Flat Plate Solar Panels

Flat Plate Solar Panels and Evacuated Tubes

Flat Plate Solar Panels installed on a roof in Dun laoghaire, Dublin.  Sun setting reflecting on the panels.

3 x Flat Plate Solar Panels heating a 300L cylinder installed in Dun Laoghaire, Co Dublin

It has been an enduring question in the solar thermal industry in Ireland for a while, which are the better solar panels? – Flat plate solar panels or Evacuated Tubes? A recent study by the Energy Trust in the UK seems to have dispelled the mystery and provided us with the answer – neither.. The reason is that although evacuated tube solar panels are more productive per m2 of aperture area, the fact that the you can install a far greater total aperture area using flat plate solar panels negates this advantage. We have been of this opinion in Glenergy for quite a while now and happily supply flat plate solar panels or evacuated tubes as required.  To help you make an informed decision we lay out some more information on flat plate solar panels and the information contained in the energy trust report.

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Flat Plate Solar Panels - Prices, pros and cons.

Flat plate solar panels are generally cheaper than evacuated tubes but produce about the same amount of energy on an annual basis -  see some guide prices on installed flat plate solar panels here

Other advantages of flat plates are:

  • Longevity - Flat plates tend to last a bit longer than tubes with design lives in excess of 30 years
  • Aesthetics - Some prefer the looks of flat plate solar panels
  • Lower stagnation temperature - flat plate solar panels do not reach the high temperatures that evacuated tubes reach.  This can result in lower maintenance requirements.

Some things to consider:

  • Seasonal production: Flat plates operate very well in the warmer months of the year (Apr - Oct) but don't dare as well as tubes in the winter.  This is fine for most systems but if winter yield is important then tubes may be the right choice.
  • In roof or on roof: Flat plates can be mounted in -roof like a velux window or on - roof on brackets like evacuated tubes.  We recommend mounting on-roof as this allows for easier and cheaper maintenance should it be needed in years to come.

The Energy Trust report.

The Energy Trust report entitled: “Here comes the sun – a field trial of solar water heating systems” is a comprehensive study of 88 solar thermal systems installed all over the UK and Ireland. 34 of these were evacuated tube systems and 54 used flat plate solar panels. The study found that there was no real difference in the total energy yield from each type of solar panel system. They said:

“There was found to be no difference in the amount of energy produced by flat-plate solar collector systems and evacuated-tube systems. The median for flat-plate collectors was 1,156kWh per year and the median for evacuated-tubes 1,140kWh per year. Although evacuated tube systems have higher insulation, flat-plate solar collectors generally have a larger working area as a proportion of the collector size. This was supported by what we found in the trial.”

There is a common tendency to assume that evacuated tubes are more productive because they are more “efficient” but as is pointed out in the study this does not mean that the system will produce more energy than a suitably sized flat plate solar panels. So be wary when someone says that their solar system is more efficient than the next guy’s as this doesn’t necessarily mean they can produce more energy – always ask for evidence of the energy yield of the entire system..

The importance of the cylinder and controls.

Some other interesting points were raised in the report including the importance of having a well insulated solar cylinder and the impact the timing of auxiliary heating had on the performance of the solar system. The main points were:
1. A well insulated solar cylinder is paramount. Heat loss from the cylinder could outstrip the energy collected form the solar panels making the solar panels useless – a suitably insulated cylinder has to be installed with a system to make it viable.
2. The timing of the back up heating. By making sure that the secondary source of heat (immersion, gas/oil boiler etc..) comes on after the solar panels have delivered as much heat as possible, increased the performance of the system. This is because solar panels require a low temperature in the bottom half of the cylinder to be as productive as possible. The study recommended that in solar panel systems it is best to wait till the evening or very early morning before putting top-up heat into the cylinder from auxiliary sources.
3. 84% of householders were either fairly or very satisfied with their systems with 50% being very satisfied! Overall customers are happy with their solar systems.

Flat Plate Solar panels field study

Energy Trust