Solar heat collectors are a good supplemental heating source that can provide homeowners with heat for their homes when the sun is shining. Solar collectors are box-like structures that capture the energy from the sun and convert it into usable energy for heating purposes. Inside the collector, solar energy is simply converted into usable thermal energy.
On the front side of the solar collectors, a clear panel or glazing material typically polycarbonate sheeting is used. Single pane glass, and double pane glass face towards the sun and allow the sunlight into the collector box. On the inside of the collector, box is a heat exchanger or an absorber. The heat exchanger or absorber is responsible for transferring the heat of the sun into a usable thermal heat source. The heat exchanger is suspended or attached inside the collector box and should be coated flat black with a high heat temperature resistant paint.
This project goes over the build of an entirely self-contained Solar Air Heater using no grid power whatsoever. The unit draws the cold air from the room and exhausts hot air into the room using 2 5V DC brushless 7 vane case fans. These fans are powered by a 16 Watt Amorphous solar panel. Both the intake and exhaust pipes are of 5-inch diameter.
STEP 1 : MAKING THE COLLECTOR BOX
The box for the Solar air heater is made of 5052 aluminum alloy sheets. The dimensions of the box are 91 inches tall and 24 inches wide. We use a one-inch flange and a metal bending brake to bend the aluminum to make the sides of the box. The top and bottom caps are bent to fit on the top and bottom of the box. When manufacturing the bottom caps, the distance between the bends is decreased by one millimeter to allow the caps to fit inside the solar airbox to facilitate drainage.
The next step is securing the aluminum box top and bottom. The procedure involves using a smaller diameter drill bit as a pilot and then drilling to the final size for the rivet only after the two pieces are mated together. The pieces are held together via Cleco fasteners. The function of the Cleco is to temporarily hold material in the exact position during the manufacturing process.
Two five inches holes are cut at both top and bottom of the box to install the plenums. The intake and the exhaust pipes for the two solar air heaters are manufactured from a single piece of five-inch HVAC plenum. These are inserted and secured into the holes using construction adhesive.
The back of the box is insulated using two sheets of half-inch foam sheet. One sheet of half-inch foam is installed on the sides. A pneumatic air file is used to cut the sheets.
We install a snap action thermostat in the interior of the exhaust manifold, constantly monitoring the temperature of the air being brought into the dwelling. The intake and exhaust manifolds need to ensure that all air travel through the interior of the cans therefore it is important to have a good seal on each can. This also means that the manifold itself needs to seal well against the interior of the heat box.
Nine holes are cut on two sheets of half-inch plywood to make the intake and the exhaust manifolds. These manifolds are secured in place against the cans using PL construction adhesives.
STEP 2 : COATING THE COLLECTOR BOX
The flat black paint helps to absorb the heat energy from the sun. It is very important to utilize a flat finish black paint inside the collector box. If the paint has a reflective coating, it will reflect the sun back outside of the collector, which results in lost potential energy. It assists with the entrapment of that heat energy rather than reflecting it away from the collector.
Once the sunlight has penetrated the collector box through the glazing, the heat exchanger material and the flat black paint will absorb that heat and begin to warm the air inside the collector. As the air inside the collector and around the absorber warms, it will expand and rise. The expansion of warm air will naturally create a convection current. As the air inside the collector rises, it will continue to pick up heat through friction with the absorber. The air passing over and through the absorber is given more opportunity to gain heat by rubbing against that surface that is being heated by the sun.
STEP 3 : INSTALLING VENTS
Now that the air is warm and picking up heat and needs a way to move through the collector box, we install two vents on the backside of the solar collector facing towards the room or space that we want to heat. Through the vent at the top of the collector, the heated air moves into the home, and the vent at the bottom allows the cooler air to return back to the collector.
Having a return event at the bottom and a supplier event at the top of the solar collector allows the natural convection process. The air inside the collector is picking up heat from the absorber and is naturally wanting to rise up and out of the collector. A natural force of air rising will induce a convection current, which will pull cooler return air from the room or condition space into the bottom of the collector box.
The collector creates a convection current inside the room.It removes cooler dense air from the bottom of the room and takes it through the collector where it is warm, and then exhausts the heated air out of the supply duct back into the room.
9 rows of 17 soda pop cans, a total of 153 355ml soda cans are used for the collector. The aluminum pop cans are painted with flat black paint to ensure all sunlight is absorbed and not reflected. Also, there is a five-inch intake and exhaust manifold at the bottom and top of the unit. This ensures that all air travels through the interior of the aluminum cans.
STEP 4 : BUILDING THE HEAT EXCHANGER
To maximize the heat transfer from the sun to air within a given space, we need to build a better heat exchanger. Solar air heating systems use air as the working fluid for absorbing and transferring solar energy. Transferring heat from one place to another by definition is a heat exchanger. When the sun heats the metal, the hot metal heats the air circulating over the metal of the heat exchanger. The job is to capture radiation from the sun and transfer this thermal energy to air via conduction heat transfer. Heat transfer output depends on the rise in temperature and the airflow.
In order to minimize heat loss through the plexiglass, we keep the absorber temperature as low as is usually possible. The cooler the absorber runs, the less heat will be lost out of the glass. A way to keep the absorber cooler while extracting the same amount of energy from the sun is to increase the airflow.
STEP 5 : INSTALLING THE AIR TUBES
The solar air tubes are held tight inside the box using two 1/16th half-inch 6063 aluminum extrudes. These lightly apply pressure on the cans holding them firmly against the back of the heating chamber.
Three separate coats of high heat black rest-oleum paint are applied to the box, all within 60 minutes of each other.
STEP 6 : IMPROVING CONDUCTION
To improve conduction heat transfer without significantly reducing airflow, we disturb the airflow within the solar air tubes. Four holes are put in some of the soda cans to create a baffle that increases the turbulence. These baffle cans are placed evenly across the tubes to distribute the airflow. We place the first baffle cans on the second row from the bottom with the intention of disturbing the airflow early. The second baffle will be located in the 10th can.
In order to stack the empty cans, we make an assembly tray “V” shaped support structure using a leftover baseboard. The cans are glued together using PL Premium construction adhesive that is water-resistant, nonshrinking, and paintable. The soda cans are positioned on the loading tray and slowly rotated to evenly distribute the construction adhesive.
The “V” channel made from baseboards holds the cans perfectly straight.
STEP 7 : INSTALLING THE PLEXIGLASS
Clear silicone adhesive will be the primary method of adhering the Plexiglas to the solar air heater. After precisely positioning the glass on top of the heating chamber, I used a 1/8 inch pilot drill to go through the plexiglass. One full tube of silicone is used around the perimeter prior to laying the glass down.
We install 2 16 Watt Sailflo Duct Exhaust fans with a capacity of moving 141 CFM (Cubic Feet per Minute) for air. These are powered by a small solar panel. One blowing air into the chamber and one sucking air out. This helps to overcome the additional internal airflow resistance built into the design.
STEP 8 : INSTALLING THE COMPLETED SOLAR AIR COLLECTOR
The completed solar air collector is installed outside facing south to maximize the exposure to the sun. Once the solar air collector is installed outside, we take the temperature rise between the incoming and outgoing air while moving 141 cubic feet of air per minute from the fans. The calculate the amount of heat transfer we multiply the CFM and Temperature rise by a factor of 1.08.
Image Credits : Dwayne Price