Solar Water

As anyone reading this can quickly find out, the cost of solar panels or collectors (for hot water generation) can vary from around £500 to over £1500 each. However, they can be very different in performance. Also, before any comparisons can be made you need to know exactly what it is you are investing your money in. There are essentially two common types i) flat plate and ii) evacuated tubes (both with or without Heat Pipes).

Flat Plate. This is the most common type of collector due to its simplicity. They are 'low tech' and therefore cheap to build (although that does not necessarily mean they are cheap to buy). They basically consist of a weatherproof box (some, but not all are double glazed) with a dark absorber plate under a transluscent top plate. Water or a heating fluid, usually a glycol mix, passes through copper piping within the box and is heated as the sun's rays hit the box. It is very simple but serious questions, in our opinion, have to be asked as to whether this type of technology is suitable for installations in our climate. They provide useful amounts of heat in the summer months but demonstrate high heat loss in cold or even windy weather and, in an average English Winter, their heat contribution can be negligible.

Evacuated Tubes. The type of technology utilised within the evacuated tubes has been around for the last 20 years or so and has a proven track record. Consequently, it is considered far more efficient than the flat plate alternative. They are available with or without heat pipes, the latter being 15-20% more efficient as a general rule. They can also come as a single or double wall vacuum with glass-metal joints or glass-glass. The glass-metal is slightly more efficient but can be susceptible to loss of vacuum over time. This is because the heat expansion and subsequent contraction of the glass and metal are different, which, when repeating on a daily basis, can cause the seal to fail. Because of this, we have opted to market the evacuated tube panels comprised of double wall, very strong, borosilicate glass with a glass-glass seal. This should give a working lifespan of at least 20 years.

Inside the evacuated tubes, on the innermost lining, is a selective absorber coating usually composed of aluminium – nitrogen – aluminium. A newer technology coating uses copper, aluminium and titanium which gives a higher heat efficiency resulting in the tubes being able to withstand up to 400c and therefore greatly extending the life of the tube. These better quality coatings have absorptance rates of 96% of the light energy that hits the surface of the tubes. The vacuum within the tubes prevents any losses of energy through conduction or convection resulting in this type of tube being extremely efficient and able to work as well at 0c as they do at 25c – ideal for all year round heat production in cooler climates such as ours. If a tube is placed in direct sunlight on a winter's day, the tip can reach temperatures of more than 100c, so this type of system can easily heat water in your cylinder to 60c, even in cooler weather.

Vacuum tubes come with or without heat pipes. The tubes without heat pipes cost less to manufacture (copper is becoming increasingly expensive) and should, therefore, be less to purchase, but this is not always the case. Our belief is that with any investment in an alternative energy system, it makes sense to obtain the most efficient with the money available. Heat pipe technology increases the efficiency of the tubes by 15-20%.

So, how do heat pipes work? The copper heat pipe within the vacuum tube is also evacuated and contains a small amount of water or a non toxic water/solar antifreeze (heating fluid) mix. When light energy hits the tube it is absorbed and transferred as heat to the heat pipe, which, because of the vacuum, boils the heating fluid within it at much lower temperatures than normal (around 30c). This causes the fluid to vaporise and rise up the pipe transferring heat to the tip (condenser) within the Manifold Header. As the heat is transferred within the condenser, thereby heating the domestic water very rapidly, the heating fluid cools, condenses and returns to the bottom of the pipe where the process begins again.

There are many evacuated tubes on the market without heat pipes and a method for heating the water, employed by some manufacturers, is known as "direct". This works by potable water being circulated through the actual tubes rather than through a manifold as with the heat pipe method. The advantage here is that there is slightly less heat loss compared with heat pipes, although a big disadvantage is that if a tube breaks, water from your cylinder will be pouring onto the roof. The system would have to be shut down until repaired (if it does not actually have to be replaced) which would obviously be very costly. The beauty of the heat pipe tubes is that if in the unlikely event of one failing it is very simple, quick and relatively cheap to replace. A bit like replacing a light bulb.

Conclusions. Back in 1999 when we were first conducting our research, it was quite easy when studying the merits of each system to dismiss the flat plate method as 'dinosaur technology' - great for other, warmer, parts of the world but generally impractical for UK installations. We have consequently decided to market evacuated tubes both with and without heat pipes (although we believe the latter heat pipe option offers the customer the best return for the money invested and this is the technology we would like to see developed within the UK).