What are the differences between Flat Plate Glass Collectors and Evacuated Tubes?

Up until the 90's there was only one type of solar hot water collector available


The two types of solar collectors available are flat plate glass and evacuated tube.
Flat Plate Glass Collectors (FPG Collectors)
 The collector box is constructed of a metal, weatherproofed and insulated box covered by a glass sheet. Within the collector box copper pipes are arrayed in parallel underneath the black metal heat absorber sheet.

Water is heated as solar energy is exchanged from the absorption sheet to the copper pipes through which water passes. As the water is heated it rises to either the storage tank above the panels or, is mechanically circulated to the storage tank on the ground. Cooler water in the storage cylinder is displaced to the collectors by returning hot water from the solar collectors.

FPG collectors were first introduced to the Australian market 60 years ago. Typically there are 2 to 3 flat plate glass collector panels required for the average household.

Urban myth: often information is cited to present FPGC’s as having a higher output however most technical performance graphs indicate the only time FPGC’s briefly outperform evacuated tubes is in summer when the noon day sun is directly above the panel.

Greatest risks: rust of the collector box, frost, internal calcification of copper manifold and risers, hail and the accumulation of dust and moss on the glass which also supports the on set of rust.

Evacuated Tubes (ET’s)
Background: invented and patented by Dr David Miller and his team at Sydney University in the mid 70’s it was initially ignored by Australian manufacturers who argued they were too efficient (!) However ET’s were soon adopted and applied by overseas companies for solar hot water systems.

In recognition to the origin of the evacuated tube it is often referred to as the Sydney Evacuated Tube.
ET’s have set new benchmark for efficiency in solar hot water systems making them the technology of choice in northern hemisphere countries where winters can be more severe than Australian winters. The adoption and acceptance of ET’s in the northern hemisphere has fuelled demand in Australia.
Versatile: easily adapted ET’s are for also used for pool, spa heating, floor heating (hydronics) and space heating.

Description: ET’s are made from two toughened borosilicate glass tubes also known as Pyrex, known for its ability to withstand high temperatures and resistance to breakage.

The outer tube is transparent allowing sunlight to pass through with minimal reflection. The inner tube is coated with a special coating, which allows for excellent solar radiation absorption and near zero reflection of solar radiation.

The tops of the two tubes are fused together and the air contained in the space between the two layers of glass is extracted (evacuated), to create a vacuum.
Insulation: an evacuated space is the principle by which the ‘Thermos Flask’ keeps the coffee hot. The very high insulation properties of a vacuum enable the evacuated tube to actually store heat like a thermos flask.

The vacuum is the most efficient insulation because heat cannot transfer across a vacuum. A vacuum cannot be heated which means that none of the sun’s energy (heat value) is lost to the atmosphere.
Efficiency: because of the vacuum radiation passes without interruption directly to the heat absorption coating with the minimum loss of energy. 97% of the sun’s radiation to passes to the absorber coating maximising the collection of solar energy which can be harvested. At the same time the vacuum insulated and prevents heat loss.

Key Points: cylindrical shape offers more useable surface area on the ET because the sun is always perpendicular to the evacuated tubes. ET’s are frost protected to minus 25 degrees Celsius, have excellent insulation properties and will not rust.