Evacuated Tubes vs. Flat Plates

In all solar thermal industry is this debate : wath panel are the best ; with evacuated tubes or flat panel. To can give you posibility to arrive to a solution to this question need first to have informations.

Flat Plate Collectors

Flat plate collectors have been on the market and in use since the early 1900's. They consists of an absorber plate – generally a painted metal – attached to copper pipes where water or a heat transfer liquid passes through. This is encased in a metal frame, surrounded by thick insulation to help retain the collected heat, and protected by a sheet of glass or glazing, which also provides an insulating air space.

Evacuated Tube Collectors

Evacuated tube collectors are more recently developed technology. Was introduced to the market in the 1970's. There are several varieties of evacuated tubes. The glass tube actually consists of two walls of glass. In between the two walls, all the air is removed, resulting in a vacuum. This vacuum is the best insulation one could ask for, and gives the evacuated tubes a much better heat retention than air space.

The heat pipe is also pressurized, allowing the liquid to boil very rapidly, at a very low temperature. It carries the collected heat to the top of the collector, where the heat is then collected by water or heat transfer liquid that flows around the top of the heat pipe, and then transferred to a storage tank.

Which is Best?

Cost is typically the primary consideration. Collector for collector, evacuated tubes can cost around 20% to 40% more than flat plate collectors. Flat plate pricing, however, is subject to greater fluctuates they use quite a bit more copper in their manufacturing process.

Shipping costs can be more with flat plates than with evacuated tubes as well. Evacuated tubes are modular, and can be shipped vertically, maximizing the usable space on a pallet.

Location is also an important consideration to cost. In some regions, it make take more or less of one type of collector vs the other to heat the same amount of water. For example in a colder climate, you could need 2 or 3 flat plate collectors to produce the same heat as 1 evacuated tube collector.

Generally, evacuated tubes perform better in colder and cloudier conditions than their flat plate counterparts. This is because of the vacuum in the glass tube, which allows tube collectors to retain a high percentage of collected heat.

However, in areas where heavy snowfall can be an issue, evacuated tube collectors will not leak heat from the collector, and therefore will not melt snow and heavy frost as quickly as a flat plate collectors. A flat plate collector, on the other hand, will collect heat through the reflected sunlight off snow & ice, and therefore melt the snow or heavy frost much quicker.

Flat plates are typically designed with an unsealed enclosure. This can make them prone to condensation over time, which can result in corrosion. Flat plate collectors – if damaged, will continue to function, and can at times be repaired. Other times, the entire flat plate must be replaced.

Evacuated tubes, on the other hand, are sealed with a vacuum. This gives them their high heat retention properties, however, without this vacuum an evacuated tube collector performs very poorly. If a tube were to lose it's vacuum, it is generally very easy to correct, and can be done easily by simply replacing the tube.

Evacuated tubes are typically less sensitive to sun angle and orientation than their flat plate counterparts. Their circular design allows sunlight to pass at an optimal angle throughout the day – from morning to night.

Flat plate collectors are more sensitive to sun angle, and may require the use of racking systems, or other elevations to maximize their production.

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