This is my first blog post how I’m starting to build solar ovens in Portugal.
It’s the beginning of May and I am staying at the Biovilla Eco farm in Setubál’s beautiful natural park Arrábida.
Biovilla is a sustainability co-operative project and holds 55 hectares inside the park. It’s open for guests and has also volunteers to keep the garden and the guesthouse. It’s less than 1 hour away from Lisbon, so if you are in the neighbourhood, pay a visit, as this is a really charming place.
There’s also workshops of all kinds hold over the weekends, so check also their facebook page for events.
I’ve been enthusiastic about green energy and it’s every-day, small-scale applications for a long time, so I finally wanted to start building them.
I couldn’t wait anymore to build something, so I just started looking for materials around trash cans and yards few days ago.
Finding the materials for solar oven was fun and quite easy
- I cycled up to Palmela’s castle and found a guy close by who was renovating an apartment and got some glass wool from his trash bag for free.
- I found a thick chipboard for a baseplate laying on a grass next to a pavement on the outskirts of the town centre.
- Then I found some fruit cases and thick black fabric next to an apartment building’s garbage cans.
When you start looking for stuff to use, your eyes open to how much stuff there is just laying around. That makes every bike trip a treasure hunt, of sorts.
The sideboards I found the next day while I was shopping for tools in Forum China. This huge Chinese store had a big backyard that had pallets and pieces of wood laying here and there. I asked the guy who helped me inside the store if I can take some and he gave me a permission. I found some nice pieces with right thickness and width, so I quickly sawed them shorter with my brand-new saw and stuffed them in my backpack and cycled back home. The pieces were a bit bent as they’ve been at the mercy of the elements. You often get recyclable stuff for free if you just dare to ask.
I found all necessary the materials, tools and accessories I was looking for, except a thermometer to use inside the oven. It’s impossible to find one!
The materials cost me about 25 euros. I had to buy the tools as well, so my total bill was close to 40 euros. You can build the oven only for 10 euros if you can find suitable parts laying around and have tools already. To make a solar oven economic and ecological, it really makes sense to use recycled or unused materials to keep the price and consumption as low as possible.
The basics are really simple. Think of heat as of water, how it seeps out of things not sealed properly, and you’re set.
Biggest expenses for the materials were the deep baking dish that cost me 10 euros and two window frames costing 11 euros.
Missing from the first picture are my multitool, tape and other miscellaneous items.
It was a sunny day and I wanted to make it more ceremonial as a new thing in my life, so I went up on the hillside to an abandoned quarry and sat there on the roof of a derelict building, left there to ruin by the long-gone quarrymen.
With my rudimentary tools and primitive workplace, I managed to build the 2-layered glass frame and the case with hinges before it got dark and burned my face in the process, too.
I didn’t make detailed notes about the build nor took pictures while working on it, as it’s the first oven I’ve built in 4 years, so I had to concentrate fully without distraction and get some confidence before showing others how to build them.
The most important feature building a solar oven is the 2-layered glass for insulation.
I sawed off 1 cm triangular wooden pieces of the fruit box and added them as spacers between the two glasses. Results of the first day:
Testing and improving the solar oven
The basics are really simple: Build a pocket of air that gains heat faster than it loses it.
So the oven worked well like this in the sun, but there was still gaps and not enough insulating material to improve the heating and storing it.
I decided to add a thin layer of the glass wool I found on the bottom. I did this with gloves to avoid getting hair-thin glass needles in my hands. On the sides I added two glass wool pads cut in size with a x-acto.
On top I added a piece of black fabric to keep the pieces in place and make cleaning of the dish easier. I cut smaller pieces on the sides and glued them in. After placing the dish back inside it was laying a bit too high, creating an air-gap on the front side.
For this I decided to use a fastener by using two screws and rubber band to make it tighter. This alone was not enough, so I also glued in some weather-stripping for sliding doors.
I tried to bake some bread with this setup, but I was not patient enough to check a recipe, so I used too much baking powder and it was stale.
After the first tries I realized I want to add a simple mirror to extend the available cooking time, as this setup only works well for about 4 hours per day. I wanted to use recycled material, so I repurposed the other picture frame I had to create a base for the mirror. I didn’t want to buy a roll of aluminum foil, so I luckily found some aluminum wrapping paper next to a trash can, which I cut in size and glued in.
To make it easier on the eyes, I painted it red and added the logo of Biovilla on the backside.
Adding a reflective screen to the oven
The wrapping surface is far from smooth, but it was free and it works anyway. Update: Use smooth surface for better efficiency.
