Solar Aided Greenhouse for Aquatic Plant Propagation

Digging a thermal collector pit.The greenhouse for Biotope One is rather small by most means.  The lack of funding makes it difficult to purchase and run a better setup at the moment but we are making do with what we have.  The greenhouse is a lean-to design with polypropylene sheeting and stick-built framing.

The previous seasons were used only in late spring to early fall as it wasn’t heated or very well insulated.  This season we decided to add a solar collector and earth mass to keep the temps more steady.  The solar collector for hot water has been around for a while and similar ideas have been utilized.  After refitting our home with radiant heat using PEX tubing I ran across an idea to pump hot air through underground tubes to heat the earth mass.  At night that heated mass released heat back into the greenhouse.  Instead of pushing hot air through, why not try water and PEX?

The greenhouse is 8 feet long and 4 feet deep.  At the peak, it is about 6 1/2 feet tall.  It already has a timer and a sprayer system to water the plants.  The racks had been taken out early in the season to redesign to get the maximum space.

Water circulates from a solar heater into the thermal mass to release warmth at night.While they were out we decided to refit the greenhouse with the solar collector and earth mass using PEX.  The 4x4x8 roughly hole was dug out.  Later we plan on insulating the rear wall and some other areas better to maximize warmth that terrarium and aquatic plants tend to love.

The hole was dug with the much-appreciated help of John who often is eager to help with such seemingly questionable tasks like this one.  The huge pile of dirt grew and grew until it consumed a huge portion of the patio.  While going to get insulation the side of the hole had caved in. About a fourth of the hole filled in, unfortunately. We dug it back out to 36 inches and shored it with a 4×8 sheet of OSB.

On the bottom I used a sheet of 2 inch styrofoam insulation. On the sides I used 1 inch insulation styro sheet. In the corners I used the waste cut off and cut one side to make a 90 degree and placed them int eh corners to help further insulate any gaps in the corners.

I put in 25 loops of PEX HDPE 1/2 inch tubing with oxygen barrier (used in the home for radiant floor heating). Then that level was covered over with 2 feet of dirt. Another serious of 25 loops were laid form the roll and covered over with 2 more feet of dirt.

The insulation and digging made much more dirt leftover. I wet the earth down and packed it the best I could without machinery. Digging the hole with a shovel was a lot of work.

Next I’ll work on the solar collectors to go along the back wall. I think I’ll use some sort of antifreeze in the tubing. I’ll add a solar pump and a controller with a 12 VDC battery. I also want to add a couple of solar fans as I don’t want to use any electrical AC if at all possible.

The first addition will be to set up the racks and growing areas while the solar collectors are being designed and built.  The fans and controller, as well as the battery, will be the next, followed by a pump to circulate the heated water to the underground tubing to warm the earth mass.

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