Are Solar Evacuated Tubes for Water Heating Worth It? Pros and Cons Explained

Thinking about installing solar evacuated tubes for water heating? Before you make your decision, it’s essential to consider a few key factors to ensure you’re choosing the best option for your home.

Solar evacuated (vacuum) tubes are incredibly efficient at converting sunlight into heat, providing you with free, eco-friendly hot water. Not only are they durable and low-maintenance, but they also help reduce your carbon footprint and lower energy costs—making them a smart choice for those looking to live sustainably.

To make an informed decision, it’s crucial to understand how they work and compare to other solar technologies, such as flat-plate collectors or photovoltaic (PV) panels. Each technology has its own strengths and weaknesses, and the right choice will depend on your specific needs, budget, and location.

What Are Solar Evacuated Tubes? Anatomy of the Vacuum Tubes

Solar evacuated tubes are cylindrical components integral to solar water heating systems, designed to efficiently capture solar energy and transfer it to a heat transfer fluid. These tubes are a key element of evacuated tube collectors (ETCs), which are specifically engineered to use the transfer fluid for heating water, space, and pools or spas.

While flat plate collectors remain the most popular, vacuum tubes have notable advantages. They perform exceptionally well for most of the day and year due to their always-perpendicular orientation to the sun’s rays.

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

Both inner and outer glass tubes are made of strong borosilicate glass, which is resistant to harsh weather conditions such as large hail. The outer tube is clear to allow efficient passage of solar rays, while the inner pipe is coated with a special selective coating to minimize reflection and maximize absorption of solar energy.

The air between the glass walls is removed to create a vacuum, which acts as an insulator, preventing heat loss from the absorber to the outside. At the bottom of the tubes is a barium getter, which removes any air from the tube to maintain the high vacuum inside.

The heat transfer fluid is transferred to the condenser bulb located at the end of the heat pump, which fits into the manifold along with the header pipe and insulation, where the accumulated solar energy is transferred.

The tubes are combined into groups called collectors, with each collector consisting of parallel rows of glass tubes to increase the exposed surface area. The collectors are supported by a frame and each glass element is connected to a header pipe or copper manifold located at the top, allowing for easy installation and replacement.

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 glass part 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.

Exploring the Various Types of Evacuated Tubes

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 device 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

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

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 Tubes 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 element reaches high temperatures. This means that the heat retention is higher, and the outside cold air won’t significantly influence the inside temperature.

Unfortunately, this can be a problem in winter because there will not be enough heat on the outer heater’s 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.

Essential Factors to Consider When Selecting Solar Evacuated Tubes

Advantages

• Works in All Climates: These heaters efficiently heat water in all weather conditions, including cloudy, winter days, windy conditions, and even during hailstorms.
• Efficient Performance: The cylindrical shape of the tubes allows them to stay perpendicular to the sun’s rays at all times, making them more efficient than flat solar panels. They remain effective year-round as long as the sun shines.
• Cold Climate Ready: The selective coating and vacuum layer help prevent heat loss, ensuring high energy absorption even in cold climates.
• High Heat Absorption: Heat pipes transfer heat in one direction only within the collector, absorbing over 92% of the heat, with less than 8% lost.
• Easy Installation and Replacement: The system is user-friendly. If a vacuum tube breaks, it can be easily replaced by simply plugging in a new one. The collector continues to work efficiently even if one or more tubes are out of service.
• Backup Compatibility: The system works well with gas and electric heaters as backups, ensuring continuous heating.
• Winter Operation: Since there is no water inside the evacuated tubes, the system can operate during winter without any freeze concerns or the need to drain the tank.
• Cost Savings: The evacuated solar tube system provides free solar water heating, significantly reducing utility bills.
• DIY Installation: The system allows for easy installation and replacement, making it ideal for DIY projects.
• High Efficiency: Heat is transferred only one way, ensuring high efficiency within the system.
• Excellent Heat Transfer: Copper materials used in the system contribute to a high transfer rate.
• Maximized Absorption: The vacuum and special absorption film enhance the system’s ability to absorb solar energy.
• Reliable in Cold Climates: These systems continue to operate efficiently even in colder environments.
• Performs in Overcast Conditions: They perform well in overcast or diffuse sunlight conditions, ensuring reliable heating.
• Low Maintenance: The system requires minimal maintenance for long-term use.
• Durability: These systems are built to last, providing long-term reliability and performance.

