostats, which can complicate temperature regulation inside the oven. They also require a voltage input that does not align with the voltage output of most solar panels, which introduces the need for an extra electronic component (a buck converter). Securely fixing commercial heating elements proved to be difficult as well, and we had trouble keeping moisture away from the electrical system; at one point, this resulted in an electrical fire.

Building your own heating element involves extra work, but it’s worth the effort.

By embedding a self-made heating element in a mortar base, we solved all these problems. A custom-made electric resistance consists of a circuit made of nichrome wire, which is an alloy of nickel and chrome. The length and thickness of the nichrome wire determine its heat dissipation and power consumption, allowing you to precisely scale the circuit according to the voltage and current produced by your solar panel. You connect the nichrome circuit to the electric cables of the solar panel, with a short section of heat-resistant electric cable in between (see our manual).

Our solar electric cooker features a thermal fuse and an internal thermostat, both of which are embedded in the mortar layer. However, these components are not necessary when operating the oven on a solar panel without a battery. Nature already provides the thermostat: once the sun goes down, the heating element stops working, making it unlikely that the oven overheats.

Image: Building your own heating element involves extra work, but it’s worth the effort. Photo by Marie Verdeil.

Image: The thermal switch and fuse are seen on the first layer of mortar. Photo by Marie Verdeil.

How to use the solar electric oven

Our cooking appliance can be used to cook raw food (vegetables, grains, meat, fish). It can also work as a (slow) microwave oven, warming up leftovers or a ready-made meal.

Food safety

Our oven reaches a maximum cooking temperature of about 120°C (248°F). To avoid food poisoning from potentially dangerous bacteria, food should either be refrigerated or heated to a minimum temperature of between 58 °C and 74°C (136°F-165°F), depending on the type of food, for at least 15 seconds. Cooked vegetables and fruits must reach a temperature of 58°C (136°F). Most meats and seafoods can be safely cooked at 63°C (145°F). Ground meats require a temperature of 71°C (160°F), and leftovers and poultry should reach 74°C (165°F). We monitor the temperature of the food using a food thermometer, which we inserted through the chimney hole at the top.

Image: Food made in the electric solar oven. Photo by Marie Verdeil.

Keep in mind that the temperature inside the oven will drop once you put the food inside. You should not put frozen food inside, because the temperature will drop spectacularly, and it may take many hours before a safe cooking temperature is restored. For the same reason, the oven should be preheated before the food is placed inside. However, because it’s connected to a solar panel, our oven will be at a sufficiently high temperature for most of the time. You should not keep food in the solar cooker overnight, unless you are sure it maintains a safe temperature until the morning (our cooker does not).

Image: Slow-cooked food (before and after cooking). Photo by Marie Verdeil.

Cooking time

It’s perfectly possible to build an insulated solar electric cooker that cooks food just as fast as a normal oven (see further). However, it makes a lot of sense to build a low-temperature, insulated “slow cooker” instead. First, it allows you to cook with a smaller solar panel. Second, slowly cooked food tastes better and retains more of its nutrients. Third, at lower cooking temperatures, food cannot burn and for that reason you don’t need to stir it either. The inconvenience of a longer cooking time is thus compensated for by a more relaxed and easier cooking process.

On average, it takes about twice as long to cook food compared to using a conventional oven. Most meals we make, cooking raw food, take between two and four hours. Heating leftovers or a ready-made meal takes about one hour. These time spans were measured in optimal weather conditions and with a preheated oven.

Cooking after sunset

Due to the cooker’s high thermal mass, it will take several hours to heat it when you first connect it to a solar panel. However, from the second day onwards, the solar panel will keep the cooker at a continuously high temperature, even for many hours after sunset. When fully charged at sunset, reaching a temperature of approximately 120°C (248°F), our electric cooker maintains a sufficiently high temperature to cook for 4-5 hours. Once the food gets in, the temperature drops but remains high enough (above 80°C/176°F) to cook food safely. The stored heat that remains at the end of the night allows us to restart the cooking process quickly in the morning - our oven is still above 40 or 50°C (104-122°F) at sunrise.

The stored heat that remains at the end of the night allows us to restart the cooking process quickly in the morning.

Draping one or more wool blankets over the oven at sunset further increases the heat storage, allowing for cooking a meal even later in the evening, or starting cooking even earlier the next day. You can also use blankets to raise the energy efficiency of the oven during the day, resulting in a higher cooking temperature.

Moisture

Depending on the food you prepare, excessive moisture in the cooking chamber can be a problem. The water in the food may evaporate and collect in the oven space. Therefore, our oven has a small chimney through which the moisture can escape. It can be closed with a cork cap if you want to keep the moisture inside. It is a good idea to leave the oven door open occasionally so that any moisture in the insulation layer can evaporate.

Alternative cooker designs

Because we shaped our cooker around an oven tray, it’s mostly suited for oven dishes. However, other designs are possible. An earlier prototype we built has a heat chamber the size of a soup pot, and so that one is better suited to prepare stews and soups. Whatever form you choose, it’s always a good idea to dimension your cooker around a specific cooking utensil. If you put a small pot into a large oven chamber, you will waste a significant amount of energy heating empty space.

Image: An earlier prototype we built has a heat chamber the size of a soup pot, and so that one is better suited to prepare stews and soups. Photo by Marie Verdeil.

Thicker insulation

The thicker the insulation layer, the more energy efficient the oven will be. A thicker insulation layer allows you to use a smaller solar panel for the same cooking time, or a faster cooking time using the same solar panel. A thicker insulation will also improve the heat storage. However, keep in mind that the device’s volume will increase exponentially. Another 5 cm of insulation on all six sides would have made our oven’s size unpractical for most kitchens.

Higher cooking temperature

If you want a solar electric cooker that cooks faster at a higher temperature, you should choose a larger solar panel and a more powerful heating element, and you should raise the setting of the thermostat and thermal fuse. Adding extra insulation also accelerates the cooking time. However, please note that we did not test our building materials at higher temperatures; therefore, proceed at your own risk.

Much depends on the local customs surrounding eating times, especially dinner. For example, European dinner times vary from about 17:00 to 19:00 in northern countries to between 21:00 and 23:00 in southern countries. The early dinner times in the north align with solar cooking. However, the late dinner times in the south would require more powerful cookers with higher temperatures and more thermal storage to cook after sunset, or to safely store a warm dish prepared in the morning.

