Solar Collector Flow Dynamics
Buckminster Fuller once remarked that many of our social and economic and physical challenges can best be addressed by understanding how nature addresses them. Have you ever wondered how a mighty oak can support the weight of a horizontal limb or how or how a bird can fly or how a tiny heart can pump blood through thousands of miles in of microscopic arteries and veins and capillaries. Sometimes ideal solutions to problems are not practical but they can still supply us with fuel for thought.
There is more than enough direct sunlight energy for everyone and yet governments allow the continuous exploitation of non-renewable resources as the quality of life and the health of our tiny planet declines. We do have alternatives but if we are to live in harmony with nature we must learn to understand the nature of nature. All animals from the microscopic ameba to the to the macroscopic sperm wale have have some kind of circulatory system that makes life possible by transporting food energy, nutrients and oxygen. Most multicultural organisms are equipped with a heart that circulates oxygenated blood through a parallel network of capillaries. If the surface area of a person 1'x1'x6' could be spread out with a thickness of .1" their surface area would be 720 sq ft. The surface area of a solar collector this large, operating at an efficiency of 50% could harvest the heat energy equivalent of 108,000,000 BTUs per year... with a street value in excess of $2000.
This natural method of circulating fluids through a large parallel network is sometimes known as branch connected plumbing. Small hearts with low energy requirements can support large organisms with this method of blood distribution. Unfortunately full blown branch connected solar plumbing systems are not cost effective when it comes to collecting and storing the sun's heat energy.
CLOSED LOOP: Homemade collectors can be fabricated from a single copper or PEX tube bonded to an absorber plate. Since the flow rate of the fluid through the serpentine tube is uniform the heat collection process is uniform. In other words heat is collected from the entire surface area of the collector. The other nice thing about serpentine collectors has to do with the construction simplicity that avoids the multiple solder junctions of a parallel flow flat plate collector. A 3/8" copper tube is fairly easy to bend and not too expensive, BUT a single 60' long copper tube slows down the flow rate and makes an excessive power demand on the pump.
This problem can be lessened by connecting two serpentine collectors in parallel. Remember the surface area of a collector and the flow rate through the collector are major factors that determine heat collection. Both the overall flow rate and the surface area may be increased by the parallel connection of two collectors. This collector arrangement is ideal for a small closed loop systems that uses a propylene glycol mix to prevent freezing, It is also possible to use serpentine collectors in open loop, drainback systems, BUT special sloping precautions, in cold climates, must be taken to assure that all the water drains when the pump is off.
OPEN LOOP: Flat plate collectors with parallel flow tubes or risers connected to headers may be used in place of a single serpentine flow tube for both closed loop and open loop systems. Parallel flow tubes drain rapidly when the pump is off. The bottom header will also drain rapidly when sloped properly. BUT serpentine collectors may also drain properly if they are well constructed and properly installed.
PARALLEL PIPE SYSTEM
There is a minor problem with the flow dynamics that causes the fluid pressure to be higher at the beginning and end of the connection junctions. This causes the water to flow more rapidly at the end risers where there is less collector heat but according to Gary Reysa this imbalance in flow rate is minor and the advantages of a reliable drainback far outweighs the minor drop in collector efficiency due to a 10% flow rate discrepancy among the riser's non-uniform flow. To improve flow tube flow rates header pipes are made larger than the vertical riser pipes. This lowers the pressure demands on the circulator system and improves flow rate. 3/4"x 1/2" x 3/4" Ts OR 1"x 1/2"x1" Ts may be used to join headers to risers.
Risers may also be connected directly to risers in the same manner that commercial collectors are manufacturers BUT connecting risers directly to flow tubes requires the use of jigs and some soldering skills. This 3/4" header pipe has 1/4" flow tubes installed without the use of T connectors. These 1/4" flow tubes are only 1" long spaced 2" apart. Commercial collectors would use 3/8" flow tubes spaced about 6" apart.
Gary Reysa used copper Ts to connect his horizontal headers to vertical flow tubes. This 8x11parallel flow collector is mounted on the side of his Montana home. Click on the picture to the left for more information.
Thom from Iowa built a variation on Gary's parallel flow vertical collector design BUT Thom decided to connect two 12'x8' parallel flow collectors in parallel. Unfortunately the flow rate distribution was not as uniform as Thom had hoped so the collector efficiency was less than expected . Notice that the input manifold to Thom's collector is connected to a T at the base of the collector. This distributes half the water to the left and the other half to the right. So far so good. Now all Thom has to do is connect the top manifold outputs to a common junction and return the water to the heat storage tank and the pump. Unfortunately this is a problem
The 3/4" return pipes are joined to a T at the bottom of the collector but the flow rates through the individual floe tubes in each branch vary greatly. Branch connected parallel plumbing works great as long as all the branches as well as the individual flow tubes have a uniform flow.
A parallel flow horizontal collector may have been a better option for Thom
There are times collectors are too high to fit under a porch or on the side of a house with south facing windows. To address this problem a horizontal collector has been be developed with horizontal flow tubes called hisars. Gary Reysa has done some preliminary testing on a small model and now Bob Allen has built a large 4'x16' collector to confirm the practical application of a horizontal, parallel flow collector.
Bob Allen built his 4'x16' horizontal collector under the patio deck of his Canada home. This is what it looked like before he installed the plate glass glazing.
So far so good. We'll keep an eye on Bob's work
to see how well it holds up and performs. We have many alternatives this is one
that worked great for Bob.
In all the examples we illustrated so far water enters through the bottom and exits through the top because if water were to enter from the top of the collector there is a good chance air would be trapped and the flow rate would be erratic... BUT what if water could be trickled from the top in a uniform manner on a thin black platform that distributes the water as a uniform film?
If we could trickle water from the top manifold through hundreds of flow tubes .1" apart we could do away with an absorber plate... But many tubes connected to top and bottom manifolds could get expensive and maintaining uniform flow throughout a system like this would be impossible.
BUT what if we could replace the flow tubes with a material that disperses water naturally?
If we could do this all we'd have to do is supply a few trickle down spigots and allow the material to do the work of uniform dispersal. Freezing would not be a problem because freezing water clinging to the material would be free to expand without causing damage. The only other thing we'd have to worry about is evaporation, but this problem is easily solved with an inner film.
Trickle Down Solar Heating
I hope some of these ideas about collector flow dynamics have been helpful. BLOG