All About Solar Water Geysers > Evacuated Tubes or Flat Panels?

What could be easier? A solar collector sits in the sun, absorbing free energy, and gets hot (Very hot on the Highveld) and transfers this free energy it to a suitable water tank. Solar collectors are designed to be robust and efficient, and they absorb a huge volume of sunlight every day. The two main technologies (Flat Plate Collectors and Evacuated Tubes) that we are discussing are time tested and will accomplish this objective very efficiently and the big question “which is better” is going to come down to various factors and the interpretation thereof.

1)      Cost – this is always a KEY consideration

2)      Efficiency – this may have some surprising twists, but of course efficiency will affect the expected payback period

3)      Quality – Should always be the main consideration

4)      Warranty – check the warranty claims and make sure you fully understand the implications

5)      Expected Life – Any Solar solution is a costly exercise and you don’t want to have to repeat the exercise in a few years’ time

Flat Panel Collectors

Flat-plate collectors were developed by Hottel and Whillier in the 1950s, and therefore have a very long proven track record. They consist of:

1)      A dark flat-plate absorber

2)      A transparent tempered glass cover that reduces heat losses

3)      A heat-transport fluid (air, antifreeze or water) to remove heat from the absorber and transfer the energy to a specially designed tank of water.

  1. On the Highveld, it is critical to select Indirect Systems which contain Propylene (food grade) glycol.

4)      A heat insulating backing. The absorber consists of a thin absorber sheet (of thermally stable polymers, aluminum, steel or copper to which a matte black or selective coating is applied) backed by a grid or coil of fluid tubing placed in an insulated casing with a glass or polycarbonate cover.

 

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 in the same manner as a clear Thermos bottle would function. 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.

So?? Which is Best?

Cost

Cost is typically the primary consideration.


In general, Evacuated tubes cost less in the South African market. However, it is very important to understand that Evacuated tubes are designed for cooler climates and when subjected to the harsh African sun, they can fail relatively easily.

Whereas Flat Plate collectors are very robust and are able to withstand the harsh conditions more easily.

Performance

Generally, evacuated tubes perform better in colder and/or cloudier conditions than their flat panel counterparts. This is because of the vacuum in the glass tube, which allows tube collectors to retain a high percentage of collected heat. They work well in freezing conditions where flat panels will not work.

Design

Flat panels 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 panel unless corrosion results, and is mainly a cosmetic downfall.

Flat panel collectors – if damaged, will continue to function, and can at times be repaired. Other times, the entire flat panel 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.

Sizing

This is another very important consideration as incorrect sizing will affect the efficiency of the system. In general, it is important to allow about 70L per person per day. A household with 3 people would be able to manage with a 200L Solar Geyser, but 4 or more people will require a 300L option. If the volume is too small, the backup element will be used more frequently, whereas bigger water volume will be easier to manage.

Installation

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

Flat panel 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.

Summary

Flat panel collectors are best for users in the Highveld Area, whereas evacuated tube collectors are best for areas where there is much higher cloudy conditions.