All About Solar > Evacuated Tubes or Flat Plates?

The solar heating collector's job is simple – it sits in the sun, absorbs the heat, and transfers it to where you need it. To do this efficiently, solar heating collectors need to absorb – and retain – a large volume of sunlight every day. There are two main technologies that have both been time tested and accomplish this, and as a result there has been a practically endless debate over which is considered “best”. These two main technologies are the Solar Flat Plates, and Solar Evacuated Tubes.

As an American manufacture of solar heating products, Solar Panels Plus has tested, designed, and supplies both flat plate and evacuated tube systems.

Flat Plate Collectors

Flat plate collectors have been on the market and in use since the early 1900's, and are one of the most time tested and well known technologies. They consists of an absorber plate – generally a painted metal, such as copper – 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 a slightly more recently developed technology, introduced to the market in the 1970's. There are several varieties of evacuated tubes, however the most commonly used type employs the use of a heat pipe surrounded by a glass tube that is under a vacuum. 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 (usually water) to boil very rapidly, at a very low temperature (usually between 75F and 80F). As the water boils, 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 or elsewhere in the system.

Evacuated Tubes vs. Flat Plates: Which is Best?

Cost

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, especially when ordering a package system. 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.

Performance

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.

Design

Flat plates are typically designed with an unsealed enclosure. This can make them prone to condensation over time, which can result in corrosion. However, this largely does not impact the actual performance of a flat plate unless corrosion results, and is mainly a cosmetic downfall.

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.

Installation

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.

When considering which technology to use, consult your local dealer, or contact us directly. We will be glad to look at both technologies and see which is the best fit for your specific application.