What type of solar panels do we use?
We use the evacuated tube solar panels, supplied by Ecologics.
While the alternative flat plate technology is perfectly satisfactory, evacuated tubes have the following advantages:
- Better performance in sub-optimal conditions so characteristic of Irish weather (hazy cloud, intermittent sunshine, wind)
- Better performance off-peak, when the sun is lower in the sky; more information
- It is straightforward and non-disruptive to replace tubes individually (subject only to access)
- The panels are easier and therefore cheaper to install
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 graphically 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%; even at due east or due west performance can be acceptable
- 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
- Evacuated tubes perform somewhat better than flat plates at sub-optimal angles; more information
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 controller are needed
- Mount the panel on a flat roof or on the ground, using a freestanding frame to tilt the tubes
- Mount the panels vertically on a south facing wall
What size of solar panel is needed?
There are now two sizes of evacuated tubes, 47mm and 58mm; the 58mm size is also longer, and offers approximately 50% better performance – per tube. Solar panels are available in 10, 20 or 30 tube frames. A solar panel system with one or two panels, in any combination – for a system with between 20 and 60 tubes – depending on the size of tube used – is suitable for a normal domestic installation, with a hot-water cylinder of 150 – 300 litres. For a given panel size, a smaller cylinder will deliver a smaller quantity of hotter water; the larger the cylinder the more often the backup heating system (electricity or oil / gas boiler) will be needed. 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.