More or less heat storage

While a solar electric cooker always needs insulation, you can build it with little or no thermal mass. The choice depends on how you want to use the device. With little to no thermal mass, the cooking appliance will heat up and cool down relatively quickly, and it will be somewhat lighter. But it won’t be able to cook after sunset. Many of the ISECs built by others are of this type.

On the other hand, it’s also possible to create a larger version of our solar cooker that can be used to cook for 24 hours a day. Add more thermal mass, insulation, and consider a higher oven temperature, as well as using a larger solar panel and a more powerful electric resistance. Such a cooking device would always be ready to use immediately, without any need for electricity storage, and it could work in industral kitchens or as a community cooking appliance.

You could also build a solar electric cooker with a heat storage consisting of metal rather than mortar. It does not allow you to cook after sunset, but it does enable you to reach higher cooking temperatures for a short time. That makes it possible to bake and fry food.

Step by step buiding guide

Image: Exploded isometric drawing of our insulated solar electric oven. Illustration by Marie Verdeil.

What you need

Cooking utensil

Oven tray. To hold the food that you are cooking in the oven. This tray, which can be made of metal, ceramics, or heat-resistant glass, is the first thing to obtain, as you will dimension the oven around it.

Electric heating element & electrical system (see our separate manual)

100W solar panel.
Nichrome wire.
Heat-resistant electric cable.
Thermal switch.
Thermal fuse.

Structural materials

Wood boards. The oven is built around a wooden structure. You can reuse an existing box or make it from scratch. Reclaimed wood or chipboard is fine, since none of it will be visible.
Tiles. We use tiles for both the interior of the cooking chamber and the exterior of the oven.
Wood screws.
Hinges and hooks. To attach the oven door.
Feet for the oven. We made these out of wood. Feet make it easier to lift and move the oven, and they protect the oven against water damage from below.
Handle for the oven door. We made one out of wood.

Insulation materials

Expanded cork boards. We used 5 cm thick expanded cork boards as insulation on all sides. We used roughly 1 m2 of expanded cork. You can also use regular cork or wool insulation. Avoid flammable materials such as cotton, wood chips, or any oil-based insulation material. Cork and wool are fire-resistant materials.
Thin cork sheets (4 mm). You place them as a sealant between the oven door and body. You also use them to fill in the height differences between the expanded cork layers above the oven chamber.

Heat storage material

Construction mortar. We use mortar to provide thermal mass for heat storage and to embed the electric resistance heater.

Fixing & filling materials

Adhesive mortar. To fix the tiles to wood and cork surfaces.
Grout. To fill up the space between the tiles.

Extra components

Food thermometer. You need one with a long sensor, as it will have to travel through the thick insulation layer at the top to reach the food.
Chimney pipe. Roll a tube of thin aluminum sheet, which you can cut out of a soda can. Alternatively, buy a metal tube of the correct size.

Tools

Screwdriver
Wood saw
Drill (you need a concrete drill bit for the chimney opening)
Soldering iron and tin
Tile cutter (optional if you find the right size of tiles)
Measuring tools
Utility knife
Mortar trowel and mixing container

Step 1: Build the structure

Image: Step-by-step instructions (fig 1. to 4.). Illustration by Marie Verdeil.

Obtain an oven tray and measure it. We dimensioned our cooking device around a stainless steel tray approximately the size of an A4 sheet: 20 cm x 27 cm.
fig 1. — Using tiles, create a box around the tray with enough room to slide in and out easily. The box will become the inner chamber of the solar cooker. For now, keep the structure together with tape. When determining the dimensions of the heating chamber, leave some space at the top of the oven tray to allow for heat circulation. Ideally, you find tiles that have the correct dimensions. Otherwise, cut the tiles to the correct dimensions using a tile cutter.
Measure the exterior dimensions of the tiled oven chamber to calculate the dimensions of the wooden box that will surround it. Add 5 cm of space on all six sides to fit the expanded cork layer. At the bottom, add about 2-3 cm extra to account for the mortar, which will embed the heating element. Add 5 mm to the dimensions on all sides to ensure everything fits.
fig 2. — Build the wooden box according to the calculated dimensions (don’t forget to add the wood thickness). Screw the wood together in a way that allows for the removal of the upper part later in the building process (see step 3). Measure everything a few times before you start cutting the wood, as it’s easy to make mistakes.
fig 2-3. — To make sure that the door neatly aligns with the rest of the wood box, build the structure as a whole and then cut away (saw) the door part off the box. The door part needs to be 6 cm deep to fit in the insulating cork layer and the oven chamber tile on that side.
fig 4. — Once you have cut the wood, unscrew the top board to gain better access inside.

Image: The oven chamber inside the wood structure. The space in between will be filled up with cork insulation and a mortar layer at the bottom. Photo by Marie Verdeil.

Step 2: Make the electric heat resistance

Create a resistive heating element using Nichrome wire. See our separate manual for instructions.

Step 3: Add insulation, create the heat storage and add the electric heat resistance

Image: Step-by-step instructions (fig 5. to 8.). Illustration by Marie Verdeil.

fig 5. — Using a thin saw or utility knife, cut and glue the expanded cork insulation boards to cover all sides of the area. You can use wood glue or hot glue. Keep the top board apart to add it later.
fig 6. — Mix some construction mortar with water and create a layer of about 10-15 mm on the bottom cork layer. Leave it to set for a couple of hours.
fig 6. — At the back of the box, about 10mm above the mortar layer, pierce a hole through the cork and wood to channel the electric cables through.
fig 7. — Place the resistance circuit on top of the mortar bed and drive the heat-resistant cable endings through the hole in the back of the box. Make sure the nichrome wires don’t cross or touch, and that the (optional) fuse and thermal switch are also lying on top of the mortar.
fig 8. — Pour another 10-15 mm of mortar to cover the circuit.

Image: The first layer of mortar with the nichrome circuit, thermal switch, and thermal fuse on top. All these components will be hidden in the second layer of mortar. Photo by Marie Verdeil.

