Cardboard Passive Solar House

In this post, I will walk you through how to create a passive solar house out of cardboard. Passive solar homes are designed to retain and lose heat without the use of electricity. Instead, they use things like overhangs, thermal mass, insulation, window size and direction, and natural airflow/ventilation to keep the house at the desired temperature. The model will also contain solar panels that are connected to and charge a battery, both of which connect to loads of your choice (I used two fans and three LEDs, which I would recommend) and power them. This is a great project for anyone to try, and it will teach you a lot about troubleshooting, solar power, passive solar homes, soldering, and much more.

Supplies you will need:

Materials: Cardboard, hot glue sticks, solder

Supply: 2 small (12 volts, 0.2 amps) solar panels, 1 big (20 volts, 0.4 amps) solar panel, portable battery, modified USB switch, portable battery charging cord, 16 gauge wire to connect or lengthen any pieces if necessary

Load: modified USB switch, 2 pin euro splicer, 2 small wire-end switches, 2 fans, 3 LEDs, one resistor, and wire for any necessary connections (particularly connecting the LEDs)

Tools: Box cutter, hot glue gun, soldering iron, digital multimeter (not required but strongly encouraged), wire cutter/stripper

Look at this slideshow to learn more about passive solar house principles. The most important things to look at are the importance of window directions, ventilation strategies, and overhangs and shade, specifically south-facing windows, cross and stack ventilation, and the direction and angles of your overhangs. Things like insulation, heat pumps, and thermal mass can't really be implemented with cardboard, so they are nice to learn about, but you don't need to figure out how to include them in your house.

Once you have an idea of some of the components you want to implement to increase the functionality of your house before even adding any electrical components, then you can start thinking about how they will materialize. How can you show these elements using cardboard? I would recommend drawing out a rough floorplan sketch of your house before you start cutting. It doesn't even have to have measurements, though they might be helpful, and it's fine if you decide to stray from your drawing and add or take out a room, for example, but it is really nice to have a plan of action to guide you before you start.

Now that you have an idea of some elements to include in your house and maybe even a design for it, you can start building it with your cardboard, hot glue, and a box cutter. Even if you do have a design, chances are it will change as you start to bring it to life, and that's totally fine. I would recommend trying to have a scale of about 1” in your cardboard house = 1 foot for a real house (if your house theoretically had a 15-foot wall, your cardboard house would have a 15-inch wall).

Be sure to either make a removable roof**, leave a wall or two open, or something else like that so that when you get to wiring your circuit into your house, you can easily access the spaces you need to. Another option that I ended up going with is using masking tape as a hinge to connect the roof to your house and some string on the sides to stop it from opening too far to make it so that you can open up your roof for access, but it will always stay connected.

If you want to and have time, you can also add furniture and decorations like I did to make it clear which room is which.

**You are later going to have to attach your solar panels to your house somewhere so that they can collect sunlight. Usually, your rooftop is the most efficient place to put them, but, as I figured out the hard way, you can't have your solar panels on a removable roof because their wires keep them connected to the house (making the removable feature of the roof no longer functional). Just keep this in mind when designing your roof and if your panels are going to go on one side of the roof, make sure you don't make that part fully removable.

There are a couple of different main ways that you can wire a circuit. One is in series, and one is in parallel.

Wiring in series is pretty straightforward. You connect your supplies and loads in one continuous loop, connecting negative ends to positive ends in a line. Parallel is a little bit trickier but still very doable. You connect all of the negative ends of the supplies and loads together and all of the positive ends of the supplies and loads together.

Each wiring strategy has its benefits and drawbacks, but to decide which kind to use for our purposes, we are focusing on the amperage and voltage of the loads. In series, voltage is additive, meaning the volts from each load add together, so you end up with high voltage. In parallel, amperage is additive, meaning the amps from each load add together, so you end up with high amperage. For this project, our loads don’t need more than five volts, and the solar panels produce twenty volts even in series, so we will wire our solar panels in parallel to create more amperage instead.

Now that we know how we want to wire our solar panels, we have to figure out where to put them.

Where we are located (in California), the best direction to face solar panels to maximize their energy is South. Throughout the day, the South gets the most usable sunlight. With that in mind, we can figure out which direction to face our cardboard house and which roof the solar panels will sit on. As I said, we want our solar panels to face south in order to generate the most energy, so depending on which roof panel you put them on, you want that side of your house to face South.

Another thing to consider while you are constructing the roof that your panels are going on is the angle of the roof. Depending on where you live, the angle will be different, but you can simply google "What is the best solar panel tilt angle for [location]?". As this is only a cardboard house, like everything else, it doesn't need to be perfect, but it is something to think about.

Next, it is time to figure out how your circuit is going to be laid out (not in your house, just out on your workspace). You need to put together a circuit that takes power from the battery and solar panels and distributes it to all of your loads. You will need to connect all of your power sources together on one end of the circuit and all of your loads together on the other. You can split this into two separate circuits that connect in the middle to make it easier to understand: the supply side (solar panels and battery) and the load side (fans and LEDs).

For our load-side, we want to have our LEDs wired in parallel and our fans wired in series. Wiring the fans in series is pretty straightforward, just wire them in a loop, but wiring the LEDs in parallel is a little bit trickier. You will need a soldering iron, solder, 16 gauge wire, and a resistor, and you can watch this video and see the image above to see what that should look like. I would also recommend adding a wire-ended switch to each of your load-side circuits so that you have more control and can individually turn each on and off. Once you have your LEDs and fans or other loads ready, you can start working on the supply side.

This is definitely one of the trickier parts of the project, but I encourage you to try to wire it on your own first. You will need all of the electrical components for this step, and if you can't figure it out, no worries, I couldn't at first, either. The drawing above shows how I wired it, which I think is a pretty good way to do it. The only thing missing from that drawing is that I added a small wire-ended switch into each of the load circuits, which I mentioned above already. Just to give a quick overview of that sketch, you can see the solar panels in the bottom right-hand corner wired together in parallel. They then connect to the first modified USB switch using alligator clips, which then connects to the battery using the battery charger.** This means that when the solar panels are not being used to power the loads, they can charge the portable charger so it can be used when sunlight is unavailable. The USB end of the battery then connects to another modified USB switch, which connects to the euro splicer (the one in my sketch is a four-pin, but a two or three-pin works just as well). Connected to the other end of the euro splicer are the two load-side circuits.

**The battery is also important because the wattage from the supply side is way too high for the load side, and everything would just burn out if all of the power was used. The battery acts as a form of resistor and brings the wattage down to a manageable amount for the loads.

The final step is putting your circuit into your house.

Now that you have laid it out and you know how everything needs to connect, you will probably need to take it back apart to get it into your house. Make sure to either take a picture of it or draw it out before you take it apart (I recommend drawing and labeling because it is easier to understand/see and helps you digest it better) to ensure you put everything back together correctly.

As you start incorporating the circuit into your house, you will notice that there are lots of wires everywhere. I would strongly recommend dedicating a corner of your house (shown above - mine is semi-removable) or adding an “electrical box” (also shown above) onto your house to hide these wires and keep everything nice and neat (you may need multiple - I used one of each).

Once you have done that, you will have to decide which rooms your loads will be in and how they will be attached. For example, I added a beam across my living room/kitchen to hang my fans from. During this process, make sure you make all of the switches throughout your circuit available as well (you can see a couple of my switches in the image of the "electrical corner"). I have 4 total switches, 2 USB and 2 wire-ended, so I made 4 holes in different places to the outside of my house so that I can easily control the power supplies or loads. I also labeled each of the switches to make everything less confusing.

Congragulations! You finished your house, enjoy!