The energy transfer system in a solar water heater is nothing more than good old fashioned plumbing. We move heat energy from the source (the absorber plate in the collector) to the storage area (the hot water tank) through pipes. This transfer system may be regulated in some cases by a control system, which usually takes the form of a pump, some valves and a differential thermostat, but this is not nearly so complicated as it sounds. (All of these last things will be discussed in Chapter 9.)
There is nothing fancy or exotic about the plumbing layout. If you've had any experience at all with measuring, cutting pipe, and soldering joints, you should be able to do all or most of the work yourself.
Needless to say, you should plan as much as you can in advance. Remember the advice of the Wright Brothers, who said in effect, "If you get it right on paper before you start, it'll be right when it'i built"
If you're totally inexperienced, you can always consult a licensed plumber whenever your confidence falters. For most plumbers, solar water heating is still something of a novelty. If you call one you know, he may be interested to learn about what you're doing. Who knows, you may even get some free advice — as we have from time to time.
You may be very tempted to use plastic tubing because it produces very little in-line friction and because it's so easy to cut and install (also because some other sources will recommend that you use polyvinyl chloride — PVC — or chlorinated polyvinyl chloride plumbing that you can glue together). Don't.
Plastic plumbing can't stand the heat in a high-performance solar water heater (although we do recommend plastic tubing for a solar system to heat swimming pool water — see Chapter 10). Most of the joints and fittings will be fine until one blistering hot day when there's lots of sun. When the water temperature gets very high — say 200 degrees — something will give out, and suddenly you'll have a serious leak and a major repair job. Sometime soon there will be plastic plumbing systems that can withstand these temperatures, but they're not here yet. Stick with copper for now.
Copper tubing comes in two basic forms. Normal household plumbing is usually made up of the first — rigid copper tube. You might want to use this if you're putting a solar hot water system in a new house. It's straight easy to work with and more attractive than the second type — if it's exposed to view. Rigid copper tubing is normally sold in 10- or 20-foot lengths.
Soft copper tubing is sold in coils, 15,30 or 60 feet long. Use this if you're planning to retrofit an old house. It's much easier to thread through old walls and other tight places because it's flexible. And because it comes in such long pieces, you don't have to worry about making a joint in some hard-to-get-at spot.
It's a good idea to be sure of the grade of tubing you're buying. The outside diameter (O.D.) is always 1/8 inch larger than the stated diameter. That is, 1-inch pipe is actually 1-1/8
inches in O.D. But the inside diameter varies, depending on the grade (Table 10).
Type K has the thickest wall. It may last longer, but it will transport less fluid because the inside is smaller. The price is also exorbitant. Type L has medium thickness. Type M has the thinnest tube wall, but it's very difficult to find in soft copper rolls. In a simple solar water heater with natural water circulation, soft Type L tubing is usually recommended because of the corrosion factor with plain water. But if you're planning a forced circulation system, rigid Type M, the lower grade tubing, should be perfectly adequate because you'll be using a low corrosion heat transfer fluid — the antifreeze solution.
The flow rate of liquid through a pipe is expressed in gallons per minute (gpm). Flow rate, of course, is reduced when there's lots of friction within the tube. (Plumbers use other terms to describe friction, by the way. They call it "pressure drop" or "head loss.")
Mormal home plumbing rarely uses tubing that's less than 3/4 inch. That should be just about right for most forced circulation solar water heaters — although for a very long run or for a
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