Phase 4: Fix the cooking chamber in place and complete the insulation

Image: Step-by-step instructions (fig 9. to 16.). Illustration by Marie Verdeil.

fig 9. — Take the tiles you prepared for the inner chamber. Place some mortar at the back of the bottom tiles and press them onto the mortar bed.
fig 10. — Using adhesive mortar, fix the remaining tiles to the sides and back of the cork boards, recreating the oven chamber that you taped together in step 1.
fig 11. — Using a drill with a concrete drill bit, make a 10-12 mm hole in the center top tile to fit an air vent chimney.
fig 11. — Lay the remaining top tiles to rest on the edges of the side tiles with a bit more mortar. It is a good idea to tilt the top tiles slightly to one side to guide condensation moisture away from the food tray.
fig 12. — Before placing back the top cork board on top of the inner tile chamber, mark the position of the chimney and drill through the cork and wooden boards.
fig 12-13. — Place the top corkboard on the top tiles with a bit of adhesive mortar and screw back the top wooden board to close the box. If there are some air gaps, fill them in with cork scraps or sheets to prevent heat leakage.
fig 14. — Glue 4 mm thick cork sheets to the box insulation that surrounds the oven chamber on the front. Use wood glue. This extra layer helps to close the door tightly and prevents any heat from escaping.
fig 15. — Insulate the door by fitting a 5 cm expanded cork board inside with wood glue. Using a bit more adhesive mortar, place the last tile on the door, making sure it aligns and closes the inner chamber when the wood box is closed.
fig 16. — Glue another 4mm corksheet to mirror the chamber’s edge.

The box is now mostly finished. Let everything dry/cure for at least 48 hours.

Image: Fixing the cooking chamber. Photo by Marie Verdeil.

Step 5: Finishing touches

Image: Step-by-step instructions (fig 17. to 19.). Illustration by Marie Verdeil.

fig 17. — Tile the box top to make it waterproof and heat-resistant (you can put the oven tray there when it comes out of the oven). Ensure that you leave a hole for the chimney.
Make and insert the chimney. Fit it into the hole you made.
Grouting. Seal the inner tile chamber with grout to prevent moisture from entering the cork insulation. Do the same for the exterior tiles and for the joint with the chimney. We also added plaster on the sides to protect the wood.
fig 18. — Add a handle to the door.
fig 19. — Add hinges to attach the door to the oven body using screws. Place a metal latch on each side to tightly lock the door during operation.
Add small feet to the oven to make it easier to lift and to protect it against water damage.
The oven is finished!
Connect the heat-resistant cables sticking out of the oven to the wires of the solar panel. Insert an on/off switch between them (on the positive wire).

Image: Some assembly steps for the oven (from left to right): Fig.1: Drilling a hole for the chimney, with all insulation in place. Fig.2: Extra layer of cork to cover the door and box insulation. Fig.3: Tiling the exterior of the solar electric oven. Fig.4: Grouting. Photos by Marie Verdeil.

Credits

Concept: Kris De Decker, with input from Marie Verdeil.
Design: Marie Verdeil, with input from Anna Mareschal de Charentenay.
Construction & documentation: Marie Verdeil, with assistance from Hugo Lopez.
Design & construction of two earlier prototypes: Vaiva Vinskaité, with input from Kris De Decker and Marie Verdeil.
Thanks to: Samira Allaouat & Alexandra Tollefsrud for the tiles. AkashaHub Barcelona for the workspace. Living Energy Farm & Cal Poly for their pioneering work on insulated solar electric cookers.

In affluent, industrialised societies, cooking is rarely seen as a problem when it comes to resource use and carbon emissions. For example, in the US, only around 5% of a household’s energy use is attributed to cooking devices (such as stovetops, ovens, microwaves, and water kettles). However, while cooking requires relatively little energy, it requires a lot of power. Energy consumption equals power consumption multiplied by time. Since cooking devices are only used for a short time during the day, their energy consumption is relatively low. However, their high power use makes it challenging to operate them on an off-grid solar PV system. ↩︎
Direct Solar Power: Off-Grid Without Batteries, Kris De Decker, Low-tech Magazine, August 2023. https://qelnixcor.cloud/2023/08/direct-solar-power-off-grid-without-batteries/ ↩︎ ↩︎ ↩︎
Too Much Combustion, Too Little Fire, Kris De Decker, Low-tech Magazine, December 2019. https://qelnixcor.cloud/2019/12/too-much-combustion-too-little-fire/ ↩︎
How to Get Your Apartment Off the Grid, Kris De Decker, Low-tech Magazine, May 2016. https://qelnixcor.cloud/2016/05/how-to-get-your-apartment-off-the-grid/ ↩︎
The first recorded use of the solar box cooker goes back to the eighteenth century when the increased use of glass made people aware of its ability to trap solar heat. For more information, see Hirst, Eric. “A golden thread: 2500 years of solar architecture and technology: by Ken Butti and John Perlin Cheshire Books, distributed by Van Nostrand Reinhold Company, New York and London, 1980, 304 pp,£ 11.95.” (1981): 167. /// Daniels, Farrington. Direct use of the sun’s energy. Yale University Press, 1964. /// Telkes, Maria. “Solar cooking ovens.” Solar Energy 3.1 (1959): 1-11]. ↩︎
If We Insulate Our Houses, Why Not Our Cooking Pots?, Kris De Decker, Low-tech Magazine, July 2014. https://qelnixcor.cloud/2014/07/if-we-insulate-our-houses-why-not-our-cooking-pots/ ↩︎
Find an example of a manual here: https://reclaimdesign.org/diy-solar-oven ↩︎
Although glass plates reduce the heat losses from the oven interior somewhat, they cannot offer the same level of thermal resistance as a thick layer of wool or cork. ↩︎
See: https://sharedcurriculum.peteschwartz.net/isecooker-construction/ ↩︎
See: https://livingenergyfarm.org/insulated-solar-electric-cooker/ ↩︎
It may seem unusual to build an oven from materials such as wood, cork, or wool. However, the temperature reached inside our oven does not pose any risk for these materials. Cork and wool are thermally stable up to approximately 200°C and start to degrade above that temperature. They are fire-resistant materials: they don’t ignite and won’t spread fire. Wood does not ignite at temperatures below 250ºC. Furthermore, all these materials are separated from the heating element and the interior oven chamber by mortar and tiles, which resist much higher temperatures. When we had an electrical fire in one of our first prototypes, the fire did not spread. ↩︎
See: https://conev.org/ISECmanual14.pdf ↩︎

Direct Solar Power: Off-Grid Without Batteries

Fri, 25 Aug 2023 00:00:00 +0000

Image: a laptop running on direct solar power. Photo: Marie Verdeil.

Conventional solar installations do not question our dependence on fossil fuels and the energy-guzzling lifestyle that results. Both rooftop solar panels and large-scale solar farms provide us with all the power we want, even when the sun is not shining. That is because these systems use the central power grid, which largely runs on fossil fuels, as a kind of battery to cope with power shortages.

