The Passive Solar Greenhouse
A passive solar greenhouse is more a description of the chosen greenhouse heating method than it is a type of greenhouse for vegetable gardening.
As the name implies, you're passively gathering heat from the sun. This means that collection of thermal energy is done without the aid of active mechanical or electrical devices - it happens naturally.
With this type of greenhouse heating method, you design and install it, and then leave it alone.
It operate on its own without interference from you. This type of system tends to be very reliable, although it does have it's limitations.
First, let's discuss some basic thermal energy (heat) properties that you'll need to keep in mind in order to understand how passive solar greenhouse heating works. Once you know how it works, you can determine how best to use it.
Here are the four main points you need to keep in mind during this discussion:
- Thermal energy is always trying to move to a lower state. In other words, warm things are always trying to cool down (give off their heat to something else).
- Dark colored objects easily absorb heat (warm up). Light colored objects do not readily absorb heat.
- Light colored objects give off (emit) heat slowly. Dark colored objects give off heat quickly.
- To effectively store thermal energy, we need to use a medium that has considerable thermal mass. Objects with thermal mass are those that are dense (heavy for the space they take up), and have low thermal conductivity (warms up and cools down slowly). Objects with good thermal mass include concrete, stone, moist soil and water.
Water is perhaps the best thermal storage medium we can use in our passive solar greenhouse. It's cheap, easy to work with, readily available, and will fit into any shaped container we might imagine. The only two drawbacks of water are that it rusts metal objects and it can freeze and break the container we have it stored in.
Passive Solar Greenhouse Heating Techniques
Okay, now that we know a little about the behavior of heat in relation to objects that have thermal mass, let's take a look at how these understandings can be applied in the greenhouse to absorb heat from the sun, store it, and release it when the sun isn't shining.
The four most common techniques for capture and storage of heat in a passive solar greenhouse are as follows:
- Barrels - perhaps the most common and effective means of creating thermal mass inside the greenhouse. If you place 55 gallon drums in the greenhouse against the north wall and fill them with water, you'll have one of the best ways of passively capturing excess thermal energy during the day.
When the sun goes down and the temperature in the greenhouse starts to cool, the warm water in the barrels will start to give off its heat to the surroundings, thus keeping the greenhouse warmer.
Painting the barrels black on the south side exposed to the sun will help the barrel absorb energy more readily. Painting them white on the north side that faces away from the sun will help keep the energy from being released in that direction (where there is nothing to keep warm).
The configuration of the barrels (a tall cylinder) isn't naturally conducive to freezing, and that's a good thing. When you think about it, water in the barrel isn't isolated in a shallow pond or narrow pipe; it's next to or surrounded by the entire mass of water inside the barrel. Therefore, it's difficult to have a 55 gallon drum freeze solid except under an extended duration of the most extreme of conditions.
- Raised beds - another common method for absorbing heat and keeping it in the greenhouse. Instead of barrels filled with water, the planting beds are filled with moist soil. The combination of soil and moisture offers good thermal mass.
The raised beds warm up quicker than the surrounding soil because they offer more surface area to warm up. The warmed soil also gives off heat after the sun goes down, thus keeping the greenhouse temperatures elevated.
Our "kitchen" greenhouse has many passive solar greenhouse features including raised beds made from wood and 55 gallon steel drums cut in half width wise. All of the planting beds are above ground, and the majority of them are barrels as high as 1.5 feet above ground, so that provides an exceptionally large amount of soil with all sides exposed to the warmth inside the greenhouse during the day.
- Containers - another common approach to planting in the greenhouse. It might seem a little bit of a hassle to put every plant in its own container, but it allows you to have 100% of your planting soil (good thermal mass) above ground. This in itself provides a form of passive solar greenhouse heating.
Heat inside the greenhouse will transfer to the soil in the containers. The higher temperatures in the root zones of the plants will have them responding very well to this comfortable environment. When it comes to vegetable gardening, generally the plants care more about soil temperature than air temperature.
The disadvantage here is similar to raised beds, the soil above ground level also gives up its heat to the environment after sundown, so it's prone to cooling off nearly as fast as it heats up.
- Pavement - a form of passive solar greenhouse heating that can easily be overlooked by those of us fixated on barrels filled with water. When I speak of pavement, I include anything with good thermal mass that you use as a walkway in the greenhouse.
Things like pavers, poured concrete, stone and gravel paths, stepping stones, bricks and even poured concrete curbing for your planting beds can all serve as thermal mass for your passive solar greenhouse design.
My three greenhouses incorporate moist sand for the walkway. I find it to be cleaner and more comfortable than dirt walkways, and easily changed if I desire. Moist sand also has relatively good thermal mass. It can hold heat well into the evening after the sun goes down.
The above discussion focuses on large objects for thermal mass. Your passive solar greenhouse objectives can also be achieved using smaller versions of thermal mass. Think bricks and concrete blocks.
Bricks can be used inside your planting beds to add thermal mass right next to what you're growing. Bricks are dense and poor conductors of energy, so they'll make for good thermal mass in your greenhouse to supplement your main approach to achieving a passive solar greenhouse.
Not only can you put bricks in your planting areas, but concrete blocks can also be used as support for shelves and steps into and out of the greenhouse. Bricks and concrete blocks can be used to create platforms and the edges of raised beds too. You're only limited by your imagination and budget.
Create A Passive Solar Greenhouse
So, there you have it. A passive solar greenhouse is one that relies on thermal mass to absorb, store and release thermal energy without any assistance from you, except for initial design and installation. It's reliable and nearly foolproof.
There are a wide range of things that you can use to support heating your passive solar greenhouse. Many items such as steel barrels and bricks can be picked up as scrap or for little cost.
If you integrate stones, rocks and concrete into the design of your greenhouse walkways and raised beds, your environment for vegetable gardening will be attractive as well as functional.
Done with Passive Solar Greenhouse, take me back to Greenhouses