A Homemade solar Collector
by John Canivan
Commercial solar collectors are almost impossible to build without the aid of expensive, technical, automated, machine shop equipment. Quality low cost homemade solar collectors can be built by anyone with basic carpentry and plumbing skills. All the materials needed to build this collector, except for the glazing, are available at your local construction supply shop. The glazing may be purchased through mail order. If you have no handyperson skills at all you can still build this unique solar collector with a little determination. This tiny article has been written to introduce theory and design concepts of a solar collector we can all learn to build. If cost has prevented you from getting involved with renewable resources in the past this serpentine collector will surely shed new light on the subject.
Most flat plate collectors have small parallel tubes connected to larger main carrier pipes. I like to call these small parallel tubes the riser tubes because this is where the collector fluid rises to harvest the heat from the sun. A simplified demonstration of how they work is demonstrated below.
PARALLEL FLOW DYNAMICS
This parallel riser tube type of collector is designed to transport collector fluid from the bottom of the collector to the top via a network of parallel pipes. Notice that the top and bottom pipes are larger than the vertical pipes. There is a reason for this.
Fluid mechanics favors an increased flow rate for end pipes. This is because fluid pressure is greatest at the base of the incoming fluid and least by the output pipe. If the top and bottom pipes are large the pressure difference is moderated and the flow rate in each of the parallel pipes is more uniform. These collectors may be connected in series because the top and bottom distribution pipes are so large. It is unfortunate that the flow rate is minimal at the center of the collector where most of the heat is concentrated. Other problems associated with homemade parallel riser type collectors include cost and leaks. Half inch and two inch copper tubing is expensive, not to mention the dozens of special T fittings and all that solder. One small, undetected leak on one of those T fittings could become a catastrophic mess.
SERPENTINE CONFIGURATION
Fortunately there are other options for the homeowner without the special automated equipment used to manufacture the parallel riser type collector. This type of collector is easier to construct, less prone to leaks, less expensive and I believe more efficient than the parallel tube riser tube collector. I call this the Serpentine Collector.
SERPENTINE FLOW DYNAMICS
The serpentine collector consists of one long continuous flexible tube so there is no problem with uniform flow rate. The size of this flexible tubing is an important consideration. Quarter inch copper tubing is inexpensive, however it restricts the flow rate too much. Half inch flexible tubing is difficult to bend and fairly expensive. 3/8 inch tubing is just right for the money. It has a reasonable flow capacity, low cost, and ease of fabrication. What more could one ask for?
The main problem with a serpentine collector is flow rate restriction. Flow rate is restricted too much and puts too much burden on the circulator pump. Connecting the serpentine collectors in parallel alleviates this problem. A two-collector system cuts the flow rate restriction in half. A four-collector system eases the burden on the circulator pump even more. It is very important to bend this 3/8 copper tubing carefully to avoid kinks. This will insure that the flow rate is uniform throughout the serpentine collector array hooked in parallel. To bend the flexible 3/8 ID tube into the serpentine configuration illustrated above it is first best to construct a bending jig. This jig will guarantee that the spacing between the loops will be in the correct position to fit into the absorber plate groves.
ABSORBER PLATE
The purpose of the absorber plate is to transform light energy into heat energy and transfer this heat to the collector plumbing. Glazing keeps rain and snow out of the collector and also traps the heat radiation (IR) that is made when visible and UV radiation strikes the absorber plate.
Some absorber plates are made from expensive sheets of copper and bonded to the collector plumbing with solder. This process is expensive, time consuming and unnecessary. Aluminum, a better conductor of heat than copper, makes a fine absorber plate. Once groves are hammered into an 8-foot long sheet of aluminum a thin bead of tar is painted into the groves and the serpentine tube is pressed into this to make intimate contact with the absorber plate. Supports are screwed into the inside walls of the collector to hold the serpentine tube in position.
INSULATION
Two-inch polystyrene insulation pressed into the bottom of the collector will conserve a lot of heat that might otherwise be lost to the atmosphere.
PARALLEL HOOK-UP
The parallel branching hook-up of serpentine collectors is essential for best performance. Commercial flat-plate collectors are connected in series, however this homegrown serpentine collector requires parallel connections. To better understand why this is important animal capillary plumbing will be compared to the serpentine plumbing hook-up.
Buckminister Fuller once said we should look to nature to solve our problems. Let's see how nature solves the problem of energy transfer to the cells. Oxygen carried by our arteries is transferred at the capillaries. It is here that CO2 is picked up and transferred to our veins. Let's take a close look at capillary plumbing and see if we can apply this to our serpentine collector hook-up.
CAPILLARY PLUMBING
Notice the network of parallel connections. This is how nature solves the problem of transferring energy to every cell in our bodies in a matter of seconds with a tiny heart. If our capillaries were connected in series our heart would have to be the size of Montana to pump enough blood through it to keep us alive.
Now lets hook our serpentine collectors in parallel to reduce the amount of pressure in our carrier pipes and also reduce the demand on our circulator pump. The hookup would look like this for a four-collector plumbing hook-up.
Four Collector Plumbing Hook-up
These four 4X8 collectors occupy a combined surface area of more than 120 sq feet. You'll need more than a few collectors to supply you with solar hot water year round. A heat storage vault and an automated circulator system controlled by differential sensors would be nice. With a little perseverance you should have the sun pay this project off for you in less than three years.
Details of how to bend the serpentine tube and how to construct a high quality low cost absorber plate, and how to build a multi-tank heat storage vault are available.
For more information go to www.JC-SolarHomes.com/how_to.htm