Although grid-connected solar panels can reduce the fossil fuel consumption of thermal power plants, these savings are at least partly offset by the additional fossil fuels required to build and maintain what is essentially a dual energy infrastructure. Combining solar and wind power can further increase the share of renewable energy in the power grid, but this requires further infrastructure development. Apart from energy, this also demands a lot of money and time.

Replacing fossil-fuel-fired power plants with energy storage, so that surplus electricity generated on sunny days can be stored for when there is no or insufficient sun, encounters the same problem. Energy storage, whether integrated into a power grid or located at individual households (off-grid systems), is very expensive and carbon-intensive to build and maintain.

Autonomous solar installation

The production of solar panels obviously costs money and energy. However, the financial and energy costs of the associated back-up infrastructure are many times higher. For grid-connected solar installations, these costs are very difficult to calculate precisely, but for autonomous solar installations (without grid connection and with their own energy storage) it is a lot easier. As an example, I will therefore take the small autonomous solar installation that powers my living room in Barcelona.

This system consists of two 50W solar panels on the balcony, a 100 Ah lead-acid battery and a 10A charge controller. The energy generated is used for lighting, the music system, and charging laptops and other electronic devices, among other things. The initial financial investment was 340 euros: 120 euros for the solar panels, 170 euros for the battery and 50 euros for the charge controller.

But while the solar panels should last 30 years and the charge controller about 10 years, I have to replace the lead battery on average every three to five years. ¹ Over a 30-year lifespan, the costs then amount to €120 for the solar panels, €150 for the charge controllers and – in the best case scenario – €1,020 for the batteries. The batteries (and associated charge controllers) therefore account for about 90% of the total lifetime costs.

Energy storage also dominates the plant’s “embedded” energy (and resulting carbon emissions). Producing my lead-acid battery took 1,200 megajoules (MJ) of energy. ² Over a 30-year lifetime (six batteries at best), that equates to 7,200 MJ. The three charge controllers add another 360 MJ over a 30-year lifetime, bringing the total energy consumption for the battery system to 7,560 MJ. ³ In contrast, the production of the solar panels costs only 2,275 MJ out of a total of 9,835 MJ. ⁴ Conclusion: more than 75% of total fossil energy consumption is due to energy storage.

Image: To the right on the balcony are the two 50W solar panels that power my flat's living room. Next to it is the 30W solar panel that makes this website work. Photo: Marie Verdeil.

Image: The structure for the solar panels, built from waste wood. Photo: Kris De Decker.

Image: The 100 Ah lead-acid battery powering the living room after sunset. Photo: Kris De Decker.

Other types of batteries would not significantly change this conclusion. For a comparable off-grid system with lithium-ion batteries, energy storage would account for about 95% of the total lifetime cost (which is almost double that of a system with lead-acid batteries). Assuming an optimistic lifetime (10 years) and including charge controllers, lithium energy storage accounts for some 70% of the energy invested in a solar grid system. ⁵ ⁶ For nickel-iron batteries, energy storage would account for 85% of the total lifetime cost (there are no energy cost data). ⁷

The scale and location of the solar installation also make no difference. A larger system needs more solar panels, but also larger batteries and more expensive and powerful charge controllers. The ratios remain the same. ⁸ The only factor that may give the solar panels a slightly larger share of the total cost is the structures on which they are mounted. I don’t take this into account because I built them myself from waste wood. However, if the solar panels are mounted on a roof, a DIY solution is less obvious. But even in that case, the cost of energy storage remains by far the biggest consideration.

Direct solar energy: much cheaper and more sustainable

Unlike fossil fuels, the sun and wind are not available on demand. The problem with our approach to renewable energy is that we insist that power should always be infinitely available, regardless of the weather, seasons or time of day. Matching energy demand to supply – as was done in the past – would lead to dramatic reductions in the cost and use of fossil fuels.

For example, if I omitted the battery storage of my solar installation, my system would become about 10 times cheaper: 120 euros instead of 1,290 euros over a 30-year lifetime. Alternatively, I could spend 1,290 euros on solar panels alone, which would give me a solar system of 1,075 watts. That’s ten times the capacity of the setup with batteries, more than what would fit on the balcony.

Without the battery and charge controller, the energy cost of the installation also drops from 9,835 MJ to 2,275 MJ. In other words, I could generate at least four times as much solar energy with the same investment in fossil fuels.

How can direct solar power be practical?

All well and good, but the sun does not shine after sunset and the amount of solar energy varies throughout the day and year. So how then can using solar panels without batteries (or other back-up infrastructure in the case of grid-connected installations) be practical?

To answer that question, we look at a pioneer of “direct solar power”: the Living Energy Farm. This environmental education community in the US state of Virginia is completely “off-the-grid” thanks to solar power, but only 10% of the solar power generated passes through a (nickel-iron) battery. However, the solar panels provide power for several homes, a communal kitchen, a metal workshop, and a farm. ⁹ ¹⁰

Image: direct solar power at the Living Energy Farm.

The solar installation has been in operation since 2011 and consists of separate systems with a total peak power of 1,400 watts. ¹¹ In comparison, the average peak power of a residential solar installation in the UK and the US – for one household – is 4,000 watts and 6,500 watts, respectively. As in my flat, the Living Energy Farm uses energy sparingly, but the fact that hardly any batteries are used has other reasons.

Some appliances are only used during the day

A first reason is obvious: some electrical appliances and machines are only used during the day. This is true, for example, of all machines in the metal workshop, including a band saw, compressor, grinder, circular saw, lathe, milling machine and drilling machine. It also applies to agricultural machinery such as a grain mill and a deep well pump. Linked directly to solar panels, these machines offer all the capabilities of modern grid-powered technology, with the exception that they can only be used during the day. ¹⁰

On a much smaller scale, I have used direct solar power for a soldering iron, glue gun and irrigation pump (for the balcony) at home. Other examples of appliances and machines that could be used only during the day include hoovers, sewing machines, washing machines, game consoles, laser cutters and 3D printers. It is not so difficult to imagine a modern society where activities such as vacuuming and DIY chores only take place during the day. It is certainly not a return to the Middle Ages.

Image: several workshop tools at the Living Energy Farm, most of them run on direct solar power. Image: Alexis Zeigler.

Image: Metal lathe running on direct solar power, Living Energy Farm. Image: Alexis Zeigler.

Image: Soldering with direct solar power. Photo: Marie Verdeil. [Watch the video](https://www.youtube.com/watch?v=qozZCJU4IOc).

