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Passive Solar Hot Water: The Thermosiphon

Solar hot water refers to the use of solar energy to create potable hot water. That is to say, the hot water can be used for cooking and human consumption. There is only one passive solar hot water system and it is called a thermosiphon (thermosyphon is also used).

The thermosiphon is based on the principle of convection. When water is heated, it will rise and flow. If the water is being pulled from a tank into the solar collector to be heated, then it will leave behind a vacuum (or empty space) in the water tank. If an open source of fresh, cool water is available, then it will be drawn into the tank to replace what is being heated.

As the water is heated, it is returned to the top of the tank. The cooler water on the bottom is then pulled back through again to be heated. Only when water is removed from the tank is it replaced with cool water from elsewhere. These are the basics of how a thermosiphon works, but there are a few specifics that are determined by whether the system is opened or closed.

Before looking at the difference between open and closed thermosiphons, it is prudent to point out one potential drawback to these systems. Because hot water rises, the storage tank must be located about 2 meters ABOVE the solar collector. This configuration usually means the solar collector is located on the ground. There are roof-top configurations, but they are more costly.

Open Thermosiphon

The only liquid to be found in an open loop system is water. These systems are the simplest, most reliable, and most efficient. Water is heated via a solar collector to create the convection current as mentioned. Hot water is then returned to the storage tank and is continually heated throughout the day. When needed, the water is transferred to an on-demand hot water tank where it is heated more if necessary and then sent off to where it is needed. Hot water that is removed from the tank is replaced by fresh water.

These systems are the simplest and most efficient, but they do have drawbacks. The chief drawback is that they can only be used in climates where freezing temperatures never occur. That means open systems are limited to tropical and subtropical locals. In these places, there is little need for additional heating, so these systems may rely purely on solar energy.

If the system cannot rely purely on solar to get the water hot enough, a secondary heating systems must be installed. This is rare with open systems as they are generally in warm environments. If secondary heating is necessary, it decreases the overall efficiency of the system.

Closed Thermosiphon

In a closed loop thermosiphon there are two separate loops and two liquids. Of course, one of the liquids is water. The other liquid is nontoxic antifreeze (usually propylene glycol). Only the antifreeze circulates through the solar collector and it does so in a closed loop.

The closed loop system still operates on the principle of convection, but only the antifreeze is subject to this. The heat gained by the antifreeze is transferred to water via a heat exchanger. These types of systems are necessary for cooler climates where water cold freeze in the pipes before reaching the solar collector to be heated.

There are several major drawbacks to closed thermosiphons. The first drawback is complexity. The heat exchanger and need to keep the two liquids separate makes these systems more complex and thus more prone to failure. The increased complexity also leads to higher costs.

The second problem these systems have is maintenance. Over time, as propylene glycol is exposed to heat, it becomes thick and viscous and does not flow well. This means that closed thermosiphons must have the antifreeze in them periodically drained, flushed, and replaced. This not only adds to the expense, but is rather complex and usually requires a professional to complete.

The final problem with close thermosiphons is that they often cannot fully heat the water on their own. Because they are used in cooler climates, it is often necessary for there to be a secondary water heating system to ensure that the water is hot enough. Of course, this is more efficient in terms of energy use than standard hot water systems, but the combined cost of the thermosiphon and the traditional hot water system usually makes these systems prohibitively expensive.
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