Are Evacuated Tubes for Solar Water Heating Worth the Investment? Advantages and Disadvantages

Evacuated tubes for solar water heating
(photo: amazon.com)

Things to consider when buying and installing evacuated tubes for the free solar water heating. See the advantages and benefits of tubular solar water heaters, how do they work, and compare to solar panels?

Evacuated tubes: A buyer's guide for the cost-effective water heating

Evacuated tube collectors (ETC) are designed to absorb solar energy, convert it into heat, and used to heat air or water. They can be utilized in various residential and commercial applications, including domestic water heating, space heating, and pool/spa heating.

While the flat plate collectors are the most popular, vacuum tubes also have some great advantages, including the great performance for most of the day and year. This is possible since the tubes are always perpendicular to the sun's rays.

Another advantage is easy handling and installation as they don't have to deal with large amounts of water like solar panels. I would add here an easy replacement if one or more tubes break.

What are the evacuated vacuum tubes? 
Anatomy of the solar tubes

Typical evacuated tubes are cylindrical in shape, designed to absorb the solar energy and heat the liquid using the heat pipe and vacuum glass tubes.

These vacuum tubes typically have two layers of clear glass joined together and a flat or curved absorbing plate attached to the copper heat pipe. The heat pipe gets the heat from the absorber plate and glass walls.

Both inner and outer glass tubes are made of strong borosilicate glass, making them resistant even to harsh weather, such as large hail. The outer tube is clear to allow an efficient pass of the solar rays.

An inner pipe is coated with a special selective coating to allow minimal reflection and high absorption of solar energy.

The air between the glass walls is removed because the vacuum acts as an insulator, preventing the heat loss from the absorber to the outside.

At the bottom of the vacuum tubes is the barium getter (a coating applied to an inner surface) which removes any air from the tube, maintaining the high vacuum in them.

At the end of the heat pump is the condenser bulb, which fits into the manifold together with the header pipe and insulation. This is where the accumulated solar energy is transferred to the heat transfer fluid.

These single solar tubes are combined into groups - collectors, and each collector consists of parallel rows of glass tubes to increase the exposed surface area. Collectors are supported by the frame.

Each tube is connected to a header pipe - a copper manifold (or heat exchanger), located at the top. Such a setup also allows easy installation and replacement (plug-and-play).

Tubes come in different sizes; up to 8 feet in length and up to 3 inches in diameter.

How does the evacuated tube collector work?

The evacuated tube collectors work by absorbing the solar energy inside the vacuum tube, increasing the temperature of the heat pipe, and transferring the heat energy to the heating fluid.

When the evacuated solar tube is exposed to the sun, it absorbs the heat. The copper heat pipe located inside the evacuated glass tube gets warmer, so the inside liquid (water, alcohol, or acetone) quickly turns to hot vapor. And because the heat pipe is in a vacuum where the pressure is reduced, the liquid can boil at a much lower temperature (around 90 F).

The steam or hot vapor then rises to the top of the heat pipe and condenser bulb, where it exchanges heat with the cold water or glycol flowing through the manifold.

Once it transfers the heat, the fluid in the heat pipe cools down and condenses (turns into the liquid), and returns to the base of the heat pipe. The cycle starts again and continues until the solar tube system has enough sunlight or diffuse light.

For the best performance, the recommended tilt angle of the collector should be maintained to allow a condensed liquid to return to the hot absorber.

These systems can achieve inside temperatures above 400 F, while the outside surface is cool to the touch due to the high level of insulation.

Benefits

• The evacuated solar tube system provides free solar water heating, reducing your utility bills significantly.
• Tubular solar water heaters provide an easy DIY installation and replacement.
• High efficiency as the heat is transferred only one way.
• High transfer rate with the use of copper tubes.
• High absorption when using the vacuum and special absorption film.
• It can operate in colder climates efficiently.
• Works excellent in overcast or diffuse sunlight conditions.
• An evacuated solar tube needs minimal maintenance.
• Broken tubes can be easily replaced.
• They are built to last.

Types

There are a few different vacuum tube configurations available making the solar water heating more or less efficient:

• Single wall glass tubes
• Double-wall or twin glass tubes
• Direct flow
• Heat pipe

Single-wall glass tubes

The air is entirely removed from inside the tube in a single-wall tube, and the metallic top and heat pipe cannot be removed. If something is wrong with the tube, the whole element needs to be replaced. When compared to the double-wall system, this system gets more sun exposure making it more efficient and better in colder climates.

Double-wall glass tubes

Double-wall evacuated tubes come with two layers of glass where the air between them is removed, forming a vacuum (an insulator) to reduce the heat loss. Each inner tube contains a metal heat pipe connected to an absorber fin and condenser bulb at the end, attached to the manifold.

