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Active Solar Hot Water

Heating water uses a lot of energy, so employing solar, even if for only part of the job, can greatly reduce energy use. Passive solar hot water is generally only suitable for warm climates where freezing is not much of an issue. In cooler climates, active systems for heating water are usually necessary. There are three types of active solar hot water and they can be divided into direct and indirect systems as well as drainback.

Direct vs. Indirect

Direct or open-loop systems directly heat potable water in with solar energy. Indirect systems or closed-loop systems heat another fluid with solar energy and then use that fluid to heat potable water. The two fluids never come into direct contact with each other.

The major advantage of a direct system is efficiency. The water is directly heated and used, so there is no loss during the heat transfer process as there is in an indirect system. Direct systems are also simpler and thus cheaper and easier to maintain.

The biggest drawback to a direct system is that it offers no protection against extremes of temperature. The biggest concern is freezing, which renders the system useless and could damage it. However, overheated water could cause burns. The other disadvantage to these systems is that the solar collectors will develop scale from hard water over time. The build of calcium and lime on the solar collector will reduce its efficiency over time and it will need to be cleaned or replaced. Of course, this can be easily prevented by treating the water before it enters the system.

Indirect systems are not as efficient as direct systems, but they have several benefits. First, antifreeze solutions are more resistant to temperature extremes. Additionally, because there is a heat transfer step, sensors can be employed to ensure that the appropriate temperature is reached during the transfer process. These systems are most commonly used in cold climates where freezing of water is a possibility. The water could freeze in the pipes that supply the solar collector, before it warmed, damaging the system and preventing its operation.

Besides their reduced efficiency, indirect systems also suffer from one other problem. The antifreeze, when subjected to repeated warming and cooling cycles, becomes viscous and has to be replaced. This usually needs to be performed by a professional, which increases maintenance costs. The complexity of indirect systems also tends to increase maintenance costs and reduces overall operating lifetime to some extent.

Direct systems do not necessarily have to use antifreeze, some of them simply use purified water in the solar loop. This eliminates many of the problems with scaling and sludge as mentioned above, though the system can still freeze. The happy medium is usually to add some antifreeze to water and have a combination in the solar loop.

Drainback

When only water is used in the solar loop of an indirect system, there is an option that can eliminate the need for antifreeze called drainback. In this system, the heat transfer fluid (usually water) is circulated through the solar collector by a pump and is not pressurized. When the pump shuts off in a drainback system, the fluid in the collector drains back into a reservoir where it is safe from freezing and from overheating. These systems are usually the lowest maintenance of all, though their efficiency is lower and complexity higher than direct loop systems.
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