It was always my dream to have a workshop that had a clear view of the southern sky so that I could experiment with solar. About 8 years ago my dream came true and I turned my barn into a solar-powered playground.
The front of the Barn had about 6 solar hours on a good day, directly on an industrial-sized garage door. The side of the Barn had a white steel door with a full glass storm door in front of it. This door would receive about 3 hours in the morning of direct sunlight.
My plan was to use passive solar techniques to assist with the heating because there was none other than the propane gas heater I installed to remodel the space. The other part of my plan was to make the entire building run off electric, at least for a portion of the day.
I set out to convert my garage door into a passive solar furnace using the first 2′ behind the door. This was my initial plan:
- The garage door was to stay closed through the night, trapping the heat inside.
- The door would be controlled by a PLC (Programmable Logic Controller)
- The Door would only open if the temperature outside was colder than 50 degrees and the temperature between the polycarbonate and the front face of the door was at least 90 degrees.
- A thermostat in the space controlled a supply fan and (2) cold air returns.
- When the door closed the cold air returns closed preventing a reverse siphon effect.
- If the temperature was too hot the door would close,
Pictured below was the first iteration of the passive solar furnace. The final design is far more attractive, but strangely not as efficient.
Constructing the Facade
The extrusion I used had a lip to seat the panel. The panels were 1/4″ thick so they ended up flush with the face of the extrusion. I used waterproof caulking and I riveted the panels in place. I installed a storm door in the middle of the wall so that I could walk through it.
Even before I finished it, and even with the white garage door, the temperature bouncing off the door and through the poly was incredible. Most folks might be wondering why I did not just paint the garage door black and have it open just a few inches, but there are a few reasons why I didn’t and allowing light to come in was one of them.
Constructing the Absorber
The absorber is a critical component of a solar furnace. The absorber should be copper or steel, aluminum dissipates heat too quickly. If the absorber is too thick it will tend to radiate heat back through the poly. My absorber was made with flat black metal roofing panels. I constructed a plywood wall about 24″ behind the garage door and installed 1″ thick styrofoam to the front, its purpose is to create a temperature difference to promote air flow. I snaked PEX tubing behind the flat black steel panels for a future modification that would allow me to install baseboard heat.
In the middle of the absorber was an opening of about 6′ x 8′ to allow light in.
Creating a Path For Air
The absorber was only 8′ high, the same height as the ceiling in the workspace. The purpose of this was to create a “void” for air to accumulate and build up heat.
It is worth mentioning that when I remodeled the Barn I built the walls away from the side of the Barn by a foot. I then had blown cellulose forced into every cavity. The Barn was like a thermos and it retained temperature very well.
The void temperature was monitored by the PLC but it never got so hot that the door was commanded shut by the PLC. This is mainly because the air was constantly circulating. This was accomplished with a powerful circulation fan in the ceiling and (2) motor controlled return dampers.
Function of the PLC
The PLC had a number of tasks to perform and it did a very good job. It was fairly easy to program, after hours of learning and trial and error. The PLC performed the following:
- Monitor air temperature outside
- Monitor temperature between the poly and garage door.
- Monitor temperature in the void
- Monitor temperature in the workspace.
- Monitor temperature on the absorber.
- Open/Close garage door when conditions were met.
- Open/close return dampers when conditions were met.
A Typical Solar Day
When the air temperature outside was around 50 degrees and the temperature of the air between the poly and the garage door was at least 90 degrees. The PLC sent a signal to the garage door opener to open all the way up.
The PLC only opened the motorized air returns when the temperature in the void reached 120 degrees. When the temperature reached setpoint the PLC opened the air returns which allowed for air to flow using natural convection.
If the temperature in the space was set for 75 degrees a circ fan turned on to raise the temperature. When the temperature was reached it turned the fan off.
The garage door would close automatically if the temperature in the void was to ever exceed 140 degrees…it never did.
When Sun started to set the temperature of the absorber would cool down and the door would close. This was to trap the heat in the space. At the same time, the air returns closed to prevent a reverse siphon effect.
Introducing Active Solar
The other solar elements of my build was installing dual-axis solar tracking panels. I installed 350 watts and installed a 400 AH lead-acid battery bank.
I also had a 3000-watt pure-sinewave inverter and a 40 amp MPPT Charge Controller. I used this to operate a 55″ LCD TV and my power tools, one at a time of course.
I had city power coming to the building but I wanted to have the option of being able to work when we lost power, and we did. I installed an MBT (Manual Bus Transfer) so that I could switch between the two sources.
Pictured below is after I removed the polycarbonate panels and constructed an insulated wall with windows. This made it a lot more attractive but I actually decreased the efficiency of the system. I installed double-paned windows which cut down on the light transmittance. I also decreased the surface area resulting in less light striking the absorber. The solar panels in the foreground all track.