Solar Water Heating (Thermal) System Basics

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Solar power for heating water is one of the most cost effective and efficient ways to harness the suns energy. Solar thermal systems have the ability to heat water to temperatures of 120ºF or higher. They do this by absorbing the sun’s heat and transferring it into a storage tank that allows you to use the stored solar heated water as needed either during the day or at night. The most efficient systems have the ability to collect up to 70 percent of the suns energy and put it into the solar storage tank.

Solar System Types

There are basically two commonly used systems types for climates that have the potential for freezing conditions; closed loop and drain back. Their differences are outlined below.

Closed Loop

Closed loop solar systems have pipes that run from the solar collectors to the storage tank. These pipes contain antifreeze that prevents them from freezing during cold conditions. The antifreeze transfers the solar energy through a heat exchanger to the storage tank. This is a pressurized system that is virtually silent and has very little limitations on sizing and placement of collectors. When purchasing this type of a system you need to beware that the antifreeze needs to be checked and or changed every 5 to 7 years. So there is some additional maintenance cost associated with this type of system.

Drain Back

Drain back solar systems also have pipes that run from the solar collectors to the storage tank. The pipes on this type of system contain water (not antifreeze) only when the system is running. When the system shuts down the water in the collectors and collector piping drains back into the solar storage tank, thus preventing freezing during colder conditions. This is a non-pressurized system that can have some noise associated with it while it is running. The system is somewhat limiting because of the requirement of the collectors and piping needing to drain into conditioned (non freezing) space. One advantage the drain back system has over the closed loop is there is virtually no maintenance on the system because it does not require changing antifreeze.

Collector Types

There are two types that are commonly used in areas that have a potential for freezing conditions: flat plate and evacuated tube. Their differences are outlined below.

Flat Plate

Flat plate collectors are one of the oldest solar thermal collector types and have not made any significant changes over the past 30 years. There is really no room for improvement or reason to change them. They have been the workhorse of the industry for years and should continue to be for years to come. The outside of the collector has an aluminum frame with a tempered glass top. The inside has a thin copper plate that has copper tubing attached to the plate and is surrounded by insulation on the bottom and sides. The collector fluid runs through the copper tubing to the solar storage tank. This type of collector can reach temperatures of 330º F if no fluid is running through them and have the ability to maintain storage tank temperature between 90º in the winter to 180º F in the summer. These temperatures vary with seasons, collector square foot to storage tank gallons ratio and weather conditions. Flat plate collectors can be used with either a closed loop or drain back system and typically do not require a heat dump unless they are larger closed loop type systems. This type of collector is ideal for water heating of domestic, radiant floor heat (if it is concrete), year round pools and spas.

Evacuated Tube

Evacuated Tube collectors have been around for at least 30 years and have been making design changes to increase their reliability. This was mostly done by increasing the ability to maintain the vacuum in the tube. The collector is constructed like a large glass test tube with the solar absorber on the inside of the tube. The open end of the test tube is sealed off and all the air is removed so the absorber is in the vacuum. Since a vacuum is a perfect insulation, these collectors are not affected by outside temperatures and work well in cold and cloudy climates. The tubes are usually mounted to a stainless steel frame and because they are tubes, wind does not have much affect on them structurally. The evacuated tube collector can reach temperatures of 475º F if no fluid is running through them and have the ability to maintain storage tank temperatures between 140º in the winter to 200º F in the summer. These temperatures vary with seasons, collector square foot to storage tank gallons and weather conditions. Due to the potential for high temperatures within the system, evacuated tube collectors should be used in closed loop systems. In most cases they will need a heat dump for systems larger than a domestic hot water heating system. This collector is ideal for higher temperature applications such as radiant floor heat that is stapled up under a wood sub floor or that has wood floor above the radiant floor, forced air heating and commercial domestic hot water heating.

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