There are two types of solar thermal collectors.  These are flat plate collectors and evacuated tube arrays.  

Flat Plate Collectors

Flat plate collectors were developed by Hottel and Whillier in the 1950s and are the most common type.  They consist of a dark flat plate absorber of solar energy and a transparent cover that allows solar energy to pass through but reduces heat loss.  There is also a heat transport fluid which can be air, glycol or water which removes heat from the absorber and a heat insulating backing.  The fluid often use is commonly known as glycol which is a type of anti-freeze.  Most flat plate collectors have a life expectancy of over 25 years.

The flat plate collectors, whilst a completely different technology, look similar to photovoltaic models to the untrained eye in respect of shape and mounting as they canbe mounted either on roof, in roof or on A-Frames.  They are however quite different and are also larger in size and heavier in weight.  You do not need as many solar thermal collectors in comparison to solar photovoltaics and most domestic installations will only require one or two collectors.  The amount of collectors will depend upon the amount of people occupying your household, your hot water usage and if it is to be connected to a heating supply. The image on the right shows flat plate collectors on display in our showroom.

Evacuated Tube Collectors

Evacuated tube collectors come in arrays of varying numbers of tubes often ranging from six to twelve.  This means that you may only need one tube array.  Evacuated tube collectors use heat pipes for their core instead of passing liquid directly through them.  Evacuated heat pipe tubes are composed of multiple evacuated glass tubes each containing an absorber plate fused to a heat pipe.  The heat from the hot end of the heat pipes is transferred to the transfer fluid (water or but typically glycol) of a domestic hot water or space heating system.  The vacuum that surrounds the outside of the tube greatly reduces convection and conduction heat loss to the outside.  This achieves greater efficiency especially in colder conditions.  The image on the left shows an evacuated tube collector on display in our showroom.

Comparisons of Flat Plate and Evacuated Tube Collectors

Aside from the differences in how the two types of collectors work, the two types of collectors still provide the same result - hot water heating and space heating.  However comparisons between flat plates and evacuated tubes are heavily debated and which type is the best is certainly a moot point.

It is argued that evacuated tubes are more efficient than flat plate collectors but flat plate collector supporters will certainly contest this point.  The choice between which collectors are chosen is often based down to personal preference and aesthetics playing a large part in the decision.  However given the larger absorber area of the evacuated tubes, they are more likely to maintain their efficiency over a wide range of ambient temperatures and heating requirements.  In constantly sunny climates flat plate collectors are more efficient whereas in more cloudy conditions their energy output drops off rapidly in comparison with evacuated tubes.

We have however prepared a table below of some of the comparisons so you can join the debate..

Flat Plate Collectors

Heat Distribution - Direct or Indirect

Solar heating primary circuits transfer heat from the solar collectors to the pre-heat cylinder.  This can be 'direct', or most commonly 'indirect'.

Direct

Direct provides simplicity and increased efficiency over secondary circuits by way of reduction of heat transfer loss.  However a direct circulation system is prone to freezing unless the water is drained back when the pump switches off.  This can put constraints on the positioning of the collectors in relation to the feed tank.  Additionally, as new water continually flows through the collectors, they can be prone to 'furring' in the collector waterways resulting in a loss of efficiency.

Indirect

The majority of circulations in the UK are indirect.  These circuits use a separate 'heat transfer fluid' circuit to transfer the heat from the collectors to the pre-heat cylinder.  The main advantage of this type of circuit is that they can employ a wide range of materials and fluids as part of the circulation and there are different types of circulation that can be used:

Passive circulation (or Gravity circulation) - relies on gravity and tendency for water to naturally circulate as it is heated.  They contain no electric components and are generally more reliable, easier to maintain and possibly longer lasting than active systems.  The downside is that careful planning is needed to optimise performance, sluggish performance is likely if there is poor control of heating and the hot water storage tank must be located above the collector level.

Active circulation (or Pumped circulation) - a pumped indirect circuit incorporating a heat transfer fluid including anti-freeze and corrosion inhibitor.  This is the most popular type in the UK and the most commonly installed by Ecofurb (UK) Ltd.  The pump, controlled by a differential temperature controller circulates the fluid from the collector panels through the heat exchanger in the hot water cylinder and back to the solar collectors for re-heating.  This type of circulation requires electricity, is more complex and is more expensive.  In comparison it provides integral protection against freezing, controls overheating, the heat is delivered at the optimal rate, heat loss is reduced through pipes and there is a greater choice of collector and pipe layout.

Storage Tank Configurations

The pre-heat configuration for most solar water heating systems can be achieved in two ways.  Either a separate pre-heat cylinder may be placed between the existing cold water feed and the normal hot water storage.  Alternatively, the existing hot water storage cylinder can be replaced with a larger double heat exchange coil cylinder.  Whichever option is chosen extra storage volume is required and this is often the determining factor.