Disadvantages

• Overheating Risks: The evacuated tubes can get much hotter than other solar collectors, which may lead to overheating issues for domestic water and space heating.
• Fragile Tubes: The tubes, especially those made of annealed glass, can be fragile. Tempered glass is a better option for improved durability.
• Higher Initial Cost: The upfront cost of evacuated solar tube systems tends to be higher compared to other solar heating systems.

Can I Use Evacuated Tube Collectors for Pool Heating?

Evacuated tubes can be used for pool heating, but some modifications may be required to prevent deterioration of copper pipes and gaskets. To avoid damage from chlorinated water, it is recommended to use stainless steel or titanium heat exchangers for heat transfer.

They are efficient in colder weather, allowing homeowners to extend their pool season by a few months. However, some experts suggest that since the vacuum tubes work best for higher temperature differentials and pools don’t require high heat, unglazed flat plate collectors may be a more suitable option.

How Long Do They 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 life expectancy.

These are not only replaced when they break but also when the vacuum inside the pipe leaks. Unfortunately, leaks cannot be repaired, so the defective element must be replaced entirely, which can happen at any time.

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

The warranty is approximately ten years.

How to Enhance the Lifespan and Performance

Proper maintenance is crucial for ensuring the long-term efficiency and reliability of evacuated tube collectors. This is recommended:

• Inspection and cleaning: Evacuated tube collectors should be inspected semi-annually for any dust, debris, or snow that may accumulate on the tubes. The external surface of the tubes can be gently cleaned with a soft cloth or brush and mild detergent if necessary.
• Checking the seals and insulation: Regularly inspect the seals and insulation around the tubes and manifold to ensure they are intact and providing adequate insulation. This helps in maintaining efficiency and preventing heat loss.
• Monitoring system pressure: The pressure in the system, including the heat transfer fluid, should be checked periodically. Fluctuations in pressure can indicate leaks or other issues that need attention.
• Assessing the heat transfer fluid: The quality and level of the heat transfer fluid should be checked annually. Over time, this fluid can degrade, necessitating replacement to maintain system efficiency.
• Checking the piping and connections: Ensure that all pipes and connections are secure and leak-free. Look for any signs of corrosion or wear.

Common Issues Faced By Homeowners and How to Address Them

When it comes to troubleshooting evacuated tube collectors, homeowners commonly encounter a few key issues, such as:

• Reduced efficiency due to snow or debris: As mentioned, accumulated snow or debris can affect efficiency. Regular cleaning and proper installation angle can mitigate this issue.
• Vacuum loss in tubes: If a tube loses its vacuum, it will not function efficiently. A tell-tale sign is the formation of condensation inside the tube. These tubes need to be replaced.
• Overheating: In very hot climates or during periods of low water usage, the system can overheat. This can be addressed by installing a heat dump system or using an automatic controller to regulate the temperature.
• Freezing problems: In extremely cold climates, the system can freeze, damaging the tubes. Anti-freeze solutions or drain-back systems can prevent this issue.

Conclusion

Solar evacuated tubes provide an innovative and reliable option for homeowners seeking to utilize renewable energy sources to heat their homes and pools.

While their initial cost may be higher than flat panels, they offer greater efficiency, particularly during colder and cloudier weather conditions, ultimately resulting in long-term cost savings.

Additionally, the ability to replace individual tubes without interrupting the entire heating system provides added convenience and ease of maintenance.

Overall, these systems are an excellent investment for those looking to reduce their carbon footprint and maximize their energy savings.