Solar Thermal

Making Hot Water From the Sun

Have you ever left the garden hose out in the sun and felt how hot the water in it gets? Imagine taking the same scenario and specifically designing a system with high efficiency collectors, modern plumbing, pumps, and adequate storage to harness that hot water and use it at your convenience. Solar thermal systems do just that. Energy is absorbed from the sun with collectors (either flat plate or evacuated tube in our case) and is converted into usable concentrated heat. There are a multiple ways systems accomplish this along with multiple uses.

Passive vs. Active, Direct vs. Indirect

A passive vs. an active system refers to whether the water or heat transfer fluid is circulated by pumps or by thermodynamics. Passive systems are most common in regions where below freezing temperatures are not a concern. Water will naturally circulate by convection, known as thermosiphoning and eliminate the need for a pump. Active systems use pumps to move the fluids through the system.

Direct systems refer to the water being heated by the sun as the same potable water that is used and consumed. A direct system is also common in regions where below freezing temperatures are not a concern. They are sometimes referred to as “open loop systems”. Indirect systems also known as “closed loop systems”refer to solar water heating where the domestic hot water is not heated in the collectors but rather using a heat transfer fluid and a heat exchanger. This system is common in climates with extended periods of below freezing temperatures. This way, the heat transfer fluid can be an antifreeze solution that won’t be affected if it is left in the piping outside.

Evacuated Tubes

This style collector is often around 2” in diameter and 6’ long, and come in racks ranging from 10 to 30 tubes. They contain a vacuum between two layers of glass thus reducing convection and conduction heat losses and ultimately allowing them to reach considerably higher temperatures than flat plates. These style collectors perform well in colder climates or in situations where very hot water is in demand.

Flat Plates

This is the most common and widely used style collector. Most collectors are 4’ wide, 8’ long, and a few inches deep. Flat plate collectors can range in size as well, and some companies even offer custom design when necessary. They are often considered more aesthetically pleasing since they can look like skylights to the untrained eye. Regarding efficiency, flat plates don’t produce as much heat as evacuated tubes. They don’t have a vacuum either so there is some radiant heat loss to the outside. This can be helpful in some situations though such as when snow and ice cover the collector, the outward radiating heat loss can melt the collector clean.

Storage and Transfer

Solar storage tanks are used to keep the heat the collectors make until it is needed. Small domestic hot water systems may only have an 80 gallon storage tank, while large commercial applications may have tanks up to several thousand gallons. Often, these tanks contain internal heat exchangers which are coils of copper piping that are used to transfer the heat. Radiant concrete floors are also an excellent storage medium.


“Every watt or BTU not used is a watt or BTU that doesn’t have to be produced, processed or stored.”

Adam Solar Resources / 1912 Mayview Road Bridgeville, PA 15017 / Office 412-220-1900 / Fax 412-220-9600 / Email: