Flat plate collectors

A flat-plate solar collector is one of two main types of solar collectors, which are key components of a solar water heating systems. The other type is an evacuated tube collector which we will discuss later.

Flat-plate collectors are the most common solar collectors for use in solar water-heating systems in homes. A flat-plate collector consists of an insulated metal box with a glass or plastic cover (the glazing) and a dark-coloured absorber plate.

Solar radiation is absorbed by the absorber plate and transferred to a fluid that circulates through the collector in tubes.

Flat-plate collectors heat the circulating fluid to a temperature considerably less than that of the boiling point of water and are best suited to applications where the demand temperature is 30-70°C and/or for applications that require heat during the winter months.

Flat collectors can be mounted in a variety of ways, depending on the type of building, application, and size of collector. Options include mounting on a roof or free-standing.

Flat plate collector
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geyser heating options

Evacuated Tubes

This system comprises a series of cylindrical glass tubes are attached to a frame and installed on top of a roof. Each tube is approx 1.9m long and inside each tube is a copper rod which extends from the bottom to the top of the tube and then into a manifold. 

A special chemical formula (glycol based) inside each tube starts to boil between 30 and 40 degrees Celsius. This quickly heats up the copper rods in each tube which in turns heats the water which passes through the manifold. The manifold in turn is connected to a geyser through a pump and differential solar controller.

The vacuum inside of the evacuated tube collectors have been proven to last more than 25 years, the reflective coating for the design is encapsulated in the vacuum inside of the tube, which will not degrade until the vacuum is lost. 

The vacuum that surrounds the outside of the tube greatly reduces convection and conduction heat loss, therefore achieving greater efficiency than flat-plate collectors, especially in colder conditions. The high temperatures that can occur may require special design to prevent overheating.

Evacuated tubes  vs. Flat Plate Collectors

Flat-plate collectors usually lose more heat to the environment than evacuated tubes due to their construction and design. They are inappropriate for high temperature applications such as process steam production. 

Evacuated tube collectors have a lower absorber plate area to gross area ratio (typically 60–80% of gross area) compared to flat plates.

Based on absorber plate area, most evacuated tube systems are more efficient per square meter than equivalent flat plate systems. This makes them suitable where roof space is limiting, for example where the number of occupants of a building is higher than the number of square metres of suitable and available roof space. 

In general, per installed square metre, evacuated tubes deliver marginally more energy when the ambient temperature is low (e.g. during winter) or when the sky is overcast. However even in areas without much sunshine and solar heat, some low cost flat plate collectors can be more cost efficient than evacuated tube collectors. 

Although several European companies manufacture evacuated tube collectors, the evacuated tube market is dominated by manufacturers in the East. Several Chinese companies have track records of 15–30 years. There is no unambiguous evidence that the two designs differ in long term reliability.

However, evacuated tube technology is younger and (especially for newer variants with sealed heat pipes) still need to demonstrate competitive lifetimes. The modularity of evacuated tubes can be advantageous in terms of extensibility and maintenance, for example if the vacuum in one tube diminishes. For a given absorber area, evacuated tubes can therefore maintain their efficiency over a wide range of ambient temperatures and heating requirements. In most climates, flat-plate collectors will generally be more cost-effective than evacuated tubes.

For a given absorber area, evacuated tubes can therefore maintain their efficiency over a wide range of ambient temperatures and heating requirements. In most climates, flat-plate collectors will generally be more cost-effective than evacuated tubes.

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