Solar Panels

What type of solar panels do we use?

We use the evacuated tube solar panels

While the alternative flat plate technology is perfectly satisfactory, evacuated tubes have the following advantages:

  • The panels are easier (lighter) to handle and therefore easier and cheaper to install
  • It is usually possible to avoid the use of Glycol anti-freeze, see Control Systems
    • Using water avoids problems (of glycol degradation) with stagnation; this in turn allows the use of larger solar panels (more tubes), hence improving performance
    • Using water avoids the need for (and cost of) regular replacement of Glycol
  • The panels are smaller (per kWh of yield, than flat plates), therefore it is easier to find the necessary roof space
    • Many Irish houses have dormer and Velux windows, along with chimneys, which can significantly reduce available roof space
  • Slightly better performance in sub-optimal conditions so characteristic of Irish weather (hazy cloud, intermittent sunshine, wind)
  • It is straightforward and non-disruptive to replace tubes individually (subject only to access)

How should the solar panel be oriented?

There are actually two angles to consider, the horizontal orientation and the angle of tilt.

  • The ideal horizontal orientation is obviously south; in practice, anything between southeast and southwest is satisfactory
  • The ideal angle of tilt is approximately equal to the latitude i.e. about 55 degrees in Ireland; the angle of tilt of the average roof is acceptable and is normal practice
  • Non-optimal orientation can be compensated for by a larger panel area

This diagram shows how performance is affected by the angles of orientation and tilt, it can be seen that:

  • Deviation from south by 45 degrees (i.e. SE or SW) reduces performance by only about 20%; however, performance declines steeply outside 45 degrees
  • A shallower tilt favours a non-optimal orientation; however, because the winter sun is lower in the sky, a steeper tilt favours winter performance over summer
  • There is a marginal preference of west over east

In practice, simplicity is more important than angles and panels are usually put on the the best available roof; other possible solutions are:

  • On an east-west facing roof, mount panels on the west roof; alternatively put panels on both sides; more plumbing and a dual pump / controller are needed – which is more expensive
    • The steeper the roof the stronger the case for a dual system
  • Mount the panel on a flat roof or on the ground, using a free-standing frame to tilt the tubes
  • Mount the panels vertically, or – with a frame – at an angle, on a south facing wall

What size of solar panel is needed?

The standard tube is 58mm (in diameter); panels are available with 20 or 30 tubes; one, or more likely two, panels will be needed. The older evacuated tubes were 47mm in diameter, and yielded about 2/3 of the energy, per tube. See also Facts & Figures.

How does a solar panel work?

A solar panel consists of a number of separate tubes; these are glass tubes about 2m long, somewhat thicker than a standard fluorescent tube. A tube consists of three basic components: a twin-walled evacuated glass envelope, blackened to absorb sunlight on the inside, aluminium fins to conduct the heat, and a copper pipe running down the centre, which has a protruding bulb at one end. Water inside the copper pipe transfers the heat up the tube to the bulb; the copper pipe is partially evacuated, which means the water boils at a low temperature; this optimises the heat transfer to the bulb. The evacuated glass tube significantly reduces convective heat loss. A well-insulated heat exchange manifold collects the heat from the bulb and transfers it to the circulating water. Water does not flow through the tube itself. See also Solar Overview.

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