Today I would like to compare two greenhouses with similar structures – in both cases a double-layer cavity is formed.
In fact I made a long video doing a case and comparison study of the two here but somehow did not get expected feedback from viewers. Maybe because the video was too long or maybe because I did not do a good job explaining things in the video. So I now want to present it in words and images.
First, let’s take look at this massively built greenhouse located in Olds, Alberta by Mr. Jianyi Dong (screenshot from his video explaining its performance here):
The doulbe layer separation is huge – at a place near the top, it is as large as two meters which is 79 inches! We can see a roll of blanket is there at the top within the cavity.
So we see that Mr. Dong has put in a lot of money and work in building such a greenhouse. In fact, it is very close to the soap bubble foam filled greenhouse that we suggest, just without using a blanket and without making the double-layer cavity so thick – half of that is enough for his case.
Anyway, how did his greenhouse perform? Let’s look at the temperature curve he gives in his video:
The orange red curve shows that, although he could keep his greenhouse’s inside temperature mostly above zero during the night, we see huge swing up to 30 degrees C during the day. He says that he has to open windows and cool it off, otherwise it can get to more than 40 degrees C – it is true and in my single layer greenhouse I can get 30 degrees C in Kelowna BC on a totally clear day in winter.
This proves what I have been saying all along about a conventional greenhouse: too much temperature swing. It is so with single layer covering. The double layer makes it even worse, not less! Covering the greenhouse with blanket during the night does not help much, only keep it slightly warmer at night time.
For that I thank Mr. Dong for his honesty and great effort in keeping and revealing the record.
Now the question, how to solve this problem?
When it is too hot during the day, it is not a good idea to ventilate as we need that heat for night time. So a better way is to use water to chill off the double cavity and transfer the extra heat from air into water.
Since after sunset we start to loose the heat captured, we need to insulate the cavity. There is no insulation that is better than soap bubble foam. Blanket is too clumpsy and expensive. Air alone does not help much as air’s heat absorbing capacity is too low.
Let us see the solid proof with a soap bubble foam filled greenhouse that Richard Nelson built in his backyard 30 years ago. It is literally a tent – see below:
Want to watch a video about it, here it is:
How did this tent-like greenhouse perform? Let’s see two charts showing four temperatures measured for exterior, inside, bubbles and soap water tank:
It shows the data collected on the 30th of November 1993 when the lowest outside temperature was -10 C. You can see the temperature swing for the greenhouse’s inside temperature was reduced with chilling effect of soap bubbles during the day and their insulating function at night.
The second chart shows even a better performance on January 29 of 1994 when it was really cold at night (-20 C), the lowest temperatuer inside was 5 degrees C. Again the inside temperature swing is greatly reduced and a relatively steady temperature curve is seen.
The bubble temperature swing took place after sunset when the outside temperature was below zero. It makes sense as old bubbles were cold but when new bubbles were formed they were warm, so that the temperature jumped toward the liquid tank temperature and then dropped down quickly, forming those peaks.
OK, lastly we show an over-engineered double-layer greenhouse that was built in Norway when Richard Nelson lived there for three years helping them to initiate a Food2Waste2Food program:
What is the performance of this greenhosue? See the following chart showing a 30-day recording of CO2 concentration (black), inside temperature (blue) and humudity (green), taken from a 2015 Master Thesis:
So we see the inside temperature is relatively steady, swing between 20 and 25 degrees C after some improvment work was done to the system.
Note that certain areas of the cavity were not filled completely with bubbles due to the fact the Norway company did not fully follow Richard Nelson’s guidance – that was the sad part of the whole project.
P.S. For those who want to access to the Master Thesis and Richard’s Commentary, I have prepared the following page:
2.bitypod.com/channel/SolaRoof_historic_projects
You need you register and login to access that page.
