Thursday, 25 October 2012

Thermal mass - Greenhouse Part 2

Oh my, it has been awhile. I wanted to post this a lot sooner, but there has been a lot of stuff happening in the meanwhile.

In this post I am going to go into thermal mass and why it is so important. I will also show and explain the measures we have implemented so far into our greenhouse and also what we are still planning to do. There is a LOT to do yet.

Thermal mass is essentially everything that can retain heat well and release it over a long period of time. Air does not store heat well. This is why I never understood north american heating systems. All that forced air seems to be hot air to me. Heating the air is like heating the outside. You will have to continually throw fuel at it. Once you have it nice and warm in the house and want to open a window, you will have to reheat everything again by adding more fuel, because all the heat escaped and nothing stayed behind.
Thermal mass stores the heat and if you open a window it will get cold in the house too. But then you can close the window and without adding more fuel to the system the room will heat up again through the heat released from the thermal mass. This is the beauty of thermal mass.

Dense and heavy materials generally make good thermal storage (also called heat sinks). In a house, this can be a clay brick wall that is sunlit during the day. In a greenhouse it can be barrels of water that are painted black for maximum heat absorption, but also raised beds packed with rock in the bottom and soil on top. Any of these measures belong to the category of "passive heat". Passive heat is really great, because it is free. And who ever turns down free heat?

For an example of extreme passive heat building have a look at earthships

In our greenhouse we have (so far) three 55 Gallon barrels filled with water and painted black. I could never really feel these drums heating up over the day, which kinda confused me for a while. But now that fall has come and the nights are getting below the freezing mark I can see the effect of these drums. Although they are not keeping the air temperature in the greenhouse up by much we consistently find that the temperature is 3 degrees above the outside temperature at the lowest point.  The most tell tale sign that there is a positive effect is that we have two tomato plants that are growing right in front of the barrel. Those plants are still growing and producing while the lowest temperature we have measured inside the greenhouse was -2 C. I call that a success to some extend.

We also started to add plastic bottles to the back of the greenhouse wall, all filled with water and painted black as well. These containers get really hot on a sunny day and we hope to eventually fill the whole wall top to bottom left to right with more bottles. These are all bottles that are not recyclable in the conventional way. At least they are not wasted and put on the landfill this way.

Lately we added some boxes filled with earth that are placed in the front of the greenhouse to absorb maximum light. These boxes double up as growing beds for winter hardy crops such as cabbage and spinach. We also grow some cold hardy herbs there such as thyme and sage (Really looking forward to using those fresh herbs in our christmas dinner).

We are planning to built another mini garden bed at the back wall that can be converted into a cold frame by enclosing it with windows. A greenhouse inside the greenhouse so to speak. 

In the future we also want to add more insulation to the back of the north wall and the small parts of the side walls to retain more heat in the greenhouse. I also want to have a thick curtain behind the door to prevent drafts. However, I am still looking for the necessary materials. Insulation is so important but unfortunately so expensive too.

I talked a lot about passive heat but I also want to mention active heat. Active heat means basically heating the greenhouse by means of a heat source that is somehow fuelled. And this is where the cost factor comes in. Commercial greenhouses are running a major cost just for heating in the winter times. For a small privately owned greenhouse I don't think this is necessary. However, there are ways of getting active heat for free. If your greenhouse is spacious enough you could encorporate a compost heap inside the greenhouse. The heat produced by the composting organisms can really be excessive and will throw a good bit of heat into the greenhouse. Some people have a section for their chickes in their greenhouse in the wintertime. This works both ways. The chicken have a nice warm temperature during the daytime and the heat of their bodys also helps heating the greenhouse at night.

Other possibilities are a rocket mass heater. Although these heaters run with firewood, small sticks are sufficient and only very little firewood is necessary. Maybe this is a good way to get rid of those apple tree prunings. For more info on rocket mass heaters check out this website

or simply google the term. There is a lot of information out there.

At this stage I would like to invite people to leave some comments to this article and maybe share some alternative ideas about thermal mass and how to heat greenhouses in general.

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