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Heating Your Greenhouse With Nothing But The Sun

Updated on January 25, 2015

A Passive Solar Greenhouse

A Passive Solar Greenhouse
A Passive Solar Greenhouse

For several years now a green revolution is taking over the world. Every day more people start to think about their ecological footprint and what they can do to change this. An easy way to help, is to start growing your own fruits and vegetables in your backyard, like many of us are doing already. Every tomato that does not have to be transported on planes and trucks is another big step forward. But if you live in a cold climate, you often have to resort to growing food indoors or to build a greenhouse. How do you keep a greenhouse warm in the winter? Do you put in a wood-burning stove? Or maybe an electrical heater? This is the problem many people are facing when they want to build their own greenhouse. They're helping the environment in one way by growing their own produce, but they're also harming the environment by burning fossil fuels, only to keep their greenhouse warm in the winter.

There is another way though, and that is to completely rethink the concept 'Greenhouse'. We will take a look what changes can be made to transform a normal greenhouse to a passive solar greenhouse.

What is a Greenhouse?

A greenhouse is a structure designed to grow plants. They are often made from glass or plastic sheets attached to a metal or wood frame. The heat from incoming sunlight is absorbed inside the structure. The surfaces inside the greenhouse release this heat to the air while the roof of the building prevents this heat from escaping. But there's one thing I can tell you, a greenhouse is not as green as you would think.

Rethinking The Greenhouse Idea

When we take a step back and study the traditional greenhouse, we can see all kinds of problems that allow the heat to escape easily. Greenhouses were designed decades, or even centuries ago and besides a few improvements here and there, they haven't been updated since then.

Replacing The North Wall

We all know glass is not a good insulator, but in most cases, greenhouses are made entirely out of glass. If we want heat to stay inside of the building, we should remove the glass wall on the north side and replace it with a thick insulated wall. There is absolutely no reason to have glass on the north side of the greenhouse, since the sunlight will never come in from this direction. The new wall can be made out of wood, straw bales, or any other material that has good insulating capabilities.

Insulating The Floor

If we want to trap heat more efficiently inside the building, we can take a look at other area's we could insulate. How about the floor? You could insulate the bottom of the greenhouse and move the plants to raised growing beds. Now that we've done that we will find that the greenhouse stays warmer for a longer period of time. As an added benefit, our plants are now much easier to access, making harvest a breeze.

Thinking About the East and West Walls

When you start thinking about it, if we can maximize the incoming sunlight, it's probably better to rethink the glass on the east and west wall as well. We can go ahead and remove these walls and replace them with highly insulated ones. It's a good idea to make the door in one of these walls as well, make sure there aren't too many cracks and openings between the door and the wall, this will allow heat to escape easily.

Optimizing the Incoming Sunlight

We should really think about why we want a greenhouse. Most people will say: 'I want to extend the growing period of the plants'. While this is true, in my opinion it's even better to try to grow plants all year long. The hardest part is the winter, because that's when it's coldest. That's why on the shortest day of the year, we would want the greenhouse to capture the sunlight most efficiently. But how can we do this? A great way is to adjust the angle of the south wall, so that on the shortest day of the year, it stands in a 90° angle to the incoming sunlight. An easy way to calculate this, is to take the latitude of where you live and add 10°. Let's say you live on the 50° N line. Just add 10° and now you know you should build your south wall at a 60° angle. This way you maximize the potential of capturing the sunlight, even on the coldest days of the year.

Storing the Heat

Even with all these tweaks, there will still be some problems. A cloudy week can cause the greenhouse to lose it's heat eventually. That's why we need a way to store the heat. Air is not good for storing heat, but water is. Water is able to store a huge amount of energy and is often used in solar systems, for just that reason. In our case, we're trying to store heat in a greenhouse without blocking the incoming sunlight. The solution is to use the entire north wall as heat storage. Stack a bunch of water containers against the back wall and paint them black. When the sun shines on to the containers, the water inside warms up. When the sun is gone at night or on cloudy days, the energy stored in the water is slowly released to the greenhouse, keeping it at a steady temperature. In the winter, the sun stands at a low position and is able to shine deeper into the greenhouse, maximizing our thermal storage in the winter. In the summer, the water containers do not get any sunlight because the sun is at a higher position, now they act as a cooling device, preventing our greenhouse from overheating.

A Passive Solar Greenhouse Design

A Passive Solar Greenhouse Design
A Passive Solar Greenhouse Design

Some Additional Thoughts

A few things to consider when building a passive solar greenhouse: - Ventilation is important. You don't want any condensation on the windows, because this will block the incoming sunlight. Explore some heat-recovering ventilation systems and see what works best for you. Solar powered ventilation is the greenest option.

  • Our north wall is covered by water containers but what do we do with our west and east wall? A good solution is to add a reflective material, to redirect the incoming sunlight that falls on these wall, giving extra light to the plants and the thermal storage.
  • Building a passive solar greenhouse does not have to be more expensive than a normal greenhouse. Try to use recycled materials like scrap lumber, old windows, or even straw bales to insulate the walls.
  • Digging in the north wall will help protect the greenhouse from cold winter winds.

Conclusion

The advantages of a passive solar greenhouse are endless. When you compare it to a classic greenhouse, there is no reason why you wouldn't rather build a passive greenhouse, even more so when you're as concerned about the environment as I am. Keep in mind that these tips aren't the only options you have. There are many other changes you can make to have an even better greenhouse. Have fun building and enjoy your own fresh fruits and vegetables.

An Example of a Good Passive Solar Greenhouse

© 2015 Jochen Englicky

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