Moreover, not all electrical appliances require constant attention. Washing machines or dishwashers that trigger automatically when the sun shines are often cited example applications of a “smart” power grid. But that approach relies on an extensive infrastructure of electricity transmission, communication networks, and electronics-packed appliances.

In contrast, in a decentralised direct solar approach, the intelligence is provided by the sun and the rotation of the planet. A direct solar-powered washing machine or dishwasher can be fully charged and switched on in the evening. The machine then starts up “automatically” in the morning. You can even use timers (electronic or mechanical) to run different appliances one after the other.

Whether clouds pose an additional limit to a direct solar installation, and to what extent, depends on the size of the solar panels. Doubling the area of solar panels guarantees sufficient solar power during moderate cloud cover, while the installation remains much cheaper and more sustainable than a system with batteries or other backup infrastructure.

An even larger area of solar panels could provide sufficient energy even during heavy cloud cover, but increasing the size of the system tenfold brings the cost back to the level of an autonomous system with batteries. Quadrupling the area makes the system equally dependent on fossil fuels again.

Many appliances already have batteries

Direct solar power does not rule out the use of electrical appliances after sunset either. As mentioned, the Living Energy Farm has a modest battery system, providing power for lights, fans, and electronic devices after sunset, among other things. ¹⁰ In addition, many modern appliances already have built-in energy storage. This is the case for all kinds of electric vehicles, for most electronic gadgets, and for older electrical appliances with AA batteries.

Consequently, these types of devices can be charged with direct solar energy during the day and then used for several hours after sunset thanks to the built-in battery. Combined with a lithium-ion power bank, a direct solar panel can also make it possible to charge USB devices after sunset. This strategy can even work for lighting, as there are many battery-powered lamps that you can use as modern torches, hung in different parts of rooms and buildings.

Image: A mobile phone on direct solar power. Photo: Marie Verdeil.

Of course, outsourcing chemical energy storage to the device is not the most sustainable option. The production of lithium-ion batteries requires fossil fuels, and (unlike lead-acid batteries) they are not recycled. The best solution, of course, is to reduce the use of electrical devices. But charging them with direct solar energy is a lot more sustainable and efficient than via other batteries or a fossil-fueled electricity grid. If we use high-tech devices, then preferably in the smartest way possible.

Non-electric energy storage

A third reason why direct solar power is more practical than it initially seems is that some electrical appliances can be used after sunset thanks to thermal energy storage. This is much cheaper and more sustainable than electrical energy storage. Thermal energy storage is already fairly well established for space and water heating systems, which store solar-heated water in an insulated boiler or (for space heating only) in the building envelope. It is no surprise that the Living Energy Farm has such systems, and solar thermal energy also provides hot water in my flat.

However, the same approach also works for two important household appliances that need to work after sunset and also consume a lot of electricity: the fridge and the cooker. Instead of storing electricity from a solar panel in a battery to then power a fridge or cooker after sunset, these appliances on the Living Energy Farm use thermal insulation. This keeps the heat inside (in the case of the cooker) or outside (in the case of the fridge) when there is no power supply. The thermal insulation also ensures very high energy efficiency, which means that each of these appliances can already operate on a solar panel of just 100-200 watts.

A direct solar-powered fridge

It is perfectly possible to connect a conventional fridge or freezer directly to a solar panel, but such an appliance would heat up very quickly at night. Even refrigerators with the most energy-efficient labels have a relatively limited insulation thickness (usually 2.5 cm). However, if that insulation thickness is increased to about 12.5 cm, the energy consumption of a refrigerator drops by a factor of four. ¹² ¹³ The passive cooling capacity of a refrigerator can be further increased by adding thermal mass in the form of a water tank inside the appliance. During the day, the solar panel cools the water or converts it to ice. At night, this cold water or ice slows down the heating of the refrigerator. ¹⁴

A direct solar-powered fridge also opens at the top, not at the front. Cold air is heavy, and so much less energy is lost that way when someone opens the door. All these design choices add up to spectacular energy efficiency. A study of direct solar refrigerators in very sunny regions (Texas and New Mexico, USA) showed that they maintained their cooling capacity for 6 or 7 days without power supply. The units operated year-round with solar panels of only 80W to 120W. ¹⁵ The Living Energy Farm powers its solar refrigerator with a 200W panel. ¹⁰

Image: The Sundanzer DDR165. A refrigerator designed specifically for direct solar power. Photo: Sundanzer.

Unlike solar heating, solar cooling is optimally tuned to seasonal variations in solar radiation. Cooling requires more energy in summer, when there is more solar energy. The aforementioned refrigerator in New Mexico recorded electricity consumption of 406 watt-hours per day in summer and only 230 watt-hours in winter. ¹⁶ Moreover, the technology can be used throughout the cold chain, of which the household refrigerator is only a small (but essential) part. Another application is air cooling, although this is less well researched and more challenging. ¹⁷

A direct solar electric cooker

In principle, a conventional cooker can also be connected directly to a solar panel, but as with a conventional fridge, it is not very practical. You can only cook during the day, and you have to install a lot of solar panels. A single hot plate needs 1,000 watts of electrical power. A solar electric cooker solves these problems by packing the cooktop with thermal insulation. The technology is basically a combination of an electric cooktop and a haybox.

Image: Test of an electric solar cooker. Photo: California Polytechnic State University (Cal Poly).

Thanks to thermal insulation, an electric solar cooker slowly accumulates heat during the day, which can then be used for cooking after sunset. In this way, a much lower power supply can be sufficient to achieve high temperatures. Think of it as “charging” your cooker, not with electricity but with heat.

Researchers at US California Polytechnic State University (Cal Poly) built the first solar electric cooker in 2015. Their 12-volt device, which has since been further developed, needs only a 100W solar panel to work. It boils a litre of water in an hour. With a full day of sunlight, it can cook almost 5 kg of beans, rice, stew or potatoes. ¹⁸

Cooking after sunset is possible by using a cooking pot with a much thicker bottom (5-10 kg). Cal Poly’s research team managed to bring the temperature of that solid heat storage to 250°C in five hours with a 100W solar panel. They were then able to boil a litre of water in three seconds after sunset. In another test, they stir-fried 1 kg of vegetables in two minutes. The ideal configuration consists of two cooking pots: one with and one without heat storage. Thus, an electric solar cooker can cook both slowly and quickly, depending on the time of day and the dish. ¹⁹

Image: The principle of a solar electric cooker with solid heat storage. Drawing: California Polytechnic State University (Cal Poly).

Thermal or electric?

