Solar Thermal Collectors FAQs

What is the difference between a flat plate collector and an evacuated tube type?

There are four types of evacuated tube panels, being:

Heat Pipe

This is a thin copper tube that is inserted down the vacuum tube. The heat transfers to the copper tube, up to the bulbus part. This inserts into the manifold. These collectors need to be in an upright position, to allow the heat to transfer to the manifold to gain maximum efficiency. This is an old technology and is being superceeded by the others. This system was designed for thermal siphon systems, where the heat pipe was connected directly to a hot water cylinder.

Double Manifold

Usually this is 2 x 22mm pipes running through the manifold, coming off each of the pipes are between 8-100mm copper tubes that pass down one side of the vaccum tube and back up from the tube to the other side of the copper tube creating a u-tube. As fluid passes down one side of the manifold it picks up the heat transfer from the vacuum tube and places it to the required heating source. These are very good collectors for large collector arrays. A connection is only required on one side of the collector, which reduces wasted pipe work. Also a Tichelmann arrangement can be done. Maximum collector array of 6. Also there is another collector that has three manifolds, but the third manifold was to pass the fluid back directly to the heating source.

Mirandum

This is a snake like layout that runs up and down each of the vacuum tubes. The transfer on this system is quite high, and not recommended to connect many of these collector arrays due to pressure loss within the system. This has pipe connections at either end, too many collectors can cause the system to overheat before it has heated the heating source. This type of panel is becoming more popular and is also a cheaper collector than the above. This comes in two ranges; copper and stainless steel.

Direct Heat

These collectors can be laid vertical or horizontal, and are a pipe in pipe system. The cold fluid runs within the inner pipe and picks up the heat transfer from the vacuum tube on the outer pipe. They are becoming popular in the UK as people like to have these fixed in a facade position or mounted on a flat roof so they are not visible from street view.

There are two types of Flat Plate Collectors:

Harp

This collector can have up to four connections and can be used for drainback systems. It is a very simple technology and has been around for many years. It is a very reliable product. The absorber can be made of copper or aluminium, the better of the two being copper as normally the pipe work is also made of copper and it is not good practice to mix metals i.e. electrolosis. These are normally the lower cost collector. They can be fitted on-roof, in-roof or facade, horizontal or vertical.

Mirandum

This flat plate panel is the same principle as the evacuated tube collector, but with a flat absorber and a glass covering. They are available as glazed or unglazed. Also fitted on-roof, in-roof or facade.

The efficiency between evacuated tube and flat plate collector is minimal. The running cost for an evacuated tube is much higher due to switching on and off at low level lights during the winter months.