What is an Evacuated Tube Collector and How Does it Work?

An evacuated tube collector (ETC) is a type of non-concentrating solar thermal device that uses parallel U-shaped glass tubes that are vacuum sealed and designed to trap sunlight and convert it to heat to raise the temperature of water. The water they heat using solar energy can be used for residential and commercial purposes. These solar thermal collectors have a heat-absorbing plate attached to each tube. 

When solar radiation falls on the plate, it gets absorbed. The heat from the absorbed sunlight is then transferred to a heat-transfer fluid, which can be water or alcohol, circulating inside the evacuated tube. This heated fluid can then be used to raise the temperature of water in a storage tank. 

Most modern evacuated tube solar collectors can operate at temperatures ranging from 50-250°C and achieve up to 63% annual daily thermal efficiency with minimal heat loss. Since they can generate such high temperatures while maintaining a high efficiency, they’re ideal for residential water heating, space heating, and industrial applications.

Some of the best features of evacuated tubular collectors include:

• Cold climate advantage: ETCs perform exceptionally well in temperatures below freezing.
• All-day solar collection: Their cylindrical shape allows them to capture sunlight from multiple angles.
• High efficiency performance: They can achieve up to 63% annual thermal efficiency with minimal heat loss because they are vacuum-sealed.

However, when it comes to installing them on rooftops at homes, and you happen to live in cities with a reliable grid, having an on-grid solar system makes more sense. Evacuated tube solar collectors can only supply hot water, but they can’t generate electricity to power any other home appliance. An on-grid rooftop solar panel system for homes can convert sunlight into free solar electricity, and power your entire household, including your regular geyser to supply hot water.

That being said, an evacuated tube collector is a worthwhile investment in remote areas. It can function like a solar water heater, supplying hot water for livestock watering, drinking, cooking, and cleaning, even when the grid supply is unavailable.

In this blog, we will explain what an evacuated tubular collector is, how it works, its types, benefits, disadvantages, and how it differs from flat plate collectors (FPCs), which are also a type of solar thermal collector. We will also draw a comparison between evacuated tube collectors and on-grid rooftop solar systems, and explain why solar PV systems are better for homes looking for energy independence and freedom from electricity bills.

TL;DR Summary Box: What is the Evacuated Tube Collector Working Principle?

The evacuated tube collector works on the principle of black body absorption, which states that a perfect black body absorbs all electromagnetic radiation that strikes it, regardless of angle or wavelength, and converts this energy into thermal energy. 

In evacuated tube collectors, the inner absorber tube is coated with materials that behave like a black body. It absorbs solar radiation while acting as a poor emitter of thermal radiation. This creates a thermal trap that maximizes solar energy capture while minimizing heat loss.

Here are the main topics covered in this blog in detail:

What is an Evacuated Tube Collector?

An evacuated tube collector is a solar thermal device that consists of multiple parallel glass tubes. Each tube contains a vacuum space and an inner absorber plate designed to capture and convert solar radiation into heat. 

The vacuum layer between the inner and outer glass tubes is used for insulation to reduce heat loss. It’s this vacuum sealing that allows evacuated tubular collectors to maintain high temperatures between 50-250°C even in freezing weather conditions.

What Are the Main Components of an Evacuated Tube Solar Collector?

An evacuated tubular collector is made of multiple components, including outer and inner glass tubes, vacuum sealing between the tubes, and a heat transfer fluid.

Let’s check out all the components of ETCs and understand their basic role in solar energy collection and its conversion to heat:

• Outer glass tube made of borosilicate: This transparent outer tube of the evacuated tube collector is made from strong borosilicate glass so that it can withstand thermal shock as well as hailstorms. It allows maximum solar radiation to pass through, and protects the internal components from wind, rain, and dust. 
• Inner absorber tube: This is the heat-collecting tube that works like a black body. It uses a special black coating to absorb solar radiation. This tube can be made of copper in glass-metal designs or specially coated glass in all-glass designs, and it reaches temperatures of 50-250°C during operation.
• Vacuum space: It’s the evacuated space between the outer and inner tubes that prevents heat loss. 
• Selective absorber coating: It’s a specialized coating made of aluminum nitride or titanium nitride oxide. It’s applied to the inner tube to maximize solar energy absorption and minimize thermal losses. 
• Heat transfer fluid: It’s the circulating fluid, usually water or a glycol-water mixture, that flows through the absorber tube to carry collected heat to the storage system. 
• Manifold: It connects individual tubes and provides inlet and outlet connections for the heat transfer fluid. The manifold ensures proper fluid distribution across all tubes and collects the heated fluid for delivery to the storage tank.
• Support frame structure: This is the mounting system that holds the tubes at the optimal angle to prevent shading and provides structural strength against strong winds. The frame is usually made of aluminum or galvanized steel.
• Insulation layer: It’s the insulation material around the manifold that prevents heat loss during fluid transport. This insulation layer maintains system efficiency by keeping the heated fluid at high temperatures while it’s circulating.

How Does an Evacuated Tube Solar Collector Work? A Step-by-Step Guide

Evacuated tube solar collectors use the black body absorption principle to work. The inner absorber tube captures solar radiation and converts it to heat. At the same time, the vacuum layer prevents heat loss.

Let’s check out the entire working process in simple steps: 

• Step 1 – Solar radiation hits the borosilicate glass tube: Sunlight penetrates through the outer borosilicate glass tube to reach the inner tube.
• Step 2 – Photons in the sunlight get absorbed by the black coating on the inner tube: Photons strike the absorber coating on the inner tube surface. The coating’s surface converts the majority of absorbed solar radiation into thermal energy.
• Step 3 – Thermal energy is generated: As photons are absorbed, electrons in the coating material become excited and release this energy as heat. This process rapidly heats the absorber tube surface to temperatures ranging from 50-250°C.
• Step 4 – Collected heat is transferred to the heat transfer fluid: The hot absorber tube transfers thermal energy to the heat transfer fluid. 
• Step 5 – The heat circulation begins: As the fluid heats up, its density decreases, making it rise on its own through natural convection. This creates a continuous circulation pattern where hot fluid moves upward toward the storage system while cooler fluid descends to replace it at the absorber level.
• Step 6 – Heat is collected in the manifold: The heated fluid from all individual tubes reaches the manifold, which acts as a collection chamber. The manifold is heavily insulated to prevent heat loss and includes inlet/outlet connections that send the combined heated fluid toward the storage tank or heat exchanger.
• Step 7 – The energy is stored and distributed: The hot fluid enters the storage tank, where thermal energy is either stored in a large water mass for later use or immediately transferred through a heat exchanger to the building’s hot water or space heating system. 
• Step 8 – Cooled fluid returns to the absorber tube for continuous operation: After releasing thermal energy in the storage system, the cooled fluid returns through the manifold back to the individual tubes to begin the heating process again. 

What Are the Different Types of Evacuated Tubular Collectors?

The two primary types of evacuated tube collectors are heat-pipe evacuated tube collectors and direct-flow evacuated tube collectors. 

Heat-pipe evacuated tube collectors use sealed pipes that contain a fluid that evaporates and condenses to move heat. They don’t carry water inside the tubes. Hence, they work well in freezing conditions. Direct-flow collectors, on the other hand, send water or a water-glycol mix directly through the tubes. This makes them cheaper and easier to install, but they need antifreeze protection in very cold weather.

• Heat-pipe evacuated tube collectors: In this design, each tube contains a sealed heat pipe with a small amount of working fluid. When sunlight heats the tube, the fluid quickly evaporates, carrying the heat to the top of the collector. There, the vapor condenses and transfers its heat to the water in the storage tank. This method provides very efficient heat transfer, works reliably even if some tubes are shaded, and offers strong freeze protection since no water flows inside the tubes.
• Direct-flow evacuated tube collectors: In direct-flow collectors, the heating fluid passes straight through the tubes. It enters at the bottom, absorbs heat as it rises, and comes out hotter at the top, where it flows into the storage tank. Direct-flow collectors are usually less expensive and easier to install. They perform well in areas with moderate climates, but in very cold conditions, they need antifreeze protection because water flows directly through the tubes.