Like solar water and space heating systems, cooking and cooling can work both with and without electricity – with PV panels on the one hand and solar thermal collectors on the other. But while solar space and water heating are more cost- and energy-efficient without electricity, for solar cooling and solar cooking it is just the opposite.

Space and water heating require relatively small temperature differences, which can be provided by low-cost solar thermal collectors made of glass plates and water pipes. In contrast, cooling and cooking require larger temperature differences, which require more sophisticated (vacuum tube or parabolic) solar collectors – and these are more expensive than PV panels. ²⁰ ²¹

The only exception is a simple solar cooker – an insulated box with a glass top – but it cannot achieve such high temperatures. Moreover, an electric solar cooker has some additional advantages. With a non-electric appliance, you have to cook outside, which is less practical but also less efficient, especially in winter: a thermal solar cooker will lose more heat to the environment. An electric solar cooker is also more energy-efficient because it is insulated on all sides. It also works better in cloudy weather and can be used after sunset. At the Living Energy Farm, the parabolic solar cooker is only used in optimal conditions – at full sun and high outdoor temperatures.

What are the technical challenges?

Although the Living Energy Farm is putting all these applications of direct solar energy into practice, there are some technical challenges for those who want to follow suit. Almost all our modern technology is designed to operate with a stable and uninterrupted power supply. It doesn’t have to be that way, but for now, direct solar power usually requires some tinkering. A direct solar system is much easier to build than an autonomous system with batteries, but it often requires modifications on the appliance side.

Some devices can be connected directly to a solar panel: it is enough to connect the positive and negative contacts of the solar panel and the device. For example, machines with a DC motor tolerate large fluctuations in the power supply. The metal workshop and agricultural machinery at the Living Energy Farm work this way. If clouds block the sun, the combined electrical load can become greater than the power supply from the solar panels, but this does not stop the machines. All the engines will slow down because they share the available energy, but they all continue to do useful work. ¹⁰ ²²

The same applies to all appliances that work on the basis of resistive heating elements, such as kettles, hotplates or electric heating systems. They work regardless of power or voltage, just slower or faster. A direct solar-powered fridge preferably operates on a variable DC compressor, which can adjust its speed according to the varying solar power production. ¹⁰ ²³

Many other devices need a specific and stable voltage input, which usually does not match what the solar panel produces. This can be solved by placing a DC-DC converter (a “buck” or “boost” converter) between the solar panel and the device. This is a small electronic module that converts the fluctuating voltage of a solar panel into a constant output voltage for a low-voltage device (5V, 12V or higher). ²⁴

Image: Experiments with direct solar power. Photo: Marie Verdeil.

If you use an inverter in addition to this, even mains appliances can operate directly on a solar panel. ²⁵ DC-DC converters are essential for all appliances that contain electronic components. This is the case for many appliances today, including those, such as washing machines or coffee machines, that until recently operated without electronics. That often gives you two options to run such appliances on direct solar power. You can either fit a DC-DC converter or modify the appliance by bypassing the electronics.

DIY manuals & commercial devices

Most direct solar power applications operate at low voltage, so you can safely do it yourself. Low-tech Magazine will soon publish a manual on this. However, the Living Energy Farm uses direct current with higher voltages for a number of applications. Examples are the machine tools in the metal workshop (90V) and a number of powerful electric solar cookers (48V, 180V). It is not a good idea to build these systems yourself unless you have the help of a qualified electrician, as these voltages can lead to fatal accidents.

Those wishing to build their own (low-voltage) electric solar cookers will find comprehensive manuals at both Living Energy Farm and Cal Poly. ²⁶ The devices can be made with simple materials. The insulation material should be fireproof. Example materials are rock wool, fibreglass, natural wool or clay.goed

Different technologies can be used for heating elements, but embedding nichrome wires in cement is the simplest option. These wires can be taken from a variety of appliances such as toasters, ovens and hotplates. In principle, the heating wires can be attached directly to the cooking pot, but it is more practical to make a heated “nest” in which a pot can be placed.

Image: Inspired by Cal Poly's work, Living Energy Farm also developed a number of electric solar cookers, one of which they [offer for sale through their website](https://livingenergylights.com/product/roxy-solar-electric-oven/). The Roxy Oven can be used as a hotplate or an oven, for example for baking bread. The door also remains closed when used as a hot plate. This solar cooker has no energy storage.

Image: The Roxy Oven without the door and with the glass wool insulation visible. The device - made in the metal workshop with direct solar power - runs on 48V and requires a solar panel of 200 to 500 watts. Living Energy Farm also offers Sunstar's solar refrigerator [for sale online](https://livingenergylights.com/product/sunstar-direct-drive-8-cuft-chest-style-refrigerator-freezer/).

Does direct solar power waste energy?

The sustainability of a solar installation depends not only on the energy required to produce and maintain the infrastructure, but also on the energy produced by the solar panels during their lifetime. Some people will argue that direct use of solar power is inferior to conventional grid-connected or battery-powered solar installations in this respect.

After all, the hoover, washing machine and power drill are not used every day, and if no electrical appliance is connected then a solar panel will not produce power either. Consequently, the amount of electricity produced by the panel will decrease over its lifetime, while the energy needed to manufacture the panel remains the same. This makes the power from a direct solar panel more carbon-intensive.

However, because energy storage in batteries (or the grid-connected alternative) accounts for such a large proportion of the total energy invested, a standalone solar panel can waste quite a lot of energy before it becomes less sustainable than its counterpart with battery storage or grid connection.

Moreover, direct use of solar power avoids the charging and discharging losses caused by batteries, or the energy losses in the transmission infrastructure for grid-connected systems. Both have to be offset by additional solar panels. Furthermore, solar panels connected to batteries or the grid also waste power – a consequence of the large difference in energy production between summer and winter.

Maximising direct solar power with collective services

Nevertheless, it is important to maximise the energy production of a direct solar panel. In that context, it is useful to return for a moment to the original example system located on my balcony. Direct solar power could be a nice addition to this system, especially for the fridge and cooker. It was because of these appliances that I concluded in 2016 that it was impossible to completely disconnect my flat from the grid.

However, the Living Energy Farm shows that it could be done: there is room for a further 200 watts of solar panels (4 x 50W) on the balcony, enough to power both a thermally insulated fridge and hob. Additional battery capacity would not be needed.

For other appliances, however, direct solar power is of little use in my case. It would not be very efficient to install an extra solar panel for the washing machine or the power drill, as they are only used occasionally. This seems to play into the hands of a “smart” electricity grid, because that way many households can use the same solar power – there is always someone who needs to wash clothes or drill a hole.

