The Solar Technology

How the Collector Tube Works

A double walled glass tube with a vacuum between the walls contains a heat pipe which transfers heat from the tube to the water in the manifold of the panel.  The purpose of the vacuum is to trap heat which has entered the tube as sunlight.  Heat can move about using three mechanisms, conduction, convection and radiation.  Energy enters as radiation from the sun, which can cross the vacuum without difficulty (after all, it has already crossed 150 million km of space!).  The vacuum prevents the heat leaving via conduction or convection, just like a thermos flask.  The inner tube has a special selective coating which absorbs over 93% of the radiation falling on it, effectively preventing radiation losses.

The heat builds up quickly inside the tube, and warms the heat pipe via thin aluminium fins which also hold the heat pipe in the centre of the tube.  The heat pipe is a copper tube that runs inside the glass tube.  It is held in the glass tube using a silicone rubber bung.  Note that no seal is required here, in fact a small hole allows the pressure inside the tube to be relieved as the pipe warms up.  The heat pipe contains a liquid at low pressure which boils at low temperature.  Because it becomes less dense as a gas, it rises up the heat pipe (which explains why the panel must be installed with a minimum slope of 15°).  At the top of the heat pipe is a bulb which fits into a copper socket in the manifold.  Because it is cooled by the water flowing through the manifold, the liquid condenses on the inside of the bulb, thereby transferring heat energy to the water flowing through the manifold.  Good thermal contact between the heat pipe bulb and the manifold socket is ensured by the use of thermal transfer paste supplied with the panel.

Note that the base of the glass tube has a silver coating; this is barium, used to maintain the vacuum between the glass tubes during manufacture (the same technique is used to manufacture CRT TV tubes).  Should the vacuum be lost, if, for example the tube is cracked, this silver coating turns to a white powdery material, provided a simple check on the integrity of the vacuum.

Layout Schematic

Typical Layout

1 Evacuated Tube Collectors

2 Electronic Controller

3 Hot Water Tank

4 Pump Station

5 Expansion Vessel

6 Solar Heat Exchanger

7 Boiler Heat Exchanger

8 Boiler

9 Cold Water Inlet

10 Temperature Sensors

For the sake of clarity, a heat dump circuit is not shown on this diagram. It would normally be T'd off from the solar circuit so that it bypassed the solar heat exchanger but still incorporated the other components, as shown in the following schematic.

The heat dump circuit is shown in blue.  Note that this diagram does not represent the physical positioning of the various components, the heat dump radiator is normally placed in an attic. The components provided in a typical pumping station such as a Flowcon A are shown outlined in green. The red lines represent electrical connections.  The solar plumbing is essentially the same when using an unvented, pressurised hot water cylinder, where the cylinder is fed directly from the water main and the cold water tank is absent.  In this case various components not shown on the schematic, including an additional expansion vessel, are required to ensure the safe operation of the cylinder. Note that the various plumbing components are on the return leg to the collector, which is cooler than the hot water cylinder feed (the solar fluid circulates in an anticlockwise direction here).

Positioning The Collector

The Evacuated tube collector includes a stainless steel frame which is designed to be mounted on a pitched roof.  In order for the heat pipe to work, the collector must be between 15° and 90° to the horizontal.  In order to collect the maximum amount of heat, (here in the Midlands, UK) it should be at an angle of around 30° to the horizontal.  However, up to 15° either way makes little difference, which conveniently includes most standard roof pitches.  All things being equal, a steeper inclination will enhance the winter heat collection, and a flatter one the summer.  Bear in mind though that the great majority of insolation occurs in the summer, so optimising for the winter will reduce the net energy collected through the year.  Similarly, the roof should ideally face due south, but anywhere between southeast and southwest will give satisfactory results.  Choose a position on the roof which is not shaded, and consider that shading in a particular place may be more severe in winter when the sun is lower in the sky.

Collector Arrays

Two large collectors may be plumbed together in series, but for an array of more than two (for commercial or other high volume use) it is recommended that they should be connected in groups of two in parallel to reduce pumping losses, and also reduce the risk of overheating in summer.

Two Panels Plumbed in Series

Four Panels Plumbed Two by Two in Parallel

Hot Water Cylinder

The solar hot water cylinder has two heat exchange coils within it.  The upper coil is connected to a conventional boiler, to provide hot water at times when the solar provision is inadequate.  The lower coil is connected to the solar circuit (the water in the lower half of the tank will be cooler than that in the upper half (warmer water rises to the top of the tank) which enhance the heat transfer from the solar circuit).  A standard central heating pump is used to circulate the solar fluid, usually set to its lowest setting.

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