The solar hot water panel (flat plate or evacuated tubes?)

You’ve got 2 choices when it comes to your solar hot water panel (or Collectors as they are called in the biz): Flat Plate or Evacuated Tube. Let’s describe each type.

Flat Plate Collectors

These are the most widely used used type of collector for domestic solar water heating. Flat plate collectors are shallow rectangular boxes with glass lids and usually aluminium for the body. Typically 1-1.5 meters wide by 2-2.5 metres long.

OK, so we’ve got a shallow aluminium box, on top of which will go a glass lid. Directly underneath the glass are placed copper pipes. These copper pipes carry the water through the collector and absorb the sun’s heat. A typical pattern of piping is shown here:

Now if we put these pipes in the box as they are, they will get our water hot, but they won’t be very efficient. In order to capture as much sun as possible, we want to maximise the surface area of the pipes that is exposed to the sun.

We do this by welding horizontal fins between the pipes.

Then we put some insulation in the bottom of the box to make sure the heat goes into the water, not through the panel and into our roof, and lay the copper assembly onto the insulation. We might paint the copper black so it absorbs even more heat, then pop the toughened glass on top and voila, there’s out flat panel solar hot water collector.

Here’s a cutaway of the finished panel:

And here’s the finished thing in all it’s glory:

As you can see, these are fairly simple devices which means that they are relatively cheap to manufacture and very reliable. They are also incredibly tough. A good flat plate collector would probably withstand a thermonuclear war (so at least the cockroaches could have a hot bath).

This (apart from the cockroach bit) probably explains their popularity in Australia.

The alternative to a flat plate collector is the evacuated glass tube collector. If you thought the Holden / Ford rivalry was bad – you want to see the evac tube / flat plate fans argue about which is best! I’ll just present the facts and let you decide, dear reader!

Evacuated Glass Tube Collector

Whilst flat plate collectors are all pretty similar in design, construction and performance, evacuated glass tubes can vary widely in their design and performance.

Evacuated Tube Collectors collect the heat through a number of annealed glass tubes that each have their own heat-absorbing plate inside. There are loads of different designs, but the basic principle is the same for all designs. Basically each tube is like a little greenhouse that traps sunlight inside the glass tube which is used to heat either the water directly, or a special heat transfer fluid that transfers the heat to the water.

The name comes from the fact that a vacuum (which is an excellent insulator) is created during manufacturing between the outer and inner glass tubes, this makes for a very efficient solar heater. And the fact that the tubes are round, not flat, means that they can efficiently collect heat no matter where in the sky the sun is. In contrast a flat plate collector is at its most efficient only when the sun is directly above the panel.

As you can imagine, these panels are quite tricky to manufacture, so they are typically more expensive than a similar quality flat plate collector. The are also more fragile, which is pretty obvious just by looking at them!

So why do people choose to pay more for an evacuated tube system?

Well the main reason is that good ones will give you more hot water in colder parts of Australia. They get more heat from the sun for a longer period of the day and their improved performance is most noticeable through the colder seasons and colder climates. This ultimately means that you will use less of your electric or gas booster if you live in those cllimates.

On top of that the round tubes are excellent at self cleaning when it rains also helping keep your system as efficient as possible.

So to sum up: you will get more solar heated hot water from an evacuated tube panel if you live in colder climes, and they look damn sexy (don’t ya think). But… you will pay more for a good one and you can’t ignore the fact that they are more fragile than a flat plate collector.

What should you go for? Flat Plate or Evacuated Tube?

To help you decide, here is a picture from the Alternative Technology Association (ATA) that shows how much of your hot water you can expect to be heated by the sun if you have a flat plate collector on your roof:

If you want to get more of your hot water solar heated than the percentage given on this map for your location, then you should seriously consider evacuated tubes or even a heat pump.

I would argue that in QLD, NT and WA, evac tubes are overkill. In southern NSW and SA they are worth considering and in VIC and Tassie they may be your best bet. Oh boy how much touble am I gonna get in from both the Evac Tube and Flat Plate vendors/evangelists for making that call!

OK so once you have decided which panel to plump for, then next decision is this: “How do you want to get the cold water

a) into the panel? and

b) out of the panel into the storage tank?”

This all depends on where you put the water storage tank relative to the panel…

Read next: Thermosiphon or split system (where do you want your hot water tank?)