How Do Thermosyphon Systems Work

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A syphon, as any elementary science student and efficient gasoline thief knows, is a tube that transports liquid up and out of a container at one level to a second container at a lower level — all with the help of atmospheric pressure. As kids we made syphons just by sucking air out of a piece of hose or rubber tubing to create a vacuum and get the liquid flow started (Figure 77).

In the most simple thermosyphoning solar water heater, the main body of water (the storage tank) is always located at the highest point in the system. Downward pressure from the tank displaces water in the cold water tube (which runs out near the base of the tank) and eventually forces it into the channels of the absorber plate, located in the collectors below.

At the same time, the collectors are gathering radiation, changing it over to BTG's of heat that are, in turn, conducted to the water (or fluid)

Kimray Back Pressure Regulator
Figure 77. This type of pressure-temperature valve which should be placed at the top of the storage tank automatically opens when it senses too much heat or pressure in the tank. These are normally preset to release at 150 psi.

passing through the absorber plate. So cold water goes into the bottom of the collector and hot water comes out the top (Figure 78).

Heated water, remember, becomes lighter or less dense than cold water, so it rises into the tube that runs back to a point about 2/3 of the way up the side of the storage tank. The phenomenon that causes heated liquid to run upward in a tube — in this case from the collector back to the tank — is called natural convection, as you may recall. Heat then, plus a syphoning effect, are the two keys to a free-circulating system (Figure 79).

Meanwhile back at the tank, something called stratification is taking place. Water keeps circulating slowly in the tank too, as we already know, but gradually colder water settles to the bottom of the tank while the hotter water remains near the top. So there are several different temperature levels, as there must be in any hot water tank. This why we can draw hot water from the top, and why we can always be sure that cold water will run out of the lower line.

But this stratification is also precisely why the tank must be above the collectors. If it were not, the system would run backwards at night when there was no heat from the sun. If there's nothing to keep the circulation going in the right direction, hot water will rise out of the tank and into the collectors, creating an awkward circumstance known as "reverse flow." Reverse flow not only takes hot water from the tank, it feeds cold water in, because the water will actually be cooled as it passes through uninsulated collectors at night. This is why the bottom of the tank should be at least a foot above the top of the collector. Two feet would be even better.

It's been suggested from time to time that a thermosyphon system with a tank below the collector could work if a check valve (which only permits water to run in one direction) were installed in one of the lines. This is a great idea in theory, but in a typical free-circulating system there's rarely enough pressure in the lines to allow the check valve to function properly.

Energy Gradeline Explained
Container 1

Figure 78. This simple demonstration, which you probably saw in the 5th grade, explains how the downward leg of a thermosyphoning solar water heater works. If the tube is "evacuated" (air is taken out), and container 2 is below the bottom of container 1, normal atmospheric pressure will force water into the tube. As long as the tube stays filled with water, the upper container will drain. This is half of the reason why water (or antifreeze solution) will circulate naturally in a thermosyphon.

Figure 78. This simple demonstration, which you probably saw in the 5th grade, explains how the downward leg of a thermosyphoning solar water heater works. If the tube is "evacuated" (air is taken out), and container 2 is below the bottom of container 1, normal atmospheric pressure will force water into the tube. As long as the tube stays filled with water, the upper container will drain. This is half of the reason why water (or antifreeze solution) will circulate naturally in a thermosyphon.

Selfmade Thermosiphon Solar

In a house with a steep enough roof, a tank can be located above the roof-mounted panels — in the attic beneath the ridge pole. (There's advantage in this if the attic is heated and well insulated, because there's less potential heat loss from the tank.) Many people in southern states have disguised their tanks in false chimneys. Another possibility: If your house is higher than your garage, consider putting collectors on the garage roof and the tank somewhere higher in the house. If none of these locations is possible or practical, there's always the ground (Figure 80).

There are some serious drawbacks to ther-mosyphoning solar water heaters, too:

For one thing, the hot water-producing capacity is considerably less than in a forced-circulation system. Where water is pumped

Figure 79. In any thermosyphoning water heating system the base of the storage tank must be at least a foot above the top of the collector. This is to take advantage of the syphoning effect, and to prevent reverse flow when there's no heat coming into the solar collector panel. Notice how the tank is plumbed. Hot water can be drawn from the top of the tank, while cold water comes out the bottom. The gate valves are optional, though a good idea. But the boiler drain cock at the very bottom of the system is critical. Its purpose is to empty the lines during freezing weather.

