U-Pipe Vacuum Tube Solar Collector: An Efficient Pressure-Bearing Engineering Hot Water Solution

In the field of solar energy heat utilization, the vacuum tube technology because of its excellent thermal performance, and the widely application. Among them, the U-Tube Vacuum Tube Solar Collector, as a high-end product in the vacuum tube family, perfectly combines the advantages of efficient heat collection of vacuum tubes and pressure-bearing operation of flat plate systems, and has become an important choice for large and medium-sized solar hot water projects. This article will delve deeply into the principle, characteristics and engineering application value of this technology.

I. Working Principle and System Structure 

The U-tube vacuum tube collector adopts a unique "U-shaped" metal tube design. Its core structure includes:

Vacuum tube assembly 

1. Outer tube: High borosilicate glass tube, ensuring high light transmittance (≥91%) 

2. Inner tube: Also made of glass, with a selective absorption coating (α≥0.93) coated on the surface 

3. Vacuum layer: The vacuum degree between the inner and outer tubes reaches 10⁻³Pa, effectively suppressing convective and conductive heat loss 

U-shaped metal heat exchange tubes 

1. A purple copper tube is bent into a U shape and passed through the interior of the vacuum tube 

2. The surface has undergone anti-oxidation treatment to ensure long-term thermal stability 

3. Internal circulation anti-freezing working medium (propylene glycol solution, etc.) 

Collector system 

1. Multiple U-tube vacuum tubes are connected in parallel to form a collector array 

2. Connect each U-shaped tube through the header to form a complete circulation loop 

3. Equipped with aluminum profile brackets to ensure structural stability

Working process: Sunlight → penetrates the outer glass tube → Is captured by the absorption coating → heat energy → is transferred to the U-shaped tube fins through radiation → is conducted to the working medium inside the U-shaped tube → The heated working medium is conveyed to the water tank heat exchanger by the circulating pump.

Ii. Five core advantages

1. Pressure-bearing operation capacity 

The all-metal flow channel design enables the working pressure to reach 0.6-1.0MPa 

It can be directly connected to the building water supply network, and the water outlet pressure is stable 

Supports simultaneous water supply at multiple points to meet the requirements of engineering applications 

2. High-efficiency heat collection performance

Vacuum insulation ensures minimal heat loss and a flat heat collection efficiency curve 

It can still maintain good performance even in low-sunlight and low-temperature environments 

The efficiency in winter is significantly higher than that of flat-plate collectors

3. Excellent anti-freezing ability 

The vacuum tube itself does not retain water, completely eliminating the risk of freezing

The freezing point of the antifreeze working medium can be as low as below -35℃

It is especially suitable for use in cold regions

4. Modular design

Each pipe operates independently, and local faults do not affect the system operation

Supports hot-swappable replacement, making maintenance simple and convenient

The heat collection area can be flexibly expanded according to demand

5. Resistance to direct sunlight

Vacuum tubes have excellent high-temperature resistance and can withstand exposure to sunlight above 400℃

There is no risk of pipe burst and it is highly safe and reliable

Iii. Technical Parameters and Performance Indicators

Typical performance parameters (single pipe ∅58×1800mm) :

Light-receiving area: 0.12m² per piece

Working medium capacity: 0.15-0.2L per tube

Operating temperature: -30℃ to 150℃

Instantaneous efficiency: η=0.75-0.05×(Δt/G) (Δt is the temperature difference, G is the irradiance)

Heat loss coefficient: ≤1.5W/m²K

System configuration suggestions

The water tank capacity should be 60 to 80 liters per square meter of heat collection area

The concentration of the antifreeze working medium is determined based on the local minimum ambient temperature

The head of the circulating pump needs to take into account the system resistance and elevation difference

Iv. Engineering Application Solutions

1. Large and medium-sized hot water projects

Hotel and guesthouse system: Heat collection area 200-500 square meters, equipped with 30-100 ton insulated water tanks