Here’s a tabulated snapshot of how these two types of evacuated tubular collectors differ:

What is the Difference Between a Flat Plate Collector and an Evacuated Tube Solar Collector?

While both are solar collectors that convert solar energy into thermal energy, flat plate collectors and evacuated tube collectors differ.

Flat plate collectors are built with a flat absorber plate kept inside an insulated box with a glass cover. They are sturdy, less fragile, and generally cheaper, but they lose more heat and reach lower temperatures than evacuated tube collectors. Evacuated tube solar collectors, on the other hand, use glass tubes with a vacuum around them, which reduces heat loss and allows them to reach higher temperatures. 

ETCs are more efficient in cold or cloudy conditions, while flat plates can be a cost-effective choice in warmer regions where freeze protection is less important.

Let’s check out the major differences between the two types in detail:

What Are the Uses of an Evacuated Tube Collector? 

Evacuated tubular collectors can be used for supplying hot water for residential, commercial, and industrial uses. 

Here are the primary applications of using ETCs:

• Solar water heating: They can be used for hot water supply at homes and businesses.
• Space heating: They can be used for floor heating and forced air heating systems.
• Swimming pool heating: They can help maintain comfortable pool temperatures year-round.
• Industrial process heating: They can be used for manufacturing processes that require temperatures up to 250°C.

What Are the Benefits of an Evacuated Tube Solar Collector?

Evacuated tube solar collectors achieve high temperatures, require low maintenance, and can capture sunlight all day long. Furthermore, they offer output even under cloud cover and can work in freezing weather conditions. 

Let’s check out all the major benefits of ETCs:

• They can reach high temperatures: ETCs can heat water to temperatures between 50-250°C, which is higher compared to what flat plate collectors can achieve.
• They work well even in cold and freezing weather: The vacuum layer prevents heat loss. Hence, these collectors can keep working even when the outside temperature drops below zero.
• They offer good performance in cloudy conditions: ETCs can capture direct as well as diffused sunlight. Thus, they can still produce useful heat when the sky is overcast.
• All-day sunlight capture: The cylindrical tubes collect sunlight from different angles, working efficiently from morning to evening without needing to track the sun.
• They need less maintenance: With fewer moving parts and sealed tubes, ETCs usually need minimal maintenance.
• Scalability: You can easily expand the system by adding more tubes.

What are the Disadvantages of an Evacuated Tube Collector? 

The biggest problem with evacuated tube collectors is that they can’t generate electricity like solar panels do. Hence, they can’t lower electricity bills as much as you would expect, and they can’t be used to take care of all energy requirements in a home. 

Let’s check out all the disadvantages of ETCs:

• They can’t produce electricity: Unlike solar systems that convert sunlight into electricity, ETCs can use sunlight to just provide hot water. Thus, they can’t support all energy requirements in a home. Solar panels, on the other hand, can power all household appliances, including normal geysers for hot water supply. They can reduce electricity bills by 90% or more, which is something an ETC can’t do.
• They have a higher installation cost compared to other concentrating solar collectors: ETCs are more expensive than flat plate collectors because of their complex design and glass tube construction. Plus, they are more expensive than normal geysers.
• The tubes are fragile: The glass tubes can break more easily compared to the flat glass of FPCs. Replacing tubes adds to the maintenance cost over time.
• Vacuum loss over time is a sure risk: If the vacuum seal in a tube is damaged, the tube loses efficiency and may need replacement.
• Cleaning and maintenance are tricky: The round tubes can collect dust and dirt, and cleaning them is a bit more time-consuming than wiping a flat surface.
• The tubes can overheat in hot climates: In regions with very high solar radiation and low hot water demand, ETCs can generate more heat than needed, leading to overheating issues.

Evacuated Tube Solar Collector vs On-Grid Rooftop Solar System: Which is Better for Homes, and Why?

