Sand Cube

The 'cube' is exactly that, a 12"x12"x12" inside dimension container, made from left over plywood from the job.

The cube, out in the elements in front of the Element!

The purpose is to get a control for the heat sink on the fredley job. Initially it allowed me to see how much settled between initial loose fill and packed. Then it sat outside with water and time and settled even more (almost 1/2" from packed)

I weighed the box, multiple times with multiple people for accuracy, and have used the same bathroom scale for subsequent measurements.

The fill was a mixture of existing site sand, silt, and some extra imported sand. It had mixed size particles and even a little clay and silt which have different shapes than sand, which means more material can pack in the same space for higher density which ultimately means higher heat storage capacity.

Think of a box of balls and how much space is left over. Now add smaller balls, foot balls and super balls and you can get many more balls in the same box.



The gist is that the fill I used in the heat sink is 120lbs/cubic foot. It's aver 130 when wet, as when it was installed, but many details were executed to ensure no water is ever in the heat sink. If it is, it will run through and take the heat with it. It's interesting that 10lbs of water, over a gallon, can still fit between all the particles.

When I did all my calculations, i used 120 #/sf, so I was right on. I also used .18-.20 as a range for the specific heat, the materials ability to hold heat. Units are based on water, btu's per pound per degree Fahrenheit.

To check this, I intend to insulate the box with 6" of rigid insulation, r-5/inch. I will then heat the box in an oven to 150 degrees, place it in a cellar at 50degrees, and measure the drop in temperature with time. By knowing the value of insulation and the area and the Delta T (change in temperature) I should be able to generate values as to the materials actual specific heat.

This is an indirect way to solve this problem, but works well with the tools I have. What I really want to know is how much energy it will take to heat up. That will tell me how much heat I can store.

The heat sink early on, before the guest house, pex, insulation or concrete.