Just what is a batch heater, anyway?
Ask a solar engineer, and he'll tell you it's an "integrated storage/collector unit."
Ask an historian, and he'll say it's the oldest kind of solar water heater there is.
Ask a home owner who owns one, and he'll say it's money in the bank. 
We'd like to add our own definition: Batch heaters are simplicity itself.
All four definitions are correct. A batch heater is an "integrated storage/collector," because one tank simultaneously stores and heats the water. It certainly qualifies as historical, because the first batch heater was built around 1890 by Clarence Kemp, an ingenious Baltimore businessman. People who own batch heaters swear by them be cause they're effective, they're extremely reliable, they don't cost a lot, and they quickly pay for themselves. Coupled with an energy-efficiency retrofit of a home's existing hot water system, our own batch heater design can pay for itself in as little as 18 months.
Batch heaters are really nothing more than insulated, weather-tight enclosures containing one or more black-painted water tanks. The south wall of the en closure is clear glass or plastic, and is tilted at an angle approximately equal to the geographic latitude of the site, so the sun shines directly on the tank and warms the "batch" of water within. The design is uncomplicated because batch heaters need no pumps, blowers, differential thermostats, or other externally powered devices. Instead, they're powered solely by the sun and by water pressure in the home's plumbing: As the sun-warmed water is fed directly to the home's taps, or to the existing water heater, cold water automatically enters the tank at its bottom.
Because batch heaters are so straightforward, you could probably build a serviceable unit with instructions as general as these: Build a south-facing, weatherproof, insulated wooden box; insert a black-painted water tank; add glazing; connect the tank to the supply line leading to your existing water heater; and install valves so you can fill and drain the system as needed. In fact, thousands of batch heaters have been built with instructions just that basic. 

Thousands more have been assembled by handymen using only their common sense to guide them. On the other hand, with a little more attention to detail you can build a batch heater that will rank among the very best, and rival the performance of more complex systems costing two or three times as much. Surprisingly, only a handful of factors separate the winners from the also-rans.

The Water Tank

The water tank is the heart of any batch heater. Its size, shape, and positioning within the heater's enclosure determines how well it does its job. A useful rule of thumb for sizing batch heaters suggests that the tank should hold from one to two-and-one-quarter gallons of water for every square foot of glazing on the batch heater enclosure.
This insures that the tank is large enough to provide a reasonable amount of hot water, but not so large that it requires many hours of solar heating before reaching the desired temperature of 110 -- 120 Degrees F. Our own batch design uses a 40-gallon tank with 28 square feet of collection area -- about 1.4 gallons per square foot. This seems to be nearly ideal for providing both adequate storage and high delivery temperatures. 
Regardless of the gallonage, long, narrow tanks are best because they have a large surface area relative to their volume, and thus effectively get the sun's heat into the water, where it belongs. Our batch heater utilizes a tank five feet tall but just 14 inches in diameter. This, too, seems nearly ideal.
Some batch heaters use one tank; others use two, three, or even more. (See Illustrations A through D.) Single-tank systems are usually cost-effective for average families (and our design is a single-tank model), while the multiple- tank systems' greater storage capacity works well for larger families.
In multiple-tank designs, there are two very different ways of plumbing the tanks. In the first, the tanks are connected to a shared inlet and outlet (parallel flow). In the second, the outlet of one tank is connected to the inlet of the next (series flow). The series flow produces slightly higher outlet temperatures, and usually is preferred for that reason.
Regardless of the number of tanks, you have a choice of how they're mounted. Illustrations A and B show horizontal mounts, with the tanks on their sides; C and D show vertical units, with the tanks on end. From a performance standpoint, the vertical mount seems better because it encourages "stratification"; that is, the hottest water tends to rise to the top of the tank where it easily can be drawn off. Stratification is much less pronounced in  horizontal tanks, and lower outlet temperatures can result from the mixing of the cold incoming water with the tank's stored hot water. Because of this, the design of a horizontal tank's inlet and outlet pipes is crucial. Illustration A's plumbing schematic shows one way to arrange the plumbing to minimize the harmful effects of mixing the tank's water in a horizontal single-tank design; Illustration B's schematic shows the correct technique for multi-tank units.

Preventing Heat Loss

If you used hot water only while the sun was shining, then you simply could insulate the walls of the batch heater's enclosure, and that would be that. But most families use large amounts of hot water twice a day: first around breakfast time, and again after supper. So a batch heater must be constructed to hold the day's solar heat through the evening and into the following morning. 
At night (assuming the walls of the heater's enclosure are thoroughly insulated), the glazing will be the principal cause of heat loss. Because of this, a batch heater should be double-glazed to minimize this loss. In cool climates, it's also a good idea to add some form of movable insulation (see Illustration A) that can be opened in the morning and closed at night. Movable insulation is highly effective, but it has a drawback because the owner must schedule twice-daily trips to the heater in order to operate it. If you forget to open the insulation, you'll get no heat for the day. Also, though the work involved in opening or closing insulation doors is hardly major, it's not really in keeping with the purely passive concept of hatch heating. A more elegant solution is to use triple glazing on the enclosure to minimize convective heat loss, and a "selective surface" on the tank to minimize radiant heat losses. (A selective surface is a special product that absorbs large amounts of solar energy, but reradiates very little, keeping the heat inside where

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