Direct flow evacuated tubes

By Rkraft - Own work, CC BY-SA 3.0,

Direct flow evacuated tubes come with two pipes and an absorber plate made of copper with a selective coating and vacuum-sealed inside a glass tube. One pipe is for inlet and the other for outlet heat transfer fluid, making a "U" shaped collector. It does not use the heat pipe with the condenser bulb.

They have the advantage of being installed at any angle, including horizontally. The problem with this system is that tubes are not easily replaced when they break, and the system must be drained.

Heat pipe evacuated tubes

By Rkraft - Own work, CC BY-SA 3.0,

This is the most popular system. As opposed to direct flow systems, which are filled with liquid, a heat pipe system has only a small amount of liquid and a dry section, making the replacement easier without emptying the entire system. They require a minimum tilt angle to ensure required performance.

Evacuated tube collectors vs. solar panels. Which one is better?

Performance and efficiency

According to the experts, evacuated tube collectors will outperform solar panels in cold climates because the efficiency doesn't rapidly change when the air temperature drops.

Due to the greater insulation properties, the outer surface of the evacuated tube is cool to touch even when the inner tube reaches high temperatures. This means that the heat retention is higher, and the outside cold air won't significantly influence the inside tube temperature.

Unfortunately, this can be a problem in winter because there will not be enough heat on the outer tube surface to melt the snow and frost accumulations.

Also, as the tubes have to be spaced apart, they have a lower absorber to gross area ratio (60-80%), according to Wikipedia. 

Solar panels are a more efficient and better option for many homeowners in warmer climates and regions with more sun exposure.

Design

Solar panels are more susceptible to damage from hail due to their larger contact area and their flat construction. Evacuated tubes are more durable, and because they are sealed, they are less prone to condensation and corrosion.

Installation

Solar panels must be installed at a recommended angle and facing the side with maximum sun exposure to maximize efficiency.

Evacuated tubes require less roof space and are lighter than panels making them easier to install and manage even on the roof at a steep angle. While they can produce heat with a higher temperature, they can also more often overheat.

Cost

For many homeowners, the cost is one of the most important factors when choosing between solar water heaters. Vacuum tubes tend to cost more than the standard flat type because their design is more complex and costs more to produce, and they can cost you 20 to 40% more.

Advantages

The main advantage of the evacuated solar tube water heaters is the ability to heat water in all climates, including cloudy and winter days, also windy conditions, and hail.

Because of the cylindrical shape, the evacuated tube system is facing the sun all the time with the tubes perpendicular to the sun rays, making them more efficient than flat solar panels. As long as the sun shines, the evacuated solar tube heater can be in service all year round.

Even in cold climates, you can use this system as the selective coating and vacuum layer prevents heat loss and ensures high energy absorption.

Since the heat pipes transfer the heat in one direction only and within the collector, over 92% of the heat is absorbed, and less than 8% of heat is lost.

They are also easy to install and replace. If one of the vacuum tubes breaks, you can easily replace it - just plug the new one in. What is great with this plug-and-play system is that the collector still performs well even if one or more tubes are out of service.

They are compatible with gas and electric heaters that can work as a backup.

Also, since there is no water inside the evacuated tubes, the system can work during wintertime without any freeze concern and need to drain a tank.

Disadvantages

• Evacuate tubes get much hotter than that solar connector, making domestic water and space heating issues due to overheating.
• The tubes are fragile, especially if made of annealed glass. A better option is tempered glass.
• The initial price is higher.

Can I use evacuated tube collectors for pool heating?

Evacuated tubes can be used for heating the pool, but some modifications must be made because copper pipes and gaskets, for example, can deteriorate.

Also, it is recommended to use stainless steel or titanium heat exchangers for heat transfer to prevent damage from the chlorinated water.

Since they can be used even in colder weather, homeowners can extend their pool season by a few months.

Some experts say that since the vacuum tubes work the best for higher temperature differentials and as the pools don't require high heat, the tube collectors would not be as beneficial. The best option will be to install unglazed flat plate collectors.

How long do evacuated tubes last?

Evacuated tubes have a life expectancy of over 20 years, but their longevity depends on many factors, including the system type, material quality, weather, climate, usage, etc.

According to solartubs.com, the method and material of the absorbing coating of the vacuum tubes have a major role in the tube's life expectancy.

Evacuated tubes are not only replaced when they break but also when the vacuum inside the tube leaks. Most likely, leaks in the tube cannot be repaired, so the defective tube must be replaced with the new one. And that can happen anytime.

If the leak happens during the warranty period, the manufacturer will replace it.

The warranty is approximately ten years.

Conclusion

Evacuated solar tube collectors are an excellent option for many homeowners as they offer an efficient and cost-effective solution for water, space, and pool heating throughout the year.

While they cost more than flat panels, they are also more efficient during winter and cloudy days. And if the problem occurs, each tube can be replaced without stopping the heating operation.