To attach the crude mirror to the oven, I used two hinges and a piece of string to stop it from falling backwards. Adjusting the mirror is now done with a simple clamp and two pieces of string. I couldn’t find anything to make it more solid, so this will do for now.
On hind-sight, having good tools is very important. Using less than optimal screwdriver of my multitool resulted in sketchy finish with the screws.
The oven works really well as it stores the heat in the thick baseboard. Attaching the mirror gives it another benefit, as a enthusiastic solar chef can use it as a lid to keep things hot inside when it’s cloudy or gets dark.
This way the food inside can keep warm for more than an hour!
Best way to use the oven is to leave it outside facing east, so it starts storing heat in the morning. By mid-day it’s too hot to touch, even if there’s occasional clouds blocking the sun.
Adding the mirror and extra insulation makes a notable difference in efficiency. It’s still a simple design and not too expensive. The extras I bought cost around 10 euros more, so adding all together, I paid around 50 euros to build the oven, including the tools. This being said, the oven can work for years, if it’s kept away from heavy rains and used properly, which makes using one an ecological choice.
Cooking with the Biovilla solar oven
Now for the real thing, cooking with solar oven. If the solar oven doesn’t actually cook food, then it’s just a nice trinket taking space outside without any real use, so it was time to put it to a test. I personally knew that it’s going to work, but this was also a demonstration for others to see how well it performs under sunny conditions.
One thing I really need more experience with, is the right things to cook in the oven. This time I tried it with fusilli, chopped up carrots and bell pepper with tomato sauce and some water in the bottom to cover the fusilli.
As the oven holds the moisture inside quite well, I theorized that the pasta would cook in the dish without any precooking.
For spices I added some Portuguese olive oil, paprika, black pepper and salt.
At 11:00 the oven was too hot to touch and I placed the smaller dish inside.
I estimated it would take around 1 hour 20 minutes for everything to cook well (before the sun creates a shade on the east-side).
As I had no way to measure the temperature, I had to use just visual inspection.
Obviously one should not open the oven too often to check the food.
Using the oven is dead-simple.
- Heat it up before using
- Place the food inside in a suitable cookware (the darker the better)
- Tighten the fastener
- Aim it where the sun will be in 30 minutes
- Adjust the mirror
- Check after about an hour
I checked after 1 hour and 20 minutes and a nice amount of steam rise from the food. The pasta had turned to white and everything was steaming and softening. I decided to let it cook for another 20 minutes. The temperature was around 20 degrees outside and the skies were mostly clear.
- It’s normal that the steam condensates on the lower mirror and creates a layer of droplets. You have shorter cooking time if you can keep the glass clean by using a lid or bag inside.
To make the oven more effective, it’s better to seal the food with another lid or use a roasting bag. I couldn’t find any bags or lids, so for this experiment, I’m just using the open dish. With the open dish, I assume it reaches close to 100 degrees in temperature, but not enough to visibly boil.
1 hour 40 minutes is fair amount of time to grow your appetite, so I took out the dish with oven gloves! The dish will burn the skin of the fingers when taking it out, so use protection.
There was bit too much water on the bottom and the carrots softened only a little, but the bell peppers, tomato sauce and pasta were delicious! I mixed everything up to make it more consistent and enjoyed my lunch. Another good thing about solar oven is that you can put the food back in the oven to store the rest for later and let it cook further.
You don’t have to worry about food turning cold or wasting energy to keep it hot
For future cooking, I’ll try to find some roasting bags and also give another try of baking bread. With cooking bags I’m confident that the oven will reach over 100 degrees.
The solar oven works and will work even better in Midsummer when the outside temperatures rise higher. I hope other people will give it a try as well and see for themselves that this is a viable method to cook food without spending any energy other than rays of sun.
I will still try to find a thermometer for the oven and measure the temperature to see how effective it really is.
Warning if you are building a wooden solar oven: As this type of oven is partially made of wood, you don’t want it to reach much over 150° Celsius (300° F). Prolonged periods of high temperatures can char the wood and possibly ignite a fire! This is especially true when using a smooth mirror, which can heat parts of the wooden frame too much. I’m not responsible if you burn something, so be careful and keep an eye on it. Update: This oven reached maximum temperature of 106°C in mid-May.
This being said, temperatures under 150 are well hot enough to cook anything with time. If you want to use it for drying, don’t close the fastener or leave a thin spacer under the lid to circulate air and remove the moisture.