Thermosiphon Solar Water Heaters: Advantages, Disadvantages and DIY

Thermosiphon solar water heaters use sunlight to strike tubes and fins within the collector box in which water or glycol is circulating. The inlet and outlet of the collector are plumbed, respectively, to the inlet and outlet of the storage tank forming a loop; the collector is parallel with the storage tank. The heated fluid moves from the collector to the storage tank and back to the collector through a process called thermosiphon. This is a natural convective action. Those that live in colder climates may find that the collector will freeze and burst. Changes in design can prevent this from happening, or another type of solar water heater may be your preference. However,
if you live in a climate zone without freezing temperatures, an open thermosiphon system could work well.

For thermosiphon's results, the water in the collector is heated, causing it to expand and rise, pushing cooler water into the rest of the loop to cause a flow. The cooler water is pushed out of the bottom of the tank and into the bottom of the collector. Once circulation begins, it continues throughout the day.

At night the water will cool, causing a reverse flow. The water in the collector is cooled by not moving or not being heated during the coolness of an evening, causing the water to flow in reverse. Since cold water is heavier and sinks, it pushes the entire loop into a reverse flow, moving warmer water from the tank to the collector which is, in turn, cooled. This can quickly cause the water warmed during the day to cause a cold shower in the morning.

The easiest way to avoid this is by positioning the bottom of the tank above the top of the collector.

Positioning the bottom of the tank above the collector is simple physics that will prevent the reverse flow. Even if the tank is positioned level with or a little below the tank, thermosiphon will still occur. You can also add a check valve that will prevent reverse flow. Use a gravity-type check valve instead of the standard pressure-type check valve which is too resistive to thermosiphon flow. Angle the valve with the plumbing for minimal pressure to open, minimum back flow to close.

The solar collector is one part of the solar water heater that many people have questions about. A common configuration uses a box, a grid of water tubes, insulation, and glass or plastic glazing. The box is a large shallow pan, with designs varying smaller and larger in width and length than a standard sheet of 4×8-foot plywood and 4-6 inches in depth. While most home built units use plywood, manufactured designs use stainless steel or aluminum for the boxes. If plywood units are properly glued, screwed, and sealed against weather, they can be durable.

Home built designs of thermosiphon solar water heaters:

Home built designs start with a 4×8-foot sheet of plywood ½ or ¾-inch thick. From it (or another sheet of plywood), cut two 4-6 inch strips from each dimension, creating the material for the box's four sides. Large diameter (1½-inch to 2-inch) copper header tubes at the top and bottom of the collector are oriented horizontally and plumbed together with smaller vertical tubes (i.e., ½-inch tubing) spaced 3-6 inches apart. Tin or copper fins or sheet is mechanically and thermally connected in a variety of methods to the tubes. The tubes and fins can be blackened with paint or through electrochemical processes to absorb the sun's energy. Next, fittings are added to form connections to external plumbing or other collectors. Once mounted in the box, sheet foam insulation is added behind and to all sides of this assembly.

To complete the unit, glass, greenhouse fiberglass, or some other translucent plastic glazing is added. Using recycled glass means that there are a range of sizes available. UV (ultraviolet)-resistant fiberglass is available at local hardware stores in several widths. Using certain plastics will cause crystallization in just a couple of seasons due to exposure to the UV rays of the sun. Select your glazing first. The best economy results when the box is sized to the glass you already have or can get.

Many experts don't recommend building your own collectors since there are so many used collectors available, but if you do build your own, don't use aluminum absorber plates. These types of plates will react adversely with copper tubes. Another tip is that it is best to silver solder any joints within the collector. The collector goes through huge temperature swings which can be hard on standard solder joints. But building your own collector can bring about an appreciation for how inexpensive used collectors really are. If you are convinced that you want to build your own collector, try building just one before you commit to building more by investing in materials, time, and labor.

Avoiding freezing and bursting in your thermosiphon solar water heater:

If you live in a climate with freezing temperatures and use a thermosiphon solar water heater, your collector will freeze and burst without some form of freeze protection valves. Valves that could be used are the Dole valves that are designed to open at a preset temperature, for example 34°F or 45°F. They drip water to create a flow in the collector and can prevent freezing. Experts are divided on the reliability of Dole valves and may or may not recommend them. They do require regular periodic inspections, cleaning, and maintenance. Another freezing preventative measure is using glycol and a heat exchanger for the thermosiphon loop.

Advantages to using a thermosiphon solar water heater:

• Simple
• Efficient
• Doesn't require electricity
• Unaffected by a utility blackout
• Natural way to move heat from a collector to storage or use
• Less energy used, especially in rural locations
• Pumps water and heat through a pipe without external power

Disadvantages of using a thermosiphon solar water heater:

• The plumbing must follow strict guidelines. Without the use of bigger tubing, gentle turns, no low spots, and no restrictive valves to ensure an unrestricted flow and air pocket at a high spot or a large bubble, the system will stop thermosiphon flow.
• Requires very specific planning and careful installation.

Avoiding bubble blocks in a thermosiphon system:

Unfortunately for some, thermosiphon doesn't begin until the collector reaches a critical temperature. The flow may begin rapidly, then slow to a more constant rate. If a bubble forms that is big enough to block the tubing, the flow stops immediately and often without warning. One problem with this is that the collectors can get hot enough to blow a T&P valve and still no flow. To avoid this, use steeply-pitched pipes to ensure a good flow.

In-line, centrifugal-type pumps have been used in radiant floor systems to periodically purge the thermosiphon loops of air bubbles. In theory, thermosiphon can push water through the pump when it's off, but the pump has another use. It can allow the owner to pump more heat into the floor from storage at night.

The thermosiphon solar water heating system can be, if properly planned, installed, and maintained, an energy saving and efficient form of water heating that can benefit you with your water use as well as warming your home. It is important to remember that systems are available, so if you are not a "DIY- Do it yourself-er, don't let that stop you from using solar water heating methods. The expense of having it done by a professional will more than reward you in the end.

Sources:
Interview
US Department of Energy
Arizona Solar Center
Build it solar