Hot Water to House

Hot Water to House

Figure 80. Where it's not possible to put collectors on a roof and still have the storage tank higher, collectors can be mounted on the ground outside the house. The panels should be located as close to the exterior wall as possible to keep the plumbing lines short. If the tank contains a heat-exchanger coil so the heater can run during cold weather, the plumbing will have to be carefully insulated. Some people protect outside plumbing lines with electrical heat tapes.

Figure 80. Where it's not possible to put collectors on a roof and still have the storage tank higher, collectors can be mounted on the ground outside the house. The panels should be located as close to the exterior wall as possible to keep the plumbing lines short. If the tank contains a heat-exchanger coil so the heater can run during cold weather, the plumbing will have to be carefully insulated. Some people protect outside plumbing lines with electrical heat tapes.

through solar collectors, it's reasonable to expect as much as a gallon per square foot of collector surface per hour in ideal conditions. In a natural circulation system, if you're getting more than a gallon of hot water per square foot of collector per day, your system is working extraordinarily well. In other words, a thermosyphon system not only starts up slowly, it provides about 1/5 the volume of hot water, even though water that has been circulated naturally will be hotter than water that's been pumped through the collectors at a faster rate.

A direct thermosyphon system, where no antifreeze is used, presents a problem other than the obvious one of freezing. Because there are no corrosion inhibitors circulating through the heater, impurities in the water can eventually cause mineral deposits to build up in the absorber tubes and plumbing.

Placing the storage tank above the collectors sometimes means structural problems. It's risky to have your hot water supply above your living space. Even if you reinforce a floor or attic well enough to be sure the whole works can't come crashing down on you in the middle of the night, a leaky tank in the attic can be a real pain in the neck. Repairing or replacing sheetrock and insulation, and having to repaint walls and ceilings that have been ruined by water can cost more than any savings you might make by sticking with a thermosyphon system (Figure 81).

A natural circulation system must of course be set up so there are no level spots in the lines where airlocks can form, All the tubing must slope either downward to the drain cock or upward to the storage tank. There must also be an absolute minimum of friction within the plumbing itself. In most cases this means using larger diameter and more expensive tubing — perhaps 1 inch or even 1-1/2 inch tubing instead of the normal 3/4-inch soft copper. It also means allowing more space for the plumbing — to eliminate any bends and 90-degree "ells" — and thereby minimizing resistance in the lines.

Head loss — resistance in the pipes — is enemy number one to any free-circulation solar

Selfmade Thermosiphon Solar

Figure 81. Some storage tanks for thermosyphoning systems can be installed in attics or even in false chimneys. This is a good way to preserve heat in the water — if the tank and the attic are well insulated. Don't forget that a 120 gallon water tank can weigh as much as half a ton, so be sure to beef up your roof and floor joist structure.

Figure 81. Some storage tanks for thermosyphoning systems can be installed in attics or even in false chimneys. This is a good way to preserve heat in the water — if the tank and the attic are well insulated. Don't forget that a 120 gallon water tank can weigh as much as half a ton, so be sure to beef up your roof and floor joist structure.

water heater. The place where there's likely to be the most friction is in the collector itself. Here the channels, either on or in the absorber plate, should be at least 3/4 inch in diameter. And there will be less resistance if the collector tubing is in a series rather than in a parallel configuration. What this means is that you almost have to build your own absorber. Most prefabricated absorber plates have parallel channels that are too small to allow easy water or fluid passage.

And then there's freezing. The best rule of thumb for deciding whether you can safely install a direct thermosyphon system goes something like this: If citrus fruit cannot be grown successfully where you live, forget it! Even if you can grow oranges in your back yard, you still may be taking a chance. This means that in most parts of the United States, thermosyphoning for solar hot water is impractical in a year-round home unless the system is set up with a heat exchanger.

A heat exchanger is a reasonably simple and efficient device that allows you to do a couple of things: (1) You can keep an antifreeze solution in a closed collector loop that is totally isolated from the domestic hot water supply in the storage tank, as we already know. (2) And because a heat exchanger permits two separate loops, your tank can hold whatever normal pressure exists in the home's regular water system (maybe 60 psi) while the collector loop can have considerably less pressure (say, 15 psi).

Remember that heat always wants to travel from a hotter area to a colder place. While this tendency can cause troublesome heat loss from the storage tank, this same principle makes a heat exchanger possible. Heat can be passed

Heat Selfmade Storage Solar

Figure 82. In a counterflow heat exchanger, two liquids move past each other in opposite directions. Heat is transferred from one to the other through the metal barrier. If a coil of tubing is submerged in a water tank, heat is conducted out of the coil into the surrounding water, even though the water in the tank is not moving.

