Thursday, November 10, 2011

Free Heat a Passive Solar heater

My daughter had a science project due and we never finished last years so we decided to try again.
The thought behind the passive solar heater is to reduce our utility bills and have something to assist with heating our cabin.

We started out with two Korean war era wood foot lockers.  the dimensions were 16" wide, 32", and 8" deep. We began by removing the lids which had been secured well with rivets which had to be drilled out.

Removing the well secured lids

We then had a huge decision to make. Aluminum conducts heat and cold extremely well. To test this for yourself take a hot can of soda, fill a basin with ice and spin the can in the ice rapidly. In a matter of minutes you will be rewarded with an ice cold drink. Our last project we gathered used cans. It was time consuming because each can had to be cleaned and holes cut in each end. The cans were then secured end to end in rows with silicone. As we were under a time crunch we looked for an alternative heat conductor. We looked at aluminum rain gutter which would have worked but it was pricey. As we strolled the aisles of the lumber store we came across sheets of aluminum ducting. It was flat but machined to form a tube 4" in diameter and came in multiple lengths. It was reasonably priced and although I would've preferred a smaller diameter we went ahead and chose it. We then needed to find something to connect the ends of the tubing to form a continuous network of tubing. We looked around and found flexible aluminum dryer duct and picked up some hose clamps to secure the connections. The remaining materials we purchased consisted of silicone caulking, flat black spray paint, liquid nails, thermometers, and panes of glass to fit our boxes. As this is being used for a school science project we had to build two boxes. One being the passive solar heater and the other serving as the control. The boxes were a bit deep for our needs so I started by cutting a couple of inches off all the way around. The boxes already had a 2X2" strips of wood running along the length and this gave us a platform to lay the panes of glass to when we reached that step.

We began by forming the ducting into tubes.  We then attached the flexible dryer duct to one end of one tube, formed a U shape and connected to the tubing beside it. At the opposite end of the middle tube, we used three tubes to fit the size of our box, we repeated the process by attaching the vent to the tube, made another U shape to attach the middle piece to the last duct. We made sure all of the hose clamps were all tight.

Forming the layout for the ducting

Drilling the air inlets
Next we drilled a 2" diameter hole through the base of the box in the bottom corner to serve as a cold air inlet. We then drilled a 2" hole on the top of the box on the opposite side of the cold air inlet.

In order to keep the entering cold air from escaping the tubing I built a box in the corner around the cold air
inlet and cut the wood on one side to slide over the ducting.   Use clamps until the glue has dried.

Add Silicone to seal any leaks

We cleaned out the sawdust from the boxes and then painted the interior of the solar heater with a couple of coats of black paint. We then attached a thermometer on the inside of both boxes near the hot air outlet. Once the paint was dry we laid out the ducting and put a nice sized line of liquid nails along the entire network of ducting. The ducting was then placed inside the box with the glued side down. The cold air inlet box was then put in position and clamped until the glue was dried. It is important to note the box's top came up to the level of the bottom of the pane of glass. After the glue had dried all of the ducting was painted flat black and once dry a second coat was applied ensuring all surfaces were covered.

Once all the glue had cured and paint dried we placed a bead of silicone caulking along the 2X2" strip of wood along the inside of the box and the glass was carefully laid in place. We then repeated the caulking on the top outer edge of the glass to ensure it was airtight.

Carefully place glass onto the lip inside the box
We repeated this step on the control box. Both boxes were placed in a sunny location facing south. The temperature inside the control box never rose more than a few degrees more than the ambient temperature which averaged in the low to mid 60's. The thermometer inside the passive solar heater maxed out at 140 f the first couple of days and ultimately burst due to the high temperatures we achieved in the passive solar box. The science fair ends this week. I plan on replacing the original lid to the heater to protect the glass during the summer months when not in use. The control box is going to be converted into another passive solar heater using the same technique.
The finished project

This type of heating device would be ideal in a bug out situation and you don't have to expend energy gathering fuel and there is no smoke to reveal your location

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