A Very Unusual Battery
Solar battery, thermal battery, thermal bank, or heat sink. Whatever you call it, I made one and I made it 36 barrels strong. A walipini, like a greenhouse, uses the sun for heat during the day, but because glazing materials are not great at retaining that heat, you have to find another way to do that for your plant's and fish's sake. There are tons of articles you can read about why you need thermal mass in a greenhouse and how it works, but the gist of it is, you need to keep some of the heat from the sun to release at night and some of the coolness of the night to release during the day. This mitigates large temperature swings which are not good for your plants and fish. Water barrels are a popular way to accomplish this, but even a concrete floor is a type of solar battery as it also absorbs the heat from the sun. The barrels of water get warmed by the sun during the day and then release that heat at night and become cool. By the time the greenhouse starts to warm up again the water barrels release that coolness while rewarming.
I decided to create stepped shelves along the backside of my greenhouse to hold my barrels and I'm glad I did. It amazes me how many times I need to waltz out on them for some reason or another. I can walk on half of the top of each row of barrels. It's scary, but it can be done. My concrete guy built the first step and I added two more on top of it that I built out of 4" x 4" and 2" x 8" treated lumber. I then painted the shelves, the barrels, and the wall behind them black so as to maximize the amount of heat they captured from the sun.
Now isn't that just the prettiest thing? On either side of the room at the top of the barrels, you can see two power vents that we added. These fans expel heat from the greenhouse while also drawing cool air in via geothermal cooling pipes we placed around the bottom of the walipini on the outside. It sounds fancier than it is and I hope I can explain this well enough for you to follow because I didn't take a single picture of it, unfortunately. I used 4" perforated drain tile to lay around the bottom of the structure before it was backfilled. One end of the drain tile hose is connected on the walipini side to two holes we built into the front wall of the structure when the walls were first being poured. All this consists of is two 6" pvc pipes built into and through the front wall on either side. The other end of the drain tile is connected to two straight vertical stacks (also made from 6" pvc) that rise up all the way from the base of the walipini, to about two feet in the air at ground level. Those stacks are open to the outside air and when the fans are turned on, the air is sucked from outside the structure, through those drain tile pipes in the ground, cooling to earth temperature as the air moves along the length of the drain tile. The now cooled air enters the greenhouse at the bottom of the front wall and when you stand in front of it, I kid you not, it feels just like air conditioning. Considering the size of my walipini, I wish I had built these holes the entire way around or at very least put four additional holes in the front of the structure. Live and learn, I suppose. In winter, the same is true, but the outside air is heated to earth temperature and then brought into the walipini.