Figure 82. In a counterflow heat exchanger, two liquids move past each other in opposite directions. Heat is transferred from one to the other through the metal barrier. If a coil of tubing is submerged in a water tank, heat is conducted out of the coil into the surrounding water, even though the water in the tank is not moving.

from a fluid flowing on one side of a barrier to another liquid on the opposite side without the two ever touching each other (Figure 82). The idea is to give the separating wall between the two liquids as much surface area as possible, so that a maximum amount of heat can be transferred.

The most common type of heat exchanger for a solar water heater — and probably the best — is a copper coil immersed right in the hot water storage tank. The longer the coil, the more surface area it has, and the greater the opportunity for heat to pass out of the copper tubing into the water in the surrounding tank (see Figure 68).

In any solar water heater, by the way, the surface area of the heat-exchanger coil should be at least equal to 1/4 the total surface area of the tubing in the absorber plates of the collector panels — if the system is to be truly efficient. The coil in a typical 120-gallon-tank heat exchanger is more than adequate to support several 24 square-foot collectors.

You can also buy heat exchanger units that can be set up separately outside the hot water storage tank (Figure 83). In this case there must be an extra loop to circulate water from the tank, through the exchanger, and back to the tank by natural convection. This loop must be insulated, of course.

Steel Casing

To Tank

Steel Casing

To Tank

From Tank

From Collector

Continuous Copper Coil

From Tank

From Collector

Continuous Copper Coil

To Collector

Figure 83. It's possible to buy heat exchanger units that can be installed outside an existing hot water tank. This requires a separate loop to bring water from the tank to the exchanger and back to the tank. Naturally this is not as efficient as an in-tank coil. Any separate unit like this should be heavily insulated. And so should the plumbing.

If a tank and ready-made heat exchanger combination seems like too big an investment, there are ways to build homemade heat exchangers that can be used either with thermosyphon solar water heaters or forced-circulation systems.

One of the simplest of these "homemade" heat exchangers can be made from salvaged junk. The design is by Steve Baer and his Zomeworks cohorts in New Mexico. It's no more than an old hot water tank that sits in a recycled 55-gallon oil drum filled with a non-toxic antifreeze and water mixture. Heated fluid enters the 55-gallon "jacket" surrounding the tank and heat is conducted to the water inside. In this plan (Figure 84), the hot-fluid intake for the jacket should be at least 9 inches below the top of the oil drum to insure good heat circulation within the jacket.

Neoprene Stuffing; Evaporation Barrier

Neoprene Stuffing; Evaporation Barrier

Thermosyphon Cooling Systems

Figure 84. Here is the Zomeworks heat exchanger. It's easy to make, efficient, and cheap. But it should be watched closely. If the wall of the tank ever corrodes through, the domestic hot water supply will be contaminated with antifreeze. Remove the insulation every once in a while to check on things.

Figure 84. Here is the Zomeworks heat exchanger. It's easy to make, efficient, and cheap. But it should be watched closely. If the wall of the tank ever corrodes through, the domestic hot water supply will be contaminated with antifreeze. Remove the insulation every once in a while to check on things.

Coil Thermosyphoning Homemade

Figure 85. This is another homemade heat exchanger. If you wrap the copper tubing carefully and use lots of solder to sweat the copper to the tank, you will get a fairly good thermal bond. "Thermon" cement should work just as well as solder. Try not to let the copper tubing kink, and be sure to cover both the heat exchanger and the tank with insulation. What you see here is an example of a double-wall heat exchanger — because heat must pass through two barriers.

Figure 85. This is another homemade heat exchanger. If you wrap the copper tubing carefully and use lots of solder to sweat the copper to the tank, you will get a fairly good thermal bond. "Thermon" cement should work just as well as solder. Try not to let the copper tubing kink, and be sure to cover both the heat exchanger and the tank with insulation. What you see here is an example of a double-wall heat exchanger — because heat must pass through two barriers.

Another alternative for an easy-to-make heat exchanger is to wrap about 50 feet of 3/8-inch or 1 /2-inch soft copper tubing around the lower 2/3 of the storage tank. Keep the wraps close together and free of kinks. Then use plenty of solder to make a good thermal bond between the tank and the tubing. Once the solder is in place, insulate the whole works. A heat exchanger like this can never be as good as an in-tank coil because there are two barriers the heat must pass through — the tubing itself and the wall of the tank (Figure 85).

Whether you choose a free-circulating solar water heater or a forced-circulation system is, of course, your decision. Even though it has disadvantages, the attractiveness of a thermosyphon-ing system is its simplicity and its low operating costs, even with a heat exchanger. But don't make a choice until you've read the next chapter.

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Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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