Solar Thermal Collectors

Several types of collectors are used in solar applications: they can be divided in two categories, flat plate and evacuated tube collectors.
Different technologies of evacuated tubes collectors exist:

  • Heat pipe evacuated tubes collectors
  • U-pipe evacuated tubes collectors
  • Direct flow evacuated tubes
Evacuated Tubes collectors

Evacuated tubes have high efficiency rates and a good adaptability: their 360 degree collector design allows them to gather much more sunlight in low-light conditions, earlier and later in the day when the sun is at angles that would not be utilized by a traditional Flat Plate Solar Collector. Their efficiency is also higher in cloudy climates or for non-optimal orientation and inclination.
For solar cooling application or industrial processes, evacuated tube collectors are more suitable because the water reaches higher temperatures.

Description of the technology: The tube is mode with two layers of glass with vacuum in between. A vacuum indicator at one extremity changes color from silver to white if a tube’s vacuum has been compromised. Inside the glass layers is a selective coating, which enables the use of the whole solar energy spectrum to generate heat.

Heat pipe

In a heat pipe, the captured heat within the tube is transferred through vapour in the header pipe to the water. Individual tubes can also be exchanged without emptying the entire system of it's fluid and should one tube break, there is little impact on the complete system.

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In a U-pipe, the heat is transferred to the glycol solution which enters and exits each tube. The efficiency rate is similar to heat pipe collectors.


Flat plate collectors

They consist of a dark flat-plate absorber of solar energy (thin absorber sheet of aluminium, steel or copper to which a selective coating is applied), a transparent cover that allows solar energy to pass through but reduces heat losses, a heat-transport fluid (air, antifreeze or water) to remove heat from the absorber, and a heat insulating backing. The fluid is circulated through tubing to transfer heat from the absorber to the water tank.
The life time of flat plate collectors is usually over 25 years.

Comparison between flat plate and evacuated tubes collectors

First, it is important to keep in mind that those two types of collectors are not suitable for exactly the same applications: as high vacuum tubes collectors can reach higher temperatures, they can be used for solar cooling or industrial processes. It is not the case with flat-plate collectors.

From an efficiency point of view, Flat-plate collectors suffer from the disadvantage of losing heat to the environment when the liquid in the collector reaches a temperature far above the ambient; while in comparison, evacuated tube collectors suffer the disadvantage that their absorber plate area to gross area ratio is smaller (typically 60-80% of gross area).

Besides efficiency, there are several other differences. Evacuated tubes work at a higher operating temperature which may be considered a safety feature. They have also less aerodynamic drag, their tubes can be replaced individually without shutting down the entire system and there is no condensation or corrosion within the tubes. However, over their life-time, there is the question of vacuum leakage.
Flat panels have been around much longer, and are less expensive.


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