Solar Space Heater

On December 16, 2007 in Uncategorized

Working on the solar space heater design a bit more I came across a program called CoDePro on the University of Wisconsin Madison’s website, which is a collector design program that supposedly makes it easier to design collectors, calculate efficiency curves of the collectors in various conditions, and compare this data to experimental data. I will install it later today after some Christmas shopping, and see how it works.

On the design side of things, I am leaning towards a flat plate type collector outside with the old water heater tank actually acting as a storage tank in the closet of the bedroom this heater will be going in. The cast iron radiator I have will be going in the bedroom itself.

The way the house was built, it should be possible to have the collector below the tank, and the radiator above it. Thinking of mounting the tank horizontal-ish on the floor of the closet, with the radiator elevated about 2ft above the floor. With the collector mounted on the side of the house at near ground level, thermosyphoning should take care of the fluid transfer. I’d rather not have to set up a PV panel with a pump.

Here is a crude MS Paint drawing I just whipped up:

I’ve been trying to decide what kind of collector I want to build though. Right now it is looking like a single pane glazed collector with tube/fin construction will fit the bill. While hunting for info on constructing these types of collectors I found some good information on tube spacing and header construction at www.solarexpert.com:

“Optimum tube spacing. Wide spacing of tubes reduces collector cost, while close spacing increases cost, but improves efficiency. Fin efficiency drops rather fast as the tube spacing is increased above about four inches, depending on the thickness and thermal conductivity of the fin metal, and effectiveness of thermal bond. The highest quality, most cost effective collectors have sufficient spacing typically no more than 4 inches, with large area tight bonding to the water tubes.

Another highly desirable design and construction feature is secure attachment of headers to water tubes. High quality collectors have the tubes brazed or welded to the headers and supported mechanically by insertion into sockets extracted directly from the header metal by a “t-drill”. In addition to high level insurance against leaks, and breakage at joints and from wind vibration, this method of attachment provides smooth easily balanced flow and eliminates the possibility of eddy corrosion.”

“The flow rates required in glazed collectors are lower so smaller headers are permissible without degradation of quality. Serpentine arrangement of water tubes should be avoided in both glazed and unglazed collectors, in favor of parallel grid arrangement of tubes. Efficiency is reduced in serpentine collectors because the average collector temperature will be higher from the repeated passages through the collector. For an 8 tube collector, the pressure drop will be 8 times as high in a serpentine collector as in a parallel grid collector for the same flow rate and tube size. It is also extremely difficult to purge all air from a serpentine collector, which is essential to proper fluid flow and heat transfer. Conversely, no special air purging is needed for a parallel grid collector because the air will rise to the top header of its own accord to be purged automatically by the air vent.“

The collector case will need to be highly insulated to prevent heat loss. Along with this, all pipes coming into/out of the collector will need to be insulated, and the pipes need to not come in contact with the case. If the pipes or the collector plates/tubes come in contact with the case, there can be a dramatic loss in heat through conduction, so make sure to not use heat conducting materials to support or attach the collector, as well.

I have moved away from the batch style heater, because I would like this to perform well in moderately cold winter temperatures, and the batch style heater will not suit that situation like a flat plate liquid based collector will.

Here is an image I found on backwoodshome.com

This image makes in a good point, in that you don’t want to use the standard cold water inlet on the top of the tank if you are setting up an old water heater as a tank and plan to move the fluid using a thermosiphon. On top of that, it looks like a good design and I will take it into account when putting the finishing touches on my design.

Looks like 3/8″ copper will be the tubing I use for the collector, and I will solder that to flat copper plates.

More on this later

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