On-grid rooftop solar systems for housing societies and homes are better investments than evacuated tube solar collectors in areas with a reliable grid connection because not only does the government offer financial assistance in the form of a subsidy to install them, but they can power all household appliances, including a geyser to fulfill the demand for hot water. 

Installing an evacuated tube collector on the roof is like replacing a regular geyser with a solar geyser to get hot water. Installing an on-grid solar system, on the other hand, offers free solar electricity for 25+ years, which can power your normal geyser and supply hot water while simultaneously powering all other home appliances, reducing your electricity bills by 90% or more. 

Let’s check out all the reasons that make on-grid solar better for areas with a reliable grid:

• The government offers a subsidy to install them: Homeowners and housing societies can claim a subsidy under the PM Surya Ghar Muft Bijli Yojana to install solar. However, it’s only offered when installing an on-grid rooftop solar system. You won’t get this assistance if you install off-grid solar systems, ground-mounted solar systems, or commercial rooftop systems.
• They’re cheaper than other types of solar systems: Off-grid and hybrid solar systems use lithium batteries to store excess solar energy, which makes them expensive. On-grid systems, on the other hand, don’t use batteries. Instead, they send excess electricity generated by the panels to the grid through a bidirectional net meter, which can also import electricity from the grid at night to keep the load running.
• They reduce electricity bills by 90% or more: Since they can meet all energy requirements, they reduce reliance on expensive grid electricity, which gets costlier by 3-6% annually. As a result, your electricity bills will reduce by 90% or more. In many cases, customers get zero bills after they go solar.

Let’s give you a head-on comparison between the cost of installing a 4 kW solar system in India vs the money that system saves in 25 years by lowering your electricity bills:

*Please note: The above-mentioned 4 kW solar panel price in India with subsidy is indicative as of 19th September 2025 for the SolarSquare Blue 6ft variant. The actual 4 kW solar plate price depends on your DISCOM charges, city, product variant opted for, panel type, inverter type, mounting structure height, type of after-sales service, savings guarantee, roof height, etc. Prices are subject to change. Additionally, when calculating savings, we have considered an annual tariff escalation of 3% and an annual degradation of 1%. The actual final savings from for a 4 kW solar panel system depends on the types of solar panels you’ve installed and their efficiency, intensity of sunlight your rooftop receives, orientation of the panels and tilt angle, the pollution level and weather conditions in your city, the temperature, shadow on the roof, impact of dirt/dust, and how well you maintain your panels after installation.

Can’t find details of solar savings in your city in the table above? Use SolarSquare’s free solar power calculator to find out the appropriate solar system size based on your energy consumption and the cost of installing that system in your city, vs the savings the same system will offer in 25 years. 

Conclusion

Evacuated tube collectors are one of the most advanced solar thermal collectors that convert sunlight into thermal energy, which can be used for residential water heating, space heating, and industrial applications. They generate higher temperatures than flat plate collectors, as they are vacuum sealed. They also work particularly well in cold climates, where traditional flat plate collectors struggle to maintain efficiency.

Consider installing them if you live in remote and rural areas with an unreliable grid connection to take care of hot water requirements for livestock watering, cooking, bathing, and cleaning. However, if you live in urban areas with a reliable grid, investing in an on-grid rooftop solar system is a better investment to achieve energy independence and get freedom from electricity bills. 

For any further details about installing rooftop solar systems, you can book a free solar consultation call with our experts now.

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FAQs

Q1. How long do evacuated tubes last?

Ans. Evacuated tubes can last anywhere between 15 and 20 years with proper care and maintenance.

Q2. What is the temperature of an evacuated tube collector?

Ans. An evacuated tube collector can generate high temperatures, ranging between 50-250°C.

Q3. What are the problems with evacuated tube solar collectors?

Ans. The main problem with evacuated tube solar collectors is that, unlike solar panels, they can’t generate electricity to take care of all energy requirements. Also, the glass tubes are fragile and can break easily when compared to the flat surface of flat plate collectors.

Q4. Do evacuated tubes work in the winter?

Ans. Yes, heat-pipe evacuated tube solar collectors are specially designed to offer strong freeze protection. These types can perform very well in temperatures below zero degrees.