Build a Solar Water Heater: An Intregal Passive Solar Water Heater
(Page 4 of 5)
January/February 1984
by David Bainbridge
The Six Commandments of Designing a Solar Hot Water System
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A few basic principles and considerations govern the design, installation, use and maintenance of IPSWH's. If your heater's specifications and installation follow the six commandments listed below, it will work admirably and will provide you and your family with inexpensive solar-warmed water, no matter which specific design you decide to go with.
[1] Locate your heater for maximum sunlight exposure. Find a sunny, south-facing location for your IPSWH, preferably close to the conventional gas, electric or wood backup unit to minimize piping distance. You might have to indulge in a bit of calculating to be sure your solar collector will be exposed to the sun year round, but such an orientation is, after all, the single most important consideration.
[2] Make the collector as effective as you can. First, decide on the type of tank(s) you'll use for your heater. Tanks come in a wide variety of sizes and shapes, with most of them being usable. But long, thin cylinders are the most efficient (they have the greatest ratio of surface area to water volume), and by far the cheapest and easiest of these are used electric water heater cores.
If you prefer new tanks, out-of-the-carton, glass-lined electrical water heater cores — minus the heating element, outer insulation and sheet metal cover — can be ordered from several manufacturers. But if you plan to retrieve your cylinders from cast-off electric units, be sure to test them thoroughly for leaks and rust, and patch carefully where necessary. Never use a tank that you have doubts about.
One of the most suspect parts of any used electric heater core is the "sacrificial anode," a rod made of metal that has a low resistance to corrosion and is inserted into the tank from the top to attract and absorb any corrosive elements in the water. Examine the anode, and replace it if substantial corrosion and/or consumption is evident. A new anode costs only a few bucks, and it will add measurably to the longevity of your system.
Next, determine where your IPSWH will be installed — roof, platform, wall or ground — keeping in mind that a filled three-tank system tips the scales at a hefty 1,000 pounds or so. For most applications, it's best just to plop the heater right on the ground. This makes installation easier, eliminates the weight problem, and renders the manual raising and lowering of a night lid less of a chore. Finally, reflectors can be used to increase solar exposure, with a variety of materials being suitable for this purpose. (It's usually no problem to make a hinged lid serve double duty as a reflector when it's raised to its daytime position.)
[3] Insure that your water tank(s) will retain heat. There are a number of options for glazing the top and the south-facing wall of your IPSWH, including single- and double-paned tempered glass, Du Pont Tedlar-coated fiberglass exterior with Teflon-film interior glazing, acrylic or polycarbonate plastics, and R-3.6 Heat Mirror. In most cases, you should use two layers of your chosen glazing material with an air space between, to provide maximum heat retention. Be sure to caulk and tightly seal each of the panes.
[4] Size your heater for its intended application. To determine what size IPSWH you need, allow 30 gallons of water capacity per person in your household. Davis Alternative Technology Associates suggests about 2.5 gallons of water per square foot of glazing as the maximum ratio for good heating, and a smaller water-to-glazing ratio will speed up heat gain considerably. Using those figures, you can plan around a requirement of 30 gallons of water capacity and about 12 square feet of glazing per user. But if you can't meet these desired measurements, don't lose heart. Even a drastically undersized system will preheat water well above ground or ambient air temperature, thus providing economical solar water warming, which will conserve nonrenewable energy sources and save cash.
[5] Make an efficient connection to the backup system. Try to locate your IPSWH close to its conventional mate, thus minimizing the amount of connecting pipe required. To be sure the pipe is properly sealed and insulated, consider using Armaflex and a coating, or urethane foam with aluminum jacketing. (Although it can take up to 72 hours to freeze a standard exposed water heater tank at 12°F, pipes — because of their restricted size — are far more sensitive to cold.) Any exposed pipes in particularly cold places should also be heat-taped.
Before hooking your IPSWH to its backup unit, bleed the air out of the tank. You can bleed through the temperature/pressure valve (if you install such an overheating safety feature), or just leave a plug out at the top of the tank until the system is full of water. As a final precaution when galvanized tank fittings are to be used with copper tubing, make sure that the two metals are separated with nonconducting, dielectric fittings to prevent accelerated corrosion. I've had very good luck using copper tubing, plastic dielectric connections and galvanized fittings on my tanks.
[6] Build your solar water heating system to last. Use the best materials you can afford or scrounge, and take proper care in the construction of your unit. And be safe: Make sure that the tank supports are strong enough to bear the load they'll carry, and consider adding a tempering valve near the backup heater.
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