However, such a smart grid does require a lot of infrastructure, even if direct solar power were to be used at that scale. It may not require batteries or fossil fuels as backup, but it does require transmission and communication infrastructure.

Image: A record player on direct solar power. Photo: Marie Verdeil. [Watch the video](https://www.youtube.com/watch?v=_LjSigJv0-0).

The Living Energy Farm demonstrates an alternative solution: the communal organisation of household tasks and work. Instead of a communal power grid distributing energy to many indidvidual households, we can set up collective services with decentralised energy production.

In the Living Energy Farm communal workshop, direct solar power can be used much more efficiently than in an individual workshop that is only used occasionally. A collective laundry in each street would also use direct solar power much more efficiently. Moreover, we save a lot of energy on building appliances this way, and gain a lot of space.

Direct wind power?

This strategy becomes even more important if we choose not direct solar power but direct wind power – or a combination of both. The Living Energy Farm is located in a sunny region, but the same approach could also work in windy places.

However, there is an important difference between solar power and wind power. The efficiency of a solar panel does not depend on its size, which makes solar power ideal for decentralised energy production. In contrast, the efficiency of a wind turbine increases more than proportionally as the rotor diameter increases. Much better than one wind turbine per household, therefore, is a somewhat larger wind turbine for a community of households, e.g. for powering a collective laundry or workshop.