School hospital system: It adopts a timed water supply and constant temperature control strategy

Swimming pool system: Maintains water temperature at 26-28℃, significantly reducing operating costs

2. Heating integration system

Combined with the floor heating system, it provides basic heating in winter

It is equipped with an auxiliary heat source to ensure stable heating in extreme weather

Give priority to providing domestic hot water, and use the waste heat for heating

3. Industrial preheating applications

Provide preheating for industries that require process heat at 40-80℃

Electroplating, food processing, textile printing and dyeing and other fields

The payback period of investment is usually 2 to 4 years

4. Agricultural drying system

Crop drying, Chinese medicinal material drying, etc

Provide stable hot air to improve the drying quality

Reduce the consumption of traditional fuels and lower production costs

V. Key Points of System Design 

1. Optimize the inclination Angle design

Make adjustments based on the local latitude

Emphasis on winter use: Latitude +10°-15°

Year-round use: Equal to the local latitude

Emphasis on summer use: latitude -10°-15°

2. Overheat protection

Set the upper limit of the temperature difference cycle (usually ≤90℃)

Adopt emergency measures such as radiators or underground water cooling

Consider seasonal occlusion schemes

3. Anti-cavitation design

Make sure that an exhaust device is installed at the highest point of the system

The pipeline design avoids the formation of air pockets

An automatic exhaust valve is adopted

4. Anti-scaling measures

Prepare the working medium with softened water

Regularly monitor changes in pH value

Consider adding corrosion inhibitors

Vi. Installation and Maintenance Guide 

Installation specifications

The pipe spacing should be no less than 0.8 meters to ensure that the rear seats are not blocked by the front seats

The foundation load-bearing capacity is ≥30kg/m²

Avoid installing in the air outlet area

Maintenance requirements

Check the working medium concentration and pH value every year

Clean the surface of the vacuum tube every quarter

Check the operating status of the circulating pump every month

Common Fault Handling

Efficiency decline: Check the vacuum degree of the vacuum tube and replace the failed tube

Leakage: Locate the aged part of the sealing ring and replace the sealing component

Poor circulation: Expel the gas and check the operation of the pump

Vii. Economic Analysis 

Investment composition (taking a 1000m² project as an example) :

Collector system: 45%-50%

Heat storage system: 25%-30%

Control system: 10%-15%

Installation project: 10%-15%

Earnings analysis

Annual energy savings per square meter of collector: 350-550kWh

 Payback period: 3 to 6 years (depending on local energy prices)

Return over life: 3 to 8 times the initial investment

Viii. Comparison with Traditional Vacuum Tube Collectors 

The U-tube collector features traditional all-glass vacuum tubes 

Pressure-bearing capacity: Pressure-bearing operation (0.6-1.0MPa), non-pressure-bearing 

Excellent anti-freezing performance (working medium circulation) and good (water in the pipe may freeze) 

The maintenance single branch pipe can be replaced if drainage is required 

It is highly efficient and stable, but is affected by water quality 

The cost is relatively high or low 

It is suitable for medium and large-scale household projects as well as small-scale projects

Conclusion 

The U-tube vacuum tube solar collector, through innovative structural design, has successfully overcome the limitations of traditional solar collectors in pressure-bearing operation, system maintenance and engineering applicability, and has become the preferred solution for large and medium-sized solar hot water projects. Its outstanding thermal performance, reliable operational characteristics and good scalability make it demonstrate significant advantages in places with hot water demands such as hotels, schools and factories.

With the continuous maturation of solar thermal utilization technology and the accumulation of engineering experience, U-tube vacuum tube collectors will continue to play a significant role in the energy transition, providing a reliable technical path for reducing fossil energy consumption and lowering carbon emissions. Choosing a U-tube system is not merely about selecting a hot water device; it is also about investing in a reliable, efficient and sustainable energy future.