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Flat Panel vs Evacuated Tube Solar Collectors

What are Flat Panel Solar Thermal Collectors?Solar Hot Water, Flat Plate Solar Hot Water Collector, - The Clean Energy Centre
The flat panel collector consists of an insulated box with a glass on one side and copper tubing that runs through the absorber plates. They are often connected to a water storage tank on the roof. Read more detail about how Flat Panel collectors work...


Cold water first passes through the solar collector, which preheats the water, and then continues to the conventional backup water heater


What are Evacuated Tube Solar Thermal Collectors?

Evacuated Tube Collectors collect the heat through a number of annealed glass tubes that each have their own heat-absorbing plate inside. There is an extensive array of different designs on the market, but the basic principle applies for all. Basically, each tube is like a little greenhouse that traps sunlight inside the glass tube which is used to heat either the water, or a special heat transfer fluid that transfers the heat to the water, via copper pipes and a heat exchanger.Solar Hot Water, Evacuated Tube solar Hot Water System - The Clean Energy Centre

 

Evacuated-tube solar collectors are made of parallel rows of transparent glass tubes. Each tube contains a glass outer tube and copper absorber tube attached to a fin.
A vacuum seal in the glass tube delivers a high level of insulation and eliminates conductive and convective heat loss. This vacuum seal delivers one of the biggest advantages of an Evacuated Tube system’s extremely high efficiency over a Flat Plate Collector Panel.

Copper pipes run through the centre of the glass tube, and these are connected to a common manifold. In an ‘Open Loop System’, these copper pipes carry the water through the collector and absorb the sun's heat. The manifold is connected to a circulation pump that keeps pumping water to the storage tank below. In a ‘Closed Loop System’, the copper pipes transfer a special heat transfer liquid that boils at very low temperatures. This transfer liquid heats the water via a heat exchange system.

 

How Evacuated Tube Collectors Work....

Step 1: Solar Absorption

Solar Radiation is absorbed by the evacuated tubes and converted tinto heat

 

Step 2: Solar Heat Transfer

Heat Pipes conduct the heat from within the solar tube up to the header

How Solar Evacuated Tubes Work - Clean Energy Centre

Step 3: Solar Energy Storage

Water is circulated through the header via intermittent pump cycling. Each time the water circulates through the header the temperatures is raised by a small amount. Throughout the day, the water in the storage tank is gradually heated.

 

Summary of advantages that Evacuated Tube Collectors have over Flat Plate Solar Hot Water Collectors:

  1. Evacuated Tube Solar Hot Water Collectors are more efficient than Flat Plate Solar Hot Water Collectors, even at low light intensities (e.g. cloudy days). The vacuum seal between the inner and outer layer of glass acts as an excellent insulator and extremely efficient at retaining high temperatures over long periods. Up to 163% efficiency has been demonstrated in Australian conditions.
  2. Evacuated Tubes are round, not flat. The ability to collect heat from a wide arc increases the efficiency of collecting heat from the sun, regardless of the suns angle and time of day. This is in contrast to a Flat Panel Collector, which is at its most efficient only when the sun is directly above the panel.
  3. Can be used in sub-zero temperatures
  4. Ability to achieve high temperatures
  5. Evacuated Tubes are easier to install due to their lightweight and modular design
  6. Improved aesthetics – Modern and elegant design
  7. Covers less roof area – Less visually intrusive
  8. Each Evacuated Tube is individually replaceable allowing for easy repair in case of damage.
  9. Longer life span providing an improved return on investment. Evacuated tubes do not have the corrosion problems reported in several flat-plate collectors.

Solar Hot Water, Evacuated Tube Solar Hot Water System Diagram - The Clean Energy Centre