The service life of lead-acid batteries depends on many factors. If they are discharged too deeply or are not fully charged regularly, the service life can be shorter than three years. On the other hand, a lead-acid battery that is hardly used or not discharged at all can last much longer than five years. However, the academic literature states a life expectancy of three to five years and this has also been my experience with the batteries I have used since 2016. See, for example, “Optimal Sizing and Life Cycle Assessment of Residential Photovoltaic Energy Systems With Battery Storage”, A. Celik, in “Progress in Photovoltaics: Research and Applications”, 2008. & “Energy pay-back time of photovoltaic energy systems: present status and prospects”, E.A. Alsema, in “Proceedings of the 2nd World Conference and Exhibition on photovoltaics solar energy conversion”, July 1998. ↩︎
Manufacturing a lead-acid battery (based on largely recycled materials) takes about 1 MJ of energy per watt-hour of storage capacity. My 100 amp-hour battery equates to a storage capacity of 1,200 watt-hours, and so the embedded energy equals 1,200 MJ. Over a 30-year lifespan, I need six of these batteries at best, so 7,200 MJ in total. Source: “Energy Analysis of Batteries in Photovoltaic systems. Part one (Performance and energy requirements)” and “Part two (Energy Return Factors and Overall Battery Efficiencies)” (PDF). Energy Conversion and Management 46, 2005. ↩︎
Not much research has been done on the embedded energy of charge controllers. The most relevant data I found is a value of 1 MJ per watt maximum power: Kim, Bunthern, et al. “Life cycle assessment for a solar energy system based on reuse components for developing countries.” Journal of cleaner production 208 (2019): 1459-1468. For a capacity of 120W (my charge controller has a maximum capacity of 10A x 12V = 120W), this amounts to 120 MJ. For the estimated lifetime, I found values of 7 and 12.5 years: same reference as above, as well as: Kim, Bunthern, et al. “Second life of power supply unit as charge controller in PV system and environmental benefit assessment.” IECON 2016-42nd Annual Conference of the IEEE Industrial Electronics Society. IEEE, 2016. I therefore made the calculation on an estimated lifetime of 10 years. ↩︎
Nawaz, I., and G. N. Tiwari. “Embodied energy analysis of photovoltaic (PV) system based on macro-and micro-level.” Energy Policy 34.17 (2006): 3144-3152. According to this widely quoted source, it takes 3,500 MJ to produce 1 m2 of solar panel. My two solar panels together measure 0.65 m2, representing a total energy cost of 2,275 MJ. A more recent literature review puts the energy cost for producing different types of solar panels at between 1,034 and 5,150 MJ/m2. The most recent studies of silicon solar panels in this review put the energy cost at around 1,000 MJ/m2, much lower than the figure I am using. See: Ludin, Norasikin Ahmad, et al. “Prospects of life cycle assessment of renewable energy from solar photovoltaic technologies: A review.” Renewable and Sustainable Energy Reviews 96 (2018): 11-28. ↩︎
Lithium-ion batteries are a lot more expensive than lead-acid batteries, but unlike lead-acid batteries, they can be discharged deeper (up to 15% of their total capacity) and have a longer lifespan (7 to 10 years). Consequently, fewer and smaller batteries are needed. Taking these factors into account, the lifetime cost of the battery is €750, compared with €1,020 for lead-acid batteries. On the other hand, lithium-ion batteries require a more sophisticated and more expensive charge controller: a 10A charge controller costs between 200 and 600 euros, depending on the quality. Assuming a price of 400 euros for the charge controller and a 10-year lifetime for both the battery and the charge controller, battery storage accounts for 95% of the total lifetime cost (a total of 2,070 euros, much more than the total cost for the system with lead-acid batteries). Sources: https://www.lithiumion-batteries.com/products/product/12v-50ah-lithium-ion-battery & https://www.lithiumion-batteries.com/products/12v-lithium-ion-battery-chargers/ ↩︎
Although the production of a lithium-ion battery costs more energy than the production of a lead-acid battery (1.4-1.9 MJ/Wh versus 1 MJ/Wh), this is offset by a longer lifespan and greater discharge capacity. The energy cost of lithium-ion batteries over a 30-year lifetime is then about 3,000 MJ, significantly less than a comparable lead-acid battery system. In contrast, the charge controller contains more complex electronics. Unfortunately, no data is available for the energy cost of such a charge controller. So there is no alternative but to estimate the energy cost based on the financial cost, which is four to twelve times more expensive than a charge controller for a lead-acid battery. Assuming a four times higher cost, the embedded energy of the charge controller increases to 480 MJ, or 1,440 MJ over a 30-year period. The total energy cost for the system is then 6,685 MJ, less than a comparable system with lead-acid batteries. Of this, almost 70% is attributable to battery storage. ↩︎
Nickel-iron batteries are even bigger and heavier than lead-acid batteries and they need regular maintenance. But they can be fully discharged and have a very long service life (20 years). Moreover, they can be used with the same charge controllers as lead-acid batteries. The lifetime cost over 30 years for the battery is €750, cheaper than the six lead-acid batteries of similar capacity. The total lifetime cost for a nickel-iron battery system with 100W solar panels is €1,020, of which 85% goes to energy storage. Unfortunately, nickel-iron batteries are hard to find, especially the smaller models. Sources: https://beyondoilsolar.com/product/nickel-iron-battery-industrial-series/ & https://beyondoilsolar.com/product-category/batteries/nickel-iron/ ↩︎
Actually, the price of solar panels in a somewhat larger solar installation would be proportionally even smaller. This is because solar panels with small sizes (such as 50W) are proportionally more expensive per watt of peak capacity than solar panels with more conventional sizes (from 250W onwards). More or less the same applies to the energy cost. ↩︎
https://livingenergyfarm.org ↩︎
Alexis Zeigler, founder of the Living Energy Farm, wrote a book about the project, which is available in full online: Empowering Communities. A Practical Guide to Energy Self Sufficiency and Stopping Climate Change. It can also be ordered in hard copy. ↩︎ ↩︎ ↩︎ ↩︎ ↩︎ ↩︎ ↩︎
Since direct solar power does not require a charge controller for each separate system, splitting up a solar system does not involve any additional costs or energy consumption. ↩︎
Research shows that doubling the insulation thickness from 2.5 cm (standard insulation) to 5 cm reduces the annual electricity consumption of a refrigerator (50 litre capacity) from 250 to 125 kilowatt hours. ¹³ With an insulation thickness of 10 to 12.5 cm, electricity consumption halves again to around 60 kilowatt hours per year. Even thicker insulation brings a smaller reduction in electricity consumption and is no longer attractive because thicker insulation also increases the cost and size of the refrigerator. The study concerns a solar-powered AC fridge that operates thanks to an inverter and a battery, which is less energy-efficient than a direct solar-powered fridge. ↩︎
Gupta, B. L., Mayank Bhatnagar, and Jyotirmay Mathur. “Optimum sizing of PV panel, battery capacity and insulation thickness for a photovoltaic operated domestic refrigerator.” Sustainable Energy Technologies and Assessments 7 (2014): 55-67. ↩︎ ↩︎
This thermal mass can literally be a container of water placed inside the fridge. or some water bottles for drinking. But the water can also be stored in reservoirs along the side of the appliance, behind an inner lining that keeps them in place and hides them from view. Water has a higher heat storage density than air, keeping the temperature stable for longer. ↩︎
Ewert, M., et al. “Photovoltaic direct drive, battery-free solar refrigerator field test results.” Proceedings of the solar conference. American solar energy society; American institute of architects, 2002. ↩︎ ↩︎
This advantage only applies if the fridge is set up in an unheated room. The modern habit of placing a fridge in a heated kitchen when the outside temperature in winter is equal or lower than that in the fridge is obviously absurdly wasteful. But neither is this advantage valid in tropical countries, where temperatures are high all year round. ↩︎
The use of direct solar power for space cooling has not been analysed as thoroughly as for domestic refrigerators. See: Luerssen, Christoph, et al. “Life cycle cost analysis (LCCA) of PV-powered cooling systems with thermal energy and battery storage for off-grid applications.” Applied energy 273 (2020): 115145. Moreover, it is unlikely to achieve equally large energy savings. A refrigerator is always insulated, but in the case of an air-cooled room or building, this is not necessarily the case. Moreover, a refrigerator is set up in a room where there is a stable temperature. A building is subject to greater temperature fluctuations and can also be heated by direct solar radiation. So direct solar air cooling is a lot more complicated. See: Qi, Ronghui, Lin Lu, and Yu Huang. “Parameter analysis and optimisation of the energy and economic performance of solar-assisted liquid desiccant cooling system under different climate conditions.” Energy conversion and management 106 (2015): 1387-1395. ↩︎
Solar Electric Cooking, Pete Schwartz, Cal Poly Physics. See also this PowerPoint by the same author. ↩︎
Insulated Solar Electric Cooker with Solid Thermal Storage, Andrew McCombs et al., 2022. See also this video. ↩︎
See: Ferreira, Carlos Infante, and Dong-Seon Kim. “Techno-economic review of solar cooling technologies based on location-specific data.” International Journal of Refrigeration 39 (2014): 23-37. ///// Riffat, James, et al. “Development and testing of a PCM enhanced domestic refrigerator with use of miniature DC compressor for weak/off grid locations.” International Journal of Green Energy 19.10 (2022): 1118-1131. ///// Du, Wenping, et al. “Dynamic energy efficiency characteristics analysis of a distributed solar photovoltaic direct-drive solar cold storage.” Building and Environment 206 (2021): 108324. ///// Alsagri, Ali Sulaiman. “Photovoltaic and photovoltaic thermal technologies for refrigeration purposes: an overview.” Arabian journal for science and engineering 47.7 (2022): 7911-7944. ↩︎
For lack of research, whether the same applies to embedded energy consumption is not clear. ↩︎
In both cases, however, it is necessary to bypass the device’s switch, because DC electricity produces more heat than AC electricity. Instead, a suitable external switch can help, but in doing so you bypass the device’s safety mechanism, which is obviously a risk. ¹⁰ Again, this does not have to be the case: it is technically possible to make devices suitable for direct solar power. ↩︎
A fixed-speed compressor can only use 50% of the solar power produced in a useful way, while a variable-speed compressor uses about 75% in a useful way. ¹⁵ A capacitor is needed to provide the compressor with an energy boost during the start-up phase. ↩︎
Instead of a DC-DC converter, you can also install a small “buffer battery” and a charge controller. Like a DC-DC converter, the charge controller will ensure a stable output voltage. In addition, the small battery can provide limited energy storage that can be useful to handle short spikes in power consumption. For example, some devices have a current spike when charging. The disadvantage of a buffer battery is that the cost and embedded energy increase, and additional components can fail. A capacitor is an alternative technology to absorb power peaks. ↩︎
However, using low-voltage direct current devices is a lot more energy-efficient because solar panels also produce low-voltage direct current: https://qelnixcor.cloud/2016/04/slow-electricity-the-return-of-dc-power/ ↩︎
Insulated Solar Cooker Construction Manual, Living Energy Farm. Insulated solar electric cooker manual, Pete Schwartz, Cal Poly Physics. Roxy Oven Manual, Living Energy Farm. Video presentation manual solar electric cookers, Alexis Zeigler, Living Energy Farm. Video manual for making heating wires. Thermal heat storage: Insulated Solar Electric Cooker with Solid Thermal Storage, Andrew McCombs et al., 2022. Also see this video. ↩︎